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

 /*

  * Copyright (c) 1998, 2013, Oracle and/or its affiliates. All rights reserved.

  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

  *

  * This code is free software; you can redistribute it and/or modify it

  * under the terms of the GNU General Public License version 2 only, as

  * published by the Free Software Foundation.

  *

  * This code is distributed in the hope that it will be useful, but WITHOUT

  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

  * version 2 for more details (a copy is included in the LICENSE file that

  * accompanied this code).

  *

  * You should have received a copy of the GNU General Public License version

  * 2 along with this work; if not, write to the Free Software Foundation,

  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

  *

  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

  * or visit www.oracle.com if you need additional information or have any

  * questions.

  *

  */

 #include "precompiled.hpp"

 #include "classfile/vmSymbols.hpp"

 #include "memory/resourceArea.hpp"

 #include "oops/markOop.hpp"

 #include "oops/oop.inline.hpp"

 #include "runtime/handles.inline.hpp"

 #include "runtime/interfaceSupport.hpp"

 #include "runtime/mutexLocker.hpp"

 #include "runtime/objectMonitor.hpp"

 #include "runtime/objectMonitor.inline.hpp"

 #include "runtime/osThread.hpp"

 #include "runtime/stubRoutines.hpp"

 #include "runtime/thread.inline.hpp"

 #include "services/threadService.hpp"

 #include "utilities/dtrace.hpp"

 #include "utilities/preserveException.hpp"

 #ifdef TARGET_OS_FAMILY_linux

 # include "os_linux.inline.hpp"

 #endif

 #ifdef TARGET_OS_FAMILY_solaris

 # include "os_solaris.inline.hpp"

 #endif

 #ifdef TARGET_OS_FAMILY_windows

 # include "os_windows.inline.hpp"

 #endif

 #ifdef TARGET_OS_FAMILY_bsd

 # include "os_bsd.inline.hpp"

 #endif

 #if defined(__GNUC__) && !defined(IA64)

   // Need to inhibit inlining for older versions of GCC to avoid build-time failures

   #define ATTR __attribute__((noinline))

 #else

   #define ATTR

 #endif

 #ifdef DTRACE_ENABLED

 // Only bother with this argument setup if dtrace is available

 // TODO-FIXME: probes should not fire when caller is _blocked.  assert() accordingly.

 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread)                           \

   char* bytes = NULL;                                                      \

   int len = 0;                                                             \

   jlong jtid = SharedRuntime::get_java_tid(thread);                        \

   Symbol* klassname = ((oop)obj)->klass()->name();                         \

   if (klassname != NULL) {                                                 \

     bytes = (char*)klassname->bytes();                                     \

     len = klassname->utf8_length();                                        \

   }

 #ifndef USDT2

 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_WAIT_PROBE(monitor, obj, thread, millis)       \

   {                                                                        \

     if (DTraceMonitorProbes) {                                            \

       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                       \

       HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid,                       \

                        (monitor), bytes, len, (millis));                   \

     }                                                                      \

   }

 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread)             \

   {                                                                        \

     if (DTraceMonitorProbes) {                                            \

       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                       \

       HS_DTRACE_PROBE4(hotspot, monitor__##probe, jtid,                    \

                        (uintptr_t)(monitor), bytes, len);                  \

     }                                                                      \

   }

 #else /* USDT2 */

 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis)            \

   {                                                                        \

     if (DTraceMonitorProbes) {                                            \

       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                            \

       HOTSPOT_MONITOR_WAIT(jtid,                                           \

                        (monitor), bytes, len, (millis));                   \

     }                                                                      \

   }

 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER

 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED

 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT

 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY

 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL

 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread)                  \

   {                                                                        \

     if (DTraceMonitorProbes) {                                            \

       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                            \

       HOTSPOT_MONITOR_##probe(jtid,                                               \

                        (uintptr_t)(monitor), bytes, len);                  \

     }                                                                      \

   }

 #endif /* USDT2 */

 #else //  ndef DTRACE_ENABLED

 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon)    {;}

 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon)          {;}

 #endif // ndef DTRACE_ENABLED

 // Tunables ...

 // The knob* variables are effectively final.  Once set they should

 // never be modified hence.  Consider using __read_mostly with GCC.

 int ObjectMonitor::Knob_Verbose    = 0 ;

 int ObjectMonitor::Knob_SpinLimit  = 5000 ;    // derived by an external tool -

 static int Knob_LogSpins           = 0 ;       // enable jvmstat tally for spins

 static int Knob_HandOff            = 0 ;

 static int Knob_ReportSettings     = 0 ;

 static int Knob_SpinBase           = 0 ;       // Floor AKA SpinMin

 static int Knob_SpinBackOff        = 0 ;       // spin-loop backoff

 static int Knob_CASPenalty         = -1 ;      // Penalty for failed CAS

 static int Knob_OXPenalty          = -1 ;      // Penalty for observed _owner change

 static int Knob_SpinSetSucc        = 1 ;       // spinners set the _succ field

 static int Knob_SpinEarly          = 1 ;

 static int Knob_SuccEnabled        = 1 ;       // futile wake throttling

 static int Knob_SuccRestrict       = 0 ;       // Limit successors + spinners to at-most-one

 static int Knob_MaxSpinners        = -1 ;      // Should be a function of # CPUs

 static int Knob_Bonus              = 100 ;     // spin success bonus

 static int Knob_BonusB             = 100 ;     // spin success bonus

 static int Knob_Penalty            = 200 ;     // spin failure penalty

 static int Knob_Poverty            = 1000 ;

 static int Knob_SpinAfterFutile    = 1 ;       // Spin after returning from park()

 static int Knob_FixedSpin          = 0 ;

 static int Knob_OState             = 3 ;       // Spinner checks thread state of _owner

 static int Knob_UsePause           = 1 ;

 static int Knob_ExitPolicy         = 0 ;

 static int Knob_PreSpin            = 10 ;      // 20-100 likely better

 static int Knob_ResetEvent         = 0 ;

 static int BackOffMask             = 0 ;

 static int Knob_FastHSSEC          = 0 ;

 static int Knob_MoveNotifyee       = 2 ;       // notify() - disposition of notifyee

 static int Knob_QMode              = 0 ;       // EntryList-cxq policy - queue discipline

 static volatile int InitDone       = 0 ;

 #define TrySpin TrySpin_VaryDuration

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

 // Theory of operations -- Monitors lists, thread residency, etc:

 //

 // * A thread acquires ownership of a monitor by successfully

 //   CAS()ing the _owner field from null to non-null.

 //

 // * Invariant: A thread appears on at most one monitor list --

 //   cxq, EntryList or WaitSet -- at any one time.

 //

 // * Contending threads "push" themselves onto the cxq with CAS

 //   and then spin/park.

 //

 // * After a contending thread eventually acquires the lock it must

 //   dequeue itself from either the EntryList or the cxq.

 //

 // * The exiting thread identifies and unparks an "heir presumptive"

 //   tentative successor thread on the EntryList.  Critically, the

 //   exiting thread doesn't unlink the successor thread from the EntryList.

 //   After having been unparked, the wakee will recontend for ownership of

 //   the monitor.   The successor (wakee) will either acquire the lock or

 //   re-park itself.

 //

 //   Succession is provided for by a policy of competitive handoff.

 //   The exiting thread does _not_ grant or pass ownership to the

 //   successor thread.  (This is also referred to as "handoff" succession").

 //   Instead the exiting thread releases ownership and possibly wakes

 //   a successor, so the successor can (re)compete for ownership of the lock.

 //   If the EntryList is empty but the cxq is populated the exiting

 //   thread will drain the cxq into the EntryList.  It does so by

 //   by detaching the cxq (installing null with CAS) and folding

 //   the threads from the cxq into the EntryList.  The EntryList is

 //   doubly linked, while the cxq is singly linked because of the

 //   CAS-based "push" used to enqueue recently arrived threads (RATs).

 //

 // * Concurrency invariants:

 //

 //   -- only the monitor owner may access or mutate the EntryList.

 //      The mutex property of the monitor itself protects the EntryList

 //      from concurrent interference.

 //   -- Only the monitor owner may detach the cxq.

 //

 // * The monitor entry list operations avoid locks, but strictly speaking

 //   they're not lock-free.  Enter is lock-free, exit is not.

 //   See http://j2se.east/~dice/PERSIST/040825-LockFreeQueues.html

 //

 // * The cxq can have multiple concurrent "pushers" but only one concurrent

 //   detaching thread.  This mechanism is immune from the ABA corruption.

 //   More precisely, the CAS-based "push" onto cxq is ABA-oblivious.

 //

 // * Taken together, the cxq and the EntryList constitute or form a

 //   single logical queue of threads stalled trying to acquire the lock.

 //   We use two distinct lists to improve the odds of a constant-time

 //   dequeue operation after acquisition (in the ::enter() epilog) and

 //   to reduce heat on the list ends.  (c.f. Michael Scott's "2Q" algorithm).

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

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

 //   minimize monitor lock holds times.  Note that even a small amount of

 //   fixed spinning will greatly reduce the # of enqueue-dequeue operations

 //   on EntryList|cxq.  That is, spinning relieves contention on the "inner"

 //   locks and monitor metadata.

 //

 //   Cxq points to the the set of Recently Arrived Threads attempting entry.

 //   Because we push threads onto _cxq with CAS, the RATs must take the form of

 //   a singly-linked LIFO.  We drain _cxq into EntryList  at unlock-time when

 //   the unlocking thread notices that EntryList is null but _cxq is != null.

 //

 //   The EntryList is ordered by the prevailing queue discipline and

 //   can be organized in any convenient fashion, such as a doubly-linked list or

 //   a circular doubly-linked list.  Critically, we want insert and delete operations

 //   to operate in constant-time.  If we need a priority queue then something akin

 //   to Solaris' sleepq would work nicely.  Viz.,

 //   http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.

 //   Queue discipline is enforced at ::exit() time, when the unlocking thread

 //   drains the cxq into the EntryList, and orders or reorders the threads on the

 //   EntryList accordingly.

 //

 //   Barring "lock barging", this mechanism provides fair cyclic ordering,

 //   somewhat similar to an elevator-scan.

 //

 // * The monitor synchronization subsystem avoids the use of native

 //   synchronization primitives except for the narrow platform-specific

 //   park-unpark abstraction.  See the comments in os_solaris.cpp regarding

 //   the semantics of park-unpark.  Put another way, this monitor implementation

 //   depends only on atomic operations and park-unpark.  The monitor subsystem

 //   manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the

 //   underlying OS manages the READY<->RUN transitions.

 //

 // * Waiting threads reside on the WaitSet list -- wait() puts

 //   the caller onto the WaitSet.

 //

 // * notify() or notifyAll() simply transfers threads from the WaitSet to

 //   either the EntryList or cxq.  Subsequent exit() operations will

 //   unpark the notifyee.  Unparking a notifee in notify() is inefficient -

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

 //   by the notifier.

 //

 // * An interesting alternative is to encode cxq as (List,LockByte) where

 //   the LockByte is 0 iff the monitor is owned.  _owner is simply an auxiliary

 //   variable, like _recursions, in the scheme.  The threads or Events that form

 //   the list would have to be aligned in 256-byte addresses.  A thread would

 //   try to acquire the lock or enqueue itself with CAS, but exiting threads

 //   could use a 1-0 protocol and simply STB to set the LockByte to 0.

