jdk8-mips64-public/hotspot: src/share/vm/runtime/objectMonitor.cpp@95a08233f46c (annotated)

src/share/vm/runtime/objectMonitor.cpp@95a08233f46c (annotated)

src/share/vm/runtime/objectMonitor.cpp

Mon, 28 May 2018 10:33:52 +0800

author: aoqi
date: Mon, 28 May 2018 10:33:52 +0800
changeset 9041: 95a08233f46c
parent 8887: b55756ea22d8
parent 8856: ac27a9c85bea
child 9931: fd44df5e3bc3
permissions: -rw-r--r--

Merge

 /*
  * Copyright (c) 1998, 2015, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
  * under the terms of the GNU General Public License version 2 only, as
  * published by the Free Software Foundation.
  *
  * This code is distributed in the hope that it will be useful, but WITHOUT
  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  * version 2 for more details (a copy is included in the LICENSE file that
  * accompanied this code).
  *
  * You should have received a copy of the GNU General Public License version
  * 2 along with this work; if not, write to the Free Software Foundation,
  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  *
  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  * or visit www.oracle.com if you need additional information or have any
  * questions.
  *
  */
 #include "precompiled.hpp"
 #include "classfile/vmSymbols.hpp"
 #include "memory/resourceArea.hpp"
 #include "oops/markOop.hpp"
 #include "oops/oop.inline.hpp"
 #include "runtime/handles.inline.hpp"
 #include "runtime/interfaceSupport.hpp"
 #include "runtime/mutexLocker.hpp"
 #include "runtime/objectMonitor.hpp"
 #include "runtime/objectMonitor.inline.hpp"
 #include "runtime/orderAccess.inline.hpp"
 #include "runtime/osThread.hpp"
 #include "runtime/stubRoutines.hpp"
 #include "runtime/thread.inline.hpp"
 #include "services/threadService.hpp"
 #include "trace/tracing.hpp"
 #include "trace/traceMacros.hpp"
 #include "utilities/dtrace.hpp"
 #include "utilities/macros.hpp"
 #include "utilities/preserveException.hpp"
 #ifdef TARGET_OS_FAMILY_linux
 # include "os_linux.inline.hpp"
 #endif
 #ifdef TARGET_OS_FAMILY_solaris
 # include "os_solaris.inline.hpp"
 #endif
 #ifdef TARGET_OS_FAMILY_windows
 # include "os_windows.inline.hpp"
 #endif
 #ifdef TARGET_OS_FAMILY_bsd
 # include "os_bsd.inline.hpp"
 #endif
 #if defined(__GNUC__) && !defined(IA64) && !defined(PPC64)
   // Need to inhibit inlining for older versions of GCC to avoid build-time failures
   #define ATTR __attribute__((noinline))
 #else
   #define ATTR
 #endif
 #ifdef DTRACE_ENABLED
 // Only bother with this argument setup if dtrace is available
 // TODO-FIXME: probes should not fire when caller is _blocked.  assert() accordingly.
 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread)                           \
   char* bytes = NULL;                                                      \
   int len = 0;                                                             \
   jlong jtid = SharedRuntime::get_java_tid(thread);                        \
   Symbol* klassname = ((oop)obj)->klass()->name();                         \
   if (klassname != NULL) {                                                 \
     bytes = (char*)klassname->bytes();                                     \
     len = klassname->utf8_length();                                        \
   }
 #ifndef USDT2
 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notify,
   jlong, uintptr_t, char*, int);
 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notifyAll,
   jlong, uintptr_t, char*, int);
 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__enter,
   jlong, uintptr_t, char*, int);
 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__entered,
   jlong, uintptr_t, char*, int);
 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__exit,
   jlong, uintptr_t, char*, int);
 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis)       \
   {                                                                        \
     if (DTraceMonitorProbes) {                                            \
       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                       \
       HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid,                       \
                        (monitor), bytes, len, (millis));                   \
     }                                                                      \
   }
 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread)             \
   {                                                                        \
     if (DTraceMonitorProbes) {                                            \
       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                       \
       HS_DTRACE_PROBE4(hotspot, monitor__##probe, jtid,                    \
                        (uintptr_t)(monitor), bytes, len);                  \
     }                                                                      \
   }
 #else /* USDT2 */
 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis)            \
   {                                                                        \
     if (DTraceMonitorProbes) {                                            \
       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                            \
       HOTSPOT_MONITOR_WAIT(jtid,                                           \
                        (monitor), bytes, len, (millis));                   \
     }                                                                      \
   }
 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread)                  \
   {                                                                        \
     if (DTraceMonitorProbes) {                                            \
       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                            \
       HOTSPOT_MONITOR_##probe(jtid,                                               \
                        (uintptr_t)(monitor), bytes, len);                  \
     }                                                                      \
   }
 #endif /* USDT2 */
 #else //  ndef DTRACE_ENABLED
 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon)    {;}
 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon)          {;}
 #endif // ndef DTRACE_ENABLED
 // Tunables ...
 // The knob* variables are effectively final.  Once set they should
 // never be modified hence.  Consider using __read_mostly with GCC.
 int ObjectMonitor::Knob_Verbose    = 0 ;
 int ObjectMonitor::Knob_SpinLimit  = 5000 ;    // derived by an external tool -
 static int Knob_LogSpins           = 0 ;       // enable jvmstat tally for spins
 static int Knob_HandOff            = 0 ;
 static int Knob_ReportSettings     = 0 ;
 static int Knob_SpinBase           = 0 ;       // Floor AKA SpinMin
 static int Knob_SpinBackOff        = 0 ;       // spin-loop backoff
 static int Knob_CASPenalty         = -1 ;      // Penalty for failed CAS
 static int Knob_OXPenalty          = -1 ;      // Penalty for observed _owner change
 static int Knob_SpinSetSucc        = 1 ;       // spinners set the _succ field
 static int Knob_SpinEarly          = 1 ;
 static int Knob_SuccEnabled        = 1 ;       // futile wake throttling
 static int Knob_SuccRestrict       = 0 ;       // Limit successors + spinners to at-most-one
 static int Knob_MaxSpinners        = -1 ;      // Should be a function of # CPUs
 static int Knob_Bonus              = 100 ;     // spin success bonus
 static int Knob_BonusB             = 100 ;     // spin success bonus
 static int Knob_Penalty            = 200 ;     // spin failure penalty
 static int Knob_Poverty            = 1000 ;
 static int Knob_SpinAfterFutile    = 1 ;       // Spin after returning from park()
 static int Knob_FixedSpin          = 0 ;
 static int Knob_OState             = 3 ;       // Spinner checks thread state of _owner
 static int Knob_UsePause           = 1 ;
 static int Knob_ExitPolicy         = 0 ;
 static int Knob_PreSpin            = 10 ;      // 20-100 likely better
 static int Knob_ResetEvent         = 0 ;
 static int BackOffMask             = 0 ;
 static int Knob_FastHSSEC          = 0 ;
 static int Knob_MoveNotifyee       = 2 ;       // notify() - disposition of notifyee
 static int Knob_QMode              = 0 ;       // EntryList-cxq policy - queue discipline
 static volatile int InitDone       = 0 ;
 #define TrySpin TrySpin_VaryDuration
 // -----------------------------------------------------------------------------
 // Theory of operations -- Monitors lists, thread residency, etc:
 //
 // * A thread acquires ownership of a monitor by successfully
 //   CAS()ing the _owner field from null to non-null.
 //
 // * Invariant: A thread appears on at most one monitor list --
 //   cxq, EntryList or WaitSet -- at any one time.
 //
 // * Contending threads "push" themselves onto the cxq with CAS
 //   and then spin/park.
 //
 // * After a contending thread eventually acquires the lock it must
 //   dequeue itself from either the EntryList or the cxq.
 //
 // * The exiting thread identifies and unparks an "heir presumptive"
 //   tentative successor thread on the EntryList.  Critically, the
 //   exiting thread doesn't unlink the successor thread from the EntryList.
 //   After having been unparked, the wakee will recontend for ownership of
 //   the monitor.   The successor (wakee) will either acquire the lock or
 //   re-park itself.
 //
 //   Succession is provided for by a policy of competitive handoff.
 //   The exiting thread does _not_ grant or pass ownership to the
 //   successor thread.  (This is also referred to as "handoff" succession").
 //   Instead the exiting thread releases ownership and possibly wakes
 //   a successor, so the successor can (re)compete for ownership of the lock.
 //   If the EntryList is empty but the cxq is populated the exiting
 //   thread will drain the cxq into the EntryList.  It does so by
 //   by detaching the cxq (installing null with CAS) and folding
 //   the threads from the cxq into the EntryList.  The EntryList is
 //   doubly linked, while the cxq is singly linked because of the
 //   CAS-based "push" used to enqueue recently arrived threads (RATs).
 //
 // * Concurrency invariants:
 //
 //   -- only the monitor owner may access or mutate the EntryList.
 //      The mutex property of the monitor itself protects the EntryList
 //      from concurrent interference.
 //   -- Only the monitor owner may detach the cxq.
 //
 // * The monitor entry list operations avoid locks, but strictly speaking
 //   they're not lock-free.  Enter is lock-free, exit is not.
 //   For a description of 'Methods and apparatus providing non-blocking access
 //   to a resource,' see U.S. Pat. No. 7844973.
 //
 // * The cxq can have multiple concurrent "pushers" but only one concurrent
 //   detaching thread.  This mechanism is immune from the ABA corruption.
 //   More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
 //
 // * Taken together, the cxq and the EntryList constitute or form a
 //   single logical queue of threads stalled trying to acquire the lock.
 //   We use two distinct lists to improve the odds of a constant-time
 //   dequeue operation after acquisition (in the ::enter() epilog) and
 //   to reduce heat on the list ends.  (c.f. Michael Scott's "2Q" algorithm).
 //   A key desideratum is to minimize queue & monitor metadata manipulation
 //   that occurs while holding the monitor lock -- that is, we want to
 //   minimize monitor lock holds times.  Note that even a small amount of
 //   fixed spinning will greatly reduce the # of enqueue-dequeue operations
 //   on EntryList|cxq.  That is, spinning relieves contention on the "inner"
 //   locks and monitor metadata.
 //
 //   Cxq points to the the set of Recently Arrived Threads attempting entry.
 //   Because we push threads onto _cxq with CAS, the RATs must take the form of
 //   a singly-linked LIFO.  We drain _cxq into EntryList  at unlock-time when
 //   the unlocking thread notices that EntryList is null but _cxq is != null.
 //
 //   The EntryList is ordered by the prevailing queue discipline and
 //   can be organized in any convenient fashion, such as a doubly-linked list or
 //   a circular doubly-linked list.  Critically, we want insert and delete operations
 //   to operate in constant-time.  If we need a priority queue then something akin
 //   to Solaris' sleepq would work nicely.  Viz.,
 //   http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
 //   Queue discipline is enforced at ::exit() time, when the unlocking thread
 //   drains the cxq into the EntryList, and orders or reorders the threads on the
 //   EntryList accordingly.
 //
 //   Barring "lock barging", this mechanism provides fair cyclic ordering,
 //   somewhat similar to an elevator-scan.
 //
 // * The monitor synchronization subsystem avoids the use of native
 //   synchronization primitives except for the narrow platform-specific
 //   park-unpark abstraction.  See the comments in os_solaris.cpp regarding
 //   the semantics of park-unpark.  Put another way, this monitor implementation
 //   depends only on atomic operations and park-unpark.  The monitor subsystem
 //   manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
 //   underlying OS manages the READY<->RUN transitions.
 //
 // * Waiting threads reside on the WaitSet list -- wait() puts
 //   the caller onto the WaitSet.
 //
 // * notify() or notifyAll() simply transfers threads from the WaitSet to
 //   either the EntryList or cxq.  Subsequent exit() operations will
 //   unpark the notifyee.  Unparking a notifee in notify() is inefficient -
 //   it's likely the notifyee would simply impale itself on the lock held
 //   by the notifier.
 //
 // * An interesting alternative is to encode cxq as (List,LockByte) where
 //   the LockByte is 0 iff the monitor is owned.  _owner is simply an auxiliary
 //   variable, like _recursions, in the scheme.  The threads or Events that form
 //   the list would have to be aligned in 256-byte addresses.  A thread would
 //   try to acquire the lock or enqueue itself with CAS, but exiting threads
 //   could use a 1-0 protocol and simply STB to set the LockByte to 0.
