truffle: src/share/vm/runtime/synchronizer.cpp comparison

comparison src/share/vm/runtime/synchronizer.cpp @ 0:a61af66fc99e jdk7-b24

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author	duke
date	Sat, 01 Dec 2007 00:00:00 +0000
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children	a7fac4381b50

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+:a61af66fc99e
+/*
+* Copyright 1998-2007 Sun Microsystems, Inc.  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 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.
+*
+*/
+# include "incls/_precompiled.incl"
+# include "incls/_synchronizer.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
+// 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.
+//
+// -----------------------------------------------------------------------------
+#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_DECL5(hotspot, monitor__wait,
+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;                                                      \
+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
+// 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.
+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") ;
+}
+void wait_reenter_begin(ObjectMonitor *mon) {
+JavaThread *jt = (JavaThread *)this->_thread;
+_active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
+}
+void wait_reenter_end(ObjectMonitor *mon) {
+JavaThread *jt = (JavaThread *)this->_thread;
+JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
+}
+};
+enum ManifestConstants {
+ClearResponsibleAtSTW   = 0,
+MaximumRecheckInterval  = 1000
+} ;
+#undef TEVENT
+#define TEVENT(nom) {if (SyncVerbose) FEVENT(nom); }
+#define FEVENT(nom) { static volatile int ctr = 0 ; int v = ++ctr ; if ((v & (v-1)) == 0) { ::printf (#nom " : %d \n", v); ::fflush(stdout); }}
+#undef  TEVENT
+#define TEVENT(nom) {;}
+// Performance concern:
+// OrderAccess::storestore() calls release() which STs 0 into the global volatile
+// OrderAccess::Dummy variable.  This store is unnecessary for correctness.
+// Many threads STing into a common location causes considerable cache migration
+// or "sloshing" on large SMP system.  As such, I avoid using OrderAccess::storestore()
+// until it's repaired.  In some cases OrderAccess::fence() -- which incurs local
+// latency on the executing processor -- is a better choice as it scales on SMP
+// systems.  See http://blogs.sun.com/dave/entry/biased_locking_in_hotspot for a
+// discussion of coherency costs.  Note that all our current reference platforms
+// provide strong ST-ST order, so the issue is moot on IA32, x64, and SPARC.
+//
+// As a general policy we use "volatile" to control compiler-based reordering
+// and explicit fences (barriers) to control for architectural reordering performed
+// by the CPU(s) or platform.
+static int  MBFence (int x) { OrderAccess::fence(); return x; }
+struct SharedGlobals {
+// These are highly shared mostly-read variables.
+// To avoid false-sharing they need to be the sole occupants of a $ line.
+double padPrefix [8];
+volatile int stwRandom ;
+volatile int stwCycle ;
+// Hot RW variables -- Sequester to avoid false-sharing
+double padSuffix [16];
+volatile int hcSequence ;
+double padFinal [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 @%X\n", 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 markOop ReadStableMark (oop obj) {
+markOop mark = obj->mark() ;
+if (!mark->is_being_inflated()) {
+return mark ;       // normal fast-path return
+}
+int its = 0 ;
+for (;;) {
+markOop mark = obj->mark() ;
+if (!mark->is_being_inflated()) {
+return mark ;    // normal fast-path return
+}
+// The object is being inflated by some other thread.
+// The caller of ReadStableMark() must wait for inflation to complete.
+// Avoid live-lock
+// TODO: consider calling SafepointSynchronize::do_call_back() while
+// spinning to see if there's a safepoint pending.  If so, immediately
+// yielding or blocking would be appropriate.  Avoid spinning while
+// there is a safepoint pending.
+// TODO: add inflation contention performance counters.
+// TODO: restrict the aggregate number of spinners.
+++its ;
+if (its > 10000 || !os::is_MP()) {
+if (its & 1) {
+os::NakedYield() ;
+TEVENT (Inflate: INFLATING - yield) ;
+} else {
+// Note that the following code attenuates the livelock problem but is not
+// a complete remedy.  A more complete solution would require that the inflating
+// thread hold the associated inflation lock.  The following code simply restricts
+// the number of spinners to at most one.  We'll have N-2 threads blocked
+// on the inflationlock, 1 thread holding the inflation lock and using
+// a yield/park strategy, and 1 thread in the midst of inflation.
