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

 /*

  * Copyright 1998-2009 Sun Microsystems, Inc.  All Rights Reserved.

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

  *

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

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

  * published by the Free Software Foundation.

  *

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

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

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

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

  * accompanied this code).

  *

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

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

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

  *

  * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,

  * CA 95054 USA or visit www.sun.com if you need additional information or

  * have any questions.

  *

  */

 # include "incls/_precompiled.incl"

 # include "incls/_synchronizer.cpp.incl"

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

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

   #define ATTR __attribute__((noinline))

 #else

   #define ATTR

 #endif

 // Native markword accessors for synchronization and hashCode().

 //

 // The "core" versions of monitor enter and exit reside in this file.

 // The interpreter and compilers contain specialized transliterated

 // variants of the enter-exit fast-path operations.  See i486.ad fast_lock(),

 // for instance.  If you make changes here, make sure to modify the

 // interpreter, and both C1 and C2 fast-path inline locking code emission.

 //

 // TODO: merge the objectMonitor and synchronizer classes.

 //

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

 #ifdef DTRACE_ENABLED

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

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

 HS_DTRACE_PROBE_DECL5(hotspot, monitor__wait,

   jlong, uintptr_t, char*, int, long);

 HS_DTRACE_PROBE_DECL4(hotspot, monitor__waited,

   jlong, uintptr_t, char*, int);

 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notify,

   jlong, uintptr_t, char*, int);

 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notifyAll,

   jlong, uintptr_t, char*, int);

 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__enter,

   jlong, uintptr_t, char*, int);

 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__entered,

   jlong, uintptr_t, char*, int);

 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__exit,

   jlong, uintptr_t, char*, int);

 #define DTRACE_MONITOR_PROBE_COMMON(klassOop, thread)                      \

   char* bytes = NULL;                                                      \

   int len = 0;                                                             \

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

   symbolOop klassname = ((oop)(klassOop))->klass()->klass_part()->name();  \

   if (klassname != NULL) {                                                 \

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

     len = klassname->utf8_length();                                        \

   }

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

   {                                                                        \

     if (DTraceMonitorProbes) {                                            \

       DTRACE_MONITOR_PROBE_COMMON(klassOop, thread);                       \

       HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid,                       \

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

     }                                                                      \

   }

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

   {                                                                        \

     if (DTraceMonitorProbes) {                                            \

       DTRACE_MONITOR_PROBE_COMMON(klassOop, thread);                       \

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

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

     }                                                                      \

   }

 #else //  ndef DTRACE_ENABLED

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

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

 #endif // ndef DTRACE_ENABLED

 // ObjectWaiter serves as a "proxy" or surrogate thread.

 // TODO-FIXME: Eliminate ObjectWaiter and use the thread-specific

 // ParkEvent instead.  Beware, however, that the JVMTI code

 // knows about ObjectWaiters, so we'll have to reconcile that code.

 // See next_waiter(), first_waiter(), etc.

 class ObjectWaiter : public StackObj {

  public:

   enum TStates { TS_UNDEF, TS_READY, TS_RUN, TS_WAIT, TS_ENTER, TS_CXQ } ;

   enum Sorted  { PREPEND, APPEND, SORTED } ;

   ObjectWaiter * volatile _next;

   ObjectWaiter * volatile _prev;

   Thread*       _thread;

   ParkEvent *   _event;

   volatile int  _notified ;

   volatile TStates TState ;

   Sorted        _Sorted ;           // List placement disposition

   bool          _active ;           // Contention monitoring is enabled

  public:

   ObjectWaiter(Thread* thread) {

     _next     = NULL;

     _prev     = NULL;

     _notified = 0;

     TState    = TS_RUN ;

     _thread   = thread;

     _event    = thread->_ParkEvent ;

     _active   = false;

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

   }

   void wait_reenter_begin(ObjectMonitor *mon) {

     JavaThread *jt = (JavaThread *)this->_thread;

     _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);

   }

   void wait_reenter_end(ObjectMonitor *mon) {

     JavaThread *jt = (JavaThread *)this->_thread;

     JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);

   }

 };

 enum ManifestConstants {

     ClearResponsibleAtSTW   = 0,

     MaximumRecheckInterval  = 1000

 } ;

 #undef TEVENT

 #define TEVENT(nom) {if (SyncVerbose) FEVENT(nom); }

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

 #undef  TEVENT

 #define TEVENT(nom) {;}

 // Performance concern:

 // OrderAccess::storestore() calls release() which STs 0 into the global volatile

 // OrderAccess::Dummy variable.  This store is unnecessary for correctness.

 // Many threads STing into a common location causes considerable cache migration

 // or "sloshing" on large SMP system.  As such, I avoid using OrderAccess::storestore()

 // until it's repaired.  In some cases OrderAccess::fence() -- which incurs local

 // latency on the executing processor -- is a better choice as it scales on SMP

 // systems.  See http://blogs.sun.com/dave/entry/biased_locking_in_hotspot for a

 // discussion of coherency costs.  Note that all our current reference platforms

 // provide strong ST-ST order, so the issue is moot on IA32, x64, and SPARC.

 //

 // As a general policy we use "volatile" to control compiler-based reordering

 // and explicit fences (barriers) to control for architectural reordering performed

 // by the CPU(s) or platform.

 static int  MBFence (int x) { OrderAccess::fence(); return x; }

 struct SharedGlobals {

     // These are highly shared mostly-read variables.

     // To avoid false-sharing they need to be the sole occupants of a $ line.

     double padPrefix [8];

     volatile int stwRandom ;

     volatile int stwCycle ;

     // Hot RW variables -- Sequester to avoid false-sharing

     double padSuffix [16];

     volatile int hcSequence ;

     double padFinal [8] ;

 } ;

 static SharedGlobals GVars ;

 // Tunables ...

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

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

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

 static int Knob_HandOff            = 0 ;

 static int Knob_Verbose            = 0 ;

 static int Knob_ReportSettings     = 0 ;

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

 static int Knob_SpinBase           = 0 ;       // Floor AKA SpinMin

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

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

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

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

 static int Knob_SpinEarly          = 1 ;

 static int Knob_SuccEnabled        = 1 ;       // futile wake throttling

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

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

 static int Knob_Bonus              = 100 ;     // spin success bonus

 static int Knob_BonusB             = 100 ;     // spin success bonus

 static int Knob_Penalty            = 200 ;     // spin failure penalty

 static int Knob_Poverty            = 1000 ;

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

 static int Knob_FixedSpin          = 0 ;

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

 static int Knob_UsePause           = 1 ;

 static int Knob_ExitPolicy         = 0 ;

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

 static int Knob_ResetEvent         = 0 ;

 static int BackOffMask             = 0 ;

 static int Knob_FastHSSEC          = 0 ;

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

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

 static volatile int InitDone       = 0 ;

 // hashCode() generation :

 //

 // Possibilities:

 // * MD5Digest of {obj,stwRandom}

 // * CRC32 of {obj,stwRandom} or any linear-feedback shift register function.

 // * A DES- or AES-style SBox[] mechanism

 // * One of the Phi-based schemes, such as:

 //   2654435761 = 2^32 * Phi (golden ratio)

 //   HashCodeValue = ((uintptr_t(obj) >> 3) * 2654435761) ^ GVars.stwRandom ;

 // * A variation of Marsaglia's shift-xor RNG scheme.

 // * (obj ^ stwRandom) is appealing, but can result

 //   in undesirable regularity in the hashCode values of adjacent objects

 //   (objects allocated back-to-back, in particular).  This could potentially

 //   result in hashtable collisions and reduced hashtable efficiency.

 //   There are simple ways to "diffuse" the middle address bits over the

 //   generated hashCode values:

 //

 static inline intptr_t get_next_hash(Thread * Self, oop obj) {

   intptr_t value = 0 ;

   if (hashCode == 0) {

      // This form uses an unguarded global Park-Miller RNG,

      // so it's possible for two threads to race and generate the same RNG.

      // On MP system we'll have lots of RW access to a global, so the

      // mechanism induces lots of coherency traffic.

      value = os::random() ;

   } else

   if (hashCode == 1) {

      // This variation has the property of being stable (idempotent)

      // between STW operations.  This can be useful in some of the 1-0

      // synchronization schemes.

      intptr_t addrBits = intptr_t(obj) >> 3 ;

      value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom ;

   } else

   if (hashCode == 2) {

      value = 1 ;            // for sensitivity testing

   } else

   if (hashCode == 3) {

      value = ++GVars.hcSequence ;

   } else

   if (hashCode == 4) {

      value = intptr_t(obj) ;

   } else {

      // Marsaglia's xor-shift scheme with thread-specific state

      // This is probably the best overall implementation -- we'll

      // likely make this the default in future releases.

      unsigned t = Self->_hashStateX ;

      t ^= (t << 11) ;

      Self->_hashStateX = Self->_hashStateY ;

      Self->_hashStateY = Self->_hashStateZ ;

      Self->_hashStateZ = Self->_hashStateW ;

      unsigned v = Self->_hashStateW ;

      v = (v ^ (v >> 19)) ^ (t ^ (t >> 8)) ;

      Self->_hashStateW = v ;

      value = v ;

   }

   value &= markOopDesc::hash_mask;

   if (value == 0) value = 0xBAD ;

   assert (value != markOopDesc::no_hash, "invariant") ;

   TEVENT (hashCode: GENERATE) ;

   return value;

 }

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

   st->print("monitor");

 }

 void BasicLock::move_to(oop obj, BasicLock* dest) {

   // Check to see if we need to inflate the lock. This is only needed

   // if an object is locked using "this" lightweight monitor. In that

   // case, the displaced_header() is unlocked, because the

   // displaced_header() contains the header for the originally unlocked

   // object. However the object could have already been inflated. But it

   // does not matter, the inflation will just a no-op. For other cases,

   // the displaced header will be either 0x0 or 0x3, which are location

   // independent, therefore the BasicLock is free to move.

   //

   // During OSR we may need to relocate a BasicLock (which contains a

   // displaced word) from a location in an interpreter frame to a

   // new location in a compiled frame.  "this" refers to the source

   // basiclock in the interpreter frame.  "dest" refers to the destination

   // basiclock in the new compiled frame.  We *always* inflate in move_to().

   // The always-Inflate policy works properly, but in 1.5.0 it can sometimes

   // cause performance problems in code that makes heavy use of a small # of

   // uncontended locks.   (We'd inflate during OSR, and then sync performance

   // would subsequently plummet because the thread would be forced thru the slow-path).

   // This problem has been made largely moot on IA32 by inlining the inflated fast-path

   // operations in Fast_Lock and Fast_Unlock in i486.ad.

   //

   // Note that there is a way to safely swing the object's markword from

   // one stack location to another.  This avoids inflation.  Obviously,

   // we need to ensure that both locations refer to the current thread's stack.

   // There are some subtle concurrency issues, however, and since the benefit is

   // is small (given the support for inflated fast-path locking in the fast_lock, etc)

   // we'll leave that optimization for another time.

   if (displaced_header()->is_neutral()) {

     ObjectSynchronizer::inflate_helper(obj);

     // WARNING: We can not put check here, because the inflation

     // will not update the displaced header. Once BasicLock is inflated,

     // no one should ever look at its content.

   } else {

     // Typically the displaced header will be 0 (recursive stack lock) or

     // unused_mark.  Naively we'd like to assert that the displaced mark

     // value is either 0, neutral, or 3.  But with the advent of the

     // store-before-CAS avoidance in fast_lock/compiler_lock_object

     // we can find any flavor mark in the displaced mark.

