Mercurial > jdk8-mips64-public > hotspot / file revision

 /*

  * Copyright 1998-2008 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

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  *

  */

 # include "incls/_precompiled.incl"

 # include "incls/_mutex.cpp.incl"

 // o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o

 //

 // Native Monitor-Mutex locking - theory of operations

 //

 // * Native Monitors are completely unrelated to Java-level monitors,

 //   although the "back-end" slow-path implementations share a common lineage.

 //   See objectMonitor:: in synchronizer.cpp.

 //   Native Monitors do *not* support nesting or recursion but otherwise

 //   they're basically Hoare-flavor monitors.

 //

 // * A thread acquires ownership of a Monitor/Mutex by CASing the LockByte

 //   in the _LockWord from zero to non-zero.  Note that the _Owner field

 //   is advisory and is used only to verify that the thread calling unlock()

 //   is indeed the last thread to have acquired the lock.

 //

 // * Contending threads "push" themselves onto the front of the contention

 //   queue -- called the cxq -- with CAS and then spin/park.

 //   The _LockWord contains the LockByte as well as the pointer to the head

 //   of the cxq.  Colocating the LockByte with the cxq precludes certain races.

 //

 // * Using a separately addressable LockByte allows for CAS:MEMBAR or CAS:0

 //   idioms.  We currently use MEMBAR in the uncontended unlock() path, as

 //   MEMBAR often has less latency than CAS.  If warranted, we could switch to

 //   a CAS:0 mode, using timers to close the resultant race, as is done

 //   with Java Monitors in synchronizer.cpp.

 //

 //   See the following for a discussion of the relative cost of atomics (CAS)

 //   MEMBAR, and ways to eliminate such instructions from the common-case paths:

 //   -- http://blogs.sun.com/dave/entry/biased_locking_in_hotspot

 //   -- http://blogs.sun.com/dave/resource/MustangSync.pdf

 //   -- http://blogs.sun.com/dave/resource/synchronization-public2.pdf

 //   -- synchronizer.cpp

 //

 // * Overall goals - desiderata

 //   1. Minimize context switching

 //   2. Minimize lock migration

 //   3. Minimize CPI -- affinity and locality

 //   4. Minimize the execution of high-latency instructions such as CAS or MEMBAR

 //   5. Minimize outer lock hold times

 //   6. Behave gracefully on a loaded system

 //

 // * Thread flow and list residency:

 //

 //   Contention queue --> EntryList --> OnDeck --> Owner --> !Owner

 //   [..resident on monitor list..]

 //   [...........contending..................]

 //

 //   -- The contention queue (cxq) contains recently-arrived threads (RATs).

 //      Threads on the cxq eventually drain into the EntryList.

 //   -- Invariant: a thread appears on at most one list -- cxq, EntryList

 //      or WaitSet -- at any one time.

 //   -- For a given monitor there can be at most one "OnDeck" thread at any

 //      given time but if needbe this particular invariant could be relaxed.

 //

 // * The WaitSet and EntryList linked lists are composed of ParkEvents.

 //   I use ParkEvent instead of threads as ParkEvents are immortal and

 //   type-stable, meaning we can safely unpark() a possibly stale

 //   list element in the unlock()-path.  (That's benign).

 //

 // * Succession policy - providing for progress:

 //

 //   As necessary, the unlock()ing thread identifies, unlinks, and unparks

 //   an "heir presumptive" tentative successor thread from the EntryList.

 //   This becomes the so-called "OnDeck" thread, of which there can be only

 //   one at any given time for a given monitor.  The wakee will recontend

 //   for ownership of monitor.

 //

 //   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.

 //

 //   Competitive handoff provides excellent overall throughput at the expense

 //   of short-term fairness.  If fairness is a concern then one remedy might

 //   be to add an AcquireCounter field to the monitor.  After a thread acquires

 //   the lock it will decrement the AcquireCounter field.  When the count

 //   reaches 0 the thread would reset the AcquireCounter variable, abdicate

 //   the lock directly to some thread on the EntryList, and then move itself to the

 //   tail of the EntryList.

 //

 //   But in practice most threads engage or otherwise participate in resource

 //   bounded producer-consumer relationships, so lock domination is not usually

 //   a practical concern.  Recall too, that in general it's easier to construct

 //   a fair lock from a fast lock, but not vice-versa.

 //

 // * 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.

 //   We use OnDeck as a pseudo-lock to enforce the at-most-one detaching

 //   thread constraint.

 //

 // * 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 reduce heat on the list ends.

 //   Threads in lock() enqueue onto cxq while threads in unlock() will

 //   dequeue from the EntryList.  (c.f. Michael Scott's "2Q" algorithm).

 //   A key desideratum is to minimize queue & monitor metadata manipulation

 //   that occurs while holding the "outer" monitor lock -- that is, we want to

 //   minimize monitor lock holds times.

 //

 //   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.  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.

 //   -- http://cvs.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/uts/common/os/sleepq.c

 //   Queue discipline is enforced at ::unlock() 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.

 //

 // * OnDeck

 //   --  For a given monitor there can be at most one OnDeck thread at any given

 //       instant.  The OnDeck thread is contending for the lock, but has been

 //       unlinked from the EntryList and cxq by some previous unlock() operations.

 //       Once a thread has been designated the OnDeck thread it will remain so

 //       until it manages to acquire the lock -- being OnDeck is a stable property.

 //   --  Threads on the EntryList or cxq are _not allowed to attempt lock acquisition.

 //   --  OnDeck also serves as an "inner lock" as follows.  Threads in unlock() will, after

 //       having cleared the LockByte and dropped the outer lock,  attempt to "trylock"

 //       OnDeck by CASing the field from null to non-null.  If successful, that thread

 //       is then responsible for progress and succession and can use CAS to detach and

 //       drain the cxq into the EntryList.  By convention, only this thread, the holder of

 //       the OnDeck inner lock, can manipulate the EntryList or detach and drain the

 //       RATs on the cxq into the EntryList.  This avoids ABA corruption on the cxq as

 //       we allow multiple concurrent "push" operations but restrict detach concurrency

 //       to at most one thread.  Having selected and detached a successor, the thread then

 //       changes the OnDeck to refer to that successor, and then unparks the successor.

 //       That successor will eventually acquire the lock and clear OnDeck.  Beware

 //       that the OnDeck usage as a lock is asymmetric.  A thread in unlock() transiently

 //       "acquires" OnDeck, performs queue manipulations, passes OnDeck to some successor,

 //       and then the successor eventually "drops" OnDeck.  Note that there's never

 //       any sense of contention on the inner lock, however.  Threads never contend

 //       or wait for the inner lock.

 //   --  OnDeck provides for futile wakeup throttling a described in section 3.3 of

 //       See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf

 //       In a sense, OnDeck subsumes the ObjectMonitor _Succ and ObjectWaiter

 //       TState fields found in Java-level objectMonitors.  (See synchronizer.cpp).

 //

 // * 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 unlock() operations will eventually unpark the notifyee.

 //   Unparking a notifee in notify() proper is inefficient - if we were to do so

 //   it's likely the notifyee would simply impale itself on the lock held

 //   by the notifier.

