1 /*
   2  * Copyright (c) 1998, 2018, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.
   8  *
   9  * This code is distributed in the hope that it will be useful, but WITHOUT
  10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  12  * version 2 for more details (a copy is included in the LICENSE file that
  13  * accompanied this code).
  14  *
  15  * You should have received a copy of the GNU General Public License version
  16  * 2 along with this work; if not, write to the Free Software Foundation,
  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  18  *
  19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  20  * or visit www.oracle.com if you need additional information or have any
  21  * questions.
  22  *
  23  */
  24 
  25 #include "precompiled.hpp"
  26 #include "classfile/vmSymbols.hpp"
  27 #include "jfr/jfrEvents.hpp"
  28 #include "jfr/support/jfrThreadId.hpp"
  29 #include "memory/allocation.inline.hpp"
  30 #include "memory/resourceArea.hpp"
  31 #include "oops/markOop.hpp"
  32 #include "oops/oop.inline.hpp"
  33 #include "runtime/atomic.hpp"
  34 #include "runtime/handles.inline.hpp"
  35 #include "runtime/interfaceSupport.inline.hpp"
  36 #include "runtime/mutexLocker.hpp"
  37 #include "runtime/objectMonitor.hpp"
  38 #include "runtime/objectMonitor.inline.hpp"
  39 #include "runtime/orderAccess.hpp"
  40 #include "runtime/osThread.hpp"
  41 #include "runtime/safepointMechanism.inline.hpp"
  42 #include "runtime/sharedRuntime.hpp"
  43 #include "runtime/stubRoutines.hpp"
  44 #include "runtime/thread.inline.hpp"
  45 #include "services/threadService.hpp"
  46 #include "utilities/dtrace.hpp"
  47 #include "utilities/macros.hpp"
  48 #include "utilities/preserveException.hpp"
  49 #if INCLUDE_JFR
  50 #include "jfr/support/jfrFlush.hpp"
  51 #endif
  52 
  53 #ifdef DTRACE_ENABLED
  54 
  55 // Only bother with this argument setup if dtrace is available
  56 // TODO-FIXME: probes should not fire when caller is _blocked.  assert() accordingly.
  57 
  58 
  59 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread)                           \
  60   char* bytes = NULL;                                                      \
  61   int len = 0;                                                             \
  62   jlong jtid = SharedRuntime::get_java_tid(thread);                        \
  63   Symbol* klassname = ((oop)obj)->klass()->name();                         \
  64   if (klassname != NULL) {                                                 \
  65     bytes = (char*)klassname->bytes();                                     \
  66     len = klassname->utf8_length();                                        \
  67   }
  68 
  69 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis)            \
  70   {                                                                        \
  71     if (DTraceMonitorProbes) {                                             \
  72       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                            \
  73       HOTSPOT_MONITOR_WAIT(jtid,                                           \
  74                            (monitor), bytes, len, (millis));               \
  75     }                                                                      \
  76   }
  77 
  78 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
  79 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
  80 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
  81 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
  82 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
  83 
  84 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread)                  \
  85   {                                                                        \
  86     if (DTraceMonitorProbes) {                                             \
  87       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                            \
  88       HOTSPOT_MONITOR_##probe(jtid,                                        \
  89                               (uintptr_t)(monitor), bytes, len);           \
  90     }                                                                      \
  91   }
  92 
  93 #else //  ndef DTRACE_ENABLED
  94 
  95 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon)    {;}
  96 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon)          {;}
  97 
  98 #endif // ndef DTRACE_ENABLED
  99 
 100 // Tunables ...
 101 // The knob* variables are effectively final.  Once set they should
 102 // never be modified hence.  Consider using __read_mostly with GCC.
 103 
 104 int ObjectMonitor::Knob_SpinLimit    = 5000;    // derived by an external tool -
 105 
 106 static int Knob_MaxSpinners         = -1;      // Should be a function of # CPUs
 107 static int Knob_Bonus               = 100;     // spin success bonus
 108 static int Knob_BonusB              = 100;     // spin success bonus
 109 static int Knob_Penalty             = 200;     // spin failure penalty
 110 static int Knob_Poverty             = 1000;
 111 static int Knob_SpinAfterFutile     = 1;       // Spin after returning from park()
 112 static int Knob_FixedSpin           = 0;
 113 static int Knob_OState              = 3;       // Spinner checks thread state of _owner
 114 static int Knob_UsePause            = 1;
 115 static int Knob_ExitPolicy          = 0;
 116 static int Knob_PreSpin             = 10;      // 20-100 likely better
 117 static int Knob_ResetEvent          = 0;
 118 
 119 static int Knob_FastHSSEC           = 0;
 120 static int Knob_MoveNotifyee        = 2;       // notify() - disposition of notifyee
 121 static int Knob_QMode               = 0;       // EntryList-cxq policy - queue discipline
 122 static volatile int InitDone        = 0;
 123 
 124 // -----------------------------------------------------------------------------
 125 // Theory of operations -- Monitors lists, thread residency, etc:
 126 //
 127 // * A thread acquires ownership of a monitor by successfully
 128 //   CAS()ing the _owner field from null to non-null.
 129 //
 130 // * Invariant: A thread appears on at most one monitor list --
 131 //   cxq, EntryList or WaitSet -- at any one time.
 132 //
 133 // * Contending threads "push" themselves onto the cxq with CAS
 134 //   and then spin/park.
 135 //
 136 // * After a contending thread eventually acquires the lock it must
 137 //   dequeue itself from either the EntryList or the cxq.
 138 //
 139 // * The exiting thread identifies and unparks an "heir presumptive"
 140 //   tentative successor thread on the EntryList.  Critically, the
 141 //   exiting thread doesn't unlink the successor thread from the EntryList.
 142 //   After having been unparked, the wakee will recontend for ownership of
 143 //   the monitor.   The successor (wakee) will either acquire the lock or
 144 //   re-park itself.
 145 //
 146 //   Succession is provided for by a policy of competitive handoff.
 147 //   The exiting thread does _not_ grant or pass ownership to the
 148 //   successor thread.  (This is also referred to as "handoff" succession").
 149 //   Instead the exiting thread releases ownership and possibly wakes
 150 //   a successor, so the successor can (re)compete for ownership of the lock.
 151 //   If the EntryList is empty but the cxq is populated the exiting
 152 //   thread will drain the cxq into the EntryList.  It does so by
 153 //   by detaching the cxq (installing null with CAS) and folding
 154 //   the threads from the cxq into the EntryList.  The EntryList is
 155 //   doubly linked, while the cxq is singly linked because of the
 156 //   CAS-based "push" used to enqueue recently arrived threads (RATs).
 157 //
 158 // * Concurrency invariants:
 159 //
 160 //   -- only the monitor owner may access or mutate the EntryList.
 161 //      The mutex property of the monitor itself protects the EntryList
 162 //      from concurrent interference.
 163 //   -- Only the monitor owner may detach the cxq.
 164 //
 165 // * The monitor entry list operations avoid locks, but strictly speaking
 166 //   they're not lock-free.  Enter is lock-free, exit is not.
 167 //   For a description of 'Methods and apparatus providing non-blocking access
 168 //   to a resource,' see U.S. Pat. No. 7844973.
 169 //
 170 // * The cxq can have multiple concurrent "pushers" but only one concurrent
 171 //   detaching thread.  This mechanism is immune from the ABA corruption.
 172 //   More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
 173 //
 174 // * Taken together, the cxq and the EntryList constitute or form a
 175 //   single logical queue of threads stalled trying to acquire the lock.
 176 //   We use two distinct lists to improve the odds of a constant-time
 177 //   dequeue operation after acquisition (in the ::enter() epilogue) and
 178 //   to reduce heat on the list ends.  (c.f. Michael Scott's "2Q" algorithm).
 179 //   A key desideratum is to minimize queue & monitor metadata manipulation
 180 //   that occurs while holding the monitor lock -- that is, we want to
 181 //   minimize monitor lock holds times.  Note that even a small amount of
 182 //   fixed spinning will greatly reduce the # of enqueue-dequeue operations
 183 //   on EntryList|cxq.  That is, spinning relieves contention on the "inner"
 184 //   locks and monitor metadata.
 185 //
 186 //   Cxq points to the set of Recently Arrived Threads attempting entry.
 187 //   Because we push threads onto _cxq with CAS, the RATs must take the form of
 188 //   a singly-linked LIFO.  We drain _cxq into EntryList  at unlock-time when
 189 //   the unlocking thread notices that EntryList is null but _cxq is != null.
 190 //
 191 //   The EntryList is ordered by the prevailing queue discipline and
 192 //   can be organized in any convenient fashion, such as a doubly-linked list or
 193 //   a circular doubly-linked list.  Critically, we want insert and delete operations
 194 //   to operate in constant-time.  If we need a priority queue then something akin
 195 //   to Solaris' sleepq would work nicely.  Viz.,
 196 //   http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
 197 //   Queue discipline is enforced at ::exit() time, when the unlocking thread
 198 //   drains the cxq into the EntryList, and orders or reorders the threads on the
 199 //   EntryList accordingly.
 200 //
 201 //   Barring "lock barging", this mechanism provides fair cyclic ordering,
 202 //   somewhat similar to an elevator-scan.
 203 //
 204 // * The monitor synchronization subsystem avoids the use of native
 205 //   synchronization primitives except for the narrow platform-specific
 206 //   park-unpark abstraction.  See the comments in os_solaris.cpp regarding
 207 //   the semantics of park-unpark.  Put another way, this monitor implementation
 208 //   depends only on atomic operations and park-unpark.  The monitor subsystem
 209 //   manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
 210 //   underlying OS manages the READY<->RUN transitions.
 211 //
 212 // * Waiting threads reside on the WaitSet list -- wait() puts
 213 //   the caller onto the WaitSet.
 214 //
 215 // * notify() or notifyAll() simply transfers threads from the WaitSet to
 216 //   either the EntryList or cxq.  Subsequent exit() operations will
 217 //   unpark the notifyee.  Unparking a notifee in notify() is inefficient -
 218 //   it's likely the notifyee would simply impale itself on the lock held
 219 //   by the notifier.
 220 //
 221 // * An interesting alternative is to encode cxq as (List,LockByte) where
 222 //   the LockByte is 0 iff the monitor is owned.  _owner is simply an auxiliary
 223 //   variable, like _recursions, in the scheme.  The threads or Events that form
 224 //   the list would have to be aligned in 256-byte addresses.  A thread would
 225 //   try to acquire the lock or enqueue itself with CAS, but exiting threads
 226 //   could use a 1-0 protocol and simply STB to set the LockByte to 0.
 227 //   Note that is is *not* word-tearing, but it does presume that full-word
 228 //   CAS operations are coherent with intermix with STB operations.  That's true
 229 //   on most common processors.
