rev 51780 : imported patch syncknobs-00-base
rev 51781 : imported patch syncknobs-01-Knob_ReportSettings
rev 51782 : imported patch syncknobs-02-Knob_SpinBackOff
rev 51783 : imported patch syncknobs-03-BackOffMask
rev 51784 : imported patch syncknobs-04-Knob_ExitRelease
rev 51785 : imported patch syncknobs-05-Knob_InlineNotify
rev 51786 : imported patch syncknobs-06-Knob_Verbose
rev 51787 : imported patch syncknobs-07-Knob_VerifyInUse
rev 51788 : imported patch syncknobs-08-Knob_VerifyMatch
rev 51789 : imported patch syncknobs-09-Knob_SpinBase

   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_SpinBase            = 0;       // Floor AKA SpinMin
 107 static int Knob_CASPenalty          = -1;      // Penalty for failed CAS
 108 static int Knob_OXPenalty           = -1;      // Penalty for observed _owner change
 109 static int Knob_SpinSetSucc         = 1;       // spinners set the _succ field
 110 static int Knob_SpinEarly           = 1;
 111 static int Knob_SuccEnabled         = 1;       // futile wake throttling
 112 static int Knob_SuccRestrict        = 0;       // Limit successors + spinners to at-most-one
 113 static int Knob_MaxSpinners         = -1;      // Should be a function of # CPUs
 114 static int Knob_Bonus               = 100;     // spin success bonus
 115 static int Knob_BonusB              = 100;     // spin success bonus
 116 static int Knob_Penalty             = 200;     // spin failure penalty
 117 static int Knob_Poverty             = 1000;
 118 static int Knob_SpinAfterFutile     = 1;       // Spin after returning from park()
 119 static int Knob_FixedSpin           = 0;
 120 static int Knob_OState              = 3;       // Spinner checks thread state of _owner
 121 static int Knob_UsePause            = 1;
 122 static int Knob_ExitPolicy          = 0;
 123 static int Knob_PreSpin             = 10;      // 20-100 likely better
 124 static int Knob_ResetEvent          = 0;
 125 
 126 static int Knob_FastHSSEC           = 0;
 127 static int Knob_MoveNotifyee        = 2;       // notify() - disposition of notifyee
 128 static int Knob_QMode               = 0;       // EntryList-cxq policy - queue discipline
 129 static volatile int InitDone        = 0;
 130 
 131 // -----------------------------------------------------------------------------
 132 // Theory of operations -- Monitors lists, thread residency, etc:
 133 //
 134 // * A thread acquires ownership of a monitor by successfully
 135 //   CAS()ing the _owner field from null to non-null.
 136 //
 137 // * Invariant: A thread appears on at most one monitor list --
 138 //   cxq, EntryList or WaitSet -- at any one time.
 139 //
 140 // * Contending threads "push" themselves onto the cxq with CAS
 141 //   and then spin/park.
 142 //
 143 // * After a contending thread eventually acquires the lock it must
 144 //   dequeue itself from either the EntryList or the cxq.
 145 //
 146 // * The exiting thread identifies and unparks an "heir presumptive"
 147 //   tentative successor thread on the EntryList.  Critically, the
 148 //   exiting thread doesn't unlink the successor thread from the EntryList.
 149 //   After having been unparked, the wakee will recontend for ownership of
 150 //   the monitor.   The successor (wakee) will either acquire the lock or
 151 //   re-park itself.
 152 //
 153 //   Succession is provided for by a policy of competitive handoff.
 154 //   The exiting thread does _not_ grant or pass ownership to the
 155 //   successor thread.  (This is also referred to as "handoff" succession").
 156 //   Instead the exiting thread releases ownership and possibly wakes
 157 //   a successor, so the successor can (re)compete for ownership of the lock.
 158 //   If the EntryList is empty but the cxq is populated the exiting
 159 //   thread will drain the cxq into the EntryList.  It does so by
 160 //   by detaching the cxq (installing null with CAS) and folding
 161 //   the threads from the cxq into the EntryList.  The EntryList is
 162 //   doubly linked, while the cxq is singly linked because of the
 163 //   CAS-based "push" used to enqueue recently arrived threads (RATs).
 164 //
 165 // * Concurrency invariants:
 166 //
 167 //   -- only the monitor owner may access or mutate the EntryList.
 168 //      The mutex property of the monitor itself protects the EntryList
 169 //      from concurrent interference.
 170 //   -- Only the monitor owner may detach the cxq.
 171 //
 172 // * The monitor entry list operations avoid locks, but strictly speaking
 173 //   they're not lock-free.  Enter is lock-free, exit is not.
 174 //   For a description of 'Methods and apparatus providing non-blocking access
 175 //   to a resource,' see U.S. Pat. No. 7844973.
 176 //
 177 // * The cxq can have multiple concurrent "pushers" but only one concurrent
 178 //   detaching thread.  This mechanism is immune from the ABA corruption.
 179 //   More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
 180 //
 181 // * Taken together, the cxq and the EntryList constitute or form a
 182 //   single logical queue of threads stalled trying to acquire the lock.
 183 //   We use two distinct lists to improve the odds of a constant-time
 184 //   dequeue operation after acquisition (in the ::enter() epilogue) and
 185 //   to reduce heat on the list ends.  (c.f. Michael Scott's "2Q" algorithm).
 186 //   A key desideratum is to minimize queue & monitor metadata manipulation
 187 //   that occurs while holding the monitor lock -- that is, we want to
 188 //   minimize monitor lock holds times.  Note that even a small amount of
 189 //   fixed spinning will greatly reduce the # of enqueue-dequeue operations
 190 //   on EntryList|cxq.  That is, spinning relieves contention on the "inner"
 191 //   locks and monitor metadata.
 192 //
 193 //   Cxq points to the set of Recently Arrived Threads attempting entry.
 194 //   Because we push threads onto _cxq with CAS, the RATs must take the form of
 195 //   a singly-linked LIFO.  We drain _cxq into EntryList  at unlock-time when
 196 //   the unlocking thread notices that EntryList is null but _cxq is != null.
 197 //
 198 //   The EntryList is ordered by the prevailing queue discipline and
 199 //   can be organized in any convenient fashion, such as a doubly-linked list or
 200 //   a circular doubly-linked list.  Critically, we want insert and delete operations
 201 //   to operate in constant-time.  If we need a priority queue then something akin
 202 //   to Solaris' sleepq would work nicely.  Viz.,
 203 //   http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
 204 //   Queue discipline is enforced at ::exit() time, when the unlocking thread
 205 //   drains the cxq into the EntryList, and orders or reorders the threads on the
 206 //   EntryList accordingly.
 207 //
 208 //   Barring "lock barging", this mechanism provides fair cyclic ordering,
 209 //   somewhat similar to an elevator-scan.
 210 //
 211 // * The monitor synchronization subsystem avoids the use of native
 212 //   synchronization primitives except for the narrow platform-specific
 213 //   park-unpark abstraction.  See the comments in os_solaris.cpp regarding
 214 //   the semantics of park-unpark.  Put another way, this monitor implementation
 215 //   depends only on atomic operations and park-unpark.  The monitor subsystem
 216 //   manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
 217 //   underlying OS manages the READY<->RUN transitions.
 218 //
 219 // * Waiting threads reside on the WaitSet list -- wait() puts
 220 //   the caller onto the WaitSet.
 221 //
 222 // * notify() or notifyAll() simply transfers threads from the WaitSet to
 223 //   either the EntryList or cxq.  Subsequent exit() operations will
 224 //   unpark the notifyee.  Unparking a notifee in notify() is inefficient -
 225 //   it's likely the notifyee would simply impale itself on the lock held
 226 //   by the notifier.
 227 //
 228 // * An interesting alternative is to encode cxq as (List,LockByte) where
 229 //   the LockByte is 0 iff the monitor is owned.  _owner is simply an auxiliary
 230 //   variable, like _recursions, in the scheme.  The threads or Events that form
 231 //   the list would have to be aligned in 256-byte addresses.  A thread would
 232 //   try to acquire the lock or enqueue itself with CAS, but exiting threads
 233 //   could use a 1-0 protocol and simply STB to set the LockByte to 0.
 234 //   Note that is is *not* word-tearing, but it does presume that full-word
 235 //   CAS operations are coherent with intermix with STB operations.  That's true
 236 //   on most common processors.
 237 //
 238 // * See also http://blogs.sun.com/dave
 239 
 240 
 241 void* ObjectMonitor::operator new (size_t size) throw() {
 242   return AllocateHeap(size, mtInternal);
 243 }
 244 void* ObjectMonitor::operator new[] (size_t size) throw() {
 245   return operator new (size);
 246 }
 247 void ObjectMonitor::operator delete(void* p) {
 248   FreeHeap(p);
 249 }
 250 void ObjectMonitor::operator delete[] (void *p) {
 251   operator delete(p);
 252 }
 253 
 254 // -----------------------------------------------------------------------------
 255 // Enter support
 256 
 257 void ObjectMonitor::enter(TRAPS) {
 258   // The following code is ordered to check the most common cases first
 259   // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
 260   Thread * const Self = THREAD;
 261 
 262   void * cur = Atomic::cmpxchg(Self, &_owner, (void*)NULL);
 263   if (cur == NULL) {
 264     // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
 265     assert(_recursions == 0, "invariant");
 266     assert(_owner == Self, "invariant");
 267     return;
 268   }
 269 
 270   if (cur == Self) {
 271     // TODO-FIXME: check for integer overflow!  BUGID 6557169.
 272     _recursions++;
 273     return;
 274   }
 275 
 276   if (Self->is_lock_owned ((address)cur)) {
 277     assert(_recursions == 0, "internal state error");
 278     _recursions = 1;
 279     // Commute owner from a thread-specific on-stack BasicLockObject address to
 280     // a full-fledged "Thread *".
 281     _owner = Self;
 282     return;
 283   }
 284 
 285   // We've encountered genuine contention.
 286   assert(Self->_Stalled == 0, "invariant");
 287   Self->_Stalled = intptr_t(this);
 288 
 289   // Try one round of spinning *before* enqueueing Self
 290   // and before going through the awkward and expensive state
 291   // transitions.  The following spin is strictly optional ...
 292   // Note that if we acquire the monitor from an initial spin
 293   // we forgo posting JVMTI events and firing DTRACE probes.
 294   if (Knob_SpinEarly && TrySpin (Self) > 0) {
 295     assert(_owner == Self, "invariant");
 296     assert(_recursions == 0, "invariant");
 297     assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 298     Self->_Stalled = 0;
 299     return;
 300   }
 301 
 302   assert(_owner != Self, "invariant");
 303   assert(_succ != Self, "invariant");
 304   assert(Self->is_Java_thread(), "invariant");
 305   JavaThread * jt = (JavaThread *) Self;
 306   assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
 307   assert(jt->thread_state() != _thread_blocked, "invariant");
 308   assert(this->object() != NULL, "invariant");
 309   assert(_count >= 0, "invariant");
 310 
 311   // Prevent deflation at STW-time.  See deflate_idle_monitors() and is_busy().
