1 /*
   2  * Copyright (c) 1998, 2014, 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 "memory/resourceArea.hpp"
  28 #include "oops/markOop.hpp"
  29 #include "oops/oop.inline.hpp"
  30 #include "runtime/atomic.inline.hpp"
  31 #include "runtime/handles.inline.hpp"
  32 #include "runtime/interfaceSupport.hpp"
  33 #include "runtime/mutexLocker.hpp"
  34 #include "runtime/objectMonitor.hpp"
  35 #include "runtime/objectMonitor.inline.hpp"
  36 #include "runtime/orderAccess.inline.hpp"
  37 #include "runtime/osThread.hpp"
  38 #include "runtime/stubRoutines.hpp"
  39 #include "runtime/thread.inline.hpp"
  40 #include "services/threadService.hpp"
  41 #include "trace/tracing.hpp"
  42 #include "trace/traceMacros.hpp"
  43 #include "utilities/dtrace.hpp"
  44 #include "utilities/macros.hpp"
  45 #include "utilities/preserveException.hpp"
  46 
  47 #if defined(__GNUC__) && !defined(IA64) && !defined(PPC64)
  48 // Need to inhibit inlining for older versions of GCC to avoid build-time failures
  49   #define NOINLINE __attribute__((noinline))
  50 #else
  51   #define NOINLINE
  52 #endif
  53 
  54 
  55 #ifdef DTRACE_ENABLED
  56 
  57 // Only bother with this argument setup if dtrace is available
  58 // TODO-FIXME: probes should not fire when caller is _blocked.  assert() accordingly.
  59 
  60 
  61 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread)                           \
  62   char* bytes = NULL;                                                      \
  63   int len = 0;                                                             \
  64   jlong jtid = SharedRuntime::get_java_tid(thread);                        \
  65   Symbol* klassname = ((oop)obj)->klass()->name();                         \
  66   if (klassname != NULL) {                                                 \
  67     bytes = (char*)klassname->bytes();                                     \
  68     len = klassname->utf8_length();                                        \
  69   }
  70 
  71 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis)            \
  72   {                                                                        \
  73     if (DTraceMonitorProbes) {                                             \
  74       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                            \
  75       HOTSPOT_MONITOR_WAIT(jtid,                                           \
  76                            (monitor), bytes, len, (millis));               \
  77     }                                                                      \
  78   }
  79 
  80 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
  81 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
  82 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
  83 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
  84 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
  85 
  86 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread)                  \
  87   {                                                                        \
  88     if (DTraceMonitorProbes) {                                             \
  89       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                            \
  90       HOTSPOT_MONITOR_##probe(jtid,                                        \
  91                               (uintptr_t)(monitor), bytes, len);           \
  92     }                                                                      \
  93   }
  94 
  95 #else //  ndef DTRACE_ENABLED
  96 
  97 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon)    {;}
  98 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon)          {;}
  99 
 100 #endif // ndef DTRACE_ENABLED
 101 
 102 // Tunables ...
 103 // The knob* variables are effectively final.  Once set they should
 104 // never be modified hence.  Consider using __read_mostly with GCC.
 105 
 106 int ObjectMonitor::Knob_Verbose     = 0;
 107 int ObjectMonitor::Knob_VerifyInUse = 0;
 108 int ObjectMonitor::Knob_SpinLimit   = 5000;    // derived by an external tool -
 109 static int Knob_LogSpins            = 0;       // enable jvmstat tally for spins
 110 static int Knob_HandOff             = 0;
 111 static int Knob_ReportSettings      = 0;
 112 
 113 static int Knob_SpinBase            = 0;       // Floor AKA SpinMin
 114 static int Knob_SpinBackOff         = 0;       // spin-loop backoff
 115 static int Knob_CASPenalty          = -1;      // Penalty for failed CAS
 116 static int Knob_OXPenalty           = -1;      // Penalty for observed _owner change
 117 static int Knob_SpinSetSucc         = 1;       // spinners set the _succ field
 118 static int Knob_SpinEarly           = 1;
 119 static int Knob_SuccEnabled         = 1;       // futile wake throttling
 120 static int Knob_SuccRestrict        = 0;       // Limit successors + spinners to at-most-one
 121 static int Knob_MaxSpinners         = -1;      // Should be a function of # CPUs
 122 static int Knob_Bonus               = 100;     // spin success bonus
 123 static int Knob_BonusB              = 100;     // spin success bonus
 124 static int Knob_Penalty             = 200;     // spin failure penalty
 125 static int Knob_Poverty             = 1000;
 126 static int Knob_SpinAfterFutile     = 1;       // Spin after returning from park()
 127 static int Knob_FixedSpin           = 0;
 128 static int Knob_OState              = 3;       // Spinner checks thread state of _owner
 129 static int Knob_UsePause            = 1;
 130 static int Knob_ExitPolicy          = 0;
 131 static int Knob_PreSpin             = 10;      // 20-100 likely better
 132 static int Knob_ResetEvent          = 0;
 133 static int BackOffMask              = 0;
 134 
 135 static int Knob_FastHSSEC           = 0;
 136 static int Knob_MoveNotifyee        = 2;       // notify() - disposition of notifyee
 137 static int Knob_QMode               = 0;       // EntryList-cxq policy - queue discipline
 138 static volatile int InitDone        = 0;
 139 
 140 #define TrySpin TrySpin_VaryDuration
 141 
 142 // -----------------------------------------------------------------------------
 143 // Theory of operations -- Monitors lists, thread residency, etc:
 144 //
 145 // * A thread acquires ownership of a monitor by successfully
 146 //   CAS()ing the _owner field from null to non-null.
 147 //
 148 // * Invariant: A thread appears on at most one monitor list --
 149 //   cxq, EntryList or WaitSet -- at any one time.
 150 //
 151 // * Contending threads "push" themselves onto the cxq with CAS
 152 //   and then spin/park.
 153 //
 154 // * After a contending thread eventually acquires the lock it must
 155 //   dequeue itself from either the EntryList or the cxq.
 156 //
 157 // * The exiting thread identifies and unparks an "heir presumptive"
 158 //   tentative successor thread on the EntryList.  Critically, the
 159 //   exiting thread doesn't unlink the successor thread from the EntryList.
 160 //   After having been unparked, the wakee will recontend for ownership of
 161 //   the monitor.   The successor (wakee) will either acquire the lock or
 162 //   re-park itself.
 163 //
 164 //   Succession is provided for by a policy of competitive handoff.
 165 //   The exiting thread does _not_ grant or pass ownership to the
 166 //   successor thread.  (This is also referred to as "handoff" succession").
 167 //   Instead the exiting thread releases ownership and possibly wakes
 168 //   a successor, so the successor can (re)compete for ownership of the lock.
 169 //   If the EntryList is empty but the cxq is populated the exiting
 170 //   thread will drain the cxq into the EntryList.  It does so by
 171 //   by detaching the cxq (installing null with CAS) and folding
 172 //   the threads from the cxq into the EntryList.  The EntryList is
 173 //   doubly linked, while the cxq is singly linked because of the
 174 //   CAS-based "push" used to enqueue recently arrived threads (RATs).
 175 //
 176 // * Concurrency invariants:
 177 //
 178 //   -- only the monitor owner may access or mutate the EntryList.
 179 //      The mutex property of the monitor itself protects the EntryList
 180 //      from concurrent interference.
 181 //   -- Only the monitor owner may detach the cxq.
 182 //
 183 // * The monitor entry list operations avoid locks, but strictly speaking
 184 //   they're not lock-free.  Enter is lock-free, exit is not.
 185 //   See http://j2se.east/~dice/PERSIST/040825-LockFreeQueues.html
 186 //
 187 // * The cxq can have multiple concurrent "pushers" but only one concurrent
 188 //   detaching thread.  This mechanism is immune from the ABA corruption.
 189 //   More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
 190 //
 191 // * Taken together, the cxq and the EntryList constitute or form a
 192 //   single logical queue of threads stalled trying to acquire the lock.
 193 //   We use two distinct lists to improve the odds of a constant-time
 194 //   dequeue operation after acquisition (in the ::enter() epilogue) and
 195 //   to reduce heat on the list ends.  (c.f. Michael Scott's "2Q" algorithm).
 196 //   A key desideratum is to minimize queue & monitor metadata manipulation
 197 //   that occurs while holding the monitor lock -- that is, we want to
 198 //   minimize monitor lock holds times.  Note that even a small amount of
 199 //   fixed spinning will greatly reduce the # of enqueue-dequeue operations
 200 //   on EntryList|cxq.  That is, spinning relieves contention on the "inner"
 201 //   locks and monitor metadata.
 202 //
 203 //   Cxq points to the set of Recently Arrived Threads attempting entry.
 204 //   Because we push threads onto _cxq with CAS, the RATs must take the form of
 205 //   a singly-linked LIFO.  We drain _cxq into EntryList  at unlock-time when
 206 //   the unlocking thread notices that EntryList is null but _cxq is != null.
 207 //
 208 //   The EntryList is ordered by the prevailing queue discipline and
 209 //   can be organized in any convenient fashion, such as a doubly-linked list or
 210 //   a circular doubly-linked list.  Critically, we want insert and delete operations
 211 //   to operate in constant-time.  If we need a priority queue then something akin
 212 //   to Solaris' sleepq would work nicely.  Viz.,
 213 //   http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
 214 //   Queue discipline is enforced at ::exit() time, when the unlocking thread
 215 //   drains the cxq into the EntryList, and orders or reorders the threads on the
 216 //   EntryList accordingly.
 217 //
 218 //   Barring "lock barging", this mechanism provides fair cyclic ordering,
 219 //   somewhat similar to an elevator-scan.
 220 //
 221 // * The monitor synchronization subsystem avoids the use of native
 222 //   synchronization primitives except for the narrow platform-specific
 223 //   park-unpark abstraction.  See the comments in os_solaris.cpp regarding
 224 //   the semantics of park-unpark.  Put another way, this monitor implementation
 225 //   depends only on atomic operations and park-unpark.  The monitor subsystem
 226 //   manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
 227 //   underlying OS manages the READY<->RUN transitions.
 228 //
 229 // * Waiting threads reside on the WaitSet list -- wait() puts
 230 //   the caller onto the WaitSet.
 231 //
 232 // * notify() or notifyAll() simply transfers threads from the WaitSet to
 233 //   either the EntryList or cxq.  Subsequent exit() operations will
 234 //   unpark the notifyee.  Unparking a notifee in notify() is inefficient -
 235 //   it's likely the notifyee would simply impale itself on the lock held
 236 //   by the notifier.
 237 //
 238 // * An interesting alternative is to encode cxq as (List,LockByte) where
 239 //   the LockByte is 0 iff the monitor is owned.  _owner is simply an auxiliary
 240 //   variable, like _recursions, in the scheme.  The threads or Events that form
 241 //   the list would have to be aligned in 256-byte addresses.  A thread would
 242 //   try to acquire the lock or enqueue itself with CAS, but exiting threads
 243 //   could use a 1-0 protocol and simply STB to set the LockByte to 0.
 244 //   Note that is is *not* word-tearing, but it does presume that full-word
 245 //   CAS operations are coherent with intermix with STB operations.  That's true
 246 //   on most common processors.
 247 //
 248 // * See also http://blogs.sun.com/dave
 249 
 250 
 251 // -----------------------------------------------------------------------------
 252 // Enter support
 253 
 254 bool ObjectMonitor::try_enter(Thread* THREAD) {
 255   if (THREAD != _owner) {
 256     if (THREAD->is_lock_owned ((address)_owner)) {
 257       assert(_recursions == 0, "internal state error");
 258       _owner = THREAD;
 259       _recursions = 1;
 260       return true;
 261     }
 262     if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
 263       return false;
 264     }
 265     return true;
 266   } else {
 267     _recursions++;
 268     return true;
 269   }
 270 }
 271 
 272 void NOINLINE ObjectMonitor::enter(TRAPS) {
 273   // The following code is ordered to check the most common cases first
 274   // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
 275   Thread * const Self = THREAD;
 276 
 277   void * cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL);
 278   if (cur == NULL) {
 279     // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
 280     assert(_recursions == 0, "invariant");
 281     assert(_owner == Self, "invariant");
 282     return;
 283   }
 284 
 285   if (cur == Self) {
 286     // TODO-FIXME: check for integer overflow!  BUGID 6557169.
 287     _recursions++;
 288     return;
 289   }
 290 
 291   if (Self->is_lock_owned ((address)cur)) {
 292     assert(_recursions == 0, "internal state error");
 293     _recursions = 1;
 294     // Commute owner from a thread-specific on-stack BasicLockObject address to
 295     // a full-fledged "Thread *".
 296     _owner = Self;
 297     return;
 298   }
 299 
 300   // We've encountered genuine contention.
 301   assert(Self->_Stalled == 0, "invariant");
 302   Self->_Stalled = intptr_t(this);
 303 
 304   // Try one round of spinning *before* enqueueing Self
 305   // and before going through the awkward and expensive state
 306   // transitions.  The following spin is strictly optional ...