 230 //
 231 // * See also http://blogs.sun.com/dave
 232 
 233 
 234 void* ObjectMonitor::operator new (size_t size) throw() {
 235   return AllocateHeap(size, mtInternal);
 236 }
 237 void* ObjectMonitor::operator new[] (size_t size) throw() {
 238   return operator new (size);
 239 }
 240 void ObjectMonitor::operator delete(void* p) {
 241   FreeHeap(p);
 242 }
 243 void ObjectMonitor::operator delete[] (void *p) {
 244   operator delete(p);
 245 }
 246 
 247 // -----------------------------------------------------------------------------
 248 // Enter support
 249 
 250 void ObjectMonitor::enter(TRAPS) {
 251   // The following code is ordered to check the most common cases first
 252   // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
 253   Thread * const Self = THREAD;
 254 
 255   void * cur = Atomic::cmpxchg(Self, &_owner, (void*)NULL);
 256   if (cur == NULL) {
 257     // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
 258     assert(_recursions == 0, "invariant");
 259     assert(_owner == Self, "invariant");
 260     return;
 261   }
 262 
 263   if (cur == Self) {
 264     // TODO-FIXME: check for integer overflow!  BUGID 6557169.
 265     _recursions++;
 266     return;
 267   }
 268 
 269   if (Self->is_lock_owned ((address)cur)) {
 270     assert(_recursions == 0, "internal state error");
 271     _recursions = 1;
 272     // Commute owner from a thread-specific on-stack BasicLockObject address to
 273     // a full-fledged "Thread *".
 274     _owner = Self;
 275     return;
 276   }
 277 
 278   // We've encountered genuine contention.
 279   assert(Self->_Stalled == 0, "invariant");
 280   Self->_Stalled = intptr_t(this);
 281 
 282   // Try one round of spinning *before* enqueueing Self
 283   // and before going through the awkward and expensive state
 284   // transitions.  The following spin is strictly optional ...
 285   // Note that if we acquire the monitor from an initial spin
 286   // we forgo posting JVMTI events and firing DTRACE probes.
 287   if (TrySpin(Self) > 0) {
 288     assert(_owner == Self, "invariant");
 289     assert(_recursions == 0, "invariant");
 290     assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 291     Self->_Stalled = 0;
 292     return;
 293   }
 294 
 295   assert(_owner != Self, "invariant");
 296   assert(_succ != Self, "invariant");
 297   assert(Self->is_Java_thread(), "invariant");
 298   JavaThread * jt = (JavaThread *) Self;
 299   assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
 300   assert(jt->thread_state() != _thread_blocked, "invariant");
 301   assert(this->object() != NULL, "invariant");
 302   assert(_count >= 0, "invariant");
 303 
 304   // Prevent deflation at STW-time.  See deflate_idle_monitors() and is_busy().
 305   // Ensure the object-monitor relationship remains stable while there's contention.
 306   Atomic::inc(&_count);
 307 
 308   JFR_ONLY(JfrConditionalFlushWithStacktrace<EventJavaMonitorEnter> flush(jt);)
 309   EventJavaMonitorEnter event;
 310   if (event.should_commit()) {
 311     event.set_monitorClass(((oop)this->object())->klass());
 312     event.set_address((uintptr_t)(this->object_addr()));
 313   }
 314 
 315   { // Change java thread status to indicate blocked on monitor enter.
 316     JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
 317 
 318     Self->set_current_pending_monitor(this);
 319 
 320     DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
 321     if (JvmtiExport::should_post_monitor_contended_enter()) {
 322       JvmtiExport::post_monitor_contended_enter(jt, this);
 323 
 324       // The current thread does not yet own the monitor and does not
 325       // yet appear on any queues that would get it made the successor.
 326       // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
 327       // handler cannot accidentally consume an unpark() meant for the
 328       // ParkEvent associated with this ObjectMonitor.
 329     }
 330 
 331     OSThreadContendState osts(Self->osthread());
 332     ThreadBlockInVM tbivm(jt);
 333 
 334     // TODO-FIXME: change the following for(;;) loop to straight-line code.
 335     for (;;) {
 336       jt->set_suspend_equivalent();
 337       // cleared by handle_special_suspend_equivalent_condition()
 338       // or java_suspend_self()
 339 
 340       EnterI(THREAD);
 341 
 342       if (!ExitSuspendEquivalent(jt)) break;
 343 
 344       // We have acquired the contended monitor, but while we were
 345       // waiting another thread suspended us. We don't want to enter
 346       // the monitor while suspended because that would surprise the
 347       // thread that suspended us.
 348       //
 349       _recursions = 0;
 350       _succ = NULL;
 351       exit(false, Self);
 352 
 353       jt->java_suspend_self();
 354     }
 355     Self->set_current_pending_monitor(NULL);
 356 
 357     // We cleared the pending monitor info since we've just gotten past
 358     // the enter-check-for-suspend dance and we now own the monitor free
 359     // and clear, i.e., it is no longer pending. The ThreadBlockInVM
 360     // destructor can go to a safepoint at the end of this block. If we
 361     // do a thread dump during that safepoint, then this thread will show
 362     // as having "-locked" the monitor, but the OS and java.lang.Thread
 363     // states will still report that the thread is blocked trying to
 364     // acquire it.
 365   }
 366 
 367   Atomic::dec(&_count);
 368   assert(_count >= 0, "invariant");
 369   Self->_Stalled = 0;
 370 
 371   // Must either set _recursions = 0 or ASSERT _recursions == 0.
 372   assert(_recursions == 0, "invariant");
 373   assert(_owner == Self, "invariant");
 374   assert(_succ != Self, "invariant");
 375   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 376 
 377   // The thread -- now the owner -- is back in vm mode.
 378   // Report the glorious news via TI,DTrace and jvmstat.
 379   // The probe effect is non-trivial.  All the reportage occurs
 380   // while we hold the monitor, increasing the length of the critical
 381   // section.  Amdahl's parallel speedup law comes vividly into play.
 382   //
 383   // Another option might be to aggregate the events (thread local or
 384   // per-monitor aggregation) and defer reporting until a more opportune
 385   // time -- such as next time some thread encounters contention but has
 386   // yet to acquire the lock.  While spinning that thread could
 387   // spinning we could increment JVMStat counters, etc.
 388 
 389   DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
 390   if (JvmtiExport::should_post_monitor_contended_entered()) {
 391     JvmtiExport::post_monitor_contended_entered(jt, this);
 392 
 393     // The current thread already owns the monitor and is not going to
 394     // call park() for the remainder of the monitor enter protocol. So
 395     // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
 396     // event handler consumed an unpark() issued by the thread that
 397     // just exited the monitor.
 398   }
 399   if (event.should_commit()) {
 400     event.set_previousOwner((uintptr_t)_previous_owner_tid);
 401     event.commit();
 402   }
 403   OM_PERFDATA_OP(ContendedLockAttempts, inc());
 404 }
 405 
 406 // Caveat: TryLock() is not necessarily serializing if it returns failure.
 407 // Callers must compensate as needed.
 408 
 409 int ObjectMonitor::TryLock(Thread * Self) {
 410   void * own = _owner;
 411   if (own != NULL) return 0;
 412   if (Atomic::replace_if_null(Self, &_owner)) {
 413     // Either guarantee _recursions == 0 or set _recursions = 0.
 414     assert(_recursions == 0, "invariant");
 415     assert(_owner == Self, "invariant");
 416     return 1;
 417   }
 418   // The lock had been free momentarily, but we lost the race to the lock.
 419   // Interference -- the CAS failed.
 420   // We can either return -1 or retry.
 421   // Retry doesn't make as much sense because the lock was just acquired.
 422   return -1;
 423 }
 424 
 425 #define MAX_RECHECK_INTERVAL 1000
 426 
 427 void ObjectMonitor::EnterI(TRAPS) {
 428   Thread * const Self = THREAD;
 429   assert(Self->is_Java_thread(), "invariant");
 430   assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant");
 431 
 432   // Try the lock - TATAS
 433   if (TryLock (Self) > 0) {
 434     assert(_succ != Self, "invariant");
 435     assert(_owner == Self, "invariant");
 436     assert(_Responsible != Self, "invariant");
 437     return;
 438   }
 439 
 440   DeferredInitialize();
 441 
 442   // We try one round of spinning *before* enqueueing Self.
 443   //
 444   // If the _owner is ready but OFFPROC we could use a YieldTo()
 445   // operation to donate the remainder of this thread's quantum
 446   // to the owner.  This has subtle but beneficial affinity
 447   // effects.
 448 
 449   if (TrySpin(Self) > 0) {
 450     assert(_owner == Self, "invariant");
 451     assert(_succ != Self, "invariant");
 452     assert(_Responsible != Self, "invariant");
 453     return;
 454   }
 455 
 456   // The Spin failed -- Enqueue and park the thread ...
 457   assert(_succ != Self, "invariant");
 458   assert(_owner != Self, "invariant");
 459   assert(_Responsible != Self, "invariant");
 460 
 461   // Enqueue "Self" on ObjectMonitor's _cxq.
 462   //
 463   // Node acts as a proxy for Self.
 464   // As an aside, if were to ever rewrite the synchronization code mostly
 465   // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
 466   // Java objects.  This would avoid awkward lifecycle and liveness issues,
 467   // as well as eliminate a subset of ABA issues.
 468   // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
 469 
 470   ObjectWaiter node(Self);
 471   Self->_ParkEvent->reset();
 472   node._prev   = (ObjectWaiter *) 0xBAD;
 473   node.TState  = ObjectWaiter::TS_CXQ;
 474 
 475   // Push "Self" onto the front of the _cxq.
 476   // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
 477   // Note that spinning tends to reduce the rate at which threads
 478   // enqueue and dequeue on EntryList|cxq.
 479   ObjectWaiter * nxt;
 480   for (;;) {
 481     node._next = nxt = _cxq;
 482     if (Atomic::cmpxchg(&node, &_cxq, nxt) == nxt) break;
 483 
 484     // Interference - the CAS failed because _cxq changed.  Just retry.
 485     // As an optional optimization we retry the lock.
 486     if (TryLock (Self) > 0) {
 487       assert(_succ != Self, "invariant");
 488       assert(_owner == Self, "invariant");
 489       assert(_Responsible != Self, "invariant");
 490       return;
 491     }
 492   }
 493 
 494   // Check for cxq|EntryList edge transition to non-null.  This indicates
 495   // the onset of contention.  While contention persists exiting threads
 496   // will use a ST:MEMBAR:LD 1-1 exit protocol.  When contention abates exit
 497   // operations revert to the faster 1-0 mode.  This enter operation may interleave
 498   // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
 499   // arrange for one of the contending thread to use a timed park() operations
 500   // to detect and recover from the race.  (Stranding is form of progress failure
 501   // where the monitor is unlocked but all the contending threads remain parked).
 502   // That is, at least one of the contended threads will periodically poll _owner.
 503   // One of the contending threads will become the designated "Responsible" thread.
 504   // The Responsible thread uses a timed park instead of a normal indefinite park
 505   // operation -- it periodically wakes and checks for and recovers from potential
 506   // strandings admitted by 1-0 exit operations.   We need at most one Responsible
 507   // thread per-monitor at any given moment.  Only threads on cxq|EntryList may
 508   // be responsible for a monitor.
 509   //
 510   // Currently, one of the contended threads takes on the added role of "Responsible".
 511   // A viable alternative would be to use a dedicated "stranding checker" thread
 512   // that periodically iterated over all the threads (or active monitors) and unparked
 513   // successors where there was risk of stranding.  This would help eliminate the
 514   // timer scalability issues we see on some platforms as we'd only have one thread
 515   // -- the checker -- parked on a timer.