 //   Note that is is *not* word-tearing, but it does presume that full-word

 //   CAS operations are coherent with intermix with STB operations.  That's true

 //   on most common processors.

 //

 // * See also http://blogs.sun.com/dave

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

 // Enter support

 bool ObjectMonitor::try_enter(Thread* THREAD) {

   if (THREAD != _owner) {

     if (THREAD->is_lock_owned ((address)_owner)) {

        assert(_recursions == 0, "internal state error");

        _owner = THREAD ;

        _recursions = 1 ;

        OwnerIsThread = 1 ;

        return true;

     }

     if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {

       return false;

     }

     return true;

   } else {

     _recursions++;

     return true;

   }

 }

 void ATTR ObjectMonitor::enter(TRAPS) {

   // The following code is ordered to check the most common cases first

   // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.

   Thread * const Self = THREAD ;

   void * cur ;

   cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;

   if (cur == NULL) {

      // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.

      assert (_recursions == 0   , "invariant") ;

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

      // CONSIDER: set or assert OwnerIsThread == 1

      return ;

   }

   if (cur == Self) {

      // TODO-FIXME: check for integer overflow!  BUGID 6557169.

      _recursions ++ ;

      return ;

   }

   if (Self->is_lock_owned ((address)cur)) {

     assert (_recursions == 0, "internal state error");

     _recursions = 1 ;

     // Commute owner from a thread-specific on-stack BasicLockObject address to

     // a full-fledged "Thread *".

     _owner = Self ;

     OwnerIsThread = 1 ;

     return ;

   }

   // We've encountered genuine contention.

   assert (Self->_Stalled == 0, "invariant") ;

   Self->_Stalled = intptr_t(this) ;

   // Try one round of spinning *before* enqueueing Self

   // and before going through the awkward and expensive state

   // transitions.  The following spin is strictly optional ...

   // Note that if we acquire the monitor from an initial spin

   // we forgo posting JVMTI events and firing DTRACE probes.

   if (Knob_SpinEarly && TrySpin (Self) > 0) {

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

      assert (_recursions == 0    , "invariant") ;

      assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;

      Self->_Stalled = 0 ;

      return ;

   }

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

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

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

   JavaThread * jt = (JavaThread *) Self ;

   assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;

   assert (jt->thread_state() != _thread_blocked   , "invariant") ;

   assert (this->object() != NULL  , "invariant") ;

   assert (_count >= 0, "invariant") ;

   // Prevent deflation at STW-time.  See deflate_idle_monitors() and is_busy().

   // Ensure the object-monitor relationship remains stable while there's contention.

   Atomic::inc_ptr(&_count);

   { // Change java thread status to indicate blocked on monitor enter.

     JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);

     DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);

     if (JvmtiExport::should_post_monitor_contended_enter()) {

       JvmtiExport::post_monitor_contended_enter(jt, this);

     }

     OSThreadContendState osts(Self->osthread());

     ThreadBlockInVM tbivm(jt);

     Self->set_current_pending_monitor(this);

     // TODO-FIXME: change the following for(;;) loop to straight-line code.

     for (;;) {

       jt->set_suspend_equivalent();

       // cleared by handle_special_suspend_equivalent_condition()

       // or java_suspend_self()

       EnterI (THREAD) ;

       if (!ExitSuspendEquivalent(jt)) break ;

       //

       // We have acquired the contended monitor, but while we were

       // waiting another thread suspended us. We don't want to enter

       // the monitor while suspended because that would surprise the

       // thread that suspended us.

       //

           _recursions = 0 ;

       _succ = NULL ;

       exit (Self) ;

       jt->java_suspend_self();

     }

     Self->set_current_pending_monitor(NULL);

   }

   Atomic::dec_ptr(&_count);

   assert (_count >= 0, "invariant") ;

   Self->_Stalled = 0 ;

   // Must either set _recursions = 0 or ASSERT _recursions == 0.

   assert (_recursions == 0     , "invariant") ;

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

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

   assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;

   // The thread -- now the owner -- is back in vm mode.

   // Report the glorious news via TI,DTrace and jvmstat.

   // The probe effect is non-trivial.  All the reportage occurs

   // while we hold the monitor, increasing the length of the critical

   // section.  Amdahl's parallel speedup law comes vividly into play.

   //

   // Another option might be to aggregate the events (thread local or

   // per-monitor aggregation) and defer reporting until a more opportune

   // time -- such as next time some thread encounters contention but has

   // yet to acquire the lock.  While spinning that thread could

   // spinning we could increment JVMStat counters, etc.

   DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);

   if (JvmtiExport::should_post_monitor_contended_entered()) {

     JvmtiExport::post_monitor_contended_entered(jt, this);

   }

   if (ObjectMonitor::_sync_ContendedLockAttempts != NULL) {

      ObjectMonitor::_sync_ContendedLockAttempts->inc() ;

   }

 }

 // Caveat: TryLock() is not necessarily serializing if it returns failure.

 // Callers must compensate as needed.

 int ObjectMonitor::TryLock (Thread * Self) {

    for (;;) {

       void * own = _owner ;

       if (own != NULL) return 0 ;

       if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {

          // Either guarantee _recursions == 0 or set _recursions = 0.

          assert (_recursions == 0, "invariant") ;

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

          // CONSIDER: set or assert that OwnerIsThread == 1

          return 1 ;

       }

       // The lock had been free momentarily, but we lost the race to the lock.

       // Interference -- the CAS failed.

       // We can either return -1 or retry.

       // Retry doesn't make as much sense because the lock was just acquired.

       if (true) return -1 ;

    }

 }

 void ATTR ObjectMonitor::EnterI (TRAPS) {

     Thread * Self = THREAD ;

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

     assert (((JavaThread *) Self)->thread_state() == _thread_blocked   , "invariant") ;

     // Try the lock - TATAS

     if (TryLock (Self) > 0) {

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

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

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

         return ;

     }

     DeferredInitialize () ;

     // We try one round of spinning *before* enqueueing Self.

     //

     // If the _owner is ready but OFFPROC we could use a YieldTo()

     // operation to donate the remainder of this thread's quantum

     // to the owner.  This has subtle but beneficial affinity

     // effects.

     if (TrySpin (Self) > 0) {

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

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

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

         return ;

     }

     // The Spin failed -- Enqueue and park the thread ...

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

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

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

     // Enqueue "Self" on ObjectMonitor's _cxq.

     //

     // Node acts as a proxy for Self.

     // As an aside, if were to ever rewrite the synchronization code mostly

     // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class

     // Java objects.  This would avoid awkward lifecycle and liveness issues,

     // as well as eliminate a subset of ABA issues.

     // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.

     //

     ObjectWaiter node(Self) ;

     Self->_ParkEvent->reset() ;

     node._prev   = (ObjectWaiter *) 0xBAD ;

     node.TState  = ObjectWaiter::TS_CXQ ;

     // Push "Self" onto the front of the _cxq.

     // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.

     // Note that spinning tends to reduce the rate at which threads

     // enqueue and dequeue on EntryList|cxq.

     ObjectWaiter * nxt ;

     for (;;) {

         node._next = nxt = _cxq ;

         if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ;

         // Interference - the CAS failed because _cxq changed.  Just retry.

         // As an optional optimization we retry the lock.

         if (TryLock (Self) > 0) {

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

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

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

             return ;

         }

     }

     // Check for cxq|EntryList edge transition to non-null.  This indicates

     // the onset of contention.  While contention persists exiting threads

     // will use a ST:MEMBAR:LD 1-1 exit protocol.  When contention abates exit

     // operations revert to the faster 1-0 mode.  This enter operation may interleave

     // (race) a concurrent 1-0 exit operation, resulting in stranding, so we

     // arrange for one of the contending thread to use a timed park() operations

     // to detect and recover from the race.  (Stranding is form of progress failure

     // where the monitor is unlocked but all the contending threads remain parked).

     // That is, at least one of the contended threads will periodically poll _owner.

     // One of the contending threads will become the designated "Responsible" thread.

     // The Responsible thread uses a timed park instead of a normal indefinite park

     // operation -- it periodically wakes and checks for and recovers from potential

     // strandings admitted by 1-0 exit operations.   We need at most one Responsible

     // thread per-monitor at any given moment.  Only threads on cxq|EntryList may

     // be responsible for a monitor.

     //

     // Currently, one of the contended threads takes on the added role of "Responsible".

     // A viable alternative would be to use a dedicated "stranding checker" thread

     // that periodically iterated over all the threads (or active monitors) and unparked

     // successors where there was risk of stranding.  This would help eliminate the

     // timer scalability issues we see on some platforms as we'd only have one thread

     // -- the checker -- parked on a timer.

     if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {

         // Try to assume the role of responsible thread for the monitor.

         // CONSIDER:  ST vs CAS vs { if (Responsible==null) Responsible=Self }

         Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;

     }

     // The lock have been released while this thread was occupied queueing

     // itself onto _cxq.  To close the race and avoid "stranding" and

     // progress-liveness failure we must resample-retry _owner before parking.

     // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.

     // In this case the ST-MEMBAR is accomplished with CAS().

     //

     // TODO: Defer all thread state transitions until park-time.

     // Since state transitions are heavy and inefficient we'd like

     // to defer the state transitions until absolutely necessary,

     // and in doing so avoid some transitions ...

     TEVENT (Inflated enter - Contention) ;

     int nWakeups = 0 ;

     int RecheckInterval = 1 ;

     for (;;) {

         if (TryLock (Self) > 0) break ;

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

         if ((SyncFlags & 2) && _Responsible == NULL) {

            Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;

         }

         // park self

         if (_Responsible == Self || (SyncFlags & 1)) {

             TEVENT (Inflated enter - park TIMED) ;

             Self->_ParkEvent->park ((jlong) RecheckInterval) ;

             // Increase the RecheckInterval, but clamp the value.

             RecheckInterval *= 8 ;

             if (RecheckInterval > 1000) RecheckInterval = 1000 ;

         } else {

             TEVENT (Inflated enter - park UNTIMED) ;

             Self->_ParkEvent->park() ;

         }

         if (TryLock(Self) > 0) break ;

         // The lock is still contested.

         // Keep a tally of the # of futile wakeups.

         // Note that the counter is not protected by a lock or updated by atomics.

         // That is by design - we trade "lossy" counters which are exposed to

         // races during updates for a lower probe effect.

         TEVENT (Inflated enter - Futile wakeup) ;

         if (ObjectMonitor::_sync_FutileWakeups != NULL) {

            ObjectMonitor::_sync_FutileWakeups->inc() ;

         }

         ++ nWakeups ;

         // Assuming this is not a spurious wakeup we'll normally find _succ == Self.

         // We can defer clearing _succ until after the spin completes

         // TrySpin() must tolerate being called with _succ == Self.

         // Try yet another round of adaptive spinning.

         if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ;

         // We can find that we were unpark()ed and redesignated _succ while

         // we were spinning.  That's harmless.  If we iterate and call park(),

         // park() will consume the event and return immediately and we'll

         // just spin again.  This pattern can repeat, leaving _succ to simply

         // spin on a CPU.  Enable Knob_ResetEvent to clear pending unparks().