 307   // Note that if we acquire the monitor from an initial spin
 308   // we forgo posting JVMTI events and firing DTRACE probes.
 309   if (Knob_SpinEarly && TrySpin (Self) > 0) {
 310     assert(_owner == Self, "invariant");
 311     assert(_recursions == 0, "invariant");
 312     assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 313     Self->_Stalled = 0;
 314     return;
 315   }
 316 
 317   assert(_owner != Self, "invariant");
 318   assert(_succ != Self, "invariant");
 319   assert(Self->is_Java_thread(), "invariant");
 320   JavaThread * jt = (JavaThread *) Self;
 321   assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
 322   assert(jt->thread_state() != _thread_blocked, "invariant");
 323   assert(this->object() != NULL, "invariant");
 324   assert(_count >= 0, "invariant");
 325 
 326   // Prevent deflation at STW-time.  See deflate_idle_monitors() and is_busy().
 327   // Ensure the object-monitor relationship remains stable while there's contention.
 328   Atomic::inc(&_count);
 329 
 330   EventJavaMonitorEnter event;
 331 
 332   { // Change java thread status to indicate blocked on monitor enter.
 333     JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
 334 
 335     DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
 336     if (JvmtiExport::should_post_monitor_contended_enter()) {
 337       JvmtiExport::post_monitor_contended_enter(jt, this);
 338 
 339       // The current thread does not yet own the monitor and does not
 340       // yet appear on any queues that would get it made the successor.
 341       // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
 342       // handler cannot accidentally consume an unpark() meant for the
 343       // ParkEvent associated with this ObjectMonitor.
 344     }
 345 
 346     OSThreadContendState osts(Self->osthread());
 347     ThreadBlockInVM tbivm(jt);
 348 
 349     Self->set_current_pending_monitor(this);
 350 
 351     // TODO-FIXME: change the following for(;;) loop to straight-line code.
 352     for (;;) {
 353       jt->set_suspend_equivalent();
 354       // cleared by handle_special_suspend_equivalent_condition()
 355       // or java_suspend_self()
 356 
 357       EnterI(THREAD);
 358 
 359       if (!ExitSuspendEquivalent(jt)) break;
 360 
 361       // We have acquired the contended monitor, but while we were
 362       // waiting another thread suspended us. We don't want to enter
 363       // the monitor while suspended because that would surprise the
 364       // thread that suspended us.
 365       //
 366       _recursions = 0;
 367       _succ = NULL;
 368       exit(false, Self);
 369 
 370       jt->java_suspend_self();
 371     }
 372     Self->set_current_pending_monitor(NULL);
 373 
 374     // We cleared the pending monitor info since we've just gotten past
 375     // the enter-check-for-suspend dance and we now own the monitor free
 376     // and clear, i.e., it is no longer pending. The ThreadBlockInVM
 377     // destructor can go to a safepoint at the end of this block. If we
 378     // do a thread dump during that safepoint, then this thread will show
 379     // as having "-locked" the monitor, but the OS and java.lang.Thread
 380     // states will still report that the thread is blocked trying to
 381     // acquire it.
 382   }
 383 
 384   Atomic::dec(&_count);
 385   assert(_count >= 0, "invariant");
 386   Self->_Stalled = 0;
 387 
 388   // Must either set _recursions = 0 or ASSERT _recursions == 0.
 389   assert(_recursions == 0, "invariant");
 390   assert(_owner == Self, "invariant");
 391   assert(_succ != Self, "invariant");
 392   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 393 
 394   // The thread -- now the owner -- is back in vm mode.
 395   // Report the glorious news via TI,DTrace and jvmstat.
 396   // The probe effect is non-trivial.  All the reportage occurs
 397   // while we hold the monitor, increasing the length of the critical
 398   // section.  Amdahl's parallel speedup law comes vividly into play.
 399   //
 400   // Another option might be to aggregate the events (thread local or
 401   // per-monitor aggregation) and defer reporting until a more opportune
 402   // time -- such as next time some thread encounters contention but has
 403   // yet to acquire the lock.  While spinning that thread could
 404   // spinning we could increment JVMStat counters, etc.
 405 
 406   DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
 407   if (JvmtiExport::should_post_monitor_contended_entered()) {
 408     JvmtiExport::post_monitor_contended_entered(jt, this);
 409 
 410     // The current thread already owns the monitor and is not going to
 411     // call park() for the remainder of the monitor enter protocol. So
 412     // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
 413     // event handler consumed an unpark() issued by the thread that
 414     // just exited the monitor.
 415   }
 416 
 417   if (event.should_commit()) {
 418     event.set_klass(((oop)this->object())->klass());
 419     event.set_previousOwner((TYPE_JAVALANGTHREAD)_previous_owner_tid);
 420     event.set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
 421     event.commit();
 422   }
 423 
 424   if (ObjectMonitor::_sync_ContendedLockAttempts != NULL) {
 425     ObjectMonitor::_sync_ContendedLockAttempts->inc();
 426   }
 427 }
 428 
 429 
 430 // Caveat: TryLock() is not necessarily serializing if it returns failure.
 431 // Callers must compensate as needed.
 432 
 433 int ObjectMonitor::TryLock(Thread * Self) {
 434   void * own = _owner;
 435   if (own != NULL) return 0;
 436   if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
 437     // Either guarantee _recursions == 0 or set _recursions = 0.
 438     assert(_recursions == 0, "invariant");
 439     assert(_owner == Self, "invariant");
 440     return 1;
 441   }
 442   // The lock had been free momentarily, but we lost the race to the lock.
 443   // Interference -- the CAS failed.
 444   // We can either return -1 or retry.
 445   // Retry doesn't make as much sense because the lock was just acquired.
 446   return -1;
 447 }
 448 
 449 void NOINLINE ObjectMonitor::EnterI(TRAPS) {
 450   Thread * const Self = THREAD;
 451   assert(Self->is_Java_thread(), "invariant");
 452   assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant");
 453 
 454   // Try the lock - TATAS
 455   if (TryLock (Self) > 0) {
 456     assert(_succ != Self, "invariant");
 457     assert(_owner == Self, "invariant");
 458     assert(_Responsible != Self, "invariant");
 459     return;
 460   }
 461 
 462   DeferredInitialize();
 463 
 464   // We try one round of spinning *before* enqueueing Self.
 465   //
 466   // If the _owner is ready but OFFPROC we could use a YieldTo()
 467   // operation to donate the remainder of this thread's quantum
 468   // to the owner.  This has subtle but beneficial affinity
 469   // effects.
 470 
 471   if (TrySpin (Self) > 0) {
 472     assert(_owner == Self, "invariant");
 473     assert(_succ != Self, "invariant");
 474     assert(_Responsible != Self, "invariant");
 475     return;
 476   }
 477 
 478   // The Spin failed -- Enqueue and park the thread ...
 479   assert(_succ != Self, "invariant");
 480   assert(_owner != Self, "invariant");
 481   assert(_Responsible != Self, "invariant");
 482 
 483   // Enqueue "Self" on ObjectMonitor's _cxq.
 484   //
 485   // Node acts as a proxy for Self.
 486   // As an aside, if were to ever rewrite the synchronization code mostly
 487   // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
 488   // Java objects.  This would avoid awkward lifecycle and liveness issues,
 489   // as well as eliminate a subset of ABA issues.
 490   // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
 491 
 492   ObjectWaiter node(Self);
 493   Self->_ParkEvent->reset();
 494   node._prev   = (ObjectWaiter *) 0xBAD;
 495   node.TState  = ObjectWaiter::TS_CXQ;
 496 
 497   // Push "Self" onto the front of the _cxq.
 498   // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
 499   // Note that spinning tends to reduce the rate at which threads
 500   // enqueue and dequeue on EntryList|cxq.
 501   ObjectWaiter * nxt;
 502   for (;;) {
 503     node._next = nxt = _cxq;
 504     if (Atomic::cmpxchg_ptr(&node, &_cxq, nxt) == nxt) break;
 505 
 506     // Interference - the CAS failed because _cxq changed.  Just retry.
 507     // As an optional optimization we retry the lock.
 508     if (TryLock (Self) > 0) {
 509       assert(_succ != Self, "invariant");
 510       assert(_owner == Self, "invariant");
 511       assert(_Responsible != Self, "invariant");
 512       return;
 513     }
 514   }
 515 
 516   // Check for cxq|EntryList edge transition to non-null.  This indicates
 517   // the onset of contention.  While contention persists exiting threads
 518   // will use a ST:MEMBAR:LD 1-1 exit protocol.  When contention abates exit
 519   // operations revert to the faster 1-0 mode.  This enter operation may interleave
 520   // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
 521   // arrange for one of the contending thread to use a timed park() operations
 522   // to detect and recover from the race.  (Stranding is form of progress failure
 523   // where the monitor is unlocked but all the contending threads remain parked).
 524   // That is, at least one of the contended threads will periodically poll _owner.
 525   // One of the contending threads will become the designated "Responsible" thread.
 526   // The Responsible thread uses a timed park instead of a normal indefinite park
 527   // operation -- it periodically wakes and checks for and recovers from potential
 528   // strandings admitted by 1-0 exit operations.   We need at most one Responsible
 529   // thread per-monitor at any given moment.  Only threads on cxq|EntryList may
 530   // be responsible for a monitor.
 531   //
 532   // Currently, one of the contended threads takes on the added role of "Responsible".
 533   // A viable alternative would be to use a dedicated "stranding checker" thread
 534   // that periodically iterated over all the threads (or active monitors) and unparked
 535   // successors where there was risk of stranding.  This would help eliminate the
 536   // timer scalability issues we see on some platforms as we'd only have one thread
 537   // -- the checker -- parked on a timer.
 538 
 539   if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
 540     // Try to assume the role of responsible thread for the monitor.
 541     // CONSIDER:  ST vs CAS vs { if (Responsible==null) Responsible=Self }
 542     Atomic::cmpxchg_ptr(Self, &_Responsible, NULL);
 543   }
 544 
 545   // The lock might have been released while this thread was occupied queueing
 546   // itself onto _cxq.  To close the race and avoid "stranding" and
 547   // progress-liveness failure we must resample-retry _owner before parking.
 548   // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
 549   // In this case the ST-MEMBAR is accomplished with CAS().
 550   //
 551   // TODO: Defer all thread state transitions until park-time.
 552   // Since state transitions are heavy and inefficient we'd like
 553   // to defer the state transitions until absolutely necessary,
 554   // and in doing so avoid some transitions ...
 555 
 556   TEVENT(Inflated enter - Contention);
 557   int nWakeups = 0;
 558   int RecheckInterval = 1;
 559 
 560   for (;;) {
 561 
 562     if (TryLock(Self) > 0) break;
 563     assert(_owner != Self, "invariant");
 564 
 565     if ((SyncFlags & 2) && _Responsible == NULL) {
 566       Atomic::cmpxchg_ptr(Self, &_Responsible, NULL);
 567     }
 568 
 569     // park self
 570     if (_Responsible == Self || (SyncFlags & 1)) {
 571       TEVENT(Inflated enter - park TIMED);
 572       Self->_ParkEvent->park((jlong) RecheckInterval);
 573       // Increase the RecheckInterval, but clamp the value.
 574       RecheckInterval *= 8;
 575       if (RecheckInterval > 1000) RecheckInterval = 1000;
 576     } else {
 577       TEVENT(Inflated enter - park UNTIMED);
 578       Self->_ParkEvent->park();
 579     }
 580 
 581     if (TryLock(Self) > 0) break;
 582 
 583     // The lock is still contested.
 584     // Keep a tally of the # of futile wakeups.
 585     // Note that the counter is not protected by a lock or updated by atomics.
 586     // That is by design - we trade "lossy" counters which are exposed to
 587     // races during updates for a lower probe effect.
 588     TEVENT(Inflated enter - Futile wakeup);
 589     if (ObjectMonitor::_sync_FutileWakeups != NULL) {
 590       ObjectMonitor::_sync_FutileWakeups->inc();
 591     }
 592     ++nWakeups;
 593 
 594     // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
 595     // We can defer clearing _succ until after the spin completes
 596     // TrySpin() must tolerate being called with _succ == Self.
 597     // Try yet another round of adaptive spinning.
 598     if ((Knob_SpinAfterFutile & 1) && TrySpin(Self) > 0) break;
 599 
 600     // We can find that we were unpark()ed and redesignated _succ while
 601     // we were spinning.  That's harmless.  If we iterate and call park(),
 602     // park() will consume the event and return immediately and we'll
 603     // just spin again.  This pattern can repeat, leaving _succ to simply
 604     // spin on a CPU.  Enable Knob_ResetEvent to clear pending unparks().
 605     // Alternately, we can sample fired() here, and if set, forgo spinning
 606     // in the next iteration.