 516 
 517   if (nxt == NULL && _EntryList == NULL) {
 518     // Try to assume the role of responsible thread for the monitor.
 519     // CONSIDER:  ST vs CAS vs { if (Responsible==null) Responsible=Self }
 520     Atomic::replace_if_null(Self, &_Responsible);
 521   }
 522 
 523   // The lock might have been released while this thread was occupied queueing
 524   // itself onto _cxq.  To close the race and avoid "stranding" and
 525   // progress-liveness failure we must resample-retry _owner before parking.
 526   // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
 527   // In this case the ST-MEMBAR is accomplished with CAS().
 528   //
 529   // TODO: Defer all thread state transitions until park-time.
 530   // Since state transitions are heavy and inefficient we'd like
 531   // to defer the state transitions until absolutely necessary,
 532   // and in doing so avoid some transitions ...
 533 
 534   int nWakeups = 0;
 535   int recheckInterval = 1;
 536 
 537   for (;;) {
 538 
 539     if (TryLock(Self) > 0) break;
 540     assert(_owner != Self, "invariant");
 541 
 542     // park self
 543     if (_Responsible == Self) {
 544       Self->_ParkEvent->park((jlong) recheckInterval);
 545       // Increase the recheckInterval, but clamp the value.
 546       recheckInterval *= 8;
 547       if (recheckInterval > MAX_RECHECK_INTERVAL) {
 548         recheckInterval = MAX_RECHECK_INTERVAL;
 549       }
 550     } else {
 551       Self->_ParkEvent->park();
 552     }
 553 
 554     if (TryLock(Self) > 0) break;
 555 
 556     // The lock is still contested.
 557     // Keep a tally of the # of futile wakeups.
 558     // Note that the counter is not protected by a lock or updated by atomics.
 559     // That is by design - we trade "lossy" counters which are exposed to
 560     // races during updates for a lower probe effect.
 561 
 562     // This PerfData object can be used in parallel with a safepoint.
 563     // See the work around in PerfDataManager::destroy().
 564     OM_PERFDATA_OP(FutileWakeups, inc());
 565     ++nWakeups;
 566 
 567     // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
 568     // We can defer clearing _succ until after the spin completes
 569     // TrySpin() must tolerate being called with _succ == Self.
 570     // Try yet another round of adaptive spinning.
 571     if ((Knob_SpinAfterFutile & 1) && TrySpin(Self) > 0) break;
 572 
 573     // We can find that we were unpark()ed and redesignated _succ while
 574     // we were spinning.  That's harmless.  If we iterate and call park(),
 575     // park() will consume the event and return immediately and we'll
 576     // just spin again.  This pattern can repeat, leaving _succ to simply
 577     // spin on a CPU.  Enable Knob_ResetEvent to clear pending unparks().
 578     // Alternately, we can sample fired() here, and if set, forgo spinning
 579     // in the next iteration.
 580 
 581     if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
 582       Self->_ParkEvent->reset();
 583       OrderAccess::fence();
 584     }
 585     if (_succ == Self) _succ = NULL;
 586 
 587     // Invariant: after clearing _succ a thread *must* retry _owner before parking.
 588     OrderAccess::fence();
 589   }
 590 
 591   // Egress :
 592   // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
 593   // Normally we'll find Self on the EntryList .
 594   // From the perspective of the lock owner (this thread), the
 595   // EntryList is stable and cxq is prepend-only.
 596   // The head of cxq is volatile but the interior is stable.
 597   // In addition, Self.TState is stable.
 598 
 599   assert(_owner == Self, "invariant");
 600   assert(object() != NULL, "invariant");
 601   // I'd like to write:
 602   //   guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
 603   // but as we're at a safepoint that's not safe.
 604 
 605   UnlinkAfterAcquire(Self, &node);
 606   if (_succ == Self) _succ = NULL;
 607 
 608   assert(_succ != Self, "invariant");
 609   if (_Responsible == Self) {
 610     _Responsible = NULL;
 611     OrderAccess::fence(); // Dekker pivot-point
 612 
 613     // We may leave threads on cxq|EntryList without a designated
 614     // "Responsible" thread.  This is benign.  When this thread subsequently
 615     // exits the monitor it can "see" such preexisting "old" threads --
 616     // threads that arrived on the cxq|EntryList before the fence, above --
 617     // by LDing cxq|EntryList.  Newly arrived threads -- that is, threads
 618     // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
 619     // non-null and elect a new "Responsible" timer thread.
 620     //
 621     // This thread executes:
 622     //    ST Responsible=null; MEMBAR    (in enter epilogue - here)
 623     //    LD cxq|EntryList               (in subsequent exit)
 624     //
 625     // Entering threads in the slow/contended path execute:
 626     //    ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
 627     //    The (ST cxq; MEMBAR) is accomplished with CAS().
 628     //
 629     // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
 630     // exit operation from floating above the ST Responsible=null.
 631   }
 632 
 633   // We've acquired ownership with CAS().
 634   // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
 635   // But since the CAS() this thread may have also stored into _succ,
 636   // EntryList, cxq or Responsible.  These meta-data updates must be
 637   // visible __before this thread subsequently drops the lock.
 638   // Consider what could occur if we didn't enforce this constraint --
 639   // STs to monitor meta-data and user-data could reorder with (become
 640   // visible after) the ST in exit that drops ownership of the lock.
 641   // Some other thread could then acquire the lock, but observe inconsistent
 642   // or old monitor meta-data and heap data.  That violates the JMM.
 643   // To that end, the 1-0 exit() operation must have at least STST|LDST
 644   // "release" barrier semantics.  Specifically, there must be at least a
 645   // STST|LDST barrier in exit() before the ST of null into _owner that drops
 646   // the lock.   The barrier ensures that changes to monitor meta-data and data
 647   // protected by the lock will be visible before we release the lock, and
 648   // therefore before some other thread (CPU) has a chance to acquire the lock.
 649   // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
 650   //
 651   // Critically, any prior STs to _succ or EntryList must be visible before
 652   // the ST of null into _owner in the *subsequent* (following) corresponding
 653   // monitorexit.  Recall too, that in 1-0 mode monitorexit does not necessarily
 654   // execute a serializing instruction.
 655 
 656   return;
 657 }
 658 
 659 // ReenterI() is a specialized inline form of the latter half of the
 660 // contended slow-path from EnterI().  We use ReenterI() only for
 661 // monitor reentry in wait().
 662 //
 663 // In the future we should reconcile EnterI() and ReenterI(), adding
 664 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
 665 // loop accordingly.
 666 
 667 void ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) {
 668   assert(Self != NULL, "invariant");
 669   assert(SelfNode != NULL, "invariant");
 670   assert(SelfNode->_thread == Self, "invariant");
 671   assert(_waiters > 0, "invariant");
 672   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 673   assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
 674   JavaThread * jt = (JavaThread *) Self;
 675 
 676   int nWakeups = 0;
 677   for (;;) {
 678     ObjectWaiter::TStates v = SelfNode->TState;
 679     guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
 680     assert(_owner != Self, "invariant");
 681 
 682     if (TryLock(Self) > 0) break;
 683     if (TrySpin(Self) > 0) break;
 684 
 685     // State transition wrappers around park() ...
 686     // ReenterI() wisely defers state transitions until
 687     // it's clear we must park the thread.
 688     {
 689       OSThreadContendState osts(Self->osthread());
 690       ThreadBlockInVM tbivm(jt);
 691 
 692       // cleared by handle_special_suspend_equivalent_condition()
 693       // or java_suspend_self()
 694       jt->set_suspend_equivalent();
 695       Self->_ParkEvent->park();
 696 
 697       // were we externally suspended while we were waiting?
 698       for (;;) {
 699         if (!ExitSuspendEquivalent(jt)) break;
 700         if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
 701         jt->java_suspend_self();
 702         jt->set_suspend_equivalent();
 703       }
 704     }
 705 
 706     // Try again, but just so we distinguish between futile wakeups and
 707     // successful wakeups.  The following test isn't algorithmically
 708     // necessary, but it helps us maintain sensible statistics.
 709     if (TryLock(Self) > 0) break;
 710 
 711     // The lock is still contested.
 712     // Keep a tally of the # of futile wakeups.
 713     // Note that the counter is not protected by a lock or updated by atomics.
 714     // That is by design - we trade "lossy" counters which are exposed to
 715     // races during updates for a lower probe effect.
 716     ++nWakeups;
 717 
 718     // Assuming this is not a spurious wakeup we'll normally
 719     // find that _succ == Self.
 720     if (_succ == Self) _succ = NULL;
 721 
 722     // Invariant: after clearing _succ a contending thread
 723     // *must* retry  _owner before parking.
 724     OrderAccess::fence();
 725 
 726     // This PerfData object can be used in parallel with a safepoint.
 727     // See the work around in PerfDataManager::destroy().
 728     OM_PERFDATA_OP(FutileWakeups, inc());
 729   }
 730 
 731   // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
 732   // Normally we'll find Self on the EntryList.
 733   // Unlinking from the EntryList is constant-time and atomic-free.
 734   // From the perspective of the lock owner (this thread), the
 735   // EntryList is stable and cxq is prepend-only.
 736   // The head of cxq is volatile but the interior is stable.
 737   // In addition, Self.TState is stable.
 738 
 739   assert(_owner == Self, "invariant");
 740   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 741   UnlinkAfterAcquire(Self, SelfNode);
 742   if (_succ == Self) _succ = NULL;
 743   assert(_succ != Self, "invariant");
 744   SelfNode->TState = ObjectWaiter::TS_RUN;
 745   OrderAccess::fence();      // see comments at the end of EnterI()
 746 }
 747 
 748 // By convention we unlink a contending thread from EntryList|cxq immediately
 749 // after the thread acquires the lock in ::enter().  Equally, we could defer
 750 // unlinking the thread until ::exit()-time.
 751 
 752 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) {
 753   assert(_owner == Self, "invariant");
 754   assert(SelfNode->_thread == Self, "invariant");
 755 
 756   if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
 757     // Normal case: remove Self from the DLL EntryList .
 758     // This is a constant-time operation.
 759     ObjectWaiter * nxt = SelfNode->_next;
 760     ObjectWaiter * prv = SelfNode->_prev;
 761     if (nxt != NULL) nxt->_prev = prv;
 762     if (prv != NULL) prv->_next = nxt;
 763     if (SelfNode == _EntryList) _EntryList = nxt;
 764     assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
 765     assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
 766   } else {
 767     assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant");
 768     // Inopportune interleaving -- Self is still on the cxq.
 769     // This usually means the enqueue of self raced an exiting thread.
 770     // Normally we'll find Self near the front of the cxq, so
 771     // dequeueing is typically fast.  If needbe we can accelerate
 772     // this with some MCS/CHL-like bidirectional list hints and advisory
 773     // back-links so dequeueing from the interior will normally operate
 774     // in constant-time.
 775     // Dequeue Self from either the head (with CAS) or from the interior
 776     // with a linear-time scan and normal non-atomic memory operations.
 777     // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
 778     // and then unlink Self from EntryList.  We have to drain eventually,
 779     // so it might as well be now.