+// A more refined approach would be to change the encoding of INFLATING
+// to allow encapsulation of a native thread pointer.  Threads waiting for
+// inflation to complete would use CAS to push themselves onto a singly linked
+// list rooted at the markword.  Once enqueued, they'd loop, checking a per-thread flag
+// and calling park().  When inflation was complete the thread that accomplished inflation
+// would detach the list and set the markword to inflated with a single CAS and
+// then for each thread on the list, set the flag and unpark() the thread.
+// This is conceptually similar to muxAcquire-muxRelease, except that muxRelease
+// wakes at most one thread whereas we need to wake the entire list.
+int ix = (intptr_t(obj) >> 5) & (NINFLATIONLOCKS-1) ;
+int YieldThenBlock = 0 ;
+assert (ix >= 0 && ix < NINFLATIONLOCKS, "invariant") ;
+assert ((NINFLATIONLOCKS & (NINFLATIONLOCKS-1)) == 0, "invariant") ;
+Thread::muxAcquire (InflationLocks + ix, "InflationLock") ;
+while (obj->mark() == markOopDesc::INFLATING()) {
+// Beware: NakedYield() is advisory and has almost no effect on some platforms
+// so we periodically call Self->_ParkEvent->park(1).
+// We use a mixed spin/yield/block mechanism.
+if ((YieldThenBlock++) >= 16) {
+Thread::current()->_ParkEvent->park(1) ;
+} else {
+os::NakedYield() ;
+}
+}
+Thread::muxRelease (InflationLocks + ix ) ;
+TEVENT (Inflate: INFLATING - yield/park) ;
+}
+} else {
+SpinPause() ;       // SMP-polite spinning
+}
+}
+}
+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) ;
+// 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");
+}
+THREAD->update_highest_lock((address)lock);
+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() ;
+// 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);
+}
+// 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
+// to be taken here, in particular most operations on perm gen
+// objects. However, we only ever bias Java instances and all of
+// the call sites of identity_hash that might revoke biases have
+// been checked to make sure they can handle a safepoint. The
+// added check of the bias pattern is to avoid useless calls to
+// thread-local storage.
+if (obj->mark()->has_bias_pattern()) {
+// Box and unbox the raw reference just in case we cause a STW safepoint.
+Handle hobj (Self, obj) ;
+// Relaxing assertion for bug 6320749.
+assert (Universe::verify_in_progress() ||
+!SafepointSynchronize::is_at_safepoint(),
+"biases should not be seen by VM thread here");
+BiasedLocking::revoke_and_rebias(hobj, false, JavaThread::current());
+obj = hobj() ;
+assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
+}
+}
+// hashCode() is a heap mutator ...
+// Relaxing assertion for bug 6320749.
+assert (Universe::verify_in_progress() ||
+!SafepointSynchronize::is_at_safepoint(), "invariant") ;
+assert (Universe::verify_in_progress() ||
+Self->is_Java_thread() , "invariant") ;
+assert (Universe::verify_in_progress() ||
+((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
+ObjectMonitor* monitor = NULL;
+markOop temp, test;
+intptr_t hash;
+markOop mark = ReadStableMark (obj);
+// object should remain ineligible for biased locking
+assert (!mark->has_bias_pattern(), "invariant") ;
+if (mark->is_neutral()) {
+hash = mark->hash();              // this is a normal header
+if (hash) {                       // if it has hash, just return it
+return hash;
+}
+hash = get_next_hash(Self, obj);  // allocate a new hash code
+temp = mark->copy_set_hash(hash); // merge the hash code into header
+// use (machine word version) atomic operation to install the hash
+test = (markOop) Atomic::cmpxchg_ptr(temp, obj->mark_addr(), mark);
+if (test == mark) {
+return hash;
+}
+// If atomic operation failed, we must inflate the header
+// into heavy weight monitor. We could add more code here
+// for fast path, but it does not worth the complexity.
+} else if (mark->has_monitor()) {
+monitor = mark->monitor();
+temp = monitor->header();
+assert (temp->is_neutral(), "invariant") ;
+hash = temp->hash();
+if (hash) {
+return hash;
+}
+// Skip to the following code to reduce code size
+} else if (Self->is_lock_owned((address)mark->locker())) {
+temp = mark->displaced_mark_helper(); // this is a lightweight monitor owned
+assert (temp->is_neutral(), "invariant") ;
+hash = temp->hash();              // by current thread, check if the displaced
+if (hash) {                       // header contains hash code
+return hash;
+}
+// WARNING:
+//   The displaced header is strictly immutable.
+// It can NOT be changed in ANY cases. So we have
+// to inflate the header into heavyweight monitor
+// even the current thread owns the lock. The reason
+// is the BasicLock (stack slot) will be asynchronously
+// read by other threads during the inflate() function.