 312   // Ensure the object-monitor relationship remains stable while there's contention.
 313   Atomic::inc(&_count);
 314 
 315   JFR_ONLY(JfrConditionalFlushWithStacktrace<EventJavaMonitorEnter> flush(jt);)
 316   EventJavaMonitorEnter event;
 317   if (event.should_commit()) {
 318     event.set_monitorClass(((oop)this->object())->klass());
 319     event.set_address((uintptr_t)(this->object_addr()));
 320   }
 321 
 322   { // Change java thread status to indicate blocked on monitor enter.
 323     JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
 324 
 325     Self->set_current_pending_monitor(this);
 326 
 327     DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
 328     if (JvmtiExport::should_post_monitor_contended_enter()) {
 329       JvmtiExport::post_monitor_contended_enter(jt, this);
 330 
 331       // The current thread does not yet own the monitor and does not
 332       // yet appear on any queues that would get it made the successor.
 333       // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
 334       // handler cannot accidentally consume an unpark() meant for the
 335       // ParkEvent associated with this ObjectMonitor.
 336     }
 337 
 338     OSThreadContendState osts(Self->osthread());
 339     ThreadBlockInVM tbivm(jt);
 340 
 341     // TODO-FIXME: change the following for(;;) loop to straight-line code.
 342     for (;;) {
 343       jt->set_suspend_equivalent();
 344       // cleared by handle_special_suspend_equivalent_condition()
 345       // or java_suspend_self()
 346 
 347       EnterI(THREAD);
 348 
 349       if (!ExitSuspendEquivalent(jt)) break;
 350 
 351       // We have acquired the contended monitor, but while we were
 352       // waiting another thread suspended us. We don't want to enter
 353       // the monitor while suspended because that would surprise the
 354       // thread that suspended us.
 355       //
 356       _recursions = 0;
 357       _succ = NULL;
 358       exit(false, Self);
 359 
 360       jt->java_suspend_self();
 361     }
 362     Self->set_current_pending_monitor(NULL);
 363 
 364     // We cleared the pending monitor info since we've just gotten past
 365     // the enter-check-for-suspend dance and we now own the monitor free
 366     // and clear, i.e., it is no longer pending. The ThreadBlockInVM
 367     // destructor can go to a safepoint at the end of this block. If we
 368     // do a thread dump during that safepoint, then this thread will show
 369     // as having "-locked" the monitor, but the OS and java.lang.Thread
 370     // states will still report that the thread is blocked trying to
 371     // acquire it.
 372   }
 373 
 374   Atomic::dec(&_count);
 375   assert(_count >= 0, "invariant");
 376   Self->_Stalled = 0;
 377 
 378   // Must either set _recursions = 0 or ASSERT _recursions == 0.
 379   assert(_recursions == 0, "invariant");
 380   assert(_owner == Self, "invariant");
 381   assert(_succ != Self, "invariant");
 382   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 383 
 384   // The thread -- now the owner -- is back in vm mode.
 385   // Report the glorious news via TI,DTrace and jvmstat.
 386   // The probe effect is non-trivial.  All the reportage occurs
 387   // while we hold the monitor, increasing the length of the critical
 388   // section.  Amdahl's parallel speedup law comes vividly into play.
 389   //
 390   // Another option might be to aggregate the events (thread local or
 391   // per-monitor aggregation) and defer reporting until a more opportune
 392   // time -- such as next time some thread encounters contention but has
 393   // yet to acquire the lock.  While spinning that thread could
 394   // spinning we could increment JVMStat counters, etc.
 395 
 396   DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
 397   if (JvmtiExport::should_post_monitor_contended_entered()) {
 398     JvmtiExport::post_monitor_contended_entered(jt, this);
 399 
 400     // The current thread already owns the monitor and is not going to
 401     // call park() for the remainder of the monitor enter protocol. So
 402     // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
 403     // event handler consumed an unpark() issued by the thread that
 404     // just exited the monitor.
 405   }
 406   if (event.should_commit()) {
 407     event.set_previousOwner((uintptr_t)_previous_owner_tid);
 408     event.commit();
 409   }
 410   OM_PERFDATA_OP(ContendedLockAttempts, inc());
 411 }
 412 
 413 // Caveat: TryLock() is not necessarily serializing if it returns failure.
 414 // Callers must compensate as needed.
 415 
 416 int ObjectMonitor::TryLock(Thread * Self) {
 417   void * own = _owner;
 418   if (own != NULL) return 0;
 419   if (Atomic::replace_if_null(Self, &_owner)) {
 420     // Either guarantee _recursions == 0 or set _recursions = 0.
 421     assert(_recursions == 0, "invariant");
 422     assert(_owner == Self, "invariant");
 423     return 1;
 424   }
 425   // The lock had been free momentarily, but we lost the race to the lock.
 426   // Interference -- the CAS failed.
 427   // We can either return -1 or retry.
 428   // Retry doesn't make as much sense because the lock was just acquired.
 429   return -1;
 430 }
 431 
 432 #define MAX_RECHECK_INTERVAL 1000
 433 
 434 void ObjectMonitor::EnterI(TRAPS) {
 435   Thread * const Self = THREAD;
 436   assert(Self->is_Java_thread(), "invariant");
 437   assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant");
 438 
 439   // Try the lock - TATAS
 440   if (TryLock (Self) > 0) {
 441     assert(_succ != Self, "invariant");
 442     assert(_owner == Self, "invariant");
 443     assert(_Responsible != Self, "invariant");
 444     return;
 445   }
 446 
 447   DeferredInitialize();
 448 
 449   // We try one round of spinning *before* enqueueing Self.
 450   //
 451   // If the _owner is ready but OFFPROC we could use a YieldTo()
 452   // operation to donate the remainder of this thread's quantum
 453   // to the owner.  This has subtle but beneficial affinity
 454   // effects.
 455 
 456   if (TrySpin (Self) > 0) {
 457     assert(_owner == Self, "invariant");
 458     assert(_succ != Self, "invariant");
 459     assert(_Responsible != Self, "invariant");
 460     return;
 461   }
 462 
 463   // The Spin failed -- Enqueue and park the thread ...
 464   assert(_succ != Self, "invariant");
 465   assert(_owner != Self, "invariant");
 466   assert(_Responsible != Self, "invariant");
 467 
 468   // Enqueue "Self" on ObjectMonitor's _cxq.
 469   //
 470   // Node acts as a proxy for Self.
 471   // As an aside, if were to ever rewrite the synchronization code mostly
 472   // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
 473   // Java objects.  This would avoid awkward lifecycle and liveness issues,
 474   // as well as eliminate a subset of ABA issues.
 475   // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
 476 
 477   ObjectWaiter node(Self);
 478   Self->_ParkEvent->reset();
 479   node._prev   = (ObjectWaiter *) 0xBAD;
 480   node.TState  = ObjectWaiter::TS_CXQ;
 481 
 482   // Push "Self" onto the front of the _cxq.
 483   // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
 484   // Note that spinning tends to reduce the rate at which threads
 485   // enqueue and dequeue on EntryList|cxq.
 486   ObjectWaiter * nxt;
 487   for (;;) {
 488     node._next = nxt = _cxq;
 489     if (Atomic::cmpxchg(&node, &_cxq, nxt) == nxt) break;
 490 
 491     // Interference - the CAS failed because _cxq changed.  Just retry.
 492     // As an optional optimization we retry the lock.
 493     if (TryLock (Self) > 0) {
 494       assert(_succ != Self, "invariant");
 495       assert(_owner == Self, "invariant");
 496       assert(_Responsible != Self, "invariant");
 497       return;
 498     }
 499   }
 500 
 501   // Check for cxq|EntryList edge transition to non-null.  This indicates
 502   // the onset of contention.  While contention persists exiting threads
 503   // will use a ST:MEMBAR:LD 1-1 exit protocol.  When contention abates exit
 504   // operations revert to the faster 1-0 mode.  This enter operation may interleave
 505   // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
 506   // arrange for one of the contending thread to use a timed park() operations
 507   // to detect and recover from the race.  (Stranding is form of progress failure
 508   // where the monitor is unlocked but all the contending threads remain parked).
 509   // That is, at least one of the contended threads will periodically poll _owner.
 510   // One of the contending threads will become the designated "Responsible" thread.
 511   // The Responsible thread uses a timed park instead of a normal indefinite park
 512   // operation -- it periodically wakes and checks for and recovers from potential
 513   // strandings admitted by 1-0 exit operations.   We need at most one Responsible
 514   // thread per-monitor at any given moment.  Only threads on cxq|EntryList may
 515   // be responsible for a monitor.
 516   //
 517   // Currently, one of the contended threads takes on the added role of "Responsible".
 518   // A viable alternative would be to use a dedicated "stranding checker" thread
 519   // that periodically iterated over all the threads (or active monitors) and unparked
 520   // successors where there was risk of stranding.  This would help eliminate the
 521   // timer scalability issues we see on some platforms as we'd only have one thread
 522   // -- the checker -- parked on a timer.
 523 
 524   if (nxt == NULL && _EntryList == NULL) {
 525     // Try to assume the role of responsible thread for the monitor.
 526     // CONSIDER:  ST vs CAS vs { if (Responsible==null) Responsible=Self }
 527     Atomic::replace_if_null(Self, &_Responsible);
 528   }
 529 
 530   // The lock might have been released while this thread was occupied queueing
 531   // itself onto _cxq.  To close the race and avoid "stranding" and
 532   // progress-liveness failure we must resample-retry _owner before parking.
 533   // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
 534   // In this case the ST-MEMBAR is accomplished with CAS().
 535   //
 536   // TODO: Defer all thread state transitions until park-time.
 537   // Since state transitions are heavy and inefficient we'd like
 538   // to defer the state transitions until absolutely necessary,
 539   // and in doing so avoid some transitions ...
 540 
 541   int nWakeups = 0;
 542   int recheckInterval = 1;
 543 
 544   for (;;) {
 545 
 546     if (TryLock(Self) > 0) break;
 547     assert(_owner != Self, "invariant");
 548 
 549     // park self
 550     if (_Responsible == Self) {
 551       Self->_ParkEvent->park((jlong) recheckInterval);
 552       // Increase the recheckInterval, but clamp the value.
 553       recheckInterval *= 8;
 554       if (recheckInterval > MAX_RECHECK_INTERVAL) {
 555         recheckInterval = MAX_RECHECK_INTERVAL;
 556       }
 557     } else {
 558       Self->_ParkEvent->park();
 559     }
 560 
 561     if (TryLock(Self) > 0) break;
 562 
 563     // The lock is still contested.
 564     // Keep a tally of the # of futile wakeups.
 565     // Note that the counter is not protected by a lock or updated by atomics.
 566     // That is by design - we trade "lossy" counters which are exposed to
 567     // races during updates for a lower probe effect.
 568 
 569     // This PerfData object can be used in parallel with a safepoint.