 //

 // * The mechanism is obstruction-free in that if the holder of the transient

 //   OnDeck lock in unlock() is preempted or otherwise stalls, other threads

 //   can still acquire and release the outer lock and continue to make progress.

 //   At worst, waking of already blocked contending threads may be delayed,

 //   but nothing worse.  (We only use "trylock" operations on the inner OnDeck

 //   lock).

 //

 // * Note that thread-local storage must be initialized before a thread

 //   uses Native monitors or mutexes.  The native monitor-mutex subsystem

 //   depends on Thread::current().

 //

 // * 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.

 //

 // * The memory consistency model provide by lock()-unlock() is at least as

 //   strong or stronger than the Java Memory model defined by JSR-133.

 //   That is, we guarantee at least entry consistency, if not stronger.

 //   See http://g.oswego.edu/dl/jmm/cookbook.html.

 //

 // * Thread:: currently contains a set of purpose-specific ParkEvents:

 //   _MutexEvent, _ParkEvent, etc.  A better approach might be to do away with

 //   the purpose-specific ParkEvents and instead implement a general per-thread

 //   stack of available ParkEvents which we could provision on-demand.  The

 //   stack acts as a local cache to avoid excessive calls to ParkEvent::Allocate()

 //   and ::Release().  A thread would simply pop an element from the local stack before it

 //   enqueued or park()ed.  When the contention was over the thread would

 //   push the no-longer-needed ParkEvent back onto its stack.

 //

 // * A slightly reduced form of ILock() and IUnlock() have been partially

 //   model-checked (Murphi) for safety and progress at T=1,2,3 and 4.

 //   It'd be interesting to see if TLA/TLC could be useful as well.

 //

 // * Mutex-Monitor is a low-level "leaf" subsystem.  That is, the monitor

 //   code should never call other code in the JVM that might itself need to

 //   acquire monitors or mutexes.  That's true *except* in the case of the

 //   ThreadBlockInVM state transition wrappers.  The ThreadBlockInVM DTOR handles

 //   mutator reentry (ingress) by checking for a pending safepoint in which case it will

 //   call SafepointSynchronize::block(), which in turn may call Safepoint_lock->lock(), etc.

 //   In that particular case a call to lock() for a given Monitor can end up recursively

 //   calling lock() on another monitor.   While distasteful, this is largely benign

 //   as the calls come from jacket that wraps lock(), and not from deep within lock() itself.

 //

 //   It's unfortunate that native mutexes and thread state transitions were convolved.

 //   They're really separate concerns and should have remained that way.  Melding

 //   them together was facile -- a bit too facile.   The current implementation badly

 //   conflates the two concerns.

 //

 // * TODO-FIXME:

 //

 //   -- Add DTRACE probes for contended acquire, contended acquired, contended unlock

 //      We should also add DTRACE probes in the ParkEvent subsystem for

 //      Park-entry, Park-exit, and Unpark.

 //

 //   -- We have an excess of mutex-like constructs in the JVM, namely:

 //      1. objectMonitors for Java-level synchronization (synchronizer.cpp)

 //      2. low-level muxAcquire and muxRelease

 //      3. low-level spinAcquire and spinRelease

 //      4. native Mutex:: and Monitor::

 //      5. jvm_raw_lock() and _unlock()

 //      6. JVMTI raw monitors -- distinct from (5) despite having a confusingly

 //         similar name.

 //

 // o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o

 // CASPTR() uses the canonical argument order that dominates in the literature.

 // Our internal cmpxchg_ptr() uses a bastardized ordering to accommodate Sun .il templates.

 #define CASPTR(a,c,s) intptr_t(Atomic::cmpxchg_ptr ((void *)(s),(void *)(a),(void *)(c)))

 #define UNS(x) (uintptr_t(x))

 #define TRACE(m) { static volatile int ctr = 0 ; int x = ++ctr ; if ((x & (x-1))==0) { ::printf ("%d:%s\n", x, #m); ::fflush(stdout); }}

 // Simplistic low-quality Marsaglia SHIFT-XOR RNG.

 // Bijective except for the trailing mask operation.

 // Useful for spin loops as the compiler can't optimize it away.

 static inline jint MarsagliaXORV (jint x) {

   if (x == 0) x = 1|os::random() ;

   x ^= x << 6;

   x ^= ((unsigned)x) >> 21;

   x ^= x << 7 ;

   return x & 0x7FFFFFFF ;

 }

 static inline jint MarsagliaXOR (jint * const a) {

   jint x = *a ;

   if (x == 0) x = UNS(a)|1 ;

   x ^= x << 6;

   x ^= ((unsigned)x) >> 21;

   x ^= x << 7 ;

   *a = x ;

   return x & 0x7FFFFFFF ;

 }

 static int Stall (int its) {

   static volatile jint rv = 1 ;

   volatile int OnFrame = 0 ;

   jint v = rv ^ UNS(OnFrame) ;

   while (--its >= 0) {

     v = MarsagliaXORV (v) ;

   }

   // Make this impossible for the compiler to optimize away,

   // but (mostly) avoid W coherency sharing on MP systems.

   if (v == 0x12345) rv = v ;

   return v ;

 }

 int Monitor::TryLock () {

   intptr_t v = _LockWord.FullWord ;

   for (;;) {

     if ((v & _LBIT) != 0) return 0 ;

     const intptr_t u = CASPTR (&_LockWord, v, v|_LBIT) ;

     if (v == u) return 1 ;

     v = u ;

   }

 }

 int Monitor::TryFast () {

   // Optimistic fast-path form ...

   // Fast-path attempt for the common uncontended case.

   // Avoid RTS->RTO $ coherence upgrade on typical SMP systems.

   intptr_t v = CASPTR (&_LockWord, 0, _LBIT) ;  // agro ...

   if (v == 0) return 1 ;

   for (;;) {

     if ((v & _LBIT) != 0) return 0 ;

     const intptr_t u = CASPTR (&_LockWord, v, v|_LBIT) ;

     if (v == u) return 1 ;

     v = u ;

   }

 }

 int Monitor::ILocked () {

   const intptr_t w = _LockWord.FullWord & 0xFF ;

   assert (w == 0 || w == _LBIT, "invariant") ;

   return w == _LBIT ;

 }

 // Polite TATAS spinlock with exponential backoff - bounded spin.

 // Ideally we'd use processor cycles, time or vtime to control

 // the loop, but we currently use iterations.

 // All the constants within were derived empirically but work over

 // over the spectrum of J2SE reference platforms.

 // On Niagara-class systems the back-off is unnecessary but

 // is relatively harmless.  (At worst it'll slightly retard

 // acquisition times).  The back-off is critical for older SMP systems

 // where constant fetching of the LockWord would otherwise impair

 // scalability.

 //

 // Clamp spinning at approximately 1/2 of a context-switch round-trip.

