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   ADIM_guarantee(_ref_count > 0, "must be positive: ref_count=%d", _ref_count);
 243 
 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, "must be Self: owner=" INTPTR_FORMAT, p2i(_owner));
 282     assert(_recursions == 0, "must be 0: recursions=" INTPTR_FORMAT,
 283            _recursions);
 284     assert(((oop)object())->mark() == markOopDesc::encode(this),
 285            "object mark must match encoded this: mark=" INTPTR_FORMAT
 286            ", encoded this=" INTPTR_FORMAT, p2i(((oop)object())->mark()),
 287            p2i(markOopDesc::encode(this)));
 288     Self->_Stalled = 0;
 289     return;
 290   }
 291 
 292   assert(_owner != Self, "invariant");
 293   assert(_succ != Self, "invariant");
 294   assert(Self->is_Java_thread(), "invariant");
 295   JavaThread * jt = (JavaThread *) Self;
 296   assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
 297   assert(jt->thread_state() != _thread_blocked, "invariant");
 298   assert(AsyncDeflateIdleMonitors || this->object() != NULL, "invariant");
 299   assert(_contentions >= 0, "must not be negative: contentions=%d", _contentions);
 300 
 301   // Prevent deflation. See ObjectSynchronizer::deflate_monitor(),
 302   // ObjectSynchronizer::deflate_monitor_using_JT() and is_busy().
 303   // Ensure the object <-> monitor relationship remains stable while
 304   // there's contention.
 305   Atomic::add(1, &_contentions);
 306 
 307   JFR_ONLY(JfrConditionalFlushWithStacktrace<EventJavaMonitorEnter> flush(jt);)
 308   EventJavaMonitorEnter event;
 309   if (event.should_commit()) {
 310     event.set_monitorClass(((oop)this->object())->klass());
 311     event.set_address((uintptr_t)(this->object_addr()));
 312   }
 313 
 314   { // Change java thread status to indicate blocked on monitor enter.
 315     JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
 316 
 317     Self->set_current_pending_monitor(this);
 318 
 319     DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
 320     if (JvmtiExport::should_post_monitor_contended_enter()) {
 321       JvmtiExport::post_monitor_contended_enter(jt, this);
 322 
 323       // The current thread does not yet own the monitor and does not
 324       // yet appear on any queues that would get it made the successor.
 325       // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
 326       // handler cannot accidentally consume an unpark() meant for the
 327       // ParkEvent associated with this ObjectMonitor.
 328     }
 329 
 330     OSThreadContendState osts(Self->osthread());
 331     ThreadBlockInVM tbivm(jt);
 332 
 333     // TODO-FIXME: change the following for(;;) loop to straight-line code.
 334     for (;;) {
 335       jt->set_suspend_equivalent();
 336       // cleared by handle_special_suspend_equivalent_condition()
 337       // or java_suspend_self()
 338 
 339       EnterI(THREAD);
 340 
 341       if (!ExitSuspendEquivalent(jt)) break;
 342 
 343       // We have acquired the contended monitor, but while we were
 344       // waiting another thread suspended us. We don't want to enter
 345       // the monitor while suspended because that would surprise the
 346       // thread that suspended us.
 347       //
 348       _recursions = 0;
 349       _succ = NULL;
 350       exit(false, Self);
 351 
 352       jt->java_suspend_self();
 353     }
 354     Self->set_current_pending_monitor(NULL);
 355 
 356     // We cleared the pending monitor info since we've just gotten past
 357     // the enter-check-for-suspend dance and we now own the monitor free
 358     // and clear, i.e., it is no longer pending. The ThreadBlockInVM
 359     // destructor can go to a safepoint at the end of this block. If we
 360     // do a thread dump during that safepoint, then this thread will show
 361     // as having "-locked" the monitor, but the OS and java.lang.Thread
 362     // states will still report that the thread is blocked trying to
 363     // acquire it.
 364   }
 365 
 366   Atomic::dec(&_contentions);
 367   assert(_contentions >= 0, "must not be negative: contentions=%d", _contentions);
 368   Self->_Stalled = 0;
 369 
 370   // Must either set _recursions = 0 or ASSERT _recursions == 0.
 371   assert(_recursions == 0, "invariant");
 372   assert(_owner == Self, "invariant");
 373   assert(_succ != Self, "invariant");
 374   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 375 
 376   // The thread -- now the owner -- is back in vm mode.
 377   // Report the glorious news via TI,DTrace and jvmstat.
 378   // The probe effect is non-trivial.  All the reportage occurs
 379   // while we hold the monitor, increasing the length of the critical
 380   // section.  Amdahl's parallel speedup law comes vividly into play.
 381   //
 382   // Another option might be to aggregate the events (thread local or
 383   // per-monitor aggregation) and defer reporting until a more opportune
 384   // time -- such as next time some thread encounters contention but has
 385   // yet to acquire the lock.  While spinning that thread could
 386   // spinning we could increment JVMStat counters, etc.
 387 
 388   DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
 389   if (JvmtiExport::should_post_monitor_contended_entered()) {
 390     JvmtiExport::post_monitor_contended_entered(jt, this);
 391 
 392     // The current thread already owns the monitor and is not going to
 393     // call park() for the remainder of the monitor enter protocol. So
 394     // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
 395     // event handler consumed an unpark() issued by the thread that
 396     // just exited the monitor.