 //   Note that is is *not* word-tearing, but it does presume that full-word
 //   CAS operations are coherent with intermix with STB operations.  That's true
 //   on most common processors.
 //
 // * See also http://blogs.sun.com/dave
 // -----------------------------------------------------------------------------
 // Enter support
 bool ObjectMonitor::try_enter(Thread* THREAD) {
   if (THREAD != _owner) {
     if (THREAD->is_lock_owned ((address)_owner)) {
        assert(_recursions == 0, "internal state error");
        _owner = THREAD ;
        _recursions = 1 ;
        OwnerIsThread = 1 ;
        return true;
     }
     if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
       return false;
     }
     return true;
   } else {
     _recursions++;
     return true;
   }
 }
 void ATTR ObjectMonitor::enter(TRAPS) {
   // The following code is ordered to check the most common cases first
   // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
   Thread * const Self = THREAD ;
   void * cur ;
   cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
   if (cur == NULL) {
      // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
      assert (_recursions == 0   , "invariant") ;
      assert (_owner      == Self, "invariant") ;
      // CONSIDER: set or assert OwnerIsThread == 1
      return ;
   }
   if (cur == Self) {
      // TODO-FIXME: check for integer overflow!  BUGID 6557169.
      _recursions ++ ;
      return ;
   }
   if (Self->is_lock_owned ((address)cur)) {
     assert (_recursions == 0, "internal state error");
     _recursions = 1 ;
     // Commute owner from a thread-specific on-stack BasicLockObject address to
     // a full-fledged "Thread *".
     _owner = Self ;
     OwnerIsThread = 1 ;
     return ;
   }
   // We've encountered genuine contention.
   assert (Self->_Stalled == 0, "invariant") ;
   Self->_Stalled = intptr_t(this) ;
   // Try one round of spinning *before* enqueueing Self
   // and before going through the awkward and expensive state
   // transitions.  The following spin is strictly optional ...
   // Note that if we acquire the monitor from an initial spin
   // we forgo posting JVMTI events and firing DTRACE probes.
   if (Knob_SpinEarly && TrySpin (Self) > 0) {
      assert (_owner == Self      , "invariant") ;
      assert (_recursions == 0    , "invariant") ;
      assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
      Self->_Stalled = 0 ;
      return ;
   }
   assert (_owner != Self          , "invariant") ;
   assert (_succ  != Self          , "invariant") ;
   assert (Self->is_Java_thread()  , "invariant") ;
   JavaThread * jt = (JavaThread *) Self ;
   assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
   assert (jt->thread_state() != _thread_blocked   , "invariant") ;
   assert (this->object() != NULL  , "invariant") ;
   assert (_count >= 0, "invariant") ;
   // Prevent deflation at STW-time.  See deflate_idle_monitors() and is_busy().
   // Ensure the object-monitor relationship remains stable while there's contention.
   Atomic::inc_ptr(&_count);
   EventJavaMonitorEnter event;
   { // Change java thread status to indicate blocked on monitor enter.
     JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
     Self->set_current_pending_monitor(this);
     DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
     if (JvmtiExport::should_post_monitor_contended_enter()) {
       JvmtiExport::post_monitor_contended_enter(jt, this);
       // The current thread does not yet own the monitor and does not
       // yet appear on any queues that would get it made the successor.
       // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
       // handler cannot accidentally consume an unpark() meant for the
       // ParkEvent associated with this ObjectMonitor.
     }
     OSThreadContendState osts(Self->osthread());
     ThreadBlockInVM tbivm(jt);
     // TODO-FIXME: change the following for(;;) loop to straight-line code.
     for (;;) {
       jt->set_suspend_equivalent();
       // cleared by handle_special_suspend_equivalent_condition()
       // or java_suspend_self()
       EnterI (THREAD) ;
       if (!ExitSuspendEquivalent(jt)) break ;
       //
       // We have acquired the contended monitor, but while we were
       // waiting another thread suspended us. We don't want to enter
       // the monitor while suspended because that would surprise the
       // thread that suspended us.
       //
           _recursions = 0 ;
       _succ = NULL ;
       exit (false, Self) ;
       jt->java_suspend_self();
     }
     Self->set_current_pending_monitor(NULL);
     // We cleared the pending monitor info since we've just gotten past
     // the enter-check-for-suspend dance and we now own the monitor free
     // and clear, i.e., it is no longer pending. The ThreadBlockInVM
     // destructor can go to a safepoint at the end of this block. If we
     // do a thread dump during that safepoint, then this thread will show
     // as having "-locked" the monitor, but the OS and java.lang.Thread
     // states will still report that the thread is blocked trying to
     // acquire it.
   }
   Atomic::dec_ptr(&_count);
   assert (_count >= 0, "invariant") ;
   Self->_Stalled = 0 ;
   // Must either set _recursions = 0 or ASSERT _recursions == 0.
   assert (_recursions == 0     , "invariant") ;
   assert (_owner == Self       , "invariant") ;
   assert (_succ  != Self       , "invariant") ;
   assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
   // The thread -- now the owner -- is back in vm mode.
   // Report the glorious news via TI,DTrace and jvmstat.
   // The probe effect is non-trivial.  All the reportage occurs
   // while we hold the monitor, increasing the length of the critical
   // section.  Amdahl's parallel speedup law comes vividly into play.
   //
   // Another option might be to aggregate the events (thread local or
   // per-monitor aggregation) and defer reporting until a more opportune
   // time -- such as next time some thread encounters contention but has
   // yet to acquire the lock.  While spinning that thread could
   // spinning we could increment JVMStat counters, etc.
   DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
   if (JvmtiExport::should_post_monitor_contended_entered()) {
     JvmtiExport::post_monitor_contended_entered(jt, this);
     // The current thread already owns the monitor and is not going to
     // call park() for the remainder of the monitor enter protocol. So
     // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
     // event handler consumed an unpark() issued by the thread that
     // just exited the monitor.
   }
   if (event.should_commit()) {
     event.set_klass(((oop)this->object())->klass());
     event.set_previousOwner((TYPE_JAVALANGTHREAD)_previous_owner_tid);
     event.set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
     event.commit();
   }
   if (ObjectMonitor::_sync_ContendedLockAttempts != NULL) {
      ObjectMonitor::_sync_ContendedLockAttempts->inc() ;
   }
 }
 // Caveat: TryLock() is not necessarily serializing if it returns failure.
 // Callers must compensate as needed.
 int ObjectMonitor::TryLock (Thread * Self) {
    for (;;) {
       void * own = _owner ;
       if (own != NULL) return 0 ;
       if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
          // Either guarantee _recursions == 0 or set _recursions = 0.
          assert (_recursions == 0, "invariant") ;
          assert (_owner == Self, "invariant") ;
          // CONSIDER: set or assert that OwnerIsThread == 1
          return 1 ;
       }
       // The lock had been free momentarily, but we lost the race to the lock.
       // Interference -- the CAS failed.
       // We can either return -1 or retry.
       // Retry doesn't make as much sense because the lock was just acquired.
       if (true) return -1 ;
    }
 }
 void ATTR ObjectMonitor::EnterI (TRAPS) {
     Thread * Self = THREAD ;
     assert (Self->is_Java_thread(), "invariant") ;
     assert (((JavaThread *) Self)->thread_state() == _thread_blocked   , "invariant") ;
     // Try the lock - TATAS
     if (TryLock (Self) > 0) {
         assert (_succ != Self              , "invariant") ;
         assert (_owner == Self             , "invariant") ;
         assert (_Responsible != Self       , "invariant") ;
         return ;
     }
     DeferredInitialize () ;
     // We try one round of spinning *before* enqueueing Self.
     //
     // If the _owner is ready but OFFPROC we could use a YieldTo()
     // operation to donate the remainder of this thread's quantum
     // to the owner.  This has subtle but beneficial affinity
     // effects.
     if (TrySpin (Self) > 0) {
         assert (_owner == Self        , "invariant") ;
         assert (_succ != Self         , "invariant") ;
         assert (_Responsible != Self  , "invariant") ;
         return ;
     }
     // The Spin failed -- Enqueue and park the thread ...
     assert (_succ  != Self            , "invariant") ;
     assert (_owner != Self            , "invariant") ;
     assert (_Responsible != Self      , "invariant") ;
     // Enqueue "Self" on ObjectMonitor's _cxq.
     //
     // Node acts as a proxy for Self.
     // As an aside, if were to ever rewrite the synchronization code mostly
     // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
     // Java objects.  This would avoid awkward lifecycle and liveness issues,
     // as well as eliminate a subset of ABA issues.
     // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
     //
     ObjectWaiter node(Self) ;
     Self->_ParkEvent->reset() ;
     node._prev   = (ObjectWaiter *) 0xBAD ;
     node.TState  = ObjectWaiter::TS_CXQ ;
     // Push "Self" onto the front of the _cxq.
     // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
     // Note that spinning tends to reduce the rate at which threads
     // enqueue and dequeue on EntryList|cxq.
     ObjectWaiter * nxt ;
     for (;;) {
         node._next = nxt = _cxq ;
         if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ;
         // Interference - the CAS failed because _cxq changed.  Just retry.
         // As an optional optimization we retry the lock.
         if (TryLock (Self) > 0) {
             assert (_succ != Self         , "invariant") ;
             assert (_owner == Self        , "invariant") ;
             assert (_Responsible != Self  , "invariant") ;
             return ;
         }
     }
     // Check for cxq|EntryList edge transition to non-null.  This indicates
     // the onset of contention.  While contention persists exiting threads
     // will use a ST:MEMBAR:LD 1-1 exit protocol.  When contention abates exit
     // operations revert to the faster 1-0 mode.  This enter operation may interleave
     // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
     // arrange for one of the contending thread to use a timed park() operations
     // to detect and recover from the race.  (Stranding is form of progress failure
     // where the monitor is unlocked but all the contending threads remain parked).
     // That is, at least one of the contended threads will periodically poll _owner.
     // One of the contending threads will become the designated "Responsible" thread.
     // The Responsible thread uses a timed park instead of a normal indefinite park
     // operation -- it periodically wakes and checks for and recovers from potential
     // strandings admitted by 1-0 exit operations.   We need at most one Responsible
     // thread per-monitor at any given moment.  Only threads on cxq|EntryList may
     // be responsible for a monitor.
     //
     // Currently, one of the contended threads takes on the added role of "Responsible".
     // A viable alternative would be to use a dedicated "stranding checker" thread
     // that periodically iterated over all the threads (or active monitors) and unparked
     // successors where there was risk of stranding.  This would help eliminate the
     // timer scalability issues we see on some platforms as we'd only have one thread
     // -- the checker -- parked on a timer.
     if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
         // Try to assume the role of responsible thread for the monitor.
         // CONSIDER:  ST vs CAS vs { if (Responsible==null) Responsible=Self }
         Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
     }
     // The lock have been released while this thread was occupied queueing
     // itself onto _cxq.  To close the race and avoid "stranding" and
     // progress-liveness failure we must resample-retry _owner before parking.
     // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
     // In this case the ST-MEMBAR is accomplished with CAS().
     //
     // TODO: Defer all thread state transitions until park-time.
     // Since state transitions are heavy and inefficient we'd like
     // to defer the state transitions until absolutely necessary,
     // and in doing so avoid some transitions ...
     TEVENT (Inflated enter - Contention) ;
     int nWakeups = 0 ;
     int RecheckInterval = 1 ;
     for (;;) {
         if (TryLock (Self) > 0) break ;
         assert (_owner != Self, "invariant") ;
         if ((SyncFlags & 2) && _Responsible == NULL) {
            Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
         }
         // park self
         if (_Responsible == Self || (SyncFlags & 1)) {
             TEVENT (Inflated enter - park TIMED) ;
             Self->_ParkEvent->park ((jlong) RecheckInterval) ;
             // Increase the RecheckInterval, but clamp the value.
             RecheckInterval *= 8 ;
             if (RecheckInterval > 1000) RecheckInterval = 1000 ;
         } else {
             TEVENT (Inflated enter - park UNTIMED) ;
             Self->_ParkEvent->park() ;
         }
         if (TryLock(Self) > 0) break ;
         // The lock is still contested.
         // Keep a tally of the # of futile wakeups.