 // See synchronizer.cpp for details and rationale.

 int Monitor::TrySpin (Thread * const Self) {

   if (TryLock())    return 1 ;

   if (!os::is_MP()) return 0 ;

   int Probes  = 0 ;

   int Delay   = 0 ;

   int Steps   = 0 ;

   int SpinMax = NativeMonitorSpinLimit ;

   int flgs    = NativeMonitorFlags ;

   for (;;) {

     intptr_t v = _LockWord.FullWord;

     if ((v & _LBIT) == 0) {

       if (CASPTR (&_LockWord, v, v|_LBIT) == v) {

         return 1 ;

       }

       continue ;

     }

     if ((flgs & 8) == 0) {

       SpinPause () ;

     }

     // Periodically increase Delay -- variable Delay form

     // conceptually: delay *= 1 + 1/Exponent

     ++ Probes;

     if (Probes > SpinMax) return 0 ;

     if ((Probes & 0x7) == 0) {

       Delay = ((Delay << 1)|1) & 0x7FF ;

       // CONSIDER: Delay += 1 + (Delay/4); Delay &= 0x7FF ;

     }

     if (flgs & 2) continue ;

     // Consider checking _owner's schedctl state, if OFFPROC abort spin.

     // If the owner is OFFPROC then it's unlike that the lock will be dropped

     // in a timely fashion, which suggests that spinning would not be fruitful

     // or profitable.

     // Stall for "Delay" time units - iterations in the current implementation.

     // Avoid generating coherency traffic while stalled.

     // Possible ways to delay:

     //   PAUSE, SLEEP, MEMBAR #sync, MEMBAR #halt,

     //   wr %g0,%asi, gethrtime, rdstick, rdtick, rdtsc, etc. ...

     // Note that on Niagara-class systems we want to minimize STs in the

     // spin loop.  N1 and brethren write-around the L1$ over the xbar into the L2$.

     // Furthermore, they don't have a W$ like traditional SPARC processors.

     // We currently use a Marsaglia Shift-Xor RNG loop.

     Steps += Delay ;

     if (Self != NULL) {

       jint rv = Self->rng[0] ;

       for (int k = Delay ; --k >= 0; ) {

         rv = MarsagliaXORV (rv) ;

         if ((flgs & 4) == 0 && SafepointSynchronize::do_call_back()) return 0 ;

       }

       Self->rng[0] = rv ;

     } else {

       Stall (Delay) ;

     }

   }

 }

 static int ParkCommon (ParkEvent * ev, jlong timo) {

   // Diagnostic support - periodically unwedge blocked threads

   intx nmt = NativeMonitorTimeout ;

   if (nmt > 0 && (nmt < timo || timo <= 0)) {

      timo = nmt ;

   }

   int err = OS_OK ;

   if (0 == timo) {

     ev->park() ;

   } else {

     err = ev->park(timo) ;

   }

   return err ;

 }

 inline int Monitor::AcquireOrPush (ParkEvent * ESelf) {

   intptr_t v = _LockWord.FullWord ;

   for (;;) {

     if ((v & _LBIT) == 0) {

       const intptr_t u = CASPTR (&_LockWord, v, v|_LBIT) ;

       if (u == v) return 1 ;        // indicate acquired

       v = u ;

     } else {

       // Anticipate success ...

       ESelf->ListNext = (ParkEvent *) (v & ~_LBIT) ;

       const intptr_t u = CASPTR (&_LockWord, v, intptr_t(ESelf)|_LBIT) ;

       if (u == v) return 0 ;        // indicate pushed onto cxq

       v = u ;

     }

     // Interference - LockWord change - just retry

   }

 }

 // ILock and IWait are the lowest level primitive internal blocking

 // synchronization functions.  The callers of IWait and ILock must have

 // performed any needed state transitions beforehand.

 // IWait and ILock may directly call park() without any concern for thread state.

 // Note that ILock and IWait do *not* access _owner.

 // _owner is a higher-level logical concept.

 void Monitor::ILock (Thread * Self) {

   assert (_OnDeck != Self->_MutexEvent, "invariant") ;

   if (TryFast()) {

  Exeunt:

     assert (ILocked(), "invariant") ;

     return ;

   }

   ParkEvent * const ESelf = Self->_MutexEvent ;

   assert (_OnDeck != ESelf, "invariant") ;

   // As an optimization, spinners could conditionally try to set ONDECK to _LBIT

   // Synchronizer.cpp uses a similar optimization.

   if (TrySpin (Self)) goto Exeunt ;

   // Slow-path - the lock is contended.

   // Either Enqueue Self on cxq or acquire the outer lock.

   // LockWord encoding = (cxq,LOCKBYTE)

   ESelf->reset() ;

   OrderAccess::fence() ;

   // Optional optimization ... try barging on the inner lock

   if ((NativeMonitorFlags & 32) && CASPTR (&_OnDeck, NULL, UNS(Self)) == 0) {

     goto OnDeck_LOOP ;

   }

   if (AcquireOrPush (ESelf)) goto Exeunt ;

   // At any given time there is at most one ondeck thread.

   // ondeck implies not resident on cxq and not resident on EntryList

   // Only the OnDeck thread can try to acquire -- contended for -- the lock.

   // CONSIDER: use Self->OnDeck instead of m->OnDeck.

   // Deschedule Self so that others may run.

   while (_OnDeck != ESelf) {

     ParkCommon (ESelf, 0) ;

   }

   // Self is now in the ONDECK position and will remain so until it

   // manages to acquire the lock.

  OnDeck_LOOP:

   for (;;) {

     assert (_OnDeck == ESelf, "invariant") ;

     if (TrySpin (Self)) break ;

     // CONSIDER: if ESelf->TryPark() && TryLock() break ...

     // It's probably wise to spin only if we *actually* blocked

     // CONSIDER: check the lockbyte, if it remains set then

     // preemptively drain the cxq into the EntryList.

     // The best place and time to perform queue operations -- lock metadata --

     // is _before having acquired the outer lock, while waiting for the lock to drop.

     ParkCommon (ESelf, 0) ;

   }

   assert (_OnDeck == ESelf, "invariant") ;

   _OnDeck = NULL ;

   // Note that we current drop the inner lock (clear OnDeck) in the slow-path

   // epilog immediately after having acquired the outer lock.

   // But instead we could consider the following optimizations:

   // A. Shift or defer dropping the inner lock until the subsequent IUnlock() operation.

   //    This might avoid potential reacquisition of the inner lock in IUlock().

   // B. While still holding the inner lock, attempt to opportunistically select

   //    and unlink the next ONDECK thread from the EntryList.

   //    If successful, set ONDECK to refer to that thread, otherwise clear ONDECK.

   //    It's critical that the select-and-unlink operation run in constant-time as

   //    it executes when holding the outer lock and may artificially increase the

   //    effective length of the critical section.

   // Note that (A) and (B) are tantamount to succession by direct handoff for

   // the inner lock.

   goto Exeunt ;

 }

 void Monitor::IUnlock (bool RelaxAssert) {

   assert (ILocked(), "invariant") ;

   _LockWord.Bytes[_LSBINDEX] = 0 ;       // drop outer lock

   OrderAccess::storeload ();

   ParkEvent * const w = _OnDeck ;

   assert (RelaxAssert || w != Thread::current()->_MutexEvent, "invariant") ;

   if (w != NULL) {

     // Either we have a valid ondeck thread or ondeck is transiently "locked"

     // by some exiting thread as it arranges for succession.  The LSBit of

     // OnDeck allows us to discriminate two cases.  If the latter, the

     // responsibility for progress and succession lies with that other thread.