 780 
 781     ObjectWaiter * v = _cxq;
 782     assert(v != NULL, "invariant");
 783     if (v != SelfNode || Atomic::cmpxchg(SelfNode->_next, &_cxq, v) != v) {
 784       // The CAS above can fail from interference IFF a "RAT" arrived.
 785       // In that case Self must be in the interior and can no longer be
 786       // at the head of cxq.
 787       if (v == SelfNode) {
 788         assert(_cxq != v, "invariant");
 789         v = _cxq;          // CAS above failed - start scan at head of list
 790       }
 791       ObjectWaiter * p;
 792       ObjectWaiter * q = NULL;
 793       for (p = v; p != NULL && p != SelfNode; p = p->_next) {
 794         q = p;
 795         assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
 796       }
 797       assert(v != SelfNode, "invariant");
 798       assert(p == SelfNode, "Node not found on cxq");
 799       assert(p != _cxq, "invariant");
 800       assert(q != NULL, "invariant");
 801       assert(q->_next == p, "invariant");
 802       q->_next = p->_next;
 803     }
 804   }
 805 
 806 #ifdef ASSERT
 807   // Diagnostic hygiene ...
 808   SelfNode->_prev  = (ObjectWaiter *) 0xBAD;
 809   SelfNode->_next  = (ObjectWaiter *) 0xBAD;
 810   SelfNode->TState = ObjectWaiter::TS_RUN;
 811 #endif
 812 }
 813 
 814 // -----------------------------------------------------------------------------
 815 // Exit support
 816 //
 817 // exit()
 818 // ~~~~~~
 819 // Note that the collector can't reclaim the objectMonitor or deflate
 820 // the object out from underneath the thread calling ::exit() as the
 821 // thread calling ::exit() never transitions to a stable state.
 822 // This inhibits GC, which in turn inhibits asynchronous (and
 823 // inopportune) reclamation of "this".
 824 //
 825 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
 826 // There's one exception to the claim above, however.  EnterI() can call
 827 // exit() to drop a lock if the acquirer has been externally suspended.
 828 // In that case exit() is called with _thread_state as _thread_blocked,
 829 // but the monitor's _count field is > 0, which inhibits reclamation.
 830 //
 831 // 1-0 exit
 832 // ~~~~~~~~
 833 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
 834 // the fast-path operators have been optimized so the common ::exit()
 835 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
 836 // The code emitted by fast_unlock() elides the usual MEMBAR.  This
 837 // greatly improves latency -- MEMBAR and CAS having considerable local
 838 // latency on modern processors -- but at the cost of "stranding".  Absent the
 839 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
 840 // ::enter() path, resulting in the entering thread being stranding
 841 // and a progress-liveness failure.   Stranding is extremely rare.
 842 // We use timers (timed park operations) & periodic polling to detect
 843 // and recover from stranding.  Potentially stranded threads periodically
 844 // wake up and poll the lock.  See the usage of the _Responsible variable.
 845 //
 846 // The CAS() in enter provides for safety and exclusion, while the CAS or
 847 // MEMBAR in exit provides for progress and avoids stranding.  1-0 locking
 848 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
 849 // We detect and recover from stranding with timers.
 850 //
 851 // If a thread transiently strands it'll park until (a) another
 852 // thread acquires the lock and then drops the lock, at which time the
 853 // exiting thread will notice and unpark the stranded thread, or, (b)
 854 // the timer expires.  If the lock is high traffic then the stranding latency
 855 // will be low due to (a).  If the lock is low traffic then the odds of
 856 // stranding are lower, although the worst-case stranding latency
 857 // is longer.  Critically, we don't want to put excessive load in the
 858 // platform's timer subsystem.  We want to minimize both the timer injection
 859 // rate (timers created/sec) as well as the number of timers active at
 860 // any one time.  (more precisely, we want to minimize timer-seconds, which is
 861 // the integral of the # of active timers at any instant over time).
 862 // Both impinge on OS scalability.  Given that, at most one thread parked on
 863 // a monitor will use a timer.
 864 //
 865 // There is also the risk of a futile wake-up. If we drop the lock
 866 // another thread can reacquire the lock immediately, and we can
 867 // then wake a thread unnecessarily. This is benign, and we've
 868 // structured the code so the windows are short and the frequency
 869 // of such futile wakups is low.
 870 
 871 void ObjectMonitor::exit(bool not_suspended, TRAPS) {
 872   Thread * const Self = THREAD;
 873   if (THREAD != _owner) {
 874     if (THREAD->is_lock_owned((address) _owner)) {
 875       // Transmute _owner from a BasicLock pointer to a Thread address.
 876       // We don't need to hold _mutex for this transition.
 877       // Non-null to Non-null is safe as long as all readers can
 878       // tolerate either flavor.
 879       assert(_recursions == 0, "invariant");
 880       _owner = THREAD;
 881       _recursions = 0;
 882     } else {
 883       // Apparent unbalanced locking ...
 884       // Naively we'd like to throw IllegalMonitorStateException.
 885       // As a practical matter we can neither allocate nor throw an
 886       // exception as ::exit() can be called from leaf routines.
 887       // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
 888       // Upon deeper reflection, however, in a properly run JVM the only
 889       // way we should encounter this situation is in the presence of
 890       // unbalanced JNI locking. TODO: CheckJNICalls.
 891       // See also: CR4414101
 892       assert(false, "Non-balanced monitor enter/exit! Likely JNI locking");
 893       return;
 894     }
 895   }
 896 
 897   if (_recursions != 0) {
 898     _recursions--;        // this is simple recursive enter
 899     return;
 900   }
 901 
 902   // Invariant: after setting Responsible=null an thread must execute
 903   // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
 904   _Responsible = NULL;
 905 
 906 #if INCLUDE_JFR
 907   // get the owner's thread id for the MonitorEnter event
 908   // if it is enabled and the thread isn't suspended
 909   if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
 910     _previous_owner_tid = JFR_THREAD_ID(Self);
 911   }
 912 #endif
 913 
 914   for (;;) {
 915     assert(THREAD == _owner, "invariant");
 916 
 917     if (Knob_ExitPolicy == 0) {
 918       // release semantics: prior loads and stores from within the critical section
 919       // must not float (reorder) past the following store that drops the lock.
 920       // On SPARC that requires MEMBAR #loadstore|#storestore.
 921       // But of course in TSO #loadstore|#storestore is not required.
 922       // I'd like to write one of the following:
 923       // A.  OrderAccess::release() ; _owner = NULL
 924       // B.  OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
 925       // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
 926       // store into a _dummy variable.  That store is not needed, but can result
 927       // in massive wasteful coherency traffic on classic SMP systems.
 928       // Instead, I use release_store(), which is implemented as just a simple
 929       // ST on x64, x86 and SPARC.
 930       OrderAccess::release_store(&_owner, (void*)NULL);   // drop the lock
 931       OrderAccess::storeload();                        // See if we need to wake a successor
 932       if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
 933         return;
 934       }
 935       // Other threads are blocked trying to acquire the lock.
 936 
 937       // Normally the exiting thread is responsible for ensuring succession,
 938       // but if other successors are ready or other entering threads are spinning
 939       // then this thread can simply store NULL into _owner and exit without
 940       // waking a successor.  The existence of spinners or ready successors
 941       // guarantees proper succession (liveness).  Responsibility passes to the
 942       // ready or running successors.  The exiting thread delegates the duty.
 943       // More precisely, if a successor already exists this thread is absolved
 944       // of the responsibility of waking (unparking) one.
 945       //
 946       // The _succ variable is critical to reducing futile wakeup frequency.
 947       // _succ identifies the "heir presumptive" thread that has been made
 948       // ready (unparked) but that has not yet run.  We need only one such
 949       // successor thread to guarantee progress.
 950       // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
 951       // section 3.3 "Futile Wakeup Throttling" for details.
 952       //
 953       // Note that spinners in Enter() also set _succ non-null.
 954       // In the current implementation spinners opportunistically set
 955       // _succ so that exiting threads might avoid waking a successor.
 956       // Another less appealing alternative would be for the exiting thread
 957       // to drop the lock and then spin briefly to see if a spinner managed
 958       // to acquire the lock.  If so, the exiting thread could exit
 959       // immediately without waking a successor, otherwise the exiting
 960       // thread would need to dequeue and wake a successor.
 961       // (Note that we'd need to make the post-drop spin short, but no
 962       // shorter than the worst-case round-trip cache-line migration time.
 963       // The dropped lock needs to become visible to the spinner, and then
 964       // the acquisition of the lock by the spinner must become visible to
 965       // the exiting thread).
 966 
 967       // It appears that an heir-presumptive (successor) must be made ready.
 968       // Only the current lock owner can manipulate the EntryList or
 969       // drain _cxq, so we need to reacquire the lock.  If we fail
 970       // to reacquire the lock the responsibility for ensuring succession
 971       // falls to the new owner.
 972       //
 973       if (!Atomic::replace_if_null(THREAD, &_owner)) {
 974         return;
 975       }
 976     } else {
 977       if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
 978         OrderAccess::release_store(&_owner, (void*)NULL);   // drop the lock
 979         OrderAccess::storeload();
 980         // Ratify the previously observed values.
 981         if (_cxq == NULL || _succ != NULL) {
 982           return;
 983         }
 984 
 985         // inopportune interleaving -- the exiting thread (this thread)
 986         // in the fast-exit path raced an entering thread in the slow-enter
 987         // path.
 988         // We have two choices:
 989         // A.  Try to reacquire the lock.
 990         //     If the CAS() fails return immediately, otherwise
 991         //     we either restart/rerun the exit operation, or simply
 992         //     fall-through into the code below which wakes a successor.
 993         // B.  If the elements forming the EntryList|cxq are TSM
 994         //     we could simply unpark() the lead thread and return
 995         //     without having set _succ.
 996         if (!Atomic::replace_if_null(THREAD, &_owner)) {
 997           return;
 998         }
 999       }
1000     }
1001 
1002     guarantee(_owner == THREAD, "invariant");
1003 
1004     ObjectWaiter * w = NULL;
1005     int QMode = Knob_QMode;
1006 
1007     if (QMode == 2 && _cxq != NULL) {
1008       // QMode == 2 : cxq has precedence over EntryList.
1009       // Try to directly wake a successor from the cxq.
1010       // If successful, the successor will need to unlink itself from cxq.
1011       w = _cxq;
1012       assert(w != NULL, "invariant");
1013       assert(w->TState == ObjectWaiter::TS_CXQ, "Invariant");
1014       ExitEpilog(Self, w);
1015       return;
1016     }
1017 
1018     if (QMode == 3 && _cxq != NULL) {
1019       // Aggressively drain cxq into EntryList at the first opportunity.
1020       // This policy ensure that recently-run threads live at the head of EntryList.
1021       // Drain _cxq into EntryList - bulk transfer.
1022       // First, detach _cxq.