         // Note that the counter is not protected by a lock or updated by atomics.
         // That is by design - we trade "lossy" counters which are exposed to
         // races during updates for a lower probe effect.
         TEVENT (Inflated enter - Futile wakeup) ;
         if (ObjectMonitor::_sync_FutileWakeups != NULL) {
            ObjectMonitor::_sync_FutileWakeups->inc() ;
         }
         ++ nWakeups ;
         // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
         // We can defer clearing _succ until after the spin completes
         // TrySpin() must tolerate being called with _succ == Self.
         // Try yet another round of adaptive spinning.
         if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ;
         // We can find that we were unpark()ed and redesignated _succ while
         // we were spinning.  That's harmless.  If we iterate and call park(),
         // park() will consume the event and return immediately and we'll
         // just spin again.  This pattern can repeat, leaving _succ to simply
         // spin on a CPU.  Enable Knob_ResetEvent to clear pending unparks().
         // Alternately, we can sample fired() here, and if set, forgo spinning
         // in the next iteration.
         if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
            Self->_ParkEvent->reset() ;
            OrderAccess::fence() ;
         }
         if (_succ == Self) _succ = NULL ;
         // Invariant: after clearing _succ a thread *must* retry _owner before parking.
         OrderAccess::fence() ;
     }
     // Egress :
     // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
     // Normally we'll find Self on the EntryList .
     // From the perspective of the lock owner (this thread), the
     // EntryList is stable and cxq is prepend-only.
     // The head of cxq is volatile but the interior is stable.
     // In addition, Self.TState is stable.
     assert (_owner == Self      , "invariant") ;
     assert (object() != NULL    , "invariant") ;
     // I'd like to write:
     //   guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
     // but as we're at a safepoint that's not safe.
     UnlinkAfterAcquire (Self, &node) ;
     if (_succ == Self) _succ = NULL ;
     assert (_succ != Self, "invariant") ;
     if (_Responsible == Self) {
         _Responsible = NULL ;
         OrderAccess::fence(); // Dekker pivot-point
         // We may leave threads on cxq|EntryList without a designated
         // "Responsible" thread.  This is benign.  When this thread subsequently
         // exits the monitor it can "see" such preexisting "old" threads --
         // threads that arrived on the cxq|EntryList before the fence, above --
         // by LDing cxq|EntryList.  Newly arrived threads -- that is, threads
         // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
         // non-null and elect a new "Responsible" timer thread.
         //
         // This thread executes:
         //    ST Responsible=null; MEMBAR    (in enter epilog - here)
         //    LD cxq|EntryList               (in subsequent exit)
         //
         // Entering threads in the slow/contended path execute:
         //    ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
         //    The (ST cxq; MEMBAR) is accomplished with CAS().
         //
         // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
         // exit operation from floating above the ST Responsible=null.
     }
     // We've acquired ownership with CAS().
     // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
     // But since the CAS() this thread may have also stored into _succ,
     // EntryList, cxq or Responsible.  These meta-data updates must be
     // visible __before this thread subsequently drops the lock.
     // Consider what could occur if we didn't enforce this constraint --
     // STs to monitor meta-data and user-data could reorder with (become
     // visible after) the ST in exit that drops ownership of the lock.
     // Some other thread could then acquire the lock, but observe inconsistent
     // or old monitor meta-data and heap data.  That violates the JMM.
     // To that end, the 1-0 exit() operation must have at least STST|LDST
     // "release" barrier semantics.  Specifically, there must be at least a
     // STST|LDST barrier in exit() before the ST of null into _owner that drops
     // the lock.   The barrier ensures that changes to monitor meta-data and data
     // protected by the lock will be visible before we release the lock, and
     // therefore before some other thread (CPU) has a chance to acquire the lock.
     // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
     //
     // Critically, any prior STs to _succ or EntryList must be visible before
     // the ST of null into _owner in the *subsequent* (following) corresponding
     // monitorexit.  Recall too, that in 1-0 mode monitorexit does not necessarily
     // execute a serializing instruction.
     if (SyncFlags & 8) {
        OrderAccess::fence() ;
     }
     return ;
 }
 // ReenterI() is a specialized inline form of the latter half of the
 // contended slow-path from EnterI().  We use ReenterI() only for
 // monitor reentry in wait().
 //
 // In the future we should reconcile EnterI() and ReenterI(), adding
 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
 // loop accordingly.
 void ATTR ObjectMonitor::ReenterI (Thread * Self, ObjectWaiter * SelfNode) {
     assert (Self != NULL                , "invariant") ;
     assert (SelfNode != NULL            , "invariant") ;
     assert (SelfNode->_thread == Self   , "invariant") ;
     assert (_waiters > 0                , "invariant") ;
     assert (((oop)(object()))->mark() == markOopDesc::encode(this) , "invariant") ;
     assert (((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
     JavaThread * jt = (JavaThread *) Self ;
     int nWakeups = 0 ;
     for (;;) {
         ObjectWaiter::TStates v = SelfNode->TState ;
         guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
         assert    (_owner != Self, "invariant") ;
         if (TryLock (Self) > 0) break ;
         if (TrySpin (Self) > 0) break ;
         TEVENT (Wait Reentry - parking) ;
         // State transition wrappers around park() ...
         // ReenterI() wisely defers state transitions until
         // it's clear we must park the thread.
         {
            OSThreadContendState osts(Self->osthread());
            ThreadBlockInVM tbivm(jt);
            // cleared by handle_special_suspend_equivalent_condition()
            // or java_suspend_self()
            jt->set_suspend_equivalent();
            if (SyncFlags & 1) {
               Self->_ParkEvent->park ((jlong)1000) ;
            } else {
               Self->_ParkEvent->park () ;
            }
            // were we externally suspended while we were waiting?
            for (;;) {
               if (!ExitSuspendEquivalent (jt)) break ;
               if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
               jt->java_suspend_self();
               jt->set_suspend_equivalent();
            }
         }
         // Try again, but just so we distinguish between futile wakeups and
         // successful wakeups.  The following test isn't algorithmically
         // necessary, but it helps us maintain sensible statistics.
         if (TryLock(Self) > 0) break ;
         // The lock is still contested.
         // Keep a tally of the # of futile wakeups.
         // Note that the counter is not protected by a lock or updated by atomics.
         // That is by design - we trade "lossy" counters which are exposed to
         // races during updates for a lower probe effect.
         TEVENT (Wait Reentry - futile wakeup) ;
         ++ nWakeups ;
         // Assuming this is not a spurious wakeup we'll normally
         // find that _succ == Self.
         if (_succ == Self) _succ = NULL ;
         // Invariant: after clearing _succ a contending thread
         // *must* retry  _owner before parking.
         OrderAccess::fence() ;
         if (ObjectMonitor::_sync_FutileWakeups != NULL) {
           ObjectMonitor::_sync_FutileWakeups->inc() ;
         }
     }
     // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
     // Normally we'll find Self on the EntryList.
     // Unlinking from the EntryList is constant-time and atomic-free.
     // From the perspective of the lock owner (this thread), the
     // EntryList is stable and cxq is prepend-only.
     // The head of cxq is volatile but the interior is stable.
     // In addition, Self.TState is stable.
     assert (_owner == Self, "invariant") ;
     assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
     UnlinkAfterAcquire (Self, SelfNode) ;
     if (_succ == Self) _succ = NULL ;
     assert (_succ != Self, "invariant") ;
     SelfNode->TState = ObjectWaiter::TS_RUN ;
     OrderAccess::fence() ;      // see comments at the end of EnterI()
 }
 // after the thread acquires the lock in ::enter().  Equally, we could defer
 // unlinking the thread until ::exit()-time.
 void ObjectMonitor::UnlinkAfterAcquire (Thread * Self, ObjectWaiter * SelfNode)
 {
     assert (_owner == Self, "invariant") ;
     assert (SelfNode->_thread == Self, "invariant") ;
     if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
         // Normal case: remove Self from the DLL EntryList .
         // This is a constant-time operation.
         ObjectWaiter * nxt = SelfNode->_next ;
         ObjectWaiter * prv = SelfNode->_prev ;
         if (nxt != NULL) nxt->_prev = prv ;
         if (prv != NULL) prv->_next = nxt ;
         if (SelfNode == _EntryList ) _EntryList = nxt ;
         assert (nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant") ;
         assert (prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant") ;
         TEVENT (Unlink from EntryList) ;
     } else {
         guarantee (SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant") ;
         // Inopportune interleaving -- Self is still on the cxq.
         // This usually means the enqueue of self raced an exiting thread.
         // Normally we'll find Self near the front of the cxq, so
         // dequeueing is typically fast.  If needbe we can accelerate
         // this with some MCS/CHL-like bidirectional list hints and advisory
         // back-links so dequeueing from the interior will normally operate
         // in constant-time.
         // Dequeue Self from either the head (with CAS) or from the interior
         // with a linear-time scan and normal non-atomic memory operations.
         // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
         // and then unlink Self from EntryList.  We have to drain eventually,
         // so it might as well be now.
         ObjectWaiter * v = _cxq ;
         assert (v != NULL, "invariant") ;
         if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) {
             // The CAS above can fail from interference IFF a "RAT" arrived.
             // In that case Self must be in the interior and can no longer be
             // at the head of cxq.
             if (v == SelfNode) {
                 assert (_cxq != v, "invariant") ;
                 v = _cxq ;          // CAS above failed - start scan at head of list
             }
             ObjectWaiter * p ;
             ObjectWaiter * q = NULL ;
             for (p = v ; p != NULL && p != SelfNode; p = p->_next) {
                 q = p ;
                 assert (p->TState == ObjectWaiter::TS_CXQ, "invariant") ;
             }
             assert (v != SelfNode,  "invariant") ;
             assert (p == SelfNode,  "Node not found on cxq") ;
             assert (p != _cxq,      "invariant") ;
             assert (q != NULL,      "invariant") ;
             assert (q->_next == p,  "invariant") ;
             q->_next = p->_next ;
         }
         TEVENT (Unlink from cxq) ;
     }
     // Diagnostic hygiene ...
     SelfNode->_prev  = (ObjectWaiter *) 0xBAD ;
     SelfNode->_next  = (ObjectWaiter *) 0xBAD ;
     SelfNode->TState = ObjectWaiter::TS_RUN ;
 }
 // -----------------------------------------------------------------------------
 // Exit support
 //
 // exit()
 // ~~~~~~
 // Note that the collector can't reclaim the objectMonitor or deflate
 // the object out from underneath the thread calling ::exit() as the
 // thread calling ::exit() never transitions to a stable state.
 // This inhibits GC, which in turn inhibits asynchronous (and
 // inopportune) reclamation of "this".
 //
 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
 // There's one exception to the claim above, however.  EnterI() can call
 // exit() to drop a lock if the acquirer has been externally suspended.
 // In that case exit() is called with _thread_state as _thread_blocked,
 // but the monitor's _count field is > 0, which inhibits reclamation.
 //
 // 1-0 exit
 // ~~~~~~~~
 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
 // the fast-path operators have been optimized so the common ::exit()
 // operation is 1-0.  See i486.ad fast_unlock(), for instance.
 // The code emitted by fast_unlock() elides the usual MEMBAR.  This
 // greatly improves latency -- MEMBAR and CAS having considerable local
 // latency on modern processors -- but at the cost of "stranding".  Absent the
 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
 // ::enter() path, resulting in the entering thread being stranding
 // and a progress-liveness failure.   Stranding is extremely rare.
 // We use timers (timed park operations) & periodic polling to detect
 // and recover from stranding.  Potentially stranded threads periodically
 // wake up and poll the lock.  See the usage of the _Responsible variable.
 //
 // The CAS() in enter provides for safety and exclusion, while the CAS or
 // MEMBAR in exit provides for progress and avoids stranding.  1-0 locking
 // eliminates the CAS/MEMBAR from the exist path, but it admits stranding.
 // We detect and recover from stranding with timers.
 //
 // If a thread transiently strands it'll park until (a) another
 // thread acquires the lock and then drops the lock, at which time the
 // exiting thread will notice and unpark the stranded thread, or, (b)
 // the timer expires.  If the lock is high traffic then the stranding latency
 // will be low due to (a).  If the lock is low traffic then the odds of
 // stranding are lower, although the worst-case stranding latency
 // is longer.  Critically, we don't want to put excessive load in the
 // platform's timer subsystem.  We want to minimize both the timer injection
 // rate (timers created/sec) as well as the number of timers active at
 // any one time.  (more precisely, we want to minimize timer-seconds, which is
 // the integral of the # of active timers at any instant over time).