 570     // See the work around in PerfDataManager::destroy().
 571     OM_PERFDATA_OP(FutileWakeups, inc());
 572     ++nWakeups;
 573 
 574     // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
 575     // We can defer clearing _succ until after the spin completes
 576     // TrySpin() must tolerate being called with _succ == Self.
 577     // Try yet another round of adaptive spinning.
 578     if ((Knob_SpinAfterFutile & 1) && TrySpin(Self) > 0) break;
 579 
 580     // We can find that we were unpark()ed and redesignated _succ while
 581     // we were spinning.  That's harmless.  If we iterate and call park(),
 582     // park() will consume the event and return immediately and we'll
 583     // just spin again.  This pattern can repeat, leaving _succ to simply
 584     // spin on a CPU.  Enable Knob_ResetEvent to clear pending unparks().
 585     // Alternately, we can sample fired() here, and if set, forgo spinning
 586     // in the next iteration.
 587 
 588     if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
 589       Self->_ParkEvent->reset();
 590       OrderAccess::fence();
 591     }
 592     if (_succ == Self) _succ = NULL;
 593 
 594     // Invariant: after clearing _succ a thread *must* retry _owner before parking.
 595     OrderAccess::fence();
 596   }
 597 
 598   // Egress :
 599   // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
 600   // Normally we'll find Self on the EntryList .
 601   // From the perspective of the lock owner (this thread), the
 602   // EntryList is stable and cxq is prepend-only.
 603   // The head of cxq is volatile but the interior is stable.
 604   // In addition, Self.TState is stable.
 605 
 606   assert(_owner == Self, "invariant");
 607   assert(object() != NULL, "invariant");
 608   // I'd like to write:
 609   //   guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
 610   // but as we're at a safepoint that's not safe.
 611 
 612   UnlinkAfterAcquire(Self, &node);
 613   if (_succ == Self) _succ = NULL;
 614 
 615   assert(_succ != Self, "invariant");
 616   if (_Responsible == Self) {
 617     _Responsible = NULL;
 618     OrderAccess::fence(); // Dekker pivot-point
 619 
 620     // We may leave threads on cxq|EntryList without a designated
 621     // "Responsible" thread.  This is benign.  When this thread subsequently
 622     // exits the monitor it can "see" such preexisting "old" threads --
 623     // threads that arrived on the cxq|EntryList before the fence, above --
 624     // by LDing cxq|EntryList.  Newly arrived threads -- that is, threads
 625     // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
 626     // non-null and elect a new "Responsible" timer thread.
 627     //
 628     // This thread executes:
 629     //    ST Responsible=null; MEMBAR    (in enter epilogue - here)
 630     //    LD cxq|EntryList               (in subsequent exit)
 631     //
 632     // Entering threads in the slow/contended path execute:
 633     //    ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
 634     //    The (ST cxq; MEMBAR) is accomplished with CAS().
 635     //
 636     // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
 637     // exit operation from floating above the ST Responsible=null.
 638   }
 639 
 640   // We've acquired ownership with CAS().
 641   // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
 642   // But since the CAS() this thread may have also stored into _succ,
 643   // EntryList, cxq or Responsible.  These meta-data updates must be
 644   // visible __before this thread subsequently drops the lock.
 645   // Consider what could occur if we didn't enforce this constraint --
 646   // STs to monitor meta-data and user-data could reorder with (become
 647   // visible after) the ST in exit that drops ownership of the lock.
 648   // Some other thread could then acquire the lock, but observe inconsistent
 649   // or old monitor meta-data and heap data.  That violates the JMM.
 650   // To that end, the 1-0 exit() operation must have at least STST|LDST
 651   // "release" barrier semantics.  Specifically, there must be at least a
 652   // STST|LDST barrier in exit() before the ST of null into _owner that drops
 653   // the lock.   The barrier ensures that changes to monitor meta-data and data
 654   // protected by the lock will be visible before we release the lock, and
 655   // therefore before some other thread (CPU) has a chance to acquire the lock.
 656   // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
 657   //
 658   // Critically, any prior STs to _succ or EntryList must be visible before
 659   // the ST of null into _owner in the *subsequent* (following) corresponding
 660   // monitorexit.  Recall too, that in 1-0 mode monitorexit does not necessarily
 661   // execute a serializing instruction.
 662 
 663   return;
 664 }
 665 
 666 // ReenterI() is a specialized inline form of the latter half of the
 667 // contended slow-path from EnterI().  We use ReenterI() only for
 668 // monitor reentry in wait().
 669 //
 670 // In the future we should reconcile EnterI() and ReenterI(), adding
 671 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
 672 // loop accordingly.
 673 
 674 void ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) {
 675   assert(Self != NULL, "invariant");
 676   assert(SelfNode != NULL, "invariant");
 677   assert(SelfNode->_thread == Self, "invariant");
 678   assert(_waiters > 0, "invariant");
 679   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 680   assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
 681   JavaThread * jt = (JavaThread *) Self;
 682 
 683   int nWakeups = 0;
 684   for (;;) {
 685     ObjectWaiter::TStates v = SelfNode->TState;
 686     guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
 687     assert(_owner != Self, "invariant");
 688 
 689     if (TryLock(Self) > 0) break;
 690     if (TrySpin(Self) > 0) break;
 691 
 692     // State transition wrappers around park() ...
 693     // ReenterI() wisely defers state transitions until
 694     // it's clear we must park the thread.
 695     {
 696       OSThreadContendState osts(Self->osthread());
 697       ThreadBlockInVM tbivm(jt);
 698 
 699       // cleared by handle_special_suspend_equivalent_condition()
 700       // or java_suspend_self()
 701       jt->set_suspend_equivalent();
 702       Self->_ParkEvent->park();
 703 
 704       // were we externally suspended while we were waiting?
 705       for (;;) {
 706         if (!ExitSuspendEquivalent(jt)) break;
 707         if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
 708         jt->java_suspend_self();
 709         jt->set_suspend_equivalent();
 710       }
 711     }
 712 
 713     // Try again, but just so we distinguish between futile wakeups and
 714     // successful wakeups.  The following test isn't algorithmically
 715     // necessary, but it helps us maintain sensible statistics.
 716     if (TryLock(Self) > 0) break;
 717 
 718     // The lock is still contested.
 719     // Keep a tally of the # of futile wakeups.
 720     // Note that the counter is not protected by a lock or updated by atomics.
 721     // That is by design - we trade "lossy" counters which are exposed to
 722     // races during updates for a lower probe effect.
 723     ++nWakeups;
 724 
 725     // Assuming this is not a spurious wakeup we'll normally
 726     // find that _succ == Self.
 727     if (_succ == Self) _succ = NULL;
 728 
 729     // Invariant: after clearing _succ a contending thread
 730     // *must* retry  _owner before parking.
 731     OrderAccess::fence();
 732 
 733     // This PerfData object can be used in parallel with a safepoint.
 734     // See the work around in PerfDataManager::destroy().
 735     OM_PERFDATA_OP(FutileWakeups, inc());
 736   }
 737 
 738   // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
 739   // Normally we'll find Self on the EntryList.
 740   // Unlinking from the EntryList is constant-time and atomic-free.
 741   // From the perspective of the lock owner (this thread), the
 742   // EntryList is stable and cxq is prepend-only.
 743   // The head of cxq is volatile but the interior is stable.
 744   // In addition, Self.TState is stable.
 745 
 746   assert(_owner == Self, "invariant");
 747   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 748   UnlinkAfterAcquire(Self, SelfNode);
 749   if (_succ == Self) _succ = NULL;
 750   assert(_succ != Self, "invariant");
 751   SelfNode->TState = ObjectWaiter::TS_RUN;
 752   OrderAccess::fence();      // see comments at the end of EnterI()
 753 }
 754 
 755 // By convention we unlink a contending thread from EntryList|cxq immediately
 756 // after the thread acquires the lock in ::enter().  Equally, we could defer
 757 // unlinking the thread until ::exit()-time.
 758 
 759 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) {
 760   assert(_owner == Self, "invariant");
 761   assert(SelfNode->_thread == Self, "invariant");
 762 
 763   if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
 764     // Normal case: remove Self from the DLL EntryList .
 765     // This is a constant-time operation.
 766     ObjectWaiter * nxt = SelfNode->_next;
 767     ObjectWaiter * prv = SelfNode->_prev;
 768     if (nxt != NULL) nxt->_prev = prv;
 769     if (prv != NULL) prv->_next = nxt;
 770     if (SelfNode == _EntryList) _EntryList = nxt;
 771     assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
 772     assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
 773   } else {
 774     assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant");
 775     // Inopportune interleaving -- Self is still on the cxq.
 776     // This usually means the enqueue of self raced an exiting thread.
 777     // Normally we'll find Self near the front of the cxq, so
 778     // dequeueing is typically fast.  If needbe we can accelerate
 779     // this with some MCS/CHL-like bidirectional list hints and advisory
 780     // back-links so dequeueing from the interior will normally operate
 781     // in constant-time.
 782     // Dequeue Self from either the head (with CAS) or from the interior
 783     // with a linear-time scan and normal non-atomic memory operations.
 784     // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
 785     // and then unlink Self from EntryList.  We have to drain eventually,
 786     // so it might as well be now.
 787 
 788     ObjectWaiter * v = _cxq;
 789     assert(v != NULL, "invariant");
 790     if (v != SelfNode || Atomic::cmpxchg(SelfNode->_next, &_cxq, v) != v) {
 791       // The CAS above can fail from interference IFF a "RAT" arrived.
 792       // In that case Self must be in the interior and can no longer be
 793       // at the head of cxq.
 794       if (v == SelfNode) {
 795         assert(_cxq != v, "invariant");
 796         v = _cxq;          // CAS above failed - start scan at head of list
 797       }
 798       ObjectWaiter * p;
 799       ObjectWaiter * q = NULL;
 800       for (p = v; p != NULL && p != SelfNode; p = p->_next) {
 801         q = p;
 802         assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
 803       }
 804       assert(v != SelfNode, "invariant");
 805       assert(p == SelfNode, "Node not found on cxq");
 806       assert(p != _cxq, "invariant");
 807       assert(q != NULL, "invariant");
 808       assert(q->_next == p, "invariant");
 809       q->_next = p->_next;
 810     }
 811   }
 812 
 813 #ifdef ASSERT
 814   // Diagnostic hygiene ...
 815   SelfNode->_prev  = (ObjectWaiter *) 0xBAD;
 816   SelfNode->_next  = (ObjectWaiter *) 0xBAD;
 817   SelfNode->TState = ObjectWaiter::TS_RUN;
 818 #endif
 819 }
 820 
 821 // -----------------------------------------------------------------------------
 822 // Exit support
 823 //
 824 // exit()
 825 // ~~~~~~
 826 // Note that the collector can't reclaim the objectMonitor or deflate
 827 // the object out from underneath the thread calling ::exit() as the
 828 // thread calling ::exit() never transitions to a stable state.
