Mercurial > hg > graal-compiler
diff src/share/vm/runtime/synchronizer.cpp @ 1930:2d26b0046e0d
Merge.
author | Thomas Wuerthinger <wuerthinger@ssw.jku.at> |
---|---|
date | Tue, 30 Nov 2010 14:53:30 +0100 |
parents | b30a2cd5e3a2 fa83ab460c54 |
children | 06f017f7daa7 |
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--- a/src/share/vm/runtime/synchronizer.cpp Mon Nov 29 18:32:30 2010 +0100 +++ b/src/share/vm/runtime/synchronizer.cpp Tue Nov 30 14:53:30 2010 +0100 @@ -1,5 +1,5 @@ /* - * Copyright 1998-2009 Sun Microsystems, Inc. All Rights Reserved. + * Copyright (c) 1998, 2009, Oracle and/or its affiliates. All rights reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it @@ -16,9 +16,9 @@ * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * - * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara, - * CA 95054 USA or visit www.sun.com if you need additional information or - * have any questions. + * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA + * or visit www.oracle.com if you need additional information or have any + * questions. * */ @@ -32,15 +32,12 @@ #define ATTR #endif -// Native markword accessors for synchronization and hashCode(). -// // The "core" versions of monitor enter and exit reside in this file. // The interpreter and compilers contain specialized transliterated // variants of the enter-exit fast-path operations. See i486.ad fast_lock(), // for instance. If you make changes here, make sure to modify the // interpreter, and both C1 and C2 fast-path inline locking code emission. // -// TODO: merge the objectMonitor and synchronizer classes. // // ----------------------------------------------------------------------------- @@ -53,16 +50,6 @@ jlong, uintptr_t, char*, int, long); HS_DTRACE_PROBE_DECL4(hotspot, monitor__waited, jlong, uintptr_t, char*, int); -HS_DTRACE_PROBE_DECL4(hotspot, monitor__notify, - jlong, uintptr_t, char*, int); -HS_DTRACE_PROBE_DECL4(hotspot, monitor__notifyAll, - jlong, uintptr_t, char*, int); -HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__enter, - jlong, uintptr_t, char*, int); -HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__entered, - jlong, uintptr_t, char*, int); -HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__exit, - jlong, uintptr_t, char*, int); #define DTRACE_MONITOR_PROBE_COMMON(klassOop, thread) \ char* bytes = NULL; \ @@ -99,61 +86,300 @@ #endif // ndef DTRACE_ENABLED -// ObjectWaiter serves as a "proxy" or surrogate thread. -// TODO-FIXME: Eliminate ObjectWaiter and use the thread-specific -// ParkEvent instead. Beware, however, that the JVMTI code -// knows about ObjectWaiters, so we'll have to reconcile that code. -// See next_waiter(), first_waiter(), etc. +// This exists only as a workaround of dtrace bug 6254741 +int dtrace_waited_probe(ObjectMonitor* monitor, Handle obj, Thread* thr) { + DTRACE_MONITOR_PROBE(waited, monitor, obj(), thr); + return 0; +} + +#define NINFLATIONLOCKS 256 +static volatile intptr_t InflationLocks [NINFLATIONLOCKS] ; + +ObjectMonitor * ObjectSynchronizer::gBlockList = NULL ; +ObjectMonitor * volatile ObjectSynchronizer::gFreeList = NULL ; +ObjectMonitor * volatile ObjectSynchronizer::gOmInUseList = NULL ; +int ObjectSynchronizer::gOmInUseCount = 0; +static volatile intptr_t ListLock = 0 ; // protects global monitor free-list cache +static volatile int MonitorFreeCount = 0 ; // # on gFreeList +static volatile int MonitorPopulation = 0 ; // # Extant -- in circulation +#define CHAINMARKER ((oop)-1) + +// ----------------------------------------------------------------------------- +// Fast Monitor Enter/Exit +// This the fast monitor enter. The interpreter and compiler use +// some assembly copies of this code. Make sure update those code +// if the following function is changed. The implementation is +// extremely sensitive to race condition. Be careful. + +void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock, bool attempt_rebias, TRAPS) { + if (UseBiasedLocking) { + if (!SafepointSynchronize::is_at_safepoint()) { + BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD); + if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) { + return; + } + } else { + assert(!attempt_rebias, "can not rebias toward VM thread"); + BiasedLocking::revoke_at_safepoint(obj); + } + assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); + } + + slow_enter (obj, lock, THREAD) ; +} + +void ObjectSynchronizer::fast_exit(oop object, BasicLock* lock, TRAPS) { + assert(!object->mark()->has_bias_pattern(), "should not see bias pattern here"); + // if displaced header is null, the previous enter is recursive enter, no-op + markOop dhw = lock->displaced_header(); + markOop mark ; + if (dhw == NULL) { + // Recursive stack-lock. + // Diagnostics -- Could be: stack-locked, inflating, inflated. + mark = object->mark() ; + assert (!mark->is_neutral(), "invariant") ; + if (mark->has_locker() && mark != markOopDesc::INFLATING()) { + assert(THREAD->is_lock_owned((address)mark->locker()), "invariant") ; + } + if (mark->has_monitor()) { + ObjectMonitor * m = mark->monitor() ; + assert(((oop)(m->object()))->mark() == mark, "invariant") ; + assert(m->is_entered(THREAD), "invariant") ; + } + return ; + } + + mark = object->mark() ; -class ObjectWaiter : public StackObj { - public: - enum TStates { TS_UNDEF, TS_READY, TS_RUN, TS_WAIT, TS_ENTER, TS_CXQ } ; - enum Sorted { PREPEND, APPEND, SORTED } ; - ObjectWaiter * volatile _next; - ObjectWaiter * volatile _prev; - Thread* _thread; - ParkEvent * _event; - volatile int _notified ; - volatile TStates TState ; - Sorted _Sorted ; // List placement disposition - bool _active ; // Contention monitoring is enabled - public: - ObjectWaiter(Thread* thread) { - _next = NULL; - _prev = NULL; - _notified = 0; - TState = TS_RUN ; - _thread = thread; - _event = thread->_ParkEvent ; - _active = false; - assert (_event != NULL, "invariant") ; + // If the object is stack-locked by the current thread, try to + // swing the displaced header from the box back to the mark. + if (mark == (markOop) lock) { + assert (dhw->is_neutral(), "invariant") ; + if ((markOop) Atomic::cmpxchg_ptr (dhw, object->mark_addr(), mark) == mark) { + TEVENT (fast_exit: release stacklock) ; + return; + } + } + + ObjectSynchronizer::inflate(THREAD, object)->exit (THREAD) ; +} + +// ----------------------------------------------------------------------------- +// Interpreter/Compiler Slow Case +// This routine is used to handle interpreter/compiler slow case +// We don't need to use fast path here, because it must have been +// failed in the interpreter/compiler code. +void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) { + markOop mark = obj->mark(); + assert(!mark->has_bias_pattern(), "should not see bias pattern here"); + + if (mark->is_neutral()) { + // Anticipate successful CAS -- the ST of the displaced mark must + // be visible <= the ST performed by the CAS. + lock->set_displaced_header(mark); + if (mark == (markOop) Atomic::cmpxchg_ptr(lock, obj()->mark_addr(), mark)) { + TEVENT (slow_enter: release stacklock) ; + return ; + } + // Fall through to inflate() ... + } else + if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) { + assert(lock != mark->locker(), "must not re-lock the same lock"); + assert(lock != (BasicLock*)obj->mark(), "don't relock with same BasicLock"); + lock->set_displaced_header(NULL); + return; + } + +#if 0 + // The following optimization isn't particularly useful. + if (mark->has_monitor() && mark->monitor()->is_entered(THREAD)) { + lock->set_displaced_header (NULL) ; + return ; + } +#endif + + // The object header will never be displaced to this lock, + // so it does not matter what the value is, except that it + // must be non-zero to avoid looking like a re-entrant lock, + // and must not look locked either. + lock->set_displaced_header(markOopDesc::unused_mark()); + ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD); +} + +// This routine is used to handle interpreter/compiler slow case +// We don't need to use fast path here, because it must have +// failed in the interpreter/compiler code. Simply use the heavy +// weight monitor should be ok, unless someone find otherwise. +void ObjectSynchronizer::slow_exit(oop object, BasicLock* lock, TRAPS) { + fast_exit (object, lock, THREAD) ; +} + +// ----------------------------------------------------------------------------- +// Class Loader support to workaround deadlocks on the class loader lock objects +// Also used by GC +// complete_exit()/reenter() are used to wait on a nested lock +// i.e. to give up an outer lock completely and then re-enter +// Used when holding nested locks - lock acquisition order: lock1 then lock2 +// 1) complete_exit lock1 - saving recursion count +// 2) wait on lock2 +// 3) when notified on lock2, unlock lock2 +// 4) reenter lock1 with original recursion count +// 5) lock lock2 +// NOTE: must use heavy weight monitor to handle complete_exit/reenter() +intptr_t ObjectSynchronizer::complete_exit(Handle obj, TRAPS) { + TEVENT (complete_exit) ; + if (UseBiasedLocking) { + BiasedLocking::revoke_and_rebias(obj, false, THREAD); + assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); } - void wait_reenter_begin(ObjectMonitor *mon) { - JavaThread *jt = (JavaThread *)this->_thread; - _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon); + ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj()); + + return monitor->complete_exit(THREAD); +} + +// NOTE: must use heavy weight monitor to handle complete_exit/reenter() +void ObjectSynchronizer::reenter(Handle obj, intptr_t recursion, TRAPS) { + TEVENT (reenter) ; + if (UseBiasedLocking) { + BiasedLocking::revoke_and_rebias(obj, false, THREAD); + assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); + } + + ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj()); + + monitor->reenter(recursion, THREAD); +} +// ----------------------------------------------------------------------------- +// JNI locks on java objects +// NOTE: must use heavy weight monitor to handle jni monitor enter +void ObjectSynchronizer::jni_enter(Handle obj, TRAPS) { // possible entry from jni enter + // the current locking is from JNI instead of Java code + TEVENT (jni_enter) ; + if (UseBiasedLocking) { + BiasedLocking::revoke_and_rebias(obj, false, THREAD); + assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); + } + THREAD->set_current_pending_monitor_is_from_java(false); + ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD); + THREAD->set_current_pending_monitor_is_from_java(true); +} + +// NOTE: must use heavy weight monitor to handle jni monitor enter +bool ObjectSynchronizer::jni_try_enter(Handle obj, Thread* THREAD) { + if (UseBiasedLocking) { + BiasedLocking::revoke_and_rebias(obj, false, THREAD); + assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); } - void wait_reenter_end(ObjectMonitor *mon) { - JavaThread *jt = (JavaThread *)this->_thread; - JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active); + ObjectMonitor* monitor = ObjectSynchronizer::inflate_helper(obj()); + return monitor->try_enter(THREAD); +} + + +// NOTE: must use heavy weight monitor to handle jni monitor exit +void ObjectSynchronizer::jni_exit(oop obj, Thread* THREAD) { + TEVENT (jni_exit) ; + if (UseBiasedLocking) { + BiasedLocking::revoke_and_rebias(obj, false, THREAD); + } + assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); + + ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj); + // If this thread has locked the object, exit the monitor. Note: can't use + // monitor->check(CHECK); must exit even if an exception is pending. + if (monitor->check(THREAD)) { + monitor->exit(THREAD); } -}; +} + +// ----------------------------------------------------------------------------- +// Internal VM locks on java objects +// standard constructor, allows locking failures +ObjectLocker::ObjectLocker(Handle obj, Thread* thread, bool doLock) { + _dolock = doLock; + _thread = thread; + debug_only(if (StrictSafepointChecks) _thread->check_for_valid_safepoint_state(false);) + _obj = obj; -enum ManifestConstants { - ClearResponsibleAtSTW = 0, - MaximumRecheckInterval = 1000 -} ; + if (_dolock) { + TEVENT (ObjectLocker) ; + + ObjectSynchronizer::fast_enter(_obj, &_lock, false, _thread); + } +} + +ObjectLocker::~ObjectLocker() { + if (_dolock) { + ObjectSynchronizer::fast_exit(_obj(), &_lock, _thread); + } +} -#undef TEVENT -#define TEVENT(nom) {if (SyncVerbose) FEVENT(nom); } +// ----------------------------------------------------------------------------- +// Wait/Notify/NotifyAll +// NOTE: must use heavy weight monitor to handle wait() +void ObjectSynchronizer::wait(Handle obj, jlong millis, TRAPS) { + if (UseBiasedLocking) { + BiasedLocking::revoke_and_rebias(obj, false, THREAD); + assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); + } + if (millis < 0) { + TEVENT (wait - throw IAX) ; + THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative"); + } + ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj()); + DTRACE_MONITOR_WAIT_PROBE(monitor, obj(), THREAD, millis); + monitor->wait(millis, true, THREAD); + + /* This dummy call is in place to get around dtrace bug 6254741. Once + that's fixed we can uncomment the following line and remove the call */ + // DTRACE_MONITOR_PROBE(waited, monitor, obj(), THREAD); + dtrace_waited_probe(monitor, obj, THREAD); +} -#define FEVENT(nom) { static volatile int ctr = 0 ; int v = ++ctr ; if ((v & (v-1)) == 0) { ::printf (#nom " : %d \n", v); ::fflush(stdout); }} +void ObjectSynchronizer::waitUninterruptibly (Handle obj, jlong millis, TRAPS) { + if (UseBiasedLocking) { + BiasedLocking::revoke_and_rebias(obj, false, THREAD); + assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); + } + if (millis < 0) { + TEVENT (wait - throw IAX) ; + THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative"); + } + ObjectSynchronizer::inflate(THREAD, obj()) -> wait(millis, false, THREAD) ; +} + +void ObjectSynchronizer::notify(Handle obj, TRAPS) { + if (UseBiasedLocking) { + BiasedLocking::revoke_and_rebias(obj, false, THREAD); + assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); + } -#undef TEVENT -#define TEVENT(nom) {;} + markOop mark = obj->mark(); + if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) { + return; + } + ObjectSynchronizer::inflate(THREAD, obj())->notify(THREAD); +} +// NOTE: see comment of notify() +void ObjectSynchronizer::notifyall(Handle obj, TRAPS) { + if (UseBiasedLocking) { + BiasedLocking::revoke_and_rebias(obj, false, THREAD); + assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); + } + + markOop mark = obj->mark(); + if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) { + return; + } + ObjectSynchronizer::inflate(THREAD, obj())->notifyAll(THREAD); +} + +// ----------------------------------------------------------------------------- +// Hash Code handling +// // Performance concern: // OrderAccess::storestore() calls release() which STs 0 into the global volatile // OrderAccess::Dummy variable. This store is unnecessary for correctness. @@ -185,756 +411,8 @@ } ; static SharedGlobals GVars ; - - -// Tunables ... -// The knob* variables are effectively final. Once set they should -// never be modified hence. Consider using __read_mostly with GCC. - -static int Knob_LogSpins = 0 ; // enable jvmstat tally for spins -static int Knob_HandOff = 0 ; -static int Knob_Verbose = 0 ; -static int Knob_ReportSettings = 0 ; - -static int Knob_SpinLimit = 5000 ; // derived by an external tool - -static int Knob_SpinBase = 0 ; // Floor AKA SpinMin -static int Knob_SpinBackOff = 0 ; // spin-loop backoff -static int Knob_CASPenalty = -1 ; // Penalty for failed CAS -static int Knob_OXPenalty = -1 ; // Penalty for observed _owner change -static int Knob_SpinSetSucc = 1 ; // spinners set the _succ field -static int Knob_SpinEarly = 1 ; -static int Knob_SuccEnabled = 1 ; // futile wake throttling -static int Knob_SuccRestrict = 0 ; // Limit successors + spinners to at-most-one -static int Knob_MaxSpinners = -1 ; // Should be a function of # CPUs -static int Knob_Bonus = 100 ; // spin success bonus -static int Knob_BonusB = 100 ; // spin success bonus -static int Knob_Penalty = 200 ; // spin failure penalty -static int Knob_Poverty = 1000 ; -static int Knob_SpinAfterFutile = 1 ; // Spin after returning from park() -static int Knob_FixedSpin = 0 ; -static int Knob_OState = 3 ; // Spinner checks thread state of _owner -static int Knob_UsePause = 1 ; -static int Knob_ExitPolicy = 0 ; -static int Knob_PreSpin = 10 ; // 20-100 likely better -static int Knob_ResetEvent = 0 ; -static int BackOffMask = 0 ; - -static int Knob_FastHSSEC = 0 ; -static int Knob_MoveNotifyee = 2 ; // notify() - disposition of notifyee -static int Knob_QMode = 0 ; // EntryList-cxq policy - queue discipline -static volatile int InitDone = 0 ; - - -// hashCode() generation : -// -// Possibilities: -// * MD5Digest of {obj,stwRandom} -// * CRC32 of {obj,stwRandom} or any linear-feedback shift register function. -// * A DES- or AES-style SBox[] mechanism -// * One of the Phi-based schemes, such as: -// 2654435761 = 2^32 * Phi (golden ratio) -// HashCodeValue = ((uintptr_t(obj) >> 3) * 2654435761) ^ GVars.stwRandom ; -// * A variation of Marsaglia's shift-xor RNG scheme. -// * (obj ^ stwRandom) is appealing, but can result -// in undesirable regularity in the hashCode values of adjacent objects -// (objects allocated back-to-back, in particular). This could potentially -// result in hashtable collisions and reduced hashtable efficiency. -// There are simple ways to "diffuse" the middle address bits over the -// generated hashCode values: -// - -static inline intptr_t get_next_hash(Thread * Self, oop obj) { - intptr_t value = 0 ; - if (hashCode == 0) { - // This form uses an unguarded global Park-Miller RNG, - // so it's possible for two threads to race and generate the same RNG. - // On MP system we'll have lots of RW access to a global, so the - // mechanism induces lots of coherency traffic. - value = os::random() ; - } else - if (hashCode == 1) { - // This variation has the property of being stable (idempotent) - // between STW operations. This can be useful in some of the 1-0 - // synchronization schemes. - intptr_t addrBits = intptr_t(obj) >> 3 ; - value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom ; - } else - if (hashCode == 2) { - value = 1 ; // for sensitivity testing - } else - if (hashCode == 3) { - value = ++GVars.hcSequence ; - } else - if (hashCode == 4) { - value = intptr_t(obj) ; - } else { - // Marsaglia's xor-shift scheme with thread-specific state - // This is probably the best overall implementation -- we'll - // likely make this the default in future releases. - unsigned t = Self->_hashStateX ; - t ^= (t << 11) ; - Self->_hashStateX = Self->_hashStateY ; - Self->_hashStateY = Self->_hashStateZ ; - Self->_hashStateZ = Self->_hashStateW ; - unsigned v = Self->_hashStateW ; - v = (v ^ (v >> 19)) ^ (t ^ (t >> 8)) ; - Self->_hashStateW = v ; - value = v ; - } - - value &= markOopDesc::hash_mask; - if (value == 0) value = 0xBAD ; - assert (value != markOopDesc::no_hash, "invariant") ; - TEVENT (hashCode: GENERATE) ; - return value; -} - -void BasicLock::print_on(outputStream* st) const { - st->print("monitor"); -} - -void BasicLock::move_to(oop obj, BasicLock* dest) { - // Check to see if we need to inflate the lock. This is only needed - // if an object is locked using "this" lightweight monitor. In that - // case, the displaced_header() is unlocked, because the - // displaced_header() contains the header for the originally unlocked - // object. However the object could have already been inflated. But it - // does not matter, the inflation will just a no-op. For other cases, - // the displaced header will be either 0x0 or 0x3, which are location - // independent, therefore the BasicLock is free to move. - // - // During OSR we may need to relocate a BasicLock (which contains a - // displaced word) from a location in an interpreter frame to a - // new location in a compiled frame. "this" refers to the source - // basiclock in the interpreter frame. "dest" refers to the destination - // basiclock in the new compiled frame. We *always* inflate in move_to(). - // The always-Inflate policy works properly, but in 1.5.0 it can sometimes - // cause performance problems in code that makes heavy use of a small # of - // uncontended locks. (We'd inflate during OSR, and then sync performance - // would subsequently plummet because the thread would be forced thru the slow-path). - // This problem has been made largely moot on IA32 by inlining the inflated fast-path - // operations in Fast_Lock and Fast_Unlock in i486.ad. - // - // Note that there is a way to safely swing the object's markword from - // one stack location to another. This avoids inflation. Obviously, - // we need to ensure that both locations refer to the current thread's stack. - // There are some subtle concurrency issues, however, and since the benefit is - // is small (given the support for inflated fast-path locking in the fast_lock, etc) - // we'll leave that optimization for another time. - - if (displaced_header()->is_neutral()) { - ObjectSynchronizer::inflate_helper(obj); - // WARNING: We can not put check here, because the inflation - // will not update the displaced header. Once BasicLock is inflated, - // no one should ever look at its content. - } else { - // Typically the displaced header will be 0 (recursive stack lock) or - // unused_mark. Naively we'd like to assert that the displaced mark - // value is either 0, neutral, or 3. But with the advent of the - // store-before-CAS avoidance in fast_lock/compiler_lock_object - // we can find any flavor mark in the displaced mark. - } -// [RGV] The next line appears to do nothing! - intptr_t dh = (intptr_t) displaced_header(); - dest->set_displaced_header(displaced_header()); -} - -// ----------------------------------------------------------------------------- - -// standard constructor, allows locking failures -ObjectLocker::ObjectLocker(Handle obj, Thread* thread, bool doLock) { - _dolock = doLock; - _thread = thread; - debug_only(if (StrictSafepointChecks) _thread->check_for_valid_safepoint_state(false);) - _obj = obj; - - if (_dolock) { - TEVENT (ObjectLocker) ; - - ObjectSynchronizer::fast_enter(_obj, &_lock, false, _thread); - } -} - -ObjectLocker::~ObjectLocker() { - if (_dolock) { - ObjectSynchronizer::fast_exit(_obj(), &_lock, _thread); - } -} - -// ----------------------------------------------------------------------------- - - -PerfCounter * ObjectSynchronizer::_sync_Inflations = NULL ; -PerfCounter * ObjectSynchronizer::_sync_Deflations = NULL ; -PerfCounter * ObjectSynchronizer::_sync_ContendedLockAttempts = NULL ; -PerfCounter * ObjectSynchronizer::_sync_FutileWakeups = NULL ; -PerfCounter * ObjectSynchronizer::_sync_Parks = NULL ; -PerfCounter * ObjectSynchronizer::_sync_EmptyNotifications = NULL ; -PerfCounter * ObjectSynchronizer::_sync_Notifications = NULL ; -PerfCounter * ObjectSynchronizer::_sync_PrivateA = NULL ; -PerfCounter * ObjectSynchronizer::_sync_PrivateB = NULL ; -PerfCounter * ObjectSynchronizer::_sync_SlowExit = NULL ; -PerfCounter * ObjectSynchronizer::_sync_SlowEnter = NULL ; -PerfCounter * ObjectSynchronizer::_sync_SlowNotify = NULL ; -PerfCounter * ObjectSynchronizer::_sync_SlowNotifyAll = NULL ; -PerfCounter * ObjectSynchronizer::_sync_FailedSpins = NULL ; -PerfCounter * ObjectSynchronizer::_sync_SuccessfulSpins = NULL ; -PerfCounter * ObjectSynchronizer::_sync_MonInCirculation = NULL ; -PerfCounter * ObjectSynchronizer::_sync_MonScavenged = NULL ; -PerfLongVariable * ObjectSynchronizer::_sync_MonExtant = NULL ; - -// One-shot global initialization for the sync subsystem. -// We could also defer initialization and initialize on-demand -// the first time we call inflate(). Initialization would -// be protected - like so many things - by the MonitorCache_lock. - -void ObjectSynchronizer::Initialize () { - static int InitializationCompleted = 0 ; - assert (InitializationCompleted == 0, "invariant") ; - InitializationCompleted = 1 ; - if (UsePerfData) { - EXCEPTION_MARK ; - #define NEWPERFCOUNTER(n) {n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,CHECK); } - #define NEWPERFVARIABLE(n) {n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,CHECK); } - NEWPERFCOUNTER(_sync_Inflations) ; - NEWPERFCOUNTER(_sync_Deflations) ; - NEWPERFCOUNTER(_sync_ContendedLockAttempts) ; - NEWPERFCOUNTER(_sync_FutileWakeups) ; - NEWPERFCOUNTER(_sync_Parks) ; - NEWPERFCOUNTER(_sync_EmptyNotifications) ; - NEWPERFCOUNTER(_sync_Notifications) ; - NEWPERFCOUNTER(_sync_SlowEnter) ; - NEWPERFCOUNTER(_sync_SlowExit) ; - NEWPERFCOUNTER(_sync_SlowNotify) ; - NEWPERFCOUNTER(_sync_SlowNotifyAll) ; - NEWPERFCOUNTER(_sync_FailedSpins) ; - NEWPERFCOUNTER(_sync_SuccessfulSpins) ; - NEWPERFCOUNTER(_sync_PrivateA) ; - NEWPERFCOUNTER(_sync_PrivateB) ; - NEWPERFCOUNTER(_sync_MonInCirculation) ; - NEWPERFCOUNTER(_sync_MonScavenged) ; - NEWPERFVARIABLE(_sync_MonExtant) ; - #undef NEWPERFCOUNTER - } -} - -// Compile-time asserts -// When possible, it's better to catch errors deterministically at -// compile-time than at runtime. The down-side to using compile-time -// asserts is that error message -- often something about negative array -// indices -- is opaque. - -#define CTASSERT(x) { int tag[1-(2*!