rev 54612 : Checkpoint latest preliminary review patches for full OpenJDK review; merge with 8222295.patch.
rev 54613 : imported patch dcubed.monitor_deflate_conc.v2.01
rev 54614 : imported patch dcubed.monitor_deflate_conc.v2.02
rev 54615 : imported patch dcubed.monitor_deflate_conc.v2.03

   1 /*
   2  * Copyright (c) 1998, 2019, 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 DEBUG_ONLY(static volatile bool InitDone = false;)
 114 
 115 // -----------------------------------------------------------------------------
 116 // Theory of operations -- Monitors lists, thread residency, etc:
 117 //
 118 // * A thread acquires ownership of a monitor by successfully
 119 //   CAS()ing the _owner field from null to non-null.
 120 //
 121 // * Invariant: A thread appears on at most one monitor list --
 122 //   cxq, EntryList or WaitSet -- at any one time.
 123 //
 124 // * Contending threads "push" themselves onto the cxq with CAS
 125 //   and then spin/park.
 126 //
 127 // * After a contending thread eventually acquires the lock it must
 128 //   dequeue itself from either the EntryList or the cxq.
 129 //
 130 // * The exiting thread identifies and unparks an "heir presumptive"
 131 //   tentative successor thread on the EntryList.  Critically, the
 132 //   exiting thread doesn't unlink the successor thread from the EntryList.
 133 //   After having been unparked, the wakee will recontend for ownership of
 134 //   the monitor.   The successor (wakee) will either acquire the lock or
 135 //   re-park itself.
 136 //
 137 //   Succession is provided for by a policy of competitive handoff.
 138 //   The exiting thread does _not_ grant or pass ownership to the
 139 //   successor thread.  (This is also referred to as "handoff" succession").
 140 //   Instead the exiting thread releases ownership and possibly wakes
 141 //   a successor, so the successor can (re)compete for ownership of the lock.
 142 //   If the EntryList is empty but the cxq is populated the exiting
 143 //   thread will drain the cxq into the EntryList.  It does so by
 144 //   by detaching the cxq (installing null with CAS) and folding
 145 //   the threads from the cxq into the EntryList.  The EntryList is
 146 //   doubly linked, while the cxq is singly linked because of the
 147 //   CAS-based "push" used to enqueue recently arrived threads (RATs).
 148 //
 149 // * Concurrency invariants:
 150 //
 151 //   -- only the monitor owner may access or mutate the EntryList.
 152 //      The mutex property of the monitor itself protects the EntryList
 153 //      from concurrent interference.
 154 //   -- Only the monitor owner may detach the cxq.
 155 //
 156 // * The monitor entry list operations avoid locks, but strictly speaking
 157 //   they're not lock-free.  Enter is lock-free, exit is not.
 158 //   For a description of 'Methods and apparatus providing non-blocking access
 159 //   to a resource,' see U.S. Pat. No. 7844973.
 160 //
 161 // * The cxq can have multiple concurrent "pushers" but only one concurrent
 162 //   detaching thread.  This mechanism is immune from the ABA corruption.
 163 //   More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
 164 //
 165 // * Taken together, the cxq and the EntryList constitute or form a
 166 //   single logical queue of threads stalled trying to acquire the lock.
 167 //   We use two distinct lists to improve the odds of a constant-time
 168 //   dequeue operation after acquisition (in the ::enter() epilogue) and
 169 //   to reduce heat on the list ends.  (c.f. Michael Scott's "2Q" algorithm).
 170 //   A key desideratum is to minimize queue & monitor metadata manipulation
 171 //   that occurs while holding the monitor lock -- that is, we want to
 172 //   minimize monitor lock holds times.  Note that even a small amount of
 173 //   fixed spinning will greatly reduce the # of enqueue-dequeue operations
 174 //   on EntryList|cxq.  That is, spinning relieves contention on the "inner"
 175 //   locks and monitor metadata.
 176 //
 177 //   Cxq points to the set of Recently Arrived Threads attempting entry.
 178 //   Because we push threads onto _cxq with CAS, the RATs must take the form of
 179 //   a singly-linked LIFO.  We drain _cxq into EntryList  at unlock-time when
 180 //   the unlocking thread notices that EntryList is null but _cxq is != null.
 181 //
 182 //   The EntryList is ordered by the prevailing queue discipline and
 183 //   can be organized in any convenient fashion, such as a doubly-linked list or
 184 //   a circular doubly-linked list.  Critically, we want insert and delete operations
 185 //   to operate in constant-time.  If we need a priority queue then something akin
 186 //   to Solaris' sleepq would work nicely.  Viz.,
 187 //   http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
 188 //   Queue discipline is enforced at ::exit() time, when the unlocking thread
 189 //   drains the cxq into the EntryList, and orders or reorders the threads on the
 190 //   EntryList accordingly.
 191 //
 192 //   Barring "lock barging", this mechanism provides fair cyclic ordering,
 193 //   somewhat similar to an elevator-scan.
 194 //
 195 // * The monitor synchronization subsystem avoids the use of native
 196 //   synchronization primitives except for the narrow platform-specific
 197 //   park-unpark abstraction.  See the comments in os_solaris.cpp regarding
 198 //   the semantics of park-unpark.  Put another way, this monitor implementation
 199 //   depends only on atomic operations and park-unpark.  The monitor subsystem
 200 //   manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
 201 //   underlying OS manages the READY<->RUN transitions.
 202 //
 203 // * Waiting threads reside on the WaitSet list -- wait() puts
 204 //   the caller onto the WaitSet.
 205 //
 206 // * notify() or notifyAll() simply transfers threads from the WaitSet to
 207 //   either the EntryList or cxq.  Subsequent exit() operations will
 208 //   unpark the notifyee.  Unparking a notifee in notify() is inefficient -
 209 //   it's likely the notifyee would simply impale itself on the lock held
 210 //   by the notifier.
 211 //
 212 // * An interesting alternative is to encode cxq as (List,LockByte) where
 213 //   the LockByte is 0 iff the monitor is owned.  _owner is simply an auxiliary
 214 //   variable, like _recursions, in the scheme.  The threads or Events that form
 215 //   the list would have to be aligned in 256-byte addresses.  A thread would
 216 //   try to acquire the lock or enqueue itself with CAS, but exiting threads
 217 //   could use a 1-0 protocol and simply STB to set the LockByte to 0.
 218 //   Note that is is *not* word-tearing, but it does presume that full-word
 219 //   CAS operations are coherent with intermix with STB operations.  That's true
 220 //   on most common processors.
 221 //
 222 // * See also http://blogs.sun.com/dave
 223 
 224 
 225 void* ObjectMonitor::operator new (size_t size) throw() {
 226   return AllocateHeap(size, mtInternal);
 227 }
 228 void* ObjectMonitor::operator new[] (size_t size) throw() {
 229   return operator new (size);
 230 }
 231 void ObjectMonitor::operator delete(void* p) {
 232   FreeHeap(p);
 233 }
 234 void ObjectMonitor::operator delete[] (void *p) {
 235   operator delete(p);
 236 }
 237 
 238 // -----------------------------------------------------------------------------
 239 // Enter support
 240 
 241 void ObjectMonitor::enter(TRAPS) {


 242   // The following code is ordered to check the most common cases first
 243   // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
 244   Thread * const Self = THREAD;
 245 
 246   void * cur = Atomic::cmpxchg(Self, &_owner, (void*)NULL);
 247   if (cur == NULL) {
 248     // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
 249     assert(_recursions == 0, "invariant");
 250     assert(_owner == Self, "invariant");
 251     return;
 252   }
 253 
 254   if (cur == Self) {
 255     // TODO-FIXME: check for integer overflow!  BUGID 6557169.
 256     _recursions++;
 257     return;
 258   }
 259 
 260   if (Self->is_lock_owned ((address)cur)) {
 261     assert(_recursions == 0, "internal state error");
 262     _recursions = 1;
 263     // Commute owner from a thread-specific on-stack BasicLockObject address to
 264     // a full-fledged "Thread *".
 265     _owner = Self;
 266     return;
 267   }
 268 
 269   // We've encountered genuine contention.
 270   assert(Self->_Stalled == 0, "invariant");
 271   Self->_Stalled = intptr_t(this);
 272 
 273   // Try one round of spinning *before* enqueueing Self
 274   // and before going through the awkward and expensive state
 275   // transitions.  The following spin is strictly optional ...
 276   // Note that if we acquire the monitor from an initial spin
 277   // we forgo posting JVMTI events and firing DTRACE probes.
 278   if (TrySpin(Self) > 0) {
 279     assert(_owner == Self, "must be Self: owner=" INTPTR_FORMAT, p2i(_owner));
 280     assert(_recursions == 0, "must be 0: recursions=" INTPTR_FORMAT,
 281            _recursions);
 282     assert(((oop)object())->mark() == markOopDesc::encode(this),
 283            "object mark must match encoded this: mark=" INTPTR_FORMAT
 284            ", encoded this=" INTPTR_FORMAT, p2i(((oop)object())->mark()),
 285            p2i(markOopDesc::encode(this)));
 286     Self->_Stalled = 0;
 287     return;
 288   }
 289 
 290   assert(_owner != Self, "invariant");
 291   assert(_succ != Self, "invariant");
 292   assert(Self->is_Java_thread(), "invariant");
 293   JavaThread * jt = (JavaThread *) Self;
 294   assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
 295   assert(jt->thread_state() != _thread_blocked, "invariant");
 296   assert(this->object() != NULL, "invariant");
 297   assert(_contentions >= 0, "invariant");
 298 
 299   // Prevent deflation at STW-time.  See deflate_idle_monitors() and is_busy().