 607 
 608     if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
 609       Self->_ParkEvent->reset();
 610       OrderAccess::fence();
 611     }
 612     if (_succ == Self) _succ = NULL;
 613 
 614     // Invariant: after clearing _succ a thread *must* retry _owner before parking.
 615     OrderAccess::fence();
 616   }
 617 
 618   // Egress :
 619   // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
 620   // Normally we'll find Self on the EntryList .
 621   // From the perspective of the lock owner (this thread), the
 622   // EntryList is stable and cxq is prepend-only.
 623   // The head of cxq is volatile but the interior is stable.
 624   // In addition, Self.TState is stable.
 625 
 626   assert(_owner == Self, "invariant");
 627   assert(object() != NULL, "invariant");
 628   // I'd like to write:
 629   //   guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
 630   // but as we're at a safepoint that's not safe.
 631 
 632   UnlinkAfterAcquire(Self, &node);
 633   if (_succ == Self) _succ = NULL;
 634 
 635   assert(_succ != Self, "invariant");
 636   if (_Responsible == Self) {
 637     _Responsible = NULL;
 638     OrderAccess::fence(); // Dekker pivot-point
 639 
 640     // We may leave threads on cxq|EntryList without a designated
 641     // "Responsible" thread.  This is benign.  When this thread subsequently
 642     // exits the monitor it can "see" such preexisting "old" threads --
 643     // threads that arrived on the cxq|EntryList before the fence, above --
 644     // by LDing cxq|EntryList.  Newly arrived threads -- that is, threads
 645     // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
 646     // non-null and elect a new "Responsible" timer thread.
 647     //
 648     // This thread executes:
 649     //    ST Responsible=null; MEMBAR    (in enter epilogue - here)
 650     //    LD cxq|EntryList               (in subsequent exit)
 651     //
 652     // Entering threads in the slow/contended path execute:
 653     //    ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
 654     //    The (ST cxq; MEMBAR) is accomplished with CAS().
 655     //
 656     // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
 657     // exit operation from floating above the ST Responsible=null.
 658   }
 659 
 660   // We've acquired ownership with CAS().
 661   // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
 662   // But since the CAS() this thread may have also stored into _succ,
 663   // EntryList, cxq or Responsible.  These meta-data updates must be
 664   // visible __before this thread subsequently drops the lock.
 665   // Consider what could occur if we didn't enforce this constraint --
 666   // STs to monitor meta-data and user-data could reorder with (become
 667   // visible after) the ST in exit that drops ownership of the lock.
 668   // Some other thread could then acquire the lock, but observe inconsistent
 669   // or old monitor meta-data and heap data.  That violates the JMM.
 670   // To that end, the 1-0 exit() operation must have at least STST|LDST
 671   // "release" barrier semantics.  Specifically, there must be at least a
 672   // STST|LDST barrier in exit() before the ST of null into _owner that drops
 673   // the lock.   The barrier ensures that changes to monitor meta-data and data
 674   // protected by the lock will be visible before we release the lock, and
 675   // therefore before some other thread (CPU) has a chance to acquire the lock.
 676   // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
 677   //
 678   // Critically, any prior STs to _succ or EntryList must be visible before
 679   // the ST of null into _owner in the *subsequent* (following) corresponding
 680   // monitorexit.  Recall too, that in 1-0 mode monitorexit does not necessarily
 681   // execute a serializing instruction.
 682 
 683   if (SyncFlags & 8) {
 684     OrderAccess::fence();
 685   }
 686   return;
 687 }
 688 
 689 // ReenterI() is a specialized inline form of the latter half of the
 690 // contended slow-path from EnterI().  We use ReenterI() only for
 691 // monitor reentry in wait().
 692 //
 693 // In the future we should reconcile EnterI() and ReenterI(), adding
 694 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
 695 // loop accordingly.
 696 
 697 void NOINLINE ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) {
 698   assert(Self != NULL, "invariant");
 699   assert(SelfNode != NULL, "invariant");
 700   assert(SelfNode->_thread == Self, "invariant");
 701   assert(_waiters > 0, "invariant");
 702   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 703   assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
 704   JavaThread * jt = (JavaThread *) Self;
 705 
 706   int nWakeups = 0;
 707   for (;;) {
 708     ObjectWaiter::TStates v = SelfNode->TState;
 709     guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
 710     assert(_owner != Self, "invariant");
 711 
 712     if (TryLock(Self) > 0) break;
 713     if (TrySpin(Self) > 0) break;
 714 
 715     TEVENT(Wait Reentry - parking);
 716 
 717     // State transition wrappers around park() ...
 718     // ReenterI() wisely defers state transitions until
 719     // it's clear we must park the thread.
 720     {
 721       OSThreadContendState osts(Self->osthread());
 722       ThreadBlockInVM tbivm(jt);
 723 
 724       // cleared by handle_special_suspend_equivalent_condition()
 725       // or java_suspend_self()
 726       jt->set_suspend_equivalent();
 727       if (SyncFlags & 1) {
 728         Self->_ParkEvent->park((jlong)1000);
 729       } else {
 730         Self->_ParkEvent->park();
 731       }
 732 
 733       // were we externally suspended while we were waiting?
 734       for (;;) {
 735         if (!ExitSuspendEquivalent(jt)) break;
 736         if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
 737         jt->java_suspend_self();
 738         jt->set_suspend_equivalent();
 739       }
 740     }
 741 
 742     // Try again, but just so we distinguish between futile wakeups and
 743     // successful wakeups.  The following test isn't algorithmically
 744     // necessary, but it helps us maintain sensible statistics.
 745     if (TryLock(Self) > 0) break;
 746 
 747     // The lock is still contested.
 748     // Keep a tally of the # of futile wakeups.
 749     // Note that the counter is not protected by a lock or updated by atomics.
 750     // That is by design - we trade "lossy" counters which are exposed to
 751     // races during updates for a lower probe effect.
 752     TEVENT(Wait Reentry - futile wakeup);
 753     ++nWakeups;
 754 
 755     // Assuming this is not a spurious wakeup we'll normally
 756     // find that _succ == Self.
 757     if (_succ == Self) _succ = NULL;
 758 
 759     // Invariant: after clearing _succ a contending thread
 760     // *must* retry  _owner before parking.
 761     OrderAccess::fence();
 762 
 763     if (ObjectMonitor::_sync_FutileWakeups != NULL) {
 764       ObjectMonitor::_sync_FutileWakeups->inc();
 765     }
 766   }
 767 
 768   // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
 769   // Normally we'll find Self on the EntryList.
 770   // Unlinking from the EntryList is constant-time and atomic-free.
 771   // From the perspective of the lock owner (this thread), the
 772   // EntryList is stable and cxq is prepend-only.
 773   // The head of cxq is volatile but the interior is stable.
 774   // In addition, Self.TState is stable.
 775 
 776   assert(_owner == Self, "invariant");
 777   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 778   UnlinkAfterAcquire(Self, SelfNode);
 779   if (_succ == Self) _succ = NULL;
 780   assert(_succ != Self, "invariant");
 781   SelfNode->TState = ObjectWaiter::TS_RUN;
 782   OrderAccess::fence();      // see comments at the end of EnterI()
 783 }
 784 
 785 // By convention we unlink a contending thread from EntryList|cxq immediately
 786 // after the thread acquires the lock in ::enter().  Equally, we could defer
 787 // unlinking the thread until ::exit()-time.
 788 
 789 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) {
 790   assert(_owner == Self, "invariant");
 791   assert(SelfNode->_thread == Self, "invariant");
 792 
 793   if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
 794     // Normal case: remove Self from the DLL EntryList .
 795     // This is a constant-time operation.
 796     ObjectWaiter * nxt = SelfNode->_next;
 797     ObjectWaiter * prv = SelfNode->_prev;
 798     if (nxt != NULL) nxt->_prev = prv;
 799     if (prv != NULL) prv->_next = nxt;
 800     if (SelfNode == _EntryList) _EntryList = nxt;
 801     assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
 802     assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
 803     TEVENT(Unlink from EntryList);
 804   } else {
 805     assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant");
 806     // Inopportune interleaving -- Self is still on the cxq.
 807     // This usually means the enqueue of self raced an exiting thread.
 808     // Normally we'll find Self near the front of the cxq, so
 809     // dequeueing is typically fast.  If needbe we can accelerate
 810     // this with some MCS/CHL-like bidirectional list hints and advisory
 811     // back-links so dequeueing from the interior will normally operate
 812     // in constant-time.
 813     // Dequeue Self from either the head (with CAS) or from the interior
 814     // with a linear-time scan and normal non-atomic memory operations.
 815     // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
 816     // and then unlink Self from EntryList.  We have to drain eventually,
 817     // so it might as well be now.
 818 
 819     ObjectWaiter * v = _cxq;
 820     assert(v != NULL, "invariant");
 821     if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) {
 822       // The CAS above can fail from interference IFF a "RAT" arrived.
 823       // In that case Self must be in the interior and can no longer be
 824       // at the head of cxq.
 825       if (v == SelfNode) {
 826         assert(_cxq != v, "invariant");
 827         v = _cxq;          // CAS above failed - start scan at head of list
 828       }
 829       ObjectWaiter * p;
 830       ObjectWaiter * q = NULL;
 831       for (p = v; p != NULL && p != SelfNode; p = p->_next) {
 832         q = p;
 833         assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
 834       }
 835       assert(v != SelfNode, "invariant");
 836       assert(p == SelfNode, "Node not found on cxq");
 837       assert(p != _cxq, "invariant");
 838       assert(q != NULL, "invariant");
 839       assert(q->_next == p, "invariant");
 840       q->_next = p->_next;
 841     }
 842     TEVENT(Unlink from cxq);
 843   }
 844 
 845 #ifdef ASSERT
 846   // Diagnostic hygiene ...
 847   SelfNode->_prev  = (ObjectWaiter *) 0xBAD;
 848   SelfNode->_next  = (ObjectWaiter *) 0xBAD;
 849   SelfNode->TState = ObjectWaiter::TS_RUN;
 850 #endif
 851 }
 852 
 853 // -----------------------------------------------------------------------------
 854 // Exit support
 855 //
 856 // exit()
 857 // ~~~~~~
 858 // Note that the collector can't reclaim the objectMonitor or deflate
 859 // the object out from underneath the thread calling ::exit() as the
 860 // thread calling ::exit() never transitions to a stable state.
 861 // This inhibits GC, which in turn inhibits asynchronous (and
 862 // inopportune) reclamation of "this".
 863 //
 864 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
 865 // There's one exception to the claim above, however.  EnterI() can call
 866 // exit() to drop a lock if the acquirer has been externally suspended.
 867 // In that case exit() is called with _thread_state as _thread_blocked,
 868 // but the monitor's _count field is > 0, which inhibits reclamation.
 869 //
 870 // 1-0 exit
 871 // ~~~~~~~~
 872 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
 873 // the fast-path operators have been optimized so the common ::exit()
 874 // operation is 1-0.  See i486.ad fast_unlock(), for instance.
 875 // The code emitted by fast_unlock() elides the usual MEMBAR.  This
 876 // greatly improves latency -- MEMBAR and CAS having considerable local
 877 // latency on modern processors -- but at the cost of "stranding".  Absent the
 878 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
 879 // ::enter() path, resulting in the entering thread being stranding
 880 // and a progress-liveness failure.   Stranding is extremely rare.
 881 // We use timers (timed park operations) & periodic polling to detect
 882 // and recover from stranding.  Potentially stranded threads periodically
 883 // wake up and poll the lock.  See the usage of the _Responsible variable.
 884 //
 885 // The CAS() in enter provides for safety and exclusion, while the CAS or
 886 // MEMBAR in exit provides for progress and avoids stranding.  1-0 locking
 887 // eliminates the CAS/MEMBAR from the exist path, but it admits stranding.
 888 // We detect and recover from stranding with timers.
 889 //
 890 // If a thread transiently strands it'll park until (a) another
 891 // thread acquires the lock and then drops the lock, at which time the
 892 // exiting thread will notice and unpark the stranded thread, or, (b)
 893 // the timer expires.  If the lock is high traffic then the stranding latency
 894 // will be low due to (a).  If the lock is low traffic then the odds of
 895 // stranding are lower, although the worst-case stranding latency
 896 // is longer.  Critically, we don't want to put excessive load in the
 897 // platform's timer subsystem.  We want to minimize both the timer injection
 898 // rate (timers created/sec) as well as the number of timers active at
 899 // any one time.  (more precisely, we want to minimize timer-seconds, which is
 900 // the integral of the # of active timers at any instant over time).
 901 // Both impinge on OS scalability.  Given that, at most one thread parked on
 902 // a monitor will use a timer.
 903 //
 904 // There is also the risk of a futile wake-up. If we drop the lock
 905 // another thread can reacquire the lock immediately, and we can
 906 // then wake a thread unnecessarily. This is benign, and we've
 907 // structured the code so the windows are short and the frequency
 908 // of such futile wakups is low.