     // For good performance, we also depend on the fact that redundant unpark()

     // operations are cheap.  That is, repeated Unpark()ing of the ONDECK thread

     // is inexpensive.  This approach provides implicit futile wakeup throttling.

     // Note that the referent "w" might be stale with respect to the lock.

     // In that case the following unpark() is harmless and the worst that'll happen

     // is a spurious return from a park() operation.  Critically, if "w" _is stale,

     // then progress is known to have occurred as that means the thread associated

     // with "w" acquired the lock.  In that case this thread need take no further

     // action to guarantee progress.

     if ((UNS(w) & _LBIT) == 0) w->unpark() ;

     return ;

   }

   intptr_t cxq = _LockWord.FullWord ;

   if (((cxq & ~_LBIT)|UNS(_EntryList)) == 0) {

     return ;      // normal fast-path exit - cxq and EntryList both empty

   }

   if (cxq & _LBIT) {

     // Optional optimization ...

     // Some other thread acquired the lock in the window since this

     // thread released it.  Succession is now that thread's responsibility.

     return ;

   }

  Succession:

   // Slow-path exit - this thread must ensure succession and progress.

   // OnDeck serves as lock to protect cxq and EntryList.

   // Only the holder of OnDeck can manipulate EntryList or detach the RATs from cxq.

   // Avoid ABA - allow multiple concurrent producers (enqueue via push-CAS)

   // but only one concurrent consumer (detacher of RATs).

   // Consider protecting this critical section with schedctl on Solaris.

   // Unlike a normal lock, however, the exiting thread "locks" OnDeck,

   // picks a successor and marks that thread as OnDeck.  That successor

   // thread will then clear OnDeck once it eventually acquires the outer lock.

   if (CASPTR (&_OnDeck, NULL, _LBIT) != UNS(NULL)) {

     return ;

   }

   ParkEvent * List = _EntryList ;

   if (List != NULL) {

     // Transfer the head of the EntryList to the OnDeck position.

     // Once OnDeck, a thread stays OnDeck until it acquires the lock.

     // For a given lock there is at most OnDeck thread at any one instant.

    WakeOne:

     assert (List == _EntryList, "invariant") ;

     ParkEvent * const w = List ;

     assert (RelaxAssert || w != Thread::current()->_MutexEvent, "invariant") ;

     _EntryList = w->ListNext ;

     // as a diagnostic measure consider setting w->_ListNext = BAD

     assert (UNS(_OnDeck) == _LBIT, "invariant") ;

     _OnDeck = w ;           // pass OnDeck to w.

                             // w will clear OnDeck once it acquires the outer lock

     // Another optional optimization ...

     // For heavily contended locks it's not uncommon that some other

     // thread acquired the lock while this thread was arranging succession.

     // Try to defer the unpark() operation - Delegate the responsibility

     // for unpark()ing the OnDeck thread to the current or subsequent owners

     // That is, the new owner is responsible for unparking the OnDeck thread.

     OrderAccess::storeload() ;

     cxq = _LockWord.FullWord ;

     if (cxq & _LBIT) return ;

     w->unpark() ;

     return ;

   }

   cxq = _LockWord.FullWord ;

   if ((cxq & ~_LBIT) != 0) {

     // The EntryList is empty but the cxq is populated.

     // drain RATs from cxq into EntryList

     // Detach RATs segment with CAS and then merge into EntryList

     for (;;) {

       // optional optimization - if locked, the owner is responsible for succession

       if (cxq & _LBIT) goto Punt ;

       const intptr_t vfy = CASPTR (&_LockWord, cxq, cxq & _LBIT) ;

       if (vfy == cxq) break ;

       cxq = vfy ;

       // Interference - LockWord changed - Just retry

       // We can see concurrent interference from contending threads

       // pushing themselves onto the cxq or from lock-unlock operations.

       // From the perspective of this thread, EntryList is stable and

       // the cxq is prepend-only -- the head is volatile but the interior

       // of the cxq is stable.  In theory if we encounter interference from threads

       // pushing onto cxq we could simply break off the original cxq suffix and

       // move that segment to the EntryList, avoiding a 2nd or multiple CAS attempts

       // on the high-traffic LockWord variable.   For instance lets say the cxq is "ABCD"

       // when we first fetch cxq above.  Between the fetch -- where we observed "A"

       // -- and CAS -- where we attempt to CAS null over A -- "PQR" arrive,

       // yielding cxq = "PQRABCD".  In this case we could simply set A.ListNext

       // null, leaving cxq = "PQRA" and transfer the "BCD" segment to the EntryList.

       // Note too, that it's safe for this thread to traverse the cxq

       // without taking any special concurrency precautions.

     }

     // We don't currently reorder the cxq segment as we move it onto

     // the EntryList, but it might make sense to reverse the order

     // or perhaps sort by thread priority.  See the comments in

     // synchronizer.cpp objectMonitor::exit().

     assert (_EntryList == NULL, "invariant") ;

     _EntryList = List = (ParkEvent *)(cxq & ~_LBIT) ;

     assert (List != NULL, "invariant") ;

     goto WakeOne ;

   }

   // cxq|EntryList is empty.

   // w == NULL implies that cxq|EntryList == NULL in the past.

   // Possible race - rare inopportune interleaving.

   // A thread could have added itself to cxq since this thread previously checked.

   // Detect and recover by refetching cxq.

  Punt:

   assert (UNS(_OnDeck) == _LBIT, "invariant") ;

   _OnDeck = NULL ;            // Release inner lock.

   OrderAccess::storeload();   // Dekker duality - pivot point

   // Resample LockWord/cxq to recover from possible race.

   // For instance, while this thread T1 held OnDeck, some other thread T2 might

   // acquire the outer lock.  Another thread T3 might try to acquire the outer

   // lock, but encounter contention and enqueue itself on cxq.  T2 then drops the

   // outer lock, but skips succession as this thread T1 still holds OnDeck.

   // T1 is and remains responsible for ensuring succession of T3.

   //

   // Note that we don't need to recheck EntryList, just cxq.

   // If threads moved onto EntryList since we dropped OnDeck

   // that implies some other thread forced succession.

   cxq = _LockWord.FullWord ;

   if ((cxq & ~_LBIT) != 0 && (cxq & _LBIT) == 0) {

     goto Succession ;         // potential race -- re-run succession

   }

   return ;

 }

 bool Monitor::notify() {

   assert (_owner == Thread::current(), "invariant") ;

   assert (ILocked(), "invariant") ;

   if (_WaitSet == NULL) return true ;

   NotifyCount ++ ;

   // Transfer one thread from the WaitSet to the EntryList or cxq.

   // Currently we just unlink the head of the WaitSet and prepend to the cxq.

   // And of course we could just unlink it and unpark it, too, but

   // in that case it'd likely impale itself on the reentry.