 300   // Ensure the object-monitor relationship remains stable while there's contention.
 301   Atomic::inc(&_contentions);


 302 
 303   JFR_ONLY(JfrConditionalFlushWithStacktrace<EventJavaMonitorEnter> flush(jt);)
 304   EventJavaMonitorEnter event;
 305   if (event.should_commit()) {
 306     event.set_monitorClass(((oop)this->object())->klass());
 307     event.set_address((uintptr_t)(this->object_addr()));
 308   }
 309 
 310   { // Change java thread status to indicate blocked on monitor enter.
 311     JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
 312 
 313     Self->set_current_pending_monitor(this);
 314 
 315     DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
 316     if (JvmtiExport::should_post_monitor_contended_enter()) {
 317       JvmtiExport::post_monitor_contended_enter(jt, this);
 318 
 319       // The current thread does not yet own the monitor and does not
 320       // yet appear on any queues that would get it made the successor.
 321       // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
 322       // handler cannot accidentally consume an unpark() meant for the
 323       // ParkEvent associated with this ObjectMonitor.
 324     }
 325 
 326     OSThreadContendState osts(Self->osthread());
 327     ThreadBlockInVM tbivm(jt);
 328 
 329     // TODO-FIXME: change the following for(;;) loop to straight-line code.
 330     for (;;) {
 331       jt->set_suspend_equivalent();
 332       // cleared by handle_special_suspend_equivalent_condition()
 333       // or java_suspend_self()
 334 
 335       EnterI(THREAD);
 336 
 337       if (!ExitSuspendEquivalent(jt)) break;
 338 
 339       // We have acquired the contended monitor, but while we were
 340       // waiting another thread suspended us. We don't want to enter
 341       // the monitor while suspended because that would surprise the
 342       // thread that suspended us.
 343       //
 344       _recursions = 0;
 345       _succ = NULL;
 346       exit(false, Self);
 347 
 348       jt->java_suspend_self();
 349     }
 350     Self->set_current_pending_monitor(NULL);
 351 
 352     // We cleared the pending monitor info since we've just gotten past
 353     // the enter-check-for-suspend dance and we now own the monitor free
 354     // and clear, i.e., it is no longer pending. The ThreadBlockInVM
 355     // destructor can go to a safepoint at the end of this block. If we
 356     // do a thread dump during that safepoint, then this thread will show
 357     // as having "-locked" the monitor, but the OS and java.lang.Thread
 358     // states will still report that the thread is blocked trying to
 359     // acquire it.
 360   }
 361 
 362   Atomic::dec(&_contentions);
 363   assert(_contentions >= 0, "invariant");
 364   Self->_Stalled = 0;
 365 
 366   // Must either set _recursions = 0 or ASSERT _recursions == 0.
 367   assert(_recursions == 0, "invariant");
 368   assert(_owner == Self, "invariant");
 369   assert(_succ != Self, "invariant");
 370   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 371 
 372   // The thread -- now the owner -- is back in vm mode.
 373   // Report the glorious news via TI,DTrace and jvmstat.
 374   // The probe effect is non-trivial.  All the reportage occurs
 375   // while we hold the monitor, increasing the length of the critical
 376   // section.  Amdahl's parallel speedup law comes vividly into play.
 377   //
 378   // Another option might be to aggregate the events (thread local or
 379   // per-monitor aggregation) and defer reporting until a more opportune
 380   // time -- such as next time some thread encounters contention but has
 381   // yet to acquire the lock.  While spinning that thread could
 382   // spinning we could increment JVMStat counters, etc.
 383 
 384   DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
 385   if (JvmtiExport::should_post_monitor_contended_entered()) {
 386     JvmtiExport::post_monitor_contended_entered(jt, this);
 387 
 388     // The current thread already owns the monitor and is not going to
 389     // call park() for the remainder of the monitor enter protocol. So
 390     // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
 391     // event handler consumed an unpark() issued by the thread that
 392     // just exited the monitor.
 393   }
 394   if (event.should_commit()) {
 395     event.set_previousOwner((uintptr_t)_previous_owner_tid);
 396     event.commit();
 397   }
 398   OM_PERFDATA_OP(ContendedLockAttempts, inc());
 399 }
 400 
 401 // Caveat: TryLock() is not necessarily serializing if it returns failure.
 402 // Callers must compensate as needed.
 403 
 404 int ObjectMonitor::TryLock(Thread * Self) {
 405   void * own = _owner;
 406   if (own != NULL) return 0;
 407   if (Atomic::replace_if_null(Self, &_owner)) {
 408     // Either guarantee _recursions == 0 or set _recursions = 0.
 409     assert(_recursions == 0, "invariant");
 410     assert(_owner == Self, "invariant");
 411     return 1;
 412   }
 413   // The lock had been free momentarily, but we lost the race to the lock.
 414   // Interference -- the CAS failed.
 415   // We can either return -1 or retry.
 416   // Retry doesn't make as much sense because the lock was just acquired.
 417   return -1;
 418 }
 419 















































































 420 #define MAX_RECHECK_INTERVAL 1000
 421 
 422 void ObjectMonitor::EnterI(TRAPS) {


 423   Thread * const Self = THREAD;
 424   assert(Self->is_Java_thread(), "invariant");
 425   assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant");
 426 
 427   // Try the lock - TATAS
 428   if (TryLock (Self) > 0) {
 429     assert(_succ != Self, "invariant");
 430     assert(_owner == Self, "invariant");
 431     assert(_Responsible != Self, "invariant");
 432     return;
 433   }
 434 











 435   assert(InitDone, "Unexpectedly not initialized");
 436 
 437   // We try one round of spinning *before* enqueueing Self.
 438   //
 439   // If the _owner is ready but OFFPROC we could use a YieldTo()
 440   // operation to donate the remainder of this thread's quantum
 441   // to the owner.  This has subtle but beneficial affinity
 442   // effects.
 443 
 444   if (TrySpin(Self) > 0) {
 445     assert(_owner == Self, "invariant");
 446     assert(_succ != Self, "invariant");
 447     assert(_Responsible != Self, "invariant");
 448     return;
 449   }
 450 
 451   // The Spin failed -- Enqueue and park the thread ...
 452   assert(_succ != Self, "invariant");
 453   assert(_owner != Self, "invariant");
 454   assert(_Responsible != Self, "invariant");
 455 
 456   // Enqueue "Self" on ObjectMonitor's _cxq.
 457   //
 458   // Node acts as a proxy for Self.
 459   // As an aside, if were to ever rewrite the synchronization code mostly
 460   // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
 461   // Java objects.  This would avoid awkward lifecycle and liveness issues,
 462   // as well as eliminate a subset of ABA issues.
 463   // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
 464 
 465   ObjectWaiter node(Self);
 466   Self->_ParkEvent->reset();
 467   node._prev   = (ObjectWaiter *) 0xBAD;
 468   node.TState  = ObjectWaiter::TS_CXQ;
 469 
 470   // Push "Self" onto the front of the _cxq.
 471   // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
 472   // Note that spinning tends to reduce the rate at which threads
 473   // enqueue and dequeue on EntryList|cxq.
 474   ObjectWaiter * nxt;
 475   for (;;) {
 476     node._next = nxt = _cxq;
 477     if (Atomic::cmpxchg(&node, &_cxq, nxt) == nxt) break;
 478 
 479     // Interference - the CAS failed because _cxq changed.  Just retry.
 480     // As an optional optimization we retry the lock.
 481     if (TryLock (Self) > 0) {
 482       assert(_succ != Self, "invariant");
 483       assert(_owner == Self, "invariant");
 484       assert(_Responsible != Self, "invariant");
 485       return;
 486     }
 487   }
 488 
 489   // Check for cxq|EntryList edge transition to non-null.  This indicates
 490   // the onset of contention.  While contention persists exiting threads
 491   // will use a ST:MEMBAR:LD 1-1 exit protocol.  When contention abates exit
 492   // operations revert to the faster 1-0 mode.  This enter operation may interleave
 493   // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
 494   // arrange for one of the contending thread to use a timed park() operations
 495   // to detect and recover from the race.  (Stranding is form of progress failure
 496   // where the monitor is unlocked but all the contending threads remain parked).
 497   // That is, at least one of the contended threads will periodically poll _owner.
 498   // One of the contending threads will become the designated "Responsible" thread.
 499   // The Responsible thread uses a timed park instead of a normal indefinite park
 500   // operation -- it periodically wakes and checks for and recovers from potential
 501   // strandings admitted by 1-0 exit operations.   We need at most one Responsible
 502   // thread per-monitor at any given moment.  Only threads on cxq|EntryList may
 503   // be responsible for a monitor.
 504   //
 505   // Currently, one of the contended threads takes on the added role of "Responsible".
 506   // A viable alternative would be to use a dedicated "stranding checker" thread
 507   // that periodically iterated over all the threads (or active monitors) and unparked
 508   // successors where there was risk of stranding.  This would help eliminate the
 509   // timer scalability issues we see on some platforms as we'd only have one thread
 510   // -- the checker -- parked on a timer.
 511 
 512   if (nxt == NULL && _EntryList == NULL) {
 513     // Try to assume the role of responsible thread for the monitor.
 514     // CONSIDER:  ST vs CAS vs { if (Responsible==null) Responsible=Self }
 515     Atomic::replace_if_null(Self, &_Responsible);
 516   }
 517 
 518   // The lock might have been released while this thread was occupied queueing
 519   // itself onto _cxq.  To close the race and avoid "stranding" and
 520   // progress-liveness failure we must resample-retry _owner before parking.
