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 /*

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

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

  *

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

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

  * published by the Free Software Foundation.

  *

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

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

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

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

  * accompanied this code).

  *

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

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

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

  *

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

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

  * questions.

  *

  */

 #include "precompiled.hpp"

 #include "classfile/vmSymbols.hpp"

 #include "memory/resourceArea.hpp"

 #include "oops/markOop.hpp"

 #include "oops/oop.inline.hpp"

 #include "runtime/biasedLocking.hpp"

 #include "runtime/handles.inline.hpp"

 #include "runtime/interfaceSupport.hpp"

 #include "runtime/mutexLocker.hpp"

 #include "runtime/objectMonitor.hpp"

 #include "runtime/objectMonitor.inline.hpp"

 #include "runtime/osThread.hpp"

 #include "runtime/stubRoutines.hpp"

 #include "runtime/synchronizer.hpp"

 #include "runtime/thread.inline.hpp"

 #include "utilities/dtrace.hpp"

 #include "utilities/events.hpp"

 #include "utilities/preserveException.hpp"

 #ifdef TARGET_OS_FAMILY_linux

 # include "os_linux.inline.hpp"

 #endif

 #ifdef TARGET_OS_FAMILY_solaris

 # include "os_solaris.inline.hpp"

 #endif

 #ifdef TARGET_OS_FAMILY_windows

 # include "os_windows.inline.hpp"

 #endif

 #ifdef TARGET_OS_FAMILY_bsd

 # include "os_bsd.inline.hpp"

 #endif

 #if defined(__GNUC__)

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

   #define ATTR __attribute__((noinline))

 #else

   #define ATTR

 #endif

 // 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.

 //

 //

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

 #ifdef DTRACE_ENABLED

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

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

 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread)                           \

   char* bytes = NULL;                                                      \

   int len = 0;                                                             \

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

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

   if (klassname != NULL) {                                                 \

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

     len = klassname->utf8_length();                                        \

   }

 #ifndef USDT2

 HS_DTRACE_PROBE_DECL5(hotspot, monitor__wait,

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

 HS_DTRACE_PROBE_DECL4(hotspot, monitor__waited,

   jlong, uintptr_t, char*, int);

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

   {                                                                        \

     if (DTraceMonitorProbes) {                                            \

       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                            \

       HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid,                       \

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

     }                                                                      \

   }

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

   {                                                                        \

     if (DTraceMonitorProbes) {                                            \

       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                            \

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

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

     }                                                                      \

   }

 #else /* USDT2 */

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

   {                                                                        \

     if (DTraceMonitorProbes) {                                            \

       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                            \

       HOTSPOT_MONITOR_WAIT(jtid,                                           \

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

     }                                                                      \

   }

 #define HOTSPOT_MONITOR_PROBE_waited HOTSPOT_MONITOR_PROBE_WAITED

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

   {                                                                        \

     if (DTraceMonitorProbes) {                                            \

       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                            \

       HOTSPOT_MONITOR_PROBE_##probe(jtid, /* probe = waited */             \

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

     }                                                                      \

   }

 #endif /* USDT2 */

 #else //  ndef DTRACE_ENABLED

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

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

 #endif // ndef DTRACE_ENABLED

 // 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;

 }

 #define NINFLATIONLOCKS 256

 static volatile intptr_t InflationLocks [NINFLATIONLOCKS] ;

 ObjectMonitor * ObjectSynchronizer::gBlockList = NULL ;

 ObjectMonitor * volatile ObjectSynchronizer::gFreeList  = NULL ;

 ObjectMonitor * volatile ObjectSynchronizer::gOmInUseList  = NULL ;

 int ObjectSynchronizer::gOmInUseCount = 0;

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

 static volatile int MonitorFreeCount  = 0 ;      // # on gFreeList

 static volatile int MonitorPopulation = 0 ;      // # Extant -- in circulation

 #define CHAINMARKER ((oop)-1)

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

 //  Fast Monitor Enter/Exit

 // 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) ;

 }

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

 // Interpreter/Compiler Slow Case

 // 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) ;

 }

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

 // Class Loader  support to workaround deadlocks on the class loader lock objects

 // Also used by GC

 // 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);

 }

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

 // JNI locks on java objects

 // 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) {

     Handle h_obj(THREAD, obj);

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

     obj = h_obj();

   }

   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);

   }

 }

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

 // Internal VM locks on java objects

 // 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);

   }

 }

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

 //  Wait/Notify/NotifyAll

 // 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);

 }

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

 // Hash Code handling

 //

 // 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.