   Thread::muxAcquire (_WaitLock, "notify:WaitLock") ;

   ParkEvent * nfy = _WaitSet ;

   if (nfy != NULL) {                  // DCL idiom

     _WaitSet = nfy->ListNext ;

     assert (nfy->Notified == 0, "invariant") ;

     // push nfy onto the cxq

     for (;;) {

       const intptr_t v = _LockWord.FullWord ;

       assert ((v & 0xFF) == _LBIT, "invariant") ;

       nfy->ListNext = (ParkEvent *)(v & ~_LBIT);

       if (CASPTR (&_LockWord, v, UNS(nfy)|_LBIT) == v) break;

       // interference - _LockWord changed -- just retry

     }

     // Note that setting Notified before pushing nfy onto the cxq is

     // also legal and safe, but the safety properties are much more

     // subtle, so for the sake of code stewardship ...

     OrderAccess::fence() ;

     nfy->Notified = 1;

   }

   Thread::muxRelease (_WaitLock) ;

   if (nfy != NULL && (NativeMonitorFlags & 16)) {

     // Experimental code ... light up the wakee in the hope that this thread (the owner)

     // will drop the lock just about the time the wakee comes ONPROC.

     nfy->unpark() ;

   }

   assert (ILocked(), "invariant") ;

   return true ;

 }

 // Currently notifyAll() transfers the waiters one-at-a-time from the waitset

 // to the cxq.  This could be done more efficiently with a single bulk en-mass transfer,

 // but in practice notifyAll() for large #s of threads is rare and not time-critical.

 // Beware too, that we invert the order of the waiters.  Lets say that the

 // waitset is "ABCD" and the cxq is "XYZ".  After a notifyAll() the waitset

 // will be empty and the cxq will be "DCBAXYZ".  This is benign, of course.

 bool Monitor::notify_all() {

   assert (_owner == Thread::current(), "invariant") ;

   assert (ILocked(), "invariant") ;

   while (_WaitSet != NULL) notify() ;

   return true ;

 }

 int Monitor::IWait (Thread * Self, jlong timo) {

   assert (ILocked(), "invariant") ;

   // Phases:

   // 1. Enqueue Self on WaitSet - currently prepend

   // 2. unlock - drop the outer lock

   // 3. wait for either notification or timeout

   // 4. lock - reentry - reacquire the outer lock

   ParkEvent * const ESelf = Self->_MutexEvent ;

   ESelf->Notified = 0 ;

   ESelf->reset() ;

   OrderAccess::fence() ;

   // Add Self to WaitSet

   // Ideally only the holder of the outer lock would manipulate the WaitSet -

   // That is, the outer lock would implicitly protect the WaitSet.

   // But if a thread in wait() encounters a timeout it will need to dequeue itself

   // from the WaitSet _before it becomes the owner of the lock.  We need to dequeue

   // as the ParkEvent -- which serves as a proxy for the thread -- can't reside

   // on both the WaitSet and the EntryList|cxq at the same time..  That is, a thread

   // on the WaitSet can't be allowed to compete for the lock until it has managed to

   // unlink its ParkEvent from WaitSet.  Thus the need for WaitLock.

   // Contention on the WaitLock is minimal.

   //

   // Another viable approach would be add another ParkEvent, "WaitEvent" to the

   // thread class.  The WaitSet would be composed of WaitEvents.  Only the

   // owner of the outer lock would manipulate the WaitSet.  A thread in wait()

   // could then compete for the outer lock, and then, if necessary, unlink itself

   // from the WaitSet only after having acquired the outer lock.  More precisely,

   // there would be no WaitLock.  A thread in in wait() would enqueue its WaitEvent

   // on the WaitSet; release the outer lock; wait for either notification or timeout;

   // reacquire the inner lock; and then, if needed, unlink itself from the WaitSet.

   //

   // Alternatively, a 2nd set of list link fields in the ParkEvent might suffice.

   // One set would be for the WaitSet and one for the EntryList.

   // We could also deconstruct the ParkEvent into a "pure" event and add a

   // new immortal/TSM "ListElement" class that referred to ParkEvents.

   // In that case we could have one ListElement on the WaitSet and another

   // on the EntryList, with both referring to the same pure Event.

   Thread::muxAcquire (_WaitLock, "wait:WaitLock:Add") ;

   ESelf->ListNext = _WaitSet ;

   _WaitSet = ESelf ;

   Thread::muxRelease (_WaitLock) ;

   // Release the outer lock

   // We call IUnlock (RelaxAssert=true) as a thread T1 might

   // enqueue itself on the WaitSet, call IUnlock(), drop the lock,

   // and then stall before it can attempt to wake a successor.

   // Some other thread T2 acquires the lock, and calls notify(), moving

   // T1 from the WaitSet to the cxq.  T2 then drops the lock.  T1 resumes,

   // and then finds *itself* on the cxq.  During the course of a normal

   // IUnlock() call a thread should _never find itself on the EntryList

   // or cxq, but in the case of wait() it's possible.

   // See synchronizer.cpp objectMonitor::wait().

   IUnlock (true) ;

   // Wait for either notification or timeout

   // Beware that in some circumstances we might propagate

   // spurious wakeups back to the caller.

   for (;;) {

     if (ESelf->Notified) break ;

     int err = ParkCommon (ESelf, timo) ;

     if (err == OS_TIMEOUT || (NativeMonitorFlags & 1)) break ;

   }

   // Prepare for reentry - if necessary, remove ESelf from WaitSet

   // ESelf can be:

   // 1. Still on the WaitSet.  This can happen if we exited the loop by timeout.

   // 2. On the cxq or EntryList

   // 3. Not resident on cxq, EntryList or WaitSet, but in the OnDeck position.

   OrderAccess::fence() ;

   int WasOnWaitSet = 0 ;

   if (ESelf->Notified == 0) {

     Thread::muxAcquire (_WaitLock, "wait:WaitLock:remove") ;

     if (ESelf->Notified == 0) {     // DCL idiom

       assert (_OnDeck != ESelf, "invariant") ;   // can't be both OnDeck and on WaitSet

       // ESelf is resident on the WaitSet -- unlink it.

       // A doubly-linked list would be better here so we can unlink in constant-time.

       // We have to unlink before we potentially recontend as ESelf might otherwise

       // end up on the cxq|EntryList -- it can't be on two lists at once.

       ParkEvent * p = _WaitSet ;

       ParkEvent * q = NULL ;            // classic q chases p

       while (p != NULL && p != ESelf) {

         q = p ;

         p = p->ListNext ;

       }

       assert (p == ESelf, "invariant") ;

       if (p == _WaitSet) {      // found at head

         assert (q == NULL, "invariant") ;

         _WaitSet = p->ListNext ;

       } else {                  // found in interior

         assert (q->ListNext == p, "invariant") ;

         q->ListNext = p->ListNext ;

       }

       WasOnWaitSet = 1 ;        // We were *not* notified but instead encountered timeout

     }

     Thread::muxRelease (_WaitLock) ;

   }

   // Reentry phase - reacquire the lock

   if (WasOnWaitSet) {

     // ESelf was previously on the WaitSet but we just unlinked it above

     // because of a timeout.  ESelf is not resident on any list and is not OnDeck

     assert (_OnDeck != ESelf, "invariant") ;

     ILock (Self) ;

   } else {

     // A prior notify() operation moved ESelf from the WaitSet to the cxq.