 521   // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
 522   // In this case the ST-MEMBAR is accomplished with CAS().
 523   //
 524   // TODO: Defer all thread state transitions until park-time.
 525   // Since state transitions are heavy and inefficient we'd like
 526   // to defer the state transitions until absolutely necessary,
 527   // and in doing so avoid some transitions ...
 528 
 529   int nWakeups = 0;
 530   int recheckInterval = 1;
 531 
 532   for (;;) {
 533 
 534     if (TryLock(Self) > 0) break;
 535     assert(_owner != Self, "invariant");
 536 
 537     // park self
 538     if (_Responsible == Self) {
 539       Self->_ParkEvent->park((jlong) recheckInterval);
 540       // Increase the recheckInterval, but clamp the value.
 541       recheckInterval *= 8;
 542       if (recheckInterval > MAX_RECHECK_INTERVAL) {
 543         recheckInterval = MAX_RECHECK_INTERVAL;
 544       }
 545     } else {
 546       Self->_ParkEvent->park();
 547     }
 548 
 549     if (TryLock(Self) > 0) break;
 550 









 551     // The lock is still contested.
 552     // Keep a tally of the # of futile wakeups.
 553     // Note that the counter is not protected by a lock or updated by atomics.
 554     // That is by design - we trade "lossy" counters which are exposed to
 555     // races during updates for a lower probe effect.
 556 
 557     // This PerfData object can be used in parallel with a safepoint.
 558     // See the work around in PerfDataManager::destroy().
 559     OM_PERFDATA_OP(FutileWakeups, inc());
 560     ++nWakeups;
 561 
 562     // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
 563     // We can defer clearing _succ until after the spin completes
 564     // TrySpin() must tolerate being called with _succ == Self.
 565     // Try yet another round of adaptive spinning.
 566     if (TrySpin(Self) > 0) break;
 567 
 568     // We can find that we were unpark()ed and redesignated _succ while
 569     // we were spinning.  That's harmless.  If we iterate and call park(),
 570     // park() will consume the event and return immediately and we'll
 571     // just spin again.  This pattern can repeat, leaving _succ to simply
 572     // spin on a CPU.
 573 
 574     if (_succ == Self) _succ = NULL;
 575 
 576     // Invariant: after clearing _succ a thread *must* retry _owner before parking.
 577     OrderAccess::fence();
 578   }
 579 
 580   // Egress :
 581   // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
 582   // Normally we'll find Self on the EntryList .
 583   // From the perspective of the lock owner (this thread), the
 584   // EntryList is stable and cxq is prepend-only.
 585   // The head of cxq is volatile but the interior is stable.
 586   // In addition, Self.TState is stable.
 587 
 588   assert(_owner == Self, "invariant");
 589   assert(object() != NULL, "invariant");
 590   // I'd like to write:
 591   //   guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
 592   // but as we're at a safepoint that's not safe.
 593 
 594   UnlinkAfterAcquire(Self, &node);
 595   if (_succ == Self) _succ = NULL;
 596 
 597   assert(_succ != Self, "invariant");
 598   if (_Responsible == Self) {
 599     _Responsible = NULL;
 600     OrderAccess::fence(); // Dekker pivot-point
 601 
 602     // We may leave threads on cxq|EntryList without a designated
 603     // "Responsible" thread.  This is benign.  When this thread subsequently
 604     // exits the monitor it can "see" such preexisting "old" threads --
 605     // threads that arrived on the cxq|EntryList before the fence, above --
 606     // by LDing cxq|EntryList.  Newly arrived threads -- that is, threads
 607     // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
 608     // non-null and elect a new "Responsible" timer thread.
 609     //
 610     // This thread executes:
 611     //    ST Responsible=null; MEMBAR    (in enter epilogue - here)
 612     //    LD cxq|EntryList               (in subsequent exit)
 613     //
 614     // Entering threads in the slow/contended path execute:
 615     //    ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
 616     //    The (ST cxq; MEMBAR) is accomplished with CAS().
 617     //
 618     // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
 619     // exit operation from floating above the ST Responsible=null.
 620   }
 621 
 622   // We've acquired ownership with CAS().
 623   // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
 624   // But since the CAS() this thread may have also stored into _succ,
 625   // EntryList, cxq or Responsible.  These meta-data updates must be
 626   // visible __before this thread subsequently drops the lock.
 627   // Consider what could occur if we didn't enforce this constraint --
 628   // STs to monitor meta-data and user-data could reorder with (become
 629   // visible after) the ST in exit that drops ownership of the lock.
 630   // Some other thread could then acquire the lock, but observe inconsistent
 631   // or old monitor meta-data and heap data.  That violates the JMM.
 632   // To that end, the 1-0 exit() operation must have at least STST|LDST
 633   // "release" barrier semantics.  Specifically, there must be at least a
 634   // STST|LDST barrier in exit() before the ST of null into _owner that drops
 635   // the lock.   The barrier ensures that changes to monitor meta-data and data
 636   // protected by the lock will be visible before we release the lock, and
 637   // therefore before some other thread (CPU) has a chance to acquire the lock.
 638   // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
 639   //
 640   // Critically, any prior STs to _succ or EntryList must be visible before
 641   // the ST of null into _owner in the *subsequent* (following) corresponding
 642   // monitorexit.  Recall too, that in 1-0 mode monitorexit does not necessarily
 643   // execute a serializing instruction.
 644 
 645   return;
 646 }
 647 
 648 // ReenterI() is a specialized inline form of the latter half of the
 649 // contended slow-path from EnterI().  We use ReenterI() only for
 650 // monitor reentry in wait().
 651 //
 652 // In the future we should reconcile EnterI() and ReenterI().
 653 
 654 void ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) {


 655   assert(Self != NULL, "invariant");
 656   assert(SelfNode != NULL, "invariant");
 657   assert(SelfNode->_thread == Self, "invariant");
 658   assert(_waiters > 0, "invariant");
 659   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 660   assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
 661   JavaThread * jt = (JavaThread *) Self;
 662 
 663   int nWakeups = 0;
 664   for (;;) {
 665     ObjectWaiter::TStates v = SelfNode->TState;
 666     guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
 667     assert(_owner != Self, "invariant");
 668 
 669     if (TryLock(Self) > 0) break;
 670     if (TrySpin(Self) > 0) break;
 671 









 672     // State transition wrappers around park() ...
 673     // ReenterI() wisely defers state transitions until
 674     // it's clear we must park the thread.
 675     {
 676       OSThreadContendState osts(Self->osthread());
 677       ThreadBlockInVM tbivm(jt);
 678 
 679       // cleared by handle_special_suspend_equivalent_condition()
 680       // or java_suspend_self()
 681       jt->set_suspend_equivalent();
 682       Self->_ParkEvent->park();
 683 
 684       // were we externally suspended while we were waiting?
 685       for (;;) {
 686         if (!ExitSuspendEquivalent(jt)) break;
 687         if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
 688         jt->java_suspend_self();
 689         jt->set_suspend_equivalent();
 690       }
 691     }
 692 
 693     // Try again, but just so we distinguish between futile wakeups and
 694     // successful wakeups.  The following test isn't algorithmically
 695     // necessary, but it helps us maintain sensible statistics.
 696     if (TryLock(Self) > 0) break;
 697 
 698     // The lock is still contested.
 699     // Keep a tally of the # of futile wakeups.
 700     // Note that the counter is not protected by a lock or updated by atomics.
 701     // That is by design - we trade "lossy" counters which are exposed to
 702     // races during updates for a lower probe effect.
 703     ++nWakeups;
 704 
 705     // Assuming this is not a spurious wakeup we'll normally
 706     // find that _succ == Self.
 707     if (_succ == Self) _succ = NULL;
 708 
 709     // Invariant: after clearing _succ a contending thread
 710     // *must* retry  _owner before parking.
 711     OrderAccess::fence();
 712 
 713     // This PerfData object can be used in parallel with a safepoint.
 714     // See the work around in PerfDataManager::destroy().
 715     OM_PERFDATA_OP(FutileWakeups, inc());
 716   }
 717 
 718   // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
 719   // Normally we'll find Self on the EntryList.
 720   // Unlinking from the EntryList is constant-time and atomic-free.
 721   // From the perspective of the lock owner (this thread), the
 722   // EntryList is stable and cxq is prepend-only.
 723   // The head of cxq is volatile but the interior is stable.
 724   // In addition, Self.TState is stable.
 725 
 726   assert(_owner == Self, "invariant");
 727   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 728   UnlinkAfterAcquire(Self, SelfNode);
 729   if (_succ == Self) _succ = NULL;
 730   assert(_succ != Self, "invariant");
 731   SelfNode->TState = ObjectWaiter::TS_RUN;
 732   OrderAccess::fence();      // see comments at the end of EnterI()
 733 }
 734 
 735 // By convention we unlink a contending thread from EntryList|cxq immediately
 736 // after the thread acquires the lock in ::enter().  Equally, we could defer
 737 // unlinking the thread until ::exit()-time.
 738 
 739 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) {
 740   assert(_owner == Self, "invariant");
 741   assert(SelfNode->_thread == Self, "invariant");
 742 
 743   if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
 744     // Normal case: remove Self from the DLL EntryList .
 745     // This is a constant-time operation.
