Mercurial > hg > truffle
diff src/share/vm/runtime/objectMonitor.cpp @ 1878:fa83ab460c54
6988353: refactor contended sync subsystem
Summary: reduce complexity by factoring synchronizer.cpp
Reviewed-by: dholmes, never, coleenp
author | acorn |
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date | Fri, 22 Oct 2010 15:59:34 -0400 |
parents | |
children | f95d63e2154a |
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--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/src/share/vm/runtime/objectMonitor.cpp Fri Oct 22 15:59:34 2010 -0400 @@ -0,0 +1,2421 @@ +/* + * 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 + * under the terms of the GNU General Public License version 2 only, as + * published by the Free Software Foundation. + * + * This code is distributed in the hope that it will be useful, but WITHOUT + * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or + * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License + * version 2 for more details (a copy is included in the LICENSE file that + * accompanied this code). + * + * You should have received a copy of the GNU General Public License version + * 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 Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA + * or visit www.oracle.com if you need additional information or have any + * questions. + * + */ + +# include "incls/_precompiled.incl" +# include "incls/_objectMonitor.cpp.incl" + +#if defined(__GNUC__) && !defined(IA64) + // Need to inhibit inlining for older versions of GCC to avoid build-time failures + #define ATTR __attribute__((noinline)) +#else + #define ATTR +#endif + + +#ifdef DTRACE_ENABLED + +// Only bother with this argument setup if dtrace is available +// TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly. + +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; \ + int len = 0; \ + jlong jtid = SharedRuntime::get_java_tid(thread); \ + symbolOop klassname = ((oop)(klassOop))->klass()->klass_part()->name(); \ + if (klassname != NULL) { \ + bytes = (char*)klassname->bytes(); \ + len = klassname->utf8_length(); \ + } + +#define DTRACE_MONITOR_WAIT_PROBE(monitor, klassOop, thread, millis) \ + { \ + if (DTraceMonitorProbes) { \ + DTRACE_MONITOR_PROBE_COMMON(klassOop, thread); \ + HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid, \ + (monitor), bytes, len, (millis)); \ + } \ + } + +#define DTRACE_MONITOR_PROBE(probe, monitor, klassOop, thread) \ + { \ + if (DTraceMonitorProbes) { \ + DTRACE_MONITOR_PROBE_COMMON(klassOop, thread); \ + HS_DTRACE_PROBE4(hotspot, monitor__##probe, jtid, \ + (uintptr_t)(monitor), bytes, len); \ + } \ + } + +#else // ndef DTRACE_ENABLED + +#define DTRACE_MONITOR_WAIT_PROBE(klassOop, thread, millis, mon) {;} +#define DTRACE_MONITOR_PROBE(probe, klassOop, thread, mon) {;} + +#endif // ndef DTRACE_ENABLED + +// Tunables ... +// The knob* variables are effectively final. Once set they should +// never be modified hence. Consider using __read_mostly with GCC. + +int ObjectMonitor::Knob_Verbose = 0 ; +int ObjectMonitor::Knob_SpinLimit = 5000 ; // derived by an external tool - +static int Knob_LogSpins = 0 ; // enable jvmstat tally for spins +static int Knob_HandOff = 0 ; +static int Knob_ReportSettings = 0 ; + +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 ; + +#define TrySpin TrySpin_VaryDuration + +// ----------------------------------------------------------------------------- +// 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 + + +// ----------------------------------------------------------------------------- +// Enter support + +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 (ObjectMonitor::_sync_ContendedLockAttempts != NULL) { + ObjectMonitor::_sync_ContendedLockAttempts->inc() ; + } +} + + +// 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 ; + } +} + +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 (ObjectMonitor::_sync_FutileWakeups != NULL) { + ObjectMonitor::_sync_FutileWakeups->inc() ; + } + ++ nWakeups ; + + // 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 ; +} + +// 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 (ObjectMonitor::_sync_FutileWakeups != NULL) { + ObjectMonitor::_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() +} + +// 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 ; +} + +// ----------------------------------------------------------------------------- +// Exit