         // Alternately, we can sample fired() here, and if set, forgo spinning

         // in the next iteration.

         if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {

            Self->_ParkEvent->reset() ;

            OrderAccess::fence() ;

         }

         if (_succ == Self) _succ = NULL ;

         // Invariant: after clearing _succ a thread *must* retry _owner before parking.

         OrderAccess::fence() ;

     }

     // Egress :

     // Self has acquired the lock -- Unlink Self from the cxq or EntryList.

     // Normally we'll find Self on the EntryList .

     // From the perspective of the lock owner (this thread), the

     // EntryList is stable and cxq is prepend-only.

     // The head of cxq is volatile but the interior is stable.

     // In addition, Self.TState is stable.

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

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

     // I'd like to write:

     //   guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;

     // but as we're at a safepoint that's not safe.

     UnlinkAfterAcquire (Self, &node) ;

     if (_succ == Self) _succ = NULL ;

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

     if (_Responsible == Self) {

         _Responsible = NULL ;

         OrderAccess::fence(); // Dekker pivot-point

         // 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.

     }

     // We've acquired ownership with CAS().

     // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.

     // But since the CAS() this thread may have also stored into _succ,

     // EntryList, cxq or Responsible.  These meta-data updates must be

     // visible __before this thread subsequently drops the lock.

     // Consider what could occur if we didn't enforce this constraint --

     // STs to monitor meta-data and user-data could reorder with (become

     // visible after) the ST in exit that drops ownership of the lock.

     // Some other thread could then acquire the lock, but observe inconsistent

     // or old monitor meta-data and heap data.  That violates the JMM.

     // To that end, the 1-0 exit() operation must have at least STST|LDST

     // "release" barrier semantics.  Specifically, there must be at least a

     // STST|LDST barrier in exit() before the ST of null into _owner that drops

     // the lock.   The barrier ensures that changes to monitor meta-data and data

     // protected by the lock will be visible before we release the lock, and

     // therefore before some other thread (CPU) has a chance to acquire the lock.

     // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.

     //

     // Critically, any prior STs to _succ or EntryList must be visible before

     // the ST of null into _owner in the *subsequent* (following) corresponding

     // monitorexit.  Recall too, that in 1-0 mode monitorexit does not necessarily

     // execute a serializing instruction.

     if (SyncFlags & 8) {

        OrderAccess::fence() ;

     }

     return ;

 }

 // ReenterI() is a specialized inline form of the latter half of the

 // contended slow-path from EnterI().  We use ReenterI() only for

 // monitor reentry in wait().

 //

 // In the future we should reconcile EnterI() and ReenterI(), adding

 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the

 // loop accordingly.

 void ATTR ObjectMonitor::ReenterI (Thread * Self, ObjectWaiter * SelfNode) {

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

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

     assert (SelfNode->_thread == Self   , "invariant") ;

     assert (_waiters > 0                , "invariant") ;

     assert (((oop)(object()))->mark() == markOopDesc::encode(this) , "invariant") ;

     assert (((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;

     JavaThread * jt = (JavaThread *) Self ;

     int nWakeups = 0 ;

     for (;;) {

         ObjectWaiter::TStates v = SelfNode->TState ;

         guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;

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

         if (TryLock (Self) > 0) break ;

         if (TrySpin (Self) > 0) break ;

         TEVENT (Wait Reentry - parking) ;

         // State transition wrappers around park() ...

         // ReenterI() wisely defers state transitions until

         // it's clear we must park the thread.

         {

            OSThreadContendState osts(Self->osthread());

            ThreadBlockInVM tbivm(jt);

            // cleared by handle_special_suspend_equivalent_condition()

            // or java_suspend_self()

            jt->set_suspend_equivalent();

            if (SyncFlags & 1) {

               Self->_ParkEvent->park ((jlong)1000) ;

            } else {

               Self->_ParkEvent->park () ;

            }

            // were we externally suspended while we were waiting?

            for (;;) {

               if (!ExitSuspendEquivalent (jt)) break ;

               if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }

               jt->java_suspend_self();

               jt->set_suspend_equivalent();

            }

         }

         // Try again, but just so we distinguish between futile wakeups and

         // successful wakeups.  The following test isn't algorithmically

         // necessary, but it helps us maintain sensible statistics.

         if (TryLock(Self) > 0) break ;

         // The lock is still contested.

         // Keep a tally of the # of futile wakeups.

         // Note that the counter is not protected by a lock or updated by atomics.

         // That is by design - we trade "lossy" counters which are exposed to

         // races during updates for a lower probe effect.

         TEVENT (Wait Reentry - futile wakeup) ;

         ++ nWakeups ;

         // Assuming this is not a spurious wakeup we'll normally

         // find that _succ == Self.

         if (_succ == Self) _succ = NULL ;

         // Invariant: after clearing _succ a contending thread

         // *must* retry  _owner before parking.

         OrderAccess::fence() ;

         if (ObjectMonitor::_sync_FutileWakeups != NULL) {

           ObjectMonitor::_sync_FutileWakeups->inc() ;

         }

     }

     // Self has acquired the lock -- Unlink Self from the cxq or EntryList .

     // Normally we'll find Self on the EntryList.

     // Unlinking from the EntryList is constant-time and atomic-free.

     // From the perspective of the lock owner (this thread), the

     // EntryList is stable and cxq is prepend-only.

     // The head of cxq is volatile but the interior is stable.

     // In addition, Self.TState is stable.

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

     assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;

     UnlinkAfterAcquire (Self, SelfNode) ;

     if (_succ == Self) _succ = NULL ;

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

     SelfNode->TState = ObjectWaiter::TS_RUN ;

     OrderAccess::fence() ;      // see comments at the end of EnterI()

 }

 // after the thread acquires the lock in ::enter().  Equally, we could defer

 // unlinking the thread until ::exit()-time.

 void ObjectMonitor::UnlinkAfterAcquire (Thread * Self, ObjectWaiter * SelfNode)

 {

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

     assert (SelfNode->_thread == Self, "invariant") ;

     if (SelfNode->TState == ObjectWaiter::TS_ENTER) {

         // Normal case: remove Self from the DLL EntryList .

         // This is a constant-time operation.

         ObjectWaiter * nxt = SelfNode->_next ;

         ObjectWaiter * prv = SelfNode->_prev ;

         if (nxt != NULL) nxt->_prev = prv ;

         if (prv != NULL) prv->_next = nxt ;

         if (SelfNode == _EntryList ) _EntryList = nxt ;

         assert (nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant") ;

         assert (prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant") ;

         TEVENT (Unlink from EntryList) ;

     } else {

         guarantee (SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant") ;

         // Inopportune interleaving -- Self is still on the cxq.

         // This usually means the enqueue of self raced an exiting thread.

         // Normally we'll find Self near the front of the cxq, so

         // dequeueing is typically fast.  If needbe we can accelerate

         // this with some MCS/CHL-like bidirectional list hints and advisory

         // back-links so dequeueing from the interior will normally operate

         // in constant-time.

         // Dequeue Self from either the head (with CAS) or from the interior

         // with a linear-time scan and normal non-atomic memory operations.

         // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList

         // and then unlink Self from EntryList.  We have to drain eventually,

         // so it might as well be now.

         ObjectWaiter * v = _cxq ;

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

         if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) {

             // The CAS above can fail from interference IFF a "RAT" arrived.

             // In that case Self must be in the interior and can no longer be

             // at the head of cxq.

             if (v == SelfNode) {

                 assert (_cxq != v, "invariant") ;

                 v = _cxq ;          // CAS above failed - start scan at head of list

             }

             ObjectWaiter * p ;

             ObjectWaiter * q = NULL ;

             for (p = v ; p != NULL && p != SelfNode; p = p->_next) {

                 q = p ;

                 assert (p->TState == ObjectWaiter::TS_CXQ, "invariant") ;

             }

             assert (v != SelfNode,  "invariant") ;

             assert (p == SelfNode,  "Node not found on cxq") ;

             assert (p != _cxq,      "invariant") ;

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

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

             q->_next = p->_next ;

         }

         TEVENT (Unlink from cxq) ;

     }

     // Diagnostic hygiene ...

     SelfNode->_prev  = (ObjectWaiter *) 0xBAD ;

     SelfNode->_next  = (ObjectWaiter *) 0xBAD ;

     SelfNode->TState = ObjectWaiter::TS_RUN ;

 }

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

 // Exit support

 //

 // exit()

 // ~~~~~~

 // Note that the collector can't reclaim the objectMonitor or deflate

 // the object out from underneath the thread calling ::exit() as the

 // thread calling ::exit() never transitions to a stable state.

 // This inhibits GC, which in turn inhibits asynchronous (and

 // inopportune) reclamation of "this".

 //

 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;

 // There's one exception to the claim above, however.  EnterI() can call

 // exit() to drop a lock if the acquirer has been externally suspended.

 // In that case exit() is called with _thread_state as _thread_blocked,

 // but the monitor's _count field is > 0, which inhibits reclamation.

 //

 // 1-0 exit

 // ~~~~~~~~

 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of

 // the fast-path operators have been optimized so the common ::exit()

 // operation is 1-0.  See i486.ad fast_unlock(), for instance.

 // The code emitted by fast_unlock() elides the usual MEMBAR.  This

 // greatly improves latency -- MEMBAR and CAS having considerable local

 // latency on modern processors -- but at the cost of "stranding".  Absent the

 // MEMBAR, a thread in fast_unlock() can race a thread in the slow

 // ::enter() path, resulting in the entering thread being stranding

 // and a progress-liveness failure.   Stranding is extremely rare.

 // We use timers (timed park operations) & periodic polling to detect

 // and recover from stranding.  Potentially stranded threads periodically

 // wake up and poll the lock.  See the usage of the _Responsible variable.

 //

 // The CAS() in enter provides for safety and exclusion, while the CAS or

 // MEMBAR in exit provides for progress and avoids stranding.  1-0 locking

 // eliminates the CAS/MEMBAR from the exist path, but it admits stranding.

 // We detect and recover from stranding with timers.

 //

 // If a thread transiently strands it'll park until (a) another

 // thread acquires the lock and then drops the lock, at which time the

 // exiting thread will notice and unpark the stranded thread, or, (b)

 // the timer expires.  If the lock is high traffic then the stranding latency

 // will be low due to (a).  If the lock is low traffic then the odds of

 // stranding are lower, although the worst-case stranding latency

 // is longer.  Critically, we don't want to put excessive load in the

 // platform's timer subsystem.  We want to minimize both the timer injection

 // rate (timers created/sec) as well as the number of timers active at

 // any one time.  (more precisely, we want to minimize timer-seconds, which is

 // the integral of the # of active timers at any instant over time).

 // Both impinge on OS scalability.  Given that, at most one thread parked on

 // a monitor will use a timer.

 void ATTR ObjectMonitor::exit(TRAPS) {

    Thread * Self = THREAD ;

    if (THREAD != _owner) {

      if (THREAD->is_lock_owned((address) _owner)) {

        // Transmute _owner from a BasicLock pointer to a Thread address.

        // We don't need to hold _mutex for this transition.