 // Both impinge on OS scalability.  Given that, at most one thread parked on
 // a monitor will use a timer.
 void ATTR ObjectMonitor::exit(bool not_suspended, TRAPS) {
    Thread * Self = THREAD ;
    if (THREAD != _owner) {
      if (THREAD->is_lock_owned((address) _owner)) {
        // Transmute _owner from a BasicLock pointer to a Thread address.
        // We don't need to hold _mutex for this transition.
        // Non-null to Non-null is safe as long as all readers can
        // tolerate either flavor.
        assert (_recursions == 0, "invariant") ;
        _owner = THREAD ;
        _recursions = 0 ;
        OwnerIsThread = 1 ;
      } else {
        // NOTE: we need to handle unbalanced monitor enter/exit
        // in native code by throwing an exception.
        // TODO: Throw an IllegalMonitorStateException ?
        TEVENT (Exit - Throw IMSX) ;
        assert(false, "Non-balanced monitor enter/exit!");
        if (false) {
           THROW(vmSymbols::java_lang_IllegalMonitorStateException());
        }
        return;
      }
    }
    if (_recursions != 0) {
      _recursions--;        // this is simple recursive enter
      TEVENT (Inflated exit - recursive) ;
      return ;
    }
    // Invariant: after setting Responsible=null an thread must execute
    // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
    if ((SyncFlags & 4) == 0) {
       _Responsible = NULL ;
    }
 #if INCLUDE_TRACE
    // get the owner's thread id for the MonitorEnter event
    // if it is enabled and the thread isn't suspended
    if (not_suspended && Tracing::is_event_enabled(TraceJavaMonitorEnterEvent)) {
      _previous_owner_tid = SharedRuntime::get_java_tid(Self);
    }
 #endif
    for (;;) {
       assert (THREAD == _owner, "invariant") ;
       if (Knob_ExitPolicy == 0) {
          // release semantics: prior loads and stores from within the critical section
          // must not float (reorder) past the following store that drops the lock.
          // On SPARC that requires MEMBAR #loadstore|#storestore.
          // But of course in TSO #loadstore|#storestore is not required.
          // I'd like to write one of the following:
          // A.  OrderAccess::release() ; _owner = NULL
          // B.  OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
          // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
          // store into a _dummy variable.  That store is not needed, but can result
          // in massive wasteful coherency traffic on classic SMP systems.
          // Instead, I use release_store(), which is implemented as just a simple
          // ST on x64, x86 and SPARC.
          OrderAccess::release_store_ptr (&_owner, NULL) ;   // drop the lock
          OrderAccess::storeload() ;                         // See if we need to wake a successor
          if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
             TEVENT (Inflated exit - simple egress) ;
             return ;
          }
          TEVENT (Inflated exit - complex egress) ;
          // Normally the exiting thread is responsible for ensuring succession,
          // but if other successors are ready or other entering threads are spinning
          // then this thread can simply store NULL into _owner and exit without
          // waking a successor.  The existence of spinners or ready successors
          // guarantees proper succession (liveness).  Responsibility passes to the
          // ready or running successors.  The exiting thread delegates the duty.
          // More precisely, if a successor already exists this thread is absolved
          // of the responsibility of waking (unparking) one.
          //
          // The _succ variable is critical to reducing futile wakeup frequency.
          // _succ identifies the "heir presumptive" thread that has been made
          // ready (unparked) but that has not yet run.  We need only one such
          // successor thread to guarantee progress.
          // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
          // section 3.3 "Futile Wakeup Throttling" for details.
          //
          // Note that spinners in Enter() also set _succ non-null.
          // In the current implementation spinners opportunistically set
          // _succ so that exiting threads might avoid waking a successor.
          // Another less appealing alternative would be for the exiting thread
          // to drop the lock and then spin briefly to see if a spinner managed
          // to acquire the lock.  If so, the exiting thread could exit
          // immediately without waking a successor, otherwise the exiting
          // thread would need to dequeue and wake a successor.
          // (Note that we'd need to make the post-drop spin short, but no
          // shorter than the worst-case round-trip cache-line migration time.
          // The dropped lock needs to become visible to the spinner, and then
          // the acquisition of the lock by the spinner must become visible to
          // the exiting thread).
          //
          // It appears that an heir-presumptive (successor) must be made ready.
          // Only the current lock owner can manipulate the EntryList or
          // drain _cxq, so we need to reacquire the lock.  If we fail
          // to reacquire the lock the responsibility for ensuring succession
          // falls to the new owner.
          //
          if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
             return ;
          }
          TEVENT (Exit - Reacquired) ;
       } else {
          if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
             OrderAccess::release_store_ptr (&_owner, NULL) ;   // drop the lock
             OrderAccess::storeload() ;
             // Ratify the previously observed values.
             if (_cxq == NULL || _succ != NULL) {
                 TEVENT (Inflated exit - simple egress) ;
                 return ;
             }
             // inopportune interleaving -- the exiting thread (this thread)
             // in the fast-exit path raced an entering thread in the slow-enter
             // path.
             // We have two choices:
             // A.  Try to reacquire the lock.
             //     If the CAS() fails return immediately, otherwise
             //     we either restart/rerun the exit operation, or simply
             //     fall-through into the code below which wakes a successor.
             // B.  If the elements forming the EntryList|cxq are TSM
             //     we could simply unpark() the lead thread and return
             //     without having set _succ.
             if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
                TEVENT (Inflated exit - reacquired succeeded) ;
                return ;
             }
             TEVENT (Inflated exit - reacquired failed) ;
          } else {
             TEVENT (Inflated exit - complex egress) ;
          }
       }
       guarantee (_owner == THREAD, "invariant") ;
       ObjectWaiter * w = NULL ;
       int QMode = Knob_QMode ;
       if (QMode == 2 && _cxq != NULL) {
           // QMode == 2 : cxq has precedence over EntryList.
           // Try to directly wake a successor from the cxq.
           // If successful, the successor will need to unlink itself from cxq.
           w = _cxq ;
           assert (w != NULL, "invariant") ;
           assert (w->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
           ExitEpilog (Self, w) ;
           return ;
       }
       if (QMode == 3 && _cxq != NULL) {
           // Aggressively drain cxq into EntryList at the first opportunity.
           // This policy ensure that recently-run threads live at the head of EntryList.
           // Drain _cxq into EntryList - bulk transfer.
           // First, detach _cxq.
           // The following loop is tantamount to: w = swap (&cxq, NULL)
           w = _cxq ;
           for (;;) {
              assert (w != NULL, "Invariant") ;
              ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
              if (u == w) break ;
              w = u ;
           }
           assert (w != NULL              , "invariant") ;
           ObjectWaiter * q = NULL ;
           ObjectWaiter * p ;
           for (p = w ; p != NULL ; p = p->_next) {
               guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
               p->TState = ObjectWaiter::TS_ENTER ;
               p->_prev = q ;
               q = p ;
           }
           // Append the RATs to the EntryList
           // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
           ObjectWaiter * Tail ;
           for (Tail = _EntryList ; Tail != NULL && Tail->_next != NULL ; Tail = Tail->_next) ;
           if (Tail == NULL) {
               _EntryList = w ;
           } else {
               Tail->_next = w ;
               w->_prev = Tail ;
           }
           // Fall thru into code that tries to wake a successor from EntryList
       }
       if (QMode == 4 && _cxq != NULL) {
           // Aggressively drain cxq into EntryList at the first opportunity.
           // This policy ensure that recently-run threads live at the head of EntryList.
           // Drain _cxq into EntryList - bulk transfer.
           // First, detach _cxq.
           // The following loop is tantamount to: w = swap (&cxq, NULL)
           w = _cxq ;
           for (;;) {
              assert (w != NULL, "Invariant") ;
              ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
              if (u == w) break ;
              w = u ;
           }
           assert (w != NULL              , "invariant") ;
           ObjectWaiter * q = NULL ;
           ObjectWaiter * p ;
           for (p = w ; p != NULL ; p = p->_next) {
               guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
               p->TState = ObjectWaiter::TS_ENTER ;
               p->_prev = q ;
               q = p ;
           }
           // Prepend the RATs to the EntryList
           if (_EntryList != NULL) {
               q->_next = _EntryList ;
               _EntryList->_prev = q ;
           }
           _EntryList = w ;
           // Fall thru into code that tries to wake a successor from EntryList
       }
       w = _EntryList  ;
       if (w != NULL) {
           // I'd like to write: guarantee (w->_thread != Self).
           // But in practice an exiting thread may find itself on the EntryList.
           // Lets say thread T1 calls O.wait().  Wait() enqueues T1 on O's waitset and
           // then calls exit().  Exit release the lock by setting O._owner to NULL.
           // Lets say T1 then stalls.  T2 acquires O and calls O.notify().  The
           // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
           // release the lock "O".  T2 resumes immediately after the ST of null into
           // _owner, above.  T2 notices that the EntryList is populated, so it
           // reacquires the lock and then finds itself on the EntryList.
           // Given all that, we have to tolerate the circumstance where "w" is
           // associated with Self.
           assert (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
           ExitEpilog (Self, w) ;
           return ;
       }
       // If we find that both _cxq and EntryList are null then just
       // re-run the exit protocol from the top.
       w = _cxq ;
       if (w == NULL) continue ;
       // Drain _cxq into EntryList - bulk transfer.
       // First, detach _cxq.
       // The following loop is tantamount to: w = swap (&cxq, NULL)
       for (;;) {
           assert (w != NULL, "Invariant") ;
           ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
           if (u == w) break ;
           w = u ;
       }
       TEVENT (Inflated exit - drain cxq into EntryList) ;
       assert (w != NULL              , "invariant") ;
       assert (_EntryList  == NULL    , "invariant") ;
       // Convert the LIFO SLL anchored by _cxq into a DLL.
       // The list reorganization step operates in O(LENGTH(w)) time.
       // It's critical that this step operate quickly as
       // "Self" still holds the outer-lock, restricting parallelism
       // and effectively lengthening the critical section.
       // Invariant: s chases t chases u.
       // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
       // we have faster access to the tail.
       if (QMode == 1) {
          // QMode == 1 : drain cxq to EntryList, reversing order
          // We also reverse the order of the list.
          ObjectWaiter * s = NULL ;
          ObjectWaiter * t = w ;
          ObjectWaiter * u = NULL ;
          while (t != NULL) {
              guarantee (t->TState == ObjectWaiter::TS_CXQ, "invariant") ;
              t->TState = ObjectWaiter::TS_ENTER ;
              u = t->_next ;
              t->_prev = u ;
              t->_next = s ;
              s = t;
              t = u ;
          }
          _EntryList  = s ;
          assert (s != NULL, "invariant") ;
       } else {
          // QMode == 0 or QMode == 2
          _EntryList = w ;
          ObjectWaiter * q = NULL ;
          ObjectWaiter * p ;
          for (p = w ; p != NULL ; p = p->_next) {
              guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
              p->TState = ObjectWaiter::TS_ENTER ;
              p->_prev = q ;
              q = p ;
          }
       }
       // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
       // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
       // See if we can abdicate to a spinner instead of waking a thread.
       // A primary goal of the implementation is to reduce the
       // context-switch rate.
       if (_succ != NULL) continue;
       w = _EntryList  ;
       if (w != NULL) {
           guarantee (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
           ExitEpilog (Self, w) ;
           return ;
       }
    }
 }
 // ExitSuspendEquivalent:
 // A faster alternate to handle_special_suspend_equivalent_condition()
 //
 // handle_special_suspend_equivalent_condition() unconditionally
 // acquires the SR_lock.  On some platforms uncontended MutexLocker()
 // operations have high latency.  Note that in ::enter() we call HSSEC
 // while holding the monitor, so we effectively lengthen the critical sections.
 //
 // There are a number of possible solutions:
 //
 // A.  To ameliorate the problem we might also defer state transitions
 //     to as late as possible -- just prior to parking.