 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 ;

 static int MonitorScavengeThreshold = 1000000 ;

 static volatile int ForceMonitorScavenge = 0 ; // Scavenge required and pending

 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

     }

   }

 }

 // 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;

 }

 //

 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) {

     // owning_thread_from_monitor_owner() may also return NULL here

     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;

 }

 // 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;

   }

 }

 // 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;

 }

 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());

       }

     }

   }

 }

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

 // 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.

 //

 // Constraining monitor pool growth via MonitorBound ...

 //

 // The monitor pool is grow-only.  We scavenge at STW safepoint-time, but the

 // the rate of scavenging is driven primarily by GC.  As such,  we can find

 // an inordinate number of monitors in circulation.

 // To avoid that scenario we can artificially induce a STW safepoint

 // if the pool appears to be growing past some reasonable bound.

 // Generally we favor time in space-time tradeoffs, but as there's no

 // natural back-pressure on the # of extant monitors we need to impose some

 // type of limit.  Beware that if MonitorBound is set to too low a value

 // we could just loop. In addition, if MonitorBound is set to a low value

 // we'll incur more safepoints, which are harmful to performance.

 // See also: GuaranteedSafepointInterval

 //

 // The current implementation uses asynchronous VM operations.

 //

 static void InduceScavenge (Thread * Self, const char * Whence) {

   // Induce STW safepoint to trim monitors

   // Ultimately, this results in a call to deflate_idle_monitors() in the near future.

   // More precisely, trigger an asynchronous STW safepoint as the number

   // of active monitors passes the specified threshold.

   // TODO: assert thread state is reasonable

   if (ForceMonitorScavenge == 0 && Atomic::xchg (1, &ForceMonitorScavenge) == 0) {

     if (ObjectMonitor::Knob_Verbose) {

       ::printf ("Monitor scavenge - Induced STW @%s (%d)\n", Whence, ForceMonitorScavenge) ;

       ::fflush(stdout) ;

     }

     // Induce a 'null' safepoint to scavenge monitors

     // Must VM_Operation instance be heap allocated as the op will be enqueue and posted

     // to the VMthread and have a lifespan longer than that of this activation record.

     // The VMThread will delete the op when completed.

     VMThread::execute (new VM_ForceAsyncSafepoint()) ;

     if (ObjectMonitor::Knob_Verbose) {

       ::printf ("Monitor scavenge - STW posted @%s (%d)\n", Whence, ForceMonitorScavenge) ;

       ::fflush(stdout) ;

     }

   }

 }

 /* Too slow for general assert or debug

 void ObjectSynchronizer::verifyInUse (Thread *Self) {

    ObjectMonitor* mid;

    int inusetally = 0;

    for (mid = Self->omInUseList; mid != NULL; mid = mid->FreeNext) {

      inusetally ++;

    }

    assert(inusetally == Self->omInUseCount, "inuse count off");

    int freetally = 0;

    for (mid = Self->omFreeList; mid != NULL; mid = mid->FreeNext) {

      freetally ++;

    }

    assert(freetally == Self->omFreeCount, "free count off");

 }

 */

 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") ;

            if (MonitorInUseLists) {

              m->FreeNext = Self->omInUseList;

              Self->omInUseList = m;

              Self->omInUseCount ++;

              // verifyInUse(Self);

            } else {

              m->FreeNext = NULL;

            }

            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; ) {

                 MonitorFreeCount --;

                 ObjectMonitor * take = gFreeList ;

                 gFreeList = take->FreeNext ;

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

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

                 take->Recycle() ;

                 omRelease (Self, take, false) ;

             }

             Thread::muxRelease (&ListLock) ;

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

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

             TEVENT (omFirst - reprovision) ;

             const int mx = MonitorBound ;

             if (mx > 0 && (MonitorPopulation-MonitorFreeCount) > mx) {

               // We can't safely induce a STW safepoint from omAlloc() as our thread

               // state may not be appropriate for such activities and callers may hold

               // naked oops, so instead we defer the action.

               InduceScavenge (Self, "omAlloc") ;

             }

             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]") ;

         MonitorPopulation += _BLOCKSIZE-1;

         MonitorFreeCount += _BLOCKSIZE-1;

         // 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.