1023       // The following loop is tantamount to: w = swap(&cxq, NULL)
1024       w = _cxq;
1025       for (;;) {
1026         assert(w != NULL, "Invariant");
1027         ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1028         if (u == w) break;
1029         w = u;
1030       }
1031       assert(w != NULL, "invariant");
1032 
1033       ObjectWaiter * q = NULL;
1034       ObjectWaiter * p;
1035       for (p = w; p != NULL; p = p->_next) {
1036         guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1037         p->TState = ObjectWaiter::TS_ENTER;
1038         p->_prev = q;
1039         q = p;
1040       }
1041 
1042       // Append the RATs to the EntryList
1043       // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
1044       ObjectWaiter * Tail;
1045       for (Tail = _EntryList; Tail != NULL && Tail->_next != NULL;
1046            Tail = Tail->_next)
1047         /* empty */;
1048       if (Tail == NULL) {
1049         _EntryList = w;
1050       } else {
1051         Tail->_next = w;
1052         w->_prev = Tail;
1053       }
1054 
1055       // Fall thru into code that tries to wake a successor from EntryList
1056     }
1057 
1058     if (QMode == 4 && _cxq != NULL) {
1059       // Aggressively drain cxq into EntryList at the first opportunity.
1060       // This policy ensure that recently-run threads live at the head of EntryList.
1061 
1062       // Drain _cxq into EntryList - bulk transfer.
1063       // First, detach _cxq.
1064       // The following loop is tantamount to: w = swap(&cxq, NULL)
1065       w = _cxq;
1066       for (;;) {
1067         assert(w != NULL, "Invariant");
1068         ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1069         if (u == w) break;
1070         w = u;
1071       }
1072       assert(w != NULL, "invariant");
1073 
1074       ObjectWaiter * q = NULL;
1075       ObjectWaiter * p;
1076       for (p = w; p != NULL; p = p->_next) {
1077         guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1078         p->TState = ObjectWaiter::TS_ENTER;
1079         p->_prev = q;
1080         q = p;
1081       }
1082 
1083       // Prepend the RATs to the EntryList
1084       if (_EntryList != NULL) {
1085         q->_next = _EntryList;
1086         _EntryList->_prev = q;
1087       }
1088       _EntryList = w;
1089 
1090       // Fall thru into code that tries to wake a successor from EntryList
1091     }
1092 
1093     w = _EntryList;
1094     if (w != NULL) {
1095       // I'd like to write: guarantee (w->_thread != Self).
1096       // But in practice an exiting thread may find itself on the EntryList.
1097       // Let's say thread T1 calls O.wait().  Wait() enqueues T1 on O's waitset and
1098       // then calls exit().  Exit release the lock by setting O._owner to NULL.
1099       // Let's say T1 then stalls.  T2 acquires O and calls O.notify().  The
1100       // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1101       // release the lock "O".  T2 resumes immediately after the ST of null into
1102       // _owner, above.  T2 notices that the EntryList is populated, so it
1103       // reacquires the lock and then finds itself on the EntryList.
1104       // Given all that, we have to tolerate the circumstance where "w" is
1105       // associated with Self.
1106       assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1107       ExitEpilog(Self, w);
1108       return;
1109     }
1110 
1111     // If we find that both _cxq and EntryList are null then just
1112     // re-run the exit protocol from the top.
1113     w = _cxq;
1114     if (w == NULL) continue;
1115 
1116     // Drain _cxq into EntryList - bulk transfer.
1117     // First, detach _cxq.
1118     // The following loop is tantamount to: w = swap(&cxq, NULL)
1119     for (;;) {
1120       assert(w != NULL, "Invariant");
1121       ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1122       if (u == w) break;
1123       w = u;
1124     }
1125 
1126     assert(w != NULL, "invariant");
1127     assert(_EntryList == NULL, "invariant");
1128 
1129     // Convert the LIFO SLL anchored by _cxq into a DLL.
1130     // The list reorganization step operates in O(LENGTH(w)) time.
1131     // It's critical that this step operate quickly as
1132     // "Self" still holds the outer-lock, restricting parallelism
1133     // and effectively lengthening the critical section.
1134     // Invariant: s chases t chases u.
1135     // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1136     // we have faster access to the tail.
1137 
1138     if (QMode == 1) {
1139       // QMode == 1 : drain cxq to EntryList, reversing order
1140       // We also reverse the order of the list.
1141       ObjectWaiter * s = NULL;
1142       ObjectWaiter * t = w;
1143       ObjectWaiter * u = NULL;
1144       while (t != NULL) {
1145         guarantee(t->TState == ObjectWaiter::TS_CXQ, "invariant");
1146         t->TState = ObjectWaiter::TS_ENTER;
1147         u = t->_next;
1148         t->_prev = u;
1149         t->_next = s;
1150         s = t;
1151         t = u;
1152       }
1153       _EntryList  = s;
1154       assert(s != NULL, "invariant");
1155     } else {
1156       // QMode == 0 or QMode == 2
1157       _EntryList = w;
1158       ObjectWaiter * q = NULL;
1159       ObjectWaiter * p;
1160       for (p = w; p != NULL; p = p->_next) {
1161         guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1162         p->TState = ObjectWaiter::TS_ENTER;
1163         p->_prev = q;
1164         q = p;
1165       }
1166     }
1167 
1168     // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1169     // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1170 
1171     // See if we can abdicate to a spinner instead of waking a thread.
1172     // A primary goal of the implementation is to reduce the
1173     // context-switch rate.
1174     if (_succ != NULL) continue;
1175 
1176     w = _EntryList;
1177     if (w != NULL) {
1178       guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1179       ExitEpilog(Self, w);
1180       return;
1181     }
1182   }
1183 }
1184 
1185 // ExitSuspendEquivalent:
1186 // A faster alternate to handle_special_suspend_equivalent_condition()
1187 //
1188 // handle_special_suspend_equivalent_condition() unconditionally
1189 // acquires the SR_lock.  On some platforms uncontended MutexLocker()
1190 // operations have high latency.  Note that in ::enter() we call HSSEC
1191 // while holding the monitor, so we effectively lengthen the critical sections.
1192 //
1193 // There are a number of possible solutions:
1194 //
1195 // A.  To ameliorate the problem we might also defer state transitions
1196 //     to as late as possible -- just prior to parking.
1197 //     Given that, we'd call HSSEC after having returned from park(),
1198 //     but before attempting to acquire the monitor.  This is only a
1199 //     partial solution.  It avoids calling HSSEC while holding the
1200 //     monitor (good), but it still increases successor reacquisition latency --
1201 //     the interval between unparking a successor and the time the successor
1202 //     resumes and retries the lock.  See ReenterI(), which defers state transitions.
1203 //     If we use this technique we can also avoid EnterI()-exit() loop
1204 //     in ::enter() where we iteratively drop the lock and then attempt
1205 //     to reacquire it after suspending.
1206 //
1207 // B.  In the future we might fold all the suspend bits into a
1208 //     composite per-thread suspend flag and then update it with CAS().
1209 //     Alternately, a Dekker-like mechanism with multiple variables
1210 //     would suffice:
1211 //       ST Self->_suspend_equivalent = false
1212 //       MEMBAR
1213 //       LD Self_>_suspend_flags
1214 //
1215 // UPDATE 2007-10-6: since I've replaced the native Mutex/Monitor subsystem
1216 // with a more efficient implementation, the need to use "FastHSSEC" has
1217 // decreased. - Dave
1218 
1219 
1220 bool ObjectMonitor::ExitSuspendEquivalent(JavaThread * jSelf) {
1221   const int Mode = Knob_FastHSSEC;
1222   if (Mode && !jSelf->is_external_suspend()) {
1223     assert(jSelf->is_suspend_equivalent(), "invariant");
1224     jSelf->clear_suspend_equivalent();
1225     if (2 == Mode) OrderAccess::storeload();
1226     if (!jSelf->is_external_suspend()) return false;
1227     // We raced a suspension -- fall thru into the slow path
1228     jSelf->set_suspend_equivalent();
1229   }
1230   return jSelf->handle_special_suspend_equivalent_condition();
1231 }
1232 
1233 
1234 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) {
1235   assert(_owner == Self, "invariant");
1236 
1237   // Exit protocol:
1238   // 1. ST _succ = wakee
1239   // 2. membar #loadstore|#storestore;
1240   // 2. ST _owner = NULL
1241   // 3. unpark(wakee)
1242 
1243   _succ = Wakee->_thread;
1244   ParkEvent * Trigger = Wakee->_event;
1245 
1246   // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1247   // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1248   // out-of-scope (non-extant).
1249   Wakee  = NULL;
1250 
1251   // Drop the lock
1252   OrderAccess::release_store(&_owner, (void*)NULL);
1253   OrderAccess::fence();                               // ST _owner vs LD in unpark()
1254 
1255   DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1256   Trigger->unpark();
1257 
1258   // Maintain stats and report events to JVMTI
1259   OM_PERFDATA_OP(Parks, inc());
1260 }
1261 
1262 
1263 // -----------------------------------------------------------------------------
1264 // Class Loader deadlock handling.
1265 //
1266 // complete_exit exits a lock returning recursion count
1267 // complete_exit/reenter operate as a wait without waiting
1268 // complete_exit requires an inflated monitor
1269 // The _owner field is not always the Thread addr even with an
1270 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1271 // thread due to contention.
1272 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1273   Thread * const Self = THREAD;
1274   assert(Self->is_Java_thread(), "Must be Java thread!");
1275   JavaThread *jt = (JavaThread *)THREAD;
1276 
1277   DeferredInitialize();
1278 
1279   if (THREAD != _owner) {
1280     if (THREAD->is_lock_owned ((address)_owner)) {
1281       assert(_recursions == 0, "internal state error");
1282       _owner = THREAD;   // Convert from basiclock addr to Thread addr
1283       _recursions = 0;
1284     }
1285   }
1286 
1287   guarantee(Self == _owner, "complete_exit not owner");
1288   intptr_t save = _recursions; // record the old recursion count
1289   _recursions = 0;        // set the recursion level to be 0
1290   exit(true, Self);           // exit the monitor
1291   guarantee(_owner != Self, "invariant");
1292   return save;
1293 }
1294 
1295 // reenter() enters a lock and sets recursion count
1296 // complete_exit/reenter operate as a wait without waiting
1297 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1298   Thread * const Self = THREAD;
1299   assert(Self->is_Java_thread(), "Must be Java thread!");
1300   JavaThread *jt = (JavaThread *)THREAD;
1301 
1302   guarantee(_owner != Self, "reenter already owner");
1303   enter(THREAD);       // enter the monitor
1304   guarantee(_recursions == 0, "reenter recursion");
1305   _recursions = recursions;
1306   return;
1307 }
1308 
1309 
1310 // -----------------------------------------------------------------------------
1311 // A macro is used below because there may already be a pending
1312 // exception which should not abort the execution of the routines
1313 // which use this (which is why we don't put this into check_slow and
1314 // call it with a CHECK argument).
1315 
1316 #define CHECK_OWNER()                                                       \
1317   do {                                                                      \
1318     if (THREAD != _owner) {                                                 \
1319       if (THREAD->is_lock_owned((address) _owner)) {                        \
1320         _owner = THREAD;  /* Convert from basiclock addr to Thread addr */  \
1321         _recursions = 0;                                                    \
1322       } else {                                                              \
1323         THROW(vmSymbols::java_lang_IllegalMonitorStateException());         \
1324       }                                                                     \
1325     }                                                                       \
1326   } while (false)
1327 
1328 // check_slow() is a misnomer.  It's called to simply to throw an IMSX exception.