 829 // This inhibits GC, which in turn inhibits asynchronous (and
 830 // inopportune) reclamation of "this".
 831 //
 832 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
 833 // There's one exception to the claim above, however.  EnterI() can call
 834 // exit() to drop a lock if the acquirer has been externally suspended.
 835 // In that case exit() is called with _thread_state as _thread_blocked,
 836 // but the monitor's _count field is > 0, which inhibits reclamation.
 837 //
 838 // 1-0 exit
 839 // ~~~~~~~~
 840 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
 841 // the fast-path operators have been optimized so the common ::exit()
 842 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
 843 // The code emitted by fast_unlock() elides the usual MEMBAR.  This
 844 // greatly improves latency -- MEMBAR and CAS having considerable local
 845 // latency on modern processors -- but at the cost of "stranding".  Absent the
 846 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
 847 // ::enter() path, resulting in the entering thread being stranding
 848 // and a progress-liveness failure.   Stranding is extremely rare.
 849 // We use timers (timed park operations) & periodic polling to detect
 850 // and recover from stranding.  Potentially stranded threads periodically
 851 // wake up and poll the lock.  See the usage of the _Responsible variable.
 852 //
 853 // The CAS() in enter provides for safety and exclusion, while the CAS or
 854 // MEMBAR in exit provides for progress and avoids stranding.  1-0 locking
 855 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
 856 // We detect and recover from stranding with timers.
 857 //
 858 // If a thread transiently strands it'll park until (a) another
 859 // thread acquires the lock and then drops the lock, at which time the
 860 // exiting thread will notice and unpark the stranded thread, or, (b)
 861 // the timer expires.  If the lock is high traffic then the stranding latency
 862 // will be low due to (a).  If the lock is low traffic then the odds of
 863 // stranding are lower, although the worst-case stranding latency
 864 // is longer.  Critically, we don't want to put excessive load in the
 865 // platform's timer subsystem.  We want to minimize both the timer injection
 866 // rate (timers created/sec) as well as the number of timers active at
 867 // any one time.  (more precisely, we want to minimize timer-seconds, which is
 868 // the integral of the # of active timers at any instant over time).
 869 // Both impinge on OS scalability.  Given that, at most one thread parked on
 870 // a monitor will use a timer.
 871 //
 872 // There is also the risk of a futile wake-up. If we drop the lock
 873 // another thread can reacquire the lock immediately, and we can
 874 // then wake a thread unnecessarily. This is benign, and we've
 875 // structured the code so the windows are short and the frequency
 876 // of such futile wakups is low.
 877 
 878 void ObjectMonitor::exit(bool not_suspended, TRAPS) {
 879   Thread * const Self = THREAD;
 880   if (THREAD != _owner) {
 881     if (THREAD->is_lock_owned((address) _owner)) {
 882       // Transmute _owner from a BasicLock pointer to a Thread address.
 883       // We don't need to hold _mutex for this transition.
 884       // Non-null to Non-null is safe as long as all readers can
 885       // tolerate either flavor.
 886       assert(_recursions == 0, "invariant");
 887       _owner = THREAD;
 888       _recursions = 0;
 889     } else {
 890       // Apparent unbalanced locking ...
 891       // Naively we'd like to throw IllegalMonitorStateException.
 892       // As a practical matter we can neither allocate nor throw an
 893       // exception as ::exit() can be called from leaf routines.
 894       // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
 895       // Upon deeper reflection, however, in a properly run JVM the only
 896       // way we should encounter this situation is in the presence of
 897       // unbalanced JNI locking. TODO: CheckJNICalls.
 898       // See also: CR4414101
 899       assert(false, "Non-balanced monitor enter/exit! Likely JNI locking");
 900       return;
 901     }
 902   }
 903 
 904   if (_recursions != 0) {
 905     _recursions--;        // this is simple recursive enter
 906     return;
 907   }
 908 
 909   // Invariant: after setting Responsible=null an thread must execute
 910   // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
 911   _Responsible = NULL;
 912 
 913 #if INCLUDE_JFR
 914   // get the owner's thread id for the MonitorEnter event
 915   // if it is enabled and the thread isn't suspended
 916   if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
 917     _previous_owner_tid = JFR_THREAD_ID(Self);
 918   }
 919 #endif
 920 
 921   for (;;) {
 922     assert(THREAD == _owner, "invariant");
 923 
 924     if (Knob_ExitPolicy == 0) {
 925       // release semantics: prior loads and stores from within the critical section
 926       // must not float (reorder) past the following store that drops the lock.
 927       // On SPARC that requires MEMBAR #loadstore|#storestore.
 928       // But of course in TSO #loadstore|#storestore is not required.
 929       // I'd like to write one of the following:
 930       // A.  OrderAccess::release() ; _owner = NULL
 931       // B.  OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
 932       // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
 933       // store into a _dummy variable.  That store is not needed, but can result
 934       // in massive wasteful coherency traffic on classic SMP systems.
 935       // Instead, I use release_store(), which is implemented as just a simple
 936       // ST on x64, x86 and SPARC.
 937       OrderAccess::release_store(&_owner, (void*)NULL);   // drop the lock
 938       OrderAccess::storeload();                        // See if we need to wake a successor
 939       if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
 940         return;
 941       }
 942       // Other threads are blocked trying to acquire the lock.
 943 
 944       // Normally the exiting thread is responsible for ensuring succession,
 945       // but if other successors are ready or other entering threads are spinning
 946       // then this thread can simply store NULL into _owner and exit without
 947       // waking a successor.  The existence of spinners or ready successors
 948       // guarantees proper succession (liveness).  Responsibility passes to the
 949       // ready or running successors.  The exiting thread delegates the duty.
 950       // More precisely, if a successor already exists this thread is absolved
 951       // of the responsibility of waking (unparking) one.
 952       //
 953       // The _succ variable is critical to reducing futile wakeup frequency.
 954       // _succ identifies the "heir presumptive" thread that has been made
 955       // ready (unparked) but that has not yet run.  We need only one such
 956       // successor thread to guarantee progress.
 957       // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
 958       // section 3.3 "Futile Wakeup Throttling" for details.
 959       //
 960       // Note that spinners in Enter() also set _succ non-null.
 961       // In the current implementation spinners opportunistically set
 962       // _succ so that exiting threads might avoid waking a successor.
 963       // Another less appealing alternative would be for the exiting thread
 964       // to drop the lock and then spin briefly to see if a spinner managed
 965       // to acquire the lock.  If so, the exiting thread could exit
 966       // immediately without waking a successor, otherwise the exiting
 967       // thread would need to dequeue and wake a successor.
 968       // (Note that we'd need to make the post-drop spin short, but no
 969       // shorter than the worst-case round-trip cache-line migration time.
 970       // The dropped lock needs to become visible to the spinner, and then
 971       // the acquisition of the lock by the spinner must become visible to
 972       // the exiting thread).
 973 
 974       // It appears that an heir-presumptive (successor) must be made ready.
 975       // Only the current lock owner can manipulate the EntryList or
 976       // drain _cxq, so we need to reacquire the lock.  If we fail
 977       // to reacquire the lock the responsibility for ensuring succession
 978       // falls to the new owner.
 979       //
 980       if (!Atomic::replace_if_null(THREAD, &_owner)) {
 981         return;
 982       }
 983     } else {
 984       if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
 985         OrderAccess::release_store(&_owner, (void*)NULL);   // drop the lock
 986         OrderAccess::storeload();
 987         // Ratify the previously observed values.
 988         if (_cxq == NULL || _succ != NULL) {
 989           return;
 990         }
 991 
 992         // inopportune interleaving -- the exiting thread (this thread)
 993         // in the fast-exit path raced an entering thread in the slow-enter
 994         // path.
 995         // We have two choices:
 996         // A.  Try to reacquire the lock.
 997         //     If the CAS() fails return immediately, otherwise
 998         //     we either restart/rerun the exit operation, or simply
 999         //     fall-through into the code below which wakes a successor.
1000         // B.  If the elements forming the EntryList|cxq are TSM
1001         //     we could simply unpark() the lead thread and return
1002         //     without having set _succ.
1003         if (!Atomic::replace_if_null(THREAD, &_owner)) {
1004           return;
1005         }
1006       }
1007     }
1008 
1009     guarantee(_owner == THREAD, "invariant");
1010 
1011     ObjectWaiter * w = NULL;
1012     int QMode = Knob_QMode;
1013 
1014     if (QMode == 2 && _cxq != NULL) {
1015       // QMode == 2 : cxq has precedence over EntryList.
1016       // Try to directly wake a successor from the cxq.
1017       // If successful, the successor will need to unlink itself from cxq.
1018       w = _cxq;
1019       assert(w != NULL, "invariant");
1020       assert(w->TState == ObjectWaiter::TS_CXQ, "Invariant");
1021       ExitEpilog(Self, w);
1022       return;
1023     }
1024 
1025     if (QMode == 3 && _cxq != NULL) {
1026       // Aggressively drain cxq into EntryList at the first opportunity.
1027       // This policy ensure that recently-run threads live at the head of EntryList.
1028       // Drain _cxq into EntryList - bulk transfer.
1029       // First, detach _cxq.
1030       // The following loop is tantamount to: w = swap(&cxq, NULL)
1031       w = _cxq;
1032       for (;;) {
1033         assert(w != NULL, "Invariant");
1034         ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1035         if (u == w) break;
1036         w = u;
1037       }
1038       assert(w != NULL, "invariant");
1039 
1040       ObjectWaiter * q = NULL;
1041       ObjectWaiter * p;
1042       for (p = w; p != NULL; p = p->_next) {
1043         guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1044         p->TState = ObjectWaiter::TS_ENTER;
1045         p->_prev = q;
1046         q = p;
1047       }
1048 
1049       // Append the RATs to the EntryList
1050       // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
1051       ObjectWaiter * Tail;
1052       for (Tail = _EntryList; Tail != NULL && Tail->_next != NULL;
1053            Tail = Tail->_next)
1054         /* empty */;
1055       if (Tail == NULL) {
1056         _EntryList = w;
1057       } else {
1058         Tail->_next = w;
1059         w->_prev = Tail;
1060       }
1061 
1062       // Fall thru into code that tries to wake a successor from EntryList
1063     }
1064 
1065     if (QMode == 4 && _cxq != NULL) {
1066       // Aggressively drain cxq into EntryList at the first opportunity.
1067       // This policy ensure that recently-run threads live at the head of EntryList.
1068 
1069       // Drain _cxq into EntryList - bulk transfer.
1070       // First, detach _cxq.