        // Non-null to Non-null is safe as long as all readers can

        // tolerate either flavor.

        assert (_recursions == 0, "invariant") ;

        _owner = THREAD ;

        _recursions = 0 ;

        OwnerIsThread = 1 ;

      } else {

        // NOTE: we need to handle unbalanced monitor enter/exit

        // in native code by throwing an exception.

        // TODO: Throw an IllegalMonitorStateException ?

        TEVENT (Exit - Throw IMSX) ;

        assert(false, "Non-balanced monitor enter/exit!");

        if (false) {

           THROW(vmSymbols::java_lang_IllegalMonitorStateException());

        }

        return;

      }

    }

    if (_recursions != 0) {

      _recursions--;        // this is simple recursive enter

      TEVENT (Inflated exit - recursive) ;

      return ;

    }

    // Invariant: after setting Responsible=null an thread must execute

    // a MEMBAR or other serializing instruction before fetching EntryList|cxq.

    if ((SyncFlags & 4) == 0) {

       _Responsible = NULL ;

    }

    for (;;) {

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

       if (Knob_ExitPolicy == 0) {

          // release semantics: prior loads and stores from within the critical section

          // must not float (reorder) past the following store that drops the lock.

          // On SPARC that requires MEMBAR #loadstore|#storestore.

          // But of course in TSO #loadstore|#storestore is not required.

          // I'd like to write one of the following:

          // A.  OrderAccess::release() ; _owner = NULL

          // B.  OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;

          // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both

          // store into a _dummy variable.  That store is not needed, but can result

          // in massive wasteful coherency traffic on classic SMP systems.

          // Instead, I use release_store(), which is implemented as just a simple

          // ST on x64, x86 and SPARC.

          OrderAccess::release_store_ptr (&_owner, NULL) ;   // drop the lock

          OrderAccess::storeload() ;                         // See if we need to wake a successor

          if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {

             TEVENT (Inflated exit - simple egress) ;

             return ;

          }

          TEVENT (Inflated exit - complex egress) ;

          // Normally the exiting thread is responsible for ensuring succession,

          // but if other successors are ready or other entering threads are spinning

          // then this thread can simply store NULL into _owner and exit without

          // waking a successor.  The existence of spinners or ready successors

          // guarantees proper succession (liveness).  Responsibility passes to the

          // ready or running successors.  The exiting thread delegates the duty.

          // More precisely, if a successor already exists this thread is absolved

          // of the responsibility of waking (unparking) one.

          //

          // The _succ variable is critical to reducing futile wakeup frequency.

          // _succ identifies the "heir presumptive" thread that has been made

          // ready (unparked) but that has not yet run.  We need only one such

          // successor thread to guarantee progress.

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

          // section 3.3 "Futile Wakeup Throttling" for details.

          //

          // Note that spinners in Enter() also set _succ non-null.

          // In the current implementation spinners opportunistically set

          // _succ so that exiting threads might avoid waking a successor.

          // Another less appealing alternative would be for the exiting thread

          // to drop the lock and then spin briefly to see if a spinner managed

          // to acquire the lock.  If so, the exiting thread could exit

          // immediately without waking a successor, otherwise the exiting

          // thread would need to dequeue and wake a successor.

          // (Note that we'd need to make the post-drop spin short, but no

          // shorter than the worst-case round-trip cache-line migration time.

          // The dropped lock needs to become visible to the spinner, and then

          // the acquisition of the lock by the spinner must become visible to

          // the exiting thread).

          //

          // It appears that an heir-presumptive (successor) must be made ready.

          // Only the current lock owner can manipulate the EntryList or

          // drain _cxq, so we need to reacquire the lock.  If we fail

          // to reacquire the lock the responsibility for ensuring succession

          // falls to the new owner.

          //

          if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {

             return ;

          }

          TEVENT (Exit - Reacquired) ;

       } else {

          if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {

             OrderAccess::release_store_ptr (&_owner, NULL) ;   // drop the lock

             OrderAccess::storeload() ;

             // Ratify the previously observed values.

             if (_cxq == NULL || _succ != NULL) {

                 TEVENT (Inflated exit - simple egress) ;

                 return ;

             }

             // inopportune interleaving -- the exiting thread (this thread)

             // in the fast-exit path raced an entering thread in the slow-enter

             // path.

             // We have two choices:

             // A.  Try to reacquire the lock.

             //     If the CAS() fails return immediately, otherwise

             //     we either restart/rerun the exit operation, or simply

             //     fall-through into the code below which wakes a successor.

             // B.  If the elements forming the EntryList|cxq are TSM

             //     we could simply unpark() the lead thread and return

             //     without having set _succ.

             if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {

                TEVENT (Inflated exit - reacquired succeeded) ;

                return ;

             }

             TEVENT (Inflated exit - reacquired failed) ;

          } else {

             TEVENT (Inflated exit - complex egress) ;

          }

       }

       guarantee (_owner == THREAD, "invariant") ;

       ObjectWaiter * w = NULL ;

       int QMode = Knob_QMode ;

       if (QMode == 2 && _cxq != NULL) {

           // QMode == 2 : cxq has precedence over EntryList.

           // Try to directly wake a successor from the cxq.

           // If successful, the successor will need to unlink itself from cxq.

           w = _cxq ;

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

           assert (w->TState == ObjectWaiter::TS_CXQ, "Invariant") ;

           ExitEpilog (Self, w) ;

           return ;

       }

       if (QMode == 3 && _cxq != NULL) {

           // Aggressively drain cxq into EntryList at the first opportunity.

           // This policy ensure that recently-run threads live at the head of EntryList.

           // Drain _cxq into EntryList - bulk transfer.

           // First, detach _cxq.

           // The following loop is tantamount to: w = swap (&cxq, NULL)

           w = _cxq ;

           for (;;) {

              assert (w != NULL, "Invariant") ;

              ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;

              if (u == w) break ;

              w = u ;

           }

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

           ObjectWaiter * q = NULL ;

           ObjectWaiter * p ;

           for (p = w ; p != NULL ; p = p->_next) {

               guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;

               p->TState = ObjectWaiter::TS_ENTER ;

               p->_prev = q ;

               q = p ;

           }

           // Append the RATs to the EntryList

           // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.

           ObjectWaiter * Tail ;

           for (Tail = _EntryList ; Tail != NULL && Tail->_next != NULL ; Tail = Tail->_next) ;

           if (Tail == NULL) {

               _EntryList = w ;

           } else {

               Tail->_next = w ;

               w->_prev = Tail ;

           }

           // Fall thru into code that tries to wake a successor from EntryList

       }

       if (QMode == 4 && _cxq != NULL) {

           // Aggressively drain cxq into EntryList at the first opportunity.

           // This policy ensure that recently-run threads live at the head of EntryList.

           // Drain _cxq into EntryList - bulk transfer.

           // First, detach _cxq.

           // The following loop is tantamount to: w = swap (&cxq, NULL)

           w = _cxq ;

           for (;;) {

              assert (w != NULL, "Invariant") ;

              ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;

              if (u == w) break ;

              w = u ;

           }

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

           ObjectWaiter * q = NULL ;

           ObjectWaiter * p ;

           for (p = w ; p != NULL ; p = p->_next) {

               guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;

               p->TState = ObjectWaiter::TS_ENTER ;

               p->_prev = q ;

               q = p ;

           }

           // Prepend the RATs to the EntryList

           if (_EntryList != NULL) {

               q->_next = _EntryList ;

               _EntryList->_prev = q ;

           }

           _EntryList = w ;

           // Fall thru into code that tries to wake a successor from EntryList

       }

       w = _EntryList  ;

       if (w != NULL) {

           // I'd like to write: guarantee (w->_thread != Self).

           // But in practice an exiting thread may find itself on the EntryList.

           // Lets say thread T1 calls O.wait().  Wait() enqueues T1 on O's waitset and

           // then calls exit().  Exit release the lock by setting O._owner to NULL.

           // Lets say T1 then stalls.  T2 acquires O and calls O.notify().  The

           // notify() operation moves T1 from O's waitset to O's EntryList. T2 then

           // release the lock "O".  T2 resumes immediately after the ST of null into

           // _owner, above.  T2 notices that the EntryList is populated, so it

           // reacquires the lock and then finds itself on the EntryList.

           // Given all that, we have to tolerate the circumstance where "w" is

           // associated with Self.

           assert (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;

           ExitEpilog (Self, w) ;

           return ;

       }

       // If we find that both _cxq and EntryList are null then just

       // re-run the exit protocol from the top.

       w = _cxq ;

       if (w == NULL) continue ;

       // Drain _cxq into EntryList - bulk transfer.

       // First, detach _cxq.

       // The following loop is tantamount to: w = swap (&cxq, NULL)

       for (;;) {

           assert (w != NULL, "Invariant") ;

           ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;

           if (u == w) break ;

           w = u ;

       }

       TEVENT (Inflated exit - drain cxq into EntryList) ;

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

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

       // Convert the LIFO SLL anchored by _cxq into a DLL.

       // The list reorganization step operates in O(LENGTH(w)) time.

       // It's critical that this step operate quickly as

       // "Self" still holds the outer-lock, restricting parallelism

       // and effectively lengthening the critical section.

       // Invariant: s chases t chases u.

       // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so

       // we have faster access to the tail.

       if (QMode == 1) {

          // QMode == 1 : drain cxq to EntryList, reversing order

          // We also reverse the order of the list.

          ObjectWaiter * s = NULL ;

          ObjectWaiter * t = w ;

          ObjectWaiter * u = NULL ;

          while (t != NULL) {

              guarantee (t->TState == ObjectWaiter::TS_CXQ, "invariant") ;

              t->TState = ObjectWaiter::TS_ENTER ;

              u = t->_next ;

              t->_prev = u ;

              t->_next = s ;

              s = t;

              t = u ;

          }

          _EntryList  = s ;

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

       } else {

          // QMode == 0 or QMode == 2

          _EntryList = w ;

          ObjectWaiter * q = NULL ;

          ObjectWaiter * p ;

          for (p = w ; p != NULL ; p = p->_next) {

              guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;

              p->TState = ObjectWaiter::TS_ENTER ;

              p->_prev = q ;

              q = p ;

          }

       }

       // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL

       // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().

       // See if we can abdicate to a spinner instead of waking a thread.

       // A primary goal of the implementation is to reduce the

       // context-switch rate.

       if (_succ != NULL) continue;

       w = _EntryList  ;

       if (w != NULL) {

           guarantee (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;

           ExitEpilog (Self, w) ;

           return ;

       }

    }

 }

 // ExitSuspendEquivalent:

 // A faster alternate to handle_special_suspend_equivalent_condition()

 //

 // handle_special_suspend_equivalent_condition() unconditionally

 // acquires the SR_lock.  On some platforms uncontended MutexLocker()

 // operations have high latency.  Note that in ::enter() we call HSSEC

 // while holding the monitor, so we effectively lengthen the critical sections.

 //

 // There are a number of possible solutions:

 //

 // A.  To ameliorate the problem we might also defer state transitions

 //     to as late as possible -- just prior to parking.

 //     Given that, we'd call HSSEC after having returned from park(),

 //     but before attempting to acquire the monitor.  This is only a

 //     partial solution.  It avoids calling HSSEC while holding the

 //     monitor (good), but it still increases successor reacquisition latency --

 //     the interval between unparking a successor and the time the successor

 //     resumes and retries the lock.  See ReenterI(), which defers state transitions.