   }

 // [RGV] The next line appears to do nothing!

   intptr_t dh = (intptr_t) displaced_header();

   dest->set_displaced_header(displaced_header());

 }

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

 // standard constructor, allows locking failures

 ObjectLocker::ObjectLocker(Handle obj, Thread* thread, bool doLock) {

   _dolock = doLock;

   _thread = thread;

   debug_only(if (StrictSafepointChecks) _thread->check_for_valid_safepoint_state(false);)

   _obj = obj;

   if (_dolock) {

     TEVENT (ObjectLocker) ;

     ObjectSynchronizer::fast_enter(_obj, &_lock, false, _thread);

   }

 }

 ObjectLocker::~ObjectLocker() {

   if (_dolock) {

     ObjectSynchronizer::fast_exit(_obj(), &_lock, _thread);

   }

 }

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

 PerfCounter * ObjectSynchronizer::_sync_Inflations                  = NULL ;

 PerfCounter * ObjectSynchronizer::_sync_Deflations                  = NULL ;

 PerfCounter * ObjectSynchronizer::_sync_ContendedLockAttempts       = NULL ;

 PerfCounter * ObjectSynchronizer::_sync_FutileWakeups               = NULL ;

 PerfCounter * ObjectSynchronizer::_sync_Parks                       = NULL ;

 PerfCounter * ObjectSynchronizer::_sync_EmptyNotifications          = NULL ;

 PerfCounter * ObjectSynchronizer::_sync_Notifications               = NULL ;

 PerfCounter * ObjectSynchronizer::_sync_PrivateA                    = NULL ;

 PerfCounter * ObjectSynchronizer::_sync_PrivateB                    = NULL ;

 PerfCounter * ObjectSynchronizer::_sync_SlowExit                    = NULL ;

 PerfCounter * ObjectSynchronizer::_sync_SlowEnter                   = NULL ;

 PerfCounter * ObjectSynchronizer::_sync_SlowNotify                  = NULL ;

 PerfCounter * ObjectSynchronizer::_sync_SlowNotifyAll               = NULL ;

 PerfCounter * ObjectSynchronizer::_sync_FailedSpins                 = NULL ;

 PerfCounter * ObjectSynchronizer::_sync_SuccessfulSpins             = NULL ;

 PerfCounter * ObjectSynchronizer::_sync_MonInCirculation            = NULL ;

 PerfCounter * ObjectSynchronizer::_sync_MonScavenged                = NULL ;

 PerfLongVariable * ObjectSynchronizer::_sync_MonExtant              = NULL ;

 // One-shot global initialization for the sync subsystem.

 // We could also defer initialization and initialize on-demand

 // the first time we call inflate().  Initialization would

 // be protected - like so many things - by the MonitorCache_lock.

 void ObjectSynchronizer::Initialize () {

   static int InitializationCompleted = 0 ;

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

   InitializationCompleted = 1 ;

   if (UsePerfData) {

       EXCEPTION_MARK ;

       #define NEWPERFCOUNTER(n)   {n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,CHECK); }

       #define NEWPERFVARIABLE(n)  {n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,CHECK); }

       NEWPERFCOUNTER(_sync_Inflations) ;

       NEWPERFCOUNTER(_sync_Deflations) ;

       NEWPERFCOUNTER(_sync_ContendedLockAttempts) ;

       NEWPERFCOUNTER(_sync_FutileWakeups) ;

       NEWPERFCOUNTER(_sync_Parks) ;

       NEWPERFCOUNTER(_sync_EmptyNotifications) ;

       NEWPERFCOUNTER(_sync_Notifications) ;

       NEWPERFCOUNTER(_sync_SlowEnter) ;

       NEWPERFCOUNTER(_sync_SlowExit) ;

       NEWPERFCOUNTER(_sync_SlowNotify) ;

       NEWPERFCOUNTER(_sync_SlowNotifyAll) ;

       NEWPERFCOUNTER(_sync_FailedSpins) ;

       NEWPERFCOUNTER(_sync_SuccessfulSpins) ;

       NEWPERFCOUNTER(_sync_PrivateA) ;

       NEWPERFCOUNTER(_sync_PrivateB) ;

       NEWPERFCOUNTER(_sync_MonInCirculation) ;

       NEWPERFCOUNTER(_sync_MonScavenged) ;

       NEWPERFVARIABLE(_sync_MonExtant) ;

       #undef NEWPERFCOUNTER

   }

 }

 // Compile-time asserts

 // When possible, it's better to catch errors deterministically at

 // compile-time than at runtime.  The down-side to using compile-time

 // asserts is that error message -- often something about negative array

 // indices -- is opaque.

 #define CTASSERT(x) { int tag[1-(2*!(x))]; printf ("Tag @" INTPTR_FORMAT "\n", (intptr_t)tag); }

 void ObjectMonitor::ctAsserts() {

   CTASSERT(offset_of (ObjectMonitor, _header) == 0);

 }

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

   int v ;

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

   return v ;

 }

 // Ad-hoc mutual exclusion primitives: SpinLock and Mux

 //

 // We employ SpinLocks _only for low-contention, fixed-length

 // short-duration critical sections where we're concerned

 // about native mutex_t or HotSpot Mutex:: latency.

 // The mux construct provides a spin-then-block mutual exclusion

 // mechanism.

 //

 // Testing has shown that contention on the ListLock guarding gFreeList

 // is common.  If we implement ListLock as a simple SpinLock it's common

 // for the JVM to devolve to yielding with little progress.  This is true

 // despite the fact that the critical sections protected by ListLock are

 // extremely short.

 //

 // TODO-FIXME: ListLock should be of type SpinLock.

 // We should make this a 1st-class type, integrated into the lock

 // hierarchy as leaf-locks.  Critically, the SpinLock structure

 // should have sufficient padding to avoid false-sharing and excessive

 // cache-coherency traffic.

 typedef volatile int SpinLockT ;

 void Thread::SpinAcquire (volatile int * adr, const char * LockName) {

   if (Atomic::cmpxchg (1, adr, 0) == 0) {

      return ;   // normal fast-path return

   }

   // Slow-path : We've encountered contention -- Spin/Yield/Block strategy.

   TEVENT (SpinAcquire - ctx) ;

   int ctr = 0 ;

   int Yields = 0 ;

   for (;;) {

      while (*adr != 0) {

         ++ctr ;

         if ((ctr & 0xFFF) == 0 || !os::is_MP()) {

            if (Yields > 5) {

              // Consider using a simple NakedSleep() instead.

              // Then SpinAcquire could be called by non-JVM threads

              Thread::current()->_ParkEvent->park(1) ;

            } else {

              os::NakedYield() ;

              ++Yields ;

            }

         } else {

            SpinPause() ;

         }

      }

      if (Atomic::cmpxchg (1, adr, 0) == 0) return ;

   }

 }

 void Thread::SpinRelease (volatile int * adr) {

   assert (*adr != 0, "invariant") ;

   OrderAccess::fence() ;      // guarantee at least release consistency.

   // Roach-motel semantics.

   // It's safe if subsequent LDs and STs float "up" into the critical section,

   // but prior LDs and STs within the critical section can't be allowed

   // to reorder or float past the ST that releases the lock.

   *adr = 0 ;

 }

 // muxAcquire and muxRelease:

 //

 // *  muxAcquire and muxRelease support a single-word lock-word construct.

 //    The LSB of the word is set IFF the lock is held.

 //    The remainder of the word points to the head of a singly-linked list

 //    of threads blocked on the lock.

 //

 // *  The current implementation of muxAcquire-muxRelease uses its own

 //    dedicated Thread._MuxEvent instance.  If we're interested in

 //    minimizing the peak number of extant ParkEvent instances then

 //    we could eliminate _MuxEvent and "borrow" _ParkEvent as long

 //    as certain invariants were satisfied.  Specifically, care would need

 //    to be taken with regards to consuming unpark() "permits".

 //    A safe rule of thumb is that a thread would never call muxAcquire()

 //    if it's enqueued (cxq, EntryList, WaitList, etc) and will subsequently

 //    park().  Otherwise the _ParkEvent park() operation in muxAcquire() could

 //    consume an unpark() permit intended for monitorenter, for instance.

 //    One way around this would be to widen the restricted-range semaphore

 //    implemented in park().  Another alternative would be to provide

 //    multiple instances of the PlatformEvent() for each thread.  One

 //    instance would be dedicated to muxAcquire-muxRelease, for instance.

 //

 // *  Usage:

 //    -- Only as leaf locks

 //    -- for short-term locking only as muxAcquire does not perform

 //       thread state transitions.

 //

 // Alternatives:

 // *  We could implement muxAcquire and muxRelease with MCS or CLH locks

 //    but with parking or spin-then-park instead of pure spinning.

 // *  Use Taura-Oyama-Yonenzawa locks.

 // *  It's possible to construct a 1-0 lock if we encode the lockword as

 //    (List,LockByte).  Acquire will CAS the full lockword while Release

 //    will STB 0 into the LockByte.  The 1-0 scheme admits stranding, so

 //    acquiring threads use timers (ParkTimed) to detect and recover from

 //    the stranding window.  Thread/Node structures must be aligned on 256-byte

 //    boundaries by using placement-new.

 // *  Augment MCS with advisory back-link fields maintained with CAS().

 //    Pictorially:  LockWord -> T1 <-> T2 <-> T3 <-> ... <-> Tn <-> Owner.

 //    The validity of the backlinks must be ratified before we trust the value.

 //    If the backlinks are invalid the exiting thread must back-track through the

 //    the forward links, which are always trustworthy.

 // *  Add a successor indication.  The LockWord is currently encoded as

 //    (List, LOCKBIT:1).  We could also add a SUCCBIT or an explicit _succ variable

 //    to provide the usual futile-wakeup optimization.

 //    See RTStt for details.

 // *  Consider schedctl.sc_nopreempt to cover the critical section.

 //

 typedef volatile intptr_t MutexT ;      // Mux Lock-word

 enum MuxBits { LOCKBIT = 1 } ;

 void Thread::muxAcquire (volatile intptr_t * Lock, const char * LockName) {

   intptr_t w = Atomic::cmpxchg_ptr (LOCKBIT, Lock, 0) ;

   if (w == 0) return ;

   if ((w & LOCKBIT) == 0 && Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {

      return ;

   }

   TEVENT (muxAcquire - Contention) ;

   ParkEvent * const Self = Thread::current()->_MuxEvent ;

   assert ((intptr_t(Self) & LOCKBIT) == 0, "invariant") ;

   for (;;) {

      int its = (os::is_MP() ? 100 : 0) + 1 ;

      // Optional spin phase: spin-then-park strategy

      while (--its >= 0) {

        w = *Lock ;

        if ((w & LOCKBIT) == 0 && Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {

           return ;

        }

      }

      Self->reset() ;

      Self->OnList = intptr_t(Lock) ;

      // The following fence() isn't _strictly necessary as the subsequent

      // CAS() both serializes execution and ratifies the fetched *Lock value.

      OrderAccess::fence();

      for (;;) {

         w = *Lock ;

         if ((w & LOCKBIT) == 0) {

             if (Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {

                 Self->OnList = 0 ;   // hygiene - allows stronger asserts

                 return ;

             }

             continue ;      // Interference -- *Lock changed -- Just retry

         }

         assert (w & LOCKBIT, "invariant") ;

         Self->ListNext = (ParkEvent *) (w & ~LOCKBIT );

         if (Atomic::cmpxchg_ptr (intptr_t(Self)|LOCKBIT, Lock, w) == w) break ;

      }

      while (Self->OnList != 0) {

         Self->park() ;

      }

   }

 }

 void Thread::muxAcquireW (volatile intptr_t * Lock, ParkEvent * ev) {

   intptr_t w = Atomic::cmpxchg_ptr (LOCKBIT, Lock, 0) ;

   if (w == 0) return ;

   if ((w & LOCKBIT) == 0 && Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {

     return ;

   }

   TEVENT (muxAcquire - Contention) ;

   ParkEvent * ReleaseAfter = NULL ;

   if (ev == NULL) {

     ev = ReleaseAfter = ParkEvent::Allocate (NULL) ;

   }

   assert ((intptr_t(ev) & LOCKBIT) == 0, "invariant") ;

   for (;;) {

     guarantee (ev->OnList == 0, "invariant") ;

     int its = (os::is_MP() ? 100 : 0) + 1 ;

     // Optional spin phase: spin-then-park strategy

     while (--its >= 0) {

       w = *Lock ;

       if ((w & LOCKBIT) == 0 && Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {

         if (ReleaseAfter != NULL) {

           ParkEvent::Release (ReleaseAfter) ;

         }

         return ;

       }

     }

     ev->reset() ;

     ev->OnList = intptr_t(Lock) ;

     // The following fence() isn't _strictly necessary as the subsequent

     // CAS() both serializes execution and ratifies the fetched *Lock value.

     OrderAccess::fence();

     for (;;) {

       w = *Lock ;

       if ((w & LOCKBIT) == 0) {

         if (Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {

           ev->OnList = 0 ;

           // We call ::Release while holding the outer lock, thus

           // artificially lengthening the critical section.