 397   }
 398   if (event.should_commit()) {
 399     event.set_previousOwner((uintptr_t)_previous_owner_tid);
 400     event.commit();
 401   }
 402   OM_PERFDATA_OP(ContendedLockAttempts, inc());
 403 }
 404 
 405 // Caveat: TryLock() is not necessarily serializing if it returns failure.
 406 // Callers must compensate as needed.
 407 
 408 int ObjectMonitor::TryLock(Thread * Self) {
 409   void * own = _owner;
 410   if (own != NULL) return 0;
 411   if (Atomic::replace_if_null(Self, &_owner)) {
 412     // Either guarantee _recursions == 0 or set _recursions = 0.
 413     assert(_recursions == 0, "invariant");
 414     assert(_owner == Self, "invariant");
 415     return 1;
 416   }
 417   // The lock had been free momentarily, but we lost the race to the lock.
 418   // Interference -- the CAS failed.
 419   // We can either return -1 or retry.
 420   // Retry doesn't make as much sense because the lock was just acquired.
 421   return -1;
 422 }
 423 
 424 // Install the displaced mark word (dmw) of a deflating ObjectMonitor
 425 // into the header of the object associated with the monitor. This
 426 // idempotent method is called by a thread that is deflating a
 427 // monitor and by other threads that have detected a race with the
 428 // deflation process.
 429 void ObjectMonitor::install_displaced_markword_in_object(const oop obj) {
 430   // This function must only be called when (owner == DEFLATER_MARKER
 431   // && ref_count <= 0), but we can't guarantee that here because
 432   // those values could change when the ObjectMonitor gets moved from
 433   // the global free list to a per-thread free list.
 434 
 435   guarantee(obj != NULL, "must be non-NULL");
 436   if (object() != obj) {
 437     // ObjectMonitor's object ref no longer refers to the target object
 438     // so the object's header has already been restored.
 439     return;
 440   }
 441 
 442   markOop dmw = header();
 443   if (dmw == NULL) {
 444     // ObjectMonitor's header/dmw has been cleared by the deflating
 445     // thread so the object's header has already been restored.
 446     return;
 447   }
 448 
 449   // A non-NULL dmw has to be either neutral (not locked and not marked)
 450   // or is already participating in this restoration protocol.
 451   assert(dmw->is_neutral() || (dmw->is_marked() && dmw->hash() == 0),
 452          "failed precondition: dmw=" INTPTR_FORMAT, p2i(dmw));
 453 
 454   markOop marked_dmw = NULL;
 455   if (!dmw->is_marked() && dmw->hash() == 0) {
 456     // This dmw has not yet started the restoration protocol so we
 457     // mark a copy of the dmw to begin the protocol.
 458     // Note: A dmw with a hashcode does not take this code path.
 459     marked_dmw = dmw->set_marked();
 460 
 461     // All of the callers to this function can be racing with each
 462     // other trying to update the _header field.
 463     dmw = (markOop) Atomic::cmpxchg(marked_dmw, &_header, dmw);
 464     if (dmw == NULL) {
 465       // ObjectMonitor's header/dmw has been cleared by the deflating
 466       // thread so the object's header has already been restored.
 467       return;
 468     }
 469     // The _header field is now marked. The winner's 'dmw' variable
 470     // contains the original, unmarked header/dmw value and any
 471     // losers have a marked header/dmw value that will be cleaned
 472     // up below.
 473   }
 474 
 475   if (dmw->is_marked()) {
 476     // Clear the mark from the header/dmw copy in preparation for
 477     // possible restoration from this thread.
 478     assert(dmw->hash() == 0, "hashcode must be 0: dmw=" INTPTR_FORMAT,
 479            p2i(dmw));
 480     dmw = dmw->set_unmarked();
 481   }
 482   assert(dmw->is_neutral(), "must be neutral: dmw=" INTPTR_FORMAT, p2i(dmw));
 483 
 484   // Install displaced mark word if the object's header still points
 485   // to this ObjectMonitor. All racing callers to this function will
 486   // reach this point, but only one can win.
 487   obj->cas_set_mark(dmw, markOopDesc::encode(this));
 488 
 489   // Note: It does not matter which thread restored the header/dmw
 490   // into the object's header. The thread deflating the monitor just
 491   // wanted the object's header restored and it is. The threads that
 492   // detected a race with the deflation process also wanted the
 493   // object's header restored before they retry their operation and
 494   // because it is restored they will only retry once.
 495 
 496   if (marked_dmw != NULL) {
 497     // Clear _header to NULL if it is still marked_dmw so a racing
 498     // install_displaced_markword_in_object() can bail out sooner.