 //     Given that, we'd call HSSEC after having returned from park(),
 //     but before attempting to acquire the monitor.  This is only a
 //     partial solution.  It avoids calling HSSEC while holding the
 //     monitor (good), but it still increases successor reacquisition latency --
 //     the interval between unparking a successor and the time the successor
 //     resumes and retries the lock.  See ReenterI(), which defers state transitions.
 //     If we use this technique we can also avoid EnterI()-exit() loop
 //     in ::enter() where we iteratively drop the lock and then attempt
 //     to reacquire it after suspending.
 //
 // B.  In the future we might fold all the suspend bits into a
 //     composite per-thread suspend flag and then update it with CAS().
 //     Alternately, a Dekker-like mechanism with multiple variables
 //     would suffice:
 //       ST Self->_suspend_equivalent = false
 //       MEMBAR
 //       LD Self_>_suspend_flags
 //
 bool ObjectMonitor::ExitSuspendEquivalent (JavaThread * jSelf) {
    int Mode = Knob_FastHSSEC ;
    if (Mode && !jSelf->is_external_suspend()) {
       assert (jSelf->is_suspend_equivalent(), "invariant") ;
       jSelf->clear_suspend_equivalent() ;
       if (2 == Mode) OrderAccess::storeload() ;
       if (!jSelf->is_external_suspend()) return false ;
       // We raced a suspension -- fall thru into the slow path
       TEVENT (ExitSuspendEquivalent - raced) ;
       jSelf->set_suspend_equivalent() ;
    }
    return jSelf->handle_special_suspend_equivalent_condition() ;
 }
 void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) {
    assert (_owner == Self, "invariant") ;
    // Exit protocol:
    // 1. ST _succ = wakee
    // 2. membar #loadstore|#storestore;
    // 2. ST _owner = NULL
    // 3. unpark(wakee)
    _succ = Knob_SuccEnabled ? Wakee->_thread : NULL ;
    ParkEvent * Trigger = Wakee->_event ;
    // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
    // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
    // out-of-scope (non-extant).
    Wakee  = NULL ;
    // Drop the lock
    OrderAccess::release_store_ptr (&_owner, NULL) ;
    OrderAccess::fence() ;                               // ST _owner vs LD in unpark()
    if (SafepointSynchronize::do_call_back()) {
       TEVENT (unpark before SAFEPOINT) ;
    }
    DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
    Trigger->unpark() ;
    // Maintain stats and report events to JVMTI
    if (ObjectMonitor::_sync_Parks != NULL) {
       ObjectMonitor::_sync_Parks->inc() ;
    }
 }
 // -----------------------------------------------------------------------------
 // Class Loader deadlock handling.
 //
 // complete_exit exits a lock returning recursion count
 // complete_exit/reenter operate as a wait without waiting
 // complete_exit requires an inflated monitor
 // The _owner field is not always the Thread addr even with an
 // inflated monitor, e.g. the monitor can be inflated by a non-owning
 // thread due to contention.
 intptr_t ObjectMonitor::complete_exit(TRAPS) {
    Thread * const Self = THREAD;
    assert(Self->is_Java_thread(), "Must be Java thread!");
    JavaThread *jt = (JavaThread *)THREAD;
    DeferredInitialize();
    if (THREAD != _owner) {
     if (THREAD->is_lock_owned ((address)_owner)) {
        assert(_recursions == 0, "internal state error");
        _owner = THREAD ;   /* Convert from basiclock addr to Thread addr */
        _recursions = 0 ;
        OwnerIsThread = 1 ;
     }
    }
    guarantee(Self == _owner, "complete_exit not owner");
    intptr_t save = _recursions; // record the old recursion count
    _recursions = 0;        // set the recursion level to be 0
    exit (true, Self) ;           // exit the monitor
    guarantee (_owner != Self, "invariant");
    return save;
 }
 // reenter() enters a lock and sets recursion count
 // complete_exit/reenter operate as a wait without waiting
 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
    Thread * const Self = THREAD;
    assert(Self->is_Java_thread(), "Must be Java thread!");
    JavaThread *jt = (JavaThread *)THREAD;
    guarantee(_owner != Self, "reenter already owner");
    enter (THREAD);       // enter the monitor
    guarantee (_recursions == 0, "reenter recursion");
    _recursions = recursions;
    return;
 }
 // -----------------------------------------------------------------------------
 // A macro is used below because there may already be a pending
 // exception which should not abort the execution of the routines
 // which use this (which is why we don't put this into check_slow and
 // call it with a CHECK argument).
 #define CHECK_OWNER()                                                             \
   do {                                                                            \
     if (THREAD != _owner) {                                                       \
       if (THREAD->is_lock_owned((address) _owner)) {                              \
         _owner = THREAD ;  /* Convert from basiclock addr to Thread addr */       \
         _recursions = 0;                                                          \
         OwnerIsThread = 1 ;                                                       \
       } else {                                                                    \
         TEVENT (Throw IMSX) ;                                                     \
         THROW(vmSymbols::java_lang_IllegalMonitorStateException());               \
       }                                                                           \
     }                                                                             \
   } while (false)
 // check_slow() is a misnomer.  It's called to simply to throw an IMSX exception.
 // TODO-FIXME: remove check_slow() -- it's likely dead.
 void ObjectMonitor::check_slow(TRAPS) {
   TEVENT (check_slow - throw IMSX) ;
   assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
   THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
 }
 static int Adjust (volatile int * adr, int dx) {
   int v ;
   for (v = *adr ; Atomic::cmpxchg (v + dx, adr, v) != v; v = *adr) ;
   return v ;
 }
 // helper method for posting a monitor wait event
 void ObjectMonitor::post_monitor_wait_event(EventJavaMonitorWait* event,
                                                            jlong notifier_tid,
                                                            jlong timeout,
                                                            bool timedout) {
   event->set_klass(((oop)this->object())->klass());
   event->set_timeout((TYPE_ULONG)timeout);
   event->set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
   event->set_notifier((TYPE_OSTHREAD)notifier_tid);
   event->set_timedOut((TYPE_BOOLEAN)timedout);
   event->commit();
 }
 // -----------------------------------------------------------------------------
 // Wait/Notify/NotifyAll
 //
 // Note: a subset of changes to ObjectMonitor::wait()
 // will need to be replicated in complete_exit above
 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
    Thread * const Self = THREAD ;
    assert(Self->is_Java_thread(), "Must be Java thread!");
    JavaThread *jt = (JavaThread *)THREAD;
    DeferredInitialize () ;
    // Throw IMSX or IEX.
    CHECK_OWNER();
    EventJavaMonitorWait event;
    // check for a pending interrupt
    if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
      // post monitor waited event.  Note that this is past-tense, we are done waiting.
      if (JvmtiExport::should_post_monitor_waited()) {
         // Note: 'false' parameter is passed here because the
         // wait was not timed out due to thread interrupt.
         JvmtiExport::post_monitor_waited(jt, this, false);
         // In this short circuit of the monitor wait protocol, the
         // current thread never drops ownership of the monitor and
         // never gets added to the wait queue so the current thread
         // cannot be made the successor. This means that the
         // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
         // consume an unpark() meant for the ParkEvent associated with
         // this ObjectMonitor.
      }
      if (event.should_commit()) {
        post_monitor_wait_event(&event, 0, millis, false);
      }
      TEVENT (Wait - Throw IEX) ;
      THROW(vmSymbols::java_lang_InterruptedException());
      return ;
    }
    TEVENT (Wait) ;
    assert (Self->_Stalled == 0, "invariant") ;
    Self->_Stalled = intptr_t(this) ;
    jt->set_current_waiting_monitor(this);
    // create a node to be put into the queue
    // Critically, after we reset() the event but prior to park(), we must check
    // for a pending interrupt.
    ObjectWaiter node(Self);
    node.TState = ObjectWaiter::TS_WAIT ;
    Self->_ParkEvent->reset() ;
    OrderAccess::fence();          // ST into Event; membar ; LD interrupted-flag
    // Enter the waiting queue, which is a circular doubly linked list in this case
    // but it could be a priority queue or any data structure.
    // _WaitSetLock protects the wait queue.  Normally the wait queue is accessed only
    // by the the owner of the monitor *except* in the case where park()
    // returns because of a timeout of interrupt.  Contention is exceptionally rare
    // so we use a simple spin-lock instead of a heavier-weight blocking lock.
    Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ;
    AddWaiter (&node) ;
    Thread::SpinRelease (&_WaitSetLock) ;
    if ((SyncFlags & 4) == 0) {
       _Responsible = NULL ;
    }
    intptr_t save = _recursions; // record the old recursion count
    _waiters++;                  // increment the number of waiters
    _recursions = 0;             // set the recursion level to be 1
    exit (true, Self) ;                    // exit the monitor
    guarantee (_owner != Self, "invariant") ;
    // The thread is on the WaitSet list - now park() it.
    // On MP systems it's conceivable that a brief spin before we park
    // could be profitable.
    //
    // TODO-FIXME: change the following logic to a loop of the form
    //   while (!timeout && !interrupted && _notified == 0) park()
    int ret = OS_OK ;
    int WasNotified = 0 ;
    { // State transition wrappers
      OSThread* osthread = Self->osthread();
      OSThreadWaitState osts(osthread, true);
      {
        ThreadBlockInVM tbivm(jt);
        // Thread is in thread_blocked state and oop access is unsafe.
        jt->set_suspend_equivalent();
        if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
            // Intentionally empty
        } else
        if (node._notified == 0) {
          if (millis <= 0) {
             Self->_ParkEvent->park () ;
          } else {
             ret = Self->_ParkEvent->park (millis) ;
          }
        }
        // were we externally suspended while we were waiting?
        if (ExitSuspendEquivalent (jt)) {
           // TODO-FIXME: add -- if succ == Self then succ = null.
           jt->java_suspend_self();
        }
      } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
      // Node may be on the WaitSet, the EntryList (or cxq), or in transition
      // from the WaitSet to the EntryList.
      // See if we need to remove Node from the WaitSet.
      // We use double-checked locking to avoid grabbing _WaitSetLock
      // if the thread is not on the wait queue.
      //
      // Note that we don't need a fence before the fetch of TState.
      // In the worst case we'll fetch a old-stale value of TS_WAIT previously
      // written by the is thread. (perhaps the fetch might even be satisfied
      // by a look-aside into the processor's own store buffer, although given
      // the length of the code path between the prior ST and this load that's
      // highly unlikely).  If the following LD fetches a stale TS_WAIT value
      // then we'll acquire the lock and then re-fetch a fresh TState value.
      // That is, we fail toward safety.
      if (node.TState == ObjectWaiter::TS_WAIT) {
          Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ;
          if (node.TState == ObjectWaiter::TS_WAIT) {
             DequeueSpecificWaiter (&node) ;       // unlink from WaitSet
             assert(node._notified == 0, "invariant");
             node.TState = ObjectWaiter::TS_RUN ;
          }
          Thread::SpinRelease (&_WaitSetLock) ;
      }
      // The thread is now either on off-list (TS_RUN),
      // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
      // The Node's TState variable is stable from the perspective of this thread.
      // No other threads will asynchronously modify TState.
      guarantee (node.TState != ObjectWaiter::TS_WAIT, "invariant") ;
      OrderAccess::loadload() ;
      if (_succ == Self) _succ = NULL ;
      WasNotified = node._notified ;
      // Reentry phase -- reacquire the monitor.
      // re-enter contended monitor after object.wait().
      // retain OBJECT_WAIT state until re-enter successfully completes
      // Thread state is thread_in_vm and oop access is again safe,
      // although the raw address of the object may have changed.
      // (Don't cache naked oops over safepoints, of course).
      // post monitor waited event. Note that this is past-tense, we are done waiting.
      if (JvmtiExport::should_post_monitor_waited()) {
        JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
        if (node._notified != 0 && _succ == Self) {
          // In this part of the monitor wait-notify-reenter protocol it
          // is possible (and normal) for another thread to do a fastpath
          // monitor enter-exit while this thread is still trying to get
          // to the reenter portion of the protocol.
          //
          // The ObjectMonitor was notified and the current thread is
          // the successor which also means that an unpark() has already
          // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
          // consume the unpark() that was done when the successor was
          // set because the same ParkEvent is shared between Java
          // monitors and JVM/TI RawMonitors (for now).