(x))]; printf ("Tag @" INTPTR_FORMAT "\n", (intptr_t)tag); } - -void ObjectMonitor::ctAsserts() { - CTASSERT(offset_of (ObjectMonitor, _header) == 0); -} - -static int Adjust (volatile int * adr, int dx) { - int v ; - for (v = *adr ; Atomic::cmpxchg (v + dx, adr, v) != v; v = *adr) ; - return v ; -} - -// Ad-hoc mutual exclusion primitives: SpinLock and Mux -// -// We employ SpinLocks _only for low-contention, fixed-length -// short-duration critical sections where we're concerned -// about native mutex_t or HotSpot Mutex:: latency. -// The mux construct provides a spin-then-block mutual exclusion -// mechanism. -// -// Testing has shown that contention on the ListLock guarding gFreeList -// is common. If we implement ListLock as a simple SpinLock it's common -// for the JVM to devolve to yielding with little progress. This is true -// despite the fact that the critical sections protected by ListLock are -// extremely short. -// -// TODO-FIXME: ListLock should be of type SpinLock. -// We should make this a 1st-class type, integrated into the lock -// hierarchy as leaf-locks. Critically, the SpinLock structure -// should have sufficient padding to avoid false-sharing and excessive -// cache-coherency traffic. - - -typedef volatile int SpinLockT ; - -void Thread::SpinAcquire (volatile int * adr, const char * LockName) { - if (Atomic::cmpxchg (1, adr, 0) == 0) { - return ; // normal fast-path return - } - - // Slow-path : We've encountered contention -- Spin/Yield/Block strategy. - TEVENT (SpinAcquire - ctx) ; - int ctr = 0 ; - int Yields = 0 ; - for (;;) { - while (*adr != 0) { - ++ctr ; - if ((ctr & 0xFFF) == 0 || !os::is_MP()) { - if (Yields > 5) { - // Consider using a simple NakedSleep() instead. - // Then SpinAcquire could be called by non-JVM threads - Thread::current()->_ParkEvent->park(1) ; - } else { - os::NakedYield() ; - ++Yields ; - } - } else { - SpinPause() ; - } - } - if (Atomic::cmpxchg (1, adr, 0) == 0) return ; - } -} - -void Thread::SpinRelease (volatile int * adr) { - assert (*adr != 0, "invariant") ; - OrderAccess::fence() ; // guarantee at least release consistency. - // Roach-motel semantics. - // It's safe if subsequent LDs and STs float "up" into the critical section, - // but prior LDs and STs within the critical section can't be allowed - // to reorder or float past the ST that releases the lock. - *adr = 0 ; -} - -// muxAcquire and muxRelease: -// -// * muxAcquire and muxRelease support a single-word lock-word construct. -// The LSB of the word is set IFF the lock is held. -// The remainder of the word points to the head of a singly-linked list -// of threads blocked on the lock. -// -// * The current implementation of muxAcquire-muxRelease uses its own -// dedicated Thread._MuxEvent instance. If we're interested in -// minimizing the peak number of extant ParkEvent instances then -// we could eliminate _MuxEvent and "borrow" _ParkEvent as long -// as certain invariants were satisfied. Specifically, care would need -// to be taken with regards to consuming unpark() "permits". -// A safe rule of thumb is that a thread would never call muxAcquire() -// if it's enqueued (cxq, EntryList, WaitList, etc) and will subsequently -// park(). Otherwise the _ParkEvent park() operation in muxAcquire() could -// consume an unpark() permit intended for monitorenter, for instance. -// One way around this would be to widen the restricted-range semaphore -// implemented in park(). Another alternative would be to provide -// multiple instances of the PlatformEvent() for each thread. One -// instance would be dedicated to muxAcquire-muxRelease, for instance. -// -// * Usage: -// -- Only as leaf locks -// -- for short-term locking only as muxAcquire does not perform -// thread state transitions. -// -// Alternatives: -// * We could implement muxAcquire and muxRelease with MCS or CLH locks -// but with parking or spin-then-park instead of pure spinning. -// * Use Taura-Oyama-Yonenzawa locks. -// * It's possible to construct a 1-0 lock if we encode the lockword as -// (List,LockByte). Acquire will CAS the full lockword while Release -// will STB 0 into the LockByte. The 1-0 scheme admits stranding, so -// acquiring threads use timers (ParkTimed) to detect and recover from -// the stranding window. Thread/Node structures must be aligned on 256-byte -// boundaries by using placement-new. -// * Augment MCS with advisory back-link fields maintained with CAS(). -// Pictorially: LockWord -> T1 <-> T2 <-> T3 <-> ... <-> Tn <-> Owner. -// The validity of the backlinks must be ratified before we trust the value. -// If the backlinks are invalid the exiting thread must back-track through the -// the forward links, which are always trustworthy. -// * Add a successor indication. The LockWord is currently encoded as -// (List, LOCKBIT:1). We could also add a SUCCBIT or an explicit _succ variable -// to provide the usual futile-wakeup optimization. -// See RTStt for details. -// * Consider schedctl.sc_nopreempt to cover the critical section. -// - - -typedef volatile intptr_t MutexT ; // Mux Lock-word -enum MuxBits { LOCKBIT = 1 } ; - -void Thread::muxAcquire (volatile intptr_t * Lock, const char * LockName) { - intptr_t w = Atomic::cmpxchg_ptr (LOCKBIT, Lock, 0) ; - if (w == 0) return ; - if ((w & LOCKBIT) == 0 && Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) { - return ; - } - - TEVENT (muxAcquire - Contention) ; - ParkEvent * const Self = Thread::current()->_MuxEvent ; - assert ((intptr_t(Self) & LOCKBIT) == 0, "invariant") ; - for (;;) { - int its = (os::is_MP() ? 100 : 0) + 1 ; - - // Optional spin phase: spin-then-park strategy - while (--its >= 0) { - w = *Lock ; - if ((w & LOCKBIT) == 0 && Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) { - return ; - } - } - - Self->reset() ; - Self->OnList = intptr_t(Lock) ; - // The following fence() isn't _strictly necessary as the subsequent - // CAS() both serializes execution and ratifies the fetched *Lock value. - OrderAccess::fence(); - for (;;) { - w = *Lock ; - if ((w & LOCKBIT) == 0) { - if (Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) { - Self->OnList = 0 ; // hygiene - allows stronger asserts - return ; - } - continue ; // Interference -- *Lock changed -- Just retry - } - assert (w & LOCKBIT, "invariant") ; - Self->ListNext = (ParkEvent *) (w & ~LOCKBIT ); - if (Atomic::cmpxchg_ptr (intptr_t(Self)|LOCKBIT, Lock, w) == w) break ; - } - - while (Self->OnList != 0) { - Self->park() ; - } - } -} - -void Thread::muxAcquireW (volatile intptr_t * Lock, ParkEvent * ev) { - intptr_t w = Atomic::cmpxchg_ptr (LOCKBIT, Lock, 0) ; - if (w == 0) return ; - if ((w & LOCKBIT) == 0 && Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) { - return ; - } - - TEVENT (muxAcquire - Contention) ; - ParkEvent * ReleaseAfter = NULL ; - if (ev == NULL) { - ev = ReleaseAfter = ParkEvent::Allocate (NULL) ; - } - assert ((intptr_t(ev) & LOCKBIT) == 0, "invariant") ; - for (;;) { - guarantee (ev->OnList == 0, "invariant") ; - int its = (os::is_MP() ? 100 : 0) + 1 ; - - // Optional spin phase: spin-then-park strategy - while (--its >= 0) { - w = *Lock ; - if ((w & LOCKBIT) == 0 && Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) { - if (ReleaseAfter != NULL) { - ParkEvent::Release (ReleaseAfter) ; - } - return ; - } - } - - ev->reset() ; - ev->OnList = intptr_t(Lock) ; - // The following fence() isn't _strictly necessary as the subsequent - // CAS() both serializes execution and ratifies the fetched *Lock value. - OrderAccess::fence(); - for (;;) { - w = *Lock ; - if ((w & LOCKBIT) == 0) { - if (Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) { - ev->OnList = 0 ; - // We call ::Release while holding the outer lock, thus - // artificially lengthening the critical section. - // Consider deferring the ::Release() until the subsequent unlock(), - // after we've dropped the outer lock. - if (ReleaseAfter != NULL) { - ParkEvent::Release (ReleaseAfter) ; - } - return ; - } - continue ; // Interference -- *Lock changed -- Just retry - } - assert (w & LOCKBIT, "invariant") ; - ev->ListNext = (ParkEvent *) (w & ~LOCKBIT ); - if (Atomic::cmpxchg_ptr (intptr_t(ev)|LOCKBIT, Lock, w) == w) break ; - } - - while (ev->OnList != 0) { - ev->park() ; - } - } -} - -// Release() must extract a successor from the list and then wake that thread. -// It can "pop" the front of the list or use a detach-modify-reattach (DMR) scheme -// similar to that used by ParkEvent::Allocate() and ::Release(). DMR-based -// Release() would : -// (A) CAS() or swap() null to *Lock, releasing the lock and detaching the list. -// (B) Extract a successor from the private list "in-hand" -// (C) attempt to CAS() the residual back into *Lock over null. -// If there were any newly arrived threads and the CAS() would fail. -// In that case Release() would detach the RATs, re-merge the list in-hand -// with the RATs and repeat as needed. Alternately, Release() might -// detach and extract a successor, but then pass the residual list to the wakee. -// The wakee would be responsible for reattaching and remerging before it -// competed for the lock. -// -// Both "pop" and DMR are immune from ABA corruption -- there can be -// multiple concurrent pushers, but only one popper or detacher. -// This implementation pops from the head of the list. This is unfair, -// but tends to provide excellent throughput as hot threads remain hot. -// (We wake recently run threads first). - -void Thread::muxRelease (volatile intptr_t * Lock) { - for (;;) { - const intptr_t w = Atomic::cmpxchg_ptr (0, Lock, LOCKBIT) ; - assert (w & LOCKBIT, "invariant") ; - if (w == LOCKBIT) return ; - ParkEvent * List = (ParkEvent *) (w & ~LOCKBIT) ; - assert (List != NULL, "invariant") ; - assert (List->OnList == intptr_t(Lock), "invariant") ; - ParkEvent * nxt = List->ListNext ; - - // The following CAS() releases the lock and pops the head element. - if (Atomic::cmpxchg_ptr (intptr_t(nxt), Lock, w) != w) { - continue ; - } - List->OnList = 0 ; - OrderAccess::fence() ; - List->unpark () ; - return ; - } -} - -// ObjectMonitor Lifecycle -// ----------------------- -// Inflation unlinks monitors from the global gFreeList and -// associates them with objects. Deflation -- which occurs at -// STW-time -- disassociates idle monitors from objects. Such -// scavenged monitors are returned to the gFreeList. -// -// The global list is protected by ListLock. All the critical sections -// are short and operate in constant-time. -// -// ObjectMonitors reside in type-stable memory (TSM) and are immortal. -// -// Lifecycle: -// -- unassigned and on the global free list -// -- unassigned and on a thread's private omFreeList -// -- assigned to an object. The object is inflated and the mark refers -// to the objectmonitor. -// -// TODO-FIXME: -// -// * We currently protect the gFreeList with a simple lock. -// An alternate lock-free scheme would be to pop elements from the gFreeList -// with CAS. This would be safe from ABA corruption as long we only -// recycled previously appearing elements onto the list in deflate_idle_monitors() -// at STW-time. Completely new elements could always be pushed onto the gFreeList -// with CAS. Elements that appeared previously on the list could only -// be installed at STW-time. -// -// * For efficiency and to help reduce the store-before-CAS penalty -// the objectmonitors on gFreeList or local free lists should be ready to install -// with the exception of _header and _object. _object can be set after inflation. -// In particular, keep all objectMonitors on a thread's private list in ready-to-install -// state with m.Owner set properly. -// -// * We could all diffuse contention by using multiple global (FreeList, Lock) -// pairs -- threads could use trylock() and a cyclic-scan strategy to search for -// an unlocked free list. -// -// * Add lifecycle tags and assert()s. -// -// * Be more consistent about when we clear an objectmonitor's fields: -// A. After extracting the objectmonitor from a free list. -// B. After adding an objectmonitor to a free list. -// - -ObjectMonitor * ObjectSynchronizer::gBlockList = NULL ; -ObjectMonitor * volatile ObjectSynchronizer::gFreeList = NULL ; -static volatile intptr_t ListLock = 0 ; // protects global monitor free-list cache -#define CHAINMARKER ((oop)-1) - -ObjectMonitor * ATTR ObjectSynchronizer::omAlloc (Thread * Self) { - // A large MAXPRIVATE value reduces both list lock contention - // and list coherency traffic, but also tends to increase the - // number of objectMonitors in circulation as well as the STW - // scavenge costs. As usual, we lean toward time in space-time - // tradeoffs. - const int MAXPRIVATE = 1024 ; - for (;;) { - ObjectMonitor * m ; - - // 1: try to allocate from the thread's local omFreeList. - // Threads will attempt to allocate first from their local list, then - // from the global list, and only after those attempts fail will the thread - // attempt to instantiate new monitors. Thread-local free lists take - // heat off the ListLock and improve allocation latency, as well as reducing - // coherency traffic on the shared global list. - m = Self->omFreeList ; - if (m != NULL) { - Self->omFreeList = m->FreeNext ; - Self->omFreeCount -- ; - // CONSIDER: set m->FreeNext = BAD -- diagnostic hygiene - guarantee (m->object() == NULL, "invariant") ; - return m ; - } - - // 2: try to allocate from the global gFreeList - // CONSIDER: use muxTry() instead of muxAcquire(). - // If the muxTry() fails then drop immediately into case 3. - // If we're using thread-local free lists then try - // to reprovision the caller's free list. - if (gFreeList != NULL) { - // Reprovision the thread's omFreeList. - // Use bulk transfers to reduce the allocation rate and heat - // on various locks. - Thread::muxAcquire (&ListLock, "omAlloc") ; - for (int i = Self->omFreeProvision; --i >= 0 && gFreeList != NULL; ) { - ObjectMonitor * take = gFreeList ; - gFreeList = take->FreeNext ; - guarantee (take->object() == NULL, "invariant") ; - guarantee (!take->is_busy(), "invariant") ; - take->Recycle() ; - omRelease (Self, take) ; - } - Thread::muxRelease (&ListLock) ; - Self->omFreeProvision += 1 + (Self->omFreeProvision/2) ; - if (Self->omFreeProvision > MAXPRIVATE ) Self->omFreeProvision = MAXPRIVATE ; - TEVENT (omFirst - reprovision) ; - continue ; - } - - // 3: allocate a block of new ObjectMonitors - // Both the local and global free lists are empty -- resort to malloc(). - // In the current implementation objectMonitors are TSM - immortal. - assert (_BLOCKSIZE > 1, "invariant") ; - ObjectMonitor * temp = new ObjectMonitor[_BLOCKSIZE]; - - // NOTE: (almost) no way to recover if allocation failed. - // We might be able to induce a STW safepoint and scavenge enough - // objectMonitors to permit progress. - if (temp == NULL) { - vm_exit_out_of_memory (sizeof (ObjectMonitor[_BLOCKSIZE]), "Allocate ObjectMonitors") ; - } - - // Format the block. - // initialize the linked list, each monitor points to its next - // forming the single linked free list, the very first monitor - // will points to next block, which forms the block list. - // The trick of using the 1st element in the block as gBlockList - // linkage should be reconsidered. A better implementation would - // look like: class Block { Block * next; int N; ObjectMonitor Body [N] ; } - - for (int i = 1; i < _BLOCKSIZE ; i++) { - temp[i].FreeNext = &temp[i+1]; - } - - // terminate the last monitor as the end of list - temp[_BLOCKSIZE - 1].FreeNext = NULL ; - - // Element [0] is reserved for global list linkage - temp[0].set_object(CHAINMARKER); - - // Consider carving out this thread's current request from the - // block in hand. This avoids some lock traffic and redundant - // list activity. - - // Acquire the ListLock to manipulate BlockList and FreeList. - // An Oyama-Taura-Yonezawa scheme might be more efficient. - Thread::muxAcquire (&ListLock, "omAlloc [2]") ; - - // Add the new block to the list of extant blocks (gBlockList). - // The very first objectMonitor in a block is reserved and dedicated. - // It serves as blocklist "next" linkage. - temp[0].FreeNext = gBlockList; - gBlockList = temp; - - // Add the new string of objectMonitors to the global free list - temp[_BLOCKSIZE - 1].FreeNext = gFreeList ; - gFreeList = temp + 1; - Thread::muxRelease (&ListLock) ; - TEVENT (Allocate block of monitors) ; - } -} - -// Place "m" on the caller's private per-thread omFreeList. -// In practice there's no need to clamp or limit the number of -// monitors on a thread's omFreeList as the only time we'll call -// omRelease is to return a monitor to the free list after a CAS -// attempt failed. This doesn't allow unbounded #s of monitors to -// accumulate on a thread's free list. -// -// In the future the usage of omRelease() might change and monitors -// could migrate between free lists. In that case to avoid excessive -// accumulation we could limit omCount to (omProvision*2), otherwise return -// the objectMonitor to the global list. We should drain (return) in reasonable chunks. -// That is, *not* one-at-a-time. - - -void ObjectSynchronizer::omRelease (Thread * Self, ObjectMonitor * m) { - guarantee (m->object() == NULL, "invariant") ; - m->FreeNext = Self->omFreeList ; - Self->omFreeList = m ; - Self->omFreeCount ++ ; -} - -// Return the monitors of a moribund thread's local free list to -// the global free list. Typically a thread calls omFlush() when -// it's dying. We could also consider having the VM thread steal -// monitors from threads that have not run java code over a few -// consecutive STW safepoints. Relatedly, we might decay -// omFreeProvision at STW safepoints. -// -// We currently call omFlush() from the Thread:: dtor _after the thread -// has been excised from the thread list and is no longer a mutator. -// That means that omFlush() can run concurrently with a safepoint and -// the scavenge operator. Calling omFlush() from JavaThread::exit() might -// be a better choice as we could safely reason that that the JVM is -// not at a safepoint at the time of the call, and thus there could -// be not inopportune interleavings between omFlush() and the scavenge -// operator. - -void ObjectSynchronizer::omFlush (Thread * Self) { - ObjectMonitor * List = Self->omFreeList ; // Null-terminated SLL - Self->omFreeList = NULL ; - if (List == NULL) return ; - ObjectMonitor * Tail = NULL ; - ObjectMonitor * s ; - for (s = List ; s != NULL ; s = s->FreeNext) { - Tail = s ; - guarantee (s->object() == NULL, "invariant") ; - guarantee (!s->is_busy(), "invariant") ; - s->set_owner (NULL) ; // redundant but good hygiene - TEVENT (omFlush - Move one) ; - } - - guarantee (Tail != NULL && List != NULL, "invariant") ; - Thread::muxAcquire (&ListLock, "omFlush") ; - Tail->FreeNext = gFreeList ; - gFreeList = List ; - Thread::muxRelease (&ListLock) ; - TEVENT (omFlush) ; -} - - -// Get the next block in the block list. -static inline ObjectMonitor* next(ObjectMonitor* block) { - assert(block->object() == CHAINMARKER, "must be a block header"); - block = block->FreeNext ; - assert(block == NULL || block->object() == CHAINMARKER, "must be a block header"); - return block; -} - -// Fast path code shared by multiple functions -ObjectMonitor* ObjectSynchronizer::inflate_helper(oop obj) { - markOop mark = obj->mark(); - if (mark->has_monitor()) { - assert(ObjectSynchronizer::verify_objmon_isinpool(mark->monitor()), "monitor is invalid"); - assert(mark->monitor()->header()->is_neutral(), "monitor must record a good object header"); - return mark->monitor(); - } - return ObjectSynchronizer::inflate(Thread::current(), obj); -} - -// Note that we could encounter some performance loss through false-sharing as -// multiple locks occupy the same $ line. Padding might be appropriate. - -#define NINFLATIONLOCKS 256 -static volatile intptr_t InflationLocks [NINFLATIONLOCKS] ; +static int MonitorScavengeThreshold = 1000000 ; +static volatile int ForceMonitorScavenge = 0 ; // Scavenge required and pending static markOop ReadStableMark (oop obj) { markOop mark = obj->mark() ; @@ -1004,445 +482,70 @@ } } -ObjectMonitor * ATTR ObjectSynchronizer::inflate (Thread * Self, oop object) { - // Inflate mutates the heap ... - // Relaxing assertion for bug 6320749. - assert (Universe::verify_in_progress() || - !SafepointSynchronize::is_at_safepoint(), "invariant") ; - - for (;;) { - const markOop mark = object->mark() ; - assert (!mark->has_bias_pattern(), "invariant") ; - - // The mark can be in one of the following states: - // * Inflated - just return - // * Stack-locked - coerce it to inflated - // * INFLATING - busy wait for conversion to complete - // * Neutral - aggressively inflate the object. - // * BIASED - Illegal. We should never see this - - // CASE: inflated - if (mark->has_monitor()) { - ObjectMonitor * inf = mark->monitor() ; - assert (inf->header()->is_neutral(), "invariant"); - assert (inf->object() == object, "invariant") ; - assert (ObjectSynchronizer::verify_objmon_isinpool(inf), "monitor is invalid"); - return inf ; - } - - // CASE: inflation in progress - inflating over a stack-lock. - // Some other thread is converting from stack-locked to inflated. - // Only that thread can complete inflation -- other threads must wait. - // The INFLATING value is transient. - // Currently, we spin/yield/park and poll the markword, waiting for inflation to finish. - // We could always eliminate polling by parking the thread on some auxiliary list. - if (mark == markOopDesc::INFLATING()) { - TEVENT (Inflate: spin while INFLATING) ; - ReadStableMark(object) ; - continue ; - } - - // CASE: stack-locked - // Could be stack-locked either by this thread or by some other thread. - // - // Note that we allocate the objectmonitor speculatively, _before_ attempting - // to install INFLATING into the mark word. We originally installed INFLATING, - // allocated the objectmonitor, and then finally STed the address of the - // objectmonitor into the mark. This was correct, but artificially lengthened - // the interval in which INFLATED appeared in the mark, thus increasing - // the odds of inflation contention. - // - // We now use per-thread private objectmonitor free lists. - // These list are reprovisioned from the global free list outside the - // critical INFLATING...ST interval. A thread can transfer - // multiple objectmonitors en-mass from the global free list to its local free list. - // This reduces coherency traffic and lock contention on the global free list. - // Using such local free lists, it doesn't matter if the omAlloc() call appears - // before or after the CAS(INFLATING) operation. - // See the comments in omAlloc(). - - if (mark->has_locker()) { - ObjectMonitor * m = omAlloc (Self) ; - // Optimistically prepare the objectmonitor - anticipate successful CAS - // We do this before the CAS in order to minimize the length of time - // in which INFLATING appears in the mark. - m->Recycle(); - m->FreeNext = NULL ; - m->_Responsible = NULL ; - m->OwnerIsThread = 0 ; - m->_recursions = 0 ; - m->_SpinDuration = Knob_SpinLimit ; // Consider: maintain by type/class - - markOop cmp = (markOop) Atomic::cmpxchg_ptr (markOopDesc::INFLATING(), object->mark_addr(), mark) ; - if (cmp != mark) { - omRelease (Self, m) ; - continue ; // Interference -- just retry - } - - // We've successfully installed INFLATING (0) into the mark-word. - // This is the only case where 0 will appear in a mark-work. - // Only the singular thread that successfully swings the mark-word - // to 0 can perform (or more precisely, complete) inflation. - // - // Why do we CAS a 0 into the mark-word instead of just CASing the - // mark-word from the stack-locked value directly to the new inflated state? - // Consider what happens when a thread unlocks a stack-locked object. - // It attempts to use CAS to swing the displaced header value from the - // on-stack basiclock back into the object header. Recall also that the - // header value (hashcode, etc) can reside in (a) the object header, or - // (b) a displaced header associated with the stack-lock, or (c) a displaced - // header in an objectMonitor. The inflate() routine must copy the header - // value from the basiclock on the owner's stack to the objectMonitor, all - // the while preserving the hashCode stability invariants. If the owner - // decides to release the lock while the value is 0, the unlock will fail - // and control will eventually pass from slow_exit() to inflate. The owner - // will then spin, waiting for the 0 value to disappear. Put another way, - // the 0 causes the owner to stall if the owner happens to try to - // drop the lock (restoring the header from the basiclock to the object) - // while inflation is in-progress. This protocol avoids races that might - // would otherwise permit hashCode values to change or "flicker" for an object. - // Critically, while object->mark is 0 mark->displaced_mark_helper() is stable. - // 0 serves as a "BUSY" inflate-in-progress indicator. - - - // fetch the displaced mark from the owner's stack. - // The owner can't die or unwind past the lock while our INFLATING - // object is in the mark. Furthermore the owner can't complete - // an unlock on the object, either. - markOop dmw = mark->displaced_mark_helper() ; - assert (dmw->is_neutral(), "invariant") ; - - // Setup monitor fields to proper values -- prepare the monitor - m->set_header(dmw) ; +// hashCode() generation : +// +// Possibilities: +// * MD5Digest of {obj,stwRandom} +// * CRC32 of {obj,stwRandom} or any linear-feedback shift register function. +// * A DES- or AES-style SBox[] mechanism +// * One of the Phi-based schemes, such as: +// 2654435761 = 2^32 * Phi (golden ratio) +// HashCodeValue = ((uintptr_t(obj) >> 3) * 2654435761) ^ GVars.stwRandom ; +// * A variation of Marsaglia's shift-xor RNG scheme. +// * (obj ^ stwRandom) is appealing, but can result +// in undesirable regularity in the hashCode values of adjacent objects +// (objects allocated back-to-back, in particular). This could potentially +// result in hashtable collisions and reduced hashtable efficiency. +// There are simple ways to "diffuse" the middle address bits over the +// generated hashCode values: +// - // Optimization: if the mark->locker stack address is associated - // with this thread we could simply set m->_owner = Self and - // m->OwnerIsThread = 1. Note that a thread can inflate an object - // that it has stack-locked -- as might happen in wait() -- directly - // with CAS. That is, we can avoid the xchg-NULL .... ST idiom. - m->set_owner(mark->locker()); - m->set_object(object); - // TODO-FIXME: assert BasicLock->dhw != 0. - - // Must preserve store ordering. The monitor state must - // be stable at the time of publishing the monitor address. - guarantee (object->mark() == markOopDesc::INFLATING(), "invariant") ; - object->release_set_mark(markOopDesc::encode(m)); - - // Hopefully the performance counters are allocated on distinct cache lines - // to avoid false sharing on MP systems ... - if (_sync_Inflations != NULL) _sync_Inflations->inc() ; - TEVENT(Inflate: overwrite stacklock) ; - if (TraceMonitorInflation) { - if (object->is_instance()) { - ResourceMark rm; - tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s", - (intptr_t) object, (intptr_t) object->mark(), - Klass::cast(object->klass())->external_name()); - } - } - return m ; - } - - // CASE: neutral - // TODO-FIXME: for entry we currently inflate and then try to CAS _owner. - // If we know we're inflating for entry it's better to inflate by swinging a - // pre-locked objectMonitor pointer into the object header. A successful - // CAS inflates the object *and* confers ownership to the inflating thread. - // In the current implementation we use a 2-step mechanism where we CAS() - // to inflate and then CAS() again to try to swing _owner from NULL to Self. - // An inflateTry() method that we could call from fast_enter() and slow_enter() - // would be useful. - - assert (mark->is_neutral(), "invariant"); - ObjectMonitor * m = omAlloc (Self) ; - // prepare m for installation - set monitor to initial state - m->Recycle(); - m->set_header(mark); - m->set_owner(NULL); - m->set_object(object); - m->OwnerIsThread = 1 ; - m->_recursions = 0 ; - m->FreeNext = NULL ; - m->_Responsible = NULL ; - m->_SpinDuration = Knob_SpinLimit ; // consider: keep metastats by type/class - - if (Atomic::cmpxchg_ptr (markOopDesc::encode(m), object->mark_addr(), mark) != mark) { - m->set_object (NULL) ; - m->set_owner (NULL) ; - m->OwnerIsThread = 0 ; - m->Recycle() ; - omRelease (Self, m) ; - m = NULL ; - continue ; - // interference - the markword changed - just retry. - // The state-transitions are one-way, so there's no chance of - // live-lock -- "Inflated" is an absorbing state. - } - - // Hopefully the performance counters are allocated on distinct - // cache lines to avoid false sharing on MP systems ... - if (_sync_Inflations != NULL) _sync_Inflations->inc() ; - TEVENT(Inflate: overwrite neutral) ; - if (TraceMonitorInflation) { - if (object->is_instance()) { - ResourceMark rm; - tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s", - (intptr_t) object, (intptr_t) object->mark(), - Klass::cast(object->klass())->external_name()); - } - } - return m ; - } -} - - -// This the fast monitor enter. The interpreter and compiler use -// some assembly copies of this code. Make sure update those code -// if the following function is changed. The implementation is -// extremely sensitive to race condition. Be careful. - -void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock, bool attempt_rebias, TRAPS) { - if (UseBiasedLocking) { - if (!SafepointSynchronize::is_at_safepoint()) { - BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD); - if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) { - return; - } - } else { - assert(!attempt_rebias, "can not rebias toward VM thread"); - BiasedLocking::revoke_at_safepoint(obj); - } - assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); - } - - slow_enter (obj, lock, THREAD) ; -} - -void ObjectSynchronizer::fast_exit(oop object, BasicLock* lock, TRAPS) { - assert(!object->mark()->has_bias_pattern(), "should not see bias pattern here"); - // if displaced header is null, the previous enter is recursive enter, no-op - markOop dhw = lock->displaced_header(); - markOop mark ; - if (dhw == NULL) { - // Recursive stack-lock. - // Diagnostics -- Could be: stack-locked, inflating, inflated. - mark = object->mark() ; - assert (!mark->is_neutral(), "invariant") ; - if (mark->has_locker() && mark != markOopDesc::INFLATING()) { - assert(THREAD->is_lock_owned((address)mark->locker()), "invariant") ; - } - if (mark->has_monitor()) { - ObjectMonitor * m = mark->monitor() ; - assert(((oop)(m->object()))->mark() == mark, "invariant") ; - assert(m->is_entered(THREAD), "invariant") ; - } - return ; +static inline intptr_t get_next_hash(Thread * Self, oop obj) { + intptr_t value = 0 ; + if (hashCode == 0) { + // This form uses an unguarded global Park-Miller RNG, + // so it's possible for two threads to race and generate the same RNG. + // On MP system we'll have lots of RW access to a global, so the + // mechanism induces lots of coherency traffic. + value = os::random() ; + } else + if (hashCode == 1) { + // This variation has the property of being stable (idempotent) + // between STW operations. This can be useful in some of the 1-0 + // synchronization schemes. + intptr_t addrBits = intptr_t(obj) >> 3 ; + value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom ; + } else + if (hashCode == 2) { + value = 1 ; // for sensitivity testing + } else + if (hashCode == 3) { + value = ++GVars.hcSequence ; + } else + if (hashCode == 4) { + value = intptr_t(obj) ; + } else { + // Marsaglia's xor-shift scheme with thread-specific state + // This is probably the best overall implementation -- we'll + // likely make this the default in future releases. + unsigned t = Self->_hashStateX ; + t ^= (t << 11) ; + Self->_hashStateX = Self->_hashStateY ; + Self->_hashStateY = Self->_hashStateZ ; + Self->_hashStateZ = Self->_hashStateW ; + unsigned v = Self->_hashStateW ; + v = (v ^ (v >> 19)) ^ (t ^ (t >> 8)) ; + Self->_hashStateW = v ; + value = v ; } - mark = object->mark() ; - - // If the object is stack-locked by the current thread, try to - // swing the displaced header from the box back to the mark. - if (mark == (markOop) lock) { - assert (dhw->is_neutral(), "invariant") ; - if ((markOop) Atomic::cmpxchg_ptr (dhw, object->mark_addr(), mark) == mark) { - TEVENT (fast_exit: release stacklock) ; - return; - } - } - - ObjectSynchronizer::inflate(THREAD, object)->exit (THREAD) ; -} - -// This routine is used to handle interpreter/compiler slow case -// We don't need to use fast path here, because it must have been -// failed in the interpreter/compiler code. -void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) { - markOop mark = obj->mark(); - assert(!mark->has_bias_pattern(), "should not see bias pattern here"); - - if (mark->is_neutral()) { - // Anticipate successful CAS -- the ST of the displaced mark must - // be visible <= the ST performed by the CAS. - lock->set_displaced_header(mark); - if (mark == (markOop) Atomic::cmpxchg_ptr(lock, obj()->mark_addr(), mark)) { - TEVENT (slow_enter: release stacklock) ; - return ; - } - // Fall through to inflate() ... - } else - if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) { - assert(lock != mark->locker(), "must not re-lock the same lock"); - assert(lock != (BasicLock*)obj->mark(), "don't relock with same BasicLock"); - lock->set_displaced_header(NULL); - return; - } - -#if 0 - // The following optimization isn't particularly useful. - if (mark->has_monitor() && mark->monitor()->is_entered(THREAD)) { - lock->set_displaced_header (NULL) ; - return ; - } -#endif - - // The object header will never be displaced to this lock, - // so it does not matter what the value is, except that it - // must be non-zero to avoid looking like a re-entrant lock, - // and must not look locked either. - lock->set_displaced_header(markOopDesc::unused_mark()); - ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD); -} - -// This routine is used to handle interpreter/compiler slow case -// We don't need to use fast path here, because it must have -// failed in the interpreter/compiler code. Simply use the heavy -// weight monitor should be ok, unless someone find otherwise. -void ObjectSynchronizer::slow_exit(oop object, BasicLock* lock, TRAPS) { - fast_exit (object, lock, THREAD) ; -} - -// NOTE: must use heavy weight monitor to handle jni monitor enter -void ObjectSynchronizer::jni_enter(Handle obj, TRAPS) { // possible entry from jni enter - // the current locking is from JNI instead of Java code - TEVENT (jni_enter) ; - if (UseBiasedLocking) { - BiasedLocking::revoke_and_rebias(obj, false, THREAD); - assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); - } - THREAD->set_current_pending_monitor_is_from_java(false); - ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD); - THREAD->set_current_pending_monitor_is_from_java(true); -} - -// NOTE: must use heavy weight monitor to handle jni monitor enter -bool ObjectSynchronizer::jni_try_enter(Handle obj, Thread* THREAD) { - if (UseBiasedLocking) { - BiasedLocking::revoke_and_rebias(obj, false, THREAD); - assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); - } - - ObjectMonitor* monitor = ObjectSynchronizer::inflate_helper(obj()); - return monitor->try_enter(THREAD); + value &= markOopDesc::hash_mask; + if (value == 0) value = 0xBAD ; + assert (value != markOopDesc::no_hash, "invariant") ; + TEVENT (hashCode: GENERATE) ; + return value; } - - -// NOTE: must use heavy weight monitor to handle jni monitor exit -void ObjectSynchronizer::jni_exit(oop obj, Thread* THREAD) { - TEVENT (jni_exit) ; - if (UseBiasedLocking) { - BiasedLocking::revoke_and_rebias(obj, false, THREAD); - } - assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); - - ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj); - // If this thread has locked the object, exit the monitor. Note: can't use - // monitor->check(CHECK); must exit even if an exception is pending. - if (monitor->check(THREAD)) { - monitor->exit(THREAD); - } -} - -// complete_exit()/reenter() are used to wait on a nested lock -// i.e. to give up an outer lock completely and then re-enter -// Used when holding nested locks - lock acquisition order: lock1 then lock2 -// 1) complete_exit lock1 - saving recursion count -// 2) wait on lock2 -// 3) when notified on lock2, unlock lock2 -// 4) reenter lock1 with original recursion count -// 5) lock lock2 -// NOTE: must use heavy weight monitor to handle complete_exit/reenter() -intptr_t ObjectSynchronizer::complete_exit(Handle obj, TRAPS) { - TEVENT (complete_exit) ; - if (UseBiasedLocking) { - BiasedLocking::revoke_and_rebias(obj, false, THREAD); - assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); - } - - ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj()); - - return monitor->complete_exit(THREAD); -} - -// NOTE: must use heavy weight monitor to handle complete_exit/reenter() -void ObjectSynchronizer::reenter(Handle obj, intptr_t recursion, TRAPS) { - TEVENT (reenter) ; - if (UseBiasedLocking) { - BiasedLocking::revoke_and_rebias(obj, false, THREAD); - assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); - } - - ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj()); - - monitor->reenter(recursion, THREAD); -} - -// This exists only as a workaround of dtrace bug 6254741 -int dtrace_waited_probe(ObjectMonitor* monitor, Handle obj, Thread* thr) { - DTRACE_MONITOR_PROBE(waited, monitor, obj(), thr); - return 0; -} - -// NOTE: must use heavy weight monitor to handle wait() -void ObjectSynchronizer::wait(Handle obj, jlong millis, TRAPS) { - if (UseBiasedLocking) { - BiasedLocking::revoke_and_rebias(obj, false, THREAD); - assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); - } - if (millis < 0) { - TEVENT (wait - throw IAX) ; - THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative"); - } - ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj()); - DTRACE_MONITOR_WAIT_PROBE(monitor, obj(), THREAD, millis); - monitor->wait(millis, true, THREAD); - - /* This dummy call is in place to get around dtrace bug 6254741. Once - that's fixed we can uncomment the following line and remove the call */ - // DTRACE_MONITOR_PROBE(waited, monitor, obj(), THREAD); - dtrace_waited_probe(monitor, obj, THREAD); -} - -void ObjectSynchronizer::waitUninterruptibly (Handle obj, jlong millis, TRAPS) { - if (UseBiasedLocking) { - BiasedLocking::revoke_and_rebias(obj, false, THREAD); - assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); - } - if (millis < 0) { - TEVENT (wait - throw IAX) ; - THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative"); - } - ObjectSynchronizer::inflate(THREAD, obj()) -> wait(millis, false, THREAD) ; -} - -void ObjectSynchronizer::notify(Handle obj, TRAPS) { - if (UseBiasedLocking) { - BiasedLocking::revoke_and_rebias(obj, false, THREAD); - assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); - } - - markOop mark = obj->mark(); - if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) { - return; - } - ObjectSynchronizer::inflate(THREAD, obj())->notify(THREAD); -} - -// NOTE: see comment of notify() -void ObjectSynchronizer::notifyall(Handle obj, TRAPS) { - if (UseBiasedLocking) { - BiasedLocking::revoke_and_rebias(obj, false, THREAD); - assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now"); - } - - markOop mark = obj->mark(); - if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) { - return; - } - ObjectSynchronizer::inflate(THREAD, obj())->notifyAll(THREAD); -} - +// intptr_t ObjectSynchronizer::FastHashCode (Thread * Self, oop obj) { if (UseBiasedLocking) { // NOTE: many places throughout the JVM do not expect a safepoint @@ -1554,6 +657,7 @@ return FastHashCode (Thread::current(), obj()) ; } + bool ObjectSynchronizer::current_thread_holds_lock(JavaThread* thread, Handle h_obj) { if (UseBiasedLocking) { @@ -1665,6 +769,774 @@ return NULL; } +// Visitors ... + +void ObjectSynchronizer::monitors_iterate(MonitorClosure* closure) { + ObjectMonitor* block = gBlockList; + ObjectMonitor* mid; + while (block) { + assert(block->object() == CHAINMARKER, "must be a block header"); + for (int i = _BLOCKSIZE - 1; i > 0; i--) { + mid = block + i; + oop object = (oop) mid->object(); + if (object != NULL) { + closure->do_monitor(mid); + } + } + block = (ObjectMonitor*) block->FreeNext; + } +} + +// Get the next block in the block list. +static inline ObjectMonitor* next(ObjectMonitor* block) { + assert(block->object() == CHAINMARKER, "must be a block header"); + block = block->FreeNext ; + assert(block == NULL || block->object() == CHAINMARKER, "must be a block header"); + return block; +} + + +void ObjectSynchronizer::oops_do(OopClosure* f) { + assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint"); + for (ObjectMonitor* block = gBlockList; block != NULL; block = next(block)) { + assert(block->object() == CHAINMARKER, "must be a block header"); + for (int i = 1; i < _BLOCKSIZE; i++) { + ObjectMonitor* mid = &block[i]; + if (mid->object() != NULL) { + f->do_oop((oop*)mid->object_addr()); + } + } + } +} + + +// ----------------------------------------------------------------------------- +// ObjectMonitor Lifecycle +// ----------------------- +// Inflation unlinks monitors from the global gFreeList and +// associates them with objects. Deflation -- which occurs at +// STW-time -- disassociates idle monitors from objects. Such +// scavenged monitors are returned to the gFreeList. +// +// The global list is protected by ListLock. All the critical sections +// are short and operate in constant-time. +// +// ObjectMonitors reside in type-stable memory (TSM) and are immortal. +// +// Lifecycle: +// -- unassigned and on the global free list +// -- unassigned and on a thread's private omFreeList +// -- assigned to an object. The object is inflated and the mark refers +// to the objectmonitor. +// + + +// Constraining monitor pool growth via MonitorBound ... +// +// The monitor pool is grow-only. We scavenge at STW safepoint-time, but the +// the rate of scavenging is driven primarily by GC. As such, we can find +// an inordinate number of monitors in circulation. +// To avoid that scenario we can artificially induce a STW safepoint +// if the pool appears to be growing past some reasonable bound. +// Generally we favor time in space-time tradeoffs, but as there's no +// natural back-pressure on the # of extant monitors we need to impose some +// type of limit. Beware that if MonitorBound is set to too low a value +// we could just loop. In addition, if MonitorBound is set to a low value +// we'll incur more safepoints, which are harmful to performance. +// See also: GuaranteedSafepointInterval +// +// The current implementation uses asynchronous VM operations. +// + +static void InduceScavenge (Thread * Self, const char * Whence) { + // Induce STW safepoint to trim monitors + // Ultimately, this results in a call to deflate_idle_monitors() in the near future. + // More precisely, trigger an asynchronous STW safepoint as the number + // of active monitors passes the specified threshold. + // TODO: assert thread state is reasonable + + if (ForceMonitorScavenge == 0 && Atomic::xchg (1, &ForceMonitorScavenge) == 0) { + if (ObjectMonitor::Knob_Verbose) { + ::printf ("Monitor scavenge - Induced STW @%s (%d)\n", Whence, ForceMonitorScavenge) ; + ::fflush(stdout) ; + } + // Induce a 'null' safepoint to scavenge monitors + // Must VM_Operation instance be heap allocated as the op will be enqueue and posted + // to the VMthread and have a lifespan longer than that of this activation record. + // The VMThread will delete the op when completed. + VMThread::execute (new VM_ForceAsyncSafepoint()) ; + + if (ObjectMonitor::Knob_Verbose) { + ::printf ("Monitor scavenge - STW posted @%s (%d)\n", Whence, ForceMonitorScavenge) ; + ::fflush(stdout) ; + } + } +} +/* Too slow for general assert or debug +void ObjectSynchronizer::verifyInUse (Thread *Self) { + ObjectMonitor* mid; + int inusetally = 0; + for (mid = Self->omInUseList; mid != NULL; mid = mid->FreeNext) { + inusetally ++; + } + assert(inusetally == Self->omInUseCount, "inuse count off"); + + int freetally = 0; + for (mid = Self->omFreeList; mid != NULL; mid = mid->FreeNext) { + freetally ++; + } + assert(freetally == Self->omFreeCount, "free count off"); +} +*/ +ObjectMonitor * ATTR ObjectSynchronizer::omAlloc (Thread * Self) { + // A large MAXPRIVATE value reduces both list lock contention + // and list coherency traffic, but also tends to increase the + // number of objectMonitors in circulation as well as the STW + // scavenge costs. As usual, we lean toward time in space-time + // tradeoffs. + const int MAXPRIVATE = 1024 ; + for (;;) { + ObjectMonitor * m ; + + // 1: try to allocate from the thread's local omFreeList. + // Threads will attempt to allocate first from their local list, then + // from the global list, and only after those attempts fail will the thread + // attempt to instantiate new monitors. Thread-local free lists take + // heat off the ListLock and improve allocation latency, as well as reducing + // coherency traffic on the shared global list. + m = Self->omFreeList ; + if (m != NULL) { + Self->omFreeList = m->FreeNext ; + Self->omFreeCount -- ; + // CONSIDER: set m->FreeNext = BAD -- diagnostic hygiene + guarantee (m->object() == NULL, "invariant") ; + if (MonitorInUseLists) { + m->FreeNext = Self->omInUseList; + Self->omInUseList = m; + Self->omInUseCount ++; + // verifyInUse(Self); + } else { + m->FreeNext = NULL; + } + return m ; + } + + // 2: try to allocate from the global gFreeList + // CONSIDER: use muxTry() instead of muxAcquire(). + // If the muxTry() fails then drop immediately into case 3. + // If we're using thread-local free lists then try + // to reprovision the caller's free list. + if (gFreeList != NULL) { + // Reprovision the thread's omFreeList. + // Use bulk transfers to reduce the allocation rate and heat + // on various locks. + Thread::muxAcquire (&ListLock, "omAlloc") ; + for (int i = Self->omFreeProvision; --i >= 0 && gFreeList != NULL; ) { + MonitorFreeCount --; + ObjectMonitor * take = gFreeList ; + gFreeList = take->FreeNext ; + guarantee (take->object() == NULL, "invariant") ; + guarantee (!take->is_busy(), "invariant") ; + take->Recycle() ; + omRelease (Self, take, false) ; + } + Thread::muxRelease (&ListLock) ; + Self->omFreeProvision += 1 + (Self->omFreeProvision/2) ; + if (Self->omFreeProvision > MAXPRIVATE ) Self->omFreeProvision = MAXPRIVATE ; + TEVENT (omFirst - reprovision) ; + + const int mx = MonitorBound ; + if (mx > 0 && (MonitorPopulation-MonitorFreeCount) > mx) { + // We can't safely induce a STW safepoint from omAlloc() as our thread + // state may not be appropriate for such activities and callers may hold + // naked oops, so instead we defer the action. + InduceScavenge (Self, "omAlloc") ; + } + continue; + } + + // 3: allocate a block of new ObjectMonitors + // Both the local and global free lists are empty -- resort to malloc(). + // In the current implementation objectMonitors are TSM - immortal. + assert (_BLOCKSIZE > 1, "invariant") ; + ObjectMonitor * temp = new ObjectMonitor[_BLOCKSIZE]; + + // NOTE: (almost) no way to recover if allocation failed. + // We might be able to induce a STW safepoint and scavenge enough + // objectMonitors to permit progress. + if (temp == NULL) { + vm_exit_out_of_memory (sizeof (ObjectMonitor[_BLOCKSIZE]), "Allocate ObjectMonitors") ; + } + + // Format the block. + // initialize the linked list, each monitor points to its next + // forming the single linked free list, the very first monitor + // will points to next block, which forms the block list. + // The trick of using the 1st element in the block as gBlockList + // linkage should be reconsidered. A better implementation would + // look like: class Block { Block * next; int N; ObjectMonitor Body [N] ; } + + for (int i = 1; i < _BLOCKSIZE ; i++) { + temp[i].FreeNext = &temp[i+1]; + } + + // terminate the last monitor as the end of list + temp[_BLOCKSIZE - 1].FreeNext = NULL ; + + // Element [0] is reserved for global list linkage + temp[0].set_object(CHAINMARKER); + + // Consider carving out this thread's current request from the + // block in hand. This avoids some lock traffic and redundant + // list activity. + + // Acquire the ListLock to manipulate BlockList and FreeList. + // An Oyama-Taura-Yonezawa scheme might be more efficient. + Thread::muxAcquire (&ListLock, "omAlloc [2]") ; + MonitorPopulation += _BLOCKSIZE-1; + MonitorFreeCount += _BLOCKSIZE-1; + + // Add the new block to the list of extant blocks (gBlockList). + // The very first objectMonitor in a block is reserved and dedicated. + // It serves as blocklist "next" linkage. + temp[0].FreeNext = gBlockList; + gBlockList = temp; + + // Add the new string of objectMonitors to the global free list + temp[_BLOCKSIZE - 1].FreeNext = gFreeList ; + gFreeList = temp + 1; + Thread::muxRelease (&ListLock) ; + TEVENT (Allocate block of monitors) ; + } +} + +// Place "m" on the caller's private per-thread omFreeList. +// In practice there's no need to clamp or limit the number of +// monitors on a thread's omFreeList as the only time we'll call +// omRelease is to return a monitor to the free list after a CAS +// attempt failed. This doesn't allow unbounded #s of monitors to +// accumulate on a thread's free list. +// + +void ObjectSynchronizer::omRelease (Thread * Self, ObjectMonitor * m, bool fromPerThreadAlloc) { + guarantee (m->object() == NULL, "invariant") ; + + // Remove from omInUseList + if (MonitorInUseLists && fromPerThreadAlloc) { + ObjectMonitor* curmidinuse = NULL; + for (ObjectMonitor* mid = Self->omInUseList; mid != NULL; ) { + if (m == mid) { + // extract from per-thread in-use-list + if (mid == Self->omInUseList) { + Self->omInUseList = mid->FreeNext; + } else if (curmidinuse != NULL) { + curmidinuse->FreeNext = mid->FreeNext; // maintain the current thread inuselist + } + Self->omInUseCount --; + // verifyInUse(Self); + break; + } else { + curmidinuse = mid; + mid = mid->FreeNext; + } + } + } + + // FreeNext is used for both onInUseList and omFreeList, so clear old before setting new + m->FreeNext = Self->omFreeList ; + Self->omFreeList = m ; + Self->omFreeCount ++ ; +} + +// Return the monitors of a moribund thread's local free list to +// the global free list. Typically a thread calls omFlush() when +// it's dying. We could also consider having the VM thread steal +// monitors from threads that have not run java code over a few +// consecutive STW safepoints. Relatedly, we might decay +// omFreeProvision at STW safepoints. +// +// Also return the monitors of a moribund thread"s omInUseList to +// a global gOmInUseList under the global list lock so these +// will continue to be scanned. +// +// We currently call omFlush() from the Thread:: dtor _after the thread +// has been excised from the thread list and is no longer a mutator. +// That means that omFlush() can run concurrently with a safepoint and +// the scavenge operator. Calling omFlush() from JavaThread::exit() might +// be a better choice as we could safely reason that that the JVM is +// not at a safepoint at the time of the call, and thus there could +// be not inopportune interleavings between omFlush() and the scavenge +// operator. + +void ObjectSynchronizer::omFlush (Thread * Self) { + ObjectMonitor * List = Self->omFreeList ; // Null-terminated SLL + Self->omFreeList = NULL ; + ObjectMonitor * Tail = NULL ; + int Tally = 0; + if (List != NULL) { + ObjectMonitor * s ; + for (s = List ; s != NULL ; s = s->FreeNext) { + Tally ++ ; + Tail = s ; + guarantee (s->object() == NULL, "invariant") ; + guarantee (!s->is_busy(), "invariant") ; + s->set_owner (NULL) ; // redundant but good hygiene + TEVENT (omFlush - Move one) ; + } + guarantee (Tail != NULL && List != NULL, "invariant") ; + } + + ObjectMonitor * InUseList = Self->omInUseList; + ObjectMonitor * InUseTail = NULL ; + int InUseTally = 0; + if (InUseList != NULL) { + Self->omInUseList = NULL; + ObjectMonitor *curom; + for (curom = InUseList; curom != NULL; curom = curom->FreeNext) { + InUseTail = curom; + InUseTally++; + } +// TODO debug + assert(Self->omInUseCount == InUseTally, "inuse count off"); + Self->omInUseCount = 0; + guarantee (InUseTail != NULL && InUseList != NULL, "invariant"); + } + + Thread::muxAcquire (&ListLock, "omFlush") ; + if (Tail != NULL) { + Tail->FreeNext = gFreeList ; + gFreeList = List ; + MonitorFreeCount += Tally; + } + + if (InUseTail != NULL) { + InUseTail->FreeNext = gOmInUseList; + gOmInUseList = InUseList; + gOmInUseCount += InUseTally; + } + + Thread::muxRelease (&ListLock) ; + TEVENT (omFlush) ; +} + +// Fast path code shared by multiple functions +ObjectMonitor* ObjectSynchronizer::inflate_helper(oop obj) { + markOop mark = obj->mark(); + if (mark->has_monitor()) { + assert(ObjectSynchronizer::verify_objmon_isinpool(mark->monitor()), "monitor is invalid"); + assert(mark->monitor()->header()->is_neutral(), "monitor must record a good object header"); + return mark->monitor(); + } + return ObjectSynchronizer::inflate(Thread::current(), obj); +} + + +// Note that we could encounter some performance loss through false-sharing as +// multiple locks occupy the same $ line. Padding might be appropriate. + + +ObjectMonitor * ATTR ObjectSynchronizer::inflate (Thread * Self, oop object) { + // Inflate mutates the heap ... + // Relaxing assertion for bug 6320749. + assert (Universe::verify_in_progress() || + !SafepointSynchronize::is_at_safepoint(), "invariant") ; + + for (;;) { + const markOop mark = object->mark() ; + assert (!mark->has_bias_pattern(), "invariant") ; + + // The mark can be in one of the following states: + // * Inflated - just return + // * Stack-locked - coerce it to inflated + // * INFLATING - busy wait for conversion to complete + // * Neutral - aggressively inflate the object. + // * BIASED - Illegal. We should never see this + + // CASE: inflated + if (mark->has_monitor()) { + ObjectMonitor * inf = mark->monitor() ; + assert (inf->header()->is_neutral(), "invariant"); + assert (inf->object() == object, "invariant") ; + assert (ObjectSynchronizer::verify_objmon_isinpool(inf), "monitor is invalid"); + return inf ; + } + + // CASE: inflation in progress - inflating over a stack-lock. + // Some other thread is converting from stack-locked to inflated. + // Only that thread can complete inflation -- other threads must wait. + // The INFLATING value is transient. + // Currently, we spin/yield/park and poll the markword, waiting for inflation to finish. + // We could always eliminate polling by parking the thread on some auxiliary list. + if (mark == markOopDesc::INFLATING()) { + TEVENT (Inflate: spin while INFLATING) ; + ReadStableMark(object) ; + continue ; + } + + // CASE: stack-locked + // Could be stack-locked either by this thread or by some other thread. + // + // Note that we allocate the objectmonitor speculatively, _before_ attempting + // to install INFLATING into the mark word. We originally installed INFLATING, + // allocated the objectmonitor, and then finally STed the address of the + // objectmonitor into the mark. This was correct, but artificially lengthened + // the interval in which INFLATED appeared in the mark, thus increasing + // the odds of inflation contention. + // + // We now use per-thread private objectmonitor free lists. + // These list are reprovisioned from the global free list outside the + // critical INFLATING...ST interval. A thread can transfer + // multiple objectmonitors en-mass from the global free list to its local free list. + // This reduces coherency traffic and lock contention on the global free list. + // Using such local free lists, it doesn't matter if the omAlloc() call appears + // before or after the CAS(INFLATING) operation. + // See the comments in omAlloc(). + + if (mark->has_locker()) { + ObjectMonitor * m = omAlloc (Self) ; + // Optimistically prepare the objectmonitor - anticipate successful CAS + // We do this before the CAS in order to minimize the length of time + // in which INFLATING appears in the mark. + m->Recycle(); + m->_Responsible = NULL ; + m->OwnerIsThread = 0 ; + m->_recursions = 0 ; + m->_SpinDuration = ObjectMonitor::Knob_SpinLimit ; // Consider: maintain by type/class + + markOop cmp = (markOop) Atomic::cmpxchg_ptr (markOopDesc::INFLATING(), object->mark_addr(), mark) ; + if (cmp != mark) { + omRelease (Self, m, true) ; + continue ; // Interference -- just retry + } + + // We've successfully installed INFLATING (0) into the mark-word. + // This is the only case where 0 will appear in a mark-work. + // Only the singular thread that successfully swings the mark-word + // to 0 can perform (or more precisely, complete) inflation. + // + // Why do we CAS a 0 into the mark-word instead of just CASing the + // mark-word from the stack-locked value directly to the new inflated state? + // Consider what happens when a thread unlocks a stack-locked object. + // It attempts to use CAS to swing the displaced header value from the + // on-stack basiclock back into the object header. Recall also that the + // header value (hashcode, etc) can reside in (a) the object header, or + // (b) a displaced header associated with the stack-lock, or (c) a displaced + // header in an objectMonitor. The inflate() routine must copy the header + // value from the basiclock on the owner's stack to the objectMonitor, all + // the while preserving the hashCode stability invariants. If the owner + // decides to release the lock while the value is 0, the unlock will fail + // and control will eventually pass from slow_exit() to inflate. The owner + // will then spin, waiting for the 0 value to disappear. Put another way, + // the 0 causes the owner to stall if the owner happens to try to + // drop the lock (restoring the header from the basiclock to the object) + // while inflation is in-progress. This protocol avoids races that might + // would otherwise permit hashCode values to change or "flicker" for an object. + // Critically, while object->mark is 0 mark->displaced_mark_helper() is stable. + // 0 serves as a "BUSY" inflate-in-progress indicator. + + + // fetch the displaced mark from the owner's stack. + // The owner can't die or unwind past the lock while our INFLATING + // object is in the mark. Furthermore the owner can't complete + // an unlock on the object, either. + markOop dmw = mark->displaced_mark_helper() ; + assert (dmw->is_neutral(), "invariant") ; + + // Setup monitor fields to proper values -- prepare the monitor + m->set_header(dmw) ; + + // Optimization: if the mark->locker stack address is associated + // with this thread we could simply set m->_owner = Self and + // m->OwnerIsThread = 1. Note that a thread can inflate an object + // that it has stack-locked -- as might happen in wait() -- directly + // with CAS. That is, we can avoid the xchg-NULL .... ST idiom. + m->set_owner(mark->locker()); + m->set_object(object); + // TODO-FIXME: assert BasicLock->dhw != 0. + + // Must preserve store ordering. The monitor state must + // be stable at the time of publishing the monitor address. + guarantee (object->mark() == markOopDesc::INFLATING(), "invariant") ; + object->release_set_mark(markOopDesc::encode(m)); + + // Hopefully the performance counters are allocated on distinct cache lines + // to avoid false sharing on MP systems ... + if (ObjectMonitor::_sync_Inflations != NULL) ObjectMonitor::_sync_Inflations->inc() ; + TEVENT(Inflate: overwrite stacklock) ; + if (TraceMonitorInflation) { + if (object->is_instance()) { + ResourceMark rm; + tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s", + (intptr_t) object, (intptr_t) object->mark(), + Klass::cast(object->klass())->external_name()); + } + } + return m ; + } + + // CASE: neutral + // TODO-FIXME: for entry we currently inflate and then try to CAS _owner. + // If we know we're inflating for entry it's better to inflate by swinging a + // pre-locked objectMonitor pointer into the object header. A successful + // CAS inflates the object *and* confers ownership to the inflating thread. + // In the current implementation we use a 2-step mechanism where we CAS() + // to inflate and then CAS() again to try to swing _owner from NULL to Self. + // An inflateTry() method that we could call from fast_enter() and slow_enter() + // would be useful. + + assert (mark->is_neutral(), "invariant"); + ObjectMonitor * m = omAlloc (Self) ; + // prepare m for installation - set monitor to initial state + m->Recycle(); + m->set_header(mark); + m->set_owner(NULL); + m->set_object(object); + m->OwnerIsThread = 1 ; + m->_recursions = 0 ; + m->_Responsible = NULL ; + m->_SpinDuration = ObjectMonitor::Knob_SpinLimit ; // consider: keep metastats by type/class + + if (Atomic::cmpxchg_ptr (markOopDesc::encode(m), object->mark_addr(), mark) != mark) { + m->set_object (NULL) ; + m->set_owner (NULL) ; + m->OwnerIsThread = 0 ; + m->Recycle() ; + omRelease (Self, m, true) ; + m = NULL ; + continue ; + // interference - the markword changed - just retry. + // The state-transitions are one-way, so there's no chance of + // live-lock -- "Inflated" is an absorbing state. + } + + // Hopefully the performance counters are allocated on distinct + // cache lines to avoid false sharing on MP systems ... + if (ObjectMonitor::_sync_Inflations != NULL) ObjectMonitor::_sync_Inflations->inc() ; + TEVENT(Inflate: overwrite neutral) ; + if (TraceMonitorInflation) { + if (object->is_instance()) { + ResourceMark rm; + tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s", + (intptr_t) object, (intptr_t) object->mark(), + Klass::cast(object->klass())->external_name()); + } + } + return m ; + } +} + +// Note that we could encounter some performance loss through false-sharing as +// multiple locks occupy the same $ line. Padding might be appropriate. + + +// Deflate_idle_monitors() is called at all safepoints, immediately +// after all mutators are stopped, but before any objects have moved. +// It traverses the list of known monitors, deflating where possible. +// The scavenged monitor are returned to the monitor free list. +// +// Beware that we scavenge at *every* stop-the-world point. +// Having a large number of monitors in-circulation negatively +// impacts the performance of some applications (e.g., PointBase). +// Broadly, we want to minimize the # of monitors in circulation. +// +// We have added a flag, MonitorInUseLists, which creates a list +// of active monitors for each thread. deflate_idle_monitors() +// only scans the per-thread inuse lists. omAlloc() puts all +// assigned monitors on the per-thread list. deflate_idle_monitors() +// returns the non-busy monitors to the global free list. +// When a thread dies, omFlush() adds the list of active monitors for +// that thread to a global gOmInUseList acquiring the +// global list lock. deflate_idle_monitors() acquires the global +// list lock to scan for non-busy monitors to the global free list. +// An alternative could have used a single global inuse list. The +// downside would have been the additional cost of acquiring the global list lock +// for every omAlloc(). +// +// Perversely, the heap size -- and thus the STW safepoint rate -- +// typically drives the scavenge rate. Large heaps can mean infrequent GC, +// which in turn can mean large(r) numbers of objectmonitors in circulation. +// This is an unfortunate aspect of this design. +// + +enum ManifestConstants { + ClearResponsibleAtSTW = 0, + MaximumRecheckInterval = 1000 +} ; + +// Deflate a single monitor if not in use +// Return true if deflated, false if in use +bool ObjectSynchronizer::deflate_monitor(ObjectMonitor* mid, oop obj, + ObjectMonitor** FreeHeadp, ObjectMonitor** FreeTailp) { + bool deflated; + // Normal case ... The monitor is associated with obj. + guarantee (obj->mark() == markOopDesc::encode(mid), "invariant") ; + guarantee (mid == obj->mark()->monitor(), "invariant"); + guarantee (mid->header()->is_neutral(), "invariant"); + + if (mid->is_busy()) { + if (ClearResponsibleAtSTW) mid->_Responsible = NULL ; + deflated = false; + } else { + // Deflate the monitor if it is no longer being used + // It's idle - scavenge and return to the global free list + // plain old deflation ... + TEVENT (deflate_idle_monitors - scavenge1) ; + if (TraceMonitorInflation) { + if (obj->is_instance()) { + ResourceMark rm; + tty->print_cr("Deflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s", + (intptr_t) obj, (intptr_t) obj->mark(), Klass::cast(obj->klass())->external_name()); + } + } + + // Restore the header back to obj + obj->release_set_mark(mid->header()); + mid->clear(); + + assert (mid->object() == NULL, "invariant") ; + + // Move the object to the working free list defined by FreeHead,FreeTail. + if (*FreeHeadp == NULL) *FreeHeadp = mid; + if (*FreeTailp != NULL) { + ObjectMonitor * prevtail = *FreeTailp; + assert(prevtail->FreeNext == NULL, "cleaned up deflated?"); // TODO KK + prevtail->FreeNext = mid; + } + *FreeTailp = mid; + deflated = true; + } + return deflated; +} + +// Caller acquires ListLock +int ObjectSynchronizer::walk_monitor_list(ObjectMonitor** listheadp, + ObjectMonitor** FreeHeadp, ObjectMonitor** FreeTailp) { + ObjectMonitor* mid; + ObjectMonitor* next; + ObjectMonitor* curmidinuse = NULL; + int deflatedcount = 0; + + for (mid = *listheadp; mid != NULL; ) { + oop obj = (oop) mid->object(); + bool deflated = false; + if (obj != NULL) { + deflated = deflate_monitor(mid, obj, FreeHeadp, FreeTailp); + } + if (deflated) { + // extract from per-thread in-use-list + if (mid == *listheadp) { + *listheadp = mid->FreeNext; + } else if (curmidinuse != NULL) { + curmidinuse->FreeNext = mid->FreeNext; // maintain the current thread inuselist + } + next = mid->FreeNext; + mid->FreeNext = NULL; // This mid is current tail in the FreeHead list + mid = next; + deflatedcount++; + } else { + curmidinuse = mid; + mid = mid->FreeNext; + } + } + return deflatedcount; +} + +void ObjectSynchronizer::deflate_idle_monitors() { + assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint"); + int nInuse = 0 ; // currently associated with objects + int nInCirculation = 0 ; // extant + int nScavenged = 0 ; // reclaimed + bool deflated = false; + + ObjectMonitor * FreeHead = NULL ; // Local SLL of scavenged monitors + ObjectMonitor * FreeTail = NULL ; + + TEVENT (deflate_idle_monitors) ; + // Prevent omFlush from changing mids in Thread dtor's during deflation + // And in case the vm thread is acquiring a lock during a safepoint + // See e.g. 6320749 + Thread::muxAcquire (&ListLock, "scavenge - return") ; + + if (MonitorInUseLists) { + int inUse = 0; + for (JavaThread* cur = Threads::first(); cur != NULL; cur = cur->next()) { + nInCirculation+= cur->omInUseCount; + int deflatedcount = walk_monitor_list(cur->omInUseList_addr(), &FreeHead, &FreeTail); + cur->omInUseCount-= deflatedcount; + // verifyInUse(cur); + nScavenged += deflatedcount; + nInuse += cur->omInUseCount; + } + + // For moribund threads, scan gOmInUseList + if (gOmInUseList) { + nInCirculation += gOmInUseCount; + int deflatedcount = walk_monitor_list((ObjectMonitor **)&gOmInUseList, &FreeHead, &FreeTail); + gOmInUseCount-= deflatedcount; + nScavenged += deflatedcount; + nInuse += gOmInUseCount; + } + + } else for (ObjectMonitor* block = gBlockList; block != NULL; block = next(block)) { + // Iterate over all extant monitors - Scavenge all idle monitors. + assert(block->object() == CHAINMARKER, "must be a block header"); + nInCirculation += _BLOCKSIZE ; + for (int i = 1 ; i < _BLOCKSIZE; i++) { + ObjectMonitor* mid = &block[i]; + oop obj = (oop) mid->object(); + + if (obj == NULL) { + // The monitor is not associated with an object. + // The monitor should either be a thread-specific private + // free list or the global free list. + // obj == NULL IMPLIES mid->is_busy() == 0 + guarantee (!mid->is_busy(), "invariant") ; + continue ; + } + deflated = deflate_monitor(mid, obj, &FreeHead, &FreeTail); + + if (deflated) { + mid->FreeNext = NULL ; + nScavenged ++ ; + } else { + nInuse ++; + } + } + } + + MonitorFreeCount += nScavenged; + + // Consider: audit gFreeList to ensure that MonitorFreeCount and list agree. + + if (ObjectMonitor::Knob_Verbose) { + ::printf ("Deflate: InCirc=%d InUse=%d Scavenged=%d ForceMonitorScavenge=%d : pop=%d free=%d\n", + nInCirculation, nInuse, nScavenged, ForceMonitorScavenge, + MonitorPopulation, MonitorFreeCount) ; + ::fflush(stdout) ; + } + + ForceMonitorScavenge = 0; // Reset + + // Move the scavenged monitors back to the global free list. + if (FreeHead != NULL) { + guarantee (FreeTail != NULL && nScavenged > 0, "invariant") ; + assert (FreeTail->FreeNext == NULL, "invariant") ; + // constant-time list splice - prepend scavenged segment to gFreeList + FreeTail->FreeNext = gFreeList ; + gFreeList = FreeHead ; + } + Thread::muxRelease (&ListLock) ; + + if (ObjectMonitor::_sync_Deflations != NULL) ObjectMonitor::_sync_Deflations->inc(nScavenged) ; + if (ObjectMonitor::_sync_MonExtant != NULL) ObjectMonitor::_sync_MonExtant ->set_value(nInCirculation); + + // TODO: Add objectMonitor leak detection. + // Audit/inventory the objectMonitors -- make sure they're all accounted for. + GVars.stwRandom = os::random() ; + GVars.stwCycle ++ ; +} + +// Monitor cleanup on JavaThread::exit // Iterate through monitor cache and attempt to release thread's monitors // Gives up on a particular monitor if an exception occurs, but continues @@ -1707,2967 +1579,6 @@ THREAD->clear_pending_exception(); } -// Visitors ... - -void ObjectSynchronizer::monitors_iterate(MonitorClosure* closure) { - ObjectMonitor* block = gBlockList; - ObjectMonitor* mid; - while (block) { - assert(block->object() == CHAINMARKER, "must be a block header"); - for (int i = _BLOCKSIZE - 1; i > 0; i--) { - mid = block + i; - oop object = (oop) mid->object(); - if (object != NULL) { - closure->do_monitor(mid); - } - } - block = (ObjectMonitor*) block->FreeNext; - } -} - -void ObjectSynchronizer::oops_do(OopClosure* f) { - assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint"); - for (ObjectMonitor* block = gBlockList; block != NULL; block = next(block)) { - assert(block->object() == CHAINMARKER, "must be a block header"); - for (int i = 1; i < _BLOCKSIZE; i++) { - ObjectMonitor* mid = &block[i]; - if (mid->object() != NULL) { - f->do_oop((oop*)mid->object_addr()); - } - } - } -} - -// Deflate_idle_monitors() is called at all safepoints, immediately -// after all mutators are stopped, but before any objects have moved. -// It traverses the list of known monitors, deflating where possible. -// The scavenged monitor are returned to the monitor free list. -// -// Beware that we scavenge at *every* stop-the-world point. -// Having a large number of monitors in-circulation negatively -// impacts the performance of some applications (e.g., PointBase). -// Broadly, we want to minimize the # of monitors in circulation. -// Alternately, we could partition the active monitors into sub-lists -// of those that need scanning and those that do not. -// Specifically, we would add a new sub-list of objectmonitors -// that are in-circulation and potentially active. deflate_idle_monitors() -// would scan only that list. Other monitors could reside on a quiescent -// list. Such sequestered monitors wouldn't need to be scanned by -// deflate_idle_monitors(). omAlloc() would first check the global free list, -// then the quiescent list, and, failing those, would allocate a new block. -// Deflate_idle_monitors() would scavenge and move monitors to the -// quiescent list. -// -// Perversely, the heap size -- and thus the STW safepoint rate -- -// typically drives the scavenge rate. Large heaps can mean infrequent GC, -// which in turn can mean large(r) numbers of objectmonitors in circulation. -// This is an unfortunate aspect of this design. -// -// Another refinement would be to refrain from calling deflate_idle_monitors() -// except at stop-the-world points associated with garbage collections. -// -// An even better solution would be to deflate on-the-fly, aggressively, -// at monitorexit-time as is done in EVM's metalock or Relaxed Locks. - -void ObjectSynchronizer::deflate_idle_monitors() { - assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint"); - int nInuse = 0 ; // currently associated with objects - int nInCirculation = 0 ; // extant - int nScavenged = 0 ; // reclaimed - - ObjectMonitor * FreeHead = NULL ; // Local SLL of scavenged monitors - ObjectMonitor * FreeTail = NULL ; - - // Iterate over all extant monitors - Scavenge all idle monitors. - TEVENT (deflate_idle_monitors) ; - for (ObjectMonitor* block = gBlockList; block != NULL; block = next(block)) { - assert(block->object() == CHAINMARKER, "must be a block header"); - nInCirculation += _BLOCKSIZE ; - for (int i = 1 ; i < _BLOCKSIZE; i++) { - ObjectMonitor* mid = &block[i]; - oop obj = (oop) mid->object(); - - if (obj == NULL) { - // The monitor is not associated with an object. - // The monitor should either be a thread-specific private - // free list or the global free list. - // obj == NULL IMPLIES mid->is_busy() == 0 - guarantee (!mid->is_busy(), "invariant") ; - continue ; - } - - // Normal case ... The monitor is associated with obj. - guarantee (obj->mark() == markOopDesc::encode(mid), "invariant") ; - guarantee (mid == obj->mark()->monitor(), "invariant"); - guarantee (mid->header()->is_neutral(), "invariant"); - - if (mid->is_busy()) { - if (ClearResponsibleAtSTW) mid->_Responsible = NULL ; - nInuse ++ ; - } else { - // Deflate the monitor if it is no longer being used - // It's idle - scavenge and return to the global free list - // plain old deflation ... - TEVENT (deflate_idle_monitors - scavenge1) ; - if (TraceMonitorInflation) { - if (obj->is_instance()) { - ResourceMark rm; - tty->print_cr("Deflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s", - (intptr_t) obj, (intptr_t) obj->mark(), Klass::cast(obj->klass())->external_name()); - } - } - - // Restore the header back to obj - obj->release_set_mark(mid->header()); - mid->clear(); - - assert (mid->object() == NULL, "invariant") ; - - // Move the object to the working free list defined by FreeHead,FreeTail. - mid->FreeNext = NULL ; - if (FreeHead == NULL) FreeHead = mid ; - if (FreeTail != NULL) FreeTail->FreeNext = mid ; - FreeTail = mid ; - nScavenged ++ ; - } - } - } - - // Move the scavenged monitors back to the global free list. - // In theory we don't need the freelist lock as we're at a STW safepoint. - // omAlloc() and omFree() can only be called while a thread is _not in safepoint state. - // But it's remotely possible that omFlush() or release_monitors_owned_by_thread() - // might be called while not at a global STW safepoint. In the interest of - // safety we protect the following access with ListLock. - // An even more conservative and prudent approach would be to guard - // the main loop in scavenge_idle_monitors() with ListLock. - if (FreeHead != NULL) { - guarantee (FreeTail != NULL && nScavenged > 0, "invariant") ; - assert (FreeTail->FreeNext == NULL, "invariant") ; - // constant-time list splice - prepend scavenged segment to gFreeList - Thread::muxAcquire (&ListLock, "scavenge - return") ; - FreeTail->FreeNext = gFreeList ; - gFreeList = FreeHead ; - Thread::muxRelease (&ListLock) ; - } - - if (_sync_Deflations != NULL) _sync_Deflations->inc(nScavenged) ; - if (_sync_MonExtant != NULL) _sync_MonExtant ->set_value(nInCirculation); - - // TODO: Add objectMonitor leak detection. - // Audit/inventory the objectMonitors -- make sure they're all accounted for. - GVars.stwRandom = os::random() ; - GVars.stwCycle ++ ; -} - -// A macro is used below because there may already be a pending -// exception which should not abort the execution of the routines -// which use this (which is why we don't put this into check_slow and -// call it with a CHECK argument). - -#define CHECK_OWNER() \ - do { \ - if (THREAD != _owner) { \ - if (THREAD->is_lock_owned((address) _owner)) { \ - _owner = THREAD ; /* Convert from basiclock addr to Thread addr */ \ - _recursions = 0; \ - OwnerIsThread = 1 ; \ - } else { \ - TEVENT (Throw IMSX) ; \ - THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \ - } \ - } \ - } while (false) - -// TODO-FIXME: eliminate ObjectWaiters. Replace this visitor/enumerator -// interface with a simple FirstWaitingThread(), NextWaitingThread() interface. - -ObjectWaiter* ObjectMonitor::first_waiter() { - return _WaitSet; -} - -ObjectWaiter* ObjectMonitor::next_waiter(ObjectWaiter* o) { - return o->_next; -} - -Thread* ObjectMonitor::thread_of_waiter(ObjectWaiter* o) { - return o->_thread; -} - -// initialize the monitor, exception the semaphore, all other fields -// are simple integers or pointers -ObjectMonitor::ObjectMonitor() { - _header = NULL; - _count = 0; - _waiters = 0, - _recursions = 0; - _object = NULL; - _owner = NULL; - _WaitSet = NULL; - _WaitSetLock = 0 ; - _Responsible = NULL ; - _succ = NULL ; - _cxq = NULL ; - FreeNext = NULL ; - _EntryList = NULL ; - _SpinFreq = 0 ; - _SpinClock = 0 ; - OwnerIsThread = 0 ; -} - -ObjectMonitor::~ObjectMonitor() { - // TODO: Add asserts ... - // _cxq == 0 _succ == NULL _owner == NULL _waiters == 0 - // _count == 0 _EntryList == NULL etc -} - -intptr_t ObjectMonitor::is_busy() const { - // TODO-FIXME: merge _count and _waiters. - // TODO-FIXME: assert _owner == null implies _recursions = 0 - // TODO-FIXME: assert _WaitSet != null implies _count > 