 499     Atomic::cmpxchg((markOop)NULL, &_header, marked_dmw);
 500   }
 501 }
 502 
 503 #define MAX_RECHECK_INTERVAL 1000
 504 
 505 void ObjectMonitor::EnterI(TRAPS) {
 506   ADIM_guarantee(_ref_count > 0, "must be positive: ref_count=%d", _ref_count);
 507 
 508   Thread * const Self = THREAD;
 509   assert(Self->is_Java_thread(), "invariant");
 510   assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant");
 511 
 512   // Try the lock - TATAS
 513   if (TryLock (Self) > 0) {
 514     assert(_succ != Self, "invariant");
 515     assert(_owner == Self, "invariant");
 516     assert(_Responsible != Self, "invariant");
 517     return;
 518   }
 519 
 520   if (_owner == DEFLATER_MARKER) {
 521     // The deflation protocol finished the first part (setting owner), but
 522     // it failed the second part (making ref_count negative) and bailed.
 523     if (Atomic::cmpxchg(Self, &_owner, DEFLATER_MARKER) == DEFLATER_MARKER) {
 524       // Acquired the monitor.
 525       assert(_succ != Self, "invariant");
 526       assert(_Responsible != Self, "invariant");
 527       return;
 528     }
 529   }
 530 
 531   assert(InitDone, "Unexpectedly not initialized");
 532 
 533   // We try one round of spinning *before* enqueueing Self.
 534   //
 535   // If the _owner is ready but OFFPROC we could use a YieldTo()
 536   // operation to donate the remainder of this thread's quantum
 537   // to the owner.  This has subtle but beneficial affinity
 538   // effects.
 539 
 540   if (TrySpin(Self) > 0) {
 541     assert(_owner == Self, "invariant");
 542     assert(_succ != Self, "invariant");
 543     assert(_Responsible != Self, "invariant");
 544     return;
 545   }
 546 
 547   // The Spin failed -- Enqueue and park the thread ...
 548   assert(_succ != Self, "invariant");
 549   assert(_owner != Self, "invariant");
 550   assert(_Responsible != Self, "invariant");
 551 
 552   // Enqueue "Self" on ObjectMonitor's _cxq.
 553   //
 554   // Node acts as a proxy for Self.
 555   // As an aside, if were to ever rewrite the synchronization code mostly
 556   // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
 557   // Java objects.  This would avoid awkward lifecycle and liveness issues,
 558   // as well as eliminate a subset of ABA issues.
 559   // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
 560 
 561   ObjectWaiter node(Self);
 562   Self->_ParkEvent->reset();
 563   node._prev   = (ObjectWaiter *) 0xBAD;
 564   node.TState  = ObjectWaiter::TS_CXQ;
 565 
 566   // Push "Self" onto the front of the _cxq.
 567   // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
 568   // Note that spinning tends to reduce the rate at which threads
 569   // enqueue and dequeue on EntryList|cxq.
 570   ObjectWaiter * nxt;
 571   for (;;) {
 572     node._next = nxt = _cxq;
 573     if (Atomic::cmpxchg(&node, &_cxq, nxt) == nxt) break;
 574 
 575     // Interference - the CAS failed because _cxq changed.  Just retry.
 576     // As an optional optimization we retry the lock.
 577     if (TryLock (Self) > 0) {
 578       assert(_succ != Self, "invariant");
 579       assert(_owner == Self, "invariant");
 580       assert(_Responsible != Self, "invariant");
 581       return;
 582     }
 583   }
 584 
 585   // Check for cxq|EntryList edge transition to non-null.  This indicates
 586   // the onset of contention.  While contention persists exiting threads
 587   // will use a ST:MEMBAR:LD 1-1 exit protocol.  When contention abates exit
 588   // operations revert to the faster 1-0 mode.  This enter operation may interleave
 589   // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
 590   // arrange for one of the contending thread to use a timed park() operations
 591   // to detect and recover from the race.  (Stranding is form of progress failure
 592   // where the monitor is unlocked but all the contending threads remain parked).
 593   // That is, at least one of the contended threads will periodically poll _owner.
 594   // One of the contending threads will become the designated "Responsible" thread.
 595   // The Responsible thread uses a timed park instead of a normal indefinite park
 596   // operation -- it periodically wakes and checks for and recovers from potential
 597   // strandings admitted by 1-0 exit operations.   We need at most one Responsible
 598   // thread per-monitor at any given moment.  Only threads on cxq|EntryList may
 599   // be responsible for a monitor.
 600   //
 601   // Currently, one of the contended threads takes on the added role of "Responsible".
 602   // A viable alternative would be to use a dedicated "stranding checker" thread
 603   // that periodically iterated over all the threads (or active monitors) and unparked
 604   // successors where there was risk of stranding.  This would help eliminate the
 605   // timer scalability issues we see on some platforms as we'd only have one thread
 606   // -- the checker -- parked on a timer.
 607 
 608   if (nxt == NULL && _EntryList == NULL) {
 609     // Try to assume the role of responsible thread for the monitor.
 610     // CONSIDER:  ST vs CAS vs { if (Responsible==null) Responsible=Self }
 611     Atomic::replace_if_null(Self, &_Responsible);
 612   }
 613 
 614   // The lock might have been released while this thread was occupied queueing
 615   // itself onto _cxq.  To close the race and avoid "stranding" and
 616   // progress-liveness failure we must resample-retry _owner before parking.
 617   // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
 618   // In this case the ST-MEMBAR is accomplished with CAS().
 619   //
 620   // TODO: Defer all thread state transitions until park-time.