 746     ObjectWaiter * nxt = SelfNode->_next;
 747     ObjectWaiter * prv = SelfNode->_prev;
 748     if (nxt != NULL) nxt->_prev = prv;
 749     if (prv != NULL) prv->_next = nxt;
 750     if (SelfNode == _EntryList) _EntryList = nxt;
 751     assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
 752     assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
 753   } else {
 754     assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant");
 755     // Inopportune interleaving -- Self is still on the cxq.
 756     // This usually means the enqueue of self raced an exiting thread.
 757     // Normally we'll find Self near the front of the cxq, so
 758     // dequeueing is typically fast.  If needbe we can accelerate
 759     // this with some MCS/CHL-like bidirectional list hints and advisory
 760     // back-links so dequeueing from the interior will normally operate
 761     // in constant-time.
 762     // Dequeue Self from either the head (with CAS) or from the interior
 763     // with a linear-time scan and normal non-atomic memory operations.
 764     // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
 765     // and then unlink Self from EntryList.  We have to drain eventually,
 766     // so it might as well be now.
 767 
 768     ObjectWaiter * v = _cxq;
 769     assert(v != NULL, "invariant");
 770     if (v != SelfNode || Atomic::cmpxchg(SelfNode->_next, &_cxq, v) != v) {
 771       // The CAS above can fail from interference IFF a "RAT" arrived.
 772       // In that case Self must be in the interior and can no longer be
 773       // at the head of cxq.
 774       if (v == SelfNode) {
 775         assert(_cxq != v, "invariant");
 776         v = _cxq;          // CAS above failed - start scan at head of list
 777       }
 778       ObjectWaiter * p;
 779       ObjectWaiter * q = NULL;
 780       for (p = v; p != NULL && p != SelfNode; p = p->_next) {
 781         q = p;
 782         assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
 783       }
 784       assert(v != SelfNode, "invariant");
 785       assert(p == SelfNode, "Node not found on cxq");
 786       assert(p != _cxq, "invariant");
 787       assert(q != NULL, "invariant");
 788       assert(q->_next == p, "invariant");
 789       q->_next = p->_next;
 790     }
 791   }
 792 
 793 #ifdef ASSERT
 794   // Diagnostic hygiene ...
 795   SelfNode->_prev  = (ObjectWaiter *) 0xBAD;
 796   SelfNode->_next  = (ObjectWaiter *) 0xBAD;
 797   SelfNode->TState = ObjectWaiter::TS_RUN;
 798 #endif
 799 }
 800 
 801 // -----------------------------------------------------------------------------
 802 // Exit support
 803 //
 804 // exit()
 805 // ~~~~~~
 806 // Note that the collector can't reclaim the objectMonitor or deflate
 807 // the object out from underneath the thread calling ::exit() as the
 808 // thread calling ::exit() never transitions to a stable state.
 809 // This inhibits GC, which in turn inhibits asynchronous (and
 810 // inopportune) reclamation of "this".
 811 //
 812 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
 813 // There's one exception to the claim above, however.  EnterI() can call
 814 // exit() to drop a lock if the acquirer has been externally suspended.
 815 // In that case exit() is called with _thread_state as _thread_blocked,
 816 // but the monitor's _contentions field is > 0, which inhibits reclamation.
 817 //
 818 // 1-0 exit
 819 // ~~~~~~~~
 820 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
 821 // the fast-path operators have been optimized so the common ::exit()
 822 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
 823 // The code emitted by fast_unlock() elides the usual MEMBAR.  This
 824 // greatly improves latency -- MEMBAR and CAS having considerable local
 825 // latency on modern processors -- but at the cost of "stranding".  Absent the
 826 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
 827 // ::enter() path, resulting in the entering thread being stranding
 828 // and a progress-liveness failure.   Stranding is extremely rare.
 829 // We use timers (timed park operations) & periodic polling to detect
 830 // and recover from stranding.  Potentially stranded threads periodically
 831 // wake up and poll the lock.  See the usage of the _Responsible variable.
 832 //
 833 // The CAS() in enter provides for safety and exclusion, while the CAS or
 834 // MEMBAR in exit provides for progress and avoids stranding.  1-0 locking
 835 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
 836 // We detect and recover from stranding with timers.
 837 //
 838 // If a thread transiently strands it'll park until (a) another
 839 // thread acquires the lock and then drops the lock, at which time the
 840 // exiting thread will notice and unpark the stranded thread, or, (b)
 841 // the timer expires.  If the lock is high traffic then the stranding latency
 842 // will be low due to (a).  If the lock is low traffic then the odds of
 843 // stranding are lower, although the worst-case stranding latency
 844 // is longer.  Critically, we don't want to put excessive load in the
 845 // platform's timer subsystem.  We want to minimize both the timer injection
 846 // rate (timers created/sec) as well as the number of timers active at
 847 // any one time.  (more precisely, we want to minimize timer-seconds, which is
 848 // the integral of the # of active timers at any instant over time).
 849 // Both impinge on OS scalability.  Given that, at most one thread parked on
 850 // a monitor will use a timer.
 851 //
 852 // There is also the risk of a futile wake-up. If we drop the lock
 853 // another thread can reacquire the lock immediately, and we can
 854 // then wake a thread unnecessarily. This is benign, and we've
 855 // structured the code so the windows are short and the frequency
 856 // of such futile wakups is low.
 857 
 858 void ObjectMonitor::exit(bool not_suspended, TRAPS) {
 859   Thread * const Self = THREAD;
 860   if (THREAD != _owner) {
 861     if (THREAD->is_lock_owned((address) _owner)) {
 862       // Transmute _owner from a BasicLock pointer to a Thread address.
 863       // We don't need to hold _mutex for this transition.
 864       // Non-null to Non-null is safe as long as all readers can
 865       // tolerate either flavor.
 866       assert(_recursions == 0, "invariant");
 867       _owner = THREAD;
 868       _recursions = 0;
 869     } else {
 870       // Apparent unbalanced locking ...
 871       // Naively we'd like to throw IllegalMonitorStateException.
 872       // As a practical matter we can neither allocate nor throw an
 873       // exception as ::exit() can be called from leaf routines.
 874       // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
 875       // Upon deeper reflection, however, in a properly run JVM the only
 876       // way we should encounter this situation is in the presence of
 877       // unbalanced JNI locking. TODO: CheckJNICalls.
 878       // See also: CR4414101
 879       assert(false, "Non-balanced monitor enter/exit! Likely JNI locking");

 880       return;
 881     }
 882   }
 883 
 884   if (_recursions != 0) {
 885     _recursions--;        // this is simple recursive enter
 886     return;
 887   }
 888 
 889   // Invariant: after setting Responsible=null an thread must execute
 890   // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
 891   _Responsible = NULL;
 892 
 893 #if INCLUDE_JFR
 894   // get the owner's thread id for the MonitorEnter event
 895   // if it is enabled and the thread isn't suspended
 896   if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
 897     _previous_owner_tid = JFR_THREAD_ID(Self);
 898   }
 899 #endif
 900 
 901   for (;;) {
 902     assert(THREAD == _owner, "invariant");
 903 
 904     // release semantics: prior loads and stores from within the critical section
 905     // must not float (reorder) past the following store that drops the lock.
 906     // On SPARC that requires MEMBAR #loadstore|#storestore.
 907     // But of course in TSO #loadstore|#storestore is not required.
 908     OrderAccess::release_store(&_owner, (void*)NULL);   // drop the lock
 909     OrderAccess::storeload();                        // See if we need to wake a successor
 910     if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
 911       return;
 912     }
 913     // Other threads are blocked trying to acquire the lock.
 914 
 915     // Normally the exiting thread is responsible for ensuring succession,
 916     // but if other successors are ready or other entering threads are spinning
 917     // then this thread can simply store NULL into _owner and exit without
 918     // waking a successor.  The existence of spinners or ready successors
 919     // guarantees proper succession (liveness).  Responsibility passes to the
 920     // ready or running successors.  The exiting thread delegates the duty.
 921     // More precisely, if a successor already exists this thread is absolved
 922     // of the responsibility of waking (unparking) one.
 923     //
 924     // The _succ variable is critical to reducing futile wakeup frequency.
 925     // _succ identifies the "heir presumptive" thread that has been made
 926     // ready (unparked) but that has not yet run.  We need only one such
 927     // successor thread to guarantee progress.
 928     // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
 929     // section 3.3 "Futile Wakeup Throttling" for details.
 930     //
 931     // Note that spinners in Enter() also set _succ non-null.
 932     // In the current implementation spinners opportunistically set
 933     // _succ so that exiting threads might avoid waking a successor.
 934     // Another less appealing alternative would be for the exiting thread
 935     // to drop the lock and then spin briefly to see if a spinner managed
 936     // to acquire the lock.  If so, the exiting thread could exit
 937     // immediately without waking a successor, otherwise the exiting
 938     // thread would need to dequeue and wake a successor.
 939     // (Note that we'd need to make the post-drop spin short, but no
 940     // shorter than the worst-case round-trip cache-line migration time.
 941     // The dropped lock needs to become visible to the spinner, and then
 942     // the acquisition of the lock by the spinner must become visible to
 943     // the exiting thread).
 944 
 945     // It appears that an heir-presumptive (successor) must be made ready.
 946     // Only the current lock owner can manipulate the EntryList or
 947     // drain _cxq, so we need to reacquire the lock.  If we fail
 948     // to reacquire the lock the responsibility for ensuring succession
 949     // falls to the new owner.
 950     //
 951     if (!Atomic::replace_if_null(THREAD, &_owner)) {
 952       return;
 953     }
 954 
 955     guarantee(_owner == THREAD, "invariant");
 956 
 957     ObjectWaiter * w = NULL;
 958 
 959     w = _EntryList;
 960     if (w != NULL) {
 961       // I'd like to write: guarantee (w->_thread != Self).