           // Consider deferring the ::Release() until the subsequent unlock(),

           // after we've dropped the outer lock.

           if (ReleaseAfter != NULL) {

             ParkEvent::Release (ReleaseAfter) ;

           }

           return ;

         }

         continue ;      // Interference -- *Lock changed -- Just retry

       }

       assert (w & LOCKBIT, "invariant") ;

       ev->ListNext = (ParkEvent *) (w & ~LOCKBIT );

       if (Atomic::cmpxchg_ptr (intptr_t(ev)|LOCKBIT, Lock, w) == w) break ;

     }

     while (ev->OnList != 0) {

       ev->park() ;

     }

   }

 }

 // Release() must extract a successor from the list and then wake that thread.

 // It can "pop" the front of the list or use a detach-modify-reattach (DMR) scheme

 // similar to that used by ParkEvent::Allocate() and ::Release().  DMR-based

 // Release() would :

 // (A) CAS() or swap() null to *Lock, releasing the lock and detaching the list.

 // (B) Extract a successor from the private list "in-hand"

 // (C) attempt to CAS() the residual back into *Lock over null.

 //     If there were any newly arrived threads and the CAS() would fail.

 //     In that case Release() would detach the RATs, re-merge the list in-hand

 //     with the RATs and repeat as needed.  Alternately, Release() might

 //     detach and extract a successor, but then pass the residual list to the wakee.

 //     The wakee would be responsible for reattaching and remerging before it

 //     competed for the lock.

 //

 // Both "pop" and DMR are immune from ABA corruption -- there can be

 // multiple concurrent pushers, but only one popper or detacher.

 // This implementation pops from the head of the list.  This is unfair,

 // but tends to provide excellent throughput as hot threads remain hot.

 // (We wake recently run threads first).

 void Thread::muxRelease (volatile intptr_t * Lock)  {

   for (;;) {

     const intptr_t w = Atomic::cmpxchg_ptr (0, Lock, LOCKBIT) ;

     assert (w & LOCKBIT, "invariant") ;

     if (w == LOCKBIT) return ;

     ParkEvent * List = (ParkEvent *) (w & ~LOCKBIT) ;

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

     assert (List->OnList == intptr_t(Lock), "invariant") ;

     ParkEvent * nxt = List->ListNext ;

     // The following CAS() releases the lock and pops the head element.

     if (Atomic::cmpxchg_ptr (intptr_t(nxt), Lock, w) != w) {

       continue ;

     }

     List->OnList = 0 ;

     OrderAccess::fence() ;

     List->unpark () ;

     return ;

   }

 }

 // ObjectMonitor Lifecycle

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

 // Inflation unlinks monitors from the global gFreeList and

 // associates them with objects.  Deflation -- which occurs at

 // STW-time -- disassociates idle monitors from objects.  Such

 // scavenged monitors are returned to the gFreeList.

 //

 // The global list is protected by ListLock.  All the critical sections

 // are short and operate in constant-time.

 //

 // ObjectMonitors reside in type-stable memory (TSM) and are immortal.

 //

 // Lifecycle:

 // --   unassigned and on the global free list

 // --   unassigned and on a thread's private omFreeList

 // --   assigned to an object.  The object is inflated and the mark refers

 //      to the objectmonitor.

 //

 // TODO-FIXME:

 //

 // *  We currently protect the gFreeList with a simple lock.

 //    An alternate lock-free scheme would be to pop elements from the gFreeList

 //    with CAS.  This would be safe from ABA corruption as long we only

 //    recycled previously appearing elements onto the list in deflate_idle_monitors()

 //    at STW-time.  Completely new elements could always be pushed onto the gFreeList

 //    with CAS.  Elements that appeared previously on the list could only

 //    be installed at STW-time.

 //

 // *  For efficiency and to help reduce the store-before-CAS penalty

 //    the objectmonitors on gFreeList or local free lists should be ready to install

 //    with the exception of _header and _object.  _object can be set after inflation.

 //    In particular, keep all objectMonitors on a thread's private list in ready-to-install

 //    state with m.Owner set properly.

 //

 // *  We could all diffuse contention by using multiple global (FreeList, Lock)

 //    pairs -- threads could use trylock() and a cyclic-scan strategy to search for

 //    an unlocked free list.

 //

 // *  Add lifecycle tags and assert()s.

 //

 // *  Be more consistent about when we clear an objectmonitor's fields:

 //    A.  After extracting the objectmonitor from a free list.

 //    B.  After adding an objectmonitor to a free list.

 //

 ObjectMonitor * ObjectSynchronizer::gBlockList = NULL ;

 ObjectMonitor * volatile ObjectSynchronizer::gFreeList  = NULL ;

 static volatile intptr_t ListLock = 0 ;      // protects global monitor free-list cache

 #define CHAINMARKER ((oop)-1)

 ObjectMonitor * ATTR ObjectSynchronizer::omAlloc (Thread * Self) {

     // A large MAXPRIVATE value reduces both list lock contention

     // and list coherency traffic, but also tends to increase the

     // number of objectMonitors in circulation as well as the STW

     // scavenge costs.  As usual, we lean toward time in space-time

     // tradeoffs.

     const int MAXPRIVATE = 1024 ;

     for (;;) {

         ObjectMonitor * m ;

         // 1: try to allocate from the thread's local omFreeList.

         // Threads will attempt to allocate first from their local list, then

         // from the global list, and only after those attempts fail will the thread

         // attempt to instantiate new monitors.   Thread-local free lists take

         // heat off the ListLock and improve allocation latency, as well as reducing

         // coherency traffic on the shared global list.

         m = Self->omFreeList ;

         if (m != NULL) {

            Self->omFreeList = m->FreeNext ;

            Self->omFreeCount -- ;

            // CONSIDER: set m->FreeNext = BAD -- diagnostic hygiene

            guarantee (m->object() == NULL, "invariant") ;

            return m ;

         }

         // 2: try to allocate from the global gFreeList

         // CONSIDER: use muxTry() instead of muxAcquire().

         // If the muxTry() fails then drop immediately into case 3.

         // If we're using thread-local free lists then try

         // to reprovision the caller's free list.

         if (gFreeList != NULL) {

             // Reprovision the thread's omFreeList.

             // Use bulk transfers to reduce the allocation rate and heat

             // on various locks.

             Thread::muxAcquire (&ListLock, "omAlloc") ;

             for (int i = Self->omFreeProvision; --i >= 0 && gFreeList != NULL; ) {

                 ObjectMonitor * take = gFreeList ;

                 gFreeList = take->FreeNext ;

                 guarantee (take->object() == NULL, "invariant") ;

                 guarantee (!take->is_busy(), "invariant") ;

                 take->Recycle() ;

                 omRelease (Self, take) ;

             }

             Thread::muxRelease (&ListLock) ;

             Self->omFreeProvision += 1 + (Self->omFreeProvision/2) ;

             if (Self->omFreeProvision > MAXPRIVATE ) Self->omFreeProvision = MAXPRIVATE ;

             TEVENT (omFirst - reprovision) ;

             continue ;

         }

         // 3: allocate a block of new ObjectMonitors

         // Both the local and global free lists are empty -- resort to malloc().

         // In the current implementation objectMonitors are TSM - immortal.

         assert (_BLOCKSIZE > 1, "invariant") ;

         ObjectMonitor * temp = new ObjectMonitor[_BLOCKSIZE];

         // NOTE: (almost) no way to recover if allocation failed.

         // We might be able to induce a STW safepoint and scavenge enough

         // objectMonitors to permit progress.

         if (temp == NULL) {

             vm_exit_out_of_memory (sizeof (ObjectMonitor[_BLOCKSIZE]), "Allocate ObjectMonitors") ;

         }

         // Format the block.

         // initialize the linked list, each monitor points to its next

         // forming the single linked free list, the very first monitor

         // will points to next block, which forms the block list.

         // The trick of using the 1st element in the block as gBlockList

         // linkage should be reconsidered.  A better implementation would

         // look like: class Block { Block * next; int N; ObjectMonitor Body [N] ; }

         for (int i = 1; i < _BLOCKSIZE ; i++) {

            temp[i].FreeNext = &temp[i+1];

         }

         // terminate the last monitor as the end of list

         temp[_BLOCKSIZE - 1].FreeNext = NULL ;

         // Element [0] is reserved for global list linkage

         temp[0].set_object(CHAINMARKER);

         // Consider carving out this thread's current request from the

         // block in hand.  This avoids some lock traffic and redundant

         // list activity.

         // Acquire the ListLock to manipulate BlockList and FreeList.

         // An Oyama-Taura-Yonezawa scheme might be more efficient.

         Thread::muxAcquire (&ListLock, "omAlloc [2]") ;

         // Add the new block to the list of extant blocks (gBlockList).

         // The very first objectMonitor in a block is reserved and dedicated.

         // It serves as blocklist "next" linkage.

         temp[0].FreeNext = gBlockList;

         gBlockList = temp;

         // Add the new string of objectMonitors to the global free list

         temp[_BLOCKSIZE - 1].FreeNext = gFreeList ;

         gFreeList = temp + 1;

         Thread::muxRelease (&ListLock) ;

         TEVENT (Allocate block of monitors) ;

     }

 }

 // Place "m" on the caller's private per-thread omFreeList.

 // In practice there's no need to clamp or limit the number of

 // monitors on a thread's omFreeList as the only time we'll call

 // omRelease is to return a monitor to the free list after a CAS

 // attempt failed.  This doesn't allow unbounded #s of monitors to

 // accumulate on a thread's free list.

 //

 // In the future the usage of omRelease() might change and monitors

 // could migrate between free lists.  In that case to avoid excessive

 // accumulation we could  limit omCount to (omProvision*2), otherwise return

 // the objectMonitor to the global list.  We should drain (return) in reasonable chunks.

 // That is, *not* one-at-a-time.

 void ObjectSynchronizer::omRelease (Thread * Self, ObjectMonitor * m) {

     guarantee (m->object() == NULL, "invariant") ;

     m->FreeNext = Self->omFreeList ;

     Self->omFreeList = m ;

     Self->omFreeCount ++ ;

 }

 // Return the monitors of a moribund thread's local free list to

 // the global free list.  Typically a thread calls omFlush() when

 // it's dying.  We could also consider having the VM thread steal

 // monitors from threads that have not run java code over a few

 // consecutive STW safepoints.  Relatedly, we might decay

 // omFreeProvision at STW safepoints.

 //

 // We currently call omFlush() from the Thread:: dtor _after the thread

 // has been excised from the thread list and is no longer a mutator.

 // That means that omFlush() can run concurrently with a safepoint and

 // the scavenge operator.  Calling omFlush() from JavaThread::exit() might

 // be a better choice as we could safely reason that that the JVM is

 // not at a safepoint at the time of the call, and thus there could

 // be not inopportune interleavings between omFlush() and the scavenge

 // operator.

 void ObjectSynchronizer::omFlush (Thread * Self) {

     ObjectMonitor * List = Self->omFreeList ;  // Null-terminated SLL

     Self->omFreeList = NULL ;

     if (List == NULL) return ;

     ObjectMonitor * Tail = NULL ;

     ObjectMonitor * s ;

     for (s = List ; s != NULL ; s = s->FreeNext) {

         Tail = s ;

         guarantee (s->object() == NULL, "invariant") ;

         guarantee (!s->is_busy(), "invariant") ;

         s->set_owner (NULL) ;   // redundant but good hygiene

         TEVENT (omFlush - Move one) ;

     }

     guarantee (Tail != NULL && List != NULL, "invariant") ;

     Thread::muxAcquire (&ListLock, "omFlush") ;

     Tail->FreeNext = gFreeList ;

     gFreeList = List ;

     Thread::muxRelease (&ListLock) ;

     TEVENT (omFlush) ;

 }

 // Get the next block in the block list.

 static inline ObjectMonitor* next(ObjectMonitor* block) {

   assert(block->object() == CHAINMARKER, "must be a block header");

   block = block->FreeNext ;

   assert(block == NULL || block->object() == CHAINMARKER, "must be a block header");

   return block;

 }

 // Fast path code shared by multiple functions

 ObjectMonitor* ObjectSynchronizer::inflate_helper(oop obj) {

   markOop mark = obj->mark();

   if (mark->has_monitor()) {

     assert(ObjectSynchronizer::verify_objmon_isinpool(mark->monitor()), "monitor is invalid");

     assert(mark->monitor()->header()->is_neutral(), "monitor must record a good object header");

     return mark->monitor();

   }

   return ObjectSynchronizer::inflate(Thread::current(), obj);

 }

 // Note that we could encounter some performance loss through false-sharing as

 // multiple locks occupy the same $ line.  Padding might be appropriate.