1071       // The following loop is tantamount to: w = swap(&cxq, NULL)
1072       w = _cxq;
1073       for (;;) {
1074         assert(w != NULL, "Invariant");
1075         ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1076         if (u == w) break;
1077         w = u;
1078       }
1079       assert(w != NULL, "invariant");
1080 
1081       ObjectWaiter * q = NULL;
1082       ObjectWaiter * p;
1083       for (p = w; p != NULL; p = p->_next) {
1084         guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1085         p->TState = ObjectWaiter::TS_ENTER;
1086         p->_prev = q;
1087         q = p;
1088       }
1089 
1090       // Prepend the RATs to the EntryList
1091       if (_EntryList != NULL) {
1092         q->_next = _EntryList;
1093         _EntryList->_prev = q;
1094       }
1095       _EntryList = w;
1096 
1097       // Fall thru into code that tries to wake a successor from EntryList
1098     }
1099 
1100     w = _EntryList;
1101     if (w != NULL) {
1102       // I'd like to write: guarantee (w->_thread != Self).
1103       // But in practice an exiting thread may find itself on the EntryList.
1104       // Let's say thread T1 calls O.wait().  Wait() enqueues T1 on O's waitset and
1105       // then calls exit().  Exit release the lock by setting O._owner to NULL.
1106       // Let's say T1 then stalls.  T2 acquires O and calls O.notify().  The
1107       // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1108       // release the lock "O".  T2 resumes immediately after the ST of null into
1109       // _owner, above.  T2 notices that the EntryList is populated, so it
1110       // reacquires the lock and then finds itself on the EntryList.
1111       // Given all that, we have to tolerate the circumstance where "w" is
1112       // associated with Self.
1113       assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1114       ExitEpilog(Self, w);
1115       return;
1116     }
1117 
1118     // If we find that both _cxq and EntryList are null then just
1119     // re-run the exit protocol from the top.
1120     w = _cxq;
1121     if (w == NULL) continue;
1122 
1123     // Drain _cxq into EntryList - bulk transfer.
1124     // First, detach _cxq.
1125     // The following loop is tantamount to: w = swap(&cxq, NULL)
1126     for (;;) {
1127       assert(w != NULL, "Invariant");
1128       ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1129       if (u == w) break;
1130       w = u;
1131     }
1132 
1133     assert(w != NULL, "invariant");
1134     assert(_EntryList == NULL, "invariant");
1135 
1136     // Convert the LIFO SLL anchored by _cxq into a DLL.
1137     // The list reorganization step operates in O(LENGTH(w)) time.
1138     // It's critical that this step operate quickly as
1139     // "Self" still holds the outer-lock, restricting parallelism
1140     // and effectively lengthening the critical section.
1141     // Invariant: s chases t chases u.
1142     // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1143     // we have faster access to the tail.
1144 
1145     if (QMode == 1) {
1146       // QMode == 1 : drain cxq to EntryList, reversing order
1147       // We also reverse the order of the list.
1148       ObjectWaiter * s = NULL;
1149       ObjectWaiter * t = w;
1150       ObjectWaiter * u = NULL;
1151       while (t != NULL) {
1152         guarantee(t->TState == ObjectWaiter::TS_CXQ, "invariant");
1153         t->TState = ObjectWaiter::TS_ENTER;
1154         u = t->_next;
1155         t->_prev = u;
1156         t->_next = s;
1157         s = t;
1158         t = u;
1159       }
1160       _EntryList  = s;
1161       assert(s != NULL, "invariant");
1162     } else {
1163       // QMode == 0 or QMode == 2
1164       _EntryList = w;
1165       ObjectWaiter * q = NULL;
1166       ObjectWaiter * p;
1167       for (p = w; p != NULL; p = p->_next) {
1168         guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1169         p->TState = ObjectWaiter::TS_ENTER;
1170         p->_prev = q;
1171         q = p;
1172       }
1173     }
1174 
1175     // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1176     // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1177 
1178     // See if we can abdicate to a spinner instead of waking a thread.
1179     // A primary goal of the implementation is to reduce the
1180     // context-switch rate.
1181     if (_succ != NULL) continue;
1182 
1183     w = _EntryList;
1184     if (w != NULL) {
1185       guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1186       ExitEpilog(Self, w);
1187       return;
1188     }
1189   }
1190 }
1191 
1192 // ExitSuspendEquivalent:
1193 // A faster alternate to handle_special_suspend_equivalent_condition()
1194 //
1195 // handle_special_suspend_equivalent_condition() unconditionally
1196 // acquires the SR_lock.  On some platforms uncontended MutexLocker()
1197 // operations have high latency.  Note that in ::enter() we call HSSEC
1198 // while holding the monitor, so we effectively lengthen the critical sections.
1199 //
1200 // There are a number of possible solutions:
1201 //
1202 // A.  To ameliorate the problem we might also defer state transitions
1203 //     to as late as possible -- just prior to parking.
1204 //     Given that, we'd call HSSEC after having returned from park(),
1205 //     but before attempting to acquire the monitor.  This is only a
1206 //     partial solution.  It avoids calling HSSEC while holding the
1207 //     monitor (good), but it still increases successor reacquisition latency --
1208 //     the interval between unparking a successor and the time the successor
1209 //     resumes and retries the lock.  See ReenterI(), which defers state transitions.
1210 //     If we use this technique we can also avoid EnterI()-exit() loop
1211 //     in ::enter() where we iteratively drop the lock and then attempt
1212 //     to reacquire it after suspending.
1213 //
1214 // B.  In the future we might fold all the suspend bits into a
1215 //     composite per-thread suspend flag and then update it with CAS().
1216 //     Alternately, a Dekker-like mechanism with multiple variables
1217 //     would suffice:
1218 //       ST Self->_suspend_equivalent = false
1219 //       MEMBAR
1220 //       LD Self_>_suspend_flags
1221 //
1222 // UPDATE 2007-10-6: since I've replaced the native Mutex/Monitor subsystem
1223 // with a more efficient implementation, the need to use "FastHSSEC" has
1224 // decreased. - Dave
1225 
1226 
1227 bool ObjectMonitor::ExitSuspendEquivalent(JavaThread * jSelf) {
1228   const int Mode = Knob_FastHSSEC;
1229   if (Mode && !jSelf->is_external_suspend()) {
1230     assert(jSelf->is_suspend_equivalent(), "invariant");
1231     jSelf->clear_suspend_equivalent();
1232     if (2 == Mode) OrderAccess::storeload();
1233     if (!jSelf->is_external_suspend()) return false;
1234     // We raced a suspension -- fall thru into the slow path
1235     jSelf->set_suspend_equivalent();
1236   }
1237   return jSelf->handle_special_suspend_equivalent_condition();
1238 }
1239 
1240 
1241 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) {
1242   assert(_owner == Self, "invariant");
1243 
1244   // Exit protocol:
1245   // 1. ST _succ = wakee
1246   // 2. membar #loadstore|#storestore;
1247   // 2. ST _owner = NULL
1248   // 3. unpark(wakee)
1249 
1250   _succ = Knob_SuccEnabled ? Wakee->_thread : NULL;
1251   ParkEvent * Trigger = Wakee->_event;
1252 
1253   // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1254   // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1255   // out-of-scope (non-extant).
1256   Wakee  = NULL;
1257 
1258   // Drop the lock
1259   OrderAccess::release_store(&_owner, (void*)NULL);
1260   OrderAccess::fence();                               // ST _owner vs LD in unpark()
1261 
1262   DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1263   Trigger->unpark();
1264 
1265   // Maintain stats and report events to JVMTI
1266   OM_PERFDATA_OP(Parks, inc());
1267 }
1268 
1269 
1270 // -----------------------------------------------------------------------------
1271 // Class Loader deadlock handling.
1272 //
1273 // complete_exit exits a lock returning recursion count
1274 // complete_exit/reenter operate as a wait without waiting
1275 // complete_exit requires an inflated monitor
1276 // The _owner field is not always the Thread addr even with an
1277 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1278 // thread due to contention.
1279 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1280   Thread * const Self = THREAD;
1281   assert(Self->is_Java_thread(), "Must be Java thread!");
1282   JavaThread *jt = (JavaThread *)THREAD;
1283 
1284   DeferredInitialize();
1285 
1286   if (THREAD != _owner) {
1287     if (THREAD->is_lock_owned ((address)_owner)) {
1288       assert(_recursions == 0, "internal state error");
1289       _owner = THREAD;   // Convert from basiclock addr to Thread addr
1290       _recursions = 0;
1291     }
1292   }
1293 
1294   guarantee(Self == _owner, "complete_exit not owner");
1295   intptr_t save = _recursions; // record the old recursion count
1296   _recursions = 0;        // set the recursion level to be 0
1297   exit(true, Self);           // exit the monitor
1298   guarantee(_owner != Self, "invariant");
1299   return save;
1300 }
1301 
1302 // reenter() enters a lock and sets recursion count
1303 // complete_exit/reenter operate as a wait without waiting
1304 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1305   Thread * const Self = THREAD;
1306   assert(Self->is_Java_thread(), "Must be Java thread!");
1307   JavaThread *jt = (JavaThread *)THREAD;
1308 
1309   guarantee(_owner != Self, "reenter already owner");
1310   enter(THREAD);       // enter the monitor
1311   guarantee(_recursions == 0, "reenter recursion");
1312   _recursions = recursions;
1313   return;
1314 }
1315 
1316 
1317 // -----------------------------------------------------------------------------
1318 // A macro is used below because there may already be a pending
1319 // exception which should not abort the execution of the routines
1320 // which use this (which is why we don't put this into check_slow and
1321 // call it with a CHECK argument).
1322 
1323 #define CHECK_OWNER()                                                       \
1324   do {                                                                      \
1325     if (THREAD != _owner) {                                                 \
1326       if (THREAD->is_lock_owned((address) _owner)) {                        \
1327         _owner = THREAD;  /* Convert from basiclock addr to Thread addr */  \
1328         _recursions = 0;                                                    \
1329       } else {                                                              \
1330         THROW(vmSymbols::java_lang_IllegalMonitorStateException());         \
1331       }                                                                     \
1332     }                                                                       \
1333   } while (false)
1334 
1335 // check_slow() is a misnomer.  It's called to simply to throw an IMSX exception.
1336 // TODO-FIXME: remove check_slow() -- it's likely dead.
1337 
1338 void ObjectMonitor::check_slow(TRAPS) {
1339   assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1340   THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1341 }
1342 
1343 static int Adjust(volatile int * adr, int dx) {
1344   int v;
1345   for (v = *adr; Atomic::cmpxchg(v + dx, adr, v) != v; v = *adr) /* empty */;
1346   return v;
1347 }
1348 
1349 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1350                                     ObjectMonitor* monitor,
1351                                     jlong notifier_tid,
1352                                     jlong timeout,
1353                                     bool timedout) {
1354   assert(event != NULL, "invariant");
1355   assert(monitor != NULL, "invariant");
1356   event->set_monitorClass(((oop)monitor->object())->klass());
1357   event->set_timeout(timeout);
1358   event->set_address((uintptr_t)monitor->object_addr());
1359   event->set_notifier(notifier_tid);
1360   event->set_timedOut(timedout);
1361   event->commit();
1362 }
1363 
1364 // -----------------------------------------------------------------------------
1365 // Wait/Notify/NotifyAll
1366 //
1367 // Note: a subset of changes to ObjectMonitor::wait()
1368 // will need to be replicated in complete_exit
1369 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1370   Thread * const Self = THREAD;
1371   assert(Self->is_Java_thread(), "Must be Java thread!");
1372   JavaThread *jt = (JavaThread *)THREAD;
1373 
1374   DeferredInitialize();
1375 
1376   // Throw IMSX or IEX.