 962       // But in practice an exiting thread may find itself on the EntryList.
 963       // Let's say thread T1 calls O.wait().  Wait() enqueues T1 on O's waitset and
 964       // then calls exit().  Exit release the lock by setting O._owner to NULL.
 965       // Let's say T1 then stalls.  T2 acquires O and calls O.notify().  The
 966       // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
 967       // release the lock "O".  T2 resumes immediately after the ST of null into
 968       // _owner, above.  T2 notices that the EntryList is populated, so it
 969       // reacquires the lock and then finds itself on the EntryList.
 970       // Given all that, we have to tolerate the circumstance where "w" is
 971       // associated with Self.
 972       assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
 973       ExitEpilog(Self, w);
 974       return;
 975     }
 976 
 977     // If we find that both _cxq and EntryList are null then just
 978     // re-run the exit protocol from the top.
 979     w = _cxq;
 980     if (w == NULL) continue;
 981 
 982     // Drain _cxq into EntryList - bulk transfer.
 983     // First, detach _cxq.
 984     // The following loop is tantamount to: w = swap(&cxq, NULL)
 985     for (;;) {
 986       assert(w != NULL, "Invariant");
 987       ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
 988       if (u == w) break;
 989       w = u;
 990     }
 991 
 992     assert(w != NULL, "invariant");
 993     assert(_EntryList == NULL, "invariant");
 994 
 995     // Convert the LIFO SLL anchored by _cxq into a DLL.
 996     // The list reorganization step operates in O(LENGTH(w)) time.
 997     // It's critical that this step operate quickly as
 998     // "Self" still holds the outer-lock, restricting parallelism
 999     // and effectively lengthening the critical section.
1000     // Invariant: s chases t chases u.
1001     // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1002     // we have faster access to the tail.
1003 
1004     _EntryList = w;
1005     ObjectWaiter * q = NULL;
1006     ObjectWaiter * p;
1007     for (p = w; p != NULL; p = p->_next) {
1008       guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1009       p->TState = ObjectWaiter::TS_ENTER;
1010       p->_prev = q;
1011       q = p;
1012     }
1013 
1014     // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1015     // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1016 
1017     // See if we can abdicate to a spinner instead of waking a thread.
1018     // A primary goal of the implementation is to reduce the
1019     // context-switch rate.
1020     if (_succ != NULL) continue;
1021 
1022     w = _EntryList;
1023     if (w != NULL) {
1024       guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1025       ExitEpilog(Self, w);
1026       return;
1027     }
1028   }
1029 }
1030 
1031 // ExitSuspendEquivalent:
1032 // A faster alternate to handle_special_suspend_equivalent_condition()
1033 //
1034 // handle_special_suspend_equivalent_condition() unconditionally
1035 // acquires the SR_lock.  On some platforms uncontended MutexLocker()
1036 // operations have high latency.  Note that in ::enter() we call HSSEC
1037 // while holding the monitor, so we effectively lengthen the critical sections.
1038 //
1039 // There are a number of possible solutions:
1040 //
1041 // A.  To ameliorate the problem we might also defer state transitions
1042 //     to as late as possible -- just prior to parking.
1043 //     Given that, we'd call HSSEC after having returned from park(),
1044 //     but before attempting to acquire the monitor.  This is only a
1045 //     partial solution.  It avoids calling HSSEC while holding the
1046 //     monitor (good), but it still increases successor reacquisition latency --
1047 //     the interval between unparking a successor and the time the successor
1048 //     resumes and retries the lock.  See ReenterI(), which defers state transitions.
1049 //     If we use this technique we can also avoid EnterI()-exit() loop
1050 //     in ::enter() where we iteratively drop the lock and then attempt
1051 //     to reacquire it after suspending.
1052 //
1053 // B.  In the future we might fold all the suspend bits into a
1054 //     composite per-thread suspend flag and then update it with CAS().
1055 //     Alternately, a Dekker-like mechanism with multiple variables
1056 //     would suffice:
1057 //       ST Self->_suspend_equivalent = false
1058 //       MEMBAR
1059 //       LD Self_>_suspend_flags
1060 
1061 bool ObjectMonitor::ExitSuspendEquivalent(JavaThread * jSelf) {
1062   return jSelf->handle_special_suspend_equivalent_condition();
1063 }
1064 
1065 
1066 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) {
1067   assert(_owner == Self, "invariant");
1068 
1069   // Exit protocol:
1070   // 1. ST _succ = wakee
1071   // 2. membar #loadstore|#storestore;
1072   // 2. ST _owner = NULL
1073   // 3. unpark(wakee)
1074 
1075   _succ = Wakee->_thread;
1076   ParkEvent * Trigger = Wakee->_event;
1077 
1078   // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1079   // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1080   // out-of-scope (non-extant).
1081   Wakee  = NULL;
1082 
1083   // Drop the lock
1084   OrderAccess::release_store(&_owner, (void*)NULL);
1085   OrderAccess::fence();                               // ST _owner vs LD in unpark()
1086 
1087   DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1088   Trigger->unpark();
1089 
1090   // Maintain stats and report events to JVMTI
1091   OM_PERFDATA_OP(Parks, inc());
1092 }
1093 
1094 
1095 // -----------------------------------------------------------------------------
1096 // Class Loader deadlock handling.
1097 //
1098 // complete_exit exits a lock returning recursion count
1099 // complete_exit/reenter operate as a wait without waiting
1100 // complete_exit requires an inflated monitor
1101 // The _owner field is not always the Thread addr even with an
1102 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1103 // thread due to contention.
1104 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1105   Thread * const Self = THREAD;
1106   assert(Self->is_Java_thread(), "Must be Java thread!");
1107   JavaThread *jt = (JavaThread *)THREAD;
1108 
1109   assert(InitDone, "Unexpectedly not initialized");
1110 
1111   if (THREAD != _owner) {
1112     if (THREAD->is_lock_owned ((address)_owner)) {
1113       assert(_recursions == 0, "internal state error");
1114       _owner = THREAD;   // Convert from basiclock addr to Thread addr
1115       _recursions = 0;
1116     }
1117   }
1118 
1119   guarantee(Self == _owner, "complete_exit not owner");
1120   intptr_t save = _recursions; // record the old recursion count
1121   _recursions = 0;        // set the recursion level to be 0
1122   exit(true, Self);           // exit the monitor
1123   guarantee(_owner != Self, "invariant");
1124   return save;
1125 }
1126 
1127 // reenter() enters a lock and sets recursion count
1128 // complete_exit/reenter operate as a wait without waiting
1129 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1130   Thread * const Self = THREAD;
1131   assert(Self->is_Java_thread(), "Must be Java thread!");
1132   JavaThread *jt = (JavaThread *)THREAD;
1133 
1134   guarantee(_owner != Self, "reenter already owner");
1135   enter(THREAD);       // enter the monitor

1136   guarantee(_recursions == 0, "reenter recursion");
1137   _recursions = recursions;
1138   return;
1139 }
1140 
1141 
1142 // -----------------------------------------------------------------------------
1143 // A macro is used below because there may already be a pending
1144 // exception which should not abort the execution of the routines
1145 // which use this (which is why we don't put this into check_slow and
1146 // call it with a CHECK argument).
1147 
1148 #define CHECK_OWNER()                                                       \
1149   do {                                                                      \
1150     if (THREAD != _owner) {                                                 \
1151       if (THREAD->is_lock_owned((address) _owner)) {                        \
1152         _owner = THREAD;  /* Convert from basiclock addr to Thread addr */  \
1153         _recursions = 0;                                                    \
1154       } else {                                                              \
1155         THROW(vmSymbols::java_lang_IllegalMonitorStateException());         \
1156       }                                                                     \
1157     }                                                                       \
1158   } while (false)
1159 
1160 // check_slow() is a misnomer.  It's called to simply to throw an IMSX exception.
1161 // TODO-FIXME: remove check_slow() -- it's likely dead.
1162 
1163 void ObjectMonitor::check_slow(TRAPS) {
1164   assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1165   THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1166 }
1167 
1168 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1169                                     ObjectMonitor* monitor,
1170                                     jlong notifier_tid,
1171                                     jlong timeout,
1172                                     bool timedout) {
1173   assert(event != NULL, "invariant");
1174   assert(monitor != NULL, "invariant");
1175   event->set_monitorClass(((oop)monitor->object())->klass());
1176   event->set_timeout(timeout);
1177   event->set_address((uintptr_t)monitor->object_addr());
1178   event->set_notifier(notifier_tid);
1179   event->set_timedOut(timedout);
1180   event->commit();
1181 }
1182 
1183 // -----------------------------------------------------------------------------
1184 // Wait/Notify/NotifyAll
1185 //
1186 // Note: a subset of changes to ObjectMonitor::wait()
1187 // will need to be replicated in complete_exit
1188 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1189   Thread * const Self = THREAD;
1190   assert(Self->is_Java_thread(), "Must be Java thread!");
1191   JavaThread *jt = (JavaThread *)THREAD;
1192 
1193   assert(InitDone, "Unexpectedly not initialized");
1194 
1195   // Throw IMSX or IEX.
1196   CHECK_OWNER();
1197 
1198   EventJavaMonitorWait event;
1199 
1200   // check for a pending interrupt
1201   if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1202     // post monitor waited event.  Note that this is past-tense, we are done waiting.
1203     if (JvmtiExport::should_post_monitor_waited()) {
1204       // Note: 'false' parameter is passed here because the
1205       // wait was not timed out due to thread interrupt.