 909 
 910 void NOINLINE ObjectMonitor::exit(bool not_suspended, TRAPS) {
 911   Thread * const Self = THREAD;
 912   if (THREAD != _owner) {
 913     if (THREAD->is_lock_owned((address) _owner)) {
 914       // Transmute _owner from a BasicLock pointer to a Thread address.
 915       // We don't need to hold _mutex for this transition.
 916       // Non-null to Non-null is safe as long as all readers can
 917       // tolerate either flavor.
 918       assert(_recursions == 0, "invariant");
 919       _owner = THREAD;
 920       _recursions = 0;
 921     } else {
 922       // Apparent unbalanced locking ...
 923       // Naively we'd like to throw IllegalMonitorStateException.
 924       // As a practical matter we can neither allocate nor throw an
 925       // exception as ::exit() can be called from leaf routines.
 926       // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
 927       // Upon deeper reflection, however, in a properly run JVM the only
 928       // way we should encounter this situation is in the presence of
 929       // unbalanced JNI locking. TODO: CheckJNICalls.
 930       // See also: CR4414101
 931       TEVENT(Exit - Throw IMSX);
 932       assert(false, "Non-balanced monitor enter/exit! Likely JNI locking");
 933       return;
 934     }
 935   }
 936 
 937   if (_recursions != 0) {
 938     _recursions--;        // this is simple recursive enter
 939     TEVENT(Inflated exit - recursive);
 940     return;
 941   }
 942 
 943   // Invariant: after setting Responsible=null an thread must execute
 944   // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
 945   if ((SyncFlags & 4) == 0) {
 946     _Responsible = NULL;
 947   }
 948 
 949 #if INCLUDE_TRACE
 950   // get the owner's thread id for the MonitorEnter event
 951   // if it is enabled and the thread isn't suspended
 952   if (not_suspended && Tracing::is_event_enabled(TraceJavaMonitorEnterEvent)) {
 953     _previous_owner_tid = SharedRuntime::get_java_tid(Self);
 954   }
 955 #endif
 956 
 957   for (;;) {
 958     assert(THREAD == _owner, "invariant");
 959 
 960 
 961     if (Knob_ExitPolicy == 0) {
 962       // release semantics: prior loads and stores from within the critical section
 963       // must not float (reorder) past the following store that drops the lock.
 964       // On SPARC that requires MEMBAR #loadstore|#storestore.
 965       // But of course in TSO #loadstore|#storestore is not required.
 966       // I'd like to write one of the following:
 967       // A.  OrderAccess::release() ; _owner = NULL
 968       // B.  OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
 969       // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
 970       // store into a _dummy variable.  That store is not needed, but can result
 971       // in massive wasteful coherency traffic on classic SMP systems.
 972       // Instead, I use release_store(), which is implemented as just a simple
 973       // ST on x64, x86 and SPARC.
 974       OrderAccess::release_store_ptr(&_owner, NULL);   // drop the lock
 975       OrderAccess::storeload();                        // See if we need to wake a successor
 976       if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
 977         TEVENT(Inflated exit - simple egress);
 978         return;
 979       }
 980       TEVENT(Inflated exit - complex egress);
 981       // Other threads are blocked trying to acquire the lock.
 982 
 983       // Normally the exiting thread is responsible for ensuring succession,
 984       // but if other successors are ready or other entering threads are spinning
 985       // then this thread can simply store NULL into _owner and exit without
 986       // waking a successor.  The existence of spinners or ready successors
 987       // guarantees proper succession (liveness).  Responsibility passes to the
 988       // ready or running successors.  The exiting thread delegates the duty.
 989       // More precisely, if a successor already exists this thread is absolved
 990       // of the responsibility of waking (unparking) one.
 991       //
 992       // The _succ variable is critical to reducing futile wakeup frequency.
 993       // _succ identifies the "heir presumptive" thread that has been made
 994       // ready (unparked) but that has not yet run.  We need only one such
 995       // successor thread to guarantee progress.
 996       // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
 997       // section 3.3 "Futile Wakeup Throttling" for details.
 998       //
 999       // Note that spinners in Enter() also set _succ non-null.
1000       // In the current implementation spinners opportunistically set
1001       // _succ so that exiting threads might avoid waking a successor.
1002       // Another less appealing alternative would be for the exiting thread
1003       // to drop the lock and then spin briefly to see if a spinner managed
1004       // to acquire the lock.  If so, the exiting thread could exit
1005       // immediately without waking a successor, otherwise the exiting
1006       // thread would need to dequeue and wake a successor.
1007       // (Note that we'd need to make the post-drop spin short, but no
1008       // shorter than the worst-case round-trip cache-line migration time.
1009       // The dropped lock needs to become visible to the spinner, and then
1010       // the acquisition of the lock by the spinner must become visible to
1011       // the exiting thread).
1012 
1013       // It appears that an heir-presumptive (successor) must be made ready.
1014       // Only the current lock owner can manipulate the EntryList or
1015       // drain _cxq, so we need to reacquire the lock.  If we fail
1016       // to reacquire the lock the responsibility for ensuring succession
1017       // falls to the new owner.
1018       //
1019       if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1020         return;
1021       }
1022       TEVENT(Exit - Reacquired);
1023     } else {
1024       if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1025         OrderAccess::release_store_ptr(&_owner, NULL);   // drop the lock
1026         OrderAccess::storeload();
1027         // Ratify the previously observed values.
1028         if (_cxq == NULL || _succ != NULL) {
1029           TEVENT(Inflated exit - simple egress);
1030           return;
1031         }
1032 
1033         // inopportune interleaving -- the exiting thread (this thread)
1034         // in the fast-exit path raced an entering thread in the slow-enter
1035         // path.
1036         // We have two choices:
1037         // A.  Try to reacquire the lock.
1038         //     If the CAS() fails return immediately, otherwise
1039         //     we either restart/rerun the exit operation, or simply
1040         //     fall-through into the code below which wakes a successor.
1041         // B.  If the elements forming the EntryList|cxq are TSM
1042         //     we could simply unpark() the lead thread and return
1043         //     without having set _succ.
1044         if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1045           TEVENT(Inflated exit - reacquired succeeded);
1046           return;
1047         }
1048         TEVENT(Inflated exit - reacquired failed);
1049       } else {
1050         TEVENT(Inflated exit - complex egress);
1051       }
1052     }
1053 
1054     guarantee(_owner == THREAD, "invariant");
1055 
1056     ObjectWaiter * w = NULL;
1057     int QMode = Knob_QMode;
1058 
1059     if (QMode == 2 && _cxq != NULL) {
1060       // QMode == 2 : cxq has precedence over EntryList.
1061       // Try to directly wake a successor from the cxq.
1062       // If successful, the successor will need to unlink itself from cxq.
1063       w = _cxq;
1064       assert(w != NULL, "invariant");
1065       assert(w->TState == ObjectWaiter::TS_CXQ, "Invariant");
1066       ExitEpilog(Self, w);
1067       return;
1068     }
1069 
1070     if (QMode == 3 && _cxq != NULL) {
1071       // Aggressively drain cxq into EntryList at the first opportunity.
1072       // This policy ensure that recently-run threads live at the head of EntryList.
1073       // Drain _cxq into EntryList - bulk transfer.
1074       // First, detach _cxq.
1075       // The following loop is tantamount to: w = swap(&cxq, NULL)
1076       w = _cxq;
1077       for (;;) {
1078         assert(w != NULL, "Invariant");
1079         ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr(NULL, &_cxq, w);
1080         if (u == w) break;
1081         w = u;
1082       }
1083       assert(w != NULL, "invariant");
1084 
1085       ObjectWaiter * q = NULL;
1086       ObjectWaiter * p;
1087       for (p = w; p != NULL; p = p->_next) {
1088         guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1089         p->TState = ObjectWaiter::TS_ENTER;
1090         p->_prev = q;
1091         q = p;
1092       }
1093 
1094       // Append the RATs to the EntryList
1095       // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
1096       ObjectWaiter * Tail;
1097       for (Tail = _EntryList; Tail != NULL && Tail->_next != NULL;
1098            Tail = Tail->_next)
1099         /* empty */;
1100       if (Tail == NULL) {
1101         _EntryList = w;
1102       } else {
1103         Tail->_next = w;
1104         w->_prev = Tail;
1105       }
1106 
1107       // Fall thru into code that tries to wake a successor from EntryList
1108     }
1109 
1110     if (QMode == 4 && _cxq != NULL) {
1111       // Aggressively drain cxq into EntryList at the first opportunity.
1112       // This policy ensure that recently-run threads live at the head of EntryList.
1113 
1114       // Drain _cxq into EntryList - bulk transfer.
1115       // First, detach _cxq.
1116       // The following loop is tantamount to: w = swap(&cxq, NULL)
1117       w = _cxq;
1118       for (;;) {
1119         assert(w != NULL, "Invariant");
1120         ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr(NULL, &_cxq, w);
1121         if (u == w) break;
1122         w = u;
1123       }
1124       assert(w != NULL, "invariant");
1125 
1126       ObjectWaiter * q = NULL;
1127       ObjectWaiter * p;
1128       for (p = w; p != NULL; p = p->_next) {
1129         guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1130         p->TState = ObjectWaiter::TS_ENTER;
1131         p->_prev = q;
1132         q = p;
1133       }
1134 
1135       // Prepend the RATs to the EntryList
1136       if (_EntryList != NULL) {
1137         q->_next = _EntryList;
1138         _EntryList->_prev = q;
1139       }
1140       _EntryList = w;
1141 
1142       // Fall thru into code that tries to wake a successor from EntryList
1143     }
1144 
1145     w = _EntryList;
1146     if (w != NULL) {
1147       // I'd like to write: guarantee (w->_thread != Self).
1148       // But in practice an exiting thread may find itself on the EntryList.
1149       // Let's say thread T1 calls O.wait().  Wait() enqueues T1 on O's waitset and
1150       // then calls exit().  Exit release the lock by setting O._owner to NULL.
1151       // Let's say T1 then stalls.  T2 acquires O and calls O.notify().  The
1152       // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1153       // release the lock "O".  T2 resumes immediately after the ST of null into
1154       // _owner, above.  T2 notices that the EntryList is populated, so it
1155       // reacquires the lock and then finds itself on the EntryList.
1156       // Given all that, we have to tolerate the circumstance where "w" is
1157       // associated with Self.
1158       assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1159       ExitEpilog(Self, w);
1160       return;
1161     }
1162 
1163     // If we find that both _cxq and EntryList are null then just
1164     // re-run the exit protocol from the top.
1165     w = _cxq;
1166     if (w == NULL) continue;
1167 
1168     // Drain _cxq into EntryList - bulk transfer.
1169     // First, detach _cxq.
1170     // The following loop is tantamount to: w = swap(&cxq, NULL)
1171     for (;;) {
1172       assert(w != NULL, "Invariant");
1173       ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr(NULL, &_cxq, w);
1174       if (u == w) break;
1175       w = u;
1176     }
1177     TEVENT(Inflated exit - drain cxq into EntryList);
1178 
1179     assert(w != NULL, "invariant");
1180     assert(_EntryList == NULL, "invariant");
1181 
1182     // Convert the LIFO SLL anchored by _cxq into a DLL.
1183     // The list reorganization step operates in O(LENGTH(w)) time.
1184     // It's critical that this step operate quickly as
1185     // "Self" still holds the outer-lock, restricting parallelism
1186     // and effectively lengthening the critical section.
1187     // Invariant: s chases t chases u.
1188     // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1189     // we have faster access to the tail.
1190 
1191     if (QMode == 1) {
1192       // QMode == 1 : drain cxq to EntryList, reversing order
1193       // We also reverse the order of the list.
1194       ObjectWaiter * s = NULL;
1195       ObjectWaiter * t = w;
1196       ObjectWaiter * u = NULL;
1197       while (t != NULL) {
1198         guarantee(t->TState == ObjectWaiter::TS_CXQ, "invariant");
1199         t->TState = ObjectWaiter::TS_ENTER;
1200         u = t->_next;
1201         t->_prev = u;
1202         t->_next = s;
1203         s = t;
1204         t = u;
1205       }
1206       _EntryList  = s;
1207       assert(s != NULL, "invariant");
1208     } else {
1209       // QMode == 0 or QMode == 2
1210       _EntryList = w;
1211       ObjectWaiter * q = NULL;
1212       ObjectWaiter * p;
1213       for (p = w; p != NULL; p = p->_next) {
1214         guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1215         p->TState = ObjectWaiter::TS_ENTER;
1216         p->_prev = q;
1217         q = p;
1218       }
1219     }
1220 
1221     // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1222     // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1223 
1224     // See if we can abdicate to a spinner instead of waking a thread.
1225     // A primary goal of the implementation is to reduce the
1226     // context-switch rate.