          //
          // We redo the unpark() to ensure forward progress, i.e., we
          // don't want all pending threads hanging (parked) with none
          // entering the unlocked monitor.
          node._event->unpark();
        }
      }
      if (event.should_commit()) {
        post_monitor_wait_event(&event, node._notifier_tid, millis, ret == OS_TIMEOUT);
      }
      OrderAccess::fence() ;
      assert (Self->_Stalled != 0, "invariant") ;
      Self->_Stalled = 0 ;
      assert (_owner != Self, "invariant") ;
      ObjectWaiter::TStates v = node.TState ;
      if (v == ObjectWaiter::TS_RUN) {
          enter (Self) ;
      } else {
          guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
          ReenterI (Self, &node) ;
          node.wait_reenter_end(this);
      }
      // Self has reacquired the lock.
      // Lifecycle - the node representing Self must not appear on any queues.
      // Node is about to go out-of-scope, but even if it were immortal we wouldn't
      // want residual elements associated with this thread left on any lists.
      guarantee (node.TState == ObjectWaiter::TS_RUN, "invariant") ;
      assert    (_owner == Self, "invariant") ;
      assert    (_succ != Self , "invariant") ;
    } // OSThreadWaitState()
    jt->set_current_waiting_monitor(NULL);
    guarantee (_recursions == 0, "invariant") ;
    _recursions = save;     // restore the old recursion count
    _waiters--;             // decrement the number of waiters
    // Verify a few postconditions
    assert (_owner == Self       , "invariant") ;
    assert (_succ  != Self       , "invariant") ;
    assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
    if (SyncFlags & 32) {
       OrderAccess::fence() ;
    }
    // check if the notification happened
    if (!WasNotified) {
      // no, it could be timeout or Thread.interrupt() or both
      // check for interrupt event, otherwise it is timeout
      if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
        TEVENT (Wait - throw IEX from epilog) ;
        THROW(vmSymbols::java_lang_InterruptedException());
      }
    }
    // NOTE: Spurious wake up will be consider as timeout.
    // Monitor notify has precedence over thread interrupt.
 }
 // Consider:
 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
 // then instead of transferring a thread from the WaitSet to the EntryList
 // we might just dequeue a thread from the WaitSet and directly unpark() it.
 void ObjectMonitor::notify(TRAPS) {
   CHECK_OWNER();
   if (_WaitSet == NULL) {
      TEVENT (Empty-Notify) ;
      return ;
   }
   DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
   int Policy = Knob_MoveNotifyee ;
   Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ;
   ObjectWaiter * iterator = DequeueWaiter() ;
   if (iterator != NULL) {
      TEVENT (Notify1 - Transfer) ;
      guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
      guarantee (iterator->_notified == 0, "invariant") ;
      if (Policy != 4) {
         iterator->TState = ObjectWaiter::TS_ENTER ;
      }
      iterator->_notified = 1 ;
      Thread * Self = THREAD;
      iterator->_notifier_tid = Self->osthread()->thread_id();
      ObjectWaiter * List = _EntryList ;
      if (List != NULL) {
         assert (List->_prev == NULL, "invariant") ;
         assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
         assert (List != iterator, "invariant") ;
      }
      if (Policy == 0) {       // prepend to EntryList
          if (List == NULL) {
              iterator->_next = iterator->_prev = NULL ;
              _EntryList = iterator ;
          } else {
              List->_prev = iterator ;
              iterator->_next = List ;
              iterator->_prev = NULL ;
              _EntryList = iterator ;
         }
      } else
      if (Policy == 1) {      // append to EntryList
          if (List == NULL) {
              iterator->_next = iterator->_prev = NULL ;
              _EntryList = iterator ;
          } else {
             // CONSIDER:  finding the tail currently requires a linear-time walk of
             // the EntryList.  We can make tail access constant-time by converting to
             // a CDLL instead of using our current DLL.
             ObjectWaiter * Tail ;
             for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
             assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
             Tail->_next = iterator ;
             iterator->_prev = Tail ;
             iterator->_next = NULL ;
         }
      } else
      if (Policy == 2) {      // prepend to cxq
          // prepend to cxq
          if (List == NULL) {
              iterator->_next = iterator->_prev = NULL ;
              _EntryList = iterator ;
          } else {
             iterator->TState = ObjectWaiter::TS_CXQ ;
             for (;;) {
                 ObjectWaiter * Front = _cxq ;
                 iterator->_next = Front ;
                 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
                     break ;
                 }
             }
          }
      } else
      if (Policy == 3) {      // append to cxq
         iterator->TState = ObjectWaiter::TS_CXQ ;
         for (;;) {
             ObjectWaiter * Tail ;
             Tail = _cxq ;
             if (Tail == NULL) {
                 iterator->_next = NULL ;
                 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
                    break ;
                 }
             } else {
                 while (Tail->_next != NULL) Tail = Tail->_next ;
                 Tail->_next = iterator ;
                 iterator->_prev = Tail ;
                 iterator->_next = NULL ;
                 break ;
             }
         }
      } else {
         ParkEvent * ev = iterator->_event ;
         iterator->TState = ObjectWaiter::TS_RUN ;
         OrderAccess::fence() ;
         ev->unpark() ;
      }
      if (Policy < 4) {
        iterator->wait_reenter_begin(this);
      }
      // _WaitSetLock protects the wait queue, not the EntryList.  We could
      // move the add-to-EntryList operation, above, outside the critical section
      // protected by _WaitSetLock.  In practice that's not useful.  With the
      // exception of  wait() timeouts and interrupts the monitor owner
      // is the only thread that grabs _WaitSetLock.  There's almost no contention
      // on _WaitSetLock so it's not profitable to reduce the length of the
      // critical section.
   }
   Thread::SpinRelease (&_WaitSetLock) ;
   if (iterator != NULL && ObjectMonitor::_sync_Notifications != NULL) {
      ObjectMonitor::_sync_Notifications->inc() ;
   }
 }
 void ObjectMonitor::notifyAll(TRAPS) {
   CHECK_OWNER();
   ObjectWaiter* iterator;
   if (_WaitSet == NULL) {
       TEVENT (Empty-NotifyAll) ;
       return ;
   }
   DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
   int Policy = Knob_MoveNotifyee ;
   int Tally = 0 ;
   Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notifyall") ;
   for (;;) {
      iterator = DequeueWaiter () ;
      if (iterator == NULL) break ;
      TEVENT (NotifyAll - Transfer1) ;
      ++Tally ;
      // Disposition - what might we do with iterator ?
      // a.  add it directly to the EntryList - either tail or head.
      // b.  push it onto the front of the _cxq.
      // For now we use (a).
      guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
      guarantee (iterator->_notified == 0, "invariant") ;
      iterator->_notified = 1 ;
      Thread * Self = THREAD;
      iterator->_notifier_tid = Self->osthread()->thread_id();
      if (Policy != 4) {
         iterator->TState = ObjectWaiter::TS_ENTER ;
      }
      ObjectWaiter * List = _EntryList ;
      if (List != NULL) {
         assert (List->_prev == NULL, "invariant") ;
         assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
         assert (List != iterator, "invariant") ;
      }
      if (Policy == 0) {       // prepend to EntryList
          if (List == NULL) {
              iterator->_next = iterator->_prev = NULL ;
              _EntryList = iterator ;
          } else {
              List->_prev = iterator ;
              iterator->_next = List ;
              iterator->_prev = NULL ;
              _EntryList = iterator ;
         }
      } else
      if (Policy == 1) {      // append to EntryList
          if (List == NULL) {
              iterator->_next = iterator->_prev = NULL ;
              _EntryList = iterator ;
          } else {
             // CONSIDER:  finding the tail currently requires a linear-time walk of
             // the EntryList.  We can make tail access constant-time by converting to
             // a CDLL instead of using our current DLL.
             ObjectWaiter * Tail ;
             for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
             assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
             Tail->_next = iterator ;
             iterator->_prev = Tail ;
             iterator->_next = NULL ;
         }
      } else
      if (Policy == 2) {      // prepend to cxq
          // prepend to cxq
          iterator->TState = ObjectWaiter::TS_CXQ ;
          for (;;) {
              ObjectWaiter * Front = _cxq ;
              iterator->_next = Front ;
              if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
                  break ;
              }
          }
      } else
      if (Policy == 3) {      // append to cxq
         iterator->TState = ObjectWaiter::TS_CXQ ;
         for (;;) {
             ObjectWaiter * Tail ;
             Tail = _cxq ;
             if (Tail == NULL) {
                 iterator->_next = NULL ;
                 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
                    break ;
                 }
             } else {
                 while (Tail->_next != NULL) Tail = Tail->_next ;
                 Tail->_next = iterator ;
                 iterator->_prev = Tail ;
                 iterator->_next = NULL ;
                 break ;
             }
         }
      } else {
         ParkEvent * ev = iterator->_event ;
         iterator->TState = ObjectWaiter::TS_RUN ;
         OrderAccess::fence() ;
         ev->unpark() ;
      }
      if (Policy < 4) {
        iterator->wait_reenter_begin(this);
      }
      // _WaitSetLock protects the wait queue, not the EntryList.  We could
      // move the add-to-EntryList operation, above, outside the critical section
      // protected by _WaitSetLock.  In practice that's not useful.  With the
      // exception of  wait() timeouts and interrupts the monitor owner
      // is the only thread that grabs _WaitSetLock.  There's almost no contention
      // on _WaitSetLock so it's not profitable to reduce the length of the
      // critical section.
   }
   Thread::SpinRelease (&_WaitSetLock) ;
   if (Tally != 0 && ObjectMonitor::_sync_Notifications != NULL) {
      ObjectMonitor::_sync_Notifications->inc(Tally) ;
   }
 }
 // -----------------------------------------------------------------------------
 // Adaptive Spinning Support
 //
 // Adaptive spin-then-block - rational spinning
 //
 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
 // algorithm.  On high order SMP systems it would be better to start with
 // a brief global spin and then revert to spinning locally.  In the spirit of MCS/CLH,
 // a contending thread could enqueue itself on the cxq and then spin locally
 // on a thread-specific variable such as its ParkEvent._Event flag.
 // That's left as an exercise for the reader.  Note that global spinning is
 // not problematic on Niagara, as the L2$ serves the interconnect and has both
 // low latency and massive bandwidth.
 //
 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
 // acquisition attempts where we opt to spin --  at 100% and vary the spin count
 // (duration) or we can fix the count at approximately the duration of
 // a context switch and vary the frequency.   Of course we could also
 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
 // For a description of 'Adaptive spin-then-block mutual exclusion in
 // multi-threaded processing,' see U.S. Pat. No. 8046758.
 //
 // This implementation varies the duration "D", where D varies with
 // the success rate of recent spin attempts. (D is capped at approximately
 // length of a round-trip context switch).  The success rate for recent
 // spin attempts is a good predictor of the success rate of future spin
 // attempts.  The mechanism adapts automatically to varying critical
 // section length (lock modality), system load and degree of parallelism.
 // D is maintained per-monitor in _SpinDuration and is initialized
 // optimistically.  Spin frequency is fixed at 100%.
 //
 // Note that _SpinDuration is volatile, but we update it without locks
 // or atomics.  The code is designed so that _SpinDuration stays within
 // a reasonable range even in the presence of races.  The arithmetic
 // operations on _SpinDuration are closed over the domain of legal values,
 // so at worst a race will install and older but still legal value.
 // At the very worst this introduces some apparent non-determinism.
 // We might spin when we shouldn't or vice-versa, but since the spin
 // count are relatively short, even in the worst case, the effect is harmless.
 //
 // Care must be taken that a low "D" value does not become an
 // an absorbing state.  Transient spinning failures -- when spinning
 // is overall profitable -- should not cause the system to converge
 // on low "D" values.  We want spinning to be stable and predictable
 // and fairly responsive to change and at the same time we don't want
 // it to oscillate, become metastable, be "too" non-deterministic,
 // or converge on or enter undesirable stable absorbing states.
 //
 // We implement a feedback-based control system -- using past behavior
 // to predict future behavior.  We face two issues: (a) if the
 // input signal is random then the spin predictor won't provide optimal
 // results, and (b) if the signal frequency is too high then the control
 // system, which has some natural response lag, will "chase" the signal.
 // (b) can arise from multimodal lock hold times.  Transient preemption
 // can also result in apparent bimodal lock hold times.
 // Although sub-optimal, neither condition is particularly harmful, as
 // in the worst-case we'll spin when we shouldn't or vice-versa.