 #define NINFLATIONLOCKS 256

 static volatile intptr_t InflationLocks [NINFLATIONLOCKS] ;

 static markOop ReadStableMark (oop obj) {

   markOop mark = obj->mark() ;

   if (!mark->is_being_inflated()) {

     return mark ;       // normal fast-path return

   }

   int its = 0 ;

   for (;;) {

     markOop mark = obj->mark() ;

     if (!mark->is_being_inflated()) {

       return mark ;    // normal fast-path return

     }

     // The object is being inflated by some other thread.

     // The caller of ReadStableMark() must wait for inflation to complete.

     // Avoid live-lock

     // TODO: consider calling SafepointSynchronize::do_call_back() while

     // spinning to see if there's a safepoint pending.  If so, immediately

     // yielding or blocking would be appropriate.  Avoid spinning while

     // there is a safepoint pending.

     // TODO: add inflation contention performance counters.

     // TODO: restrict the aggregate number of spinners.

     ++its ;

     if (its > 10000 || !os::is_MP()) {

        if (its & 1) {

          os::NakedYield() ;

          TEVENT (Inflate: INFLATING - yield) ;

        } else {

          // Note that the following code attenuates the livelock problem but is not

          // a complete remedy.  A more complete solution would require that the inflating

          // thread hold the associated inflation lock.  The following code simply restricts

          // the number of spinners to at most one.  We'll have N-2 threads blocked

          // on the inflationlock, 1 thread holding the inflation lock and using

          // a yield/park strategy, and 1 thread in the midst of inflation.

          // A more refined approach would be to change the encoding of INFLATING

          // to allow encapsulation of a native thread pointer.  Threads waiting for

          // inflation to complete would use CAS to push themselves onto a singly linked

          // list rooted at the markword.  Once enqueued, they'd loop, checking a per-thread flag

          // and calling park().  When inflation was complete the thread that accomplished inflation

          // would detach the list and set the markword to inflated with a single CAS and

          // then for each thread on the list, set the flag and unpark() the thread.

          // This is conceptually similar to muxAcquire-muxRelease, except that muxRelease

          // wakes at most one thread whereas we need to wake the entire list.

          int ix = (intptr_t(obj) >> 5) & (NINFLATIONLOCKS-1) ;

          int YieldThenBlock = 0 ;

          assert (ix >= 0 && ix < NINFLATIONLOCKS, "invariant") ;

          assert ((NINFLATIONLOCKS & (NINFLATIONLOCKS-1)) == 0, "invariant") ;

          Thread::muxAcquire (InflationLocks + ix, "InflationLock") ;

          while (obj->mark() == markOopDesc::INFLATING()) {

            // Beware: NakedYield() is advisory and has almost no effect on some platforms

            // so we periodically call Self->_ParkEvent->park(1).

            // We use a mixed spin/yield/block mechanism.

            if ((YieldThenBlock++) >= 16) {

               Thread::current()->_ParkEvent->park(1) ;

            } else {

               os::NakedYield() ;

            }

          }

          Thread::muxRelease (InflationLocks + ix ) ;

          TEVENT (Inflate: INFLATING - yield/park) ;

        }

     } else {

        SpinPause() ;       // SMP-polite spinning

     }

   }

 }

 ObjectMonitor * ATTR ObjectSynchronizer::inflate (Thread * Self, oop object) {

   // Inflate mutates the heap ...

   // Relaxing assertion for bug 6320749.

   assert (Universe::verify_in_progress() ||

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

   for (;;) {

       const markOop mark = object->mark() ;

       assert (!mark->has_bias_pattern(), "invariant") ;

       // The mark can be in one of the following states:

       // *  Inflated     - just return

       // *  Stack-locked - coerce it to inflated

       // *  INFLATING    - busy wait for conversion to complete

       // *  Neutral      - aggressively inflate the object.

       // *  BIASED       - Illegal.  We should never see this

       // CASE: inflated

       if (mark->has_monitor()) {

           ObjectMonitor * inf = mark->monitor() ;

           assert (inf->header()->is_neutral(), "invariant");

           assert (inf->object() == object, "invariant") ;

           assert (ObjectSynchronizer::verify_objmon_isinpool(inf), "monitor is invalid");

           return inf ;

       }

       // CASE: inflation in progress - inflating over a stack-lock.

       // Some other thread is converting from stack-locked to inflated.

       // Only that thread can complete inflation -- other threads must wait.

       // The INFLATING value is transient.

       // Currently, we spin/yield/park and poll the markword, waiting for inflation to finish.

       // We could always eliminate polling by parking the thread on some auxiliary list.

       if (mark == markOopDesc::INFLATING()) {

          TEVENT (Inflate: spin while INFLATING) ;

          ReadStableMark(object) ;

          continue ;

       }

       // CASE: stack-locked

       // Could be stack-locked either by this thread or by some other thread.

       //

       // Note that we allocate the objectmonitor speculatively, _before_ attempting

       // to install INFLATING into the mark word.  We originally installed INFLATING,

       // allocated the objectmonitor, and then finally STed the address of the

       // objectmonitor into the mark.  This was correct, but artificially lengthened

       // the interval in which INFLATED appeared in the mark, thus increasing

       // the odds of inflation contention.

       //

       // We now use per-thread private objectmonitor free lists.

       // These list are reprovisioned from the global free list outside the

       // critical INFLATING...ST interval.  A thread can transfer

       // multiple objectmonitors en-mass from the global free list to its local free list.

       // This reduces coherency traffic and lock contention on the global free list.

       // Using such local free lists, it doesn't matter if the omAlloc() call appears

       // before or after the CAS(INFLATING) operation.

       // See the comments in omAlloc().

       if (mark->has_locker()) {

           ObjectMonitor * m = omAlloc (Self) ;

           // Optimistically prepare the objectmonitor - anticipate successful CAS

           // We do this before the CAS in order to minimize the length of time

           // in which INFLATING appears in the mark.

           m->Recycle();

           m->FreeNext      = NULL ;

           m->_Responsible  = NULL ;

           m->OwnerIsThread = 0 ;

           m->_recursions   = 0 ;

           m->_SpinDuration = Knob_SpinLimit ;   // Consider: maintain by type/class

           markOop cmp = (markOop) Atomic::cmpxchg_ptr (markOopDesc::INFLATING(), object->mark_addr(), mark) ;

           if (cmp != mark) {

              omRelease (Self, m) ;

              continue ;       // Interference -- just retry

           }

           // We've successfully installed INFLATING (0) into the mark-word.

           // This is the only case where 0 will appear in a mark-work.

           // Only the singular thread that successfully swings the mark-word

           // to 0 can perform (or more precisely, complete) inflation.

           //

           // Why do we CAS a 0 into the mark-word instead of just CASing the

           // mark-word from the stack-locked value directly to the new inflated state?

           // Consider what happens when a thread unlocks a stack-locked object.

           // It attempts to use CAS to swing the displaced header value from the

           // on-stack basiclock back into the object header.  Recall also that the

           // header value (hashcode, etc) can reside in (a) the object header, or

           // (b) a displaced header associated with the stack-lock, or (c) a displaced

           // header in an objectMonitor.  The inflate() routine must copy the header

           // value from the basiclock on the owner's stack to the objectMonitor, all

           // the while preserving the hashCode stability invariants.  If the owner

           // decides to release the lock while the value is 0, the unlock will fail

           // and control will eventually pass from slow_exit() to inflate.  The owner

           // will then spin, waiting for the 0 value to disappear.   Put another way,

           // the 0 causes the owner to stall if the owner happens to try to

           // drop the lock (restoring the header from the basiclock to the object)

           // while inflation is in-progress.  This protocol avoids races that might

           // would otherwise permit hashCode values to change or "flicker" for an object.

           // Critically, while object->mark is 0 mark->displaced_mark_helper() is stable.

           // 0 serves as a "BUSY" inflate-in-progress indicator.

           // fetch the displaced mark from the owner's stack.

           // The owner can't die or unwind past the lock while our INFLATING

           // object is in the mark.  Furthermore the owner can't complete

           // an unlock on the object, either.

           markOop dmw = mark->displaced_mark_helper() ;

           assert (dmw->is_neutral(), "invariant") ;

           // Setup monitor fields to proper values -- prepare the monitor

           m->set_header(dmw) ;

           // Optimization: if the mark->locker stack address is associated

           // with this thread we could simply set m->_owner = Self and

           // m->OwnerIsThread = 1. Note that a thread can inflate an object

           // that it has stack-locked -- as might happen in wait() -- directly

           // with CAS.  That is, we can avoid the xchg-NULL .... ST idiom.

           m->set_owner(mark->locker());

           m->set_object(object);

           // TODO-FIXME: assert BasicLock->dhw != 0.

           // Must preserve store ordering. The monitor state must

           // be stable at the time of publishing the monitor address.

           guarantee (object->mark() == markOopDesc::INFLATING(), "invariant") ;

           object->release_set_mark(markOopDesc::encode(m));

           // Hopefully the performance counters are allocated on distinct cache lines

           // to avoid false sharing on MP systems ...

           if (_sync_Inflations != NULL) _sync_Inflations->inc() ;

           TEVENT(Inflate: overwrite stacklock) ;

           if (TraceMonitorInflation) {

             if (object->is_instance()) {

               ResourceMark rm;

               tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",

                 (intptr_t) object, (intptr_t) object->mark(),

                 Klass::cast(object->klass())->external_name());

             }

           }

           return m ;

       }

       // CASE: neutral

       // TODO-FIXME: for entry we currently inflate and then try to CAS _owner.

       // If we know we're inflating for entry it's better to inflate by swinging a

       // pre-locked objectMonitor pointer into the object header.   A successful

       // CAS inflates the object *and* confers ownership to the inflating thread.

       // In the current implementation we use a 2-step mechanism where we CAS()

       // to inflate and then CAS() again to try to swing _owner from NULL to Self.

       // An inflateTry() method that we could call from fast_enter() and slow_enter()

       // would be useful.

       assert (mark->is_neutral(), "invariant");

       ObjectMonitor * m = omAlloc (Self) ;

       // prepare m for installation - set monitor to initial state

       m->Recycle();

       m->set_header(mark);

       m->set_owner(NULL);

       m->set_object(object);

       m->OwnerIsThread = 1 ;

       m->_recursions   = 0 ;

       m->FreeNext      = NULL ;

       m->_Responsible  = NULL ;

       m->_SpinDuration = Knob_SpinLimit ;       // consider: keep metastats by type/class

       if (Atomic::cmpxchg_ptr (markOopDesc::encode(m), object->mark_addr(), mark) != mark) {

           m->set_object (NULL) ;

           m->set_owner  (NULL) ;

           m->OwnerIsThread = 0 ;

           m->Recycle() ;

           omRelease (Self, m) ;

           m = NULL ;

           continue ;

           // interference - the markword changed - just retry.

           // The state-transitions are one-way, so there's no chance of

           // live-lock -- "Inflated" is an absorbing state.

       }

       // Hopefully the performance counters are allocated on distinct

       // cache lines to avoid false sharing on MP systems ...

       if (_sync_Inflations != NULL) _sync_Inflations->inc() ;

       TEVENT(Inflate: overwrite neutral) ;

       if (TraceMonitorInflation) {

         if (object->is_instance()) {

           ResourceMark rm;

           tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",

             (intptr_t) object, (intptr_t) object->mark(),

             Klass::cast(object->klass())->external_name());

         }

       }

       return m ;

   }

 }

 // This the fast monitor enter. The interpreter and compiler use

 // some assembly copies of this code. Make sure update those code

 // if the following function is changed. The implementation is

 // extremely sensitive to race condition. Be careful.

 void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock, bool attempt_rebias, TRAPS) {

  if (UseBiasedLocking) {

     if (!SafepointSynchronize::is_at_safepoint()) {

       BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD);

       if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) {

         return;

       }

     } else {

       assert(!attempt_rebias, "can not rebias toward VM thread");

       BiasedLocking::revoke_at_safepoint(obj);

     }

     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");

  }

  slow_enter (obj, lock, THREAD) ;

 }

 void ObjectSynchronizer::fast_exit(oop object, BasicLock* lock, TRAPS) {

   assert(!object->mark()->has_bias_pattern(), "should not see bias pattern here");

   // if displaced header is null, the previous enter is recursive enter, no-op

   markOop dhw = lock->displaced_header();

   markOop mark ;

   if (dhw == NULL) {

      // Recursive stack-lock.

