rev 51780 : imported patch syncknobs-00-base
rev 51781 : imported patch syncknobs-01-Knob_ReportSettings
rev 51782 : imported patch syncknobs-02-Knob_SpinBackOff
rev 51783 : imported patch syncknobs-03-BackOffMask
rev 51784 : imported patch syncknobs-04-Knob_ExitRelease
rev 51785 : imported patch syncknobs-05-Knob_InlineNotify
rev 51786 : imported patch syncknobs-06-Knob_Verbose
rev 51787 : imported patch syncknobs-07-Knob_VerifyInUse
rev 51788 : imported patch syncknobs-08-Knob_VerifyMatch
rev 51789 : imported patch syncknobs-09-Knob_SpinBase
rev 51790 : imported patch syncknobs-10-Knob_CASPenalty
rev 51791 : imported patch syncknobs-11-Knob_OXPenalty
rev 51792 : imported patch syncknobs-12-Knob_SpinSetSucc
rev 51793 : imported patch syncknobs-13-Knob_SpinEarly
rev 51794 : imported patch syncknobs-14-Knob_SuccEnabled
rev 51795 : imported patch syncknobs-15-Knob_SuccRestrict
rev 51796 : imported patch syncknobs-16-Knob_MaxSpinners
rev 51797 : imported patch syncknobs-17-Knob_SpinAfterFutile
rev 51798 : imported patch syncknobs-18-Knob_OState
rev 51799 : imported patch syncknobs-19-Knob_UsePause
rev 51800 : imported patch syncknobs-20-Knob_ExitPolicy
rev 51801 : imported patch syncknobs-21-Knob_ResetEvent
rev 51802 : imported patch syncknobs-22-Knob_FastHSSEC
rev 51803 : imported patch syncknobs-23-Knob_MoveNotifyee

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