 //     If we use this technique we can also avoid EnterI()-exit() loop

 //     in ::enter() where we iteratively drop the lock and then attempt

 //     to reacquire it after suspending.

 //

 // B.  In the future we might fold all the suspend bits into a

 //     composite per-thread suspend flag and then update it with CAS().

 //     Alternately, a Dekker-like mechanism with multiple variables

 //     would suffice:

 //       ST Self->_suspend_equivalent = false

 //       MEMBAR

 //       LD Self_>_suspend_flags

 //

 bool ObjectMonitor::ExitSuspendEquivalent (JavaThread * jSelf) {

    int Mode = Knob_FastHSSEC ;

    if (Mode && !jSelf->is_external_suspend()) {

       assert (jSelf->is_suspend_equivalent(), "invariant") ;

       jSelf->clear_suspend_equivalent() ;

       if (2 == Mode) OrderAccess::storeload() ;

       if (!jSelf->is_external_suspend()) return false ;

       // We raced a suspension -- fall thru into the slow path

       TEVENT (ExitSuspendEquivalent - raced) ;

       jSelf->set_suspend_equivalent() ;

    }

    return jSelf->handle_special_suspend_equivalent_condition() ;

 }

 void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) {

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

    // Exit protocol:

    // 1. ST _succ = wakee

    // 2. membar #loadstore|#storestore;

    // 2. ST _owner = NULL

    // 3. unpark(wakee)

    _succ = Knob_SuccEnabled ? Wakee->_thread : NULL ;

    ParkEvent * Trigger = Wakee->_event ;

    // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.

    // The thread associated with Wakee may have grabbed the lock and "Wakee" may be

    // out-of-scope (non-extant).

    Wakee  = NULL ;

    // Drop the lock

    OrderAccess::release_store_ptr (&_owner, NULL) ;

    OrderAccess::fence() ;                               // ST _owner vs LD in unpark()

    if (SafepointSynchronize::do_call_back()) {

       TEVENT (unpark before SAFEPOINT) ;

    }

    DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);

    Trigger->unpark() ;

    // Maintain stats and report events to JVMTI

    if (ObjectMonitor::_sync_Parks != NULL) {

       ObjectMonitor::_sync_Parks->inc() ;

    }

 }

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

 // Class Loader deadlock handling.

 //

 // complete_exit exits a lock returning recursion count

 // complete_exit/reenter operate as a wait without waiting

 // complete_exit requires an inflated monitor

 // The _owner field is not always the Thread addr even with an

 // inflated monitor, e.g. the monitor can be inflated by a non-owning

 // thread due to contention.

 intptr_t ObjectMonitor::complete_exit(TRAPS) {

    Thread * const Self = THREAD;

    assert(Self->is_Java_thread(), "Must be Java thread!");

    JavaThread *jt = (JavaThread *)THREAD;

    DeferredInitialize();

    if (THREAD != _owner) {

     if (THREAD->is_lock_owned ((address)_owner)) {

        assert(_recursions == 0, "internal state error");

        _owner = THREAD ;   /* Convert from basiclock addr to Thread addr */

        _recursions = 0 ;

        OwnerIsThread = 1 ;

     }

    }

    guarantee(Self == _owner, "complete_exit not owner");

    intptr_t save = _recursions; // record the old recursion count

    _recursions = 0;        // set the recursion level to be 0

    exit (Self) ;           // exit the monitor

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

    return save;

 }

 // reenter() enters a lock and sets recursion count

 // complete_exit/reenter operate as a wait without waiting

 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {

    Thread * const Self = THREAD;

    assert(Self->is_Java_thread(), "Must be Java thread!");

    JavaThread *jt = (JavaThread *)THREAD;

    guarantee(_owner != Self, "reenter already owner");

    enter (THREAD);       // enter the monitor

    guarantee (_recursions == 0, "reenter recursion");

    _recursions = recursions;

    return;

 }

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

 // A macro is used below because there may already be a pending

 // exception which should not abort the execution of the routines

 // which use this (which is why we don't put this into check_slow and

 // call it with a CHECK argument).

 #define CHECK_OWNER()                                                             \

   do {                                                                            \

     if (THREAD != _owner) {                                                       \

       if (THREAD->is_lock_owned((address) _owner)) {                              \

         _owner = THREAD ;  /* Convert from basiclock addr to Thread addr */       \

         _recursions = 0;                                                          \

         OwnerIsThread = 1 ;                                                       \

       } else {                                                                    \

         TEVENT (Throw IMSX) ;                                                     \

         THROW(vmSymbols::java_lang_IllegalMonitorStateException());               \

       }                                                                           \

     }                                                                             \

   } while (false)

 // check_slow() is a misnomer.  It's called to simply to throw an IMSX exception.

 // TODO-FIXME: remove check_slow() -- it's likely dead.

 void ObjectMonitor::check_slow(TRAPS) {

   TEVENT (check_slow - throw IMSX) ;

   assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");

   THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");

 }

 static int Adjust (volatile int * adr, int dx) {

   int v ;

   for (v = *adr ; Atomic::cmpxchg (v + dx, adr, v) != v; v = *adr) ;

   return v ;

 }

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

 // Wait/Notify/NotifyAll

 //

 // Note: a subset of changes to ObjectMonitor::wait()

 // will need to be replicated in complete_exit above

 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {

    Thread * const Self = THREAD ;

    assert(Self->is_Java_thread(), "Must be Java thread!");

    JavaThread *jt = (JavaThread *)THREAD;

    DeferredInitialize () ;

    // Throw IMSX or IEX.

    CHECK_OWNER();

    // check for a pending interrupt

    if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {

      // post monitor waited event.  Note that this is past-tense, we are done waiting.

      if (JvmtiExport::should_post_monitor_waited()) {

         // Note: 'false' parameter is passed here because the

         // wait was not timed out due to thread interrupt.

         JvmtiExport::post_monitor_waited(jt, this, false);

      }

      TEVENT (Wait - Throw IEX) ;

      THROW(vmSymbols::java_lang_InterruptedException());

      return ;

    }

    TEVENT (Wait) ;

    assert (Self->_Stalled == 0, "invariant") ;

    Self->_Stalled = intptr_t(this) ;

    jt->set_current_waiting_monitor(this);

    // create a node to be put into the queue

    // Critically, after we reset() the event but prior to park(), we must check

    // for a pending interrupt.

    ObjectWaiter node(Self);

    node.TState = ObjectWaiter::TS_WAIT ;

    Self->_ParkEvent->reset() ;

    OrderAccess::fence();          // ST into Event; membar ; LD interrupted-flag

    // Enter the waiting queue, which is a circular doubly linked list in this case

    // but it could be a priority queue or any data structure.

    // _WaitSetLock protects the wait queue.  Normally the wait queue is accessed only

    // by the the owner of the monitor *except* in the case where park()

    // returns because of a timeout of interrupt.  Contention is exceptionally rare

    // so we use a simple spin-lock instead of a heavier-weight blocking lock.

    Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ;

    AddWaiter (&node) ;

    Thread::SpinRelease (&_WaitSetLock) ;

    if ((SyncFlags & 4) == 0) {

       _Responsible = NULL ;

    }

    intptr_t save = _recursions; // record the old recursion count

    _waiters++;                  // increment the number of waiters

    _recursions = 0;             // set the recursion level to be 1

    exit (Self) ;                    // exit the monitor

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

    // As soon as the ObjectMonitor's ownership is dropped in the exit()

    // call above, another thread can enter() the ObjectMonitor, do the

    // notify(), and exit() the ObjectMonitor. If the other thread's

    // exit() call chooses this thread as the successor and the unpark()

    // call happens to occur while this thread is posting a

    // MONITOR_CONTENDED_EXIT event, then we run the risk of the event

    // handler using RawMonitors and consuming the unpark().

    //

    // To avoid the problem, we re-post the event. This does no harm

    // even if the original unpark() was not consumed because we are the

    // chosen successor for this monitor.

    if (node._notified != 0 && _succ == Self) {

       node._event->unpark();

    }

    // The thread is on the WaitSet list - now park() it.

    // On MP systems it's conceivable that a brief spin before we park

    // could be profitable.

    //

    // TODO-FIXME: change the following logic to a loop of the form

    //   while (!timeout && !interrupted && _notified == 0) park()

    int ret = OS_OK ;

    int WasNotified = 0 ;

    { // State transition wrappers

      OSThread* osthread = Self->osthread();

      OSThreadWaitState osts(osthread, true);

      {

        ThreadBlockInVM tbivm(jt);

        // Thread is in thread_blocked state and oop access is unsafe.

        jt->set_suspend_equivalent();

        if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {

            // Intentionally empty

        } else

        if (node._notified == 0) {

          if (millis <= 0) {

             Self->_ParkEvent->park () ;

          } else {

             ret = Self->_ParkEvent->park (millis) ;

          }

        }

        // were we externally suspended while we were waiting?

        if (ExitSuspendEquivalent (jt)) {

           // TODO-FIXME: add -- if succ == Self then succ = null.

           jt->java_suspend_self();

        }

      } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm

      // Node may be on the WaitSet, the EntryList (or cxq), or in transition

      // from the WaitSet to the EntryList.

      // See if we need to remove Node from the WaitSet.

      // We use double-checked locking to avoid grabbing _WaitSetLock

      // if the thread is not on the wait queue.

      //

      // Note that we don't need a fence before the fetch of TState.

      // In the worst case we'll fetch a old-stale value of TS_WAIT previously

      // written by the is thread. (perhaps the fetch might even be satisfied

      // by a look-aside into the processor's own store buffer, although given

      // the length of the code path between the prior ST and this load that's

      // highly unlikely).  If the following LD fetches a stale TS_WAIT value

      // then we'll acquire the lock and then re-fetch a fresh TState value.

      // That is, we fail toward safety.

      if (node.TState == ObjectWaiter::TS_WAIT) {

          Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ;

          if (node.TState == ObjectWaiter::TS_WAIT) {

             DequeueSpecificWaiter (&node) ;       // unlink from WaitSet

             assert(node._notified == 0, "invariant");

             node.TState = ObjectWaiter::TS_RUN ;

          }

          Thread::SpinRelease (&_WaitSetLock) ;

      }

      // The thread is now either on off-list (TS_RUN),

      // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).

      // The Node's TState variable is stable from the perspective of this thread.

      // No other threads will asynchronously modify TState.

      guarantee (node.TState != ObjectWaiter::TS_WAIT, "invariant") ;

      OrderAccess::loadload() ;

      if (_succ == Self) _succ = NULL ;

      WasNotified = node._notified ;

      // Reentry phase -- reacquire the monitor.

      // re-enter contended monitor after object.wait().

      // retain OBJECT_WAIT state until re-enter successfully completes

      // Thread state is thread_in_vm and oop access is again safe,

      // although the raw address of the object may have changed.

      // (Don't cache naked oops over safepoints, of course).

      // post monitor waited event. Note that this is past-tense, we are done waiting.

      if (JvmtiExport::should_post_monitor_waited()) {

        JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);

      }

      OrderAccess::fence() ;

      assert (Self->_Stalled != 0, "invariant") ;

      Self->_Stalled = 0 ;

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

      ObjectWaiter::TStates v = node.TState ;

      if (v == ObjectWaiter::TS_RUN) {

          enter (Self) ;

      } else {

          guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;

          ReenterI (Self, &node) ;

          node.wait_reenter_end(this);

      }

      // Self has reacquired the lock.