1377   CHECK_OWNER();
1378 
1379   EventJavaMonitorWait event;
1380 
1381   // check for a pending interrupt
1382   if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1383     // post monitor waited event.  Note that this is past-tense, we are done waiting.
1384     if (JvmtiExport::should_post_monitor_waited()) {
1385       // Note: 'false' parameter is passed here because the
1386       // wait was not timed out due to thread interrupt.
1387       JvmtiExport::post_monitor_waited(jt, this, false);
1388 
1389       // In this short circuit of the monitor wait protocol, the
1390       // current thread never drops ownership of the monitor and
1391       // never gets added to the wait queue so the current thread
1392       // cannot be made the successor. This means that the
1393       // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1394       // consume an unpark() meant for the ParkEvent associated with
1395       // this ObjectMonitor.
1396     }
1397     if (event.should_commit()) {
1398       post_monitor_wait_event(&event, this, 0, millis, false);
1399     }
1400     THROW(vmSymbols::java_lang_InterruptedException());
1401     return;
1402   }
1403 
1404   assert(Self->_Stalled == 0, "invariant");
1405   Self->_Stalled = intptr_t(this);
1406   jt->set_current_waiting_monitor(this);
1407 
1408   // create a node to be put into the queue
1409   // Critically, after we reset() the event but prior to park(), we must check
1410   // for a pending interrupt.
1411   ObjectWaiter node(Self);
1412   node.TState = ObjectWaiter::TS_WAIT;
1413   Self->_ParkEvent->reset();
1414   OrderAccess::fence();          // ST into Event; membar ; LD interrupted-flag
1415 
1416   // Enter the waiting queue, which is a circular doubly linked list in this case
1417   // but it could be a priority queue or any data structure.
1418   // _WaitSetLock protects the wait queue.  Normally the wait queue is accessed only
1419   // by the the owner of the monitor *except* in the case where park()
1420   // returns because of a timeout of interrupt.  Contention is exceptionally rare
1421   // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1422 
1423   Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1424   AddWaiter(&node);
1425   Thread::SpinRelease(&_WaitSetLock);
1426 
1427   _Responsible = NULL;
1428 
1429   intptr_t save = _recursions; // record the old recursion count
1430   _waiters++;                  // increment the number of waiters
1431   _recursions = 0;             // set the recursion level to be 1
1432   exit(true, Self);                    // exit the monitor
1433   guarantee(_owner != Self, "invariant");
1434 
1435   // The thread is on the WaitSet list - now park() it.
1436   // On MP systems it's conceivable that a brief spin before we park
1437   // could be profitable.
1438   //
1439   // TODO-FIXME: change the following logic to a loop of the form
1440   //   while (!timeout && !interrupted && _notified == 0) park()
1441 
1442   int ret = OS_OK;
1443   int WasNotified = 0;
1444   { // State transition wrappers
1445     OSThread* osthread = Self->osthread();
1446     OSThreadWaitState osts(osthread, true);
1447     {
1448       ThreadBlockInVM tbivm(jt);
1449       // Thread is in thread_blocked state and oop access is unsafe.
1450       jt->set_suspend_equivalent();
1451 
1452       if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1453         // Intentionally empty
1454       } else if (node._notified == 0) {
1455         if (millis <= 0) {
1456           Self->_ParkEvent->park();
1457         } else {
1458           ret = Self->_ParkEvent->park(millis);
1459         }
1460       }
1461 
1462       // were we externally suspended while we were waiting?
1463       if (ExitSuspendEquivalent (jt)) {
1464         // TODO-FIXME: add -- if succ == Self then succ = null.
1465         jt->java_suspend_self();
1466       }
1467 
1468     } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1469 
1470     // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1471     // from the WaitSet to the EntryList.
1472     // See if we need to remove Node from the WaitSet.
1473     // We use double-checked locking to avoid grabbing _WaitSetLock
1474     // if the thread is not on the wait queue.
1475     //
1476     // Note that we don't need a fence before the fetch of TState.
1477     // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1478     // written by the is thread. (perhaps the fetch might even be satisfied
1479     // by a look-aside into the processor's own store buffer, although given
1480     // the length of the code path between the prior ST and this load that's
1481     // highly unlikely).  If the following LD fetches a stale TS_WAIT value
1482     // then we'll acquire the lock and then re-fetch a fresh TState value.
1483     // That is, we fail toward safety.
1484 
1485     if (node.TState == ObjectWaiter::TS_WAIT) {
1486       Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1487       if (node.TState == ObjectWaiter::TS_WAIT) {
1488         DequeueSpecificWaiter(&node);       // unlink from WaitSet
1489         assert(node._notified == 0, "invariant");
1490         node.TState = ObjectWaiter::TS_RUN;
1491       }
1492       Thread::SpinRelease(&_WaitSetLock);
1493     }
1494 
1495     // The thread is now either on off-list (TS_RUN),
1496     // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1497     // The Node's TState variable is stable from the perspective of this thread.
1498     // No other threads will asynchronously modify TState.
1499     guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1500     OrderAccess::loadload();
1501     if (_succ == Self) _succ = NULL;
1502     WasNotified = node._notified;
1503 
1504     // Reentry phase -- reacquire the monitor.
1505     // re-enter contended monitor after object.wait().
1506     // retain OBJECT_WAIT state until re-enter successfully completes
1507     // Thread state is thread_in_vm and oop access is again safe,
1508     // although the raw address of the object may have changed.
1509     // (Don't cache naked oops over safepoints, of course).
1510 
1511     // post monitor waited event. Note that this is past-tense, we are done waiting.
1512     if (JvmtiExport::should_post_monitor_waited()) {
1513       JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1514 
1515       if (node._notified != 0 && _succ == Self) {
1516         // In this part of the monitor wait-notify-reenter protocol it
1517         // is possible (and normal) for another thread to do a fastpath
1518         // monitor enter-exit while this thread is still trying to get
1519         // to the reenter portion of the protocol.
1520         //
1521         // The ObjectMonitor was notified and the current thread is
1522         // the successor which also means that an unpark() has already
1523         // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1524         // consume the unpark() that was done when the successor was
1525         // set because the same ParkEvent is shared between Java
1526         // monitors and JVM/TI RawMonitors (for now).
1527         //
1528         // We redo the unpark() to ensure forward progress, i.e., we
1529         // don't want all pending threads hanging (parked) with none
1530         // entering the unlocked monitor.
1531         node._event->unpark();
1532       }
1533     }
1534 
1535     if (event.should_commit()) {
1536       post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1537     }
1538 
1539     OrderAccess::fence();
1540 
1541     assert(Self->_Stalled != 0, "invariant");
1542     Self->_Stalled = 0;
1543 
1544     assert(_owner != Self, "invariant");
1545     ObjectWaiter::TStates v = node.TState;
1546     if (v == ObjectWaiter::TS_RUN) {
1547       enter(Self);
1548     } else {
1549       guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1550       ReenterI(Self, &node);
1551       node.wait_reenter_end(this);
1552     }
1553 
1554     // Self has reacquired the lock.
1555     // Lifecycle - the node representing Self must not appear on any queues.
1556     // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1557     // want residual elements associated with this thread left on any lists.
1558     guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1559     assert(_owner == Self, "invariant");
1560     assert(_succ != Self, "invariant");
1561   } // OSThreadWaitState()
1562 
1563   jt->set_current_waiting_monitor(NULL);
1564 
1565   guarantee(_recursions == 0, "invariant");
1566   _recursions = save;     // restore the old recursion count
1567   _waiters--;             // decrement the number of waiters
1568 
1569   // Verify a few postconditions
1570   assert(_owner == Self, "invariant");
1571   assert(_succ != Self, "invariant");
1572   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
1573 
1574   // check if the notification happened
1575   if (!WasNotified) {
1576     // no, it could be timeout or Thread.interrupt() or both
1577     // check for interrupt event, otherwise it is timeout
1578     if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1579       THROW(vmSymbols::java_lang_InterruptedException());
1580     }
1581   }
1582 
1583   // NOTE: Spurious wake up will be consider as timeout.
1584   // Monitor notify has precedence over thread interrupt.
1585 }
1586 
1587 
1588 // Consider:
1589 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1590 // then instead of transferring a thread from the WaitSet to the EntryList
1591 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1592 
1593 void ObjectMonitor::INotify(Thread * Self) {
1594   const int policy = Knob_MoveNotifyee;
1595 
1596   Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1597   ObjectWaiter * iterator = DequeueWaiter();
1598   if (iterator != NULL) {
1599     guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1600     guarantee(iterator->_notified == 0, "invariant");
1601     // Disposition - what might we do with iterator ?
1602     // a.  add it directly to the EntryList - either tail (policy == 1)
1603     //     or head (policy == 0).
1604     // b.  push it onto the front of the _cxq (policy == 2).
1605     // For now we use (b).
1606     if (policy != 4) {
1607       iterator->TState = ObjectWaiter::TS_ENTER;
1608     }
1609     iterator->_notified = 1;
1610     iterator->_notifier_tid = JFR_THREAD_ID(Self);
1611 
1612     ObjectWaiter * list = _EntryList;
1613     if (list != NULL) {
1614       assert(list->_prev == NULL, "invariant");
1615       assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1616       assert(list != iterator, "invariant");
1617     }
1618 
1619     if (policy == 0) {       // prepend to EntryList
1620       if (list == NULL) {
1621         iterator->_next = iterator->_prev = NULL;
1622         _EntryList = iterator;
1623       } else {
1624         list->_prev = iterator;
1625         iterator->_next = list;
1626         iterator->_prev = NULL;
1627         _EntryList = iterator;
1628       }
1629     } else if (policy == 1) {      // append to EntryList
1630       if (list == NULL) {
1631         iterator->_next = iterator->_prev = NULL;
1632         _EntryList = iterator;
1633       } else {
1634         // CONSIDER:  finding the tail currently requires a linear-time walk of
1635         // the EntryList.  We can make tail access constant-time by converting to
1636         // a CDLL instead of using our current DLL.