1206       JvmtiExport::post_monitor_waited(jt, this, false);
1207 
1208       // In this short circuit of the monitor wait protocol, the
1209       // current thread never drops ownership of the monitor and
1210       // never gets added to the wait queue so the current thread
1211       // cannot be made the successor. This means that the
1212       // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1213       // consume an unpark() meant for the ParkEvent associated with
1214       // this ObjectMonitor.
1215     }
1216     if (event.should_commit()) {
1217       post_monitor_wait_event(&event, this, 0, millis, false);
1218     }
1219     THROW(vmSymbols::java_lang_InterruptedException());
1220     return;
1221   }
1222 
1223   assert(Self->_Stalled == 0, "invariant");
1224   Self->_Stalled = intptr_t(this);
1225   jt->set_current_waiting_monitor(this);
1226 
1227   // create a node to be put into the queue
1228   // Critically, after we reset() the event but prior to park(), we must check
1229   // for a pending interrupt.
1230   ObjectWaiter node(Self);
1231   node.TState = ObjectWaiter::TS_WAIT;
1232   Self->_ParkEvent->reset();
1233   OrderAccess::fence();          // ST into Event; membar ; LD interrupted-flag
1234 
1235   // Enter the waiting queue, which is a circular doubly linked list in this case
1236   // but it could be a priority queue or any data structure.
1237   // _WaitSetLock protects the wait queue.  Normally the wait queue is accessed only
1238   // by the the owner of the monitor *except* in the case where park()
1239   // returns because of a timeout of interrupt.  Contention is exceptionally rare
1240   // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1241 
1242   Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1243   AddWaiter(&node);
1244   Thread::SpinRelease(&_WaitSetLock);
1245 
1246   _Responsible = NULL;
1247 
1248   intptr_t save = _recursions; // record the old recursion count
1249   _waiters++;                  // increment the number of waiters
1250   _recursions = 0;             // set the recursion level to be 1
1251   exit(true, Self);                    // exit the monitor
1252   guarantee(_owner != Self, "invariant");
1253 
1254   // The thread is on the WaitSet list - now park() it.
1255   // On MP systems it's conceivable that a brief spin before we park
1256   // could be profitable.
1257   //
1258   // TODO-FIXME: change the following logic to a loop of the form
1259   //   while (!timeout && !interrupted && _notified == 0) park()
1260 
1261   int ret = OS_OK;
1262   int WasNotified = 0;
1263   { // State transition wrappers
1264     OSThread* osthread = Self->osthread();
1265     OSThreadWaitState osts(osthread, true);
1266     {
1267       ThreadBlockInVM tbivm(jt);
1268       // Thread is in thread_blocked state and oop access is unsafe.
1269       jt->set_suspend_equivalent();
1270 
1271       if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1272         // Intentionally empty
1273       } else if (node._notified == 0) {
1274         if (millis <= 0) {
1275           Self->_ParkEvent->park();
1276         } else {
1277           ret = Self->_ParkEvent->park(millis);
1278         }
1279       }
1280 
1281       // were we externally suspended while we were waiting?
1282       if (ExitSuspendEquivalent (jt)) {
1283         // TODO-FIXME: add -- if succ == Self then succ = null.
1284         jt->java_suspend_self();
1285       }
1286 
1287     } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1288 
1289     // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1290     // from the WaitSet to the EntryList.
1291     // See if we need to remove Node from the WaitSet.
1292     // We use double-checked locking to avoid grabbing _WaitSetLock
1293     // if the thread is not on the wait queue.
1294     //
1295     // Note that we don't need a fence before the fetch of TState.
1296     // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1297     // written by the is thread. (perhaps the fetch might even be satisfied
1298     // by a look-aside into the processor's own store buffer, although given
1299     // the length of the code path between the prior ST and this load that's
1300     // highly unlikely).  If the following LD fetches a stale TS_WAIT value
1301     // then we'll acquire the lock and then re-fetch a fresh TState value.
1302     // That is, we fail toward safety.
1303 
1304     if (node.TState == ObjectWaiter::TS_WAIT) {
1305       Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1306       if (node.TState == ObjectWaiter::TS_WAIT) {
1307         DequeueSpecificWaiter(&node);       // unlink from WaitSet
1308         assert(node._notified == 0, "invariant");
1309         node.TState = ObjectWaiter::TS_RUN;
1310       }
1311       Thread::SpinRelease(&_WaitSetLock);
1312     }
1313 
1314     // The thread is now either on off-list (TS_RUN),
1315     // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1316     // The Node's TState variable is stable from the perspective of this thread.
1317     // No other threads will asynchronously modify TState.
1318     guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1319     OrderAccess::loadload();
1320     if (_succ == Self) _succ = NULL;
1321     WasNotified = node._notified;
1322 
1323     // Reentry phase -- reacquire the monitor.
1324     // re-enter contended monitor after object.wait().
1325     // retain OBJECT_WAIT state until re-enter successfully completes
1326     // Thread state is thread_in_vm and oop access is again safe,
1327     // although the raw address of the object may have changed.
1328     // (Don't cache naked oops over safepoints, of course).
1329 
1330     // post monitor waited event. Note that this is past-tense, we are done waiting.
1331     if (JvmtiExport::should_post_monitor_waited()) {
1332       JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1333 
1334       if (node._notified != 0 && _succ == Self) {
1335         // In this part of the monitor wait-notify-reenter protocol it
1336         // is possible (and normal) for another thread to do a fastpath
1337         // monitor enter-exit while this thread is still trying to get
1338         // to the reenter portion of the protocol.
1339         //
1340         // The ObjectMonitor was notified and the current thread is
1341         // the successor which also means that an unpark() has already
1342         // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1343         // consume the unpark() that was done when the successor was
1344         // set because the same ParkEvent is shared between Java
1345         // monitors and JVM/TI RawMonitors (for now).
1346         //
1347         // We redo the unpark() to ensure forward progress, i.e., we
1348         // don't want all pending threads hanging (parked) with none
1349         // entering the unlocked monitor.
1350         node._event->unpark();
1351       }
1352     }
1353 
1354     if (event.should_commit()) {
1355       post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1356     }
1357 
1358     OrderAccess::fence();
1359 
1360     assert(Self->_Stalled != 0, "invariant");
1361     Self->_Stalled = 0;
1362 
1363     assert(_owner != Self, "invariant");
1364     ObjectWaiter::TStates v = node.TState;
1365     if (v == ObjectWaiter::TS_RUN) {
1366       enter(Self);
1367     } else {
1368       guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1369       ReenterI(Self, &node);
1370       node.wait_reenter_end(this);
1371     }
1372 
1373     // Self has reacquired the lock.
1374     // Lifecycle - the node representing Self must not appear on any queues.
1375     // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1376     // want residual elements associated with this thread left on any lists.
1377     guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1378     assert(_owner == Self, "invariant");
1379     assert(_succ != Self, "invariant");
1380   } // OSThreadWaitState()
1381 
1382   jt->set_current_waiting_monitor(NULL);
1383 
1384   guarantee(_recursions == 0, "invariant");
1385   _recursions = save;     // restore the old recursion count
1386   _waiters--;             // decrement the number of waiters
1387 
1388   // Verify a few postconditions
1389   assert(_owner == Self, "invariant");
1390   assert(_succ != Self, "invariant");
1391   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
1392 
1393   // check if the notification happened
1394   if (!WasNotified) {
1395     // no, it could be timeout or Thread.interrupt() or both
1396     // check for interrupt event, otherwise it is timeout
1397     if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1398       THROW(vmSymbols::java_lang_InterruptedException());
1399     }
1400   }
1401 
1402   // NOTE: Spurious wake up will be consider as timeout.
1403   // Monitor notify has precedence over thread interrupt.
1404 }
1405 
1406 
1407 // Consider:
1408 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1409 // then instead of transferring a thread from the WaitSet to the EntryList
1410 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1411 
1412 void ObjectMonitor::INotify(Thread * Self) {
1413   Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1414   ObjectWaiter * iterator = DequeueWaiter();
1415   if (iterator != NULL) {
1416     guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1417     guarantee(iterator->_notified == 0, "invariant");
1418     // Disposition - what might we do with iterator ?
1419     // a.  add it directly to the EntryList - either tail (policy == 1)
1420     //     or head (policy == 0).
1421     // b.  push it onto the front of the _cxq (policy == 2).
1422     // For now we use (b).
1423 
1424     iterator->TState = ObjectWaiter::TS_ENTER;
1425 
1426     iterator->_notified = 1;
1427     iterator->_notifier_tid = JFR_THREAD_ID(Self);
1428 
1429     ObjectWaiter * list = _EntryList;
1430     if (list != NULL) {
1431       assert(list->_prev == NULL, "invariant");
1432       assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1433       assert(list != iterator, "invariant");
1434     }
1435 
1436     // prepend to cxq
1437     if (list == NULL) {
1438       iterator->_next = iterator->_prev = NULL;
1439       _EntryList = iterator;
1440     } else {
1441       iterator->TState = ObjectWaiter::TS_CXQ;
1442       for (;;) {
1443         ObjectWaiter * front = _cxq;
1444         iterator->_next = front;
1445         if (Atomic::cmpxchg(iterator, &_cxq, front) == front) {
1446           break;
1447         }
1448       }
1449     }
1450 
1451     // _WaitSetLock protects the wait queue, not the EntryList.  We could
1452     // move the add-to-EntryList operation, above, outside the critical section
1453     // protected by _WaitSetLock.  In practice that's not useful.  With the
1454     // exception of  wait() timeouts and interrupts the monitor owner
1455     // is the only thread that grabs _WaitSetLock.  There's almost no contention
1456     // on _WaitSetLock so it's not profitable to reduce the length of the
1457     // critical section.