      // Lifecycle - the node representing Self must not appear on any queues.

      // Node is about to go out-of-scope, but even if it were immortal we wouldn't

      // want residual elements associated with this thread left on any lists.

      guarantee (node.TState == ObjectWaiter::TS_RUN, "invariant") ;

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

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

    } // OSThreadWaitState()

    jt->set_current_waiting_monitor(NULL);

    guarantee (_recursions == 0, "invariant") ;

    _recursions = save;     // restore the old recursion count

    _waiters--;             // decrement the number of waiters

    // Verify a few postconditions

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

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

    assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;

    if (SyncFlags & 32) {

       OrderAccess::fence() ;

    }

    // check if the notification happened

    if (!WasNotified) {

      // no, it could be timeout or Thread.interrupt() or both

      // check for interrupt event, otherwise it is timeout

      if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {

        TEVENT (Wait - throw IEX from epilog) ;

        THROW(vmSymbols::java_lang_InterruptedException());

      }

    }

    // NOTE: Spurious wake up will be consider as timeout.

    // Monitor notify has precedence over thread interrupt.

 }

 // Consider:

 // If the lock is cool (cxq == null && succ == null) and we're on an MP system

 // then instead of transferring a thread from the WaitSet to the EntryList

 // we might just dequeue a thread from the WaitSet and directly unpark() it.

 void ObjectMonitor::notify(TRAPS) {

   CHECK_OWNER();

   if (_WaitSet == NULL) {

      TEVENT (Empty-Notify) ;

      return ;

   }

   DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);

   int Policy = Knob_MoveNotifyee ;

   Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ;

   ObjectWaiter * iterator = DequeueWaiter() ;

   if (iterator != NULL) {

      TEVENT (Notify1 - Transfer) ;

      guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;

      guarantee (iterator->_notified == 0, "invariant") ;

      if (Policy != 4) {

         iterator->TState = ObjectWaiter::TS_ENTER ;

      }

      iterator->_notified = 1 ;

      ObjectWaiter * List = _EntryList ;

      if (List != NULL) {

         assert (List->_prev == NULL, "invariant") ;

         assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;

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

      }

      if (Policy == 0) {       // prepend to EntryList

          if (List == NULL) {

              iterator->_next = iterator->_prev = NULL ;

              _EntryList = iterator ;

          } else {

              List->_prev = iterator ;

              iterator->_next = List ;

              iterator->_prev = NULL ;

              _EntryList = iterator ;

         }

      } else

      if (Policy == 1) {      // append to EntryList

          if (List == NULL) {

              iterator->_next = iterator->_prev = NULL ;

              _EntryList = iterator ;

          } else {

             // CONSIDER:  finding the tail currently requires a linear-time walk of

             // the EntryList.  We can make tail access constant-time by converting to

             // a CDLL instead of using our current DLL.

             ObjectWaiter * Tail ;

             for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;

             assert (Tail != NULL && Tail->_next == NULL, "invariant") ;

             Tail->_next = iterator ;

             iterator->_prev = Tail ;

             iterator->_next = NULL ;

         }

      } else

      if (Policy == 2) {      // prepend to cxq

          // prepend to cxq

          if (List == NULL) {

              iterator->_next = iterator->_prev = NULL ;

              _EntryList = iterator ;

          } else {

             iterator->TState = ObjectWaiter::TS_CXQ ;

             for (;;) {

                 ObjectWaiter * Front = _cxq ;

                 iterator->_next = Front ;

                 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {

                     break ;

                 }

             }

          }

      } else

      if (Policy == 3) {      // append to cxq

         iterator->TState = ObjectWaiter::TS_CXQ ;

         for (;;) {

             ObjectWaiter * Tail ;

             Tail = _cxq ;

             if (Tail == NULL) {

                 iterator->_next = NULL ;

                 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {

                    break ;

                 }

             } else {

                 while (Tail->_next != NULL) Tail = Tail->_next ;

                 Tail->_next = iterator ;

                 iterator->_prev = Tail ;

                 iterator->_next = NULL ;

                 break ;

             }

         }

      } else {

         ParkEvent * ev = iterator->_event ;

         iterator->TState = ObjectWaiter::TS_RUN ;

         OrderAccess::fence() ;

         ev->unpark() ;

      }

      if (Policy < 4) {

        iterator->wait_reenter_begin(this);

      }

      // _WaitSetLock protects the wait queue, not the EntryList.  We could

      // move the add-to-EntryList operation, above, outside the critical section

      // protected by _WaitSetLock.  In practice that's not useful.  With the

      // exception of  wait() timeouts and interrupts the monitor owner

      // is the only thread that grabs _WaitSetLock.  There's almost no contention

      // on _WaitSetLock so it's not profitable to reduce the length of the

      // critical section.

   }

   Thread::SpinRelease (&_WaitSetLock) ;

   if (iterator != NULL && ObjectMonitor::_sync_Notifications != NULL) {

      ObjectMonitor::_sync_Notifications->inc() ;

   }

 }

 void ObjectMonitor::notifyAll(TRAPS) {

   CHECK_OWNER();

   ObjectWaiter* iterator;

   if (_WaitSet == NULL) {

       TEVENT (Empty-NotifyAll) ;

       return ;

   }

   DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);

   int Policy = Knob_MoveNotifyee ;

   int Tally = 0 ;

   Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notifyall") ;

   for (;;) {

      iterator = DequeueWaiter () ;

      if (iterator == NULL) break ;

      TEVENT (NotifyAll - Transfer1) ;

      ++Tally ;

      // Disposition - what might we do with iterator ?

      // a.  add it directly to the EntryList - either tail or head.

      // b.  push it onto the front of the _cxq.

      // For now we use (a).

      guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;

      guarantee (iterator->_notified == 0, "invariant") ;

      iterator->_notified = 1 ;

      if (Policy != 4) {

         iterator->TState = ObjectWaiter::TS_ENTER ;

      }

      ObjectWaiter * List = _EntryList ;

      if (List != NULL) {

         assert (List->_prev == NULL, "invariant") ;

         assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;

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

      }

      if (Policy == 0) {       // prepend to EntryList

          if (List == NULL) {

              iterator->_next = iterator->_prev = NULL ;

              _EntryList = iterator ;

          } else {

              List->_prev = iterator ;

              iterator->_next = List ;

              iterator->_prev = NULL ;

              _EntryList = iterator ;

         }

      } else

      if (Policy == 1) {      // append to EntryList

          if (List == NULL) {

              iterator->_next = iterator->_prev = NULL ;

              _EntryList = iterator ;

          } else {

             // CONSIDER:  finding the tail currently requires a linear-time walk of

             // the EntryList.  We can make tail access constant-time by converting to

             // a CDLL instead of using our current DLL.

             ObjectWaiter * Tail ;

             for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;

             assert (Tail != NULL && Tail->_next == NULL, "invariant") ;

             Tail->_next = iterator ;

             iterator->_prev = Tail ;

             iterator->_next = NULL ;

         }

      } else

      if (Policy == 2) {      // prepend to cxq

          // prepend to cxq

          iterator->TState = ObjectWaiter::TS_CXQ ;

          for (;;) {

              ObjectWaiter * Front = _cxq ;

              iterator->_next = Front ;

              if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {

                  break ;

              }

          }

      } else

      if (Policy == 3) {      // append to cxq

         iterator->TState = ObjectWaiter::TS_CXQ ;

         for (;;) {

             ObjectWaiter * Tail ;

             Tail = _cxq ;

             if (Tail == NULL) {

                 iterator->_next = NULL ;

                 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {

                    break ;

                 }

             } else {

                 while (Tail->_next != NULL) Tail = Tail->_next ;

                 Tail->_next = iterator ;

                 iterator->_prev = Tail ;

                 iterator->_next = NULL ;

                 break ;

             }

         }

      } else {

         ParkEvent * ev = iterator->_event ;

         iterator->TState = ObjectWaiter::TS_RUN ;

         OrderAccess::fence() ;

         ev->unpark() ;

      }

      if (Policy < 4) {

        iterator->wait_reenter_begin(this);

      }

      // _WaitSetLock protects the wait queue, not the EntryList.  We could

      // move the add-to-EntryList operation, above, outside the critical section

      // protected by _WaitSetLock.  In practice that's not useful.  With the

      // exception of  wait() timeouts and interrupts the monitor owner

      // is the only thread that grabs _WaitSetLock.  There's almost no contention

      // on _WaitSetLock so it's not profitable to reduce the length of the

      // critical section.

   }

   Thread::SpinRelease (&_WaitSetLock) ;

   if (Tally != 0 && ObjectMonitor::_sync_Notifications != NULL) {

      ObjectMonitor::_sync_Notifications->inc(Tally) ;

   }

 }

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

 // Adaptive Spinning Support

 //

 // Adaptive spin-then-block - rational spinning

 //

 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS

 // algorithm.  On high order SMP systems it would be better to start with

 // a brief global spin and then revert to spinning locally.  In the spirit of MCS/CLH,

 // a contending thread could enqueue itself on the cxq and then spin locally

 // on a thread-specific variable such as its ParkEvent._Event flag.

 // That's left as an exercise for the reader.  Note that global spinning is

 // not problematic on Niagara, as the L2$ serves the interconnect and has both

 // low latency and massive bandwidth.

 //

 // Broadly, we can fix the spin frequency -- that is, the % of contended lock

 // acquisition attempts where we opt to spin --  at 100% and vary the spin count

 // (duration) or we can fix the count at approximately the duration of

 // a context switch and vary the frequency.   Of course we could also

 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.

 // See http://j2se.east/~dice/PERSIST/040824-AdaptiveSpinning.html.

 //

 // This implementation varies the duration "D", where D varies with

 // the success rate of recent spin attempts. (D is capped at approximately

 // length of a round-trip context switch).  The success rate for recent

 // spin attempts is a good predictor of the success rate of future spin

 // attempts.  The mechanism adapts automatically to varying critical

 // section length (lock modality), system load and degree of parallelism.

 // D is maintained per-monitor in _SpinDuration and is initialized

 // optimistically.  Spin frequency is fixed at 100%.

 //

 // Note that _SpinDuration is volatile, but we update it without locks

 // or atomics.  The code is designed so that _SpinDuration stays within

 // a reasonable range even in the presence of races.  The arithmetic

 // operations on _SpinDuration are closed over the domain of legal values,

 // so at worst a race will install and older but still legal value.

 // At the very worst this introduces some apparent non-determinism.

 // We might spin when we shouldn't or vice-versa, but since the spin

 // count are relatively short, even in the worst case, the effect is harmless.

 //

 // Care must be taken that a low "D" value does not become an

 // an absorbing state.  Transient spinning failures -- when spinning

 // is overall profitable -- should not cause the system to converge

 // on low "D" values.  We want spinning to be stable and predictable

 // and fairly responsive to change and at the same time we don't want

 // it to oscillate, become metastable, be "too" non-deterministic,

 // or converge on or enter undesirable stable absorbing states.