1637         ObjectWaiter * tail;
1638         for (tail = list; tail->_next != NULL; tail = tail->_next) {}
1639         assert(tail != NULL && tail->_next == NULL, "invariant");
1640         tail->_next = iterator;
1641         iterator->_prev = tail;
1642         iterator->_next = NULL;
1643       }
1644     } else if (policy == 2) {      // prepend to cxq
1645       if (list == NULL) {
1646         iterator->_next = iterator->_prev = NULL;
1647         _EntryList = iterator;
1648       } else {
1649         iterator->TState = ObjectWaiter::TS_CXQ;
1650         for (;;) {
1651           ObjectWaiter * front = _cxq;
1652           iterator->_next = front;
1653           if (Atomic::cmpxchg(iterator, &_cxq, front) == front) {
1654             break;
1655           }
1656         }
1657       }
1658     } else if (policy == 3) {      // append to cxq
1659       iterator->TState = ObjectWaiter::TS_CXQ;
1660       for (;;) {
1661         ObjectWaiter * tail = _cxq;
1662         if (tail == NULL) {
1663           iterator->_next = NULL;
1664           if (Atomic::replace_if_null(iterator, &_cxq)) {
1665             break;
1666           }
1667         } else {
1668           while (tail->_next != NULL) tail = tail->_next;
1669           tail->_next = iterator;
1670           iterator->_prev = tail;
1671           iterator->_next = NULL;
1672           break;
1673         }
1674       }
1675     } else {
1676       ParkEvent * ev = iterator->_event;
1677       iterator->TState = ObjectWaiter::TS_RUN;
1678       OrderAccess::fence();
1679       ev->unpark();
1680     }
1681 
1682     // _WaitSetLock protects the wait queue, not the EntryList.  We could
1683     // move the add-to-EntryList operation, above, outside the critical section
1684     // protected by _WaitSetLock.  In practice that's not useful.  With the
1685     // exception of  wait() timeouts and interrupts the monitor owner
1686     // is the only thread that grabs _WaitSetLock.  There's almost no contention
1687     // on _WaitSetLock so it's not profitable to reduce the length of the
1688     // critical section.
1689 
1690     if (policy < 4) {
1691       iterator->wait_reenter_begin(this);
1692     }
1693   }
1694   Thread::SpinRelease(&_WaitSetLock);
1695 }
1696 
1697 // Consider: a not-uncommon synchronization bug is to use notify() when
1698 // notifyAll() is more appropriate, potentially resulting in stranded
1699 // threads; this is one example of a lost wakeup. A useful diagnostic
1700 // option is to force all notify() operations to behave as notifyAll().
1701 //
1702 // Note: We can also detect many such problems with a "minimum wait".
1703 // When the "minimum wait" is set to a small non-zero timeout value
1704 // and the program does not hang whereas it did absent "minimum wait",
1705 // that suggests a lost wakeup bug.
1706 
1707 void ObjectMonitor::notify(TRAPS) {
1708   CHECK_OWNER();
1709   if (_WaitSet == NULL) {
1710     return;
1711   }
1712   DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1713   INotify(THREAD);
1714   OM_PERFDATA_OP(Notifications, inc(1));
1715 }
1716 
1717 
1718 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1719 // from the waitset to the EntryList. This could be done more efficiently with a
1720 // single bulk transfer but in practice it's not time-critical. Beware too,
1721 // that in prepend-mode we invert the order of the waiters. Let's say that the
1722 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1723 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1724 
1725 void ObjectMonitor::notifyAll(TRAPS) {
1726   CHECK_OWNER();
1727   if (_WaitSet == NULL) {
1728     return;
1729   }
1730 
1731   DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1732   int tally = 0;
1733   while (_WaitSet != NULL) {
1734     tally++;
1735     INotify(THREAD);
1736   }
1737 
1738   OM_PERFDATA_OP(Notifications, inc(tally));
1739 }
1740 
1741 // -----------------------------------------------------------------------------
1742 // Adaptive Spinning Support
1743 //
1744 // Adaptive spin-then-block - rational spinning
1745 //
1746 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1747 // algorithm.  On high order SMP systems it would be better to start with
1748 // a brief global spin and then revert to spinning locally.  In the spirit of MCS/CLH,
1749 // a contending thread could enqueue itself on the cxq and then spin locally
1750 // on a thread-specific variable such as its ParkEvent._Event flag.
1751 // That's left as an exercise for the reader.  Note that global spinning is
1752 // not problematic on Niagara, as the L2 cache serves the interconnect and
1753 // has both low latency and massive bandwidth.
1754 //
1755 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1756 // acquisition attempts where we opt to spin --  at 100% and vary the spin count
1757 // (duration) or we can fix the count at approximately the duration of
1758 // a context switch and vary the frequency.   Of course we could also
1759 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1760 // For a description of 'Adaptive spin-then-block mutual exclusion in
1761 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1762 //
1763 // This implementation varies the duration "D", where D varies with
1764 // the success rate of recent spin attempts. (D is capped at approximately
1765 // length of a round-trip context switch).  The success rate for recent
1766 // spin attempts is a good predictor of the success rate of future spin
1767 // attempts.  The mechanism adapts automatically to varying critical
1768 // section length (lock modality), system load and degree of parallelism.
1769 // D is maintained per-monitor in _SpinDuration and is initialized
1770 // optimistically.  Spin frequency is fixed at 100%.
1771 //
1772 // Note that _SpinDuration is volatile, but we update it without locks
1773 // or atomics.  The code is designed so that _SpinDuration stays within
1774 // a reasonable range even in the presence of races.  The arithmetic
1775 // operations on _SpinDuration are closed over the domain of legal values,
1776 // so at worst a race will install and older but still legal value.
1777 // At the very worst this introduces some apparent non-determinism.
1778 // We might spin when we shouldn't or vice-versa, but since the spin
1779 // count are relatively short, even in the worst case, the effect is harmless.
1780 //
1781 // Care must be taken that a low "D" value does not become an
1782 // an absorbing state.  Transient spinning failures -- when spinning
1783 // is overall profitable -- should not cause the system to converge
1784 // on low "D" values.  We want spinning to be stable and predictable
1785 // and fairly responsive to change and at the same time we don't want
1786 // it to oscillate, become metastable, be "too" non-deterministic,
1787 // or converge on or enter undesirable stable absorbing states.
1788 //
1789 // We implement a feedback-based control system -- using past behavior
1790 // to predict future behavior.  We face two issues: (a) if the
1791 // input signal is random then the spin predictor won't provide optimal
1792 // results, and (b) if the signal frequency is too high then the control
1793 // system, which has some natural response lag, will "chase" the signal.
1794 // (b) can arise from multimodal lock hold times.  Transient preemption
1795 // can also result in apparent bimodal lock hold times.
1796 // Although sub-optimal, neither condition is particularly harmful, as
1797 // in the worst-case we'll spin when we shouldn't or vice-versa.
1798 // The maximum spin duration is rather short so the failure modes aren't bad.
1799 // To be conservative, I've tuned the gain in system to bias toward
1800 // _not spinning.  Relatedly, the system can sometimes enter a mode where it
1801 // "rings" or oscillates between spinning and not spinning.  This happens
1802 // when spinning is just on the cusp of profitability, however, so the
1803 // situation is not dire.  The state is benign -- there's no need to add
1804 // hysteresis control to damp the transition rate between spinning and
1805 // not spinning.
1806 
1807 // Spinning: Fixed frequency (100%), vary duration
1808 int ObjectMonitor::TrySpin(Thread * Self) {
1809   // Dumb, brutal spin.  Good for comparative measurements against adaptive spinning.
1810   int ctr = Knob_FixedSpin;
1811   if (ctr != 0) {
1812     while (--ctr >= 0) {
1813       if (TryLock(Self) > 0) return 1;
1814       SpinPause();
1815     }
1816     return 0;
1817   }
1818 
1819   for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1820     if (TryLock(Self) > 0) {
1821       // Increase _SpinDuration ...
1822       // Note that we don't clamp SpinDuration precisely at SpinLimit.
1823       // Raising _SpurDuration to the poverty line is key.
1824       int x = _SpinDuration;
1825       if (x < Knob_SpinLimit) {
1826         if (x < Knob_Poverty) x = Knob_Poverty;
1827         _SpinDuration = x + Knob_BonusB;
1828       }
1829       return 1;
1830     }
1831     SpinPause();
1832   }
1833 
1834   // Admission control - verify preconditions for spinning
1835   //
1836   // We always spin a little bit, just to prevent _SpinDuration == 0 from
1837   // becoming an absorbing state.  Put another way, we spin briefly to
1838   // sample, just in case the system load, parallelism, contention, or lock
1839   // modality changed.
1840   //
1841   // Consider the following alternative:
1842   // Periodically set _SpinDuration = _SpinLimit and try a long/full
1843   // spin attempt.  "Periodically" might mean after a tally of
1844   // the # of failed spin attempts (or iterations) reaches some threshold.
1845   // This takes us into the realm of 1-out-of-N spinning, where we
1846   // hold the duration constant but vary the frequency.
1847 
1848   ctr = _SpinDuration;
1849   if (ctr < Knob_SpinBase) ctr = Knob_SpinBase;
1850   if (ctr <= 0) return 0;
1851 
1852   if (Knob_SuccRestrict && _succ != NULL) return 0;
1853   if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
1854     return 0;
1855   }
1856 
1857   int MaxSpin = Knob_MaxSpinners;
1858   if (MaxSpin >= 0) {
1859     if (_Spinner > MaxSpin) {
1860       return 0;
1861     }
1862     // Slightly racy, but benign ...
1863     Adjust(&_Spinner, 1);
1864   }
1865 
1866   // We're good to spin ... spin ingress.
1867   // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1868   // when preparing to LD...CAS _owner, etc and the CAS is likely
1869   // to succeed.
1870   int caspty  = Knob_CASPenalty;
1871   int oxpty   = Knob_OXPenalty;
1872   int sss     = Knob_SpinSetSucc;
1873   if (sss && _succ == NULL) _succ = Self;
1874   Thread * prv = NULL;
1875 
1876   // There are three ways to exit the following loop:
1877   // 1.  A successful spin where this thread has acquired the lock.
1878   // 2.  Spin failure with prejudice
1879   // 3.  Spin failure without prejudice
1880 
1881   while (--ctr >= 0) {
1882 
1883     // Periodic polling -- Check for pending GC
1884     // Threads may spin while they're unsafe.
1885     // We don't want spinning threads to delay the JVM from reaching
1886     // a stop-the-world safepoint or to steal cycles from GC.
1887     // If we detect a pending safepoint we abort in order that
1888     // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1889     // this thread, if safe, doesn't steal cycles from GC.
1890     // This is in keeping with the "no loitering in runtime" rule.
1891     // We periodically check to see if there's a safepoint pending.
1892     if ((ctr & 0xFF) == 0) {
1893       if (SafepointMechanism::poll(Self)) {
1894         goto Abort;           // abrupt spin egress
1895       }
1896       if (Knob_UsePause & 1) SpinPause();
1897     }
1898 
1899     if (Knob_UsePause & 2) SpinPause();
1900 
1901     // Probe _owner with TATAS
1902     // If this thread observes the monitor transition or flicker
1903     // from locked to unlocked to locked, then the odds that this
1904     // thread will acquire the lock in this spin attempt go down
1905     // considerably.  The same argument applies if the CAS fails
1906     // or if we observe _owner change from one non-null value to
1907     // another non-null value.   In such cases we might abort
1908     // the spin without prejudice or apply a "penalty" to the
1909     // spin count-down variable "ctr", reducing it by 100, say.