1227     if (_succ != NULL) continue;
1228 
1229     w = _EntryList;
1230     if (w != NULL) {
1231       guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1232       ExitEpilog(Self, w);
1233       return;
1234     }
1235   }
1236 }
1237 
1238 // ExitSuspendEquivalent:
1239 // A faster alternate to handle_special_suspend_equivalent_condition()
1240 //
1241 // handle_special_suspend_equivalent_condition() unconditionally
1242 // acquires the SR_lock.  On some platforms uncontended MutexLocker()
1243 // operations have high latency.  Note that in ::enter() we call HSSEC
1244 // while holding the monitor, so we effectively lengthen the critical sections.
1245 //
1246 // There are a number of possible solutions:
1247 //
1248 // A.  To ameliorate the problem we might also defer state transitions
1249 //     to as late as possible -- just prior to parking.
1250 //     Given that, we'd call HSSEC after having returned from park(),
1251 //     but before attempting to acquire the monitor.  This is only a
1252 //     partial solution.  It avoids calling HSSEC while holding the
1253 //     monitor (good), but it still increases successor reacquisition latency --
1254 //     the interval between unparking a successor and the time the successor
1255 //     resumes and retries the lock.  See ReenterI(), which defers state transitions.
1256 //     If we use this technique we can also avoid EnterI()-exit() loop
1257 //     in ::enter() where we iteratively drop the lock and then attempt
1258 //     to reacquire it after suspending.
1259 //
1260 // B.  In the future we might fold all the suspend bits into a
1261 //     composite per-thread suspend flag and then update it with CAS().
1262 //     Alternately, a Dekker-like mechanism with multiple variables
1263 //     would suffice:
1264 //       ST Self->_suspend_equivalent = false
1265 //       MEMBAR
1266 //       LD Self_>_suspend_flags
1267 //
1268 // UPDATE 2007-10-6: since I've replaced the native Mutex/Monitor subsystem
1269 // with a more efficient implementation, the need to use "FastHSSEC" has
1270 // decreased. - Dave
1271 
1272 
1273 bool ObjectMonitor::ExitSuspendEquivalent(JavaThread * jSelf) {
1274   const int Mode = Knob_FastHSSEC;
1275   if (Mode && !jSelf->is_external_suspend()) {
1276     assert(jSelf->is_suspend_equivalent(), "invariant");
1277     jSelf->clear_suspend_equivalent();
1278     if (2 == Mode) OrderAccess::storeload();
1279     if (!jSelf->is_external_suspend()) return false;
1280     // We raced a suspension -- fall thru into the slow path
1281     TEVENT(ExitSuspendEquivalent - raced);
1282     jSelf->set_suspend_equivalent();
1283   }
1284   return jSelf->handle_special_suspend_equivalent_condition();
1285 }
1286 
1287 
1288 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) {
1289   assert(_owner == Self, "invariant");
1290 
1291   // Exit protocol:
1292   // 1. ST _succ = wakee
1293   // 2. membar #loadstore|#storestore;
1294   // 2. ST _owner = NULL
1295   // 3. unpark(wakee)
1296 
1297   _succ = Knob_SuccEnabled ? Wakee->_thread : NULL;
1298   ParkEvent * Trigger = Wakee->_event;
1299 
1300   // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1301   // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1302   // out-of-scope (non-extant).
1303   Wakee  = NULL;
1304 
1305   // Drop the lock
1306   OrderAccess::release_store_ptr(&_owner, NULL);
1307   OrderAccess::fence();                               // ST _owner vs LD in unpark()
1308 
1309   if (SafepointSynchronize::do_call_back()) {
1310     TEVENT(unpark before SAFEPOINT);
1311   }
1312 
1313   DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1314   Trigger->unpark();
1315 
1316   // Maintain stats and report events to JVMTI
1317   if (ObjectMonitor::_sync_Parks != NULL) {
1318     ObjectMonitor::_sync_Parks->inc();
1319   }
1320 }
1321 
1322 
1323 // -----------------------------------------------------------------------------
1324 // Class Loader deadlock handling.
1325 //
1326 // complete_exit exits a lock returning recursion count
1327 // complete_exit/reenter operate as a wait without waiting
1328 // complete_exit requires an inflated monitor
1329 // The _owner field is not always the Thread addr even with an
1330 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1331 // thread due to contention.
1332 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1333   Thread * const Self = THREAD;
1334   assert(Self->is_Java_thread(), "Must be Java thread!");
1335   JavaThread *jt = (JavaThread *)THREAD;
1336 
1337   DeferredInitialize();
1338 
1339   if (THREAD != _owner) {
1340     if (THREAD->is_lock_owned ((address)_owner)) {
1341       assert(_recursions == 0, "internal state error");
1342       _owner = THREAD;   // Convert from basiclock addr to Thread addr
1343       _recursions = 0;
1344     }
1345   }
1346 
1347   guarantee(Self == _owner, "complete_exit not owner");
1348   intptr_t save = _recursions; // record the old recursion count
1349   _recursions = 0;        // set the recursion level to be 0
1350   exit(true, Self);           // exit the monitor
1351   guarantee(_owner != Self, "invariant");
1352   return save;
1353 }
1354 
1355 // reenter() enters a lock and sets recursion count
1356 // complete_exit/reenter operate as a wait without waiting
1357 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1358   Thread * const Self = THREAD;
1359   assert(Self->is_Java_thread(), "Must be Java thread!");
1360   JavaThread *jt = (JavaThread *)THREAD;
1361 
1362   guarantee(_owner != Self, "reenter already owner");
1363   enter(THREAD);       // enter the monitor
1364   guarantee(_recursions == 0, "reenter recursion");
1365   _recursions = recursions;
1366   return;
1367 }
1368 
1369 
1370 // -----------------------------------------------------------------------------
1371 // A macro is used below because there may already be a pending
1372 // exception which should not abort the execution of the routines
1373 // which use this (which is why we don't put this into check_slow and
1374 // call it with a CHECK argument).
1375 
1376 #define CHECK_OWNER()                                                       \
1377   do {                                                                      \
1378     if (THREAD != _owner) {                                                 \
1379       if (THREAD->is_lock_owned((address) _owner)) {                        \
1380         _owner = THREAD;  /* Convert from basiclock addr to Thread addr */  \
1381         _recursions = 0;                                                    \
1382       } else {                                                              \
1383         TEVENT(Throw IMSX);                                                 \
1384         THROW(vmSymbols::java_lang_IllegalMonitorStateException());         \
1385       }                                                                     \
1386     }                                                                       \
1387   } while (false)
1388 
1389 // check_slow() is a misnomer.  It's called to simply to throw an IMSX exception.
1390 // TODO-FIXME: remove check_slow() -- it's likely dead.
1391 
1392 void ObjectMonitor::check_slow(TRAPS) {
1393   TEVENT(check_slow - throw IMSX);
1394   assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1395   THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1396 }
1397 
1398 static int Adjust(volatile int * adr, int dx) {
1399   int v;
1400   for (v = *adr; Atomic::cmpxchg(v + dx, adr, v) != v; v = *adr) /* empty */;
1401   return v;
1402 }
1403 
1404 // helper method for posting a monitor wait event
1405 void ObjectMonitor::post_monitor_wait_event(EventJavaMonitorWait* event,
1406                                             jlong notifier_tid,
1407                                             jlong timeout,
1408                                             bool timedout) {
1409   event->set_klass(((oop)this->object())->klass());
1410   event->set_timeout((TYPE_ULONG)timeout);
1411   event->set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
1412   event->set_notifier((TYPE_OSTHREAD)notifier_tid);
1413   event->set_timedOut((TYPE_BOOLEAN)timedout);
1414   event->commit();
1415 }
1416 
1417 // -----------------------------------------------------------------------------
1418 // Wait/Notify/NotifyAll
1419 //
1420 // Note: a subset of changes to ObjectMonitor::wait()
1421 // will need to be replicated in complete_exit
1422 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1423   Thread * const Self = THREAD;
1424   assert(Self->is_Java_thread(), "Must be Java thread!");
1425   JavaThread *jt = (JavaThread *)THREAD;
1426 
1427   DeferredInitialize();
1428 
1429   // Throw IMSX or IEX.
1430   CHECK_OWNER();
1431 
1432   EventJavaMonitorWait event;
1433 
1434   // check for a pending interrupt
1435   if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1436     // post monitor waited event.  Note that this is past-tense, we are done waiting.
1437     if (JvmtiExport::should_post_monitor_waited()) {
1438       // Note: 'false' parameter is passed here because the
1439       // wait was not timed out due to thread interrupt.
1440       JvmtiExport::post_monitor_waited(jt, this, false);
1441 
1442       // In this short circuit of the monitor wait protocol, the
1443       // current thread never drops ownership of the monitor and
1444       // never gets added to the wait queue so the current thread
1445       // cannot be made the successor. This means that the
1446       // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1447       // consume an unpark() meant for the ParkEvent associated with
1448       // this ObjectMonitor.
1449     }
1450     if (event.should_commit()) {
1451       post_monitor_wait_event(&event, 0, millis, false);
1452     }
1453     TEVENT(Wait - Throw IEX);
1454     THROW(vmSymbols::java_lang_InterruptedException());
1455     return;
1456   }
1457 
1458   TEVENT(Wait);
1459 
1460   assert(Self->_Stalled == 0, "invariant");
1461   Self->_Stalled = intptr_t(this);
1462   jt->set_current_waiting_monitor(this);
1463 
1464   // create a node to be put into the queue
1465   // Critically, after we reset() the event but prior to park(), we must check
1466   // for a pending interrupt.
1467   ObjectWaiter node(Self);
1468   node.TState = ObjectWaiter::TS_WAIT;
1469   Self->_ParkEvent->reset();
1470   OrderAccess::fence();          // ST into Event; membar ; LD interrupted-flag
1471 
1472   // Enter the waiting queue, which is a circular doubly linked list in this case
1473   // but it could be a priority queue or any data structure.
1474   // _WaitSetLock protects the wait queue.  Normally the wait queue is accessed only
1475   // by the the owner of the monitor *except* in the case where park()
1476   // returns because of a timeout of interrupt.  Contention is exceptionally rare
1477   // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1478 
1479   Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1480   AddWaiter(&node);
1481   Thread::SpinRelease(&_WaitSetLock);
1482 
1483   if ((SyncFlags & 4) == 0) {
1484     _Responsible = NULL;
1485   }
1486   intptr_t save = _recursions; // record the old recursion count
1487   _waiters++;                  // increment the number of waiters
1488   _recursions = 0;             // set the recursion level to be 1
1489   exit(true, Self);                    // exit the monitor
1490   guarantee(_owner != Self, "invariant");
1491 
1492   // The thread is on the WaitSet list - now park() it.
1493   // On MP systems it's conceivable that a brief spin before we park
1494   // could be profitable.
1495   //
1496   // TODO-FIXME: change the following logic to a loop of the form
1497   //   while (!timeout && !interrupted && _notified == 0) park()
1498 
1499   int ret = OS_OK;
1500   int WasNotified = 0;
1501   { // State transition wrappers
1502     OSThread* osthread = Self->osthread();
1503     OSThreadWaitState osts(osthread, true);
1504     {
1505       ThreadBlockInVM tbivm(jt);
1506       // Thread is in thread_blocked state and oop access is unsafe.
1507       jt->set_suspend_equivalent();
1508 
1509       if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1510         // Intentionally empty
1511       } else if (node._notified == 0) {
1512         if (millis <= 0) {
1513           Self->_ParkEvent->park();
1514         } else {
1515           ret = Self->_ParkEvent->park(millis);
1516         }
1517       }
1518 
1519       // were we externally suspended while we were waiting?
1520       if (ExitSuspendEquivalent (jt)) {
1521         // TODO-FIXME: add -- if succ == Self then succ = null.
1522         jt->java_suspend_self();
1523       }
1524 
1525     } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1526 
1527     // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1528     // from the WaitSet to the EntryList.
1529     // See if we need to remove Node from the WaitSet.
1530     // We use double-checked locking to avoid grabbing _WaitSetLock
1531     // if the thread is not on the wait queue.
1532     //
1533     // Note that we don't need a fence before the fetch of TState.
1534     // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1535     // written by the is thread. (perhaps the fetch might even be satisfied
1536     // by a look-aside into the processor's own store buffer, although given
1537     // the length of the code path between the prior ST and this load that's
1538     // highly unlikely).  If the following LD fetches a stale TS_WAIT value
1539     // then we'll acquire the lock and then re-fetch a fresh TState value.
1540     // That is, we fail toward safety.
1541 
1542     if (node.TState == ObjectWaiter::TS_WAIT) {
1543       Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1544       if (node.TState == ObjectWaiter::TS_WAIT) {
1545         DequeueSpecificWaiter(&node);       // unlink from WaitSet
1546         assert(node._notified == 0, "invariant");
1547         node.TState = ObjectWaiter::TS_RUN;
1548       }
1549       Thread::SpinRelease(&_WaitSetLock);
1550     }
1551 
1552     // The thread is now either on off-list (TS_RUN),
1553     // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1554     // The Node's TState variable is stable from the perspective of this thread.