 //

 // We implement a feedback-based control system -- using past behavior

 // to predict future behavior.  We face two issues: (a) if the

 // input signal is random then the spin predictor won't provide optimal

 // results, and (b) if the signal frequency is too high then the control

 // system, which has some natural response lag, will "chase" the signal.

 // (b) can arise from multimodal lock hold times.  Transient preemption

 // can also result in apparent bimodal lock hold times.

 // Although sub-optimal, neither condition is particularly harmful, as

 // in the worst-case we'll spin when we shouldn't or vice-versa.

 // The maximum spin duration is rather short so the failure modes aren't bad.

 // To be conservative, I've tuned the gain in system to bias toward

 // _not spinning.  Relatedly, the system can sometimes enter a mode where it

 // "rings" or oscillates between spinning and not spinning.  This happens

 // when spinning is just on the cusp of profitability, however, so the

 // situation is not dire.  The state is benign -- there's no need to add

 // hysteresis control to damp the transition rate between spinning and

 // not spinning.

 //

 intptr_t ObjectMonitor::SpinCallbackArgument = 0 ;

 int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL ;

 // Spinning: Fixed frequency (100%), vary duration

 int ObjectMonitor::TrySpin_VaryDuration (Thread * Self) {

     // Dumb, brutal spin.  Good for comparative measurements against adaptive spinning.

     int ctr = Knob_FixedSpin ;

     if (ctr != 0) {

         while (--ctr >= 0) {

             if (TryLock (Self) > 0) return 1 ;

             SpinPause () ;

         }

         return 0 ;

     }

     for (ctr = Knob_PreSpin + 1; --ctr >= 0 ; ) {

       if (TryLock(Self) > 0) {

         // Increase _SpinDuration ...

         // Note that we don't clamp SpinDuration precisely at SpinLimit.

         // Raising _SpurDuration to the poverty line is key.

         int x = _SpinDuration ;

         if (x < Knob_SpinLimit) {

            if (x < Knob_Poverty) x = Knob_Poverty ;

            _SpinDuration = x + Knob_BonusB ;

         }

         return 1 ;

       }

       SpinPause () ;

     }

     // Admission control - verify preconditions for spinning

     //

     // We always spin a little bit, just to prevent _SpinDuration == 0 from

     // becoming an absorbing state.  Put another way, we spin briefly to

     // sample, just in case the system load, parallelism, contention, or lock

     // modality changed.

     //

     // Consider the following alternative:

     // Periodically set _SpinDuration = _SpinLimit and try a long/full

     // spin attempt.  "Periodically" might mean after a tally of

     // the # of failed spin attempts (or iterations) reaches some threshold.

     // This takes us into the realm of 1-out-of-N spinning, where we

     // hold the duration constant but vary the frequency.

     ctr = _SpinDuration  ;

     if (ctr < Knob_SpinBase) ctr = Knob_SpinBase ;

     if (ctr <= 0) return 0 ;

     if (Knob_SuccRestrict && _succ != NULL) return 0 ;

     if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {

        TEVENT (Spin abort - notrunnable [TOP]);

        return 0 ;

     }

     int MaxSpin = Knob_MaxSpinners ;

     if (MaxSpin >= 0) {

        if (_Spinner > MaxSpin) {

           TEVENT (Spin abort -- too many spinners) ;

           return 0 ;

        }

        // Slighty racy, but benign ...

        Adjust (&_Spinner, 1) ;

     }

     // We're good to spin ... spin ingress.

     // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades

     // when preparing to LD...CAS _owner, etc and the CAS is likely

     // to succeed.

     int hits    = 0 ;

     int msk     = 0 ;

     int caspty  = Knob_CASPenalty ;

     int oxpty   = Knob_OXPenalty ;

     int sss     = Knob_SpinSetSucc ;

     if (sss && _succ == NULL ) _succ = Self ;

     Thread * prv = NULL ;

     // There are three ways to exit the following loop:

     // 1.  A successful spin where this thread has acquired the lock.

     // 2.  Spin failure with prejudice

     // 3.  Spin failure without prejudice

     while (--ctr >= 0) {

       // Periodic polling -- Check for pending GC

       // Threads may spin while they're unsafe.

       // We don't want spinning threads to delay the JVM from reaching

       // a stop-the-world safepoint or to steal cycles from GC.

       // If we detect a pending safepoint we abort in order that

       // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)

       // this thread, if safe, doesn't steal cycles from GC.

       // This is in keeping with the "no loitering in runtime" rule.

       // We periodically check to see if there's a safepoint pending.

       if ((ctr & 0xFF) == 0) {

          if (SafepointSynchronize::do_call_back()) {

             TEVENT (Spin: safepoint) ;

             goto Abort ;           // abrupt spin egress

          }

          if (Knob_UsePause & 1) SpinPause () ;

          int (*scb)(intptr_t,int) = SpinCallbackFunction ;

          if (hits > 50 && scb != NULL) {

             int abend = (*scb)(SpinCallbackArgument, 0) ;

          }

       }

       if (Knob_UsePause & 2) SpinPause() ;

       // Exponential back-off ...  Stay off the bus to reduce coherency traffic.

       // This is useful on classic SMP systems, but is of less utility on

       // N1-style CMT platforms.

       //

       // Trade-off: lock acquisition latency vs coherency bandwidth.

       // Lock hold times are typically short.  A histogram

       // of successful spin attempts shows that we usually acquire

       // the lock early in the spin.  That suggests we want to

       // sample _owner frequently in the early phase of the spin,

       // but then back-off and sample less frequently as the spin

       // progresses.  The back-off makes a good citizen on SMP big

       // SMP systems.  Oversampling _owner can consume excessive

       // coherency bandwidth.  Relatedly, if we _oversample _owner we

       // can inadvertently interfere with the the ST m->owner=null.

       // executed by the lock owner.

       if (ctr & msk) continue ;

       ++hits ;

       if ((hits & 0xF) == 0) {

         // The 0xF, above, corresponds to the exponent.

         // Consider: (msk+1)|msk

         msk = ((msk << 2)|3) & BackOffMask ;

       }

       // Probe _owner with TATAS

       // If this thread observes the monitor transition or flicker

       // from locked to unlocked to locked, then the odds that this

       // thread will acquire the lock in this spin attempt go down

       // considerably.  The same argument applies if the CAS fails

       // or if we observe _owner change from one non-null value to

       // another non-null value.   In such cases we might abort

       // the spin without prejudice or apply a "penalty" to the

       // spin count-down variable "ctr", reducing it by 100, say.

       Thread * ox = (Thread *) _owner ;

       if (ox == NULL) {

          ox = (Thread *) Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;

          if (ox == NULL) {

             // The CAS succeeded -- this thread acquired ownership

             // Take care of some bookkeeping to exit spin state.

             if (sss && _succ == Self) {

                _succ = NULL ;

             }

             if (MaxSpin > 0) Adjust (&_Spinner, -1) ;

             // Increase _SpinDuration :

             // The spin was successful (profitable) so we tend toward

             // longer spin attempts in the future.

             // CONSIDER: factor "ctr" into the _SpinDuration adjustment.

             // If we acquired the lock early in the spin cycle it

             // makes sense to increase _SpinDuration proportionally.

             // Note that we don't clamp SpinDuration precisely at SpinLimit.

             int x = _SpinDuration ;

             if (x < Knob_SpinLimit) {

                 if (x < Knob_Poverty) x = Knob_Poverty ;

                 _SpinDuration = x + Knob_Bonus ;

             }

             return 1 ;

          }

          // The CAS failed ... we can take any of the following actions:

          // * penalize: ctr -= Knob_CASPenalty

          // * exit spin with prejudice -- goto Abort;

          // * exit spin without prejudice.

          // * Since CAS is high-latency, retry again immediately.

          prv = ox ;

          TEVENT (Spin: cas failed) ;

          if (caspty == -2) break ;

          if (caspty == -1) goto Abort ;

          ctr -= caspty ;

          continue ;

       }

       // Did lock ownership change hands ?

       if (ox != prv && prv != NULL ) {

           TEVENT (spin: Owner changed)

           if (oxpty == -2) break ;

           if (oxpty == -1) goto Abort ;

           ctr -= oxpty ;

       }

       prv = ox ;

       // Abort the spin if the owner is not executing.

       // The owner must be executing in order to drop the lock.

       // Spinning while the owner is OFFPROC is idiocy.

       // Consider: ctr -= RunnablePenalty ;

       if (Knob_OState && NotRunnable (Self, ox)) {

          TEVENT (Spin abort - notrunnable);

          goto Abort ;

       }

       if (sss && _succ == NULL ) _succ = Self ;

    }

    // Spin failed with prejudice -- reduce _SpinDuration.

    // TODO: Use an AIMD-like policy to adjust _SpinDuration.

    // AIMD is globally stable.

    TEVENT (Spin failure) ;

    {

      int x = _SpinDuration ;

      if (x > 0) {

         // Consider an AIMD scheme like: x -= (x >> 3) + 100

         // This is globally sample and tends to damp the response.

         x -= Knob_Penalty ;

         if (x < 0) x = 0 ;

         _SpinDuration = x ;

      }

    }

  Abort:

    if (MaxSpin >= 0) Adjust (&_Spinner, -1) ;

    if (sss && _succ == Self) {

       _succ = NULL ;

       // Invariant: after setting succ=null a contending thread

       // must recheck-retry _owner before parking.  This usually happens

       // in the normal usage of TrySpin(), but it's safest

       // to make TrySpin() as foolproof as possible.

       OrderAccess::fence() ;

       if (TryLock(Self) > 0) return 1 ;

    }

    return 0 ;

 }

 // NotRunnable() -- informed spinning

 //

 // Don't bother spinning if the owner is not eligible to drop the lock.

 // Peek at the owner's schedctl.sc_state and Thread._thread_values and

 // spin only if the owner thread is _thread_in_Java or _thread_in_vm.

 // The thread must be runnable in order to drop the lock in timely fashion.

 // If the _owner is not runnable then spinning will not likely be

 // successful (profitable).

 //

 // Beware -- the thread referenced by _owner could have died

 // so a simply fetch from _owner->_thread_state might trap.

 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.

 // Because of the lifecycle issues the schedctl and _thread_state values

 // observed by NotRunnable() might be garbage.  NotRunnable must

 // tolerate this and consider the observed _thread_state value

 // as advisory.

 //

 // Beware too, that _owner is sometimes a BasicLock address and sometimes

 // a thread pointer.  We differentiate the two cases with OwnerIsThread.

 // Alternately, we might tag the type (thread pointer vs basiclock pointer)

 // with the LSB of _owner.  Another option would be to probablistically probe

 // the putative _owner->TypeTag value.

 //

 // Checking _thread_state isn't perfect.  Even if the thread is

 // in_java it might be blocked on a page-fault or have been preempted

 // and sitting on a ready/dispatch queue.  _thread state in conjunction

 // with schedctl.sc_state gives us a good picture of what the

 // thread is doing, however.

 //

 // TODO: check schedctl.sc_state.

 // We'll need to use SafeFetch32() to read from the schedctl block.

 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/

 //

 // The return value from NotRunnable() is *advisory* -- the

 // result is based on sampling and is not necessarily coherent.

 // The caller must tolerate false-negative and false-positive errors.