1910 
1911     Thread * ox = (Thread *) _owner;
1912     if (ox == NULL) {
1913       ox = (Thread*)Atomic::cmpxchg(Self, &_owner, (void*)NULL);
1914       if (ox == NULL) {
1915         // The CAS succeeded -- this thread acquired ownership
1916         // Take care of some bookkeeping to exit spin state.
1917         if (sss && _succ == Self) {
1918           _succ = NULL;
1919         }
1920         if (MaxSpin > 0) Adjust(&_Spinner, -1);
1921 
1922         // Increase _SpinDuration :
1923         // The spin was successful (profitable) so we tend toward
1924         // longer spin attempts in the future.
1925         // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
1926         // If we acquired the lock early in the spin cycle it
1927         // makes sense to increase _SpinDuration proportionally.
1928         // Note that we don't clamp SpinDuration precisely at SpinLimit.
1929         int x = _SpinDuration;
1930         if (x < Knob_SpinLimit) {
1931           if (x < Knob_Poverty) x = Knob_Poverty;
1932           _SpinDuration = x + Knob_Bonus;
1933         }
1934         return 1;
1935       }
1936 
1937       // The CAS failed ... we can take any of the following actions:
1938       // * penalize: ctr -= Knob_CASPenalty
1939       // * exit spin with prejudice -- goto Abort;
1940       // * exit spin without prejudice.
1941       // * Since CAS is high-latency, retry again immediately.
1942       prv = ox;
1943       if (caspty == -2) break;
1944       if (caspty == -1) goto Abort;
1945       ctr -= caspty;
1946       continue;
1947     }
1948 
1949     // Did lock ownership change hands ?
1950     if (ox != prv && prv != NULL) {
1951       if (oxpty == -2) break;
1952       if (oxpty == -1) goto Abort;
1953       ctr -= oxpty;
1954     }
1955     prv = ox;
1956 
1957     // Abort the spin if the owner is not executing.
1958     // The owner must be executing in order to drop the lock.
1959     // Spinning while the owner is OFFPROC is idiocy.
1960     // Consider: ctr -= RunnablePenalty ;
1961     if (Knob_OState && NotRunnable (Self, ox)) {
1962       goto Abort;
1963     }
1964     if (sss && _succ == NULL) _succ = Self;
1965   }
1966 
1967   // Spin failed with prejudice -- reduce _SpinDuration.
1968   // TODO: Use an AIMD-like policy to adjust _SpinDuration.
1969   // AIMD is globally stable.
1970   {
1971     int x = _SpinDuration;
1972     if (x > 0) {
1973       // Consider an AIMD scheme like: x -= (x >> 3) + 100
1974       // This is globally sample and tends to damp the response.
1975       x -= Knob_Penalty;
1976       if (x < 0) x = 0;
1977       _SpinDuration = x;
1978     }
1979   }
1980 
1981  Abort:
1982   if (MaxSpin >= 0) Adjust(&_Spinner, -1);
1983   if (sss && _succ == Self) {
1984     _succ = NULL;
1985     // Invariant: after setting succ=null a contending thread
1986     // must recheck-retry _owner before parking.  This usually happens
1987     // in the normal usage of TrySpin(), but it's safest
1988     // to make TrySpin() as foolproof as possible.
1989     OrderAccess::fence();
1990     if (TryLock(Self) > 0) return 1;
1991   }
1992   return 0;
1993 }
1994 
1995 // NotRunnable() -- informed spinning
1996 //
1997 // Don't bother spinning if the owner is not eligible to drop the lock.
1998 // Spin only if the owner thread is _thread_in_Java or _thread_in_vm.
1999 // The thread must be runnable in order to drop the lock in timely fashion.
2000 // If the _owner is not runnable then spinning will not likely be
2001 // successful (profitable).
2002 //
2003 // Beware -- the thread referenced by _owner could have died
2004 // so a simply fetch from _owner->_thread_state might trap.
2005 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2006 // Because of the lifecycle issues, the _thread_state values
2007 // observed by NotRunnable() might be garbage.  NotRunnable must
2008 // tolerate this and consider the observed _thread_state value
2009 // as advisory.
2010 //
2011 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2012 // a thread pointer.
2013 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2014 // with the LSB of _owner.  Another option would be to probabilistically probe
2015 // the putative _owner->TypeTag value.
2016 //
2017 // Checking _thread_state isn't perfect.  Even if the thread is
2018 // in_java it might be blocked on a page-fault or have been preempted
2019 // and sitting on a ready/dispatch queue.
2020 //
2021 // The return value from NotRunnable() is *advisory* -- the
2022 // result is based on sampling and is not necessarily coherent.
2023 // The caller must tolerate false-negative and false-positive errors.
2024 // Spinning, in general, is probabilistic anyway.
2025 
2026 
2027 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) {
2028   // Check ox->TypeTag == 2BAD.
2029   if (ox == NULL) return 0;
2030 
2031   // Avoid transitive spinning ...
2032   // Say T1 spins or blocks trying to acquire L.  T1._Stalled is set to L.
2033   // Immediately after T1 acquires L it's possible that T2, also
2034   // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2035   // This occurs transiently after T1 acquired L but before
2036   // T1 managed to clear T1.Stalled.  T2 does not need to abort
2037   // its spin in this circumstance.
2038   intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
2039 
2040   if (BlockedOn == 1) return 1;
2041   if (BlockedOn != 0) {
2042     return BlockedOn != intptr_t(this) && _owner == ox;
2043   }
2044 
2045   assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant");
2046   int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);;
2047   // consider also: jst != _thread_in_Java -- but that's overspecific.
2048   return jst == _thread_blocked || jst == _thread_in_native;
2049 }
2050 
2051 
2052 // -----------------------------------------------------------------------------
2053 // WaitSet management ...
2054 
2055 ObjectWaiter::ObjectWaiter(Thread* thread) {
2056   _next     = NULL;
2057   _prev     = NULL;
2058   _notified = 0;
2059   _notifier_tid = 0;
2060   TState    = TS_RUN;
2061   _thread   = thread;
2062   _event    = thread->_ParkEvent;
2063   _active   = false;
2064   assert(_event != NULL, "invariant");
2065 }
2066 
2067 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2068   JavaThread *jt = (JavaThread *)this->_thread;
2069   _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2070 }
2071 
2072 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2073   JavaThread *jt = (JavaThread *)this->_thread;
2074   JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2075 }
2076 
2077 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2078   assert(node != NULL, "should not add NULL node");
2079   assert(node->_prev == NULL, "node already in list");
2080   assert(node->_next == NULL, "node already in list");
2081   // put node at end of queue (circular doubly linked list)
2082   if (_WaitSet == NULL) {
2083     _WaitSet = node;
2084     node->_prev = node;
2085     node->_next = node;
2086   } else {
2087     ObjectWaiter* head = _WaitSet;
2088     ObjectWaiter* tail = head->_prev;
2089     assert(tail->_next == head, "invariant check");
2090     tail->_next = node;
2091     head->_prev = node;
2092     node->_next = head;
2093     node->_prev = tail;
2094   }
2095 }
2096 
2097 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2098   // dequeue the very first waiter
2099   ObjectWaiter* waiter = _WaitSet;
2100   if (waiter) {
2101     DequeueSpecificWaiter(waiter);
2102   }
2103   return waiter;
2104 }
2105 
2106 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2107   assert(node != NULL, "should not dequeue NULL node");
2108   assert(node->_prev != NULL, "node already removed from list");
2109   assert(node->_next != NULL, "node already removed from list");
2110   // when the waiter has woken up because of interrupt,
2111   // timeout or other spurious wake-up, dequeue the
2112   // waiter from waiting list
2113   ObjectWaiter* next = node->_next;
2114   if (next == node) {
2115     assert(node->_prev == node, "invariant check");
2116     _WaitSet = NULL;
2117   } else {
2118     ObjectWaiter* prev = node->_prev;
2119     assert(prev->_next == node, "invariant check");
2120     assert(next->_prev == node, "invariant check");
2121     next->_prev = prev;
2122     prev->_next = next;
2123     if (_WaitSet == node) {
2124       _WaitSet = next;
2125     }
2126   }
2127   node->_next = NULL;
2128   node->_prev = NULL;
2129 }
2130 
2131 // -----------------------------------------------------------------------------
2132 // PerfData support
2133 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts       = NULL;
2134 PerfCounter * ObjectMonitor::_sync_FutileWakeups               = NULL;
2135 PerfCounter * ObjectMonitor::_sync_Parks                       = NULL;
2136 PerfCounter * ObjectMonitor::_sync_Notifications               = NULL;
2137 PerfCounter * ObjectMonitor::_sync_Inflations                  = NULL;
2138 PerfCounter * ObjectMonitor::_sync_Deflations                  = NULL;
2139 PerfLongVariable * ObjectMonitor::_sync_MonExtant              = NULL;
2140 
2141 // One-shot global initialization for the sync subsystem.
2142 // We could also defer initialization and initialize on-demand
2143 // the first time we call inflate().  Initialization would
2144 // be protected - like so many things - by the MonitorCache_lock.
2145 
2146 void ObjectMonitor::Initialize() {
2147   static int InitializationCompleted = 0;
2148   assert(InitializationCompleted == 0, "invariant");
2149   InitializationCompleted = 1;
2150   if (UsePerfData) {
2151     EXCEPTION_MARK;
2152 #define NEWPERFCOUNTER(n)                                                \
2153   {                                                                      \
2154     n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,  \
2155                                         CHECK);                          \
2156   }
2157 #define NEWPERFVARIABLE(n)                                                \
2158   {                                                                       \
2159     n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,  \
2160                                          CHECK);                          \
2161   }
2162     NEWPERFCOUNTER(_sync_Inflations);
2163     NEWPERFCOUNTER(_sync_Deflations);
2164     NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2165     NEWPERFCOUNTER(_sync_FutileWakeups);
2166     NEWPERFCOUNTER(_sync_Parks);
2167     NEWPERFCOUNTER(_sync_Notifications);
2168     NEWPERFVARIABLE(_sync_MonExtant);
2169 #undef NEWPERFCOUNTER
2170 #undef NEWPERFVARIABLE
2171   }
2172 }
2173 
2174 void ObjectMonitor::DeferredInitialize() {
2175   if (InitDone > 0) return;
2176   if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2177     while (InitDone != 1) /* empty */;
2178     return;
2179   }
2180 
2181   // One-shot global initialization ...
2182   // The initialization is idempotent, so we don't need locks.
2183   // In the future consider doing this via os::init_2().
2184 
2185   if (!os::is_MP()) {
2186     Knob_SpinLimit = 0;
2187     Knob_SpinBase  = 0;
2188     Knob_PreSpin   = 0;
2189     Knob_FixedSpin = -1;
2190   }
2191 
2192   OrderAccess::fence();
2193   InitDone = 1;
2194 }
2195 
--- EOF ---