     // ESelf is now on the cxq, EntryList or at the OnDeck position.

     // The following fragment is extracted from Monitor::ILock()

     for (;;) {

       if (_OnDeck == ESelf && TrySpin(Self)) break ;

       ParkCommon (ESelf, 0) ;

     }

     assert (_OnDeck == ESelf, "invariant") ;

     _OnDeck = NULL ;

   }

   assert (ILocked(), "invariant") ;

   return WasOnWaitSet != 0 ;        // return true IFF timeout

 }

 // ON THE VMTHREAD SNEAKING PAST HELD LOCKS:

 // In particular, there are certain types of global lock that may be held

 // by a Java thread while it is blocked at a safepoint but before it has

 // written the _owner field. These locks may be sneakily acquired by the

 // VM thread during a safepoint to avoid deadlocks. Alternatively, one should

 // identify all such locks, and ensure that Java threads never block at

 // safepoints while holding them (_no_safepoint_check_flag). While it

 // seems as though this could increase the time to reach a safepoint

 // (or at least increase the mean, if not the variance), the latter

 // approach might make for a cleaner, more maintainable JVM design.

 //

 // Sneaking is vile and reprehensible and should be excised at the 1st

 // opportunity.  It's possible that the need for sneaking could be obviated

 // as follows.  Currently, a thread might (a) while TBIVM, call pthread_mutex_lock

 // or ILock() thus acquiring the "physical" lock underlying Monitor/Mutex.

 // (b) stall at the TBIVM exit point as a safepoint is in effect.  Critically,

 // it'll stall at the TBIVM reentry state transition after having acquired the

 // underlying lock, but before having set _owner and having entered the actual

 // critical section.  The lock-sneaking facility leverages that fact and allowed the

 // VM thread to logically acquire locks that had already be physically locked by mutators

 // but where mutators were known blocked by the reentry thread state transition.

 //

 // If we were to modify the Monitor-Mutex so that TBIVM state transitions tightly

 // wrapped calls to park(), then we could likely do away with sneaking.  We'd

 // decouple lock acquisition and parking.  The critical invariant  to eliminating

 // sneaking is to ensure that we never "physically" acquire the lock while TBIVM.

 // An easy way to accomplish this is to wrap the park calls in a narrow TBIVM jacket.

 // One difficulty with this approach is that the TBIVM wrapper could recurse and

 // call lock() deep from within a lock() call, while the MutexEvent was already enqueued.

 // Using a stack (N=2 at minimum) of ParkEvents would take care of that problem.

 //

 // But of course the proper ultimate approach is to avoid schemes that require explicit

 // sneaking or dependence on any any clever invariants or subtle implementation properties

 // of Mutex-Monitor and instead directly address the underlying design flaw.

 void Monitor::lock (Thread * Self) {

 #ifdef CHECK_UNHANDLED_OOPS

   // Clear unhandled oops so we get a crash right away.  Only clear for non-vm

   // or GC threads.

   if (Self->is_Java_thread()) {

     Self->clear_unhandled_oops();

   }

 #endif // CHECK_UNHANDLED_OOPS

   debug_only(check_prelock_state(Self));

   assert (_owner != Self              , "invariant") ;

   assert (_OnDeck != Self->_MutexEvent, "invariant") ;

   if (TryFast()) {

  Exeunt:

     assert (ILocked(), "invariant") ;

     assert (owner() == NULL, "invariant");

     set_owner (Self);

     return ;

   }

   // The lock is contended ...

   bool can_sneak = Self->is_VM_thread() && SafepointSynchronize::is_at_safepoint();

   if (can_sneak && _owner == NULL) {

     // a java thread has locked the lock but has not entered the

     // critical region -- let's just pretend we've locked the lock

     // and go on.  we note this with _snuck so we can also

     // pretend to unlock when the time comes.

     _snuck = true;

     goto Exeunt ;

   }

   // Try a brief spin to avoid passing thru thread state transition ...

   if (TrySpin (Self)) goto Exeunt ;

   check_block_state(Self);

   if (Self->is_Java_thread()) {

     // Horribile dictu - we suffer through a state transition

     assert(rank() > Mutex::special, "Potential deadlock with special or lesser rank mutex");

     ThreadBlockInVM tbivm ((JavaThread *) Self) ;

     ILock (Self) ;

   } else {

     // Mirabile dictu

     ILock (Self) ;

   }

   goto Exeunt ;

 }

 void Monitor::lock() {

   this->lock(Thread::current());

 }

 // Lock without safepoint check - a degenerate variant of lock().

 // Should ONLY be used by safepoint code and other code

 // that is guaranteed not to block while running inside the VM. If this is called with

 // thread state set to be in VM, the safepoint synchronization code will deadlock!

 void Monitor::lock_without_safepoint_check (Thread * Self) {

   assert (_owner != Self, "invariant") ;

   ILock (Self) ;

   assert (_owner == NULL, "invariant");

   set_owner (Self);

 }

 void Monitor::lock_without_safepoint_check () {

   lock_without_safepoint_check (Thread::current()) ;

 }

 // Returns true if thread succeceed [sic] in grabbing the lock, otherwise false.

 bool Monitor::try_lock() {

   Thread * const Self = Thread::current();

   debug_only(check_prelock_state(Self));

   // assert(!thread->is_inside_signal_handler(), "don't lock inside signal handler");

   // Special case, where all Java threads are stopped.

   // The lock may have been acquired but _owner is not yet set.

   // In that case the VM thread can safely grab the lock.

   // It strikes me this should appear _after the TryLock() fails, below.

   bool can_sneak = Self->is_VM_thread() && SafepointSynchronize::is_at_safepoint();

   if (can_sneak && _owner == NULL) {

     set_owner(Self); // Do not need to be atomic, since we are at a safepoint

     _snuck = true;

     return true;

   }

   if (TryLock()) {

     // We got the lock

     assert (_owner == NULL, "invariant");

     set_owner (Self);

     return true;

   }

   return false;

 }

 void Monitor::unlock() {

   assert (_owner  == Thread::current(), "invariant") ;

   assert (_OnDeck != Thread::current()->_MutexEvent , "invariant") ;

   set_owner (NULL) ;

   if (_snuck) {

     assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak");

     _snuck = false;

     return ;

   }

   IUnlock (false) ;

 }

 // Yet another degenerate version of Monitor::lock() or lock_without_safepoint_check()

 // jvm_raw_lock() and _unlock() can be called by non-Java threads via JVM_RawMonitorEnter.

 //

 // There's no expectation that JVM_RawMonitors will interoperate properly with the native

 // Mutex-Monitor constructs.  We happen to implement JVM_RawMonitors in terms of

 // native Mutex-Monitors simply as a matter of convenience.  A simple abstraction layer

 // over a pthread_mutex_t would work equally as well, but require more platform-specific

 // code -- a "PlatformMutex".  Alternatively, a simply layer over muxAcquire-muxRelease

 // would work too.