 // The maximum spin duration is rather short so the failure modes aren't bad.
 // To be conservative, I've tuned the gain in system to bias toward
 // _not spinning.  Relatedly, the system can sometimes enter a mode where it
 // "rings" or oscillates between spinning and not spinning.  This happens
 // when spinning is just on the cusp of profitability, however, so the
 // situation is not dire.  The state is benign -- there's no need to add
 // hysteresis control to damp the transition rate between spinning and
 // not spinning.
 //
 intptr_t ObjectMonitor::SpinCallbackArgument = 0 ;
 int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL ;
 // Spinning: Fixed frequency (100%), vary duration
 int ObjectMonitor::TrySpin_VaryDuration (Thread * Self) {
     // Dumb, brutal spin.  Good for comparative measurements against adaptive spinning.
     int ctr = Knob_FixedSpin ;
     if (ctr != 0) {
         while (--ctr >= 0) {
             if (TryLock (Self) > 0) return 1 ;
             SpinPause () ;
         }
         return 0 ;
     }
     for (ctr = Knob_PreSpin + 1; --ctr >= 0 ; ) {
       if (TryLock(Self) > 0) {
         // Increase _SpinDuration ...
         // Note that we don't clamp SpinDuration precisely at SpinLimit.
         // Raising _SpurDuration to the poverty line is key.
         int x = _SpinDuration ;
         if (x < Knob_SpinLimit) {
            if (x < Knob_Poverty) x = Knob_Poverty ;
            _SpinDuration = x + Knob_BonusB ;
         }
         return 1 ;
       }
       SpinPause () ;
     }
     // Admission control - verify preconditions for spinning
     //
     // We always spin a little bit, just to prevent _SpinDuration == 0 from
     // becoming an absorbing state.  Put another way, we spin briefly to
     // sample, just in case the system load, parallelism, contention, or lock
     // modality changed.
     //
     // Consider the following alternative:
     // Periodically set _SpinDuration = _SpinLimit and try a long/full
     // spin attempt.  "Periodically" might mean after a tally of
     // the # of failed spin attempts (or iterations) reaches some threshold.
     // This takes us into the realm of 1-out-of-N spinning, where we
     // hold the duration constant but vary the frequency.
     ctr = _SpinDuration  ;
     if (ctr < Knob_SpinBase) ctr = Knob_SpinBase ;
     if (ctr <= 0) return 0 ;
     if (Knob_SuccRestrict && _succ != NULL) return 0 ;
     if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
        TEVENT (Spin abort - notrunnable [TOP]);
        return 0 ;
     }
     int MaxSpin = Knob_MaxSpinners ;
     if (MaxSpin >= 0) {
        if (_Spinner > MaxSpin) {
           TEVENT (Spin abort -- too many spinners) ;
           return 0 ;
        }
        // Slighty racy, but benign ...
        Adjust (&_Spinner, 1) ;
     }
     // We're good to spin ... spin ingress.
     // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
     // when preparing to LD...CAS _owner, etc and the CAS is likely
     // to succeed.
     int hits    = 0 ;
     int msk     = 0 ;
     int caspty  = Knob_CASPenalty ;
     int oxpty   = Knob_OXPenalty ;
     int sss     = Knob_SpinSetSucc ;
     if (sss && _succ == NULL ) _succ = Self ;
     Thread * prv = NULL ;
     // There are three ways to exit the following loop:
     // 1.  A successful spin where this thread has acquired the lock.
     // 2.  Spin failure with prejudice
     // 3.  Spin failure without prejudice
     while (--ctr >= 0) {
       // Periodic polling -- Check for pending GC
       // Threads may spin while they're unsafe.
       // We don't want spinning threads to delay the JVM from reaching
       // a stop-the-world safepoint or to steal cycles from GC.
       // If we detect a pending safepoint we abort in order that
       // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
       // this thread, if safe, doesn't steal cycles from GC.
       // This is in keeping with the "no loitering in runtime" rule.
       // We periodically check to see if there's a safepoint pending.
       if ((ctr & 0xFF) == 0) {
          if (SafepointSynchronize::do_call_back()) {
             TEVENT (Spin: safepoint) ;
             goto Abort ;           // abrupt spin egress
          }
          if (Knob_UsePause & 1) SpinPause () ;
          int (*scb)(intptr_t,int) = SpinCallbackFunction ;
          if (hits > 50 && scb != NULL) {
             int abend = (*scb)(SpinCallbackArgument, 0) ;
          }
       }
       if (Knob_UsePause & 2) SpinPause() ;
       // Exponential back-off ...  Stay off the bus to reduce coherency traffic.
       // This is useful on classic SMP systems, but is of less utility on
       // N1-style CMT platforms.
       //
       // Trade-off: lock acquisition latency vs coherency bandwidth.
       // Lock hold times are typically short.  A histogram
       // of successful spin attempts shows that we usually acquire
       // the lock early in the spin.  That suggests we want to
       // sample _owner frequently in the early phase of the spin,
       // but then back-off and sample less frequently as the spin
       // progresses.  The back-off makes a good citizen on SMP big
       // SMP systems.  Oversampling _owner can consume excessive
       // coherency bandwidth.  Relatedly, if we _oversample _owner we
       // can inadvertently interfere with the the ST m->owner=null.
       // executed by the lock owner.
       if (ctr & msk) continue ;
       ++hits ;
       if ((hits & 0xF) == 0) {
         // The 0xF, above, corresponds to the exponent.
         // Consider: (msk+1)|msk
         msk = ((msk << 2)|3) & BackOffMask ;
       }
       // Probe _owner with TATAS
       // If this thread observes the monitor transition or flicker
       // from locked to unlocked to locked, then the odds that this
       // thread will acquire the lock in this spin attempt go down
       // considerably.  The same argument applies if the CAS fails
       // or if we observe _owner change from one non-null value to
       // another non-null value.   In such cases we might abort
       // the spin without prejudice or apply a "penalty" to the
       // spin count-down variable "ctr", reducing it by 100, say.
       Thread * ox = (Thread *) _owner ;
       if (ox == NULL) {
          ox = (Thread *) Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
          if (ox == NULL) {
             // The CAS succeeded -- this thread acquired ownership
             // Take care of some bookkeeping to exit spin state.
             if (sss && _succ == Self) {
                _succ = NULL ;
             }
             if (MaxSpin > 0) Adjust (&_Spinner, -1) ;
             // Increase _SpinDuration :
             // The spin was successful (profitable) so we tend toward
             // longer spin attempts in the future.
             // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
             // If we acquired the lock early in the spin cycle it
             // makes sense to increase _SpinDuration proportionally.
             // Note that we don't clamp SpinDuration precisely at SpinLimit.
             int x = _SpinDuration ;
             if (x < Knob_SpinLimit) {
                 if (x < Knob_Poverty) x = Knob_Poverty ;
                 _SpinDuration = x + Knob_Bonus ;
             }
             return 1 ;
          }
          // The CAS failed ... we can take any of the following actions:
          // * penalize: ctr -= Knob_CASPenalty
          // * exit spin with prejudice -- goto Abort;
          // * exit spin without prejudice.
          // * Since CAS is high-latency, retry again immediately.
          prv = ox ;
          TEVENT (Spin: cas failed) ;
          if (caspty == -2) break ;
          if (caspty == -1) goto Abort ;
          ctr -= caspty ;
          continue ;
       }
       // Did lock ownership change hands ?
       if (ox != prv && prv != NULL ) {
           TEVENT (spin: Owner changed)
           if (oxpty == -2) break ;
           if (oxpty == -1) goto Abort ;
           ctr -= oxpty ;
       }
       prv = ox ;
       // Abort the spin if the owner is not executing.
       // The owner must be executing in order to drop the lock.
       // Spinning while the owner is OFFPROC is idiocy.
       // Consider: ctr -= RunnablePenalty ;
       if (Knob_OState && NotRunnable (Self, ox)) {
          TEVENT (Spin abort - notrunnable);
          goto Abort ;
       }
       if (sss && _succ == NULL ) _succ = Self ;
    }
    // Spin failed with prejudice -- reduce _SpinDuration.
    // TODO: Use an AIMD-like policy to adjust _SpinDuration.
    // AIMD is globally stable.
    TEVENT (Spin failure) ;
    {
      int x = _SpinDuration ;
      if (x > 0) {
         // Consider an AIMD scheme like: x -= (x >> 3) + 100
         // This is globally sample and tends to damp the response.
         x -= Knob_Penalty ;
         if (x < 0) x = 0 ;
         _SpinDuration = x ;
      }
    }
  Abort:
    if (MaxSpin >= 0) Adjust (&_Spinner, -1) ;
    if (sss && _succ == Self) {
       _succ = NULL ;
       // Invariant: after setting succ=null a contending thread
       // must recheck-retry _owner before parking.  This usually happens
       // in the normal usage of TrySpin(), but it's safest
       // to make TrySpin() as foolproof as possible.
       OrderAccess::fence() ;
       if (TryLock(Self) > 0) return 1 ;
    }
    return 0 ;
 }
 // NotRunnable() -- informed spinning
 //
 // Don't bother spinning if the owner is not eligible to drop the lock.
 // Peek at the owner's schedctl.sc_state and Thread._thread_values and
 // spin only if the owner thread is _thread_in_Java or _thread_in_vm.
 // The thread must be runnable in order to drop the lock in timely fashion.
 // If the _owner is not runnable then spinning will not likely be
 // successful (profitable).
 //
 // Beware -- the thread referenced by _owner could have died
 // so a simply fetch from _owner->_thread_state might trap.
 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
 // Because of the lifecycle issues the schedctl and _thread_state values
 // observed by NotRunnable() might be garbage.  NotRunnable must
 // tolerate this and consider the observed _thread_state value
 // as advisory.
 //
 // Beware too, that _owner is sometimes a BasicLock address and sometimes
 // a thread pointer.  We differentiate the two cases with OwnerIsThread.
 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
 // with the LSB of _owner.  Another option would be to probablistically probe
 // the putative _owner->TypeTag value.
 //
 // Checking _thread_state isn't perfect.  Even if the thread is
 // in_java it might be blocked on a page-fault or have been preempted
 // and sitting on a ready/dispatch queue.  _thread state in conjunction
 // with schedctl.sc_state gives us a good picture of what the
 // thread is doing, however.
 //
 // TODO: check schedctl.sc_state.
 // We'll need to use SafeFetch32() to read from the schedctl block.
 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/
 //
 // The return value from NotRunnable() is *advisory* -- the
 // result is based on sampling and is not necessarily coherent.
 // The caller must tolerate false-negative and false-positive errors.
 // Spinning, in general, is probabilistic anyway.
 int ObjectMonitor::NotRunnable (Thread * Self, Thread * ox) {
     // Check either OwnerIsThread or ox->TypeTag == 2BAD.
     if (!OwnerIsThread) return 0 ;
     if (ox == NULL) return 0 ;
     // Avoid transitive spinning ...
     // Say T1 spins or blocks trying to acquire L.  T1._Stalled is set to L.
     // Immediately after T1 acquires L it's possible that T2, also
     // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
     // This occurs transiently after T1 acquired L but before
     // T1 managed to clear T1.Stalled.  T2 does not need to abort
     // its spin in this circumstance.
     intptr_t BlockedOn = SafeFetchN ((intptr_t *) &ox->_Stalled, intptr_t(1)) ;
     if (BlockedOn == 1) return 1 ;
     if (BlockedOn != 0) {
       return BlockedOn != intptr_t(this) && _owner == ox ;
     }
     assert (sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant") ;
     int jst = SafeFetch32 ((int *) &((JavaThread *) ox)->_thread_state, -1) ; ;
     // consider also: jst != _thread_in_Java -- but that's overspecific.
     return jst == _thread_blocked || jst == _thread_in_native ;
 }
 // -----------------------------------------------------------------------------
 // WaitSet management ...