      // Diagnostics -- Could be: stack-locked, inflating, inflated.

      mark = object->mark() ;

      assert (!mark->is_neutral(), "invariant") ;

      if (mark->has_locker() && mark != markOopDesc::INFLATING()) {

         assert(THREAD->is_lock_owned((address)mark->locker()), "invariant") ;

      }

      if (mark->has_monitor()) {

         ObjectMonitor * m = mark->monitor() ;

         assert(((oop)(m->object()))->mark() == mark, "invariant") ;

         assert(m->is_entered(THREAD), "invariant") ;

      }

      return ;

   }

   mark = object->mark() ;

   // If the object is stack-locked by the current thread, try to

   // swing the displaced header from the box back to the mark.

   if (mark == (markOop) lock) {

      assert (dhw->is_neutral(), "invariant") ;

      if ((markOop) Atomic::cmpxchg_ptr (dhw, object->mark_addr(), mark) == mark) {

         TEVENT (fast_exit: release stacklock) ;

         return;

      }

   }

   ObjectSynchronizer::inflate(THREAD, object)->exit (THREAD) ;

 }

 // This routine is used to handle interpreter/compiler slow case

 // We don't need to use fast path here, because it must have been

 // failed in the interpreter/compiler code.

 void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) {

   markOop mark = obj->mark();

   assert(!mark->has_bias_pattern(), "should not see bias pattern here");

   if (mark->is_neutral()) {

     // Anticipate successful CAS -- the ST of the displaced mark must

     // be visible <= the ST performed by the CAS.

     lock->set_displaced_header(mark);

     if (mark == (markOop) Atomic::cmpxchg_ptr(lock, obj()->mark_addr(), mark)) {

       TEVENT (slow_enter: release stacklock) ;

       return ;

     }

     // Fall through to inflate() ...

   } else

   if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {

     assert(lock != mark->locker(), "must not re-lock the same lock");

     assert(lock != (BasicLock*)obj->mark(), "don't relock with same BasicLock");

     lock->set_displaced_header(NULL);

     return;

   }

 #if 0

   // The following optimization isn't particularly useful.

   if (mark->has_monitor() && mark->monitor()->is_entered(THREAD)) {

     lock->set_displaced_header (NULL) ;

     return ;

   }

 #endif

   // The object header will never be displaced to this lock,

   // so it does not matter what the value is, except that it

   // must be non-zero to avoid looking like a re-entrant lock,

   // and must not look locked either.

   lock->set_displaced_header(markOopDesc::unused_mark());

   ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);

 }

 // This routine is used to handle interpreter/compiler slow case

 // We don't need to use fast path here, because it must have

 // failed in the interpreter/compiler code. Simply use the heavy

 // weight monitor should be ok, unless someone find otherwise.

 void ObjectSynchronizer::slow_exit(oop object, BasicLock* lock, TRAPS) {

   fast_exit (object, lock, THREAD) ;

 }

 // NOTE: must use heavy weight monitor to handle jni monitor enter

 void ObjectSynchronizer::jni_enter(Handle obj, TRAPS) { // possible entry from jni enter

   // the current locking is from JNI instead of Java code

   TEVENT (jni_enter) ;

   if (UseBiasedLocking) {

     BiasedLocking::revoke_and_rebias(obj, false, THREAD);

     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");

   }

   THREAD->set_current_pending_monitor_is_from_java(false);

   ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);

   THREAD->set_current_pending_monitor_is_from_java(true);

 }

 // NOTE: must use heavy weight monitor to handle jni monitor enter

 bool ObjectSynchronizer::jni_try_enter(Handle obj, Thread* THREAD) {

   if (UseBiasedLocking) {

     BiasedLocking::revoke_and_rebias(obj, false, THREAD);

     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");

   }

   ObjectMonitor* monitor = ObjectSynchronizer::inflate_helper(obj());

   return monitor->try_enter(THREAD);

 }

 // NOTE: must use heavy weight monitor to handle jni monitor exit

 void ObjectSynchronizer::jni_exit(oop obj, Thread* THREAD) {

   TEVENT (jni_exit) ;

   if (UseBiasedLocking) {

     BiasedLocking::revoke_and_rebias(obj, false, THREAD);

   }

   assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");

   ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj);

   // If this thread has locked the object, exit the monitor.  Note:  can't use

   // monitor->check(CHECK); must exit even if an exception is pending.

   if (monitor->check(THREAD)) {

      monitor->exit(THREAD);

   }

 }

 // complete_exit()/reenter() are used to wait on a nested lock

 // i.e. to give up an outer lock completely and then re-enter

 // Used when holding nested locks - lock acquisition order: lock1 then lock2

 //  1) complete_exit lock1 - saving recursion count

 //  2) wait on lock2

 //  3) when notified on lock2, unlock lock2

 //  4) reenter lock1 with original recursion count

 //  5) lock lock2

 // NOTE: must use heavy weight monitor to handle complete_exit/reenter()

 intptr_t ObjectSynchronizer::complete_exit(Handle obj, TRAPS) {

   TEVENT (complete_exit) ;

   if (UseBiasedLocking) {

     BiasedLocking::revoke_and_rebias(obj, false, THREAD);

     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");

   }

   ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());

   return monitor->complete_exit(THREAD);

 }

 // NOTE: must use heavy weight monitor to handle complete_exit/reenter()

 void ObjectSynchronizer::reenter(Handle obj, intptr_t recursion, TRAPS) {

   TEVENT (reenter) ;

   if (UseBiasedLocking) {

     BiasedLocking::revoke_and_rebias(obj, false, THREAD);

     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");

   }

   ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());

   monitor->reenter(recursion, THREAD);

 }

 // This exists only as a workaround of dtrace bug 6254741

 int dtrace_waited_probe(ObjectMonitor* monitor, Handle obj, Thread* thr) {

   DTRACE_MONITOR_PROBE(waited, monitor, obj(), thr);

   return 0;

 }

 // NOTE: must use heavy weight monitor to handle wait()

 void ObjectSynchronizer::wait(Handle obj, jlong millis, TRAPS) {

   if (UseBiasedLocking) {

     BiasedLocking::revoke_and_rebias(obj, false, THREAD);

     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");

   }

   if (millis < 0) {

     TEVENT (wait - throw IAX) ;

     THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");

   }

   ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());

   DTRACE_MONITOR_WAIT_PROBE(monitor, obj(), THREAD, millis);

   monitor->wait(millis, true, THREAD);

   /* This dummy call is in place to get around dtrace bug 6254741.  Once

      that's fixed we can uncomment the following line and remove the call */

   // DTRACE_MONITOR_PROBE(waited, monitor, obj(), THREAD);

   dtrace_waited_probe(monitor, obj, THREAD);

 }

 void ObjectSynchronizer::waitUninterruptibly (Handle obj, jlong millis, TRAPS) {

   if (UseBiasedLocking) {

     BiasedLocking::revoke_and_rebias(obj, false, THREAD);

     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");

   }

   if (millis < 0) {

     TEVENT (wait - throw IAX) ;

     THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");

   }

   ObjectSynchronizer::inflate(THREAD, obj()) -> wait(millis, false, THREAD) ;

 }

 void ObjectSynchronizer::notify(Handle obj, TRAPS) {

  if (UseBiasedLocking) {

     BiasedLocking::revoke_and_rebias(obj, false, THREAD);

     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");

   }

   markOop mark = obj->mark();

   if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {

     return;

   }

   ObjectSynchronizer::inflate(THREAD, obj())->notify(THREAD);

 }

 // NOTE: see comment of notify()

 void ObjectSynchronizer::notifyall(Handle obj, TRAPS) {

   if (UseBiasedLocking) {

     BiasedLocking::revoke_and_rebias(obj, false, THREAD);

     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");

   }

   markOop mark = obj->mark();

   if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {

     return;

   }

   ObjectSynchronizer::inflate(THREAD, obj())->notifyAll(THREAD);

 }

 intptr_t ObjectSynchronizer::FastHashCode (Thread * Self, oop obj) {

   if (UseBiasedLocking) {

     // NOTE: many places throughout the JVM do not expect a safepoint

     // to be taken here, in particular most operations on perm gen

     // objects. However, we only ever bias Java instances and all of

     // the call sites of identity_hash that might revoke biases have

     // been checked to make sure they can handle a safepoint. The

     // added check of the bias pattern is to avoid useless calls to

     // thread-local storage.

     if (obj->mark()->has_bias_pattern()) {

       // Box and unbox the raw reference just in case we cause a STW safepoint.

       Handle hobj (Self, obj) ;

       // Relaxing assertion for bug 6320749.

       assert (Universe::verify_in_progress() ||

               !SafepointSynchronize::is_at_safepoint(),

              "biases should not be seen by VM thread here");

       BiasedLocking::revoke_and_rebias(hobj, false, JavaThread::current());

       obj = hobj() ;

       assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");

     }

   }

   // hashCode() is a heap mutator ...

   // Relaxing assertion for bug 6320749.

   assert (Universe::verify_in_progress() ||

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

   assert (Universe::verify_in_progress() ||

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

   assert (Universe::verify_in_progress() ||

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

   ObjectMonitor* monitor = NULL;

   markOop temp, test;

   intptr_t hash;

   markOop mark = ReadStableMark (obj);

   // object should remain ineligible for biased locking

   assert (!mark->has_bias_pattern(), "invariant") ;

   if (mark->is_neutral()) {

     hash = mark->hash();              // this is a normal header

     if (hash) {                       // if it has hash, just return it

       return hash;

     }

     hash = get_next_hash(Self, obj);  // allocate a new hash code

     temp = mark->copy_set_hash(hash); // merge the hash code into header

     // use (machine word version) atomic operation to install the hash

     test = (markOop) Atomic::cmpxchg_ptr(temp, obj->mark_addr(), mark);

     if (test == mark) {

       return hash;

     }

     // If atomic operation failed, we must inflate the header

     // into heavy weight monitor. We could add more code here

     // for fast path, but it does not worth the complexity.

   } else if (mark->has_monitor()) {

     monitor = mark->monitor();

     temp = monitor->header();

     assert (temp->is_neutral(), "invariant") ;

     hash = temp->hash();

     if (hash) {

       return hash;

     }

     // Skip to the following code to reduce code size

   } else if (Self->is_lock_owned((address)mark->locker())) {

     temp = mark->displaced_mark_helper(); // this is a lightweight monitor owned

     assert (temp->is_neutral(), "invariant") ;

     hash = temp->hash();              // by current thread, check if the displaced

     if (hash) {                       // header contains hash code

       return hash;

     }

     // WARNING:

     //   The displaced header is strictly immutable.

     // It can NOT be changed in ANY cases. So we have

     // to inflate the header into heavyweight monitor

     // even the current thread owns the lock. The reason

     // is the BasicLock (stack slot) will be asynchronously

     // read by other threads during the inflate() function.

     // Any change to stack may not propagate to other threads

     // correctly.

   }

   // Inflate the monitor to set hash code

   monitor = ObjectSynchronizer::inflate(Self, obj);

   // Load displaced header and check it has hash code

   mark = monitor->header();

   assert (mark->is_neutral(), "invariant") ;

   hash = mark->hash();

   if (hash == 0) {

     hash = get_next_hash(Self, obj);

     temp = mark->copy_set_hash(hash); // merge hash code into header

     assert (temp->is_neutral(), "invariant") ;

     test = (markOop) Atomic::cmpxchg_ptr(temp, monitor, mark);

     if (test != mark) {

       // The only update to the header in the monitor (outside GC)

       // is install the hash code. If someone add new usage of

       // displaced header, please update this code

       hash = test->hash();

       assert (test->is_neutral(), "invariant") ;

       assert (hash != 0, "Trivial unexpected object/monitor header usage.");

     }

   }

   // We finally get the hash

   return hash;

 }

 // Deprecated -- use FastHashCode() instead.

 intptr_t ObjectSynchronizer::identity_hash_value_for(Handle obj) {

   return FastHashCode (Thread::current(), obj()) ;

 }

 bool ObjectSynchronizer::current_thread_holds_lock(JavaThread* thread,

                                                    Handle h_obj) {

   if (UseBiasedLocking) {

     BiasedLocking::revoke_and_rebias(h_obj, false, thread);

     assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");

   }

   assert(thread == JavaThread::current(), "Can only be called on current thread");

   oop obj = h_obj();

   markOop mark = ReadStableMark (obj) ;

   // Uncontended case, header points to stack

   if (mark->has_locker()) {

     return thread->is_lock_owned((address)mark->locker());

   }

   // Contended case, header points to ObjectMonitor (tagged pointer)

   if (mark->has_monitor()) {

     ObjectMonitor* monitor = mark->monitor();

     return monitor->is_entered(thread) != 0 ;

   }

   // Unlocked case, header in place

   assert(mark->is_neutral(), "sanity check");

   return false;

 }

 // Be aware of this method could revoke bias of the lock object.