 //

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

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

     // Remove from omInUseList

     if (MonitorInUseLists && fromPerThreadAlloc) {

       ObjectMonitor* curmidinuse = NULL;

       for (ObjectMonitor* mid = Self->omInUseList; mid != NULL; ) {

        if (m == mid) {

          // extract from per-thread in-use-list

          if (mid == Self->omInUseList) {

            Self->omInUseList = mid->FreeNext;

          } else if (curmidinuse != NULL) {

            curmidinuse->FreeNext = mid->FreeNext; // maintain the current thread inuselist

          }

          Self->omInUseCount --;

          // verifyInUse(Self);

          break;

        } else {

          curmidinuse = mid;

          mid = mid->FreeNext;

       }

     }

   }

   // FreeNext is used for both onInUseList and omFreeList, so clear old before setting new

   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.

 //

 // Also return the monitors of a moribund thread"s omInUseList to

 // a global gOmInUseList under the global list lock so these

 // will continue to be scanned.

 //

 // 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 ;

     ObjectMonitor * Tail = NULL ;

     int Tally = 0;

     if (List != NULL) {

       ObjectMonitor * s ;

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

           Tally ++ ;

           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") ;

     }

     ObjectMonitor * InUseList = Self->omInUseList;

     ObjectMonitor * InUseTail = NULL ;

     int InUseTally = 0;

     if (InUseList != NULL) {

       Self->omInUseList = NULL;

       ObjectMonitor *curom;

       for (curom = InUseList; curom != NULL; curom = curom->FreeNext) {

         InUseTail = curom;

         InUseTally++;

       }

 // TODO debug

       assert(Self->omInUseCount == InUseTally, "inuse count off");

       Self->omInUseCount = 0;

       guarantee (InUseTail != NULL && InUseList != NULL, "invariant");

     }

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

     if (Tail != NULL) {

       Tail->FreeNext = gFreeList ;

       gFreeList = List ;

       MonitorFreeCount += Tally;

     }

     if (InUseTail != NULL) {

       InUseTail->FreeNext = gOmInUseList;

       gOmInUseList = InUseList;

       gOmInUseCount += InUseTally;

     }

     Thread::muxRelease (&ListLock) ;

     TEVENT (omFlush) ;

 }

 // 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.

 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->_Responsible  = NULL ;

           m->OwnerIsThread = 0 ;

           m->_recursions   = 0 ;

           m->_SpinDuration = ObjectMonitor::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, true) ;

              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 (ObjectMonitor::_sync_Inflations != NULL) ObjectMonitor::_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(),

                 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->_Responsible  = NULL ;

       m->_SpinDuration = ObjectMonitor::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, true) ;

           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 (ObjectMonitor::_sync_Inflations != NULL) ObjectMonitor::_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(),

             object->klass()->external_name());

         }

       }

       return m ;

   }

 }

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

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

 // 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.

 //

 // We have added a flag, MonitorInUseLists, which creates a list

 // of active monitors for each thread. deflate_idle_monitors()

 // only scans the per-thread inuse lists. omAlloc() puts all

 // assigned monitors on the per-thread list. deflate_idle_monitors()

 // returns the non-busy monitors to the global free list.

 // When a thread dies, omFlush() adds the list of active monitors for

 // that thread to a global gOmInUseList acquiring the

 // global list lock. deflate_idle_monitors() acquires the global

 // list lock to scan for non-busy monitors to the global free list.

 // An alternative could have used a single global inuse list. The

 // downside would have been the additional cost of acquiring the global list lock

 // for every omAlloc().

 //

 // 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.

 //

 enum ManifestConstants {

     ClearResponsibleAtSTW   = 0,

     MaximumRecheckInterval  = 1000

 } ;

 // Deflate a single monitor if not in use

 // Return true if deflated, false if in use

 bool ObjectSynchronizer::deflate_monitor(ObjectMonitor* mid, oop obj,

                                          ObjectMonitor** FreeHeadp, ObjectMonitor** FreeTailp) {

   bool deflated;

   // 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 ;

      deflated = false;

   } 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(), 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.

      if (*FreeHeadp == NULL) *FreeHeadp = mid;

      if (*FreeTailp != NULL) {

        ObjectMonitor * prevtail = *FreeTailp;

        assert(prevtail->FreeNext == NULL, "cleaned up deflated?"); // TODO KK

        prevtail->FreeNext = mid;

       }

      *FreeTailp = mid;

      deflated = true;

   }

   return deflated;

 }

 // Caller acquires ListLock

 int ObjectSynchronizer::walk_monitor_list(ObjectMonitor** listheadp,

                                           ObjectMonitor** FreeHeadp, ObjectMonitor** FreeTailp) {

   ObjectMonitor* mid;