 ObjectWaiter::ObjectWaiter(Thread* thread) {
   _next     = NULL;
   _prev     = NULL;
   _notified = 0;
   TState    = TS_RUN ;
   _thread   = thread;
   _event    = thread->_ParkEvent ;
   _active   = false;
   assert (_event != NULL, "invariant") ;
 }
 void ObjectWaiter::wait_reenter_begin(ObjectMonitor *mon) {
   JavaThread *jt = (JavaThread *)this->_thread;
   _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
 }
 void ObjectWaiter::wait_reenter_end(ObjectMonitor *mon) {
   JavaThread *jt = (JavaThread *)this->_thread;
   JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
 }
 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
   assert(node != NULL, "should not dequeue NULL node");
   assert(node->_prev == NULL, "node already in list");
   assert(node->_next == NULL, "node already in list");
   // put node at end of queue (circular doubly linked list)
   if (_WaitSet == NULL) {
     _WaitSet = node;
     node->_prev = node;
     node->_next = node;
   } else {
     ObjectWaiter* head = _WaitSet ;
     ObjectWaiter* tail = head->_prev;
     assert(tail->_next == head, "invariant check");
     tail->_next = node;
     head->_prev = node;
     node->_next = head;
     node->_prev = tail;
   }
 }
 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
   // dequeue the very first waiter
   ObjectWaiter* waiter = _WaitSet;
   if (waiter) {
     DequeueSpecificWaiter(waiter);
   }
   return waiter;
 }
 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
   assert(node != NULL, "should not dequeue NULL node");
   assert(node->_prev != NULL, "node already removed from list");
   assert(node->_next != NULL, "node already removed from list");
   // when the waiter has woken up because of interrupt,
   // timeout or other spurious wake-up, dequeue the
   // waiter from waiting list
   ObjectWaiter* next = node->_next;
   if (next == node) {
     assert(node->_prev == node, "invariant check");
     _WaitSet = NULL;
   } else {
     ObjectWaiter* prev = node->_prev;
     assert(prev->_next == node, "invariant check");
     assert(next->_prev == node, "invariant check");
     next->_prev = prev;
     prev->_next = next;
     if (_WaitSet == node) {
       _WaitSet = next;
     }
   }
   node->_next = NULL;
   node->_prev = NULL;
 }
 // -----------------------------------------------------------------------------
 // PerfData support
 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts       = NULL ;
 PerfCounter * ObjectMonitor::_sync_FutileWakeups               = NULL ;
 PerfCounter * ObjectMonitor::_sync_Parks                       = NULL ;
 PerfCounter * ObjectMonitor::_sync_EmptyNotifications          = NULL ;
 PerfCounter * ObjectMonitor::_sync_Notifications               = NULL ;
 PerfCounter * ObjectMonitor::_sync_PrivateA                    = NULL ;
 PerfCounter * ObjectMonitor::_sync_PrivateB                    = NULL ;
 PerfCounter * ObjectMonitor::_sync_SlowExit                    = NULL ;
 PerfCounter * ObjectMonitor::_sync_SlowEnter                   = NULL ;
 PerfCounter * ObjectMonitor::_sync_SlowNotify                  = NULL ;
 PerfCounter * ObjectMonitor::_sync_SlowNotifyAll               = NULL ;
 PerfCounter * ObjectMonitor::_sync_FailedSpins                 = NULL ;
 PerfCounter * ObjectMonitor::_sync_SuccessfulSpins             = NULL ;
 PerfCounter * ObjectMonitor::_sync_MonInCirculation            = NULL ;
 PerfCounter * ObjectMonitor::_sync_MonScavenged                = NULL ;
 PerfCounter * ObjectMonitor::_sync_Inflations                  = NULL ;
 PerfCounter * ObjectMonitor::_sync_Deflations                  = NULL ;
 PerfLongVariable * ObjectMonitor::_sync_MonExtant              = NULL ;
 // One-shot global initialization for the sync subsystem.
 // We could also defer initialization and initialize on-demand
 // the first time we call inflate().  Initialization would
 // be protected - like so many things - by the MonitorCache_lock.
 void ObjectMonitor::Initialize () {
   static int InitializationCompleted = 0 ;
   assert (InitializationCompleted == 0, "invariant") ;
   InitializationCompleted = 1 ;
   if (UsePerfData) {
       EXCEPTION_MARK ;
       #define NEWPERFCOUNTER(n)   {n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,CHECK); }
       #define NEWPERFVARIABLE(n)  {n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,CHECK); }
       NEWPERFCOUNTER(_sync_Inflations) ;
       NEWPERFCOUNTER(_sync_Deflations) ;
       NEWPERFCOUNTER(_sync_ContendedLockAttempts) ;
       NEWPERFCOUNTER(_sync_FutileWakeups) ;
       NEWPERFCOUNTER(_sync_Parks) ;
       NEWPERFCOUNTER(_sync_EmptyNotifications) ;
       NEWPERFCOUNTER(_sync_Notifications) ;
       NEWPERFCOUNTER(_sync_SlowEnter) ;
       NEWPERFCOUNTER(_sync_SlowExit) ;
       NEWPERFCOUNTER(_sync_SlowNotify) ;
       NEWPERFCOUNTER(_sync_SlowNotifyAll) ;
       NEWPERFCOUNTER(_sync_FailedSpins) ;
       NEWPERFCOUNTER(_sync_SuccessfulSpins) ;
       NEWPERFCOUNTER(_sync_PrivateA) ;
       NEWPERFCOUNTER(_sync_PrivateB) ;
       NEWPERFCOUNTER(_sync_MonInCirculation) ;
       NEWPERFCOUNTER(_sync_MonScavenged) ;
       NEWPERFVARIABLE(_sync_MonExtant) ;
       #undef NEWPERFCOUNTER
   }
 }
 // Compile-time asserts
 // When possible, it's better to catch errors deterministically at
 // compile-time than at runtime.  The down-side to using compile-time
 // asserts is that error message -- often something about negative array
 // indices -- is opaque.
 #define CTASSERT(x) { int tag[1-(2*!(x))]; printf ("Tag @" INTPTR_FORMAT "\n", (intptr_t)tag); }
 void ObjectMonitor::ctAsserts() {
   CTASSERT(offset_of (ObjectMonitor, _header) == 0);
 }
 static char * kvGet (char * kvList, const char * Key) {
     if (kvList == NULL) return NULL ;
     size_t n = strlen (Key) ;
     char * Search ;
     for (Search = kvList ; *Search ; Search += strlen(Search) + 1) {
         if (strncmp (Search, Key, n) == 0) {
             if (Search[n] == '=') return Search + n + 1 ;
             if (Search[n] == 0)   return (char *) "1" ;
         }
     }
     return NULL ;
 }
 static int kvGetInt (char * kvList, const char * Key, int Default) {
     char * v = kvGet (kvList, Key) ;
     int rslt = v ? ::strtol (v, NULL, 0) : Default ;
     if (Knob_ReportSettings && v != NULL) {
         ::printf ("  SyncKnob: %s %d(%d)\n", Key, rslt, Default) ;
         ::fflush (stdout) ;
     }
     return rslt ;
 }
 void ObjectMonitor::DeferredInitialize () {
   if (InitDone > 0) return ;
   if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
       while (InitDone != 1) ;
       return ;
   }
   // One-shot global initialization ...
   // The initialization is idempotent, so we don't need locks.
   // In the future consider doing this via os::init_2().
   // SyncKnobs consist of <Key>=<Value> pairs in the style
   // of environment variables.  Start by converting ':' to NUL.
   if (SyncKnobs == NULL) SyncKnobs = "" ;
   size_t sz = strlen (SyncKnobs) ;
   char * knobs = (char *) malloc (sz + 2) ;
   if (knobs == NULL) {
      vm_exit_out_of_memory (sz + 2, OOM_MALLOC_ERROR, "Parse SyncKnobs") ;
      guarantee (0, "invariant") ;
   }
   strcpy (knobs, SyncKnobs) ;
   knobs[sz+1] = 0 ;
   for (char * p = knobs ; *p ; p++) {
      if (*p == ':') *p = 0 ;
   }
   #define SETKNOB(x) { Knob_##x = kvGetInt (knobs, #x, Knob_##x); }
   SETKNOB(ReportSettings) ;
   SETKNOB(Verbose) ;
   SETKNOB(FixedSpin) ;
   SETKNOB(SpinLimit) ;
   SETKNOB(SpinBase) ;
   SETKNOB(SpinBackOff);
   SETKNOB(CASPenalty) ;
   SETKNOB(OXPenalty) ;
   SETKNOB(LogSpins) ;
   SETKNOB(SpinSetSucc) ;
   SETKNOB(SuccEnabled) ;
   SETKNOB(SuccRestrict) ;
   SETKNOB(Penalty) ;
   SETKNOB(Bonus) ;
   SETKNOB(BonusB) ;
   SETKNOB(Poverty) ;
   SETKNOB(SpinAfterFutile) ;
   SETKNOB(UsePause) ;
   SETKNOB(SpinEarly) ;
   SETKNOB(OState) ;
   SETKNOB(MaxSpinners) ;
   SETKNOB(PreSpin) ;
   SETKNOB(ExitPolicy) ;
   SETKNOB(QMode);
   SETKNOB(ResetEvent) ;
   SETKNOB(MoveNotifyee) ;
   SETKNOB(FastHSSEC) ;
   #undef SETKNOB
   if (Knob_Verbose) {
     sanity_checks();
   }
   if (os::is_MP()) {
      BackOffMask = (1 << Knob_SpinBackOff) - 1 ;
      if (Knob_ReportSettings) ::printf ("BackOffMask=%X\n", BackOffMask) ;
      // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
   } else {
      Knob_SpinLimit = 0 ;
      Knob_SpinBase  = 0 ;
      Knob_PreSpin   = 0 ;
      Knob_FixedSpin = -1 ;
   }
   if (Knob_LogSpins == 0) {
      ObjectMonitor::_sync_FailedSpins = NULL ;
   }
   free (knobs) ;
   OrderAccess::fence() ;
   InitDone = 1 ;
 }
 void ObjectMonitor::sanity_checks() {
   int error_cnt = 0;
   int warning_cnt = 0;
   bool verbose = Knob_Verbose != 0 NOT_PRODUCT(|| VerboseInternalVMTests);
   if (verbose) {
     tty->print_cr("INFO: sizeof(ObjectMonitor)=" SIZE_FORMAT,
                   sizeof(ObjectMonitor));
   }
   uint cache_line_size = VM_Version::L1_data_cache_line_size();
   if (verbose) {
     tty->print_cr("INFO: L1_data_cache_line_size=%u", cache_line_size);
   }
   ObjectMonitor dummy;
   u_char *addr_begin  = (u_char*)&dummy;
   u_char *addr_header = (u_char*)&dummy._header;
   u_char *addr_owner  = (u_char*)&dummy._owner;
   uint offset_header = (uint)(addr_header - addr_begin);
   if (verbose) tty->print_cr("INFO: offset(_header)=%u", offset_header);
   uint offset_owner = (uint)(addr_owner - addr_begin);
   if (verbose) tty->print_cr("INFO: offset(_owner)=%u", offset_owner);
   if ((uint)(addr_header - addr_begin) != 0) {
     tty->print_cr("ERROR: offset(_header) must be zero (0).");
     error_cnt++;
   }
   if (cache_line_size != 0) {
     // We were able to determine the L1 data cache line size so
     // do some cache line specific sanity checks
     if ((offset_owner - offset_header) < cache_line_size) {
       tty->print_cr("WARNING: the _header and _owner fields are closer "
                     "than a cache line which permits false sharing.");
       warning_cnt++;
     }
     if ((sizeof(ObjectMonitor) % cache_line_size) != 0) {
       tty->print_cr("WARNING: ObjectMonitor size is not a multiple of "
                     "a cache line which permits false sharing.");
       warning_cnt++;
     }
   }
   ObjectSynchronizer::sanity_checks(verbose, cache_line_size, &error_cnt,
                                     &warning_cnt);
   if (verbose || error_cnt != 0 || warning_cnt != 0) {
     tty->print_cr("INFO: error_cnt=%d", error_cnt);
     tty->print_cr("INFO: warning_cnt=%d", warning_cnt);
   }
   guarantee(error_cnt == 0,
             "Fatal error(s) found in ObjectMonitor::sanity_checks()");
 }
 #ifndef PRODUCT
 void ObjectMonitor::verify() {
 }
 void ObjectMonitor::print() {
 }
 #endif

Mercurial > jdk8-mips64-public > hotspot / annotate

src/share/vm/runtime/objectMonitor.cpp@95a08233f46c (annotated)

src/share/vm/runtime/objectMonitor.cpp