 // This method querys the ownership of the lock handle specified by 'h_obj'.

 // If the current thread owns the lock, it returns owner_self. If no

 // thread owns the lock, it returns owner_none. Otherwise, it will return

 // ower_other.

 ObjectSynchronizer::LockOwnership ObjectSynchronizer::query_lock_ownership

 (JavaThread *self, Handle h_obj) {

   // The caller must beware this method can revoke bias, and

   // revocation can result in a safepoint.

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

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

   // Possible mark states: neutral, biased, stack-locked, inflated

   if (UseBiasedLocking && h_obj()->mark()->has_bias_pattern()) {

     // CASE: biased

     BiasedLocking::revoke_and_rebias(h_obj, false, self);

     assert(!h_obj->mark()->has_bias_pattern(),

            "biases should be revoked by now");

   }

   assert(self == JavaThread::current(), "Can only be called on current thread");

   oop obj = h_obj();

   markOop mark = ReadStableMark (obj) ;

   // CASE: stack-locked.  Mark points to a BasicLock on the owner's stack.

   if (mark->has_locker()) {

     return self->is_lock_owned((address)mark->locker()) ?

       owner_self : owner_other;

   }

   // CASE: inflated. Mark (tagged pointer) points to an objectMonitor.

   // The Object:ObjectMonitor relationship is stable as long as we're

   // not at a safepoint.

   if (mark->has_monitor()) {

     void * owner = mark->monitor()->_owner ;

     if (owner == NULL) return owner_none ;

     return (owner == self ||

             self->is_lock_owned((address)owner)) ? owner_self : owner_other;

   }

   // CASE: neutral

   assert(mark->is_neutral(), "sanity check");

   return owner_none ;           // it's unlocked

 }

 // FIXME: jvmti should call this

 JavaThread* ObjectSynchronizer::get_lock_owner(Handle h_obj, bool doLock) {

   if (UseBiasedLocking) {

     if (SafepointSynchronize::is_at_safepoint()) {

       BiasedLocking::revoke_at_safepoint(h_obj);

     } else {

       BiasedLocking::revoke_and_rebias(h_obj, false, JavaThread::current());

     }

     assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");

   }

   oop obj = h_obj();

   address owner = NULL;

   markOop mark = ReadStableMark (obj) ;

   // Uncontended case, header points to stack

   if (mark->has_locker()) {

     owner = (address) mark->locker();

   }

   // Contended case, header points to ObjectMonitor (tagged pointer)

   if (mark->has_monitor()) {

     ObjectMonitor* monitor = mark->monitor();

     assert(monitor != NULL, "monitor should be non-null");

     owner = (address) monitor->owner();

   }

   if (owner != NULL) {

     return Threads::owning_thread_from_monitor_owner(owner, doLock);

   }

   // Unlocked case, header in place

   // Cannot have assertion since this object may have been

   // locked by another thread when reaching here.

   // assert(mark->is_neutral(), "sanity check");

   return NULL;

 }

 // Iterate through monitor cache and attempt to release thread's monitors

 // Gives up on a particular monitor if an exception occurs, but continues

 // the overall iteration, swallowing the exception.

 class ReleaseJavaMonitorsClosure: public MonitorClosure {

 private:

   TRAPS;

 public:

   ReleaseJavaMonitorsClosure(Thread* thread) : THREAD(thread) {}

   void do_monitor(ObjectMonitor* mid) {

     if (mid->owner() == THREAD) {

       (void)mid->complete_exit(CHECK);

     }

   }

 };

 // Release all inflated monitors owned by THREAD.  Lightweight monitors are

 // ignored.  This is meant to be called during JNI thread detach which assumes

 // all remaining monitors are heavyweight.  All exceptions are swallowed.

 // Scanning the extant monitor list can be time consuming.

 // A simple optimization is to add a per-thread flag that indicates a thread

 // called jni_monitorenter() during its lifetime.

 //

 // Instead of No_Savepoint_Verifier it might be cheaper to

 // use an idiom of the form:

 //   auto int tmp = SafepointSynchronize::_safepoint_counter ;

 //   <code that must not run at safepoint>

 //   guarantee (((tmp ^ _safepoint_counter) | (tmp & 1)) == 0) ;

 // Since the tests are extremely cheap we could leave them enabled

 // for normal product builds.

 void ObjectSynchronizer::release_monitors_owned_by_thread(TRAPS) {

   assert(THREAD == JavaThread::current(), "must be current Java thread");

   No_Safepoint_Verifier nsv ;

   ReleaseJavaMonitorsClosure rjmc(THREAD);

   Thread::muxAcquire(&ListLock, "release_monitors_owned_by_thread");

   ObjectSynchronizer::monitors_iterate(&rjmc);

   Thread::muxRelease(&ListLock);

   THREAD->clear_pending_exception();

 }

 // Visitors ...

 void ObjectSynchronizer::monitors_iterate(MonitorClosure* closure) {

   ObjectMonitor* block = gBlockList;

   ObjectMonitor* mid;

   while (block) {

     assert(block->object() == CHAINMARKER, "must be a block header");

     for (int i = _BLOCKSIZE - 1; i > 0; i--) {

       mid = block + i;

       oop object = (oop) mid->object();

       if (object != NULL) {

         closure->do_monitor(mid);

       }

     }

     block = (ObjectMonitor*) block->FreeNext;

   }

 }

 void ObjectSynchronizer::oops_do(OopClosure* f) {

   assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");

   for (ObjectMonitor* block = gBlockList; block != NULL; block = next(block)) {

     assert(block->object() == CHAINMARKER, "must be a block header");

     for (int i = 1; i < _BLOCKSIZE; i++) {

       ObjectMonitor* mid = &block[i];

       if (mid->object() != NULL) {

         f->do_oop((oop*)mid->object_addr());

       }

     }

   }

 }

 // Deflate_idle_monitors() is called at all safepoints, immediately

 // after all mutators are stopped, but before any objects have moved.

 // It traverses the list of known monitors, deflating where possible.

 // The scavenged monitor are returned to the monitor free list.

 //

 // Beware that we scavenge at *every* stop-the-world point.

 // Having a large number of monitors in-circulation negatively

 // impacts the performance of some applications (e.g., PointBase).

 // Broadly, we want to minimize the # of monitors in circulation.

 // Alternately, we could partition the active monitors into sub-lists

 // of those that need scanning and those that do not.

 // Specifically, we would add a new sub-list of objectmonitors

 // that are in-circulation and potentially active.  deflate_idle_monitors()

 // would scan only that list.  Other monitors could reside on a quiescent

 // list.  Such sequestered monitors wouldn't need to be scanned by

 // deflate_idle_monitors().  omAlloc() would first check the global free list,

 // then the quiescent list, and, failing those, would allocate a new block.

 // Deflate_idle_monitors() would scavenge and move monitors to the

 // quiescent list.

 //

 // Perversely, the heap size -- and thus the STW safepoint rate --

 // typically drives the scavenge rate.  Large heaps can mean infrequent GC,

 // which in turn can mean large(r) numbers of objectmonitors in circulation.

 // This is an unfortunate aspect of this design.

 //

 // Another refinement would be to refrain from calling deflate_idle_monitors()

 // except at stop-the-world points associated with garbage collections.

 //

 // An even better solution would be to deflate on-the-fly, aggressively,

 // at monitorexit-time as is done in EVM's metalock or Relaxed Locks.

 void ObjectSynchronizer::deflate_idle_monitors() {

   assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");

   int nInuse = 0 ;              // currently associated with objects

   int nInCirculation = 0 ;      // extant

   int nScavenged = 0 ;          // reclaimed

   ObjectMonitor * FreeHead = NULL ;  // Local SLL of scavenged monitors

   ObjectMonitor * FreeTail = NULL ;

   // Iterate over all extant monitors - Scavenge all idle monitors.

   TEVENT (deflate_idle_monitors) ;

   for (ObjectMonitor* block = gBlockList; block != NULL; block = next(block)) {

     assert(block->object() == CHAINMARKER, "must be a block header");

     nInCirculation += _BLOCKSIZE ;

     for (int i = 1 ; i < _BLOCKSIZE; i++) {

       ObjectMonitor* mid = &block[i];

       oop obj = (oop) mid->object();

       if (obj == NULL) {

         // The monitor is not associated with an object.

         // The monitor should either be a thread-specific private

         // free list or the global free list.

         // obj == NULL IMPLIES mid->is_busy() == 0

         guarantee (!mid->is_busy(), "invariant") ;

         continue ;

       }

       // Normal case ... The monitor is associated with obj.

       guarantee (obj->mark() == markOopDesc::encode(mid), "invariant") ;

       guarantee (mid == obj->mark()->monitor(), "invariant");

       guarantee (mid->header()->is_neutral(), "invariant");

       if (mid->is_busy()) {

          if (ClearResponsibleAtSTW) mid->_Responsible = NULL ;

          nInuse ++ ;

       } else {

          // Deflate the monitor if it is no longer being used

          // It's idle - scavenge and return to the global free list

          // plain old deflation ...

          TEVENT (deflate_idle_monitors - scavenge1) ;

          if (TraceMonitorInflation) {

            if (obj->is_instance()) {

              ResourceMark rm;

                tty->print_cr("Deflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",

                     (intptr_t) obj, (intptr_t) obj->mark(), Klass::cast(obj->klass())->external_name());

            }

          }

          // Restore the header back to obj

          obj->release_set_mark(mid->header());

          mid->clear();

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

          // Move the object to the working free list defined by FreeHead,FreeTail.

          mid->FreeNext = NULL ;

          if (FreeHead == NULL) FreeHead = mid ;

          if (FreeTail != NULL) FreeTail->FreeNext = mid ;

          FreeTail = mid ;

          nScavenged ++ ;

       }

     }

   }

   // Move the scavenged monitors back to the global free list.

   // In theory we don't need the freelist lock as we're at a STW safepoint.

   // omAlloc() and omFree() can only be called while a thread is _not in safepoint state.

   // But it's remotely possible that omFlush() or release_monitors_owned_by_thread()

   // might be called while not at a global STW safepoint.  In the interest of

   // safety we protect the following access with ListLock.

   // An even more conservative and prudent approach would be to guard

   // the main loop in scavenge_idle_monitors() with ListLock.

   if (FreeHead != NULL) {

      guarantee (FreeTail != NULL && nScavenged > 0, "invariant") ;

      assert (FreeTail->FreeNext == NULL, "invariant") ;

      // constant-time list splice - prepend scavenged segment to gFreeList

      Thread::muxAcquire (&ListLock, "scavenge - return") ;

      FreeTail->FreeNext = gFreeList ;

      gFreeList = FreeHead ;

      Thread::muxRelease (&ListLock) ;

   }

   if (_sync_Deflations != NULL) _sync_Deflations->inc(nScavenged) ;

   if (_sync_MonExtant  != NULL) _sync_MonExtant ->set_value(nInCirculation);

   // TODO: Add objectMonitor leak detection.

   // Audit/inventory the objectMonitors -- make sure they're all accounted for.

   GVars.stwRandom = os::random() ;

   GVars.stwCycle ++ ;

 }

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

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

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

 // call it with a CHECK argument).

 #define CHECK_OWNER()                                                             \

   do {                                                                            \

     if (THREAD != _owner) {                                                       \

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

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

         _recursions = 0;                                                          \

         OwnerIsThread = 1 ;                                                       \

       } else {                                                                    \

         TEVENT (Throw IMSX) ;                                                     \

         THROW(vmSymbols::java_lang_IllegalMonitorStateException());               \

       }                                                                           \

     }                                                                             \

   } while (false)

 // TODO-FIXME: eliminate ObjectWaiters.  Replace this visitor/enumerator

 // interface with a simple FirstWaitingThread(), NextWaitingThread() interface.