1555     // No other threads will asynchronously modify TState.
1556     guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1557     OrderAccess::loadload();
1558     if (_succ == Self) _succ = NULL;
1559     WasNotified = node._notified;
1560 
1561     // Reentry phase -- reacquire the monitor.
1562     // re-enter contended monitor after object.wait().
1563     // retain OBJECT_WAIT state until re-enter successfully completes
1564     // Thread state is thread_in_vm and oop access is again safe,
1565     // although the raw address of the object may have changed.
1566     // (Don't cache naked oops over safepoints, of course).
1567 
1568     // post monitor waited event. Note that this is past-tense, we are done waiting.
1569     if (JvmtiExport::should_post_monitor_waited()) {
1570       JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1571 
1572       if (node._notified != 0 && _succ == Self) {
1573         // In this part of the monitor wait-notify-reenter protocol it
1574         // is possible (and normal) for another thread to do a fastpath
1575         // monitor enter-exit while this thread is still trying to get
1576         // to the reenter portion of the protocol.
1577         //
1578         // The ObjectMonitor was notified and the current thread is
1579         // the successor which also means that an unpark() has already
1580         // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1581         // consume the unpark() that was done when the successor was
1582         // set because the same ParkEvent is shared between Java
1583         // monitors and JVM/TI RawMonitors (for now).
1584         //
1585         // We redo the unpark() to ensure forward progress, i.e., we
1586         // don't want all pending threads hanging (parked) with none
1587         // entering the unlocked monitor.
1588         node._event->unpark();
1589       }
1590     }
1591 
1592     if (event.should_commit()) {
1593       post_monitor_wait_event(&event, node._notifier_tid, millis, ret == OS_TIMEOUT);
1594     }
1595 
1596     OrderAccess::fence();
1597 
1598     assert(Self->_Stalled != 0, "invariant");
1599     Self->_Stalled = 0;
1600 
1601     assert(_owner != Self, "invariant");
1602     ObjectWaiter::TStates v = node.TState;
1603     if (v == ObjectWaiter::TS_RUN) {
1604       enter(Self);
1605     } else {
1606       guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1607       ReenterI(Self, &node);
1608       node.wait_reenter_end(this);
1609     }
1610 
1611     // Self has reacquired the lock.
1612     // Lifecycle - the node representing Self must not appear on any queues.
1613     // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1614     // want residual elements associated with this thread left on any lists.
1615     guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1616     assert(_owner == Self, "invariant");
1617     assert(_succ != Self, "invariant");
1618   } // OSThreadWaitState()
1619 
1620   jt->set_current_waiting_monitor(NULL);
1621 
1622   guarantee(_recursions == 0, "invariant");
1623   _recursions = save;     // restore the old recursion count
1624   _waiters--;             // decrement the number of waiters
1625 
1626   // Verify a few postconditions
1627   assert(_owner == Self, "invariant");
1628   assert(_succ != Self, "invariant");
1629   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
1630 
1631   if (SyncFlags & 32) {
1632     OrderAccess::fence();
1633   }
1634 
1635   // check if the notification happened
1636   if (!WasNotified) {
1637     // no, it could be timeout or Thread.interrupt() or both
1638     // check for interrupt event, otherwise it is timeout
1639     if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1640       TEVENT(Wait - throw IEX from epilog);
1641       THROW(vmSymbols::java_lang_InterruptedException());
1642     }
1643   }
1644 
1645   // NOTE: Spurious wake up will be consider as timeout.
1646   // Monitor notify has precedence over thread interrupt.
1647 }
1648 
1649 
1650 // Consider:
1651 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1652 // then instead of transferring a thread from the WaitSet to the EntryList
1653 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1654 
1655 int ObjectMonitor::INotify(Thread * Self) {
1656   const int policy = Knob_MoveNotifyee;
1657 
1658   Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1659   ObjectWaiter * iterator = DequeueWaiter();
1660   if (iterator != NULL) {
1661     TEVENT(Notify1 - Transfer);
1662     guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1663     guarantee(iterator->_notified == 0, "invariant");
1664     // Disposition - what might we do with iterator ?
1665     // a.  add it directly to the EntryList - either tail or head.
1666     // b.  push it onto the front of the _cxq.
1667     // For now we use (a).
1668     if (policy != 4) {
1669       iterator->TState = ObjectWaiter::TS_ENTER;
1670     }
1671     iterator->_notified = 1;
1672     iterator->_notifier_tid = Self->osthread()->thread_id();
1673 
1674     ObjectWaiter * list = _EntryList;
1675     if (list != NULL) {
1676       assert(list->_prev == NULL, "invariant");
1677       assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1678       assert(list != iterator, "invariant");
1679     }
1680 
1681     if (policy == 0) {       // prepend to EntryList
1682       if (list == NULL) {
1683         iterator->_next = iterator->_prev = NULL;
1684         _EntryList = iterator;
1685       } else {
1686         list->_prev = iterator;
1687         iterator->_next = list;
1688         iterator->_prev = NULL;
1689         _EntryList = iterator;
1690       }
1691     } else if (policy == 1) {      // append to EntryList
1692       if (list == NULL) {
1693         iterator->_next = iterator->_prev = NULL;
1694         _EntryList = iterator;
1695       } else {
1696         // CONSIDER:  finding the tail currently requires a linear-time walk of
1697         // the EntryList.  We can make tail access constant-time by converting to
1698         // a CDLL instead of using our current DLL.
1699         ObjectWaiter * tail;
1700         for (tail = list; tail->_next != NULL; tail = tail->_next) /* empty */;
1701         assert(tail != NULL && tail->_next == NULL, "invariant");
1702         tail->_next = iterator;
1703         iterator->_prev = tail;
1704         iterator->_next = NULL;
1705       }
1706     } else if (policy == 2) {      // prepend to cxq
1707       // prepend to cxq
1708       if (list == NULL) {
1709         iterator->_next = iterator->_prev = NULL;
1710         _EntryList = iterator;
1711       } else {
1712         iterator->TState = ObjectWaiter::TS_CXQ;
1713         for (;;) {
1714           ObjectWaiter * front = _cxq;
1715           iterator->_next = front;
1716           if (Atomic::cmpxchg_ptr(iterator, &_cxq, front) == front) {
1717             break;
1718           }
1719         }
1720       }
1721     } else if (policy == 3) {      // append to cxq
1722       iterator->TState = ObjectWaiter::TS_CXQ;
1723       for (;;) {
1724         ObjectWaiter * tail = _cxq;
1725         if (tail == NULL) {
1726           iterator->_next = NULL;
1727           if (Atomic::cmpxchg_ptr(iterator, &_cxq, NULL) == NULL) {
1728             break;
1729           }
1730         } else {
1731           while (tail->_next != NULL) tail = tail->_next;
1732           tail->_next = iterator;
1733           iterator->_prev = tail;
1734           iterator->_next = NULL;
1735           break;
1736         }
1737       }
1738     } else {
1739       ParkEvent * ev = iterator->_event;
1740       iterator->TState = ObjectWaiter::TS_RUN;
1741       OrderAccess::fence();
1742       ev->unpark();
1743     }
1744 
1745     // _WaitSetLock protects the wait queue, not the EntryList.  We could
1746     // move the add-to-EntryList operation, above, outside the critical section
1747     // protected by _WaitSetLock.  In practice that's not useful.  With the
1748     // exception of  wait() timeouts and interrupts the monitor owner
1749     // is the only thread that grabs _WaitSetLock.  There's almost no contention
1750     // on _WaitSetLock so it's not profitable to reduce the length of the
1751     // critical section.
1752 
1753     if (policy < 4) {
1754       iterator->wait_reenter_begin(this);
1755     }
1756   }
1757   Thread::SpinRelease(&_WaitSetLock);
1758 
1759   return 0;
1760 }
1761 
1762 // Consider: a non-uncommon synchronization bug is to use notify() when notifyAll()
1763 // is more appropriate, potentially resulting in lost wakeups and stranded threads.
1764 // A useful diagnostic option is to force all notify() operations to behave
1765 // as notifyAll().  We can also detect many such problems with MinimumWait.
1766 // When MinimumWait is set to a small non-zero timeout value and the program
1767 // does not hang whereas it did absent MininumWait, that suggests a lost wakeup bug.
1768 
1769 void ObjectMonitor::notify(TRAPS) {
1770   CHECK_OWNER();
1771   if (_WaitSet == NULL) {
1772     TEVENT(Empty-Notify);
1773     return;
1774   }
1775   DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1776   INotify(THREAD);
1777   if (ObjectMonitor::_sync_Notifications != NULL) {
1778     ObjectMonitor::_sync_Notifications->inc(1);
1779   }
1780 }
1781 
1782 
1783 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1784 // from the waitset to the EntryList. This could be done more efficiently with a
1785 // single bulk transfer but in practice it's not time-critical. Beware too,
1786 // that in prepend-mode we invert the order of the waiters. Let's say that the
1787 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1788 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1789 
1790 void ObjectMonitor::notifyAll(TRAPS) {
1791   CHECK_OWNER();
1792   if (_WaitSet == NULL) {
1793     TEVENT(Empty-NotifyAll);
1794     return;
1795   }
1796 
1797   DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1798   int tally = 0;
1799   while (_WaitSet != NULL) {
1800     tally++;
1801     INotify(THREAD);
1802   }
1803 
1804   if (tally != 0 && ObjectMonitor::_sync_Notifications != NULL) {
1805     ObjectMonitor::_sync_Notifications->inc(tally);
1806   }
1807 }
1808 
1809 // -----------------------------------------------------------------------------
1810 // Adaptive Spinning Support
1811 //
1812 // Adaptive spin-then-block - rational spinning
1813 //
1814 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1815 // algorithm.  On high order SMP systems it would be better to start with
1816 // a brief global spin and then revert to spinning locally.  In the spirit of MCS/CLH,
1817 // a contending thread could enqueue itself on the cxq and then spin locally
1818 // on a thread-specific variable such as its ParkEvent._Event flag.
1819 // That's left as an exercise for the reader.  Note that global spinning is
1820 // not problematic on Niagara, as the L2 cache serves the interconnect and
1821 // has both low latency and massive bandwidth.
1822 //
1823 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1824 // acquisition attempts where we opt to spin --  at 100% and vary the spin count
1825 // (duration) or we can fix the count at approximately the duration of
1826 // a context switch and vary the frequency.   Of course we could also
1827 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1828 // See http://j2se.east/~dice/PERSIST/040824-AdaptiveSpinning.html.
1829 //
1830 // This implementation varies the duration "D", where D varies with
1831 // the success rate of recent spin attempts. (D is capped at approximately
1832 // length of a round-trip context switch).  The success rate for recent
1833 // spin attempts is a good predictor of the success rate of future spin
1834 // attempts.  The mechanism adapts automatically to varying critical
1835 // section length (lock modality), system load and degree of parallelism.
1836 // D is maintained per-monitor in _SpinDuration and is initialized
1837 // optimistically.  Spin frequency is fixed at 100%.
1838 //
1839 // Note that _SpinDuration is volatile, but we update it without locks
1840 // or atomics.  The code is designed so that _SpinDuration stays within
1841 // a reasonable range even in the presence of races.  The arithmetic
1842 // operations on _SpinDuration are closed over the domain of legal values,
1843 // so at worst a race will install and older but still legal value.
1844 // At the very worst this introduces some apparent non-determinism.
1845 // We might spin when we shouldn't or vice-versa, but since the spin
1846 // count are relatively short, even in the worst case, the effect is harmless.
1847 //
1848 // Care must be taken that a low "D" value does not become an
1849 // an absorbing state.  Transient spinning failures -- when spinning
1850 // is overall profitable -- should not cause the system to converge
1851 // on low "D" values.  We want spinning to be stable and predictable
1852 // and fairly responsive to change and at the same time we don't want
1853 // it to oscillate, become metastable, be "too" non-deterministic,
1854 // or converge on or enter undesirable stable absorbing states.
1855 //
1856 // We implement a feedback-based control system -- using past behavior
1857 // to predict future behavior.  We face two issues: (a) if the
1858 // input signal is random then the spin predictor won't provide optimal
1859 // results, and (b) if the signal frequency is too high then the control
1860 // system, which has some natural response lag, will "chase" the signal.
1861 // (b) can arise from multimodal lock hold times.  Transient preemption
1862 // can also result in apparent bimodal lock hold times.
1863 // Although sub-optimal, neither condition is particularly harmful, as
1864 // in the worst-case we'll spin when we shouldn't or vice-versa.
1865 // The maximum spin duration is rather short so the failure modes aren't bad.