   ObjectMonitor* next;

   ObjectMonitor* curmidinuse = NULL;

   int deflatedcount = 0;

   for (mid = *listheadp; mid != NULL; ) {

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

      bool deflated = false;

      if (obj != NULL) {

        deflated = deflate_monitor(mid, obj, FreeHeadp, FreeTailp);

      }

      if (deflated) {

        // extract from per-thread in-use-list

        if (mid == *listheadp) {

          *listheadp = mid->FreeNext;

        } else if (curmidinuse != NULL) {

          curmidinuse->FreeNext = mid->FreeNext; // maintain the current thread inuselist

        }

        next = mid->FreeNext;

        mid->FreeNext = NULL;  // This mid is current tail in the FreeHead list

        mid = next;

        deflatedcount++;

      } else {

        curmidinuse = mid;

        mid = mid->FreeNext;

     }

   }

   return deflatedcount;

 }

 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

   bool deflated = false;

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

   ObjectMonitor * FreeTail = NULL ;

   TEVENT (deflate_idle_monitors) ;

   // Prevent omFlush from changing mids in Thread dtor's during deflation

   // And in case the vm thread is acquiring a lock during a safepoint

   // See e.g. 6320749

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

   if (MonitorInUseLists) {

     int inUse = 0;

     for (JavaThread* cur = Threads::first(); cur != NULL; cur = cur->next()) {

       nInCirculation+= cur->omInUseCount;

       int deflatedcount = walk_monitor_list(cur->omInUseList_addr(), &FreeHead, &FreeTail);

       cur->omInUseCount-= deflatedcount;

       // verifyInUse(cur);

       nScavenged += deflatedcount;

       nInuse += cur->omInUseCount;

      }

    // For moribund threads, scan gOmInUseList

    if (gOmInUseList) {

      nInCirculation += gOmInUseCount;

      int deflatedcount = walk_monitor_list((ObjectMonitor **)&gOmInUseList, &FreeHead, &FreeTail);

      gOmInUseCount-= deflatedcount;

      nScavenged += deflatedcount;

      nInuse += gOmInUseCount;

     }

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

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

     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 ;

       }

       deflated = deflate_monitor(mid, obj, &FreeHead, &FreeTail);

       if (deflated) {

         mid->FreeNext = NULL ;

         nScavenged ++ ;

       } else {

         nInuse ++;

       }

     }

   }

   MonitorFreeCount += nScavenged;

   // Consider: audit gFreeList to ensure that MonitorFreeCount and list agree.

   if (ObjectMonitor::Knob_Verbose) {

     ::printf ("Deflate: InCirc=%d InUse=%d Scavenged=%d ForceMonitorScavenge=%d : pop=%d free=%d\n",

         nInCirculation, nInuse, nScavenged, ForceMonitorScavenge,

         MonitorPopulation, MonitorFreeCount) ;

     ::fflush(stdout) ;

   }

   ForceMonitorScavenge = 0;    // Reset

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

   if (FreeHead != NULL) {

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

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

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

      FreeTail->FreeNext = gFreeList ;

      gFreeList = FreeHead ;

   }

   Thread::muxRelease (&ListLock) ;

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

   if (ObjectMonitor::_sync_MonExtant  != NULL) ObjectMonitor::_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 ++ ;

 }

 // Monitor cleanup on JavaThread::exit

 // 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();

 }

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

 // Non-product code

 #ifndef PRODUCT

 // Verify all monitors in the monitor cache, the verification is weak.

 void ObjectSynchronizer::verify() {

   ObjectMonitor* block = gBlockList;

   ObjectMonitor* mid;

   while (block) {

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

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

       mid = block + i;

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

       if (object != NULL) {

         mid->verify();

       }

     }

     block = (ObjectMonitor*) block->FreeNext;

   }

 }

 // Check if monitor belongs to the monitor cache

 // The list is grow-only so it's *relatively* safe to traverse

 // the list of extant blocks without taking a lock.

 int ObjectSynchronizer::verify_objmon_isinpool(ObjectMonitor *monitor) {

   ObjectMonitor* block = gBlockList;

   while (block) {

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

     if (monitor > &block[0] && monitor < &block[_BLOCKSIZE]) {

       address mon = (address) monitor;

       address blk = (address) block;

       size_t diff = mon - blk;

       assert((diff % sizeof(ObjectMonitor)) == 0, "check");

       return 1;

     }

     block = (ObjectMonitor*) block->FreeNext;

   }

   return 0;

 }

 #endif