 621   // Since state transitions are heavy and inefficient we'd like
 622   // to defer the state transitions until absolutely necessary,
 623   // and in doing so avoid some transitions ...
 624 
 625   int nWakeups = 0;
 626   int recheckInterval = 1;
 627 
 628   for (;;) {
 629 
 630     if (TryLock(Self) > 0) break;
 631     assert(_owner != Self, "invariant");
 632 
 633     // park self
 634     if (_Responsible == Self) {
 635       Self->_ParkEvent->park((jlong) recheckInterval);
 636       // Increase the recheckInterval, but clamp the value.
 637       recheckInterval *= 8;
 638       if (recheckInterval > MAX_RECHECK_INTERVAL) {
 639         recheckInterval = MAX_RECHECK_INTERVAL;
 640       }
 641     } else {
 642       Self->_ParkEvent->park();
 643     }
 644 
 645     if (TryLock(Self) > 0) break;
 646 
 647     if (_owner == DEFLATER_MARKER) {
 648       // The deflation protocol finished the first part (setting owner), but
 649       // it failed the second part (making ref_count negative) and bailed.
 650       if (Atomic::cmpxchg(Self, &_owner, DEFLATER_MARKER) == DEFLATER_MARKER) {
 651         // Acquired the monitor.
 652         break;
 653       }
 654     }
 655 
 656     // The lock is still contested.
 657     // Keep a tally of the # of futile wakeups.
 658     // Note that the counter is not protected by a lock or updated by atomics.
 659     // That is by design - we trade "lossy" counters which are exposed to
 660     // races during updates for a lower probe effect.
 661 
 662     // This PerfData object can be used in parallel with a safepoint.
 663     // See the work around in PerfDataManager::destroy().
 664     OM_PERFDATA_OP(FutileWakeups, inc());
 665     ++nWakeups;
 666 
 667     // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
 668     // We can defer clearing _succ until after the spin completes
 669     // TrySpin() must tolerate being called with _succ == Self.
 670     // Try yet another round of adaptive spinning.
 671     if (TrySpin(Self) > 0) break;
 672 
 673     // We can find that we were unpark()ed and redesignated _succ while
 674     // we were spinning.  That's harmless.  If we iterate and call park(),
 675     // park() will consume the event and return immediately and we'll
 676     // just spin again.  This pattern can repeat, leaving _succ to simply
 677     // spin on a CPU.
 678 
 679     if (_succ == Self) _succ = NULL;
 680 
 681     // Invariant: after clearing _succ a thread *must* retry _owner before parking.
 682     OrderAccess::fence();
 683   }
 684 
 685   // Egress :
 686   // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
 687   // Normally we'll find Self on the EntryList .
 688   // From the perspective of the lock owner (this thread), the
 689   // EntryList is stable and cxq is prepend-only.
 690   // The head of cxq is volatile but the interior is stable.
 691   // In addition, Self.TState is stable.
 692 
 693   assert(_owner == Self, "invariant");
 694   assert(object() != NULL, "invariant");
 695   // I'd like to write:
 696   //   guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
 697   // but as we're at a safepoint that's not safe.
 698 
 699   UnlinkAfterAcquire(Self, &node);
 700   if (_succ == Self) _succ = NULL;
 701 
 702   assert(_succ != Self, "invariant");
 703   if (_Responsible == Self) {
 704     _Responsible = NULL;
 705     OrderAccess::fence(); // Dekker pivot-point
 706 
 707     // We may leave threads on cxq|EntryList without a designated
 708     // "Responsible" thread.  This is benign.  When this thread subsequently
 709     // exits the monitor it can "see" such preexisting "old" threads --
 710     // threads that arrived on the cxq|EntryList before the fence, above --
 711     // by LDing cxq|EntryList.  Newly arrived threads -- that is, threads
 712     // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
 713     // non-null and elect a new "Responsible" timer thread.
 714     //
 715     // This thread executes:
 716     //    ST Responsible=null; MEMBAR    (in enter epilogue - here)
 717     //    LD cxq|EntryList               (in subsequent exit)
 718     //
 719     // Entering threads in the slow/contended path execute:
 720     //    ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
 721     //    The (ST cxq; MEMBAR) is accomplished with CAS().
 722     //
 723     // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
 724     // exit operation from floating above the ST Responsible=null.
 725   }
 726 
 727   // We've acquired ownership with CAS().
 728   // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
 729   // But since the CAS() this thread may have also stored into _succ,
 730   // EntryList, cxq or Responsible.  These meta-data updates must be
 731   // visible __before this thread subsequently drops the lock.
 732   // Consider what could occur if we didn't enforce this constraint --
 733   // STs to monitor meta-data and user-data could reorder with (become
 734   // visible after) the ST in exit that drops ownership of the lock.
 735   // Some other thread could then acquire the lock, but observe inconsistent
 736   // or old monitor meta-data and heap data.  That violates the JMM.
 737   // To that end, the 1-0 exit() operation must have at least STST|LDST
 738   // "release" barrier semantics.  Specifically, there must be at least a
 739   // STST|LDST barrier in exit() before the ST of null into _owner that drops
 740   // the lock.   The barrier ensures that changes to monitor meta-data and data
 741   // protected by the lock will be visible before we release the lock, and
 742   // therefore before some other thread (CPU) has a chance to acquire the lock.