 //

 // Since the caller might be a foreign thread, we don't necessarily have a Thread.MutexEvent

 // instance available.  Instead, we transiently allocate a ParkEvent on-demand if

 // we encounter contention.  That ParkEvent remains associated with the thread

 // until it manages to acquire the lock, at which time we return the ParkEvent

 // to the global ParkEvent free list.  This is correct and suffices for our purposes.

 //

 // Beware that the original jvm_raw_unlock() had a "_snuck" test but that

 // jvm_raw_lock() didn't have the corresponding test.  I suspect that's an

 // oversight, but I've replicated the original suspect logic in the new code ...

 void Monitor::jvm_raw_lock() {

   assert(rank() == native, "invariant");

   if (TryLock()) {

  Exeunt:

     assert (ILocked(), "invariant") ;

     assert (_owner == NULL, "invariant");

     // This can potentially be called by non-java Threads. Thus, the ThreadLocalStorage

     // might return NULL. Don't call set_owner since it will break on an NULL owner

     // Consider installing a non-null "ANON" distinguished value instead of just NULL.

     _owner = ThreadLocalStorage::thread();

     return ;

   }

   if (TrySpin(NULL)) goto Exeunt ;

   // slow-path - apparent contention

   // Allocate a ParkEvent for transient use.

   // The ParkEvent remains associated with this thread until

   // the time the thread manages to acquire the lock.

   ParkEvent * const ESelf = ParkEvent::Allocate(NULL) ;

   ESelf->reset() ;

   OrderAccess::storeload() ;

   // Either Enqueue Self on cxq or acquire the outer lock.

   if (AcquireOrPush (ESelf)) {

     ParkEvent::Release (ESelf) ;      // surrender the ParkEvent

     goto Exeunt ;

   }

   // At any given time there is at most one ondeck thread.

   // ondeck implies not resident on cxq and not resident on EntryList

   // Only the OnDeck thread can try to acquire -- contended for -- the lock.

   // CONSIDER: use Self->OnDeck instead of m->OnDeck.

   for (;;) {

     if (_OnDeck == ESelf && TrySpin(NULL)) break ;

     ParkCommon (ESelf, 0) ;

   }

   assert (_OnDeck == ESelf, "invariant") ;

   _OnDeck = NULL ;

   ParkEvent::Release (ESelf) ;      // surrender the ParkEvent

   goto Exeunt ;

 }

 void Monitor::jvm_raw_unlock() {

   // Nearly the same as Monitor::unlock() ...

   // directly set _owner instead of using set_owner(null)

   _owner = NULL ;

   if (_snuck) {         // ???

     assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak");

     _snuck = false;

     return ;

   }

   IUnlock(false) ;

 }

 bool Monitor::wait(bool no_safepoint_check, long timeout, bool as_suspend_equivalent) {

   Thread * const Self = Thread::current() ;

   assert (_owner == Self, "invariant") ;

   assert (ILocked(), "invariant") ;

   // as_suspend_equivalent logically implies !no_safepoint_check

   guarantee (!as_suspend_equivalent || !no_safepoint_check, "invariant") ;

   // !no_safepoint_check logically implies java_thread

   guarantee (no_safepoint_check || Self->is_Java_thread(), "invariant") ;

   #ifdef ASSERT

     Monitor * least = get_least_ranked_lock_besides_this(Self->owned_locks());

     assert(least != this, "Specification of get_least_... call above");

     if (least != NULL && least->rank() <= special) {

       tty->print("Attempting to wait on monitor %s/%d while holding"

                  " lock %s/%d -- possible deadlock",

                  name(), rank(), least->name(), least->rank());

       assert(false, "Shouldn't block(wait) while holding a lock of rank special");

     }

   #endif // ASSERT

   int wait_status ;

   // conceptually set the owner to NULL in anticipation of

   // abdicating the lock in wait

   set_owner(NULL);

   if (no_safepoint_check) {

     wait_status = IWait (Self, timeout) ;

   } else {

     assert (Self->is_Java_thread(), "invariant") ;

     JavaThread *jt = (JavaThread *)Self;

     // Enter safepoint region - ornate and Rococo ...

     ThreadBlockInVM tbivm(jt);

     OSThreadWaitState osts(Self->osthread(), false /* not Object.wait() */);

     if (as_suspend_equivalent) {

       jt->set_suspend_equivalent();

       // cleared by handle_special_suspend_equivalent_condition() or

       // java_suspend_self()

     }

     wait_status = IWait (Self, timeout) ;

     // were we externally suspended while we were waiting?

     if (as_suspend_equivalent && jt->handle_special_suspend_equivalent_condition()) {

       // Our event wait has finished and we own the lock, but

       // while we were waiting another thread suspended us. We don't

       // want to hold the lock while suspended because that

       // would surprise the thread that suspended us.

       assert (ILocked(), "invariant") ;

       IUnlock (true) ;

       jt->java_suspend_self();

       ILock (Self) ;

       assert (ILocked(), "invariant") ;

     }

   }

   // Conceptually reestablish ownership of the lock.

   // The "real" lock -- the LockByte -- was reacquired by IWait().

   assert (ILocked(), "invariant") ;

   assert (_owner == NULL, "invariant") ;

   set_owner (Self) ;

   return wait_status != 0 ;          // return true IFF timeout

 }

 Monitor::~Monitor() {

   assert ((UNS(_owner)|UNS(_LockWord.FullWord)|UNS(_EntryList)|UNS(_WaitSet)|UNS(_OnDeck)) == 0, "") ;

 }

 void Monitor::ClearMonitor (Monitor * m, const char *name) {

   m->_owner             = NULL ;

   m->_snuck             = false ;

   if (name == NULL) {

     strcpy(m->_name, "UNKNOWN") ;

   } else {

     strncpy(m->_name, name, MONITOR_NAME_LEN - 1);

     m->_name[MONITOR_NAME_LEN - 1] = '\0';

   }

   m->_LockWord.FullWord = 0 ;

   m->_EntryList         = NULL ;

   m->_OnDeck            = NULL ;

   m->_WaitSet           = NULL ;

   m->_WaitLock[0]       = 0 ;

 }

 Monitor::Monitor() { ClearMonitor(this); }

 Monitor::Monitor (int Rank, const char * name, bool allow_vm_block) {

   ClearMonitor (this, name) ;

 #ifdef ASSERT

   _allow_vm_block  = allow_vm_block;

   _rank            = Rank ;

 #endif

 }

 Mutex::~Mutex() {

   assert ((UNS(_owner)|UNS(_LockWord.FullWord)|UNS(_EntryList)|UNS(_WaitSet)|UNS(_OnDeck)) == 0, "") ;

 }

 Mutex::Mutex (int Rank, const char * name, bool allow_vm_block) {

   ClearMonitor ((Monitor *) this, name) ;

 #ifdef ASSERT

  _allow_vm_block   = allow_vm_block;

  _rank             = Rank ;

 #endif

 }

 bool Monitor::owned_by_self() const {

   bool ret = _owner == Thread::current();

   assert (!ret || _LockWord.Bytes[_LSBINDEX] != 0, "invariant") ;

   return ret;