1866 // To be conservative, I've tuned the gain in system to bias toward
1867 // _not spinning.  Relatedly, the system can sometimes enter a mode where it
1868 // "rings" or oscillates between spinning and not spinning.  This happens
1869 // when spinning is just on the cusp of profitability, however, so the
1870 // situation is not dire.  The state is benign -- there's no need to add
1871 // hysteresis control to damp the transition rate between spinning and
1872 // not spinning.
1873 
1874 intptr_t ObjectMonitor::SpinCallbackArgument = 0;
1875 int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL;
1876 
1877 // Spinning: Fixed frequency (100%), vary duration
1878 
1879 
1880 int ObjectMonitor::TrySpin_VaryDuration(Thread * Self) {
1881   // Dumb, brutal spin.  Good for comparative measurements against adaptive spinning.
1882   int ctr = Knob_FixedSpin;
1883   if (ctr != 0) {
1884     while (--ctr >= 0) {
1885       if (TryLock(Self) > 0) return 1;
1886       SpinPause();
1887     }
1888     return 0;
1889   }
1890 
1891   for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1892     if (TryLock(Self) > 0) {
1893       // Increase _SpinDuration ...
1894       // Note that we don't clamp SpinDuration precisely at SpinLimit.
1895       // Raising _SpurDuration to the poverty line is key.
1896       int x = _SpinDuration;
1897       if (x < Knob_SpinLimit) {
1898         if (x < Knob_Poverty) x = Knob_Poverty;
1899         _SpinDuration = x + Knob_BonusB;
1900       }
1901       return 1;
1902     }
1903     SpinPause();
1904   }
1905 
1906   // Admission control - verify preconditions for spinning
1907   //
1908   // We always spin a little bit, just to prevent _SpinDuration == 0 from
1909   // becoming an absorbing state.  Put another way, we spin briefly to
1910   // sample, just in case the system load, parallelism, contention, or lock
1911   // modality changed.
1912   //
1913   // Consider the following alternative:
1914   // Periodically set _SpinDuration = _SpinLimit and try a long/full
1915   // spin attempt.  "Periodically" might mean after a tally of
1916   // the # of failed spin attempts (or iterations) reaches some threshold.
1917   // This takes us into the realm of 1-out-of-N spinning, where we
1918   // hold the duration constant but vary the frequency.
1919 
1920   ctr = _SpinDuration;
1921   if (ctr < Knob_SpinBase) ctr = Knob_SpinBase;
1922   if (ctr <= 0) return 0;
1923 
1924   if (Knob_SuccRestrict && _succ != NULL) return 0;
1925   if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
1926     TEVENT(Spin abort - notrunnable [TOP]);
1927     return 0;
1928   }
1929 
1930   int MaxSpin = Knob_MaxSpinners;
1931   if (MaxSpin >= 0) {
1932     if (_Spinner > MaxSpin) {
1933       TEVENT(Spin abort -- too many spinners);
1934       return 0;
1935     }
1936     // Slightly racy, but benign ...
1937     Adjust(&_Spinner, 1);
1938   }
1939 
1940   // We're good to spin ... spin ingress.
1941   // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1942   // when preparing to LD...CAS _owner, etc and the CAS is likely
1943   // to succeed.
1944   int hits    = 0;
1945   int msk     = 0;
1946   int caspty  = Knob_CASPenalty;
1947   int oxpty   = Knob_OXPenalty;
1948   int sss     = Knob_SpinSetSucc;
1949   if (sss && _succ == NULL) _succ = Self;
1950   Thread * prv = NULL;
1951 
1952   // There are three ways to exit the following loop:
1953   // 1.  A successful spin where this thread has acquired the lock.
1954   // 2.  Spin failure with prejudice
1955   // 3.  Spin failure without prejudice
1956 
1957   while (--ctr >= 0) {
1958 
1959     // Periodic polling -- Check for pending GC
1960     // Threads may spin while they're unsafe.
1961     // We don't want spinning threads to delay the JVM from reaching
1962     // a stop-the-world safepoint or to steal cycles from GC.
1963     // If we detect a pending safepoint we abort in order that
1964     // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1965     // this thread, if safe, doesn't steal cycles from GC.
1966     // This is in keeping with the "no loitering in runtime" rule.
1967     // We periodically check to see if there's a safepoint pending.
1968     if ((ctr & 0xFF) == 0) {
1969       if (SafepointSynchronize::do_call_back()) {
1970         TEVENT(Spin: safepoint);
1971         goto Abort;           // abrupt spin egress
1972       }
1973       if (Knob_UsePause & 1) SpinPause();
1974 
1975       int (*scb)(intptr_t,int) = SpinCallbackFunction;
1976       if (hits > 50 && scb != NULL) {
1977         int abend = (*scb)(SpinCallbackArgument, 0);
1978       }
1979     }
1980 
1981     if (Knob_UsePause & 2) SpinPause();
1982 
1983     // Exponential back-off ...  Stay off the bus to reduce coherency traffic.
1984     // This is useful on classic SMP systems, but is of less utility on
1985     // N1-style CMT platforms.
1986     //
1987     // Trade-off: lock acquisition latency vs coherency bandwidth.
1988     // Lock hold times are typically short.  A histogram
1989     // of successful spin attempts shows that we usually acquire
1990     // the lock early in the spin.  That suggests we want to
1991     // sample _owner frequently in the early phase of the spin,
1992     // but then back-off and sample less frequently as the spin
1993     // progresses.  The back-off makes a good citizen on SMP big
1994     // SMP systems.  Oversampling _owner can consume excessive
1995     // coherency bandwidth.  Relatedly, if we _oversample _owner we
1996     // can inadvertently interfere with the the ST m->owner=null.
1997     // executed by the lock owner.
1998     if (ctr & msk) continue;
1999     ++hits;
2000     if ((hits & 0xF) == 0) {
2001       // The 0xF, above, corresponds to the exponent.
2002       // Consider: (msk+1)|msk
2003       msk = ((msk << 2)|3) & BackOffMask;
2004     }
2005 
2006     // Probe _owner with TATAS
2007     // If this thread observes the monitor transition or flicker
2008     // from locked to unlocked to locked, then the odds that this
2009     // thread will acquire the lock in this spin attempt go down
2010     // considerably.  The same argument applies if the CAS fails
2011     // or if we observe _owner change from one non-null value to
2012     // another non-null value.   In such cases we might abort
2013     // the spin without prejudice or apply a "penalty" to the
2014     // spin count-down variable "ctr", reducing it by 100, say.
2015 
2016     Thread * ox = (Thread *) _owner;
2017     if (ox == NULL) {
2018       ox = (Thread *) Atomic::cmpxchg_ptr(Self, &_owner, NULL);
2019       if (ox == NULL) {
2020         // The CAS succeeded -- this thread acquired ownership
2021         // Take care of some bookkeeping to exit spin state.
2022         if (sss && _succ == Self) {
2023           _succ = NULL;
2024         }
2025         if (MaxSpin > 0) Adjust(&_Spinner, -1);
2026 
2027         // Increase _SpinDuration :
2028         // The spin was successful (profitable) so we tend toward
2029         // longer spin attempts in the future.
2030         // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2031         // If we acquired the lock early in the spin cycle it
2032         // makes sense to increase _SpinDuration proportionally.
2033         // Note that we don't clamp SpinDuration precisely at SpinLimit.
2034         int x = _SpinDuration;
2035         if (x < Knob_SpinLimit) {
2036           if (x < Knob_Poverty) x = Knob_Poverty;
2037           _SpinDuration = x + Knob_Bonus;
2038         }
2039         return 1;
2040       }
2041 
2042       // The CAS failed ... we can take any of the following actions:
2043       // * penalize: ctr -= Knob_CASPenalty
2044       // * exit spin with prejudice -- goto Abort;
2045       // * exit spin without prejudice.
2046       // * Since CAS is high-latency, retry again immediately.
2047       prv = ox;
2048       TEVENT(Spin: cas failed);
2049       if (caspty == -2) break;
2050       if (caspty == -1) goto Abort;
2051       ctr -= caspty;
2052       continue;
2053     }
2054 
2055     // Did lock ownership change hands ?
2056     if (ox != prv && prv != NULL) {
2057       TEVENT(spin: Owner changed)
2058       if (oxpty == -2) break;
2059       if (oxpty == -1) goto Abort;
2060       ctr -= oxpty;
2061     }
2062     prv = ox;
2063 
2064     // Abort the spin if the owner is not executing.
2065     // The owner must be executing in order to drop the lock.
2066     // Spinning while the owner is OFFPROC is idiocy.
2067     // Consider: ctr -= RunnablePenalty ;
2068     if (Knob_OState && NotRunnable (Self, ox)) {
2069       TEVENT(Spin abort - notrunnable);
2070       goto Abort;
2071     }
2072     if (sss && _succ == NULL) _succ = Self;
2073   }
2074 
2075   // Spin failed with prejudice -- reduce _SpinDuration.
2076   // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2077   // AIMD is globally stable.
2078   TEVENT(Spin failure);
2079   {
2080     int x = _SpinDuration;
2081     if (x > 0) {
2082       // Consider an AIMD scheme like: x -= (x >> 3) + 100
2083       // This is globally sample and tends to damp the response.
2084       x -= Knob_Penalty;
2085       if (x < 0) x = 0;
2086       _SpinDuration = x;
2087     }
2088   }
2089 
2090  Abort:
2091   if (MaxSpin >= 0) Adjust(&_Spinner, -1);
2092   if (sss && _succ == Self) {
2093     _succ = NULL;
2094     // Invariant: after setting succ=null a contending thread
2095     // must recheck-retry _owner before parking.  This usually happens
2096     // in the normal usage of TrySpin(), but it's safest
2097     // to make TrySpin() as foolproof as possible.
2098     OrderAccess::fence();
2099     if (TryLock(Self) > 0) return 1;
2100   }
2101   return 0;
2102 }
2103 
2104 // NotRunnable() -- informed spinning
2105 //
2106 // Don't bother spinning if the owner is not eligible to drop the lock.
2107 // Peek at the owner's schedctl.sc_state and Thread._thread_values and
2108 // spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2109 // The thread must be runnable in order to drop the lock in timely fashion.
2110 // If the _owner is not runnable then spinning will not likely be
2111 // successful (profitable).
2112 //
2113 // Beware -- the thread referenced by _owner could have died
2114 // so a simply fetch from _owner->_thread_state might trap.
2115 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2116 // Because of the lifecycle issues the schedctl and _thread_state values
2117 // observed by NotRunnable() might be garbage.  NotRunnable must
2118 // tolerate this and consider the observed _thread_state value
2119 // as advisory.
2120 //
2121 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2122 // a thread pointer.
2123 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2124 // with the LSB of _owner.  Another option would be to probablistically probe
2125 // the putative _owner->TypeTag value.
2126 //
2127 // Checking _thread_state isn't perfect.  Even if the thread is
2128 // in_java it might be blocked on a page-fault or have been preempted
2129 // and sitting on a ready/dispatch queue.  _thread state in conjunction
2130 // with schedctl.sc_state gives us a good picture of what the
2131 // thread is doing, however.
2132 //
2133 // TODO: check schedctl.sc_state.
2134 // We'll need to use SafeFetch32() to read from the schedctl block.
2135 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/
2136 //
2137 // The return value from NotRunnable() is *advisory* -- the
2138 // result is based on sampling and is not necessarily coherent.
2139 // The caller must tolerate false-negative and false-positive errors.
2140 // Spinning, in general, is probabilistic anyway.
2141 
2142 
2143 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) {
2144   // Check ox->TypeTag == 2BAD.
2145   if (ox == NULL) return 0;
2146 
2147   // Avoid transitive spinning ...
2148   // Say T1 spins or blocks trying to acquire L.  T1._Stalled is set to L.
2149   // Immediately after T1 acquires L it's possible that T2, also
2150   // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2151   // This occurs transiently after T1 acquired L but before
2152   // T1 managed to clear T1.Stalled.  T2 does not need to abort
2153   // its spin in this circumstance.
2154   intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
2155 
2156   if (BlockedOn == 1) return 1;
2157   if (BlockedOn != 0) {
2158     return BlockedOn != intptr_t(this) && _owner == ox;
2159   }
2160 
2161   assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant");
2162   int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);;
2163   // consider also: jst != _thread_in_Java -- but that's overspecific.
2164   return jst == _thread_blocked || jst == _thread_in_native;
2165 }
2166 
2167 
2168 // -----------------------------------------------------------------------------
2169 // WaitSet management ...