1329 // TODO-FIXME: remove check_slow() -- it's likely dead.
1330 
1331 void ObjectMonitor::check_slow(TRAPS) {
1332   assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1333   THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1334 }
1335 
1336 static int Adjust(volatile int * adr, int dx) {
1337   int v;
1338   for (v = *adr; Atomic::cmpxchg(v + dx, adr, v) != v; v = *adr) /* empty */;
1339   return v;
1340 }
1341 
1342 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1343                                     ObjectMonitor* monitor,
1344                                     jlong notifier_tid,
1345                                     jlong timeout,
1346                                     bool timedout) {
1347   assert(event != NULL, "invariant");
1348   assert(monitor != NULL, "invariant");
1349   event->set_monitorClass(((oop)monitor->object())->klass());
1350   event->set_timeout(timeout);
1351   event->set_address((uintptr_t)monitor->object_addr());
1352   event->set_notifier(notifier_tid);
1353   event->set_timedOut(timedout);
1354   event->commit();
1355 }
1356 
1357 // -----------------------------------------------------------------------------
1358 // Wait/Notify/NotifyAll
1359 //
1360 // Note: a subset of changes to ObjectMonitor::wait()
1361 // will need to be replicated in complete_exit
1362 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1363   Thread * const Self = THREAD;
1364   assert(Self->is_Java_thread(), "Must be Java thread!");
1365   JavaThread *jt = (JavaThread *)THREAD;
1366 
1367   DeferredInitialize();
1368 
1369   // Throw IMSX or IEX.
1370   CHECK_OWNER();
1371 
1372   EventJavaMonitorWait event;
1373 
1374   // check for a pending interrupt
1375   if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1376     // post monitor waited event.  Note that this is past-tense, we are done waiting.
1377     if (JvmtiExport::should_post_monitor_waited()) {
1378       // Note: 'false' parameter is passed here because the
1379       // wait was not timed out due to thread interrupt.
1380       JvmtiExport::post_monitor_waited(jt, this, false);
1381 
1382       // In this short circuit of the monitor wait protocol, the
1383       // current thread never drops ownership of the monitor and
1384       // never gets added to the wait queue so the current thread
1385       // cannot be made the successor. This means that the
1386       // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1387       // consume an unpark() meant for the ParkEvent associated with
1388       // this ObjectMonitor.
1389     }
1390     if (event.should_commit()) {
1391       post_monitor_wait_event(&event, this, 0, millis, false);
1392     }
1393     THROW(vmSymbols::java_lang_InterruptedException());
1394     return;
1395   }
1396 
1397   assert(Self->_Stalled == 0, "invariant");
1398   Self->_Stalled = intptr_t(this);
1399   jt->set_current_waiting_monitor(this);
1400 
1401   // create a node to be put into the queue
1402   // Critically, after we reset() the event but prior to park(), we must check
1403   // for a pending interrupt.
1404   ObjectWaiter node(Self);
1405   node.TState = ObjectWaiter::TS_WAIT;
1406   Self->_ParkEvent->reset();
1407   OrderAccess::fence();          // ST into Event; membar ; LD interrupted-flag
1408 
1409   // Enter the waiting queue, which is a circular doubly linked list in this case
1410   // but it could be a priority queue or any data structure.
1411   // _WaitSetLock protects the wait queue.  Normally the wait queue is accessed only
1412   // by the the owner of the monitor *except* in the case where park()
1413   // returns because of a timeout of interrupt.  Contention is exceptionally rare
1414   // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1415 
1416   Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1417   AddWaiter(&node);
1418   Thread::SpinRelease(&_WaitSetLock);
1419 
1420   _Responsible = NULL;
1421 
1422   intptr_t save = _recursions; // record the old recursion count
1423   _waiters++;                  // increment the number of waiters
1424   _recursions = 0;             // set the recursion level to be 1
1425   exit(true, Self);                    // exit the monitor
1426   guarantee(_owner != Self, "invariant");
1427 
1428   // The thread is on the WaitSet list - now park() it.
1429   // On MP systems it's conceivable that a brief spin before we park
1430   // could be profitable.
1431   //
1432   // TODO-FIXME: change the following logic to a loop of the form
1433   //   while (!timeout && !interrupted && _notified == 0) park()
1434 
1435   int ret = OS_OK;
1436   int WasNotified = 0;
1437   { // State transition wrappers
1438     OSThread* osthread = Self->osthread();
1439     OSThreadWaitState osts(osthread, true);
1440     {
1441       ThreadBlockInVM tbivm(jt);
1442       // Thread is in thread_blocked state and oop access is unsafe.
1443       jt->set_suspend_equivalent();
1444 
1445       if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1446         // Intentionally empty
1447       } else if (node._notified == 0) {
1448         if (millis <= 0) {
1449           Self->_ParkEvent->park();
1450         } else {
1451           ret = Self->_ParkEvent->park(millis);
1452         }
1453       }
1454 
1455       // were we externally suspended while we were waiting?
1456       if (ExitSuspendEquivalent (jt)) {
1457         // TODO-FIXME: add -- if succ == Self then succ = null.
1458         jt->java_suspend_self();
1459       }
1460 
1461     } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1462 
1463     // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1464     // from the WaitSet to the EntryList.
1465     // See if we need to remove Node from the WaitSet.
1466     // We use double-checked locking to avoid grabbing _WaitSetLock
1467     // if the thread is not on the wait queue.
1468     //
1469     // Note that we don't need a fence before the fetch of TState.
1470     // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1471     // written by the is thread. (perhaps the fetch might even be satisfied
1472     // by a look-aside into the processor's own store buffer, although given
1473     // the length of the code path between the prior ST and this load that's
1474     // highly unlikely).  If the following LD fetches a stale TS_WAIT value
1475     // then we'll acquire the lock and then re-fetch a fresh TState value.
1476     // That is, we fail toward safety.
1477 
1478     if (node.TState == ObjectWaiter::TS_WAIT) {
1479       Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1480       if (node.TState == ObjectWaiter::TS_WAIT) {
1481         DequeueSpecificWaiter(&node);       // unlink from WaitSet
1482         assert(node._notified == 0, "invariant");
1483         node.TState = ObjectWaiter::TS_RUN;
1484       }
1485       Thread::SpinRelease(&_WaitSetLock);
1486     }
1487 
1488     // The thread is now either on off-list (TS_RUN),
1489     // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1490     // The Node's TState variable is stable from the perspective of this thread.
1491     // No other threads will asynchronously modify TState.
1492     guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1493     OrderAccess::loadload();
1494     if (_succ == Self) _succ = NULL;
1495     WasNotified = node._notified;
1496 
1497     // Reentry phase -- reacquire the monitor.
1498     // re-enter contended monitor after object.wait().
1499     // retain OBJECT_WAIT state until re-enter successfully completes
1500     // Thread state is thread_in_vm and oop access is again safe,
1501     // although the raw address of the object may have changed.
1502     // (Don't cache naked oops over safepoints, of course).
1503 
1504     // post monitor waited event. Note that this is past-tense, we are done waiting.
1505     if (JvmtiExport::should_post_monitor_waited()) {
1506       JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1507 
1508       if (node._notified != 0 && _succ == Self) {
1509         // In this part of the monitor wait-notify-reenter protocol it
1510         // is possible (and normal) for another thread to do a fastpath
1511         // monitor enter-exit while this thread is still trying to get
1512         // to the reenter portion of the protocol.
1513         //
1514         // The ObjectMonitor was notified and the current thread is
1515         // the successor which also means that an unpark() has already
1516         // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1517         // consume the unpark() that was done when the successor was
1518         // set because the same ParkEvent is shared between Java
1519         // monitors and JVM/TI RawMonitors (for now).
1520         //
1521         // We redo the unpark() to ensure forward progress, i.e., we
1522         // don't want all pending threads hanging (parked) with none
1523         // entering the unlocked monitor.
1524         node._event->unpark();
1525       }
1526     }
1527 
1528     if (event.should_commit()) {
1529       post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1530     }
1531 
1532     OrderAccess::fence();
1533 
1534     assert(Self->_Stalled != 0, "invariant");
1535     Self->_Stalled = 0;
1536 
1537     assert(_owner != Self, "invariant");
1538     ObjectWaiter::TStates v = node.TState;
1539     if (v == ObjectWaiter::TS_RUN) {
1540       enter(Self);
1541     } else {
1542       guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1543       ReenterI(Self, &node);
1544       node.wait_reenter_end(this);
1545     }
1546 
1547     // Self has reacquired the lock.
1548     // Lifecycle - the node representing Self must not appear on any queues.
1549     // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1550     // want residual elements associated with this thread left on any lists.
1551     guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1552     assert(_owner == Self, "invariant");
1553     assert(_succ != Self, "invariant");
1554   } // OSThreadWaitState()
1555 
1556   jt->set_current_waiting_monitor(NULL);
1557 
1558   guarantee(_recursions == 0, "invariant");
1559   _recursions = save;     // restore the old recursion count
1560   _waiters--;             // decrement the number of waiters
1561 
1562   // Verify a few postconditions
1563   assert(_owner == Self, "invariant");
1564   assert(_succ != Self, "invariant");
1565   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
1566 
1567   // check if the notification happened
1568   if (!WasNotified) {
1569     // no, it could be timeout or Thread.interrupt() or both
1570     // check for interrupt event, otherwise it is timeout
1571     if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1572       THROW(vmSymbols::java_lang_InterruptedException());
1573     }
1574   }
1575 
1576   // NOTE: Spurious wake up will be consider as timeout.
1577   // Monitor notify has precedence over thread interrupt.
1578 }
1579 
1580 
1581 // Consider:
1582 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1583 // then instead of transferring a thread from the WaitSet to the EntryList
1584 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1585 
1586 void ObjectMonitor::INotify(Thread * Self) {
1587   const int policy = Knob_MoveNotifyee;
1588 
1589   Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1590   ObjectWaiter * iterator = DequeueWaiter();
1591   if (iterator != NULL) {
1592     guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1593     guarantee(iterator->_notified == 0, "invariant");
1594     // Disposition - what might we do with iterator ?
1595     // a.  add it directly to the EntryList - either tail (policy == 1)
1596     //     or head (policy == 0).
1597     // b.  push it onto the front of the _cxq (policy == 2).
1598     // For now we use (b).
1599     if (policy != 4) {
1600       iterator->TState = ObjectWaiter::TS_ENTER;
1601     }
1602     iterator->_notified = 1;
1603     iterator->_notifier_tid = JFR_THREAD_ID(Self);
1604 
1605     ObjectWaiter * list = _EntryList;
1606     if (list != NULL) {
1607       assert(list->_prev == NULL, "invariant");
1608       assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1609       assert(list != iterator, "invariant");
1610     }
1611 
1612     if (policy == 0) {       // prepend to EntryList
1613       if (list == NULL) {
1614         iterator->_next = iterator->_prev = NULL;
1615         _EntryList = iterator;
1616       } else {
1617         list->_prev = iterator;
1618         iterator->_next = list;
1619         iterator->_prev = NULL;
1620         _EntryList = iterator;
1621       }
1622     } else if (policy == 1) {      // append to EntryList
1623       if (list == NULL) {
1624         iterator->_next = iterator->_prev = NULL;
1625         _EntryList = iterator;
1626       } else {
1627         // CONSIDER:  finding the tail currently requires a linear-time walk of
1628         // the EntryList.  We can make tail access constant-time by converting to
1629         // a CDLL instead of using our current DLL.