1458 
1459     iterator->wait_reenter_begin(this);
1460   }
1461   Thread::SpinRelease(&_WaitSetLock);
1462 }
1463 
1464 // Consider: a not-uncommon synchronization bug is to use notify() when
1465 // notifyAll() is more appropriate, potentially resulting in stranded
1466 // threads; this is one example of a lost wakeup. A useful diagnostic
1467 // option is to force all notify() operations to behave as notifyAll().
1468 //
1469 // Note: We can also detect many such problems with a "minimum wait".
1470 // When the "minimum wait" is set to a small non-zero timeout value
1471 // and the program does not hang whereas it did absent "minimum wait",
1472 // that suggests a lost wakeup bug.
1473 
1474 void ObjectMonitor::notify(TRAPS) {
1475   CHECK_OWNER();
1476   if (_WaitSet == NULL) {
1477     return;
1478   }
1479   DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1480   INotify(THREAD);
1481   OM_PERFDATA_OP(Notifications, inc(1));
1482 }
1483 
1484 
1485 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1486 // from the waitset to the EntryList. This could be done more efficiently with a
1487 // single bulk transfer but in practice it's not time-critical. Beware too,
1488 // that in prepend-mode we invert the order of the waiters. Let's say that the
1489 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1490 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1491 
1492 void ObjectMonitor::notifyAll(TRAPS) {
1493   CHECK_OWNER();
1494   if (_WaitSet == NULL) {
1495     return;
1496   }
1497 
1498   DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1499   int tally = 0;
1500   while (_WaitSet != NULL) {
1501     tally++;
1502     INotify(THREAD);
1503   }
1504 
1505   OM_PERFDATA_OP(Notifications, inc(tally));
1506 }
1507 
1508 // -----------------------------------------------------------------------------
1509 // Adaptive Spinning Support
1510 //
1511 // Adaptive spin-then-block - rational spinning
1512 //
1513 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1514 // algorithm.  On high order SMP systems it would be better to start with
1515 // a brief global spin and then revert to spinning locally.  In the spirit of MCS/CLH,
1516 // a contending thread could enqueue itself on the cxq and then spin locally
1517 // on a thread-specific variable such as its ParkEvent._Event flag.
1518 // That's left as an exercise for the reader.  Note that global spinning is
1519 // not problematic on Niagara, as the L2 cache serves the interconnect and
1520 // has both low latency and massive bandwidth.
1521 //
1522 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1523 // acquisition attempts where we opt to spin --  at 100% and vary the spin count
1524 // (duration) or we can fix the count at approximately the duration of
1525 // a context switch and vary the frequency.   Of course we could also
1526 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1527 // For a description of 'Adaptive spin-then-block mutual exclusion in
1528 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1529 //
1530 // This implementation varies the duration "D", where D varies with
1531 // the success rate of recent spin attempts. (D is capped at approximately
1532 // length of a round-trip context switch).  The success rate for recent
1533 // spin attempts is a good predictor of the success rate of future spin
1534 // attempts.  The mechanism adapts automatically to varying critical
1535 // section length (lock modality), system load and degree of parallelism.
1536 // D is maintained per-monitor in _SpinDuration and is initialized
1537 // optimistically.  Spin frequency is fixed at 100%.
1538 //
1539 // Note that _SpinDuration is volatile, but we update it without locks
1540 // or atomics.  The code is designed so that _SpinDuration stays within
1541 // a reasonable range even in the presence of races.  The arithmetic
1542 // operations on _SpinDuration are closed over the domain of legal values,
1543 // so at worst a race will install and older but still legal value.
1544 // At the very worst this introduces some apparent non-determinism.
1545 // We might spin when we shouldn't or vice-versa, but since the spin
1546 // count are relatively short, even in the worst case, the effect is harmless.
1547 //
1548 // Care must be taken that a low "D" value does not become an
1549 // an absorbing state.  Transient spinning failures -- when spinning
1550 // is overall profitable -- should not cause the system to converge
1551 // on low "D" values.  We want spinning to be stable and predictable
1552 // and fairly responsive to change and at the same time we don't want
1553 // it to oscillate, become metastable, be "too" non-deterministic,
1554 // or converge on or enter undesirable stable absorbing states.
1555 //
1556 // We implement a feedback-based control system -- using past behavior
1557 // to predict future behavior.  We face two issues: (a) if the
1558 // input signal is random then the spin predictor won't provide optimal
1559 // results, and (b) if the signal frequency is too high then the control
1560 // system, which has some natural response lag, will "chase" the signal.
1561 // (b) can arise from multimodal lock hold times.  Transient preemption
1562 // can also result in apparent bimodal lock hold times.
1563 // Although sub-optimal, neither condition is particularly harmful, as
1564 // in the worst-case we'll spin when we shouldn't or vice-versa.
1565 // The maximum spin duration is rather short so the failure modes aren't bad.
1566 // To be conservative, I've tuned the gain in system to bias toward
1567 // _not spinning.  Relatedly, the system can sometimes enter a mode where it
1568 // "rings" or oscillates between spinning and not spinning.  This happens
1569 // when spinning is just on the cusp of profitability, however, so the
1570 // situation is not dire.  The state is benign -- there's no need to add
1571 // hysteresis control to damp the transition rate between spinning and
1572 // not spinning.
1573 
1574 // Spinning: Fixed frequency (100%), vary duration
1575 int ObjectMonitor::TrySpin(Thread * Self) {
1576   // Dumb, brutal spin.  Good for comparative measurements against adaptive spinning.
1577   int ctr = Knob_FixedSpin;
1578   if (ctr != 0) {
1579     while (--ctr >= 0) {
1580       if (TryLock(Self) > 0) return 1;
1581       SpinPause();
1582     }
1583     return 0;
1584   }
1585 
1586   for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1587     if (TryLock(Self) > 0) {
1588       // Increase _SpinDuration ...
1589       // Note that we don't clamp SpinDuration precisely at SpinLimit.
1590       // Raising _SpurDuration to the poverty line is key.
1591       int x = _SpinDuration;
1592       if (x < Knob_SpinLimit) {
1593         if (x < Knob_Poverty) x = Knob_Poverty;
1594         _SpinDuration = x + Knob_BonusB;
1595       }
1596       return 1;
1597     }
1598     SpinPause();
1599   }
1600 
1601   // Admission control - verify preconditions for spinning
1602   //
1603   // We always spin a little bit, just to prevent _SpinDuration == 0 from
1604   // becoming an absorbing state.  Put another way, we spin briefly to
1605   // sample, just in case the system load, parallelism, contention, or lock
1606   // modality changed.
1607   //
1608   // Consider the following alternative:
1609   // Periodically set _SpinDuration = _SpinLimit and try a long/full
1610   // spin attempt.  "Periodically" might mean after a tally of
1611   // the # of failed spin attempts (or iterations) reaches some threshold.
1612   // This takes us into the realm of 1-out-of-N spinning, where we
1613   // hold the duration constant but vary the frequency.
1614 
1615   ctr = _SpinDuration;
1616   if (ctr <= 0) return 0;
1617 
1618   if (NotRunnable(Self, (Thread *) _owner)) {
1619     return 0;
1620   }
1621 
1622   // We're good to spin ... spin ingress.
1623   // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1624   // when preparing to LD...CAS _owner, etc and the CAS is likely
1625   // to succeed.
1626   if (_succ == NULL) {
1627     _succ = Self;
1628   }
1629   Thread * prv = NULL;
1630 
1631   // There are three ways to exit the following loop:
1632   // 1.  A successful spin where this thread has acquired the lock.
1633   // 2.  Spin failure with prejudice
1634   // 3.  Spin failure without prejudice
1635 
1636   while (--ctr >= 0) {
1637 
1638     // Periodic polling -- Check for pending GC
1639     // Threads may spin while they're unsafe.
1640     // We don't want spinning threads to delay the JVM from reaching
1641     // a stop-the-world safepoint or to steal cycles from GC.
1642     // If we detect a pending safepoint we abort in order that
1643     // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1644     // this thread, if safe, doesn't steal cycles from GC.
1645     // This is in keeping with the "no loitering in runtime" rule.
1646     // We periodically check to see if there's a safepoint pending.
1647     if ((ctr & 0xFF) == 0) {
1648       if (SafepointMechanism::should_block(Self)) {
1649         goto Abort;           // abrupt spin egress
1650       }
1651       SpinPause();
1652     }
1653 
1654     // Probe _owner with TATAS
1655     // If this thread observes the monitor transition or flicker
1656     // from locked to unlocked to locked, then the odds that this
1657     // thread will acquire the lock in this spin attempt go down
1658     // considerably.  The same argument applies if the CAS fails
1659     // or if we observe _owner change from one non-null value to
1660     // another non-null value.   In such cases we might abort
1661     // the spin without prejudice or apply a "penalty" to the
1662     // spin count-down variable "ctr", reducing it by 100, say.
1663 
1664     Thread * ox = (Thread *) _owner;
1665     if (ox == NULL) {
1666       ox = (Thread*)Atomic::cmpxchg(Self, &_owner, (void*)NULL);
1667       if (ox == NULL) {
1668         // The CAS succeeded -- this thread acquired ownership
1669         // Take care of some bookkeeping to exit spin state.
1670         if (_succ == Self) {
1671           _succ = NULL;
1672         }
1673 
1674         // Increase _SpinDuration :
1675         // The spin was successful (profitable) so we tend toward
1676         // longer spin attempts in the future.
1677         // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
1678         // If we acquired the lock early in the spin cycle it
1679         // makes sense to increase _SpinDuration proportionally.
1680         // Note that we don't clamp SpinDuration precisely at SpinLimit.