+// Any change to stack may not propagate to other threads
+// correctly.
+}
+// Inflate the monitor to set hash code
+monitor = ObjectSynchronizer::inflate(Self, obj);
+// Load displaced header and check it has hash code
+mark = monitor->header();
+assert (mark->is_neutral(), "invariant") ;
+hash = mark->hash();
+if (hash == 0) {
+hash = get_next_hash(Self, obj);
+temp = mark->copy_set_hash(hash); // merge hash code into header
+assert (temp->is_neutral(), "invariant") ;
+test = (markOop) Atomic::cmpxchg_ptr(temp, monitor, mark);
+if (test != mark) {
+// The only update to the header in the monitor (outside GC)
+// is install the hash code. If someone add new usage of
+// displaced header, please update this code
+hash = test->hash();
+assert (test->is_neutral(), "invariant") ;
+assert (hash != 0, "Trivial unexpected object/monitor header usage.");
+}
+}
+// We finally get the hash
+return hash;
+}
+// Deprecated -- use FastHashCode() instead.
+intptr_t ObjectSynchronizer::identity_hash_value_for(Handle obj) {
+return FastHashCode (Thread::current(), obj()) ;
+}
+bool ObjectSynchronizer::current_thread_holds_lock(JavaThread* thread,
+Handle h_obj) {
+if (UseBiasedLocking) {
+BiasedLocking::revoke_and_rebias(h_obj, false, thread);
+assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
+}
+assert(thread == JavaThread::current(), "Can only be called on current thread");
+oop obj = h_obj();
+markOop mark = ReadStableMark (obj) ;
+// Uncontended case, header points to stack
+if (mark->has_locker()) {
+return thread->is_lock_owned((address)mark->locker());
+}
+// Contended case, header points to ObjectMonitor (tagged pointer)
+if (mark->has_monitor()) {
+ObjectMonitor* monitor = mark->monitor();
+return monitor->is_entered(thread) != 0 ;
+}
+// Unlocked case, header in place
+assert(mark->is_neutral(), "sanity check");
+return false;
+}
+// Be aware of this method could revoke bias of the lock object.
+// This method querys the ownership of the lock handle specified by 'h_obj'.
+// If the current thread owns the lock, it returns owner_self. If no
+// thread owns the lock, it returns owner_none. Otherwise, it will return
+// ower_other.
+ObjectSynchronizer::LockOwnership ObjectSynchronizer::query_lock_ownership
+(JavaThread *self, Handle h_obj) {
+// The caller must beware this method can revoke bias, and
+// revocation can result in a safepoint.
+assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
+assert (self->thread_state() != _thread_blocked , "invariant") ;
+// Possible mark states: neutral, biased, stack-locked, inflated
+if (UseBiasedLocking && h_obj()->mark()->has_bias_pattern()) {
+// CASE: biased
+BiasedLocking::revoke_and_rebias(h_obj, false, self);
+assert(!h_obj->mark()->has_bias_pattern(),
+"biases should be revoked by now");
+}
+assert(self == JavaThread::current(), "Can only be called on current thread");
+oop obj = h_obj();
+markOop mark = ReadStableMark (obj) ;
+// CASE: stack-locked.  Mark points to a BasicLock on the owner's stack.
+if (mark->has_locker()) {
+return self->is_lock_owned((address)mark->locker()) ?
+owner_self : owner_other;
+}
+// CASE: inflated. Mark (tagged pointer) points to an objectMonitor.
+// The Object:ObjectMonitor relationship is stable as long as we're
+// not at a safepoint.
+if (mark->has_monitor()) {
+void * owner = mark->monitor()->_owner ;
+if (owner == NULL) return owner_none ;
+return (owner == self ||
+self->is_lock_owned((address)owner)) ? owner_self : owner_other;
+}
+// CASE: neutral
+assert(mark->is_neutral(), "sanity check");
+return owner_none ;           // it's unlocked
+}
+// FIXME: jvmti should call this
+JavaThread* ObjectSynchronizer::get_lock_owner(Handle h_obj, bool doLock) {
+if (UseBiasedLocking) {
+if (SafepointSynchronize::is_at_safepoint()) {
+BiasedLocking::revoke_at_safepoint(h_obj);
+} else {
+BiasedLocking::revoke_and_rebias(h_obj, false, JavaThread::current());
+}
+assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
+}
+oop obj = h_obj();
+address owner = NULL;
+markOop mark = ReadStableMark (obj) ;
+// Uncontended case, header points to stack
+if (mark->has_locker()) {
+owner = (address) mark->locker();
+}
+// Contended case, header points to ObjectMonitor (tagged pointer)
+if (mark->has_monitor()) {
+ObjectMonitor* monitor = mark->monitor();
+assert(monitor != NULL, "monitor should be non-null");
+owner = (address) monitor->owner();
+}
+if (owner != NULL) {
+return Threads::owning_thread_from_monitor_owner(owner, doLock);
+}
+// Unlocked case, header in place
+// Cannot have assertion since this object may have been
+// locked by another thread when reaching here.