 ObjectWaiter* ObjectMonitor::first_waiter() {

   return _WaitSet;

 }

 ObjectWaiter* ObjectMonitor::next_waiter(ObjectWaiter* o) {

   return o->_next;

 }

 Thread* ObjectMonitor::thread_of_waiter(ObjectWaiter* o) {

   return o->_thread;

 }

 // initialize the monitor, exception the semaphore, all other fields

 // are simple integers or pointers

 ObjectMonitor::ObjectMonitor() {

   _header       = NULL;

   _count        = 0;

   _waiters      = 0,

   _recursions   = 0;

   _object       = NULL;

   _owner        = NULL;

   _WaitSet      = NULL;

   _WaitSetLock  = 0 ;

   _Responsible  = NULL ;

   _succ         = NULL ;

   _cxq          = NULL ;

   FreeNext      = NULL ;

   _EntryList    = NULL ;

   _SpinFreq     = 0 ;

   _SpinClock    = 0 ;

   OwnerIsThread = 0 ;

 }

 ObjectMonitor::~ObjectMonitor() {

    // TODO: Add asserts ...

    // _cxq == 0 _succ == NULL _owner == NULL _waiters == 0

    // _count == 0 _EntryList  == NULL etc

 }

 intptr_t ObjectMonitor::is_busy() const {

   // TODO-FIXME: merge _count and _waiters.

   // TODO-FIXME: assert _owner == null implies _recursions = 0

   // TODO-FIXME: assert _WaitSet != null implies _count > 0

   return _count|_waiters|intptr_t(_owner)|intptr_t(_cxq)|intptr_t(_EntryList ) ;

 }

 void ObjectMonitor::Recycle () {

   // TODO: add stronger asserts ...

   // _cxq == 0 _succ == NULL _owner == NULL _waiters == 0

   // _count == 0 EntryList  == NULL

   // _recursions == 0 _WaitSet == NULL

   // TODO: assert (is_busy()|_recursions) == 0

   _succ          = NULL ;

   _EntryList     = NULL ;

   _cxq           = NULL ;

   _WaitSet       = NULL ;

   _recursions    = 0 ;

   _SpinFreq      = 0 ;

   _SpinClock     = 0 ;

   OwnerIsThread  = 0 ;

 }

 // WaitSet management ...

 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {

   assert(node != NULL, "should not dequeue NULL node");

   assert(node->_prev == NULL, "node already in list");

   assert(node->_next == NULL, "node already in list");

   // put node at end of queue (circular doubly linked list)

   if (_WaitSet == NULL) {

     _WaitSet = node;

     node->_prev = node;

     node->_next = node;

   } else {

     ObjectWaiter* head = _WaitSet ;

     ObjectWaiter* tail = head->_prev;

     assert(tail->_next == head, "invariant check");

     tail->_next = node;

     head->_prev = node;

     node->_next = head;

     node->_prev = tail;

   }

 }

 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {

   // dequeue the very first waiter

   ObjectWaiter* waiter = _WaitSet;

   if (waiter) {

     DequeueSpecificWaiter(waiter);

   }

   return waiter;

 }

 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {

   assert(node != NULL, "should not dequeue NULL node");

   assert(node->_prev != NULL, "node already removed from list");

   assert(node->_next != NULL, "node already removed from list");

   // when the waiter has woken up because of interrupt,

   // timeout or other spurious wake-up, dequeue the

   // waiter from waiting list

   ObjectWaiter* next = node->_next;

   if (next == node) {

     assert(node->_prev == node, "invariant check");

     _WaitSet = NULL;

   } else {

     ObjectWaiter* prev = node->_prev;

     assert(prev->_next == node, "invariant check");

     assert(next->_prev == node, "invariant check");

     next->_prev = prev;

     prev->_next = next;

     if (_WaitSet == node) {

       _WaitSet = next;

     }

   }

   node->_next = NULL;

   node->_prev = NULL;

 }

 static char * kvGet (char * kvList, const char * Key) {

     if (kvList == NULL) return NULL ;

     size_t n = strlen (Key) ;

     char * Search ;

     for (Search = kvList ; *Search ; Search += strlen(Search) + 1) {

         if (strncmp (Search, Key, n) == 0) {

             if (Search[n] == '=') return Search + n + 1 ;

             if (Search[n] == 0)   return (char *) "1" ;

         }

     }

     return NULL ;

 }

 static int kvGetInt (char * kvList, const char * Key, int Default) {

     char * v = kvGet (kvList, Key) ;

     int rslt = v ? ::strtol (v, NULL, 0) : Default ;

     if (Knob_ReportSettings && v != NULL) {

         ::printf ("  SyncKnob: %s %d(%d)\n", Key, rslt, Default) ;

         ::fflush (stdout) ;

     }

     return rslt ;

 }

 // By convention we unlink a contending thread from EntryList|cxq immediately

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

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

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

 {

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

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

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

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

         // This is a constant-time operation.

         ObjectWaiter * nxt = SelfNode->_next ;

         ObjectWaiter * prv = SelfNode->_prev ;

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

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

         if (SelfNode == _EntryList ) _EntryList = nxt ;

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

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

         TEVENT (Unlink from EntryList) ;

     } else {

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

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

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

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

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

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

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

         // in constant-time.

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

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

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

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

         // so it might as well be now.

         ObjectWaiter * v = _cxq ;

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

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

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

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

             // at the head of cxq.

             if (v == SelfNode) {

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

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

             }

             ObjectWaiter * p ;

             ObjectWaiter * q = NULL ;

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

                 q = p ;

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

             }

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

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

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

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

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

             q->_next = p->_next ;

         }

         TEVENT (Unlink from cxq) ;

     }

     // Diagnostic hygiene ...

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

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

     SelfNode->TState = ObjectWaiter::TS_RUN ;

 }

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

 // Callers must compensate as needed.

 int ObjectMonitor::TryLock (Thread * Self) {

    for (;;) {

       void * own = _owner ;

       if (own != NULL) return 0 ;

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

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

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

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

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

          return 1 ;

       }

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

       // Interference -- the CAS failed.

       // We can either return -1 or retry.

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

       if (true) return -1 ;

    }

 }

 // NotRunnable() -- informed spinning

 //

 // Don't bother spinning if the owner is not eligible to drop the lock.

 // Peek at the owner's schedctl.sc_state and Thread._thread_values and

 // spin only if the owner thread is _thread_in_Java or _thread_in_vm.

 // The thread must be runnable in order to drop the lock in timely fashion.

 // If the _owner is not runnable then spinning will not likely be

 // successful (profitable).

 //

 // Beware -- the thread referenced by _owner could have died

 // so a simply fetch from _owner->_thread_state might trap.

 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.

 // Because of the lifecycle issues the schedctl and _thread_state values

 // observed by NotRunnable() might be garbage.  NotRunnable must

 // tolerate this and consider the observed _thread_state value

 // as advisory.

 //

 // Beware too, that _owner is sometimes a BasicLock address and sometimes

 // a thread pointer.  We differentiate the two cases with OwnerIsThread.

 // Alternately, we might tag the type (thread pointer vs basiclock pointer)

 // with the LSB of _owner.  Another option would be to probablistically probe

 // the putative _owner->TypeTag value.

 //

 // Checking _thread_state isn't perfect.  Even if the thread is

 // in_java it might be blocked on a page-fault or have been preempted

 // and sitting on a ready/dispatch queue.  _thread state in conjunction

 // with schedctl.sc_state gives us a good picture of what the

 // thread is doing, however.

 //

 // TODO: check schedctl.sc_state.

 // We'll need to use SafeFetch32() to read from the schedctl block.

 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/

 //

 // The return value from NotRunnable() is *advisory* -- the

 // result is based on sampling and is not necessarily coherent.

 // The caller must tolerate false-negative and false-positive errors.

 // Spinning, in general, is probabilistic anyway.

 int ObjectMonitor::NotRunnable (Thread * Self, Thread * ox) {

     // Check either OwnerIsThread or ox->TypeTag == 2BAD.

     if (!OwnerIsThread) return 0 ;

     if (ox == NULL) return 0 ;

     // Avoid transitive spinning ...

     // Say T1 spins or blocks trying to acquire L.  T1._Stalled is set to L.

     // Immediately after T1 acquires L it's possible that T2, also

     // spinning on L, will see L.Owner=T1 and T1._Stalled=L.

     // This occurs transiently after T1 acquired L but before

     // T1 managed to clear T1.Stalled.  T2 does not need to abort

     // its spin in this circumstance.

     intptr_t BlockedOn = SafeFetchN ((intptr_t *) &ox->_Stalled, intptr_t(1)) ;

     if (BlockedOn == 1) return 1 ;

     if (BlockedOn != 0) {

       return BlockedOn != intptr_t(this) && _owner == ox ;

     }

     assert (sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant") ;

     int jst = SafeFetch32 ((int *) &((JavaThread *) ox)->_thread_state, -1) ; ;

     // consider also: jst != _thread_in_Java -- but that's overspecific.

     return jst == _thread_blocked || jst == _thread_in_native ;

 }

 // Adaptive spin-then-block - rational spinning

 //

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

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

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

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

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

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

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

 // low latency and massive bandwidth.

 //

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

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

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

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

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

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

 //

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

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

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

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

 // attempts.  The mechanism adapts automatically to varying critical

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

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

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

 //

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

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

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

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

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

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

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

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

 //

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

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

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

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

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

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

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

 //

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

 // not spinning.

 //

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

 //

 // Spin-then-block strategies ...

 //

 // Thoughts on ways to improve spinning :

 //

 // *  Periodically call {psr_}getloadavg() while spinning, and

 //    permit unbounded spinning if the load average is <

 //    the number of processors.  Beware, however, that getloadavg()

 //    is exceptionally fast on solaris (about 1/10 the cost of a full

 //    spin cycle, but quite expensive on linux.  Beware also, that

 //    multiple JVMs could "ring" or oscillate in a feedback loop.

 //    Sufficient damping would solve that problem.

 //

 // *  We currently use spin loops with iteration counters to approximate

 //    spinning for some interval.  Given the availability of high-precision

 //    time sources such as gethrtime(), %TICK, %STICK, RDTSC, etc., we should

 //    someday reimplement the spin loops to duration-based instead of iteration-based.

 //

 // *  Don't spin if there are more than N = (CPUs/2) threads

 //        currently spinning on the monitor (or globally).

 //    That is, limit the number of concurrent spinners.

 //    We might also limit the # of spinners in the JVM, globally.

 //

 // *  If a spinning thread observes _owner change hands it should

 //    abort the spin (and park immediately) or at least debit

 //    the spin counter by a large "penalty".

 //

 // *  Classically, the spin count is either K*(CPUs-1) or is a

 //        simple constant that approximates the length of a context switch.

 //    We currently use a value -- computed by a special utility -- that

 //    approximates round-trip context switch times.

 //

 // *  Normally schedctl_start()/_stop() is used to advise the kernel

 //    to avoid preempting threads that are running in short, bounded

 //    critical sections.  We could use the schedctl hooks in an inverted

 //    sense -- spinners would set the nopreempt flag, but poll the preempt

 //    pending flag.  If a spinner observed a pending preemption it'd immediately

 //    abort the spin and park.   As such, the schedctl service acts as

 //    a preemption warning mechanism.

 //

 // *  In lieu of spinning, if the system is running below saturation

 //    (that is, loadavg() << #cpus), we can instead suppress futile

 //    wakeup throttling, or even wake more than one successor at exit-time.

 //    The net effect is largely equivalent to spinning.  In both cases,

 //    contending threads go ONPROC and opportunistically attempt to acquire

 //    the lock, decreasing lock handover latency at the expense of wasted

 //    cycles and context switching.

 //

 // *  We might to spin less after we've parked as the thread will

 //    have less $ and TLB affinity with the processor.

 //    Likewise, we might spin less if we come ONPROC on a different

 //    processor or after a long period (>> rechose_interval).