support +// +// 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") ; + + + 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") ; + + 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 ; + } + } +} + +// 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() ; +} + + +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() + + if (SafepointSynchronize::do_call_back()) { + TEVENT (unpark before SAFEPOINT) ; + } + + DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self); + Trigger->unpark() ; + + // Maintain stats and report events to JVMTI + if (ObjectMonitor::_sync_Parks != NULL) { + ObjectMonitor::_sync_Parks->inc() ; + } +} + + +// ----------------------------------------------------------------------------- +// Class Loader deadlock handling. +// +// 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; +} + + +// ----------------------------------------------------------------------------- +// 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) + +// 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"); +} + +static int Adjust (volatile int * adr, int dx) { + int v ; + for (v = *adr ; Atomic::cmpxchg (v + dx, adr, v) != v; v = *adr) ; + return v ; +} +// ----------------------------------------------------------------------------- +// Wait/Notify/NotifyAll +// +// 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") ; + 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 && ObjectMonitor::_sync_Notifications != NULL) { + ObjectMonitor::_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). + + 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 && ObjectMonitor::_sync_Notifications != NULL) { + ObjectMonitor::_sync_Notifications->inc(Tally) ; + } +} + +// ----------------------------------------------------------------------------- +// Adaptive Spinning Support +// +// 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. +// + +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 ; +} + +// 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 ; +} + + +// ----------------------------------------------------------------------------- +// WaitSet management ... + +ObjectWaiter::ObjectWaiter(Thread* thread) { + _next = NULL; + _prev = NULL; + _notified = 0; + TState = TS_RUN ; + _thread = thread; + _event = thread->_ParkEvent ; + _active = false; + assert (_event != NULL, "invariant") ; +} + +void ObjectWaiter::wait_reenter_begin(ObjectMonitor *mon) { + JavaThread *jt = (JavaThread *)this->_thread; + _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon); +} + +void ObjectWaiter::wait_reenter_end(ObjectMonitor *mon) { + JavaThread *jt = (JavaThread *)this->_thread; + JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active); +} + +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; +} + +// ----------------------------------------------------------------------------- +// PerfData support +PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL ; +PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL ; +PerfCounter * ObjectMonitor::_sync_Parks = NULL ; +PerfCounter * ObjectMonitor::_sync_EmptyNotifications = NULL ; +PerfCounter * ObjectMonitor::_sync_Notifications = NULL ; +PerfCounter * ObjectMonitor::_sync_PrivateA = NULL ; +PerfCounter * ObjectMonitor::_sync_PrivateB = NULL ; +PerfCounter * ObjectMonitor::_sync_SlowExit = NULL ; +PerfCounter * ObjectMonitor::_sync_SlowEnter = NULL ; +PerfCounter * ObjectMonitor::_sync_SlowNotify = NULL ; +PerfCounter * ObjectMonitor::_sync_SlowNotifyAll = NULL ; +PerfCounter * ObjectMonitor::_sync_FailedSpins = NULL ; +PerfCounter * ObjectMonitor::_sync_SuccessfulSpins = NULL ; +PerfCounter * ObjectMonitor::_sync_MonInCirculation = NULL ; +PerfCounter * ObjectMonitor::_sync_MonScavenged = NULL ; +PerfCounter * ObjectMonitor::_sync_Inflations = NULL ; +PerfCounter * ObjectMonitor::_sync_Deflations = NULL ; +PerfLongVariable * ObjectMonitor::_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 ObjectMonitor::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 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 ; +} + +void ObjectMonitor::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) { + ObjectMonitor::_sync_FailedSpins = NULL ; + } + + free (knobs) ; + OrderAccess::fence() ; + InitDone = 1 ; +} + +#ifndef PRODUCT +void ObjectMonitor::verify() { +} + +void ObjectMonitor::print() { +} +#endif