+// assert(mark->is_neutral(), "sanity check");
+return NULL;
+}
+// Iterate through monitor cache and attempt to release thread's monitors
+// Gives up on a particular monitor if an exception occurs, but continues
+// the overall iteration, swallowing the exception.
+class ReleaseJavaMonitorsClosure: public MonitorClosure {
+private:
+TRAPS;
+public:
+ReleaseJavaMonitorsClosure(Thread* thread) : THREAD(thread) {}
+void do_monitor(ObjectMonitor* mid) {
+if (mid->owner() == THREAD) {
+(void)mid->complete_exit(CHECK);
+}
+}
+};
+// Release all inflated monitors owned by THREAD.  Lightweight monitors are
+// ignored.  This is meant to be called during JNI thread detach which assumes
+// all remaining monitors are heavyweight.  All exceptions are swallowed.
+// Scanning the extant monitor list can be time consuming.
+// A simple optimization is to add a per-thread flag that indicates a thread
+// called jni_monitorenter() during its lifetime.
+//
+// Instead of No_Savepoint_Verifier it might be cheaper to
+// use an idiom of the form:
+//   auto int tmp = SafepointSynchronize::_safepoint_counter ;
+//   <code that must not run at safepoint>
+//   guarantee (((tmp ^ _safepoint_counter) | (tmp & 1)) == 0) ;
+// Since the tests are extremely cheap we could leave them enabled
+// for normal product builds.
+void ObjectSynchronizer::release_monitors_owned_by_thread(TRAPS) {
+assert(THREAD == JavaThread::current(), "must be current Java thread");
+No_Safepoint_Verifier nsv ;
+ReleaseJavaMonitorsClosure rjmc(THREAD);
+Thread::muxAcquire(&ListLock, "release_monitors_owned_by_thread");
+ObjectSynchronizer::monitors_iterate(&rjmc);
+Thread::muxRelease(&ListLock);
+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 ;
+// 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
+Trigger->unpark() ;
+// Maintain stats and report events to JVMTI
+if (ObjectSynchronizer::_sync_Parks != NULL) {
+ObjectSynchronizer::_sync_Parks->inc() ;
+}
+DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
+}
+// 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
+#ifndef PRODUCT
+void ObjectSynchronizer::trace_locking(Handle locking_obj, bool is_compiled,
+bool is_method, bool is_locking) {
+// Don't know what to do here
+}
+// Verify all monitors in the monitor cache, the verification is weak.
+void ObjectSynchronizer::verify() {
+ObjectMonitor* block = gBlockList;
+ObjectMonitor* mid;
+while (block) {
+assert(block->object() == CHAINMARKER, "must be a block header");
+for (int i = 1; i < _BLOCKSIZE; i++) {
+mid = block + i;
+oop object = (oop) mid->object();
+if (object != NULL) {
+mid->verify();
+}
+}
+block = (ObjectMonitor*) block->FreeNext;
+}
+}
+// Check if monitor belongs to the monitor cache
+// The list is grow-only so it's *relatively* safe to traverse
+// the list of extant blocks without taking a lock.
+int ObjectSynchronizer::verify_objmon_isinpool(ObjectMonitor *monitor) {
+ObjectMonitor* block = gBlockList;
+while (block) {
+assert(block->object() == CHAINMARKER, "must be a block header");
+if (monitor > &block[0] && monitor < &block[_BLOCKSIZE]) {
+address mon = (address) monitor;
+address blk = (address) block;
+size_t diff = mon - blk;
+assert((diff % sizeof(ObjectMonitor)) == 0, "check");
+return 1;
+}
+block = (ObjectMonitor*) block->FreeNext;
+}
+return 0;
+}
+#endif

Mercurial > hg > truffle

comparison src/share/vm/runtime/synchronizer.cpp @ 0:a61af66fc99e jdk7-b24