 743   // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
 744   //
 745   // Critically, any prior STs to _succ or EntryList must be visible before
 746   // the ST of null into _owner in the *subsequent* (following) corresponding
 747   // monitorexit.  Recall too, that in 1-0 mode monitorexit does not necessarily
 748   // execute a serializing instruction.
 749 
 750   return;
 751 }
 752 
 753 // ReenterI() is a specialized inline form of the latter half of the
 754 // contended slow-path from EnterI().  We use ReenterI() only for
 755 // monitor reentry in wait().
 756 //
 757 // In the future we should reconcile EnterI() and ReenterI().
 758 
 759 void ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) {
 760   ADIM_guarantee(_ref_count > 0, "must be positive: ref_count=%d", _ref_count);
 761 
 762   assert(Self != NULL, "invariant");
 763   assert(SelfNode != NULL, "invariant");
 764   assert(SelfNode->_thread == Self, "invariant");
 765   assert(_waiters > 0, "invariant");
 766   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 767   assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
 768   JavaThread * jt = (JavaThread *) Self;
 769 
 770   int nWakeups = 0;
 771   for (;;) {
 772     ObjectWaiter::TStates v = SelfNode->TState;
 773     guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
 774     assert(_owner != Self, "invariant");
 775 
 776     if (TryLock(Self) > 0) break;
 777     if (TrySpin(Self) > 0) break;
 778 
 779     if (_owner == DEFLATER_MARKER) {
 780       // The deflation protocol finished the first part (setting owner), but
 781       // it failed the second part (making ref_count negative) and bailed.
 782       if (Atomic::cmpxchg(Self, &_owner, DEFLATER_MARKER) == DEFLATER_MARKER) {
 783         // Acquired the monitor.
 784         break;
 785       }
 786     }
 787 
 788     // State transition wrappers around park() ...
 789     // ReenterI() wisely defers state transitions until
 790     // it's clear we must park the thread.
 791     {
 792       OSThreadContendState osts(Self->osthread());
 793       ThreadBlockInVM tbivm(jt);
 794 
 795       // cleared by handle_special_suspend_equivalent_condition()
 796       // or java_suspend_self()
 797       jt->set_suspend_equivalent();
 798       Self->_ParkEvent->park();
 799 
 800       // were we externally suspended while we were waiting?
 801       for (;;) {
 802         if (!ExitSuspendEquivalent(jt)) break;
 803         if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
 804         jt->java_suspend_self();
 805         jt->set_suspend_equivalent();
 806       }
 807     }
 808 
 809     // Try again, but just so we distinguish between futile wakeups and
 810     // successful wakeups.  The following test isn't algorithmically
 811     // necessary, but it helps us maintain sensible statistics.
 812     if (TryLock(Self) > 0) break;
 813 
 814     // The lock is still contested.
 815     // Keep a tally of the # of futile wakeups.
 816     // Note that the counter is not protected by a lock or updated by atomics.
 817     // That is by design - we trade "lossy" counters which are exposed to
 818     // races during updates for a lower probe effect.
 819     ++nWakeups;
 820 
 821     // Assuming this is not a spurious wakeup we'll normally
 822     // find that _succ == Self.
 823     if (_succ == Self) _succ = NULL;
 824 
 825     // Invariant: after clearing _succ a contending thread
 826     // *must* retry  _owner before parking.
 827     OrderAccess::fence();
 828 
 829     // This PerfData object can be used in parallel with a safepoint.
 830     // See the work around in PerfDataManager::destroy().
 831     OM_PERFDATA_OP(FutileWakeups, inc());
 832   }
 833 
 834   // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
 835   // Normally we'll find Self on the EntryList.
 836   // Unlinking from the EntryList is constant-time and atomic-free.
 837   // From the perspective of the lock owner (this thread), the
 838   // EntryList is stable and cxq is prepend-only.
 839   // The head of cxq is volatile but the interior is stable.
 840   // In addition, Self.TState is stable.
 841 
 842   assert(_owner == Self, "invariant");
 843   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 844   UnlinkAfterAcquire(Self, SelfNode);
 845   if (_succ == Self) _succ = NULL;
 846   assert(_succ != Self, "invariant");
 847   SelfNode->TState = ObjectWaiter::TS_RUN;
 848   OrderAccess::fence();      // see comments at the end of EnterI()
 849 }
 850 
 851 // By convention we unlink a contending thread from EntryList|cxq immediately
 852 // after the thread acquires the lock in ::enter().  Equally, we could defer
 853 // unlinking the thread until ::exit()-time.
 854 
 855 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) {
 856   assert(_owner == Self, "invariant");
 857   assert(SelfNode->_thread == Self, "invariant");
 858 
 859   if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
 860     // Normal case: remove Self from the DLL EntryList .
 861     // This is a constant-time operation.
 862     ObjectWaiter * nxt = SelfNode->_next;
 863     ObjectWaiter * prv = SelfNode->_prev;
 864     if (nxt != NULL) nxt->_prev = prv;
 865     if (prv != NULL) prv->_next = nxt;
 866     if (SelfNode == _EntryList) _EntryList = nxt;
 867     assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
 868     assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
 869   } else {
 870     assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant");
 871     // Inopportune interleaving -- Self is still on the cxq.