 //

 // *  A table-driven state machine similar to Solaris' dispadmin scheduling

 //    tables might be a better design.  Instead of encoding information in

 //    _SpinDuration, _SpinFreq and _SpinClock we'd just use explicit,

 //    discrete states.   Success or failure during a spin would drive

 //    state transitions, and each state node would contain a spin count.

 //

 // *  If the processor is operating in a mode intended to conserve power

 //    (such as Intel's SpeedStep) or to reduce thermal output (thermal

 //    step-down mode) then the Java synchronization subsystem should

 //    forgo spinning.

 //

 // *  The minimum spin duration should be approximately the worst-case

 //    store propagation latency on the platform.  That is, the time

 //    it takes a store on CPU A to become visible on CPU B, where A and

 //    B are "distant".

 //

 // *  We might want to factor a thread's priority in the spin policy.

 //    Threads with a higher priority might spin for slightly longer.

 //    Similarly, if we use back-off in the TATAS loop, lower priority

 //    threads might back-off longer.  We don't currently use a

 //    thread's priority when placing it on the entry queue.  We may

 //    want to consider doing so in future releases.

 //

 // *  We might transiently drop a thread's scheduling priority while it spins.

 //    SCHED_BATCH on linux and FX scheduling class at priority=0 on Solaris

 //    would suffice.  We could even consider letting the thread spin indefinitely at

 //    a depressed or "idle" priority.  This brings up fairness issues, however --

 //    in a saturated system a thread would with a reduced priority could languish

 //    for extended periods on the ready queue.

 //

 // *  While spinning try to use the otherwise wasted time to help the VM make

 //    progress:

 //

 //    -- YieldTo() the owner, if the owner is OFFPROC but ready

 //       Done our remaining quantum directly to the ready thread.

 //       This helps "push" the lock owner through the critical section.

 //       It also tends to improve affinity/locality as the lock

 //       "migrates" less frequently between CPUs.

 //    -- Walk our own stack in anticipation of blocking.  Memoize the roots.

 //    -- Perform strand checking for other thread.  Unpark potential strandees.

 //    -- Help GC: trace or mark -- this would need to be a bounded unit of work.

 //       Unfortunately this will pollute our $ and TLBs.  Recall that we

 //       spin to avoid context switching -- context switching has an

 //       immediate cost in latency, a disruptive cost to other strands on a CMT

 //       processor, and an amortized cost because of the D$ and TLB cache

 //       reload transient when the thread comes back ONPROC and repopulates

 //       $s and TLBs.

 //    -- call getloadavg() to see if the system is saturated.  It'd probably

 //       make sense to call getloadavg() half way through the spin.

 //       If the system isn't at full capacity the we'd simply reset

 //       the spin counter to and extend the spin attempt.

 //    -- Doug points out that we should use the same "helping" policy

 //       in thread.yield().

 //

 // *  Try MONITOR-MWAIT on systems that support those instructions.

 //

 // *  The spin statistics that drive spin decisions & frequency are

 //    maintained in the objectmonitor structure so if we deflate and reinflate

 //    we lose spin state.  In practice this is not usually a concern

 //    as the default spin state after inflation is aggressive (optimistic)

 //    and tends toward spinning.  So in the worst case for a lock where

 //    spinning is not profitable we may spin unnecessarily for a brief

 //    period.  But then again, if a lock is contended it'll tend not to deflate

 //    in the first place.

 intptr_t ObjectMonitor::SpinCallbackArgument = 0 ;

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

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

 int ObjectMonitor::TrySpin_VaryDuration (Thread * Self) {

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

     int ctr = Knob_FixedSpin ;

     if (ctr != 0) {

         while (--ctr >= 0) {

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

             SpinPause () ;

         }

         return 0 ;

     }

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

       if (TryLock(Self) > 0) {

         // Increase _SpinDuration ...

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

         // Raising _SpurDuration to the poverty line is key.

         int x = _SpinDuration ;

         if (x < Knob_SpinLimit) {

            if (x < Knob_Poverty) x = Knob_Poverty ;

            _SpinDuration = x + Knob_BonusB ;

         }

         return 1 ;

       }

       SpinPause () ;

     }

     // Admission control - verify preconditions for spinning

     //

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

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

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

     // modality changed.

     //

     // Consider the following alternative:

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

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

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

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

     // hold the duration constant but vary the frequency.

     ctr = _SpinDuration  ;

     if (ctr < Knob_SpinBase) ctr = Knob_SpinBase ;

     if (ctr <= 0) return 0 ;

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

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

        TEVENT (Spin abort - notrunnable [TOP]);

        return 0 ;

     }

     int MaxSpin = Knob_MaxSpinners ;

     if (MaxSpin >= 0) {

        if (_Spinner > MaxSpin) {

           TEVENT (Spin abort -- too many spinners) ;

           return 0 ;

        }

        // Slighty racy, but benign ...

        Adjust (&_Spinner, 1) ;

     }

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

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

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

     // to succeed.

     int hits    = 0 ;

     int msk     = 0 ;

     int caspty  = Knob_CASPenalty ;

     int oxpty   = Knob_OXPenalty ;

     int sss     = Knob_SpinSetSucc ;

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

     Thread * prv = NULL ;

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

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

     // 2.  Spin failure with prejudice

     // 3.  Spin failure without prejudice

     while (--ctr >= 0) {

       // Periodic polling -- Check for pending GC

       // Threads may spin while they're unsafe.

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

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

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

       // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)

       // this thread, if safe, doesn't steal cycles from GC.

       // This is in keeping with the "no loitering in runtime" rule.

       // We periodically check to see if there's a safepoint pending.

       if ((ctr & 0xFF) == 0) {

          if (SafepointSynchronize::do_call_back()) {

             TEVENT (Spin: safepoint) ;

             goto Abort ;           // abrupt spin egress

          }

          if (Knob_UsePause & 1) SpinPause () ;

          int (*scb)(intptr_t,int) = SpinCallbackFunction ;

          if (hits > 50 && scb != NULL) {

             int abend = (*scb)(SpinCallbackArgument, 0) ;

          }

       }

       if (Knob_UsePause & 2) SpinPause() ;

       // Exponential back-off ...  Stay off the bus to reduce coherency traffic.

       // This is useful on classic SMP systems, but is of less utility on

       // N1-style CMT platforms.

       //

       // Trade-off: lock acquisition latency vs coherency bandwidth.

       // Lock hold times are typically short.  A histogram

       // of successful spin attempts shows that we usually acquire

       // the lock early in the spin.  That suggests we want to

       // sample _owner frequently in the early phase of the spin,

       // but then back-off and sample less frequently as the spin

       // progresses.  The back-off makes a good citizen on SMP big

       // SMP systems.  Oversampling _owner can consume excessive

       // coherency bandwidth.  Relatedly, if we _oversample _owner we

       // can inadvertently interfere with the the ST m->owner=null.

       // executed by the lock owner.

       if (ctr & msk) continue ;

       ++hits ;

       if ((hits & 0xF) == 0) {

         // The 0xF, above, corresponds to the exponent.

         // Consider: (msk+1)|msk

         msk = ((msk << 2)|3) & BackOffMask ;

       }

       // Probe _owner with TATAS

       // If this thread observes the monitor transition or flicker

       // from locked to unlocked to locked, then the odds that this

       // thread will acquire the lock in this spin attempt go down

       // considerably.  The same argument applies if the CAS fails

       // or if we observe _owner change from one non-null value to

       // another non-null value.   In such cases we might abort

       // the spin without prejudice or apply a "penalty" to the

       // spin count-down variable "ctr", reducing it by 100, say.

       Thread * ox = (Thread *) _owner ;

       if (ox == NULL) {

          ox = (Thread *) Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;

          if (ox == NULL) {

             // The CAS succeeded -- this thread acquired ownership

             // Take care of some bookkeeping to exit spin state.

             if (sss && _succ == Self) {

                _succ = NULL ;

             }

             if (MaxSpin > 0) Adjust (&_Spinner, -1) ;

             // Increase _SpinDuration :

             // The spin was successful (profitable) so we tend toward

             // longer spin attempts in the future.

             // CONSIDER: factor "ctr" into the _SpinDuration adjustment.

             // If we acquired the lock early in the spin cycle it

             // makes sense to increase _SpinDuration proportionally.

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

             int x = _SpinDuration ;

             if (x < Knob_SpinLimit) {

                 if (x < Knob_Poverty) x = Knob_Poverty ;

                 _SpinDuration = x + Knob_Bonus ;

             }

             return 1 ;

          }

          // The CAS failed ... we can take any of the following actions:

          // * penalize: ctr -= Knob_CASPenalty

          // * exit spin with prejudice -- goto Abort;

          // * exit spin without prejudice.

          // * Since CAS is high-latency, retry again immediately.

          prv = ox ;

          TEVENT (Spin: cas failed) ;

          if (caspty == -2) break ;

          if (caspty == -1) goto Abort ;

          ctr -= caspty ;

          continue ;

       }

       // Did lock ownership change hands ?

       if (ox != prv && prv != NULL ) {

           TEVENT (spin: Owner changed)

           if (oxpty == -2) break ;

           if (oxpty == -1) goto Abort ;

           ctr -= oxpty ;

       }

       prv = ox ;

       // Abort the spin if the owner is not executing.

       // The owner must be executing in order to drop the lock.

       // Spinning while the owner is OFFPROC is idiocy.

       // Consider: ctr -= RunnablePenalty ;

       if (Knob_OState && NotRunnable (Self, ox)) {

          TEVENT (Spin abort - notrunnable);

          goto Abort ;

       }

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

    }

    // Spin failed with prejudice -- reduce _SpinDuration.

    // TODO: Use an AIMD-like policy to adjust _SpinDuration.

    // AIMD is globally stable.

    TEVENT (Spin failure) ;

    {

      int x = _SpinDuration ;

      if (x > 0) {

         // Consider an AIMD scheme like: x -= (x >> 3) + 100

         // This is globally sample and tends to damp the response.

         x -= Knob_Penalty ;

         if (x < 0) x = 0 ;

         _SpinDuration = x ;

      }

    }

  Abort:

    if (MaxSpin >= 0) Adjust (&_Spinner, -1) ;

    if (sss && _succ == Self) {

       _succ = NULL ;

       // Invariant: after setting succ=null a contending thread

       // must recheck-retry _owner before parking.  This usually happens

       // in the normal usage of TrySpin(), but it's safest

       // to make TrySpin() as foolproof as possible.

       OrderAccess::fence() ;

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

    }

    return 0 ;

 }

 #define TrySpin TrySpin_VaryDuration

 static void DeferredInitialize () {

   if (InitDone > 0) return ;

   if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {

       while (InitDone != 1) ;

       return ;

   }

   // One-shot global initialization ...

   // The initialization is idempotent, so we don't need locks.

   // In the future consider doing this via os::init_2().

   // SyncKnobs consist of <Key>=<Value> pairs in the style

   // of environment variables.  Start by converting ':' to NUL.

   if (SyncKnobs == NULL) SyncKnobs = "" ;

   size_t sz = strlen (SyncKnobs) ;

   char * knobs = (char *) malloc (sz + 2) ;

   if (knobs == NULL) {

      vm_exit_out_of_memory (sz + 2, "Parse SyncKnobs") ;

      guarantee (0, "invariant") ;

   }

   strcpy (knobs, SyncKnobs) ;

   knobs[sz+1] = 0 ;

   for (char * p = knobs ; *p ; p++) {

      if (*p == ':') *p = 0 ;

   }

   #define SETKNOB(x) { Knob_##x = kvGetInt (knobs, #x, Knob_##x); }

   SETKNOB(ReportSettings) ;

   SETKNOB(Verbose) ;

   SETKNOB(FixedSpin) ;

   SETKNOB(SpinLimit) ;

   SETKNOB(SpinBase) ;

   SETKNOB(SpinBackOff);

   SETKNOB(CASPenalty) ;

   SETKNOB(OXPenalty) ;

   SETKNOB(LogSpins) ;

   SETKNOB(SpinSetSucc) ;

   SETKNOB(SuccEnabled) ;

   SETKNOB(SuccRestrict) ;

   SETKNOB(Penalty) ;

   SETKNOB(Bonus) ;

   SETKNOB(BonusB) ;

   SETKNOB(Poverty) ;

   SETKNOB(SpinAfterFutile) ;

   SETKNOB(UsePause) ;

   SETKNOB(SpinEarly) ;

   SETKNOB(OState) ;

   SETKNOB(MaxSpinners) ;