 }

 void Monitor::print_on_error(outputStream* st) const {

   st->print("[" PTR_FORMAT, this);

   st->print("] %s", _name);

   st->print(" - owner thread: " PTR_FORMAT, _owner);

 }

 // ----------------------------------------------------------------------------------

 // Non-product code

 #ifndef PRODUCT

 void Monitor::print_on(outputStream* st) const {

   st->print_cr("Mutex: [0x%lx/0x%lx] %s - owner: 0x%lx", this, _LockWord.FullWord, _name, _owner);

 }

 #endif

 #ifndef PRODUCT

 #ifdef ASSERT

 Monitor * Monitor::get_least_ranked_lock(Monitor * locks) {

   Monitor *res, *tmp;

   for (res = tmp = locks; tmp != NULL; tmp = tmp->next()) {

     if (tmp->rank() < res->rank()) {

       res = tmp;

     }

   }

   if (!SafepointSynchronize::is_at_safepoint()) {

     // In this case, we expect the held locks to be

     // in increasing rank order (modulo any native ranks)

     for (tmp = locks; tmp != NULL; tmp = tmp->next()) {

       if (tmp->next() != NULL) {

         assert(tmp->rank() == Mutex::native ||

                tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?");

       }

     }

   }

   return res;

 }

 Monitor* Monitor::get_least_ranked_lock_besides_this(Monitor* locks) {

   Monitor *res, *tmp;

   for (res = NULL, tmp = locks; tmp != NULL; tmp = tmp->next()) {

     if (tmp != this && (res == NULL || tmp->rank() < res->rank())) {

       res = tmp;

     }

   }

   if (!SafepointSynchronize::is_at_safepoint()) {

     // In this case, we expect the held locks to be

     // in increasing rank order (modulo any native ranks)

     for (tmp = locks; tmp != NULL; tmp = tmp->next()) {

       if (tmp->next() != NULL) {

         assert(tmp->rank() == Mutex::native ||

                tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?");

       }

     }

   }

   return res;

 }

 bool Monitor::contains(Monitor* locks, Monitor * lock) {

   for (; locks != NULL; locks = locks->next()) {

     if (locks == lock)

       return true;

   }

   return false;

 }

 #endif

 // Called immediately after lock acquisition or release as a diagnostic

 // to track the lock-set of the thread and test for rank violations that

 // might indicate exposure to deadlock.

 // Rather like an EventListener for _owner (:>).

 void Monitor::set_owner_implementation(Thread *new_owner) {

   // This function is solely responsible for maintaining

   // and checking the invariant that threads and locks

   // are in a 1/N relation, with some some locks unowned.

   // It uses the Mutex::_owner, Mutex::_next, and

   // Thread::_owned_locks fields, and no other function

   // changes those fields.

   // It is illegal to set the mutex from one non-NULL

   // owner to another--it must be owned by NULL as an

   // intermediate state.

   if (new_owner != NULL) {

     // the thread is acquiring this lock

     assert(new_owner == Thread::current(), "Should I be doing this?");

     assert(_owner == NULL, "setting the owner thread of an already owned mutex");

     _owner = new_owner; // set the owner

     // link "this" into the owned locks list

     #ifdef ASSERT  // Thread::_owned_locks is under the same ifdef

       Monitor* locks = get_least_ranked_lock(new_owner->owned_locks());

                     // Mutex::set_owner_implementation is a friend of Thread

       assert(this->rank() >= 0, "bad lock rank");

       if (LogMultipleMutexLocking && locks != NULL) {

         Events::log("thread " INTPTR_FORMAT " locks %s, already owns %s", new_owner, name(), locks->name());

       }

       // Deadlock avoidance rules require us to acquire Mutexes only in

       // a global total order. For example m1 is the lowest ranked mutex

       // that the thread holds and m2 is the mutex the thread is trying

       // to acquire, then  deadlock avoidance rules require that the rank

       // of m2 be less  than the rank of m1.

       // The rank Mutex::native  is an exception in that it is not subject

       // to the verification rules.

       // Here are some further notes relating to mutex acquisition anomalies:

       // . under Solaris, the interrupt lock gets acquired when doing

       //   profiling, so any lock could be held.

       // . it is also ok to acquire Safepoint_lock at the very end while we

       //   already hold Terminator_lock - may happen because of periodic safepoints

       if (this->rank() != Mutex::native &&

           this->rank() != Mutex::suspend_resume &&

           locks != NULL && locks->rank() <= this->rank() &&

           !SafepointSynchronize::is_at_safepoint() &&

           this != Interrupt_lock && this != ProfileVM_lock &&

           !(this == Safepoint_lock && contains(locks, Terminator_lock) &&

             SafepointSynchronize::is_synchronizing())) {

         new_owner->print_owned_locks();

         fatal(err_msg("acquiring lock %s/%d out of order with lock %s/%d -- "

                       "possible deadlock", this->name(), this->rank(),

                       locks->name(), locks->rank()));

       }

       this->_next = new_owner->_owned_locks;

       new_owner->_owned_locks = this;

     #endif

   } else {

     // the thread is releasing this lock

     Thread* old_owner = _owner;

     debug_only(_last_owner = old_owner);

     assert(old_owner != NULL, "removing the owner thread of an unowned mutex");

     assert(old_owner == Thread::current(), "removing the owner thread of an unowned mutex");

     _owner = NULL; // set the owner

     #ifdef ASSERT

       Monitor *locks = old_owner->owned_locks();

       if (LogMultipleMutexLocking && locks != this) {

         Events::log("thread " INTPTR_FORMAT " unlocks %s, still owns %s", old_owner, this->name(), locks->name());

       }

       // remove "this" from the owned locks list

       Monitor *prev = NULL;

       bool found = false;

       for (; locks != NULL; prev = locks, locks = locks->next()) {

         if (locks == this) {

           found = true;

           break;

         }

       }

       assert(found, "Removing a lock not owned");

       if (prev == NULL) {

         old_owner->_owned_locks = _next;

       } else {

         prev->_next = _next;

       }

       _next = NULL;

     #endif

   }

 }

 // Factored out common sanity checks for locking mutex'es. Used by lock() and try_lock()

 void Monitor::check_prelock_state(Thread *thread) {

   assert((!thread->is_Java_thread() || ((JavaThread *)thread)->thread_state() == _thread_in_vm)

          || rank() == Mutex::special, "wrong thread state for using locks");

   if (StrictSafepointChecks) {

     if (thread->is_VM_thread() && !allow_vm_block()) {

       fatal(err_msg("VM thread using lock %s (not allowed to block on)",

                     name()));

     }

     debug_only(if (rank() != Mutex::special) \

       thread->check_for_valid_safepoint_state(false);)

   }

 }

 void Monitor::check_block_state(Thread *thread) {

   if (!_allow_vm_block && thread->is_VM_thread()) {

     warning("VM thread blocked on lock");

     print();

     BREAKPOINT;

   }

   assert(_owner != thread, "deadlock: blocking on monitor owned by current thread");

 }

 #endif // PRODUCT