1630         ObjectWaiter * tail;
1631         for (tail = list; tail->_next != NULL; tail = tail->_next) {}
1632         assert(tail != NULL && tail->_next == NULL, "invariant");
1633         tail->_next = iterator;
1634         iterator->_prev = tail;
1635         iterator->_next = NULL;
1636       }
1637     } else if (policy == 2) {      // prepend to cxq
1638       if (list == NULL) {
1639         iterator->_next = iterator->_prev = NULL;
1640         _EntryList = iterator;
1641       } else {
1642         iterator->TState = ObjectWaiter::TS_CXQ;
1643         for (;;) {
1644           ObjectWaiter * front = _cxq;
1645           iterator->_next = front;
1646           if (Atomic::cmpxchg(iterator, &_cxq, front) == front) {
1647             break;
1648           }
1649         }
1650       }
1651     } else if (policy == 3) {      // append to cxq
1652       iterator->TState = ObjectWaiter::TS_CXQ;
1653       for (;;) {
1654         ObjectWaiter * tail = _cxq;
1655         if (tail == NULL) {
1656           iterator->_next = NULL;
1657           if (Atomic::replace_if_null(iterator, &_cxq)) {
1658             break;
1659           }
1660         } else {
1661           while (tail->_next != NULL) tail = tail->_next;
1662           tail->_next = iterator;
1663           iterator->_prev = tail;
1664           iterator->_next = NULL;
1665           break;
1666         }
1667       }
1668     } else {
1669       ParkEvent * ev = iterator->_event;
1670       iterator->TState = ObjectWaiter::TS_RUN;
1671       OrderAccess::fence();
1672       ev->unpark();
1673     }
1674 
1675     // _WaitSetLock protects the wait queue, not the EntryList.  We could
1676     // move the add-to-EntryList operation, above, outside the critical section
1677     // protected by _WaitSetLock.  In practice that's not useful.  With the
1678     // exception of  wait() timeouts and interrupts the monitor owner
1679     // is the only thread that grabs _WaitSetLock.  There's almost no contention
1680     // on _WaitSetLock so it's not profitable to reduce the length of the
1681     // critical section.
1682 
1683     if (policy < 4) {
1684       iterator->wait_reenter_begin(this);
1685     }
1686   }
1687   Thread::SpinRelease(&_WaitSetLock);
1688 }
1689 
1690 // Consider: a not-uncommon synchronization bug is to use notify() when
1691 // notifyAll() is more appropriate, potentially resulting in stranded
1692 // threads; this is one example of a lost wakeup. A useful diagnostic
1693 // option is to force all notify() operations to behave as notifyAll().
1694 //
1695 // Note: We can also detect many such problems with a "minimum wait".
1696 // When the "minimum wait" is set to a small non-zero timeout value
1697 // and the program does not hang whereas it did absent "minimum wait",
1698 // that suggests a lost wakeup bug.
1699 
1700 void ObjectMonitor::notify(TRAPS) {
1701   CHECK_OWNER();
1702   if (_WaitSet == NULL) {
1703     return;
1704   }
1705   DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1706   INotify(THREAD);
1707   OM_PERFDATA_OP(Notifications, inc(1));
1708 }
1709 
1710 
1711 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1712 // from the waitset to the EntryList. This could be done more efficiently with a
1713 // single bulk transfer but in practice it's not time-critical. Beware too,
1714 // that in prepend-mode we invert the order of the waiters. Let's say that the
1715 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1716 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1717 
1718 void ObjectMonitor::notifyAll(TRAPS) {
1719   CHECK_OWNER();
1720   if (_WaitSet == NULL) {
1721     return;
1722   }
1723 
1724   DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1725   int tally = 0;
1726   while (_WaitSet != NULL) {
1727     tally++;
1728     INotify(THREAD);
1729   }
1730 
1731   OM_PERFDATA_OP(Notifications, inc(tally));
1732 }
1733 
1734 // -----------------------------------------------------------------------------
1735 // Adaptive Spinning Support
1736 //
1737 // Adaptive spin-then-block - rational spinning
1738 //
1739 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1740 // algorithm.  On high order SMP systems it would be better to start with
1741 // a brief global spin and then revert to spinning locally.  In the spirit of MCS/CLH,
1742 // a contending thread could enqueue itself on the cxq and then spin locally
1743 // on a thread-specific variable such as its ParkEvent._Event flag.
1744 // That's left as an exercise for the reader.  Note that global spinning is
1745 // not problematic on Niagara, as the L2 cache serves the interconnect and
1746 // has both low latency and massive bandwidth.
1747 //
1748 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1749 // acquisition attempts where we opt to spin --  at 100% and vary the spin count
1750 // (duration) or we can fix the count at approximately the duration of
1751 // a context switch and vary the frequency.   Of course we could also
1752 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1753 // For a description of 'Adaptive spin-then-block mutual exclusion in
1754 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1755 //
1756 // This implementation varies the duration "D", where D varies with
1757 // the success rate of recent spin attempts. (D is capped at approximately
1758 // length of a round-trip context switch).  The success rate for recent
1759 // spin attempts is a good predictor of the success rate of future spin
1760 // attempts.  The mechanism adapts automatically to varying critical
1761 // section length (lock modality), system load and degree of parallelism.
1762 // D is maintained per-monitor in _SpinDuration and is initialized
1763 // optimistically.  Spin frequency is fixed at 100%.
1764 //
1765 // Note that _SpinDuration is volatile, but we update it without locks
1766 // or atomics.  The code is designed so that _SpinDuration stays within
1767 // a reasonable range even in the presence of races.  The arithmetic
1768 // operations on _SpinDuration are closed over the domain of legal values,
1769 // so at worst a race will install and older but still legal value.
1770 // At the very worst this introduces some apparent non-determinism.
1771 // We might spin when we shouldn't or vice-versa, but since the spin
1772 // count are relatively short, even in the worst case, the effect is harmless.
1773 //
1774 // Care must be taken that a low "D" value does not become an
1775 // an absorbing state.  Transient spinning failures -- when spinning
1776 // is overall profitable -- should not cause the system to converge
1777 // on low "D" values.  We want spinning to be stable and predictable
1778 // and fairly responsive to change and at the same time we don't want
1779 // it to oscillate, become metastable, be "too" non-deterministic,
1780 // or converge on or enter undesirable stable absorbing states.
1781 //
1782 // We implement a feedback-based control system -- using past behavior
1783 // to predict future behavior.  We face two issues: (a) if the
1784 // input signal is random then the spin predictor won't provide optimal
1785 // results, and (b) if the signal frequency is too high then the control
1786 // system, which has some natural response lag, will "chase" the signal.
1787 // (b) can arise from multimodal lock hold times.  Transient preemption
1788 // can also result in apparent bimodal lock hold times.
1789 // Although sub-optimal, neither condition is particularly harmful, as
1790 // in the worst-case we'll spin when we shouldn't or vice-versa.
1791 // The maximum spin duration is rather short so the failure modes aren't bad.
1792 // To be conservative, I've tuned the gain in system to bias toward
1793 // _not spinning.  Relatedly, the system can sometimes enter a mode where it
1794 // "rings" or oscillates between spinning and not spinning.  This happens
1795 // when spinning is just on the cusp of profitability, however, so the
1796 // situation is not dire.  The state is benign -- there's no need to add
1797 // hysteresis control to damp the transition rate between spinning and
1798 // not spinning.
1799 
1800 // Spinning: Fixed frequency (100%), vary duration
1801 int ObjectMonitor::TrySpin(Thread * Self) {
1802   // Dumb, brutal spin.  Good for comparative measurements against adaptive spinning.
1803   int ctr = Knob_FixedSpin;
1804   if (ctr != 0) {
1805     while (--ctr >= 0) {
1806       if (TryLock(Self) > 0) return 1;
1807       SpinPause();
1808     }
1809     return 0;
1810   }
1811 
1812   for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1813     if (TryLock(Self) > 0) {
1814       // Increase _SpinDuration ...
1815       // Note that we don't clamp SpinDuration precisely at SpinLimit.
1816       // Raising _SpurDuration to the poverty line is key.
1817       int x = _SpinDuration;
1818       if (x < Knob_SpinLimit) {
1819         if (x < Knob_Poverty) x = Knob_Poverty;
1820         _SpinDuration = x + Knob_BonusB;
1821       }
1822       return 1;
1823     }
1824     SpinPause();
1825   }
1826 
1827   // Admission control - verify preconditions for spinning
1828   //
1829   // We always spin a little bit, just to prevent _SpinDuration == 0 from
1830   // becoming an absorbing state.  Put another way, we spin briefly to
1831   // sample, just in case the system load, parallelism, contention, or lock
1832   // modality changed.
1833   //
1834   // Consider the following alternative:
1835   // Periodically set _SpinDuration = _SpinLimit and try a long/full
1836   // spin attempt.  "Periodically" might mean after a tally of
1837   // the # of failed spin attempts (or iterations) reaches some threshold.
1838   // This takes us into the realm of 1-out-of-N spinning, where we
1839   // hold the duration constant but vary the frequency.
1840 
1841   ctr = _SpinDuration;
1842   if (ctr <= 0) return 0;
1843 
1844   if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
1845     return 0;
1846   }
1847 
1848   int MaxSpin = Knob_MaxSpinners;
1849   if (MaxSpin >= 0) {
1850     if (_Spinner > MaxSpin) {
1851       return 0;
1852     }
1853     // Slightly racy, but benign ...
1854     Adjust(&_Spinner, 1);
1855   }
1856 
1857   // We're good to spin ... spin ingress.
1858   // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1859   // when preparing to LD...CAS _owner, etc and the CAS is likely
1860   // to succeed.
1861   if (_succ == NULL) {
1862     _succ = Self;
1863   }
1864   Thread * prv = NULL;
1865 
1866   // There are three ways to exit the following loop:
1867   // 1.  A successful spin where this thread has acquired the lock.
1868   // 2.  Spin failure with prejudice
1869   // 3.  Spin failure without prejudice
1870 
1871   while (--ctr >= 0) {
1872 
1873     // Periodic polling -- Check for pending GC
1874     // Threads may spin while they're unsafe.
1875     // We don't want spinning threads to delay the JVM from reaching
1876     // a stop-the-world safepoint or to steal cycles from GC.
1877     // If we detect a pending safepoint we abort in order that
1878     // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1879     // this thread, if safe, doesn't steal cycles from GC.
1880     // This is in keeping with the "no loitering in runtime" rule.
1881     // We periodically check to see if there's a safepoint pending.