 872     // This usually means the enqueue of self raced an exiting thread.
 873     // Normally we'll find Self near the front of the cxq, so
 874     // dequeueing is typically fast.  If needbe we can accelerate
 875     // this with some MCS/CHL-like bidirectional list hints and advisory
 876     // back-links so dequeueing from the interior will normally operate
 877     // in constant-time.
 878     // Dequeue Self from either the head (with CAS) or from the interior
 879     // with a linear-time scan and normal non-atomic memory operations.
 880     // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
 881     // and then unlink Self from EntryList.  We have to drain eventually,
 882     // so it might as well be now.
 883 
 884     ObjectWaiter * v = _cxq;
 885     assert(v != NULL, "invariant");
 886     if (v != SelfNode || Atomic::cmpxchg(SelfNode->_next, &_cxq, v) != v) {
 887       // The CAS above can fail from interference IFF a "RAT" arrived.
 888       // In that case Self must be in the interior and can no longer be
 889       // at the head of cxq.
 890       if (v == SelfNode) {
 891         assert(_cxq != v, "invariant");
 892         v = _cxq;          // CAS above failed - start scan at head of list
 893       }
 894       ObjectWaiter * p;
 895       ObjectWaiter * q = NULL;
 896       for (p = v; p != NULL && p != SelfNode; p = p->_next) {
 897         q = p;
 898         assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
 899       }
 900       assert(v != SelfNode, "invariant");
 901       assert(p == SelfNode, "Node not found on cxq");
 902       assert(p != _cxq, "invariant");
 903       assert(q != NULL, "invariant");
 904       assert(q->_next == p, "invariant");
 905       q->_next = p->_next;
 906     }
 907   }
 908 
 909 #ifdef ASSERT
 910   // Diagnostic hygiene ...
 911   SelfNode->_prev  = (ObjectWaiter *) 0xBAD;
 912   SelfNode->_next  = (ObjectWaiter *) 0xBAD;
 913   SelfNode->TState = ObjectWaiter::TS_RUN;
 914 #endif
 915 }
 916 
 917 // -----------------------------------------------------------------------------
 918 // Exit support
 919 //
 920 // exit()
 921 // ~~~~~~
 922 // Note that the collector can't reclaim the objectMonitor or deflate
 923 // the object out from underneath the thread calling ::exit() as the
 924 // thread calling ::exit() never transitions to a stable state.
 925 // This inhibits GC, which in turn inhibits asynchronous (and
 926 // inopportune) reclamation of "this".
 927 //
 928 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
 929 // There's one exception to the claim above, however.  EnterI() can call
 930 // exit() to drop a lock if the acquirer has been externally suspended.
 931 // In that case exit() is called with _thread_state as _thread_blocked,
 932 // but the monitor's _contentions field is > 0, which inhibits reclamation.
 933 //
 934 // 1-0 exit
 935 // ~~~~~~~~
 936 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
 937 // the fast-path operators have been optimized so the common ::exit()
 938 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
 939 // The code emitted by fast_unlock() elides the usual MEMBAR.  This
 940 // greatly improves latency -- MEMBAR and CAS having considerable local
 941 // latency on modern processors -- but at the cost of "stranding".  Absent the
 942 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
 943 // ::enter() path, resulting in the entering thread being stranding
 944 // and a progress-liveness failure.   Stranding is extremely rare.
 945 // We use timers (timed park operations) & periodic polling to detect
 946 // and recover from stranding.  Potentially stranded threads periodically
 947 // wake up and poll the lock.  See the usage of the _Responsible variable.
 948 //
 949 // The CAS() in enter provides for safety and exclusion, while the CAS or
 950 // MEMBAR in exit provides for progress and avoids stranding.  1-0 locking
 951 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
 952 // We detect and recover from stranding with timers.
 953 //
 954 // If a thread transiently strands it'll park until (a) another
 955 // thread acquires the lock and then drops the lock, at which time the
 956 // exiting thread will notice and unpark the stranded thread, or, (b)
 957 // the timer expires.  If the lock is high traffic then the stranding latency
 958 // will be low due to (a).  If the lock is low traffic then the odds of
 959 // stranding are lower, although the worst-case stranding latency
 960 // is longer.  Critically, we don't want to put excessive load in the
 961 // platform's timer subsystem.  We want to minimize both the timer injection
 962 // rate (timers created/sec) as well as the number of timers active at
 963 // any one time.  (more precisely, we want to minimize timer-seconds, which is
 964 // the integral of the # of active timers at any instant over time).
 965 // Both impinge on OS scalability.  Given that, at most one thread parked on
 966 // a monitor will use a timer.
 967 //
 968 // There is also the risk of a futile wake-up. If we drop the lock
 969 // another thread can reacquire the lock immediately, and we can
 970 // then wake a thread unnecessarily. This is benign, and we've
 971 // structured the code so the windows are short and the frequency
 972 // of such futile wakups is low.
 973 
 974 void ObjectMonitor::exit(bool not_suspended, TRAPS) {
 975   Thread * const Self = THREAD;
 976   if (THREAD != _owner) {
 977     if (THREAD->is_lock_owned((address) _owner)) {
 978       // Transmute _owner from a BasicLock pointer to a Thread address.