0 - return _count|_waiters|intptr_t(_owner)|intptr_t(_cxq)|intptr_t(_EntryList ) ; -} - -void ObjectMonitor::Recycle () { - // TODO: add stronger asserts ... - // _cxq == 0 _succ == NULL _owner == NULL _waiters == 0 - // _count == 0 EntryList == NULL - // _recursions == 0 _WaitSet == NULL - // TODO: assert (is_busy()|_recursions) == 0 - _succ = NULL ; - _EntryList = NULL ; - _cxq = NULL ; - _WaitSet = NULL ; - _recursions = 0 ; - _SpinFreq = 0 ; - _SpinClock = 0 ; - OwnerIsThread = 0 ; -} - -// WaitSet management ... - -inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) { - assert(node != NULL, "should not dequeue NULL node"); - assert(node->_prev == NULL, "node already in list"); - assert(node->_next == NULL, "node already in list"); - // put node at end of queue (circular doubly linked list) - if (_WaitSet == NULL) { - _WaitSet = node; - node->_prev = node; - node->_next = node; - } else { - ObjectWaiter* head = _WaitSet ; - ObjectWaiter* tail = head->_prev; - assert(tail->_next == head, "invariant check"); - tail->_next = node; - head->_prev = node; - node->_next = head; - node->_prev = tail; - } -} - -inline ObjectWaiter* ObjectMonitor::DequeueWaiter() { - // dequeue the very first waiter - ObjectWaiter* waiter = _WaitSet; - if (waiter) { - DequeueSpecificWaiter(waiter); - } - return waiter; -} - -inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) { - assert(node != NULL, "should not dequeue NULL node"); - assert(node->_prev != NULL, "node already removed from list"); - assert(node->_next != NULL, "node already removed from list"); - // when the waiter has woken up because of interrupt, - // timeout or other spurious wake-up, dequeue the - // waiter from waiting list - ObjectWaiter* next = node->_next; - if (next == node) { - assert(node->_prev == node, "invariant check"); - _WaitSet = NULL; - } else { - ObjectWaiter* prev = node->_prev; - assert(prev->_next == node, "invariant check"); - assert(next->_prev == node, "invariant check"); - next->_prev = prev; - prev->_next = next; - if (_WaitSet == node) { - _WaitSet = next; - } - } - node->_next = NULL; - node->_prev = NULL; -} - -static char * kvGet (char * kvList, const char * Key) { - if (kvList == NULL) return NULL ; - size_t n = strlen (Key) ; - char * Search ; - for (Search = kvList ; *Search ; Search += strlen(Search) + 1) { - if (strncmp (Search, Key, n) == 0) { - if (Search[n] == '=') return Search + n + 1 ; - if (Search[n] == 0) return (char *) "1" ; - } - } - return NULL ; -} - -static int kvGetInt (char * kvList, const char * Key, int Default) { - char * v = kvGet (kvList, Key) ; - int rslt = v ? ::strtol (v, NULL, 0) : Default ; - if (Knob_ReportSettings && v != NULL) { - ::printf (" SyncKnob: %s %d(%d)\n", Key, rslt, Default) ; - ::fflush (stdout) ; - } - return rslt ; -} - -// By convention we unlink a contending thread from EntryList|cxq immediately -// after the thread acquires the lock in ::enter(). Equally, we could defer -// unlinking the thread until ::exit()-time. - -void ObjectMonitor::UnlinkAfterAcquire (Thread * Self, ObjectWaiter * SelfNode) -{ - assert (_owner == Self, "invariant") ; - assert (SelfNode->_thread == Self, "invariant") ; - - if (SelfNode->TState == ObjectWaiter::TS_ENTER) { - // Normal case: remove Self from the DLL EntryList . - // This is a constant-time operation. - ObjectWaiter * nxt = SelfNode->_next ; - ObjectWaiter * prv = SelfNode->_prev ; - if (nxt != NULL) nxt->_prev = prv ; - if (prv != NULL) prv->_next = nxt ; - if (SelfNode == _EntryList ) _EntryList = nxt ; - assert (nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant") ; - assert (prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant") ; - TEVENT (Unlink from EntryList) ; - } else { - guarantee (SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant") ; - // Inopportune interleaving -- Self is still on the cxq. - // This usually means the enqueue of self raced an exiting thread. - // Normally we'll find Self near the front of the cxq, so - // dequeueing is typically fast. If needbe we can accelerate - // this with some MCS/CHL-like bidirectional list hints and advisory - // back-links so dequeueing from the interior will normally operate - // in constant-time. - // Dequeue Self from either the head (with CAS) or from the interior - // with a linear-time scan and normal non-atomic memory operations. - // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList - // and then unlink Self from EntryList. We have to drain eventually, - // so it might as well be now. - - ObjectWaiter * v = _cxq ; - assert (v != NULL, "invariant") ; - if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) { - // The CAS above can fail from interference IFF a "RAT" arrived. - // In that case Self must be in the interior and can no longer be - // at the head of cxq. - if (v == SelfNode) { - assert (_cxq != v, "invariant") ; - v = _cxq ; // CAS above failed - start scan at head of list - } - ObjectWaiter * p ; - ObjectWaiter * q = NULL ; - for (p = v ; p != NULL && p != SelfNode; p = p->_next) { - q = p ; - assert (p->TState == ObjectWaiter::TS_CXQ, "invariant") ; - } - assert (v != SelfNode, "invariant") ; - assert (p == SelfNode, "Node not found on cxq") ; - assert (p != _cxq, "invariant") ; - assert (q != NULL, "invariant") ; - assert (q->_next == p, "invariant") ; - q->_next = p->_next ; - } - TEVENT (Unlink from cxq) ; - } - - // Diagnostic hygiene ... - SelfNode->_prev = (ObjectWaiter *) 0xBAD ; - SelfNode->_next = (ObjectWaiter *) 0xBAD ; - SelfNode->TState = ObjectWaiter::TS_RUN ; -} - -// Caveat: TryLock() is not necessarily serializing if it returns failure. -// Callers must compensate as needed. - -int ObjectMonitor::TryLock (Thread * Self) { - for (;;) { - void * own = _owner ; - if (own != NULL) return 0 ; - if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) { - // Either guarantee _recursions == 0 or set _recursions = 0. - assert (_recursions == 0, "invariant") ; - assert (_owner == Self, "invariant") ; - // CONSIDER: set or assert that OwnerIsThread == 1 - return 1 ; - } - // The lock had been free momentarily, but we lost the race to the lock. - // Interference -- the CAS failed. - // We can either return -1 or retry. - // Retry doesn't make as much sense because the lock was just acquired. - if (true) return -1 ; - } -} - -// NotRunnable() -- informed spinning -// -// Don't bother spinning if the owner is not eligible to drop the lock. -// Peek at the owner's schedctl.sc_state and Thread._thread_values and -// spin only if the owner thread is _thread_in_Java or _thread_in_vm. -// The thread must be runnable in order to drop the lock in timely fashion. -// If the _owner is not runnable then spinning will not likely be -// successful (profitable). -// -// Beware -- the thread referenced by _owner could have died -// so a simply fetch from _owner->_thread_state might trap. -// Instead, we use SafeFetchXX() to safely LD _owner->_thread_state. -// Because of the lifecycle issues the schedctl and _thread_state values -// observed by NotRunnable() might be garbage. NotRunnable must -// tolerate this and consider the observed _thread_state value -// as advisory. -// -// Beware too, that _owner is sometimes a BasicLock address and sometimes -// a thread pointer. We differentiate the two cases with OwnerIsThread. -// Alternately, we might tag the type (thread pointer vs basiclock pointer) -// with the LSB of _owner. Another option would be to probablistically probe -// the putative _owner->TypeTag value. -// -// Checking _thread_state isn't perfect. Even if the thread is -// in_java it might be blocked on a page-fault or have been preempted -// and sitting on a ready/dispatch queue. _thread state in conjunction -// with schedctl.sc_state gives us a good picture of what the -// thread is doing, however. -// -// TODO: check schedctl.sc_state. -// We'll need to use SafeFetch32() to read from the schedctl block. -// See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/ -// -// The return value from NotRunnable() is *advisory* -- the -// result is based on sampling and is not necessarily coherent. -// The caller must tolerate false-negative and false-positive errors. -// Spinning, in general, is probabilistic anyway. - - -int ObjectMonitor::NotRunnable (Thread * Self, Thread * ox) { - // Check either OwnerIsThread or ox->TypeTag == 2BAD. - if (!OwnerIsThread) return 0 ; - - if (ox == NULL) return 0 ; - - // Avoid transitive spinning ... - // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L. - // Immediately after T1 acquires L it's possible that T2, also - // spinning on L, will see L.Owner=T1 and T1._Stalled=L. - // This occurs transiently after T1 acquired L but before - // T1 managed to clear T1.Stalled. T2 does not need to abort - // its spin in this circumstance. - intptr_t BlockedOn = SafeFetchN ((intptr_t *) &ox->_Stalled, intptr_t(1)) ; - - if (BlockedOn == 1) return 1 ; - if (BlockedOn != 0) { - return BlockedOn != intptr_t(this) && _owner == ox ; - } - - assert (sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant") ; - int jst = SafeFetch32 ((int *) &((JavaThread *) ox)->_thread_state, -1) ; ; - // consider also: jst != _thread_in_Java -- but that's overspecific. - return jst == _thread_blocked || jst == _thread_in_native ; -} - - -// Adaptive spin-then-block - rational spinning -// -// Note that we spin "globally" on _owner with a classic SMP-polite TATAS -// algorithm. On high order SMP systems it would be better to start with -// a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH, -// a contending thread could enqueue itself on the cxq and then spin locally -// on a thread-specific variable such as its ParkEvent._Event flag. -// That's left as an exercise for the reader. Note that global spinning is -// not problematic on Niagara, as the L2$ serves the interconnect and has both -// low latency and massive bandwidth. -// -// Broadly, we can fix the spin frequency -- that is, the % of contended lock -// acquisition attempts where we opt to spin -- at 100% and vary the spin count -// (duration) or we can fix the count at approximately the duration of -// a context switch and vary the frequency. Of course we could also -// vary both satisfying K == Frequency * Duration, where K is adaptive by monitor. -// See http://j2se.east/~dice/PERSIST/040824-AdaptiveSpinning.html. -// -// This implementation varies the duration "D", where D varies with -// the success rate of recent spin attempts. (D is capped at approximately -// length of a round-trip context switch). The success rate for recent -// spin attempts is a good predictor of the success rate of future spin -// attempts. The mechanism adapts automatically to varying critical -// section length (lock modality), system load and degree of parallelism. -// D is maintained per-monitor in _SpinDuration and is initialized -// optimistically. Spin frequency is fixed at 100%. -// -// Note that _SpinDuration is volatile, but we update it without locks -// or atomics. The code is designed so that _SpinDuration stays within -// a reasonable range even in the presence of races. The arithmetic -// operations on _SpinDuration are closed over the domain of legal values, -// so at worst a race will install and older but still legal value. -// At the very worst this introduces some apparent non-determinism. -// We might spin when we shouldn't or vice-versa, but since the spin -// count are relatively short, even in the worst case, the effect is harmless. -// -// Care must be taken that a low "D" value does not become an -// an absorbing state. Transient spinning failures -- when spinning -// is overall profitable -- should not cause the system to converge -// on low "D" values. We want spinning to be stable and predictable -// and fairly responsive to change and at the same time we don't want -// it to oscillate, become metastable, be "too" non-deterministic, -// or converge on or enter undesirable stable absorbing states. -// -// We implement a feedback-based control system -- using past behavior -// to predict future behavior. We face two issues: (a) if the -// input signal is random then the spin predictor won't provide optimal -// results, and (b) if the signal frequency is too high then the control -// system, which has some natural response lag, will "chase" the signal. -// (b) can arise from multimodal lock hold times. Transient preemption -// can also result in apparent bimodal lock hold times. -// Although sub-optimal, neither condition is particularly harmful, as -// in the worst-case we'll spin when we shouldn't or vice-versa. -// The maximum spin duration is rather short so the failure modes aren't bad. -// To be conservative, I've tuned the gain in system to bias toward -// _not spinning. Relatedly, the system can sometimes enter a mode where it -// "rings" or oscillates between spinning and not spinning. This happens -// when spinning is just on the cusp of profitability, however, so the -// situation is not dire. The state is benign -- there's no need to add -// hysteresis control to damp the transition rate between spinning and -// not spinning. -// -// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -// -// Spin-then-block strategies ... -// -// Thoughts on ways to improve spinning : -// -// * Periodically call {psr_}getloadavg() while spinning, and -// permit unbounded spinning if the load average is < -// the number of processors. Beware, however, that getloadavg() -// is exceptionally fast on solaris (about 1/10 the cost of a full -// spin cycle, but quite expensive on linux. Beware also, that -// multiple JVMs could "ring" or oscillate in a feedback loop. -// Sufficient damping would solve that problem. -// -// * We currently use spin loops with iteration counters to approximate -// spinning for some interval. Given the availability of high-precision -// time sources such as gethrtime(), %TICK, %STICK, RDTSC, etc., we should -// someday reimplement the spin loops to duration-based instead of iteration-based. -// -// * Don't spin if there are more than N = (CPUs/2) threads -// currently spinning on the monitor (or globally). -// That is, limit the number of concurrent spinners. -// We might also limit the # of spinners in the JVM, globally. -// -// * If a spinning thread observes _owner change hands it should -// abort the spin (and park immediately) or at least debit -// the spin counter by a large "penalty". -// -// * Classically, the spin count is either K*(CPUs-1) or is a -// simple constant that approximates the length of a context switch. -// We currently use a value -- computed by a special utility -- that -// approximates round-trip context switch times. -// -// * Normally schedctl_start()/_stop() is used to advise the kernel -// to avoid preempting threads that are running in short, bounded -// critical sections. We could use the schedctl hooks in an inverted -// sense -- spinners would set the nopreempt flag, but poll the preempt -// pending flag. If a spinner observed a pending preemption it'd immediately -// abort the spin and park. As such, the schedctl service acts as -// a preemption warning mechanism. -// -// * In lieu of spinning, if the system is running below saturation -// (that is, loadavg() << #cpus), we can instead suppress futile -// wakeup throttling, or even wake more than one successor at exit-time. -// The net effect is largely equivalent to spinning. In both cases, -// contending threads go ONPROC and opportunistically attempt to acquire -// the lock, decreasing lock handover latency at the expense of wasted -// cycles and context switching. -// -// * We might to spin less after we've parked as the thread will -// have less $ and TLB affinity with the processor. -// Likewise, we might spin less if we come ONPROC on a different -// processor or after a long period (>> rechose_interval). -// -// * A table-driven state machine similar to Solaris' dispadmin scheduling -// tables might be a better design. Instead of encoding information in -// _SpinDuration, _SpinFreq and _SpinClock we'd just use explicit, -// discrete states. Success or failure during a spin would drive -// state transitions, and each state node would contain a spin count. -// -// * If the processor is operating in a mode intended to conserve power -// (such as Intel's SpeedStep) or to reduce thermal output (thermal -// step-down mode) then the Java synchronization subsystem should -// forgo spinning. -// -// * The minimum spin duration should be approximately the worst-case -// store propagation latency on the platform. That is, the time -// it takes a store on CPU A to become visible on CPU B, where A and -// B are "distant". -// -// * We might want to factor a thread's priority in the spin policy. -// Threads with a higher priority might spin for slightly longer. -// Similarly, if we use back-off in the TATAS loop, lower priority -// threads might back-off longer. We don't currently use a -// thread's priority when placing it on the entry queue. We may -// want to consider doing so in future releases. -// -// * We might transiently drop a thread's scheduling priority while it spins. -// SCHED_BATCH on linux and FX scheduling class at priority=0 on Solaris -// would suffice. We could even consider letting the thread spin indefinitely at -// a depressed or "idle" priority. This brings up fairness issues, however -- -// in a saturated system a thread would with a reduced priority could languish -// for extended periods on the ready queue. -// -// * While spinning try to use the otherwise wasted time to help the VM make -// progress: -// -// -- YieldTo() the owner, if the owner is OFFPROC but ready -// Done our remaining quantum directly to the ready thread. -// This helps "push" the lock owner through the critical section. -// It also tends to improve affinity/locality as the lock -// "migrates" less frequently between CPUs. -// -- Walk our own stack in anticipation of blocking. Memoize the roots. -// -- Perform strand checking for other thread. Unpark potential strandees. -// -- Help GC: trace or mark -- this would need to be a bounded unit of work. -// Unfortunately this will pollute our $ and TLBs. Recall that we -// spin to avoid context switching -- context switching has an -// immediate cost in latency, a disruptive cost to other strands on a CMT -// processor, and an amortized cost because of the D$ and TLB cache -// reload transient when the thread comes back ONPROC and repopulates -// $s and TLBs. -// -- call getloadavg() to see if the system is saturated. It'd probably -// make sense to call getloadavg() half way through the spin. -// If the system isn't at full capacity the we'd simply reset -// the spin counter to and extend the spin attempt. -// -- Doug points out that we should use the same "helping" policy -// in thread.yield(). -// -// * Try MONITOR-MWAIT on systems that support those instructions. -// -// * The spin statistics that drive spin decisions & frequency are -// maintained in the objectmonitor structure so if we deflate and reinflate -// we lose spin state. In practice this is not usually a concern -// as the default spin state after inflation is aggressive (optimistic) -// and tends toward spinning. So in the worst case for a lock where -// spinning is not profitable we may spin unnecessarily for a brief -// period. But then again, if a lock is contended it'll tend not to deflate -// in the first place. - - -intptr_t ObjectMonitor::SpinCallbackArgument = 0 ; -int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL ; - -// Spinning: Fixed frequency (100%), vary duration - -int ObjectMonitor::TrySpin_VaryDuration (Thread * Self) { - - // Dumb, brutal spin. Good for comparative measurements against adaptive spinning. - int ctr = Knob_FixedSpin ; - if (ctr != 0) { - while (--ctr >= 0) { - if (TryLock (Self) > 0) return 1 ; - SpinPause () ; - } - return 0 ; - } - - for (ctr = Knob_PreSpin + 1; --ctr >= 0 ; ) { - if (TryLock(Self) > 0) { - // Increase _SpinDuration ... - // Note that we don't clamp SpinDuration precisely at SpinLimit. - // Raising _SpurDuration to the poverty line is key. - int x = _SpinDuration ; - if (x < Knob_SpinLimit) { - if (x < Knob_Poverty) x = Knob_Poverty ; - _SpinDuration = x + Knob_BonusB ; - } - return 1 ; - } - SpinPause () ; - } - - // Admission control - verify preconditions for spinning - // - // We always spin a little bit, just to prevent _SpinDuration == 0 from - // becoming an absorbing state. Put another way, we spin briefly to - // sample, just in case the system load, parallelism, contention, or lock - // modality changed. - // - // Consider the following alternative: - // Periodically set _SpinDuration = _SpinLimit and try a long/full - // spin attempt. "Periodically" might mean after a tally of - // the # of failed spin attempts (or iterations) reaches some threshold. - // This takes us into the realm of 1-out-of-N spinning, where we - // hold the duration constant but vary the frequency. - - ctr = _SpinDuration ; - if (ctr < Knob_SpinBase) ctr = Knob_SpinBase ; - if (ctr <= 0) return 0 ; - - if (Knob_SuccRestrict && _succ != NULL) return 0 ; - if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) { - TEVENT (Spin abort - notrunnable [TOP]); - return 0 ; - } - - int MaxSpin = Knob_MaxSpinners ; - if (MaxSpin >= 0) { - if (_Spinner > MaxSpin) { - TEVENT (Spin abort -- too many spinners) ; - return 0 ; - } - // Slighty racy, but benign ... - Adjust (&_Spinner, 1) ; - } - - // We're good to spin ... spin ingress. - // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades - // when preparing to LD...CAS _owner, etc and the CAS is likely - // to succeed. - int hits = 0 ; - int msk = 0 ; - int caspty = Knob_CASPenalty ; - int oxpty = Knob_OXPenalty ; - int sss = Knob_SpinSetSucc ; - if (sss && _succ == NULL ) _succ = Self ; - Thread * prv = NULL ; - - // There are three ways to exit the following loop: - // 1. A successful spin where this thread has acquired the lock. - // 2. Spin failure with prejudice - // 3. Spin failure without prejudice - - while (--ctr >= 0) { - - // Periodic polling -- Check for pending GC - // Threads may spin while they're unsafe. - // We don't want spinning threads to delay the JVM from reaching - // a stop-the-world safepoint or to steal cycles from GC. - // If we detect a pending safepoint we abort in order that - // (a) this thread, if unsafe, doesn't delay the safepoint, and (b) - // this thread, if safe, doesn't steal cycles from GC. - // This is in keeping with the "no loitering in runtime" rule. - // We periodically check to see if there's a safepoint pending. - if ((ctr & 0xFF) == 0) { - if (SafepointSynchronize::do_call_back()) { - TEVENT (Spin: safepoint) ; - goto Abort ; // abrupt spin egress - } - if (Knob_UsePause & 1) SpinPause () ; - - int (*scb)(intptr_t,int) = SpinCallbackFunction ; - if (hits > 50 && scb != NULL) { - int abend = (*scb)(SpinCallbackArgument, 0) ; - } - } - - if (Knob_UsePause & 2) SpinPause() ; - - // Exponential back-off ... Stay off the bus to reduce coherency traffic. - // This is useful on classic SMP systems, but is of less utility on - // N1-style CMT platforms. - // - // Trade-off: lock acquisition latency vs coherency bandwidth. - // Lock hold times are typically short. A histogram - // of successful spin attempts shows that we usually acquire - // the lock early in the spin. That suggests we want to - // sample _owner frequently in the early phase of the spin, - // but then back-off and sample less frequently as the spin - // progresses. The back-off makes a good citizen on SMP big - // SMP systems. Oversampling _owner can consume excessive - // coherency bandwidth. Relatedly, if we _oversample _owner we - // can inadvertently interfere with the the ST m->owner=null. - // executed by the lock owner. - if (ctr & msk) continue ; - ++hits ; - if ((hits & 0xF) == 0) { - // The 0xF, above, corresponds to the exponent. - // Consider: (msk+1)|msk - msk = ((msk << 2)|3) & BackOffMask ; - } - - // Probe _owner with TATAS - // If this thread observes the monitor transition or flicker - // from locked to unlocked to locked, then the odds that this - // thread will acquire the lock in this spin attempt go down - // considerably. The same argument applies if the CAS fails - // or if we observe _owner change from one non-null value to - // another non-null value. In such cases we might abort - // the spin without prejudice or apply a "penalty" to the - // spin count-down variable "ctr", reducing it by 100, say. - - Thread * ox = (Thread *) _owner ; - if (ox == NULL) { - ox = (Thread *) Atomic::cmpxchg_ptr (Self, &_owner, NULL) ; - if (ox == NULL) { - // The CAS succeeded -- this thread acquired ownership - // Take care of some bookkeeping to exit spin state. - if (sss && _succ == Self) { - _succ = NULL ; - } - if (MaxSpin > 0) Adjust (&_Spinner, -1) ; - - // Increase _SpinDuration : - // The spin was successful (profitable) so we tend toward - // longer spin attempts in the future. - // CONSIDER: factor "ctr" into the _SpinDuration adjustment. - // If we acquired the lock early in the spin cycle it - // makes sense to increase _SpinDuration proportionally. - // Note that we don't clamp SpinDuration precisely at SpinLimit. - int x = _SpinDuration ; - if (x < Knob_SpinLimit) { - if (x < Knob_Poverty) x = Knob_Poverty ; - _SpinDuration = x + Knob_Bonus ; - } - return 1 ; - } - - // The CAS failed ... we can take any of the following actions: - // * penalize: ctr -= Knob_CASPenalty - // * exit spin with prejudice -- goto Abort; - // * exit spin without prejudice. - // * Since CAS is high-latency, retry again immediately. - prv = ox ; - TEVENT (Spin: cas failed) ; - if (caspty == -2) break ; - if (caspty == -1) goto Abort ; - ctr -= caspty ; - continue ; - } - - // Did lock ownership change hands ? - if (ox != prv && prv != NULL ) { - TEVENT (spin: Owner changed) - if (oxpty == -2) break ; - if (oxpty == -1) goto Abort ; - ctr -= oxpty ; - } - prv = ox ; - - // Abort the spin if the owner is not executing. - // The owner must be executing in order to drop the lock. - // Spinning while the owner is OFFPROC is idiocy. - // Consider: ctr -= RunnablePenalty ; - if (Knob_OState && NotRunnable (Self, ox)) { - TEVENT (Spin abort - notrunnable); - goto Abort ; - } - if (sss && _succ == NULL ) _succ = Self ; - } - - // Spin failed with prejudice -- reduce _SpinDuration. - // TODO: Use an AIMD-like policy to adjust _SpinDuration. - // AIMD is globally stable. - TEVENT (Spin failure) ; - { - int x = _SpinDuration ; - if (x > 0) { - // Consider an AIMD scheme like: x -= (x >> 3) + 100 - // This is globally sample and tends to damp the response. - x -= Knob_Penalty ; - if (x < 0) x = 0 ; - _SpinDuration = x ; - } - } - - Abort: - if (MaxSpin >= 0) Adjust (&_Spinner, -1) ; - if (sss && _succ == Self) { - _succ = NULL ; - // Invariant: after setting succ=null a contending thread - // must recheck-retry _owner before parking. This usually happens - // in the normal usage of TrySpin(), but it's safest - // to make TrySpin() as foolproof as possible. - OrderAccess::fence() ; - if (TryLock(Self) > 0) return 1 ; - } - return 0 ; -} - -#define TrySpin TrySpin_VaryDuration - -static void DeferredInitialize () { - if (InitDone > 0) return ; - if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) { - while (InitDone != 1) ; - return ; - } - - // One-shot global initialization ... - // The initialization is idempotent, so we don't need locks. - // In the future consider doing this via os::init_2(). - // SyncKnobs consist of <Key>=<Value> pairs in the style - // of environment variables. Start by converting ':' to NUL. - - if (SyncKnobs == NULL) SyncKnobs = "" ; - - size_t sz = strlen (SyncKnobs) ; - char * knobs = (char *) malloc (sz + 2) ; - if (knobs == NULL) { - vm_exit_out_of_memory (sz + 2, "Parse SyncKnobs") ; - guarantee (0, "invariant") ; - } - strcpy (knobs, SyncKnobs) ; - knobs[sz+1] = 0 ; - for (char * p = knobs ; *p ; p++) { - if (*p == ':') *p = 0 ; - } - - #define SETKNOB(x) { Knob_##x = kvGetInt (knobs, #x, Knob_##x); } - SETKNOB(ReportSettings) ; - SETKNOB(Verbose) ; - SETKNOB(FixedSpin) ; - SETKNOB(SpinLimit) ; - SETKNOB(SpinBase) ; - SETKNOB(SpinBackOff); - SETKNOB(CASPenalty) ; - SETKNOB(OXPenalty) ; - SETKNOB(LogSpins) ; - SETKNOB(SpinSetSucc) ; - SETKNOB(SuccEnabled) ; - SETKNOB(SuccRestrict) ; - SETKNOB(Penalty) ; - SETKNOB(Bonus) ; - SETKNOB(BonusB) ; - SETKNOB(Poverty) ; - SETKNOB(SpinAfterFutile) ; - SETKNOB(UsePause) ; - SETKNOB(SpinEarly) ; - SETKNOB(OState) ; - SETKNOB(MaxSpinners) ; - SETKNOB(PreSpin) ; - SETKNOB(ExitPolicy) ; - SETKNOB(QMode); - SETKNOB(ResetEvent) ; - SETKNOB(MoveNotifyee) ; - SETKNOB(FastHSSEC) ; - #undef SETKNOB - - if (os::is_MP()) { - BackOffMask = (1 << Knob_SpinBackOff) - 1 ; - if (Knob_ReportSettings) ::printf ("BackOffMask=%X\n", BackOffMask) ; - // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1) - } else { - Knob_SpinLimit = 0 ; - Knob_SpinBase = 0 ; - Knob_PreSpin = 0 ; - Knob_FixedSpin = -1 ; - } - - if (Knob_LogSpins == 0) { - ObjectSynchronizer::_sync_FailedSpins = NULL ; - } - - free (knobs) ; - OrderAccess::fence() ; - InitDone = 1 ; -} - -// Theory of operations -- Monitors lists, thread residency, etc: -// -// * A thread acquires ownership of a monitor by successfully -// CAS()ing the _owner field from null to non-null. -// -// * Invariant: A thread appears on at most one monitor list -- -// cxq, EntryList or WaitSet -- at any one time. -// -// * Contending threads "push" themselves onto the cxq with CAS -// and then spin/park. -// -// * After a contending thread eventually acquires the lock it must -// dequeue itself from either the EntryList or the cxq. -// -// * The exiting thread identifies and unparks an "heir presumptive" -// tentative successor thread on the EntryList. Critically, the -// exiting thread doesn't unlink the successor thread from the EntryList. -// After having been unparked, the wakee will recontend for ownership of -// the monitor. The successor (wakee) will either acquire the lock or -// re-park itself. -// -// Succession is provided for by a policy of competitive handoff. -// The exiting thread does _not_ grant or pass ownership to the -// successor thread. (This is also referred to as "handoff" succession"). -// Instead the exiting thread releases ownership and possibly wakes -// a successor, so the successor can (re)compete for ownership of the lock. -// If the EntryList is empty but the cxq is populated the exiting -// thread will drain the cxq into the EntryList. It does so by -// by detaching the cxq (installing null with CAS) and folding -// the threads from the cxq into the EntryList. The EntryList is -// doubly linked, while the cxq is singly linked because of the -// CAS-based "push" used to enqueue recently arrived threads (RATs). -// -// * Concurrency invariants: -// -// -- only the monitor owner may access or mutate the EntryList. -// The mutex property of the monitor itself protects the EntryList -// from concurrent interference. -// -- Only the monitor owner may detach the cxq. -// -// * The monitor entry list operations avoid locks, but strictly speaking -// they're not lock-free. Enter is lock-free, exit is not. -// See http://j2se.east/~dice/PERSIST/040825-LockFreeQueues.html -// -// * The cxq can have multiple concurrent "pushers" but only one concurrent -// detaching thread. This mechanism is immune from the ABA corruption. -// More precisely, the CAS-based "push" onto cxq is ABA-oblivious. -// -// * Taken together, the cxq and the EntryList constitute or form a -// single logical queue of threads stalled trying to acquire the lock. -// We use two distinct lists to improve the odds of a constant-time -// dequeue operation after acquisition (in the ::enter() epilog) and -// to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm). -// A key desideratum is to minimize queue & monitor metadata manipulation -// that occurs while holding the monitor lock -- that is, we want to -// minimize monitor lock holds times. Note that even a small amount of -// fixed spinning will greatly reduce the # of enqueue-dequeue operations -// on EntryList|cxq. That is, spinning relieves contention on the "inner" -// locks and monitor metadata. -// -// Cxq points to the the set of Recently Arrived Threads attempting entry. -// Because we push threads onto _cxq with CAS, the RATs must take the form of -// a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when -// the unlocking thread notices that EntryList is null but _cxq is != null. -// -// The EntryList is ordered by the prevailing queue discipline and -// can be organized in any convenient fashion, such as a doubly-linked list or -// a circular doubly-linked list. Critically, we want insert and delete operations -// to operate in constant-time. If we need a priority queue then something akin -// to Solaris' sleepq would work nicely. Viz., -// http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c. -// Queue discipline is enforced at ::exit() time, when the unlocking thread -// drains the cxq into the EntryList, and orders or reorders the threads on the -// EntryList accordingly. -// -// Barring "lock barging", this mechanism provides fair cyclic ordering, -// somewhat similar to an elevator-scan. -// -// * The monitor synchronization subsystem avoids the use of native -// synchronization primitives except for the narrow platform-specific -// park-unpark abstraction. See the comments in os_solaris.cpp regarding -// the semantics of park-unpark. Put another way, this monitor implementation -// depends only on atomic operations and park-unpark. The monitor subsystem -// manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the -// underlying OS manages the READY<->RUN transitions. -// -// * Waiting threads reside on the WaitSet list -- wait() puts -// the caller onto the WaitSet. -// -// * notify() or notifyAll() simply transfers threads from the WaitSet to -// either the EntryList or cxq. Subsequent exit() operations will -// unpark the notifyee. Unparking a notifee in notify() is inefficient - -// it's likely the notifyee would simply impale itself on the lock held -// by the notifier. -// -// * An interesting alternative is to encode cxq as (List,LockByte) where -// the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary -// variable, like _recursions, in the scheme. The threads or Events that form -// the list would have to be aligned in 256-byte addresses. A thread would -// try to acquire the lock or enqueue itself with CAS, but exiting threads -// could use a 1-0 protocol and simply STB to set the LockByte to 0. -// Note that is is *not* word-tearing, but it does presume that full-word -// CAS operations are coherent with intermix with STB operations. That's true -// on most common processors. -// -// * See also http://blogs.sun.com/dave - - -void ATTR ObjectMonitor::EnterI (TRAPS) { - Thread * Self = THREAD ; - assert (Self->is_Java_thread(), "invariant") ; - assert (((JavaThread *) Self)->thread_state() == _thread_blocked , "invariant") ; - - // Try the lock - TATAS - if (TryLock (Self) > 0) { - assert (_succ != Self , "invariant") ; - assert (_owner == Self , "invariant") ; - assert (_Responsible != Self , "invariant") ; - return ; - } - - DeferredInitialize () ; - - // We try one round of spinning *before* enqueueing Self. - // - // If the _owner is ready but OFFPROC we could use a YieldTo() - // operation to donate the remainder of this thread's quantum - // to the owner. This has subtle but beneficial affinity - // effects. - - if (TrySpin (Self) > 0) { - assert (_owner == Self , "invariant") ; - assert (_succ != Self , "invariant") ; - assert (_Responsible != Self , "invariant") ; - return ; - } - - // The Spin failed -- Enqueue and park the thread ... - assert (_succ != Self , "invariant") ; - assert (_owner != Self , "invariant") ; - assert (_Responsible != Self , "invariant") ; - - // Enqueue "Self" on ObjectMonitor's _cxq. - // - // Node acts as a proxy for Self. - // As an aside, if were to ever rewrite the synchronization code mostly - // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class - // Java objects. This would avoid awkward lifecycle and liveness issues, - // as well as eliminate a subset of ABA issues. - // TODO: eliminate ObjectWaiter and enqueue either Threads or Events. - // - - ObjectWaiter node(Self) ; - Self->_ParkEvent->reset() ; - node._prev = (ObjectWaiter *) 0xBAD ; - node.TState = ObjectWaiter::TS_CXQ ; - - // Push "Self" onto the front of the _cxq. - // Once on cxq/EntryList, Self stays on-queue until it acquires the lock. - // Note that spinning tends to reduce the rate at which threads - // enqueue and dequeue on EntryList|cxq. - ObjectWaiter * nxt ; - for (;;) { - node._next = nxt = _cxq ; - if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ; - - // Interference - the CAS failed because _cxq changed. Just retry. - // As an optional optimization we retry the lock. - if (TryLock (Self) > 0) { - assert (_succ != Self , "invariant") ; - assert (_owner == Self , "invariant") ; - assert (_Responsible != Self , "invariant") ; - return ; - } - } - - // Check for cxq|EntryList edge transition to non-null. This indicates - // the onset of contention. While contention persists exiting threads - // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit - // operations revert to the faster 1-0 mode. This enter operation may interleave - // (race) a concurrent 1-0 exit operation, resulting in stranding, so we - // arrange for one of the contending thread to use a timed park() operations - // to detect and recover from the race. (Stranding is form of progress failure - // where the monitor is unlocked but all the contending threads remain parked). - // That is, at least one of the contended threads will periodically poll _owner. - // One of the contending threads will become the designated "Responsible" thread. - // The Responsible thread uses a timed park instead of a normal indefinite park - // operation -- it periodically wakes and checks for and recovers from potential - // strandings admitted by 1-0 exit operations. We need at most one Responsible - // thread per-monitor at any given moment. Only threads on cxq|EntryList may - // be responsible for a monitor. - // - // Currently, one of the contended threads takes on the added role of "Responsible". - // A viable alternative would be to use a dedicated "stranding checker" thread - // that periodically iterated over all the threads (or active monitors) and unparked - // successors where there was risk of stranding. This would help eliminate the - // timer scalability issues we see on some platforms as we'd only have one thread - // -- the checker -- parked on a timer. - - if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) { - // Try to assume the role of responsible thread for the monitor. - // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self } - Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ; - } - - // The lock have been released while this thread was occupied queueing - // itself onto _cxq. To close the race and avoid "stranding" and - // progress-liveness failure we must resample-retry _owner before parking. - // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner. - // In this case the ST-MEMBAR is accomplished with CAS(). - // - // TODO: Defer all thread state transitions until park-time. - // Since state transitions are heavy and inefficient we'd like - // to defer the state transitions until absolutely necessary, - // and in doing so avoid some transitions ... - - TEVENT (Inflated enter - Contention) ; - int nWakeups = 0 ; - int RecheckInterval = 1 ; - - for (;;) { - - if (TryLock (Self) > 0) break ; - assert (_owner != Self, "invariant") ; - - if ((SyncFlags & 2) && _Responsible == NULL) { - Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ; - } - - // park self - if (_Responsible == Self || (SyncFlags & 1)) { - TEVENT (Inflated enter - park TIMED) ; - Self->_ParkEvent->park ((jlong) RecheckInterval) ; - // Increase the RecheckInterval, but clamp the value. - RecheckInterval *= 8 ; - if (RecheckInterval > 1000) RecheckInterval = 1000 ; - } else { - TEVENT (Inflated enter - park UNTIMED) ; - Self->_ParkEvent->park() ; - } - - if (TryLock(Self) > 0) break ; - - // The lock is still contested. - // Keep a tally of the # of futile wakeups. - // Note that the counter is not protected by a lock or updated by atomics. - // That is by design - we trade "lossy" counters which are exposed to - // races during updates for a lower probe effect. - TEVENT (Inflated enter - Futile wakeup) ; - if (ObjectSynchronizer::_sync_FutileWakeups != NULL) { - ObjectSynchronizer::_sync_FutileWakeups->inc() ; - } - ++ nWakeups ; - - if (THREAD->is_Compiler_thread() && nWakeups >= 5) { - assert(false, "Compiler thread blocked by lock"); - } - - // Assuming this is not a spurious wakeup we'll normally find _succ == Self. - // We can defer clearing _succ until after the spin completes - // TrySpin() must tolerate being called with _succ == Self. - // Try yet another round of adaptive spinning. - if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ; - - // We can find that we were unpark()ed and redesignated _succ while - // we were spinning. That's harmless. If we iterate and call park(), - // park() will consume the event and return immediately and we'll - // just spin again. This pattern can repeat, leaving _succ to simply - // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks(). - // Alternately, we can sample fired() here, and if set, forgo spinning - // in the next iteration. - - if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) { - Self->_ParkEvent->reset() ; - OrderAccess::fence() ; - } - if (_succ == Self) _succ = NULL ; - - // Invariant: after clearing _succ a thread *must* retry _owner before parking. - OrderAccess::fence() ; - } - - // Egress : - // Self has acquired the lock -- Unlink Self from the cxq or EntryList. - // Normally we'll find Self on the EntryList . - // From the perspective of the lock owner (this thread), the - // EntryList is stable and cxq is prepend-only. - // The head of cxq is volatile but the interior is stable. - // In addition, Self.TState is stable. - - assert (_owner == Self , "invariant") ; - assert (object() != NULL , "invariant") ; - // I'd like to write: - // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ; - // but as we're at a safepoint that's not safe. - - UnlinkAfterAcquire (Self, &node) ; - if (_succ == Self) _succ = NULL ; - - assert (_succ != Self, "invariant") ; - if (_Responsible == Self) { - _Responsible = NULL ; - // Dekker pivot-point. - // Consider OrderAccess::storeload() here - - // We may leave threads on cxq|EntryList without a designated - // "Responsible" thread. This is benign. When this thread subsequently - // exits the monitor it can "see" such preexisting "old" threads -- - // threads that arrived on the cxq|EntryList before the fence, above -- - // by LDing cxq|EntryList. Newly arrived threads -- that is, threads - // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible - // non-null and elect a new "Responsible" timer thread. - // - // This thread executes: - // ST Responsible=null; MEMBAR (in enter epilog - here) - // LD cxq|EntryList (in subsequent exit) - // - // Entering threads in the slow/contended path execute: - // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog) - // The (ST cxq; MEMBAR) is accomplished with CAS(). - // - // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent - // exit operation from floating above the ST Responsible=null. - // - // In *practice* however, EnterI() is always followed by some atomic - // operation such as the decrement of _count in ::enter(). Those atomics - // obviate the need for the explicit MEMBAR, above. - } - - // We've acquired ownership with CAS(). - // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics. - // But since the CAS() this thread may have also stored into _succ, - // EntryList, cxq or Responsible. These meta-data updates must be - // visible __before this thread subsequently drops the lock. - // Consider what could occur if we didn't enforce this constraint -- - // STs to monitor meta-data and user-data could reorder with (become - // visible after) the ST in exit that drops ownership of the lock. - // Some other thread could then acquire the lock, but observe inconsistent - // or old monitor meta-data and heap data. That violates the JMM. - // To that end, the 1-0 exit() operation must have at least STST|LDST - // "release" barrier semantics. Specifically, there must be at least a - // STST|LDST barrier in exit() before the ST of null into _owner that drops - // the lock. The barrier ensures that changes to monitor meta-data and data - // protected by the lock will be visible before we release the lock, and - // therefore before some other thread (CPU) has a chance to acquire the lock. - // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html. - // - // Critically, any prior STs to _succ or EntryList must be visible before - // the ST of null into _owner in the *subsequent* (following) corresponding - // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily - // execute a serializing instruction. - - if (SyncFlags & 8) { - OrderAccess::fence() ; - } - return ; -} - -// ExitSuspendEquivalent: -// A faster alternate to handle_special_suspend_equivalent_condition() -// -// handle_special_suspend_equivalent_condition() unconditionally -// acquires the SR_lock. On some platforms uncontended MutexLocker() -// operations have high latency. Note that in ::enter() we call HSSEC -// while holding the monitor, so we effectively lengthen the critical sections. -// -// There are a number of possible solutions: -// -// A. To ameliorate the problem we might also defer state transitions -// to as late as possible -- just prior to parking. -// Given that, we'd call HSSEC after having returned from park(), -// but before attempting to acquire the monitor. This is only a -// partial solution. It avoids calling HSSEC while holding the -// monitor (good), but it still increases successor reacquisition latency -- -// the interval between unparking a successor and the time the successor -// resumes and retries the lock. See ReenterI(), which defers state transitions. -// If we use this technique we can also avoid EnterI()-exit() loop -// in ::enter() where we iteratively drop the lock and then attempt -// to reacquire it after suspending. -// -// B. In the future we might fold all the suspend bits into a -// composite per-thread suspend flag and then update it with CAS(). -// Alternately, a Dekker-like mechanism with multiple variables -// would suffice: -// ST Self->_suspend_equivalent = false -// MEMBAR -// LD Self_>_suspend_flags -// - - -bool ObjectMonitor::ExitSuspendEquivalent (JavaThread * jSelf) { - int Mode = Knob_FastHSSEC ; - if (Mode && !jSelf->is_external_suspend()) { - assert (jSelf->is_suspend_equivalent(), "invariant") ; - jSelf->clear_suspend_equivalent() ; - if (2 == Mode) OrderAccess::storeload() ; - if (!jSelf->is_external_suspend()) return false ; - // We raced a suspension -- fall thru into the slow path - TEVENT (ExitSuspendEquivalent - raced) ; - jSelf->set_suspend_equivalent() ; - } - return jSelf->handle_special_suspend_equivalent_condition() ; -} - - -// ReenterI() is a specialized inline form of the latter half of the -// contended slow-path from EnterI(). We use ReenterI() only for -// monitor reentry in wait(). -// -// In the future we should reconcile EnterI() and ReenterI(), adding -// Knob_Reset and Knob_SpinAfterFutile support and restructuring the -// loop accordingly. - -void ATTR ObjectMonitor::ReenterI (Thread * Self, ObjectWaiter * SelfNode) { - assert (Self != NULL , "invariant") ; - assert (SelfNode != NULL , "invariant") ; - assert (SelfNode->_thread == Self , "invariant") ; - assert (_waiters > 0 , "invariant") ; - assert (((oop)(object()))->mark() == markOopDesc::encode(this) , "invariant") ; - assert (((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ; - JavaThread * jt = (JavaThread *) Self ; - - int nWakeups = 0 ; - for (;;) { - ObjectWaiter::TStates v = SelfNode->TState ; - guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ; - assert (_owner != Self, "invariant") ; - - if (TryLock (Self) > 0) break ; - if (TrySpin (Self) > 0) break ; - - TEVENT (Wait Reentry - parking) ; - - // State transition wrappers around park() ... - // ReenterI() wisely defers state transitions until - // it's clear we must park the thread. - { - OSThreadContendState osts(Self->osthread()); - ThreadBlockInVM tbivm(jt); - - // cleared by handle_special_suspend_equivalent_condition() - // or java_suspend_self() - jt->set_suspend_equivalent(); - if (SyncFlags & 1) { - Self->_ParkEvent->park ((jlong)1000) ; - } else { - Self->_ParkEvent->park () ; - } - - // were we externally suspended while we were waiting? - for (;;) { - if (!ExitSuspendEquivalent (jt)) break ; - if (_succ == Self) { _succ = NULL; OrderAccess::fence(); } - jt->java_suspend_self(); - jt->set_suspend_equivalent(); - } - } - - // Try again, but just so we distinguish between futile wakeups and - // successful wakeups. The following test isn't algorithmically - // necessary, but it helps us maintain sensible statistics. - if (TryLock(Self) > 0) break ; - - // The lock is still contested. - // Keep a tally of the # of futile wakeups. - // Note that the counter is not protected by a lock or updated by atomics. - // That is by design - we trade "lossy" counters which are exposed to - // races during updates for a lower probe effect. - TEVENT (Wait Reentry - futile wakeup) ; - ++ nWakeups ; - - // Assuming this is not a spurious wakeup we'll normally - // find that _succ == Self. - if (_succ == Self) _succ = NULL ; - - // Invariant: after clearing _succ a contending thread - // *must* retry _owner before parking. - OrderAccess::fence() ; - - if (ObjectSynchronizer::_sync_FutileWakeups != NULL) { - ObjectSynchronizer::_sync_FutileWakeups->inc() ; - } - } - - // Self has acquired the lock -- Unlink Self from the cxq or EntryList . - // Normally we'll find Self on the EntryList. - // Unlinking from the EntryList is constant-time and atomic-free. - // From the perspective of the lock owner (this thread), the - // EntryList is stable and cxq is prepend-only. - // The head of cxq is volatile but the interior is stable. - // In addition, Self.TState is stable. - - assert (_owner == Self, "invariant") ; - assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ; - UnlinkAfterAcquire (Self, SelfNode) ; - if (_succ == Self) _succ = NULL ; - assert (_succ != Self, "invariant") ; - SelfNode->TState = ObjectWaiter::TS_RUN ; - OrderAccess::fence() ; // see comments at the end of EnterI() -} - -bool ObjectMonitor::try_enter(Thread* THREAD) { - if (THREAD != _owner) { - if (THREAD->is_lock_owned ((address)_owner)) { - assert(_recursions == 0, "internal state error"); - _owner = THREAD ; - _recursions = 1 ; - OwnerIsThread = 1 ; - return true; - } - if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) { - return false; - } - return true; - } else { - _recursions++; - return true; - } -} - -void ATTR ObjectMonitor::enter(TRAPS) { - // The following code is ordered to check the most common cases first - // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors. - Thread * const Self = THREAD ; - void * cur ; - - cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ; - if (cur == NULL) { - // Either ASSERT _recursions == 0 or explicitly set _recursions = 0. - assert (_recursions == 0 , "invariant") ; - assert (_owner == Self, "invariant") ; - // CONSIDER: set or assert OwnerIsThread == 1 - return ; - } - - if (cur == Self) { - // TODO-FIXME: check for integer overflow! BUGID 6557169. - _recursions ++ ; - return ; - } - - if (Self->is_lock_owned ((address)cur)) { - assert (_recursions == 0, "internal state error"); - _recursions = 1 ; - // Commute owner from a thread-specific on-stack BasicLockObject address to - // a full-fledged "Thread *". - _owner = Self ; - OwnerIsThread = 1 ; - return ; - } - - // We've encountered genuine contention. - assert (Self->_Stalled == 0, "invariant") ; - Self->_Stalled = intptr_t(this) ; - - // Try one round of spinning *before* enqueueing Self - // and before going through the awkward and expensive state - // transitions. The following spin is strictly optional ... - // Note that if we acquire the monitor from an initial spin - // we forgo posting JVMTI events and firing DTRACE probes. - if (Knob_SpinEarly && TrySpin (Self) > 0) { - assert (_owner == Self , "invariant") ; - assert (_recursions == 0 , "invariant") ; - assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ; - Self->_Stalled = 0 ; - return ; - } - - assert (_owner != Self , "invariant") ; - assert (_succ != Self , "invariant") ; - assert (Self->is_Java_thread() , "invariant") ; - JavaThread * jt = (JavaThread *) Self ; - assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ; - assert (jt->thread_state() != _thread_blocked , "invariant") ; - assert (this->object() != NULL , "invariant") ; - assert (_count >= 0, "invariant") ; - - // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy(). - // Ensure the object-monitor relationship remains stable while there's contention. - Atomic::inc_ptr(&_count); - - { // Change java thread status to indicate blocked on monitor enter. - JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this); - - DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt); - if (JvmtiExport::should_post_monitor_contended_enter()) { - JvmtiExport::post_monitor_contended_enter(jt, this); - } - - OSThreadContendState osts(Self->osthread()); - ThreadBlockInVM tbivm(jt); - - Self->set_current_pending_monitor(this); - - // TODO-FIXME: change the following for(;;) loop to straight-line code. - for (;;) { - jt->set_suspend_equivalent(); - // cleared by handle_special_suspend_equivalent_condition() - // or java_suspend_self() - - EnterI (THREAD) ; - - if (!ExitSuspendEquivalent(jt)) break ; - - // - // We have acquired the contended monitor, but while we were - // waiting another thread suspended us. We don't want to enter - // the monitor while suspended because that would surprise the - // thread that suspended us. - // - _recursions = 0 ; - _succ = NULL ; - exit (Self) ; - - jt->java_suspend_self(); - } - Self->set_current_pending_monitor(NULL); - } - - Atomic::dec_ptr(&_count); - assert (_count >= 0, "invariant") ; - Self->_Stalled = 0 ; - - // Must either set _recursions = 0 or ASSERT _recursions == 0. - assert (_recursions == 0 , "invariant") ; - assert (_owner == Self , "invariant") ; - assert (_succ != Self , "invariant") ; - assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ; - - // The thread -- now the owner -- is back in vm mode. - // Report the glorious news via TI,DTrace and jvmstat. - // The probe effect is non-trivial. All the reportage occurs - // while we hold the monitor, increasing the length of the critical - // section. Amdahl's parallel speedup law comes vividly into play. - // - // Another option might be to aggregate the events (thread local or - // per-monitor aggregation) and defer reporting until a more opportune - // time -- such as next time some thread encounters contention but has - // yet to acquire the lock. While spinning that thread could - // spinning we could increment JVMStat counters, etc. - - DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt); - if (JvmtiExport::should_post_monitor_contended_entered()) { - JvmtiExport::post_monitor_contended_entered(jt, this); - } - if (ObjectSynchronizer::_sync_ContendedLockAttempts != NULL) { - ObjectSynchronizer::_sync_ContendedLockAttempts->inc() ; - } -} - -void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) { - assert (_owner == Self, "invariant") ; - - // Exit protocol: - // 1. ST _succ = wakee - // 2. membar #loadstore|#storestore; - // 2. ST _owner = NULL - // 3. unpark(wakee) - - _succ = Knob_SuccEnabled ? Wakee->_thread : NULL ; - ParkEvent * Trigger = Wakee->_event ; - - // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again. - // The thread associated with Wakee may have grabbed the lock and "Wakee" may be - // out-of-scope (non-extant). - Wakee = NULL ; - - // Drop the lock - OrderAccess::release_store_ptr (&_owner, NULL) ; - OrderAccess::fence() ; // ST _owner vs LD in unpark() - - // TODO-FIXME: - // If there's a safepoint pending the best policy would be to - // get _this thread to a safepoint and only wake the successor - // after the safepoint completed. monitorexit uses a "leaf" - // state transition, however, so this thread can't become - // safe at this point in time. (Its stack isn't walkable). - // The next best thing is to defer waking the successor by - // adding to a list of thread to be unparked after at the - // end of the forthcoming STW). - if (SafepointSynchronize::do_call_back()) { - TEVENT (unpark before SAFEPOINT) ; - } - - // Possible optimizations ... - // - // * Consider: set Wakee->UnparkTime = timeNow() - // When the thread wakes up it'll compute (timeNow() - Self->UnparkTime()). - // By measuring recent ONPROC latency we can approximate the - // system load. In turn, we can feed that information back - // into the spinning & succession policies. - // (ONPROC latency correlates strongly with load). - // - // * Pull affinity: - // If the wakee is cold then transiently setting it's affinity - // to the current CPU is a good idea. - // See http://j2se.east/~dice/PERSIST/050624-PullAffinity.txt - DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self); - Trigger->unpark() ; - - // Maintain stats and report events to JVMTI - if (ObjectSynchronizer::_sync_Parks != NULL) { - ObjectSynchronizer::_sync_Parks->inc() ; - } -} - - -// exit() -// ~~~~~~ -// Note that the collector can't reclaim the objectMonitor or deflate -// the object out from underneath the thread calling ::exit() as the -// thread calling ::exit() never transitions to a stable state. -// This inhibits GC, which in turn inhibits asynchronous (and -// inopportune) reclamation of "this". -// -// We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ; -// There's one exception to the claim above, however. EnterI() can call -// exit() to drop a lock if the acquirer has been externally suspended. -// In that case exit() is called with _thread_state as _thread_blocked, -// but the monitor's _count field is > 0, which inhibits reclamation. -// -// 1-0 exit -// ~~~~~~~~ -// ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of -// the fast-path operators have been optimized so the common ::exit() -// operation is 1-0. See i486.ad fast_unlock(), for instance. -// The code emitted by fast_unlock() elides the usual MEMBAR. This -// greatly improves latency -- MEMBAR and CAS having considerable local -// latency on modern processors -- but at the cost of "stranding". Absent the -// MEMBAR, a thread in fast_unlock() can race a thread in the slow -// ::enter() path, resulting in the entering thread being stranding -// and a progress-liveness failure. Stranding is extremely rare. -// We use timers (timed park operations) & periodic polling to detect -// and recover from stranding. Potentially stranded threads periodically -// wake up and poll the lock. See the usage of the _Responsible variable. -// -// The CAS() in enter provides for safety and exclusion, while the CAS or -// MEMBAR in exit provides for progress and avoids stranding. 1-0 locking -// eliminates the CAS/MEMBAR from the exist path, but it admits stranding. -// We detect and recover from stranding with timers. -// -// If a thread transiently strands it'll park until (a) another -// thread acquires the lock and then drops the lock, at which time the -// exiting thread will notice and unpark the stranded thread, or, (b) -// the timer expires. If the lock is high traffic then the stranding latency -// will be low due to (a). If the lock is low traffic then the odds of -// stranding are lower, although the worst-case stranding latency -// is longer. Critically, we don't want to put excessive load in the -// platform's timer subsystem. We want to minimize both the timer injection -// rate (timers created/sec) as well as the number of timers active at -// any one time. (more precisely, we want to minimize timer-seconds, which is -// the integral of the # of active timers at any instant over time). -// Both impinge on OS scalability. Given that, at most one thread parked on -// a monitor will use a timer. - -void ATTR ObjectMonitor::exit(TRAPS) { - Thread * Self = THREAD ; - if (THREAD != _owner) { - if (THREAD->is_lock_owned((address) _owner)) { - // Transmute _owner from a BasicLock pointer to a Thread address. - // We don't need to hold _mutex for this transition. - // Non-null to Non-null is safe as long as all readers can - // tolerate either flavor. - assert (_recursions == 0, "invariant") ; - _owner = THREAD ; - _recursions = 0 ; - OwnerIsThread = 1 ; - } else { - // NOTE: we need to handle unbalanced monitor enter/exit - // in native code by throwing an exception. - // TODO: Throw an IllegalMonitorStateException ? - TEVENT (Exit - Throw IMSX) ; - assert(false, "Non-balanced monitor enter/exit!"); - if (false) { - THROW(vmSymbols::java_lang_IllegalMonitorStateException()); - } - return; - } - } - - if (_recursions != 0) { - _recursions--; // this is simple recursive enter - TEVENT (Inflated exit - recursive) ; - return ; - } - - // Invariant: after setting Responsible=null an thread must execute - // a MEMBAR or other serializing instruction before fetching EntryList|cxq. - if ((SyncFlags & 4) == 0) { - _Responsible = NULL ; - } - - for (;;) { - assert (THREAD == _owner, "invariant") ; - - // Fast-path monitor exit: - // - // Observe the Dekker/Lamport duality: - // A thread in ::exit() executes: - // ST Owner=null; MEMBAR; LD EntryList|cxq. - // A thread in the contended ::enter() path executes the complementary: - // ST EntryList|cxq = nonnull; MEMBAR; LD Owner. - // - // Note that there's a benign race in the exit path. We can drop the - // lock, another thread can reacquire the lock immediately, and we can - // then wake a thread unnecessarily (yet another flavor of futile wakeup). - // This is benign, and we've structured the code so the windows are short - // and the frequency of such futile wakeups is low. - // - // We could eliminate the race by encoding both the "LOCKED" state and - // the queue head in a single word. Exit would then use either CAS to - // clear the LOCKED bit/byte. This precludes the desirable 1-0 optimization, - // however. - // - // Possible fast-path ::exit() optimization: - // The current fast-path exit implementation fetches both cxq and EntryList. - // See also i486.ad fast_unlock(). Testing has shown that two LDs - // isn't measurably slower than a single LD on any platforms. - // Still, we could reduce the 2 LDs to one or zero by one of the following: - // - // - Use _count instead of cxq|EntryList - // We intend to eliminate _count, however, when we switch - // to on-the-fly deflation in ::exit() as is used in - // Metalocks and RelaxedLocks. - // - // - Establish the invariant that cxq == null implies EntryList == null. - // set cxq == EMPTY (1) to encode the state where cxq is empty - // by EntryList != null. EMPTY is a distinguished value. - // The fast-path exit() would fetch cxq but not EntryList. - // - // - Encode succ as follows: - // succ = t : Thread t is the successor -- t is ready or is spinning. - // Exiting thread does not need to wake a successor. - // succ = 0 : No successor required -> (EntryList|cxq) == null - // Exiting thread does not need to wake a successor - // succ = 1 : Successor required -> (EntryList|cxq) != null and - // logically succ == null. - // Exiting thread must wake a successor. - // - // The 1-1 fast-exit path would appear as : - // _owner = null ; membar ; - // if (_succ == 1 && CAS (&_owner, null, Self) == null) goto SlowPath - // goto FastPathDone ; - // - // and the 1-0 fast-exit path would appear as: - // if (_succ == 1) goto SlowPath - // Owner = null ; - // goto FastPathDone - // - // - Encode the LSB of _owner as 1 to indicate that exit() - // must use the slow-path and make a successor ready. - // (_owner & 1) == 0 IFF succ != null || (EntryList|cxq) == null - // (_owner & 1) == 0 IFF succ == null && (EntryList|cxq) != null (obviously) - // The 1-0 fast exit path would read: - // if (_owner != Self) goto SlowPath - // _owner = null - // goto FastPathDone - - if (Knob_ExitPolicy == 0) { - // release semantics: prior loads and stores from within the critical section - // must not float (reorder) past the following store that drops the lock. - // On SPARC that requires MEMBAR #loadstore|#storestore. - // But of course in TSO #loadstore|#storestore is not required. - // I'd like to write one of the following: - // A. OrderAccess::release() ; _owner = NULL - // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL; - // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both - // store into a _dummy variable. That store is not needed, but can result - // in massive wasteful coherency traffic on classic SMP systems. - // Instead, I use release_store(), which is implemented as just a simple - // ST on x64, x86 and SPARC. - OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock - OrderAccess::storeload() ; // See if we need to wake a successor - if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) { - TEVENT (Inflated exit - simple egress) ; - return ; - } - TEVENT (Inflated exit - complex egress) ; - - // Normally the exiting thread is responsible for ensuring succession, - // but if other successors are ready or other entering threads are spinning - // then this thread can simply store NULL into _owner and exit without - // waking a successor. The existence of spinners or ready successors - // guarantees proper succession (liveness). Responsibility passes to the - // ready or running successors. The exiting thread delegates the duty. - // More precisely, if a successor already exists this thread is absolved - // of the responsibility of waking (unparking) one. - // - // The _succ variable is critical to reducing futile wakeup frequency. - // _succ identifies the "heir presumptive" thread that has been made - // ready (unparked) but that has not yet run. We need only one such - // successor thread to guarantee progress. - // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf - // section 3.3 "Futile Wakeup Throttling" for details. - // - // Note that spinners in Enter() also set _succ non-null. - // In the current implementation spinners opportunistically set - // _succ so that exiting threads might avoid waking a successor. - // Another less appealing alternative would be for the exiting thread - // to drop the lock and then spin briefly to see if a spinner managed - // to acquire the lock. If so, the exiting thread could exit - // immediately without waking a successor, otherwise the exiting - // thread would need to dequeue and wake a successor. - // (Note that we'd need to make the post-drop spin short, but no - // shorter than the worst-case round-trip cache-line migration time. - // The dropped lock needs to become visible to the spinner, and then - // the acquisition of the lock by the spinner must become visible to - // the exiting thread). - // - - // It appears that an heir-presumptive (successor) must be made ready. - // Only the current lock owner can manipulate the EntryList or - // drain _cxq, so we need to reacquire the lock. If we fail - // to reacquire the lock the responsibility for ensuring succession - // falls to the new owner. - // - if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) { - return ; - } - TEVENT (Exit - Reacquired) ; - } else { - if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) { - OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock - OrderAccess::storeload() ; - // Ratify the previously observed values. - if (_cxq == NULL || _succ != NULL) { - TEVENT (Inflated exit - simple egress) ; - return ; - } - - // inopportune interleaving -- the exiting thread (this thread) - // in the fast-exit path raced an entering thread in the slow-enter - // path. - // We have two choices: - // A. Try to reacquire the lock. - // If the CAS() fails return immediately, otherwise - // we either restart/rerun the exit operation, or simply - // fall-through into the code below which wakes a successor. - // B. If the elements forming the EntryList|cxq are TSM - // we could simply unpark() the lead thread and return - // without having set _succ. - if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) { - TEVENT (Inflated exit - reacquired succeeded) ; - return ; - } - TEVENT (Inflated exit - reacquired failed) ; - } else { - TEVENT (Inflated exit - complex egress) ; - } - } - - guarantee (_owner == THREAD, "invariant") ; - - // Select an appropriate successor ("heir presumptive") from the EntryList - // and make it ready. Generally we just wake the head of EntryList . - // There's no algorithmic constraint that we use the head - it's just - // a policy decision. Note that the thread at head of the EntryList - // remains at the head until it acquires the lock. This means we'll - // repeatedly wake the same thread until it manages to grab the lock. - // This is generally a good policy - if we're seeing lots of futile wakeups - // at least we're waking/rewaking a thread that's like to be hot or warm - // (have residual D$ and TLB affinity). - // - // "Wakeup locality" optimization: - // http://j2se.east/~dice/PERSIST/040825-WakeLocality.txt - // In the future we'll try to bias the selection mechanism - // to preferentially pick a thread that recently ran on - // a processor element that shares cache with the CPU on which - // the exiting thread is running. We need access to Solaris' - // schedctl.sc_cpu to make that work. - // - ObjectWaiter * w = NULL ; - int QMode = Knob_QMode ; - - if (QMode == 2 && _cxq != NULL) { - // QMode == 2 : cxq has precedence over EntryList. - // Try to directly wake a successor from the cxq. - // If successful, the successor will need to unlink itself from cxq. - w = _cxq ; - assert (w != NULL, "invariant") ; - assert (w->TState == ObjectWaiter::TS_CXQ, "Invariant") ; - ExitEpilog (Self, w) ; - return ; - } - - if (QMode == 3 && _cxq != NULL) { - // Aggressively drain cxq into EntryList at the first opportunity. - // This policy ensure that recently-run threads live at the head of EntryList. - // Drain _cxq into EntryList - bulk transfer. - // First, detach _cxq. - // The following loop is tantamount to: w = swap (&cxq, NULL) - w = _cxq ; - for (;;) { - assert (w != NULL, "Invariant") ; - ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ; - if (u == w) break ; - w = u ; - } - assert (w != NULL , "invariant") ; - - ObjectWaiter * q = NULL ; - ObjectWaiter * p ; - for (p = w ; p != NULL ; p = p->_next) { - guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ; - p->TState = ObjectWaiter::TS_ENTER ; - p->_prev = q ; - q = p ; - } - - // Append the RATs to the EntryList - // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time. - ObjectWaiter * Tail ; - for (Tail = _EntryList ; Tail != NULL && Tail->_next != NULL ; Tail = Tail->_next) ; - if (Tail == NULL) { - _EntryList = w ; - } else { - Tail->_next = w ; - w->_prev = Tail ; - } - - // Fall thru into code that tries to wake a successor from EntryList - } - - if (QMode == 4 && _cxq != NULL) { - // Aggressively drain cxq into EntryList at the first opportunity. - // This policy ensure that recently-run threads live at the head of EntryList. - - // Drain _cxq into EntryList - bulk transfer. - // First, detach _cxq. - // The following loop is tantamount to: w = swap (&cxq, NULL) - w = _cxq ; - for (;;) { - assert (w != NULL, "Invariant") ; - ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ; - if (u == w) break ; - w = u ; - } - assert (w != NULL , "invariant") ; - - ObjectWaiter * q = NULL ; - ObjectWaiter * p ; - for (p = w ; p != NULL ; p = p->_next) { - guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ; - p->TState = ObjectWaiter::TS_ENTER ; - p->_prev = q ; - q = p ; - } - - // Prepend the RATs to the EntryList - if (_EntryList != NULL) { - q->_next = _EntryList ; - _EntryList->_prev = q ; - } - _EntryList = w ; - - // Fall thru into code that tries to wake a successor from EntryList - } - - w = _EntryList ; - if (w != NULL) { - // I'd like to write: guarantee (w->_thread != Self). - // But in practice an exiting thread may find itself on the EntryList. - // Lets say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and - // then calls exit(). Exit release the lock by setting O._owner to NULL. - // Lets say T1 then stalls. T2 acquires O and calls O.notify(). The - // notify() operation moves T1 from O's waitset to O's EntryList. T2 then - // release the lock "O". T2 resumes immediately after the ST of null into - // _owner, above. T2 notices that the EntryList is populated, so it - // reacquires the lock and then finds itself on the EntryList. - // Given all that, we have to tolerate the circumstance where "w" is - // associated with Self. - assert (w->TState == ObjectWaiter::TS_ENTER, "invariant") ; - ExitEpilog (Self, w) ; - return ; - } - - // If we find that both _cxq and EntryList are null then just - // re-run the exit protocol from the top. - w = _cxq ; - if (w == NULL) continue ; - - // Drain _cxq into EntryList - bulk transfer. - // First, detach _cxq. - // The following loop is tantamount to: w = swap (&cxq, NULL) - for (;;) { - assert (w != NULL, "Invariant") ; - ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ; - if (u == w) break ; - w = u ; - } - TEVENT (Inflated exit - drain cxq into EntryList) ; - - assert (w != NULL , "invariant") ; - assert (_EntryList == NULL , "invariant") ; - - // Convert the LIFO SLL anchored by _cxq into a DLL. - // The list reorganization step operates in O(LENGTH(w)) time. - // It's critical that this step operate quickly as - // "Self" still holds the outer-lock, restricting parallelism - // and effectively lengthening the critical section. - // Invariant: s chases t chases u. - // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so - // we have faster access to the tail. - - if (QMode == 1) { - // QMode == 1 : drain cxq to EntryList, reversing order - // We also reverse the order of the list. - ObjectWaiter * s = NULL ; - ObjectWaiter * t = w ; - ObjectWaiter * u = NULL ; - while (t != NULL) { - guarantee (t->TState == ObjectWaiter::TS_CXQ, "invariant") ; - t->TState = ObjectWaiter::TS_ENTER ; - u = t->_next ; - t->_prev = u ; - t->_next = s ; - s = t; - t = u ; - } - _EntryList = s ; - assert (s != NULL, "invariant") ; - } else { - // QMode == 0 or QMode == 2 - _EntryList = w ; - ObjectWaiter * q = NULL ; - ObjectWaiter * p ; - for (p = w ; p != NULL ; p = p->_next) { - guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ; - p->TState = ObjectWaiter::TS_ENTER ; - p->_prev = q ; - q = p ; - } - } - - // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL - // The MEMBAR is satisfied by the release_store() operation in ExitEpilog(). - - // See if we can abdicate to a spinner instead of waking a thread. - // A primary goal of the implementation is to reduce the - // context-switch rate. - if (_succ != NULL) continue; - - w = _EntryList ; - if (w != NULL) { - guarantee (w->TState == ObjectWaiter::TS_ENTER, "invariant") ; - ExitEpilog (Self, w) ; - return ; - } - } -} -// complete_exit exits a lock returning recursion count -// complete_exit/reenter operate as a wait without waiting -// complete_exit requires an inflated monitor -// The _owner field is not always the Thread addr even with an -// inflated monitor, e.g. the monitor can be inflated by a non-owning -// thread due to contention. -intptr_t ObjectMonitor::complete_exit(TRAPS) { - Thread * const Self = THREAD; - assert(Self->is_Java_thread(), "Must be Java thread!"); - JavaThread *jt = (JavaThread *)THREAD; - - DeferredInitialize(); - - if (THREAD != _owner) { - if (THREAD->is_lock_owned ((address)_owner)) { - assert(_recursions == 0, "internal state error"); - _owner = THREAD ; /* Convert from basiclock addr to Thread addr */ - _recursions = 0 ; - OwnerIsThread = 1 ; - } - } - - guarantee(Self == _owner, "complete_exit not owner"); - intptr_t save = _recursions; // record the old recursion count - _recursions = 0; // set the recursion level to be 0 - exit (Self) ; // exit the monitor - guarantee (_owner != Self, "invariant"); - return save; -} - -// reenter() enters a lock and sets recursion count -// complete_exit/reenter operate as a wait without waiting -void ObjectMonitor::reenter(intptr_t recursions, TRAPS) { - Thread * const Self = THREAD; - assert(Self->is_Java_thread(), "Must be Java thread!"); - JavaThread *jt = (JavaThread *)THREAD; - - guarantee(_owner != Self, "reenter already owner"); - enter (THREAD); // enter the monitor - guarantee (_recursions == 0, "reenter recursion"); - _recursions = recursions; - return; -} - -// Note: a subset of changes to ObjectMonitor::wait() -// will need to be replicated in complete_exit above -void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) { - Thread * const Self = THREAD ; - assert(Self->is_Java_thread(), "Must be Java thread!"); - JavaThread *jt = (JavaThread *)THREAD; - - DeferredInitialize () ; - - // Throw IMSX or IEX. - CHECK_OWNER(); - - // check for a pending interrupt - if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) { - // post monitor waited event. Note that this is past-tense, we are done waiting. - if (JvmtiExport::should_post_monitor_waited()) { - // Note: 'false' parameter is passed here because the - // wait was not timed out due to thread interrupt. - JvmtiExport::post_monitor_waited(jt, this, false); - } - TEVENT (Wait - Throw IEX) ; - THROW(vmSymbols::java_lang_InterruptedException()); - return ; - } - TEVENT (Wait) ; - - assert (Self->_Stalled == 0, "invariant") ; - Self->_Stalled = intptr_t(this) ; - jt->set_current_waiting_monitor(this); - - // create a node to be put into the queue - // Critically, after we reset() the event but prior to park(), we must check - // for a pending interrupt. - ObjectWaiter node(Self); - node.TState = ObjectWaiter::TS_WAIT ; - Self->_ParkEvent->reset() ; - OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag - - // Enter the waiting queue, which is a circular doubly linked list in this case - // but it could be a priority queue or any data structure. - // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only - // by the the owner of the monitor *except* in the case where park() - // returns because of a timeout of interrupt. Contention is exceptionally rare - // so we use a simple spin-lock instead of a heavier-weight blocking lock. - - Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ; - AddWaiter (&node) ; - Thread::SpinRelease (&_WaitSetLock) ; - - if ((SyncFlags & 4) == 0) { - _Responsible = NULL ; - } - intptr_t save = _recursions; // record the old recursion count - _waiters++; // increment the number of waiters - _recursions = 0; // set the recursion level to be 1 - exit (Self) ; // exit the monitor - guarantee (_owner != Self, "invariant") ; - - // As soon as the ObjectMonitor's ownership is dropped in the exit() - // call above, another thread can enter() the ObjectMonitor, do the - // notify(), and exit() the ObjectMonitor. If the other thread's - // exit() call chooses this thread as the successor and the unpark() - // call happens to occur while this thread is posting a - // MONITOR_CONTENDED_EXIT event, then we run the risk of the event - // handler using RawMonitors and consuming the unpark(). - // - // To avoid the problem, we re-post the event. This does no harm - // even if the original unpark() was not consumed because we are the - // chosen successor for this monitor. - if (node._notified != 0 && _succ == Self) { - node._event->unpark(); - } - - // The thread is on the WaitSet list - now park() it. - // On MP systems it's conceivable that a brief spin before we park - // could be profitable. - // - // TODO-FIXME: change the following logic to a loop of the form - // while (!timeout && !interrupted && _notified == 0) park() - - int ret = OS_OK ; - int WasNotified = 0 ; - { // State transition wrappers - OSThread* osthread = Self->osthread(); - OSThreadWaitState osts(osthread, true); - { - ThreadBlockInVM tbivm(jt); - // Thread is in thread_blocked state and oop access is unsafe. - jt->set_suspend_equivalent(); - - if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) { - // Intentionally empty - } else - if (node._notified == 0) { - if (millis <= 0) { - Self->_ParkEvent->park () ; - } else { - ret = Self->_ParkEvent->park (millis) ; - } - } - - // were we externally suspended while we were waiting? - if (ExitSuspendEquivalent (jt)) { - // TODO-FIXME: add -- if succ == Self then succ = null. - jt->java_suspend_self(); - } - - } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm - - - // Node may be on the WaitSet, the EntryList (or cxq), or in transition - // from the WaitSet to the EntryList. - // See if we need to remove Node from the WaitSet. - // We use double-checked locking to avoid grabbing _WaitSetLock - // if the thread is not on the wait queue. - // - // Note that we don't need a fence before the fetch of TState. - // In the worst case we'll fetch a old-stale value of TS_WAIT previously - // written by the is thread. (perhaps the fetch might even be satisfied - // by a look-aside into the processor's own store buffer, although given - // the length of the code path between the prior ST and this load that's - // highly unlikely). If the following LD fetches a stale TS_WAIT value - // then we'll acquire the lock and then re-fetch a fresh TState value. - // That is, we fail toward safety. - - if (node.TState == ObjectWaiter::TS_WAIT) { - Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ; - if (node.TState == ObjectWaiter::TS_WAIT) { - DequeueSpecificWaiter (&node) ; // unlink from WaitSet - assert(node._notified == 0, "invariant"); - node.TState = ObjectWaiter::TS_RUN ; - } - Thread::SpinRelease (&_WaitSetLock) ; - } - - // The thread is now either on off-list (TS_RUN), - // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ). - // The Node's TState variable is stable from the perspective of this thread. - // No other threads will asynchronously modify TState. - guarantee (node.TState != ObjectWaiter::TS_WAIT, "invariant") ; - OrderAccess::loadload() ; - if (_succ == Self) _succ = NULL ; - WasNotified = node._notified ; - - // Reentry phase -- reacquire the monitor. - // re-enter contended monitor after object.wait(). - // retain OBJECT_WAIT state until re-enter successfully completes - // Thread state is thread_in_vm and oop access is again safe, - // although the raw address of the object may have changed. - // (Don't cache naked oops over safepoints, of course). - - // post monitor waited event. Note that this is past-tense, we are done waiting. - if (JvmtiExport::should_post_monitor_waited()) { - JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT); - } - OrderAccess::fence() ; - - assert (Self->_Stalled != 0, "invariant") ; - Self->_Stalled = 0 ; - - assert (_owner != Self, "invariant") ; - ObjectWaiter::TStates v = node.TState ; - if (v == ObjectWaiter::TS_RUN) { - enter (Self) ; - } else { - guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ; - ReenterI (Self, &node) ; - node.wait_reenter_end(this); - } - - // Self has reacquired the lock. - // Lifecycle - the node representing Self must not appear on any queues. - // Node is about to go out-of-scope, but even if it were immortal we wouldn't - // want residual elements associated with this thread left on any lists. - guarantee (node.TState == ObjectWaiter::TS_RUN, "invariant") ; - assert (_owner == Self, "invariant") ; - assert (_succ != Self , "invariant") ; - } // OSThreadWaitState() - - jt->set_current_waiting_monitor(NULL); - - guarantee (_recursions == 0, "invariant") ; - _recursions = save; // restore the old recursion count - _waiters--; // decrement the number of waiters - - // Verify a few postconditions - assert (_owner == Self , "invariant") ; - assert (_succ != Self , "invariant") ; - assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ; - - if (SyncFlags & 32) { - OrderAccess::fence() ; - } - - // check if the notification happened - if (!WasNotified) { - // no, it could be timeout or Thread.interrupt() or both - // check for interrupt event, otherwise it is timeout - if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) { - TEVENT (Wait - throw IEX from epilog) ; - THROW(vmSymbols::java_lang_InterruptedException()); - } - } - - // NOTE: Spurious wake up will be consider as timeout. - // Monitor notify has precedence over thread interrupt. -} - - -// Consider: -// If the lock is cool (cxq == null && succ == null) and we're on an MP system -// then instead of transferring a thread from the WaitSet to the EntryList -// we might just dequeue a thread from the WaitSet and directly unpark() it. - -void ObjectMonitor::notify(TRAPS) { - CHECK_OWNER(); - if (_WaitSet == NULL) { - TEVENT (Empty-Notify) ; - return ; - } - DTRACE_MONITOR_PROBE(notify, this, object(), THREAD); - - int Policy = Knob_MoveNotifyee ; - - Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ; - ObjectWaiter * iterator = DequeueWaiter() ; - if (iterator != NULL) { - TEVENT (Notify1 - Transfer) ; - guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ; - guarantee (iterator->_notified == 0, "invariant") ; - // Disposition - what might we do with iterator ? - // a. add it directly to the EntryList - either tail or head. - // b. push it onto the front of the _cxq. - // For now we use (a). - if (Policy != 4) { - iterator->TState = ObjectWaiter::TS_ENTER ; - } - iterator->_notified = 1 ; - - ObjectWaiter * List = _EntryList ; - if (List != NULL) { - assert (List->_prev == NULL, "invariant") ; - assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ; - assert (List != iterator, "invariant") ; - } - - if (Policy == 0) { // prepend to EntryList - if (List == NULL) { - iterator->_next = iterator->_prev = NULL ; - _EntryList = iterator ; - } else { - List->_prev = iterator ; - iterator->_next = List ; - iterator->_prev = NULL ; - _EntryList = iterator ; - } - } else - if (Policy == 1) { // append to EntryList - if (List == NULL) { - iterator->_next = iterator->_prev = NULL ; - _EntryList = iterator ; - } else { - // CONSIDER: finding the tail currently requires a linear-time walk of - // the EntryList. We can make tail access constant-time by converting to - // a CDLL instead of using our current DLL. - ObjectWaiter * Tail ; - for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ; - assert (Tail != NULL && Tail->_next == NULL, "invariant") ; - Tail->_next = iterator ; - iterator->_prev = Tail ; - iterator->_next = NULL ; - } - } else - if (Policy == 2) { // prepend to cxq - // prepend to cxq - if (List == NULL) { - iterator->_next = iterator->_prev = NULL ; - _EntryList = iterator ; - } else { - iterator->TState = ObjectWaiter::TS_CXQ ; - for (;;) { - ObjectWaiter * Front = _cxq ; - iterator->_next = Front ; - if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) { - break ; - } - } - } - } else - if (Policy == 3) { // append to cxq - iterator->TState = ObjectWaiter::TS_CXQ ; - for (;;) { - ObjectWaiter * Tail ; - Tail = _cxq ; - if (Tail == NULL) { - iterator->_next = NULL ; - if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) { - break ; - } - } else { - while (Tail->_next != NULL) Tail = Tail->_next ; - Tail->_next = iterator ; - iterator->_prev = Tail ; - iterator->_next = NULL ; - break ; - } - } - } else { - ParkEvent * ev = iterator->_event ; - iterator->TState = ObjectWaiter::TS_RUN ; - OrderAccess::fence() ; - ev->unpark() ; - } - - if (Policy < 4) { - iterator->wait_reenter_begin(this); - } - - // _WaitSetLock protects the wait queue, not the EntryList. We could - // move the add-to-EntryList operation, above, outside the critical section - // protected by _WaitSetLock. In practice that's not useful. With the - // exception of wait() timeouts and interrupts the monitor owner - // is the only thread that grabs _WaitSetLock. There's almost no contention - // on _WaitSetLock so it's not profitable to reduce the length of the - // critical section. - } - - Thread::SpinRelease (&_WaitSetLock) ; - - if (iterator != NULL && ObjectSynchronizer::_sync_Notifications != NULL) { - ObjectSynchronizer::_sync_Notifications->inc() ; - } -} - - -void ObjectMonitor::notifyAll(TRAPS) { - CHECK_OWNER(); - ObjectWaiter* iterator; - if (_WaitSet == NULL) { - TEVENT (Empty-NotifyAll) ; - return ; - } - DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD); - - int Policy = Knob_MoveNotifyee ; - int Tally = 0 ; - Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notifyall") ; - - for (;;) { - iterator = DequeueWaiter () ; - if (iterator == NULL) break ; - TEVENT (NotifyAll - Transfer1) ; - ++Tally ; - - // Disposition - what might we do with iterator ? - // a. add it directly to the EntryList - either tail or head. - // b. push it onto the front of the _cxq. - // For now we use (a). - // - // TODO-FIXME: currently notifyAll() transfers the waiters one-at-a-time from the waitset - // to the EntryList. This could be done more efficiently with a single bulk transfer, - // but in practice it's not time-critical. Beware too, that in prepend-mode we invert the - // order of the waiters. Lets say that the waitset is "ABCD" and the EntryList is "XYZ". - // After a notifyAll() in prepend mode the waitset will be empty and the EntryList will - // be "DCBAXYZ". - - guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ; - guarantee (iterator->_notified == 0, "invariant") ; - iterator->_notified = 1 ; - if (Policy != 4) { - iterator->TState = ObjectWaiter::TS_ENTER ; - } - - ObjectWaiter * List = _EntryList ; - if (List != NULL) { - assert (List->_prev == NULL, "invariant") ; - assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ; - assert (List != iterator, "invariant") ; - } - - if (Policy == 0) { // prepend to EntryList - if (List == NULL) { - iterator->_next = iterator->_prev = NULL ; - _EntryList = iterator ; - } else { - List->_prev = iterator ; - iterator->_next = List ; - iterator->_prev = NULL ; - _EntryList = iterator ; - } - } else - if (Policy == 1) { // append to EntryList - if (List == NULL) { - iterator->_next = iterator->_prev = NULL ; - _EntryList = iterator ; - } else { - // CONSIDER: finding the tail currently requires a linear-time walk of - // the EntryList. We can make tail access constant-time by converting to - // a CDLL instead of using our current DLL. - ObjectWaiter * Tail ; - for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ; - assert (Tail != NULL && Tail->_next == NULL, "invariant") ; - Tail->_next = iterator ; - iterator->_prev = Tail ; - iterator->_next = NULL ; - } - } else - if (Policy == 2) { // prepend to cxq - // prepend to cxq - iterator->TState = ObjectWaiter::TS_CXQ ; - for (;;) { - ObjectWaiter * Front = _cxq ; - iterator->_next = Front ; - if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) { - break ; - } - } - } else - if (Policy == 3) { // append to cxq - iterator->TState = ObjectWaiter::TS_CXQ ; - for (;;) { - ObjectWaiter * Tail ; - Tail = _cxq ; - if (Tail == NULL) { - iterator->_next = NULL ; - if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) { - break ; - } - } else { - while (Tail->_next != NULL) Tail = Tail->_next ; - Tail->_next = iterator ; - iterator->_prev = Tail ; - iterator->_next = NULL ; - break ; - } - } - } else { - ParkEvent * ev = iterator->_event ; - iterator->TState = ObjectWaiter::TS_RUN ; - OrderAccess::fence() ; - ev->unpark() ; - } - - if (Policy < 4) { - iterator->wait_reenter_begin(this); - } - - // _WaitSetLock protects the wait queue, not the EntryList. We could - // move the add-to-EntryList operation, above, outside the critical section - // protected by _WaitSetLock. In practice that's not useful. With the - // exception of wait() timeouts and interrupts the monitor owner - // is the only thread that grabs _WaitSetLock. There's almost no contention - // on _WaitSetLock so it's not profitable to reduce the length of the - // critical section. - } - - Thread::SpinRelease (&_WaitSetLock) ; - - if (Tally != 0 && ObjectSynchronizer::_sync_Notifications != NULL) { - ObjectSynchronizer::_sync_Notifications->inc(Tally) ; - } -} - -// check_slow() is a misnomer. It's called to simply to throw an IMSX exception. -// TODO-FIXME: remove check_slow() -- it's likely dead. - -void ObjectMonitor::check_slow(TRAPS) { - TEVENT (check_slow - throw IMSX) ; - assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner"); - THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner"); -} - - -// ------------------------------------------------------------------------- -// The raw monitor subsystem is entirely distinct from normal -// java-synchronization or jni-synchronization. raw monitors are not -// associated with objects. They can be implemented in any manner -// that makes sense. The original implementors decided to piggy-back -// the raw-monitor implementation on the existing Java objectMonitor mechanism. -// This flaw needs to fixed. We should reimplement raw monitors as sui-generis. -// Specifically, we should not implement raw monitors via java monitors. -// Time permitting, we should disentangle and deconvolve the two implementations -// and move the resulting raw monitor implementation over to the JVMTI directories. -// Ideally, the raw monitor implementation would be built on top of -// park-unpark and nothing else. -// -// raw monitors are used mainly by JVMTI -// The raw monitor implementation borrows the ObjectMonitor structure, -// but the operators are degenerate and extremely simple. -// -// Mixed use of a single objectMonitor instance -- as both a raw monitor -// and a normal java monitor -- is not permissible. -// -// Note that we use the single RawMonitor_lock to protect queue operations for -// _all_ raw monitors. This is a scalability impediment, but since raw monitor usage -// is deprecated and rare, this is not of concern. The RawMonitor_lock can not -// be held indefinitely. The critical sections must be short and bounded. -// -// ------------------------------------------------------------------------- - -int ObjectMonitor::SimpleEnter (Thread * Self) { - for (;;) { - if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) { - return OS_OK ; - } - - ObjectWaiter Node (Self) ; - Self->_ParkEvent->reset() ; // strictly optional - Node.TState = ObjectWaiter::TS_ENTER ; - - RawMonitor_lock->lock_without_safepoint_check() ; - Node._next = _EntryList ; - _EntryList = &Node ; - OrderAccess::fence() ; - if (_owner == NULL && Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) { - _EntryList = Node._next ; - RawMonitor_lock->unlock() ; - return OS_OK ; - } - RawMonitor_lock->unlock() ; - while (Node.TState == ObjectWaiter::TS_ENTER) { - Self->_ParkEvent->park() ; - } - } -} - -int ObjectMonitor::SimpleExit (Thread * Self) { - guarantee (_owner == Self, "invariant") ; - OrderAccess::release_store_ptr (&_owner, NULL) ; - OrderAccess::fence() ; - if (_EntryList == NULL) return OS_OK ; - ObjectWaiter * w ; - - RawMonitor_lock->lock_without_safepoint_check() ; - w = _EntryList ; - if (w != NULL) { - _EntryList = w->_next ; - } - RawMonitor_lock->unlock() ; - if (w != NULL) { - guarantee (w ->TState == ObjectWaiter::TS_ENTER, "invariant") ; - ParkEvent * ev = w->_event ; - w->TState = ObjectWaiter::TS_RUN ; - OrderAccess::fence() ; - ev->unpark() ; - } - return OS_OK ; -} - -int ObjectMonitor::SimpleWait (Thread * Self, jlong millis) { - guarantee (_owner == Self , "invariant") ; - guarantee (_recursions == 0, "invariant") ; - - ObjectWaiter Node (Self) ; - Node._notified = 0 ; - Node.TState = ObjectWaiter::TS_WAIT ; - - RawMonitor_lock->lock_without_safepoint_check() ; - Node._next = _WaitSet ; - _WaitSet = &Node ; - RawMonitor_lock->unlock() ; - - SimpleExit (Self) ; - guarantee (_owner != Self, "invariant") ; - - int ret = OS_OK ; - if (millis <= 0) { - Self->_ParkEvent->park(); - } else { - ret = Self->_ParkEvent->park(millis); - } - - // If thread still resides on the waitset then unlink it. - // Double-checked locking -- the usage is safe in this context - // as we TState is volatile and the lock-unlock operators are - // serializing (barrier-equivalent). - - if (Node.TState == ObjectWaiter::TS_WAIT) { - RawMonitor_lock->lock_without_safepoint_check() ; - if (Node.TState == ObjectWaiter::TS_WAIT) { - // Simple O(n) unlink, but performance isn't critical here. - ObjectWaiter * p ; - ObjectWaiter * q = NULL ; - for (p = _WaitSet ; p != &Node; p = p->_next) { - q = p ; - } - guarantee (p == &Node, "invariant") ; - if (q == NULL) { - guarantee (p == _WaitSet, "invariant") ; - _WaitSet = p->_next ; - } else { - guarantee (p == q->_next, "invariant") ; - q->_next = p->_next ; - } - Node.TState = ObjectWaiter::TS_RUN ; - } - RawMonitor_lock->unlock() ; - } - - guarantee (Node.TState == ObjectWaiter::TS_RUN, "invariant") ; - SimpleEnter (Self) ; - - guarantee (_owner == Self, "invariant") ; - guarantee (_recursions == 0, "invariant") ; - return ret ; -} - -int ObjectMonitor::SimpleNotify (Thread * Self, bool All) { - guarantee (_owner == Self, "invariant") ; - if (_WaitSet == NULL) return OS_OK ; - - // We have two options: - // A. Transfer the threads from the WaitSet to the EntryList - // B. Remove the thread from the WaitSet and unpark() it. - // - // We use (B), which is crude and results in lots of futile - // context switching. In particular (B) induces lots of contention. - - ParkEvent * ev = NULL ; // consider using a small auto array ... - RawMonitor_lock->lock_without_safepoint_check() ; - for (;;) { - ObjectWaiter * w = _WaitSet ; - if (w == NULL) break ; - _WaitSet = w->_next ; - if (ev != NULL) { ev->unpark(); ev = NULL; } - ev = w->_event ; - OrderAccess::loadstore() ; - w->TState = ObjectWaiter::TS_RUN ; - OrderAccess::storeload(); - if (!All) break ; - } - RawMonitor_lock->unlock() ; - if (ev != NULL) ev->unpark(); - return OS_OK ; -} - -// Any JavaThread will enter here with state _thread_blocked -int ObjectMonitor::raw_enter(TRAPS) { - TEVENT (raw_enter) ; - void * Contended ; - - // don't enter raw monitor if thread is being externally suspended, it will - // surprise the suspender if a "suspended" thread can still enter monitor - JavaThread * jt = (JavaThread *)THREAD; - if (THREAD->is_Java_thread()) { - jt->SR_lock()->lock_without_safepoint_check(); - while (jt->is_external_suspend()) { - jt->SR_lock()->unlock(); - jt->java_suspend_self(); - jt->SR_lock()->lock_without_safepoint_check(); - } - // guarded by SR_lock to avoid racing with new external suspend requests. - Contended = Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) ; - jt->SR_lock()->unlock(); - } else { - Contended = Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) ; - } - - if (Contended == THREAD) { - _recursions ++ ; - return OM_OK ; - } - - if (Contended == NULL) { - guarantee (_owner == THREAD, "invariant") ; - guarantee (_recursions == 0, "invariant") ; - return OM_OK ; - } - - THREAD->set_current_pending_monitor(this); - - if (!THREAD->is_Java_thread()) { - // No other non-Java threads besides VM thread would acquire - // a raw monitor. - assert(THREAD->is_VM_thread(), "must be VM thread"); - SimpleEnter (THREAD) ; - } else { - guarantee (jt->thread_state() == _thread_blocked, "invariant") ; - for (;;) { - jt->set_suspend_equivalent(); - // cleared by handle_special_suspend_equivalent_condition() or - // java_suspend_self() - SimpleEnter (THREAD) ; - - // were we externally suspended while we were waiting? - if (!jt->handle_special_suspend_equivalent_condition()) break ; - - // This thread was externally suspended - // - // This logic isn't needed for JVMTI raw monitors, - // but doesn't hurt just in case the suspend rules change. This - // logic is needed for the ObjectMonitor.wait() reentry phase. - // We have reentered the contended monitor, but while we were - // waiting another thread suspended us. We don't want to reenter - // the monitor while suspended because that would surprise the - // thread that suspended us. - // - // Drop the lock - - SimpleExit (THREAD) ; - - jt->java_suspend_self(); - } - - assert(_owner == THREAD, "Fatal error with monitor owner!"); - assert(_recursions == 0, "Fatal error with monitor recursions!"); - } - - THREAD->set_current_pending_monitor(NULL); - guarantee (_recursions == 0, "invariant") ; - return OM_OK; -} - -// Used mainly for JVMTI raw monitor implementation -// Also used for ObjectMonitor::wait(). -int ObjectMonitor::raw_exit(TRAPS) { - TEVENT (raw_exit) ; - if (THREAD != _owner) { - return OM_ILLEGAL_MONITOR_STATE; - } - if (_recursions > 0) { - --_recursions ; - return OM_OK ; - } - - void * List = _EntryList ; - SimpleExit (THREAD) ; - - return OM_OK; -} - -// Used for JVMTI raw monitor implementation. -// All JavaThreads will enter here with state _thread_blocked - -int ObjectMonitor::raw_wait(jlong millis, bool interruptible, TRAPS) { - TEVENT (raw_wait) ; - if (THREAD != _owner) { - return OM_ILLEGAL_MONITOR_STATE; - } - - // To avoid spurious wakeups we reset the parkevent -- This is strictly optional. - // The caller must be able to tolerate spurious returns from raw_wait(). - THREAD->_ParkEvent->reset() ; - OrderAccess::fence() ; - - // check interrupt event - if (interruptible && Thread::is_interrupted(THREAD, true)) { - return OM_INTERRUPTED; - } - - intptr_t save = _recursions ; - _recursions = 0 ; - _waiters ++ ; - if (THREAD->is_Java_thread()) { - guarantee (((JavaThread *) THREAD)->thread_state() == _thread_blocked, "invariant") ; - ((JavaThread *)THREAD)->set_suspend_equivalent(); - } - int rv = SimpleWait (THREAD, millis) ; - _recursions = save ; - _waiters -- ; - - guarantee (THREAD == _owner, "invariant") ; - if (THREAD->is_Java_thread()) { - JavaThread * jSelf = (JavaThread *) THREAD ; - for (;;) { - if (!jSelf->handle_special_suspend_equivalent_condition()) break ; - SimpleExit (THREAD) ; - jSelf->java_suspend_self(); - SimpleEnter (THREAD) ; - jSelf->set_suspend_equivalent() ; - } - } - guarantee (THREAD == _owner, "invariant") ; - - if (interruptible && Thread::is_interrupted(THREAD, true)) { - return OM_INTERRUPTED; - } - return OM_OK ; -} - -int ObjectMonitor::raw_notify(TRAPS) { - TEVENT (raw_notify) ; - if (THREAD != _owner) { - return OM_ILLEGAL_MONITOR_STATE; - } - SimpleNotify (THREAD, false) ; - return OM_OK; -} - -int ObjectMonitor::raw_notifyAll(TRAPS) { - TEVENT (raw_notifyAll) ; - if (THREAD != _owner) { - return OM_ILLEGAL_MONITOR_STATE; - } - SimpleNotify (THREAD, true) ; - return OM_OK; -} - -#ifndef PRODUCT -void ObjectMonitor::verify() { -} - -void ObjectMonitor::print() { -} -#endif - //------------------------------------------------------------------------------ // Non-product code