1882     if ((ctr & 0xFF) == 0) {
1883       if (SafepointMechanism::poll(Self)) {
1884         goto Abort;           // abrupt spin egress
1885       }
1886       if (Knob_UsePause & 1) SpinPause();
1887     }
1888 
1889     if (Knob_UsePause & 2) SpinPause();
1890 
1891     // Probe _owner with TATAS
1892     // If this thread observes the monitor transition or flicker
1893     // from locked to unlocked to locked, then the odds that this
1894     // thread will acquire the lock in this spin attempt go down
1895     // considerably.  The same argument applies if the CAS fails
1896     // or if we observe _owner change from one non-null value to
1897     // another non-null value.   In such cases we might abort
1898     // the spin without prejudice or apply a "penalty" to the
1899     // spin count-down variable "ctr", reducing it by 100, say.
1900 
1901     Thread * ox = (Thread *) _owner;
1902     if (ox == NULL) {
1903       ox = (Thread*)Atomic::cmpxchg(Self, &_owner, (void*)NULL);
1904       if (ox == NULL) {
1905         // The CAS succeeded -- this thread acquired ownership
1906         // Take care of some bookkeeping to exit spin state.
1907         if (_succ == Self) {
1908           _succ = NULL;
1909         }
1910         if (MaxSpin > 0) Adjust(&_Spinner, -1);
1911 
1912         // Increase _SpinDuration :
1913         // The spin was successful (profitable) so we tend toward
1914         // longer spin attempts in the future.
1915         // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
1916         // If we acquired the lock early in the spin cycle it
1917         // makes sense to increase _SpinDuration proportionally.
1918         // Note that we don't clamp SpinDuration precisely at SpinLimit.
1919         int x = _SpinDuration;
1920         if (x < Knob_SpinLimit) {
1921           if (x < Knob_Poverty) x = Knob_Poverty;
1922           _SpinDuration = x + Knob_Bonus;
1923         }
1924         return 1;
1925       }
1926 
1927       // The CAS failed ... we can take any of the following actions:
1928       // * penalize: ctr -= CASPenalty
1929       // * exit spin with prejudice -- goto Abort;
1930       // * exit spin without prejudice.
1931       // * Since CAS is high-latency, retry again immediately.
1932       prv = ox;
1933       goto Abort;
1934     }
1935 
1936     // Did lock ownership change hands ?
1937     if (ox != prv && prv != NULL) {
1938       goto Abort;
1939     }
1940     prv = ox;
1941 
1942     // Abort the spin if the owner is not executing.
1943     // The owner must be executing in order to drop the lock.
1944     // Spinning while the owner is OFFPROC is idiocy.
1945     // Consider: ctr -= RunnablePenalty ;
1946     if (Knob_OState && NotRunnable (Self, ox)) {
1947       goto Abort;
1948     }
1949     if (_succ == NULL) {
1950       _succ = Self;
1951     }
1952   }
1953 
1954   // Spin failed with prejudice -- reduce _SpinDuration.
1955   // TODO: Use an AIMD-like policy to adjust _SpinDuration.
1956   // AIMD is globally stable.
1957   {
1958     int x = _SpinDuration;
1959     if (x > 0) {
1960       // Consider an AIMD scheme like: x -= (x >> 3) + 100
1961       // This is globally sample and tends to damp the response.
1962       x -= Knob_Penalty;
1963       if (x < 0) x = 0;
1964       _SpinDuration = x;
1965     }
1966   }
1967 
1968  Abort:
1969   if (MaxSpin >= 0) Adjust(&_Spinner, -1);
1970   if (_succ == Self) {
1971     _succ = NULL;
1972     // Invariant: after setting succ=null a contending thread
1973     // must recheck-retry _owner before parking.  This usually happens
1974     // in the normal usage of TrySpin(), but it's safest
1975     // to make TrySpin() as foolproof as possible.
1976     OrderAccess::fence();
1977     if (TryLock(Self) > 0) return 1;
1978   }
1979   return 0;
1980 }
1981 
1982 // NotRunnable() -- informed spinning
1983 //
1984 // Don't bother spinning if the owner is not eligible to drop the lock.
1985 // Spin only if the owner thread is _thread_in_Java or _thread_in_vm.
1986 // The thread must be runnable in order to drop the lock in timely fashion.
1987 // If the _owner is not runnable then spinning will not likely be
1988 // successful (profitable).
1989 //
1990 // Beware -- the thread referenced by _owner could have died
1991 // so a simply fetch from _owner->_thread_state might trap.
1992 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
1993 // Because of the lifecycle issues, the _thread_state values
1994 // observed by NotRunnable() might be garbage.  NotRunnable must
1995 // tolerate this and consider the observed _thread_state value
1996 // as advisory.
1997 //
1998 // Beware too, that _owner is sometimes a BasicLock address and sometimes
1999 // a thread pointer.
2000 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2001 // with the LSB of _owner.  Another option would be to probabilistically probe
2002 // the putative _owner->TypeTag value.
2003 //
2004 // Checking _thread_state isn't perfect.  Even if the thread is
2005 // in_java it might be blocked on a page-fault or have been preempted
2006 // and sitting on a ready/dispatch queue.
2007 //
2008 // The return value from NotRunnable() is *advisory* -- the
2009 // result is based on sampling and is not necessarily coherent.
2010 // The caller must tolerate false-negative and false-positive errors.
2011 // Spinning, in general, is probabilistic anyway.
2012 
2013 
2014 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) {
2015   // Check ox->TypeTag == 2BAD.
2016   if (ox == NULL) return 0;
2017 
2018   // Avoid transitive spinning ...
2019   // Say T1 spins or blocks trying to acquire L.  T1._Stalled is set to L.
2020   // Immediately after T1 acquires L it's possible that T2, also
2021   // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2022   // This occurs transiently after T1 acquired L but before
2023   // T1 managed to clear T1.Stalled.  T2 does not need to abort
2024   // its spin in this circumstance.
2025   intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
2026 
2027   if (BlockedOn == 1) return 1;
2028   if (BlockedOn != 0) {
2029     return BlockedOn != intptr_t(this) && _owner == ox;
2030   }
2031 
2032   assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant");
2033   int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);;
2034   // consider also: jst != _thread_in_Java -- but that's overspecific.
2035   return jst == _thread_blocked || jst == _thread_in_native;
2036 }
2037 
2038 
2039 // -----------------------------------------------------------------------------
2040 // WaitSet management ...
2041 
2042 ObjectWaiter::ObjectWaiter(Thread* thread) {
2043   _next     = NULL;
2044   _prev     = NULL;
2045   _notified = 0;
2046   _notifier_tid = 0;
2047   TState    = TS_RUN;
2048   _thread   = thread;
2049   _event    = thread->_ParkEvent;
2050   _active   = false;
2051   assert(_event != NULL, "invariant");
2052 }
2053 
2054 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2055   JavaThread *jt = (JavaThread *)this->_thread;
2056   _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2057 }
2058 
2059 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2060   JavaThread *jt = (JavaThread *)this->_thread;
2061   JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2062 }
2063 
2064 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2065   assert(node != NULL, "should not add NULL node");
2066   assert(node->_prev == NULL, "node already in list");
2067   assert(node->_next == NULL, "node already in list");
2068   // put node at end of queue (circular doubly linked list)
2069   if (_WaitSet == NULL) {
2070     _WaitSet = node;
2071     node->_prev = node;
2072     node->_next = node;
2073   } else {
2074     ObjectWaiter* head = _WaitSet;
2075     ObjectWaiter* tail = head->_prev;
2076     assert(tail->_next == head, "invariant check");
2077     tail->_next = node;
2078     head->_prev = node;
2079     node->_next = head;
2080     node->_prev = tail;
2081   }
2082 }
2083 
2084 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2085   // dequeue the very first waiter
2086   ObjectWaiter* waiter = _WaitSet;
2087   if (waiter) {
2088     DequeueSpecificWaiter(waiter);
2089   }
2090   return waiter;
2091 }
2092 
2093 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2094   assert(node != NULL, "should not dequeue NULL node");
2095   assert(node->_prev != NULL, "node already removed from list");
2096   assert(node->_next != NULL, "node already removed from list");
2097   // when the waiter has woken up because of interrupt,
2098   // timeout or other spurious wake-up, dequeue the
2099   // waiter from waiting list
2100   ObjectWaiter* next = node->_next;
2101   if (next == node) {
2102     assert(node->_prev == node, "invariant check");
2103     _WaitSet = NULL;
2104   } else {
2105     ObjectWaiter* prev = node->_prev;
2106     assert(prev->_next == node, "invariant check");
2107     assert(next->_prev == node, "invariant check");
2108     next->_prev = prev;
2109     prev->_next = next;
2110     if (_WaitSet == node) {
2111       _WaitSet = next;
2112     }
2113   }
2114   node->_next = NULL;
2115   node->_prev = NULL;
2116 }
2117 
2118 // -----------------------------------------------------------------------------
2119 // PerfData support
2120 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts       = NULL;
2121 PerfCounter * ObjectMonitor::_sync_FutileWakeups               = NULL;
2122 PerfCounter * ObjectMonitor::_sync_Parks                       = NULL;
2123 PerfCounter * ObjectMonitor::_sync_Notifications               = NULL;
2124 PerfCounter * ObjectMonitor::_sync_Inflations                  = NULL;
2125 PerfCounter * ObjectMonitor::_sync_Deflations                  = NULL;
2126 PerfLongVariable * ObjectMonitor::_sync_MonExtant              = NULL;
2127 
2128 // One-shot global initialization for the sync subsystem.
2129 // We could also defer initialization and initialize on-demand
2130 // the first time we call inflate().  Initialization would
2131 // be protected - like so many things - by the MonitorCache_lock.
2132 
2133 void ObjectMonitor::Initialize() {
2134   static int InitializationCompleted = 0;
2135   assert(InitializationCompleted == 0, "invariant");
2136   InitializationCompleted = 1;
2137   if (UsePerfData) {
2138     EXCEPTION_MARK;
2139 #define NEWPERFCOUNTER(n)                                                \
2140   {                                                                      \
2141     n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,  \
2142                                         CHECK);                          \
2143   }
2144 #define NEWPERFVARIABLE(n)                                                \
2145   {                                                                       \
2146     n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,  \
2147                                          CHECK);                          \
2148   }
2149     NEWPERFCOUNTER(_sync_Inflations);
2150     NEWPERFCOUNTER(_sync_Deflations);
2151     NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2152     NEWPERFCOUNTER(_sync_FutileWakeups);
2153     NEWPERFCOUNTER(_sync_Parks);
2154     NEWPERFCOUNTER(_sync_Notifications);
2155     NEWPERFVARIABLE(_sync_MonExtant);
2156 #undef NEWPERFCOUNTER
2157 #undef NEWPERFVARIABLE
2158   }
2159 }
2160 
2161 void ObjectMonitor::DeferredInitialize() {
2162   if (InitDone > 0) return;
2163   if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2164     while (InitDone != 1) /* empty */;
2165     return;
2166   }
2167 
2168   // One-shot global initialization ...
2169   // The initialization is idempotent, so we don't need locks.
2170   // In the future consider doing this via os::init_2().
2171 
2172   if (!os::is_MP()) {
2173     Knob_SpinLimit = 0;
2174     Knob_PreSpin   = 0;
2175     Knob_FixedSpin = -1;
2176   }
2177 
2178   OrderAccess::fence();
2179   InitDone = 1;
2180 }
2181