1681         int x = _SpinDuration;
1682         if (x < Knob_SpinLimit) {
1683           if (x < Knob_Poverty) x = Knob_Poverty;
1684           _SpinDuration = x + Knob_Bonus;
1685         }
1686         return 1;
1687       }
1688 
1689       // The CAS failed ... we can take any of the following actions:
1690       // * penalize: ctr -= CASPenalty
1691       // * exit spin with prejudice -- goto Abort;
1692       // * exit spin without prejudice.
1693       // * Since CAS is high-latency, retry again immediately.
1694       prv = ox;
1695       goto Abort;
1696     }
1697 
1698     // Did lock ownership change hands ?
1699     if (ox != prv && prv != NULL) {
1700       goto Abort;
1701     }
1702     prv = ox;
1703 
1704     // Abort the spin if the owner is not executing.
1705     // The owner must be executing in order to drop the lock.
1706     // Spinning while the owner is OFFPROC is idiocy.
1707     // Consider: ctr -= RunnablePenalty ;
1708     if (NotRunnable(Self, ox)) {
1709       goto Abort;
1710     }
1711     if (_succ == NULL) {
1712       _succ = Self;
1713     }
1714   }
1715 
1716   // Spin failed with prejudice -- reduce _SpinDuration.
1717   // TODO: Use an AIMD-like policy to adjust _SpinDuration.
1718   // AIMD is globally stable.
1719   {
1720     int x = _SpinDuration;
1721     if (x > 0) {
1722       // Consider an AIMD scheme like: x -= (x >> 3) + 100
1723       // This is globally sample and tends to damp the response.
1724       x -= Knob_Penalty;
1725       if (x < 0) x = 0;
1726       _SpinDuration = x;
1727     }
1728   }
1729 
1730  Abort:
1731   if (_succ == Self) {
1732     _succ = NULL;
1733     // Invariant: after setting succ=null a contending thread
1734     // must recheck-retry _owner before parking.  This usually happens
1735     // in the normal usage of TrySpin(), but it's safest
1736     // to make TrySpin() as foolproof as possible.
1737     OrderAccess::fence();
1738     if (TryLock(Self) > 0) return 1;
1739   }
1740   return 0;
1741 }
1742 
1743 // NotRunnable() -- informed spinning
1744 //
1745 // Don't bother spinning if the owner is not eligible to drop the lock.
1746 // Spin only if the owner thread is _thread_in_Java or _thread_in_vm.
1747 // The thread must be runnable in order to drop the lock in timely fashion.
1748 // If the _owner is not runnable then spinning will not likely be
1749 // successful (profitable).
1750 //
1751 // Beware -- the thread referenced by _owner could have died
1752 // so a simply fetch from _owner->_thread_state might trap.
1753 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
1754 // Because of the lifecycle issues, the _thread_state values
1755 // observed by NotRunnable() might be garbage.  NotRunnable must
1756 // tolerate this and consider the observed _thread_state value
1757 // as advisory.
1758 //
1759 // Beware too, that _owner is sometimes a BasicLock address and sometimes
1760 // a thread pointer.
1761 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
1762 // with the LSB of _owner.  Another option would be to probabilistically probe
1763 // the putative _owner->TypeTag value.
1764 //
1765 // Checking _thread_state isn't perfect.  Even if the thread is
1766 // in_java it might be blocked on a page-fault or have been preempted
1767 // and sitting on a ready/dispatch queue.
1768 //
1769 // The return value from NotRunnable() is *advisory* -- the
1770 // result is based on sampling and is not necessarily coherent.
1771 // The caller must tolerate false-negative and false-positive errors.
1772 // Spinning, in general, is probabilistic anyway.
1773 
1774 
1775 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) {
1776   // Check ox->TypeTag == 2BAD.
1777   if (ox == NULL) return 0;
1778 
1779   // Avoid transitive spinning ...
1780   // Say T1 spins or blocks trying to acquire L.  T1._Stalled is set to L.
1781   // Immediately after T1 acquires L it's possible that T2, also
1782   // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
1783   // This occurs transiently after T1 acquired L but before
1784   // T1 managed to clear T1.Stalled.  T2 does not need to abort
1785   // its spin in this circumstance.
1786   intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
1787 
1788   if (BlockedOn == 1) return 1;
1789   if (BlockedOn != 0) {
1790     return BlockedOn != intptr_t(this) && _owner == ox;
1791   }
1792 
1793   assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant");
1794   int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);;
1795   // consider also: jst != _thread_in_Java -- but that's overspecific.
1796   return jst == _thread_blocked || jst == _thread_in_native;
1797 }
1798 
1799 
1800 // -----------------------------------------------------------------------------
1801 // WaitSet management ...
1802 
1803 ObjectWaiter::ObjectWaiter(Thread* thread) {
1804   _next     = NULL;
1805   _prev     = NULL;
1806   _notified = 0;
1807   _notifier_tid = 0;
1808   TState    = TS_RUN;
1809   _thread   = thread;
1810   _event    = thread->_ParkEvent;
1811   _active   = false;
1812   assert(_event != NULL, "invariant");
1813 }
1814 
1815 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
1816   JavaThread *jt = (JavaThread *)this->_thread;
1817   _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
1818 }
1819 
1820 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
1821   JavaThread *jt = (JavaThread *)this->_thread;
1822   JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
1823 }
1824 
1825 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
1826   assert(node != NULL, "should not add NULL node");
1827   assert(node->_prev == NULL, "node already in list");
1828   assert(node->_next == NULL, "node already in list");
1829   // put node at end of queue (circular doubly linked list)
1830   if (_WaitSet == NULL) {
1831     _WaitSet = node;
1832     node->_prev = node;
1833     node->_next = node;
1834   } else {
1835     ObjectWaiter* head = _WaitSet;
1836     ObjectWaiter* tail = head->_prev;
1837     assert(tail->_next == head, "invariant check");
1838     tail->_next = node;
1839     head->_prev = node;
1840     node->_next = head;
1841     node->_prev = tail;
1842   }
1843 }
1844 
1845 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
1846   // dequeue the very first waiter
1847   ObjectWaiter* waiter = _WaitSet;
1848   if (waiter) {
1849     DequeueSpecificWaiter(waiter);
1850   }
1851   return waiter;
1852 }
1853 
1854 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
1855   assert(node != NULL, "should not dequeue NULL node");
1856   assert(node->_prev != NULL, "node already removed from list");
1857   assert(node->_next != NULL, "node already removed from list");
1858   // when the waiter has woken up because of interrupt,
1859   // timeout or other spurious wake-up, dequeue the
1860   // waiter from waiting list
1861   ObjectWaiter* next = node->_next;
1862   if (next == node) {
1863     assert(node->_prev == node, "invariant check");
1864     _WaitSet = NULL;
1865   } else {
1866     ObjectWaiter* prev = node->_prev;
1867     assert(prev->_next == node, "invariant check");
1868     assert(next->_prev == node, "invariant check");
1869     next->_prev = prev;
1870     prev->_next = next;
1871     if (_WaitSet == node) {
1872       _WaitSet = next;
1873     }
1874   }
1875   node->_next = NULL;
1876   node->_prev = NULL;
1877 }
1878 
1879 // -----------------------------------------------------------------------------
1880 // PerfData support
1881 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts       = NULL;
1882 PerfCounter * ObjectMonitor::_sync_FutileWakeups               = NULL;
1883 PerfCounter * ObjectMonitor::_sync_Parks                       = NULL;
1884 PerfCounter * ObjectMonitor::_sync_Notifications               = NULL;
1885 PerfCounter * ObjectMonitor::_sync_Inflations                  = NULL;
1886 PerfCounter * ObjectMonitor::_sync_Deflations                  = NULL;
1887 PerfLongVariable * ObjectMonitor::_sync_MonExtant              = NULL;
1888 
1889 // One-shot global initialization for the sync subsystem.
1890 // We could also defer initialization and initialize on-demand
1891 // the first time we call ObjectSynchronizer::inflate().
1892 // Initialization would be protected - like so many things - by
1893 // the MonitorCache_lock.
1894 
1895 void ObjectMonitor::Initialize() {
1896   assert(!InitDone, "invariant");
1897 
1898   if (!os::is_MP()) {
1899     Knob_SpinLimit = 0;
1900     Knob_PreSpin   = 0;
1901     Knob_FixedSpin = -1;
1902   }
1903 
1904   if (UsePerfData) {
1905     EXCEPTION_MARK;
1906 #define NEWPERFCOUNTER(n)                                                \
1907   {                                                                      \
1908     n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,  \
1909                                         CHECK);                          \
1910   }
1911 #define NEWPERFVARIABLE(n)                                                \
1912   {                                                                       \
1913     n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,  \
1914                                          CHECK);                          \
1915   }
1916     NEWPERFCOUNTER(_sync_Inflations);
1917     NEWPERFCOUNTER(_sync_Deflations);
1918     NEWPERFCOUNTER(_sync_ContendedLockAttempts);
1919     NEWPERFCOUNTER(_sync_FutileWakeups);
1920     NEWPERFCOUNTER(_sync_Parks);
1921     NEWPERFCOUNTER(_sync_Notifications);
1922     NEWPERFVARIABLE(_sync_MonExtant);
1923 #undef NEWPERFCOUNTER
1924 #undef NEWPERFVARIABLE
1925   }
1926 
1927   DEBUG_ONLY(InitDone = true;)










































































1928 }
--- EOF ---