2170 
2171 ObjectWaiter::ObjectWaiter(Thread* thread) {
2172   _next     = NULL;
2173   _prev     = NULL;
2174   _notified = 0;
2175   TState    = TS_RUN;
2176   _thread   = thread;
2177   _event    = thread->_ParkEvent;
2178   _active   = false;
2179   assert(_event != NULL, "invariant");
2180 }
2181 
2182 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2183   JavaThread *jt = (JavaThread *)this->_thread;
2184   _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2185 }
2186 
2187 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2188   JavaThread *jt = (JavaThread *)this->_thread;
2189   JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2190 }
2191 
2192 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2193   assert(node != NULL, "should not dequeue NULL node");
2194   assert(node->_prev == NULL, "node already in list");
2195   assert(node->_next == NULL, "node already in list");
2196   // put node at end of queue (circular doubly linked list)
2197   if (_WaitSet == NULL) {
2198     _WaitSet = node;
2199     node->_prev = node;
2200     node->_next = node;
2201   } else {
2202     ObjectWaiter* head = _WaitSet;
2203     ObjectWaiter* tail = head->_prev;
2204     assert(tail->_next == head, "invariant check");
2205     tail->_next = node;
2206     head->_prev = node;
2207     node->_next = head;
2208     node->_prev = tail;
2209   }
2210 }
2211 
2212 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2213   // dequeue the very first waiter
2214   ObjectWaiter* waiter = _WaitSet;
2215   if (waiter) {
2216     DequeueSpecificWaiter(waiter);
2217   }
2218   return waiter;
2219 }
2220 
2221 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2222   assert(node != NULL, "should not dequeue NULL node");
2223   assert(node->_prev != NULL, "node already removed from list");
2224   assert(node->_next != NULL, "node already removed from list");
2225   // when the waiter has woken up because of interrupt,
2226   // timeout or other spurious wake-up, dequeue the
2227   // waiter from waiting list
2228   ObjectWaiter* next = node->_next;
2229   if (next == node) {
2230     assert(node->_prev == node, "invariant check");
2231     _WaitSet = NULL;
2232   } else {
2233     ObjectWaiter* prev = node->_prev;
2234     assert(prev->_next == node, "invariant check");
2235     assert(next->_prev == node, "invariant check");
2236     next->_prev = prev;
2237     prev->_next = next;
2238     if (_WaitSet == node) {
2239       _WaitSet = next;
2240     }
2241   }
2242   node->_next = NULL;
2243   node->_prev = NULL;
2244 }
2245 
2246 // -----------------------------------------------------------------------------
2247 // PerfData support
2248 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts       = NULL;
2249 PerfCounter * ObjectMonitor::_sync_FutileWakeups               = NULL;
2250 PerfCounter * ObjectMonitor::_sync_Parks                       = NULL;
2251 PerfCounter * ObjectMonitor::_sync_EmptyNotifications          = NULL;
2252 PerfCounter * ObjectMonitor::_sync_Notifications               = NULL;
2253 PerfCounter * ObjectMonitor::_sync_PrivateA                    = NULL;
2254 PerfCounter * ObjectMonitor::_sync_PrivateB                    = NULL;
2255 PerfCounter * ObjectMonitor::_sync_SlowExit                    = NULL;
2256 PerfCounter * ObjectMonitor::_sync_SlowEnter                   = NULL;
2257 PerfCounter * ObjectMonitor::_sync_SlowNotify                  = NULL;
2258 PerfCounter * ObjectMonitor::_sync_SlowNotifyAll               = NULL;
2259 PerfCounter * ObjectMonitor::_sync_FailedSpins                 = NULL;
2260 PerfCounter * ObjectMonitor::_sync_SuccessfulSpins             = NULL;
2261 PerfCounter * ObjectMonitor::_sync_MonInCirculation            = NULL;
2262 PerfCounter * ObjectMonitor::_sync_MonScavenged                = NULL;
2263 PerfCounter * ObjectMonitor::_sync_Inflations                  = NULL;
2264 PerfCounter * ObjectMonitor::_sync_Deflations                  = NULL;
2265 PerfLongVariable * ObjectMonitor::_sync_MonExtant              = NULL;
2266 
2267 // One-shot global initialization for the sync subsystem.
2268 // We could also defer initialization and initialize on-demand
2269 // the first time we call inflate().  Initialization would
2270 // be protected - like so many things - by the MonitorCache_lock.
2271 
2272 void ObjectMonitor::Initialize() {
2273   static int InitializationCompleted = 0;
2274   assert(InitializationCompleted == 0, "invariant");
2275   InitializationCompleted = 1;
2276   if (UsePerfData) {
2277     EXCEPTION_MARK;
2278 #define NEWPERFCOUNTER(n)                                                \
2279   {                                                                      \
2280     n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,  \
2281                                         CHECK);                          \
2282   }
2283 #define NEWPERFVARIABLE(n)                                                \
2284   {                                                                       \
2285     n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,  \
2286                                          CHECK);                          \
2287   }
2288     NEWPERFCOUNTER(_sync_Inflations);
2289     NEWPERFCOUNTER(_sync_Deflations);
2290     NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2291     NEWPERFCOUNTER(_sync_FutileWakeups);
2292     NEWPERFCOUNTER(_sync_Parks);
2293     NEWPERFCOUNTER(_sync_EmptyNotifications);
2294     NEWPERFCOUNTER(_sync_Notifications);
2295     NEWPERFCOUNTER(_sync_SlowEnter);
2296     NEWPERFCOUNTER(_sync_SlowExit);
2297     NEWPERFCOUNTER(_sync_SlowNotify);
2298     NEWPERFCOUNTER(_sync_SlowNotifyAll);
2299     NEWPERFCOUNTER(_sync_FailedSpins);
2300     NEWPERFCOUNTER(_sync_SuccessfulSpins);
2301     NEWPERFCOUNTER(_sync_PrivateA);
2302     NEWPERFCOUNTER(_sync_PrivateB);
2303     NEWPERFCOUNTER(_sync_MonInCirculation);
2304     NEWPERFCOUNTER(_sync_MonScavenged);
2305     NEWPERFVARIABLE(_sync_MonExtant);
2306 #undef NEWPERFCOUNTER
2307 #undef NEWPERFVARIABLE
2308   }
2309 }
2310 
2311 static char * kvGet(char * kvList, const char * Key) {
2312   if (kvList == NULL) return NULL;
2313   size_t n = strlen(Key);
2314   char * Search;
2315   for (Search = kvList; *Search; Search += strlen(Search) + 1) {
2316     if (strncmp (Search, Key, n) == 0) {
2317       if (Search[n] == '=') return Search + n + 1;
2318       if (Search[n] == 0)   return(char *) "1";
2319     }
2320   }
2321   return NULL;
2322 }
2323 
2324 static int kvGetInt(char * kvList, const char * Key, int Default) {
2325   char * v = kvGet(kvList, Key);
2326   int rslt = v ? ::strtol(v, NULL, 0) : Default;
2327   if (Knob_ReportSettings && v != NULL) {
2328     ::printf ("  SyncKnob: %s %d(%d)\n", Key, rslt, Default) ;
2329     ::fflush(stdout);
2330   }
2331   return rslt;
2332 }
2333 
2334 void ObjectMonitor::DeferredInitialize() {
2335   if (InitDone > 0) return;
2336   if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2337     while (InitDone != 1) /* empty */;
2338     return;
2339   }
2340 
2341   // One-shot global initialization ...
2342   // The initialization is idempotent, so we don't need locks.
2343   // In the future consider doing this via os::init_2().
2344   // SyncKnobs consist of <Key>=<Value> pairs in the style
2345   // of environment variables.  Start by converting ':' to NUL.
2346 
2347   if (SyncKnobs == NULL) SyncKnobs = "";
2348 
2349   size_t sz = strlen(SyncKnobs);
2350   char * knobs = (char *) malloc(sz + 2);
2351   if (knobs == NULL) {
2352     vm_exit_out_of_memory(sz + 2, OOM_MALLOC_ERROR, "Parse SyncKnobs");
2353     guarantee(0, "invariant");
2354   }
2355   strcpy(knobs, SyncKnobs);
2356   knobs[sz+1] = 0;
2357   for (char * p = knobs; *p; p++) {
2358     if (*p == ':') *p = 0;
2359   }
2360 
2361   #define SETKNOB(x) { Knob_##x = kvGetInt(knobs, #x, Knob_##x); }
2362   SETKNOB(ReportSettings);
2363   SETKNOB(Verbose);
2364   SETKNOB(VerifyInUse);
2365   SETKNOB(FixedSpin);
2366   SETKNOB(SpinLimit);
2367   SETKNOB(SpinBase);
2368   SETKNOB(SpinBackOff);
2369   SETKNOB(CASPenalty);
2370   SETKNOB(OXPenalty);
2371   SETKNOB(LogSpins);
2372   SETKNOB(SpinSetSucc);
2373   SETKNOB(SuccEnabled);
2374   SETKNOB(SuccRestrict);
2375   SETKNOB(Penalty);
2376   SETKNOB(Bonus);
2377   SETKNOB(BonusB);
2378   SETKNOB(Poverty);
2379   SETKNOB(SpinAfterFutile);
2380   SETKNOB(UsePause);
2381   SETKNOB(SpinEarly);
2382   SETKNOB(OState);
2383   SETKNOB(MaxSpinners);
2384   SETKNOB(PreSpin);
2385   SETKNOB(ExitPolicy);
2386   SETKNOB(QMode);
2387   SETKNOB(ResetEvent);
2388   SETKNOB(MoveNotifyee);
2389   SETKNOB(FastHSSEC);
2390   #undef SETKNOB
2391 
2392   if (Knob_Verbose) {
2393     sanity_checks();
2394   }
2395 
2396   if (os::is_MP()) {
2397     BackOffMask = (1 << Knob_SpinBackOff) - 1;
2398     if (Knob_ReportSettings) ::printf("BackOffMask=%X\n", BackOffMask);
2399     // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
2400   } else {
2401     Knob_SpinLimit = 0;
2402     Knob_SpinBase  = 0;
2403     Knob_PreSpin   = 0;
2404     Knob_FixedSpin = -1;
2405   }
2406 
2407   if (Knob_LogSpins == 0) {
2408     ObjectMonitor::_sync_FailedSpins = NULL;
2409   }
2410 
2411   free(knobs);
2412   OrderAccess::fence();
2413   InitDone = 1;
2414 }
2415 
2416 void ObjectMonitor::sanity_checks() {
2417   int error_cnt = 0;
2418   int warning_cnt = 0;
2419   bool verbose = Knob_Verbose != 0 NOT_PRODUCT(|| VerboseInternalVMTests);
2420 
2421   if (verbose) {
2422     tty->print_cr("INFO: sizeof(ObjectMonitor)=" SIZE_FORMAT,
2423                   sizeof(ObjectMonitor));
2424     tty->print_cr("INFO: sizeof(PaddedEnd<ObjectMonitor>)=" SIZE_FORMAT,
2425                   sizeof(PaddedEnd<ObjectMonitor>));
2426   }
2427 
2428   uint cache_line_size = VM_Version::L1_data_cache_line_size();
2429   if (verbose) {
2430     tty->print_cr("INFO: L1_data_cache_line_size=%u", cache_line_size);
2431   }
2432 
2433   ObjectMonitor dummy;
2434   u_char *addr_begin  = (u_char*)&dummy;
2435   u_char *addr_header = (u_char*)&dummy._header;
2436   u_char *addr_owner  = (u_char*)&dummy._owner;
2437 
2438   uint offset_header = (uint)(addr_header - addr_begin);
2439   if (verbose) tty->print_cr("INFO: offset(_header)=%u", offset_header);
2440 
2441   uint offset_owner = (uint)(addr_owner - addr_begin);
2442   if (verbose) tty->print_cr("INFO: offset(_owner)=%u", offset_owner);
2443 
2444   if ((uint)(addr_header - addr_begin) != 0) {
2445     tty->print_cr("ERROR: offset(_header) must be zero (0).");
2446     error_cnt++;
2447   }
2448 
2449   if (cache_line_size != 0) {
2450     // We were able to determine the L1 data cache line size so
2451     // do some cache line specific sanity checks
2452 
2453     if ((offset_owner - offset_header) < cache_line_size) {
2454       tty->print_cr("WARNING: the _header and _owner fields are closer "
2455                     "than a cache line which permits false sharing.");
2456       warning_cnt++;
2457     }
2458 
2459     if ((sizeof(PaddedEnd<ObjectMonitor>) % cache_line_size) != 0) {
2460       tty->print_cr("WARNING: PaddedEnd<ObjectMonitor> size is not a "
2461                     "multiple of a cache line which permits false sharing.");
2462       warning_cnt++;
2463     }
2464   }
2465 
2466   ObjectSynchronizer::sanity_checks(verbose, cache_line_size, &error_cnt,
2467                                     &warning_cnt);
2468 
2469   if (verbose || error_cnt != 0 || warning_cnt != 0) {
2470     tty->print_cr("INFO: error_cnt=%d", error_cnt);
2471     tty->print_cr("INFO: warning_cnt=%d", warning_cnt);
2472   }
2473 
2474   guarantee(error_cnt == 0,
2475             "Fatal error(s) found in ObjectMonitor::sanity_checks()");
2476 }
2477 
2478 #ifndef PRODUCT
2479 void ObjectMonitor::verify() {
2480 }
2481 
2482 void ObjectMonitor::print() {
2483 }
2484 #endif