 979       // We don't need to hold _mutex for this transition.
 980       // Non-null to Non-null is safe as long as all readers can
 981       // tolerate either flavor.
 982       assert(_recursions == 0, "invariant");
 983       _owner = THREAD;
 984       _recursions = 0;
 985     } else {
 986       // Apparent unbalanced locking ...
 987       // Naively we'd like to throw IllegalMonitorStateException.
 988       // As a practical matter we can neither allocate nor throw an
 989       // exception as ::exit() can be called from leaf routines.
 990       // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
 991       // Upon deeper reflection, however, in a properly run JVM the only
 992       // way we should encounter this situation is in the presence of
 993       // unbalanced JNI locking. TODO: CheckJNICalls.
 994       // See also: CR4414101
 995       assert(false, "Non-balanced monitor enter/exit! Likely JNI locking: "
 996              "owner=" INTPTR_FORMAT, p2i(_owner));
 997       return;
 998     }
 999   }
1000 
1001   if (_recursions != 0) {
1002     _recursions--;        // this is simple recursive enter
1003     return;
1004   }
1005 
1006   // Invariant: after setting Responsible=null an thread must execute
1007   // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
1008   _Responsible = NULL;
1009 
1010 #if INCLUDE_JFR
1011   // get the owner's thread id for the MonitorEnter event
1012   // if it is enabled and the thread isn't suspended
1013   if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
1014     _previous_owner_tid = JFR_THREAD_ID(Self);
1015   }
1016 #endif
1017 
1018   for (;;) {
1019     assert(THREAD == _owner, "invariant");
1020 
1021     // release semantics: prior loads and stores from within the critical section
1022     // must not float (reorder) past the following store that drops the lock.
1023     // On SPARC that requires MEMBAR #loadstore|#storestore.
1024     // But of course in TSO #loadstore|#storestore is not required.
1025     OrderAccess::release_store(&_owner, (void*)NULL);   // drop the lock
1026     OrderAccess::storeload();                        // See if we need to wake a successor
1027     if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1028       return;
1029     }
1030     // Other threads are blocked trying to acquire the lock.
1031 
1032     // Normally the exiting thread is responsible for ensuring succession,
1033     // but if other successors are ready or other entering threads are spinning
1034     // then this thread can simply store NULL into _owner and exit without
1035     // waking a successor.  The existence of spinners or ready successors
1036     // guarantees proper succession (liveness).  Responsibility passes to the
1037     // ready or running successors.  The exiting thread delegates the duty.
1038     // More precisely, if a successor already exists this thread is absolved
1039     // of the responsibility of waking (unparking) one.
1040     //
1041     // The _succ variable is critical to reducing futile wakeup frequency.
1042     // _succ identifies the "heir presumptive" thread that has been made
1043     // ready (unparked) but that has not yet run.  We need only one such
1044     // successor thread to guarantee progress.
1045     // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1046     // section 3.3 "Futile Wakeup Throttling" for details.
1047     //
1048     // Note that spinners in Enter() also set _succ non-null.
1049     // In the current implementation spinners opportunistically set
1050     // _succ so that exiting threads might avoid waking a successor.
1051     // Another less appealing alternative would be for the exiting thread
1052     // to drop the lock and then spin briefly to see if a spinner managed
1053     // to acquire the lock.  If so, the exiting thread could exit
1054     // immediately without waking a successor, otherwise the exiting
1055     // thread would need to dequeue and wake a successor.
1056     // (Note that we'd need to make the post-drop spin short, but no
1057     // shorter than the worst-case round-trip cache-line migration time.
1058     // The dropped lock needs to become visible to the spinner, and then
1059     // the acquisition of the lock by the spinner must become visible to
1060     // the exiting thread).
1061 
1062     // It appears that an heir-presumptive (successor) must be made ready.
1063     // Only the current lock owner can manipulate the EntryList or
1064     // drain _cxq, so we need to reacquire the lock.  If we fail
1065     // to reacquire the lock the responsibility for ensuring succession
1066     // falls to the new owner.
1067     //
1068     if (!Atomic::replace_if_null(THREAD, &_owner)) {
1069       return;
1070     }
1071 
1072     guarantee(_owner == THREAD, "invariant");
1073 
1074     ObjectWaiter * w = NULL;
1075 
1076     w = _EntryList;
1077     if (w != NULL) {
1078       // I'd like to write: guarantee (w->_thread != Self).
1079       // But in practice an exiting thread may find itself on the EntryList.
1080       // Let's say thread T1 calls O.wait().  Wait() enqueues T1 on O's waitset and
1081       // then calls exit().  Exit release the lock by setting O._owner to NULL.
1082       // Let's say T1 then stalls.  T2 acquires O and calls O.notify().  The
1083       // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1084       // release the lock "O".  T2 resumes immediately after the ST of null into
1085       // _owner, above.  T2 notices that the EntryList is populated, so it
1086       // reacquires the lock and then finds itself on the EntryList.
1087       // Given all that, we have to tolerate the circumstance where "w" is
1088       // associated with Self.