 // Spinning, in general, is probabilistic anyway.

 int ObjectMonitor::NotRunnable (Thread * Self, Thread * ox) {

     // Check either OwnerIsThread or ox->TypeTag == 2BAD.

     if (!OwnerIsThread) return 0 ;

     if (ox == NULL) return 0 ;

     // Avoid transitive spinning ...

     // Say T1 spins or blocks trying to acquire L.  T1._Stalled is set to L.

     // Immediately after T1 acquires L it's possible that T2, also

     // spinning on L, will see L.Owner=T1 and T1._Stalled=L.

     // This occurs transiently after T1 acquired L but before

     // T1 managed to clear T1.Stalled.  T2 does not need to abort

     // its spin in this circumstance.

     intptr_t BlockedOn = SafeFetchN ((intptr_t *) &ox->_Stalled, intptr_t(1)) ;

     if (BlockedOn == 1) return 1 ;

     if (BlockedOn != 0) {

       return BlockedOn != intptr_t(this) && _owner == ox ;

     }

     assert (sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant") ;

     int jst = SafeFetch32 ((int *) &((JavaThread *) ox)->_thread_state, -1) ; ;

     // consider also: jst != _thread_in_Java -- but that's overspecific.

     return jst == _thread_blocked || jst == _thread_in_native ;

 }

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

 // WaitSet management ...

 ObjectWaiter::ObjectWaiter(Thread* thread) {

   _next     = NULL;

   _prev     = NULL;

   _notified = 0;

   TState    = TS_RUN ;

   _thread   = thread;

   _event    = thread->_ParkEvent ;

   _active   = false;

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

 }

 void ObjectWaiter::wait_reenter_begin(ObjectMonitor *mon) {

   JavaThread *jt = (JavaThread *)this->_thread;

   _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);

 }

 void ObjectWaiter::wait_reenter_end(ObjectMonitor *mon) {

   JavaThread *jt = (JavaThread *)this->_thread;

   JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);

 }

 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {

   assert(node != NULL, "should not dequeue NULL node");

   assert(node->_prev == NULL, "node already in list");

   assert(node->_next == NULL, "node already in list");

   // put node at end of queue (circular doubly linked list)

   if (_WaitSet == NULL) {

     _WaitSet = node;

     node->_prev = node;

     node->_next = node;

   } else {

     ObjectWaiter* head = _WaitSet ;

     ObjectWaiter* tail = head->_prev;

     assert(tail->_next == head, "invariant check");

     tail->_next = node;

     head->_prev = node;

     node->_next = head;

     node->_prev = tail;

   }

 }

 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {

   // dequeue the very first waiter

   ObjectWaiter* waiter = _WaitSet;

   if (waiter) {

     DequeueSpecificWaiter(waiter);

   }

   return waiter;

 }

 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {

   assert(node != NULL, "should not dequeue NULL node");

   assert(node->_prev != NULL, "node already removed from list");

   assert(node->_next != NULL, "node already removed from list");

   // when the waiter has woken up because of interrupt,

   // timeout or other spurious wake-up, dequeue the

   // waiter from waiting list

   ObjectWaiter* next = node->_next;

   if (next == node) {

     assert(node->_prev == node, "invariant check");

     _WaitSet = NULL;

   } else {

     ObjectWaiter* prev = node->_prev;

     assert(prev->_next == node, "invariant check");

     assert(next->_prev == node, "invariant check");

     next->_prev = prev;

     prev->_next = next;

     if (_WaitSet == node) {

       _WaitSet = next;

     }

   }

   node->_next = NULL;

   node->_prev = NULL;

 }

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

 // PerfData support

 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts       = NULL ;

 PerfCounter * ObjectMonitor::_sync_FutileWakeups               = NULL ;

 PerfCounter * ObjectMonitor::_sync_Parks                       = NULL ;

 PerfCounter * ObjectMonitor::_sync_EmptyNotifications          = NULL ;

 PerfCounter * ObjectMonitor::_sync_Notifications               = NULL ;

 PerfCounter * ObjectMonitor::_sync_PrivateA                    = NULL ;

 PerfCounter * ObjectMonitor::_sync_PrivateB                    = NULL ;

 PerfCounter * ObjectMonitor::_sync_SlowExit                    = NULL ;

 PerfCounter * ObjectMonitor::_sync_SlowEnter                   = NULL ;

 PerfCounter * ObjectMonitor::_sync_SlowNotify                  = NULL ;

 PerfCounter * ObjectMonitor::_sync_SlowNotifyAll               = NULL ;

 PerfCounter * ObjectMonitor::_sync_FailedSpins                 = NULL ;

 PerfCounter * ObjectMonitor::_sync_SuccessfulSpins             = NULL ;

 PerfCounter * ObjectMonitor::_sync_MonInCirculation            = NULL ;

 PerfCounter * ObjectMonitor::_sync_MonScavenged                = NULL ;

 PerfCounter * ObjectMonitor::_sync_Inflations                  = NULL ;

 PerfCounter * ObjectMonitor::_sync_Deflations                  = NULL ;

 PerfLongVariable * ObjectMonitor::_sync_MonExtant              = NULL ;

 // One-shot global initialization for the sync subsystem.

 // We could also defer initialization and initialize on-demand

 // the first time we call inflate().  Initialization would

 // be protected - like so many things - by the MonitorCache_lock.

 void ObjectMonitor::Initialize () {

   static int InitializationCompleted = 0 ;

   assert (InitializationCompleted == 0, "invariant") ;

   InitializationCompleted = 1 ;

   if (UsePerfData) {

       EXCEPTION_MARK ;

       #define NEWPERFCOUNTER(n)   {n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,CHECK); }

       #define NEWPERFVARIABLE(n)  {n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,CHECK); }

       NEWPERFCOUNTER(_sync_Inflations) ;

       NEWPERFCOUNTER(_sync_Deflations) ;

       NEWPERFCOUNTER(_sync_ContendedLockAttempts) ;

       NEWPERFCOUNTER(_sync_FutileWakeups) ;

       NEWPERFCOUNTER(_sync_Parks) ;

       NEWPERFCOUNTER(_sync_EmptyNotifications) ;

       NEWPERFCOUNTER(_sync_Notifications) ;

       NEWPERFCOUNTER(_sync_SlowEnter) ;

       NEWPERFCOUNTER(_sync_SlowExit) ;

       NEWPERFCOUNTER(_sync_SlowNotify) ;

       NEWPERFCOUNTER(_sync_SlowNotifyAll) ;

       NEWPERFCOUNTER(_sync_FailedSpins) ;

       NEWPERFCOUNTER(_sync_SuccessfulSpins) ;

       NEWPERFCOUNTER(_sync_PrivateA) ;

       NEWPERFCOUNTER(_sync_PrivateB) ;

       NEWPERFCOUNTER(_sync_MonInCirculation) ;

       NEWPERFCOUNTER(_sync_MonScavenged) ;

       NEWPERFVARIABLE(_sync_MonExtant) ;

       #undef NEWPERFCOUNTER

   }

 }

 // Compile-time asserts

 // When possible, it's better to catch errors deterministically at

 // compile-time than at runtime.  The down-side to using compile-time

 // asserts is that error message -- often something about negative array

 // indices -- is opaque.

 #define CTASSERT(x) { int tag[1-(2*!(x))]; printf ("Tag @" INTPTR_FORMAT "\n", (intptr_t)tag); }

 void ObjectMonitor::ctAsserts() {

   CTASSERT(offset_of (ObjectMonitor, _header) == 0);

 }

 static char * kvGet (char * kvList, const char * Key) {

     if (kvList == NULL) return NULL ;

     size_t n = strlen (Key) ;

     char * Search ;

     for (Search = kvList ; *Search ; Search += strlen(Search) + 1) {

         if (strncmp (Search, Key, n) == 0) {

             if (Search[n] == '=') return Search + n + 1 ;

             if (Search[n] == 0)   return (char *) "1" ;

         }

     }

     return NULL ;

 }

 static int kvGetInt (char * kvList, const char * Key, int Default) {

     char * v = kvGet (kvList, Key) ;

     int rslt = v ? ::strtol (v, NULL, 0) : Default ;

     if (Knob_ReportSettings && v != NULL) {

         ::printf ("  SyncKnob: %s %d(%d)\n", Key, rslt, Default) ;

         ::fflush (stdout) ;

     }

     return rslt ;

 }

 void ObjectMonitor::DeferredInitialize () {

   if (InitDone > 0) return ;

   if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {

       while (InitDone != 1) ;

       return ;

   }

   // One-shot global initialization ...

   // The initialization is idempotent, so we don't need locks.

   // In the future consider doing this via os::init_2().

   // SyncKnobs consist of <Key>=<Value> pairs in the style

   // of environment variables.  Start by converting ':' to NUL.

   if (SyncKnobs == NULL) SyncKnobs = "" ;

   size_t sz = strlen (SyncKnobs) ;

   char * knobs = (char *) malloc (sz + 2) ;

   if (knobs == NULL) {

      vm_exit_out_of_memory (sz + 2, "Parse SyncKnobs") ;

      guarantee (0, "invariant") ;

   }

   strcpy (knobs, SyncKnobs) ;

   knobs[sz+1] = 0 ;

   for (char * p = knobs ; *p ; p++) {

      if (*p == ':') *p = 0 ;

   }

   #define SETKNOB(x) { Knob_##x = kvGetInt (knobs, #x, Knob_##x); }

   SETKNOB(ReportSettings) ;

   SETKNOB(Verbose) ;

   SETKNOB(FixedSpin) ;

   SETKNOB(SpinLimit) ;

   SETKNOB(SpinBase) ;

   SETKNOB(SpinBackOff);

   SETKNOB(CASPenalty) ;

   SETKNOB(OXPenalty) ;

   SETKNOB(LogSpins) ;

   SETKNOB(SpinSetSucc) ;

   SETKNOB(SuccEnabled) ;

   SETKNOB(SuccRestrict) ;

   SETKNOB(Penalty) ;

   SETKNOB(Bonus) ;

   SETKNOB(BonusB) ;

   SETKNOB(Poverty) ;

   SETKNOB(SpinAfterFutile) ;

   SETKNOB(UsePause) ;

   SETKNOB(SpinEarly) ;

   SETKNOB(OState) ;

   SETKNOB(MaxSpinners) ;

   SETKNOB(PreSpin) ;

   SETKNOB(ExitPolicy) ;

   SETKNOB(QMode);

   SETKNOB(ResetEvent) ;

   SETKNOB(MoveNotifyee) ;

   SETKNOB(FastHSSEC) ;

   #undef SETKNOB

   if (os::is_MP()) {

      BackOffMask = (1 << Knob_SpinBackOff) - 1 ;

      if (Knob_ReportSettings) ::printf ("BackOffMask=%X\n", BackOffMask) ;

      // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)

   } else {

      Knob_SpinLimit = 0 ;

      Knob_SpinBase  = 0 ;

      Knob_PreSpin   = 0 ;

      Knob_FixedSpin = -1 ;

   }

   if (Knob_LogSpins == 0) {

      ObjectMonitor::_sync_FailedSpins = NULL ;

   }

   free (knobs) ;

   OrderAccess::fence() ;

   InitDone = 1 ;

 }

 #ifndef PRODUCT

 void ObjectMonitor::verify() {

 }

 void ObjectMonitor::print() {

 }

 #endif