1089       assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1090       ExitEpilog(Self, w);
1091       return;
1092     }
1093 
1094     // If we find that both _cxq and EntryList are null then just
1095     // re-run the exit protocol from the top.
1096     w = _cxq;
1097     if (w == NULL) continue;
1098 
1099     // Drain _cxq into EntryList - bulk transfer.
1100     // First, detach _cxq.
1101     // The following loop is tantamount to: w = swap(&cxq, NULL)
1102     for (;;) {
1103       assert(w != NULL, "Invariant");
1104       ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1105       if (u == w) break;
1106       w = u;
1107     }
1108 
1109     assert(w != NULL, "invariant");
1110     assert(_EntryList == NULL, "invariant");
1111 
1112     // Convert the LIFO SLL anchored by _cxq into a DLL.
1113     // The list reorganization step operates in O(LENGTH(w)) time.
1114     // It's critical that this step operate quickly as
1115     // "Self" still holds the outer-lock, restricting parallelism
1116     // and effectively lengthening the critical section.
1117     // Invariant: s chases t chases u.
1118     // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1119     // we have faster access to the tail.
1120 
1121     _EntryList = w;
1122     ObjectWaiter * q = NULL;
1123     ObjectWaiter * p;
1124     for (p = w; p != NULL; p = p->_next) {
1125       guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1126       p->TState = ObjectWaiter::TS_ENTER;
1127       p->_prev = q;
1128       q = p;
1129     }
1130 
1131     // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1132     // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1133 
1134     // See if we can abdicate to a spinner instead of waking a thread.
1135     // A primary goal of the implementation is to reduce the
1136     // context-switch rate.
1137     if (_succ != NULL) continue;
1138 
1139     w = _EntryList;
1140     if (w != NULL) {
1141       guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1142       ExitEpilog(Self, w);
1143       return;
1144     }
1145   }
1146 }
1147 
1148 // ExitSuspendEquivalent:
1149 // A faster alternate to handle_special_suspend_equivalent_condition()
1150 //
1151 // handle_special_suspend_equivalent_condition() unconditionally
1152 // acquires the SR_lock.  On some platforms uncontended MutexLocker()
1153 // operations have high latency.  Note that in ::enter() we call HSSEC
1154 // while holding the monitor, so we effectively lengthen the critical sections.
1155 //
1156 // There are a number of possible solutions:
1157 //
1158 // A.  To ameliorate the problem we might also defer state transitions
1159 //     to as late as possible -- just prior to parking.
1160 //     Given that, we'd call HSSEC after having returned from park(),
1161 //     but before attempting to acquire the monitor.  This is only a
1162 //     partial solution.  It avoids calling HSSEC while holding the
1163 //     monitor (good), but it still increases successor reacquisition latency --
1164 //     the interval between unparking a successor and the time the successor
1165 //     resumes and retries the lock.  See ReenterI(), which defers state transitions.
1166 //     If we use this technique we can also avoid EnterI()-exit() loop
1167 //     in ::enter() where we iteratively drop the lock and then attempt
1168 //     to reacquire it after suspending.
1169 //
1170 // B.  In the future we might fold all the suspend bits into a
1171 //     composite per-thread suspend flag and then update it with CAS().
1172 //     Alternately, a Dekker-like mechanism with multiple variables
1173 //     would suffice:
1174 //       ST Self->_suspend_equivalent = false
1175 //       MEMBAR
1176 //       LD Self_>_suspend_flags
1177 
1178 bool ObjectMonitor::ExitSuspendEquivalent(JavaThread * jSelf) {
1179   return jSelf->handle_special_suspend_equivalent_condition();
1180 }
1181 
1182 
1183 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) {
1184   assert(_owner == Self, "invariant");
1185 
1186   // Exit protocol:
1187   // 1. ST _succ = wakee
1188   // 2. membar #loadstore|#storestore;
1189   // 2. ST _owner = NULL
1190   // 3. unpark(wakee)
1191 
1192   _succ = Wakee->_thread;
1193   ParkEvent * Trigger = Wakee->_event;
1194 
1195   // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1196   // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1197   // out-of-scope (non-extant).
1198   Wakee  = NULL;
1199 
1200   // Drop the lock
1201   OrderAccess::release_store(&_owner, (void*)NULL);
1202   OrderAccess::fence();                               // ST _owner vs LD in unpark()
1203 
1204   DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1205   Trigger->unpark();
1206 
1207   // Maintain stats and report events to JVMTI
1208   OM_PERFDATA_OP(Parks, inc());
1209 }
1210 
1211 
1212 // -----------------------------------------------------------------------------
1213 // Class Loader deadlock handling.
1214 //
1215 // complete_exit exits a lock returning recursion count
1216 // complete_exit/reenter operate as a wait without waiting
1217 // complete_exit requires an inflated monitor
1218 // The _owner field is not always the Thread addr even with an
1219 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1220 // thread due to contention.
1221 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1222   Thread * const Self = THREAD;
1223   assert(Self->is_Java_thread(), "Must be Java thread!");
1224   JavaThread *jt = (JavaThread *)THREAD;
1225 
1226   assert(InitDone, "Unexpectedly not initialized");
1227 
1228   if (THREAD != _owner) {
1229     if (THREAD->is_lock_owned ((address)_owner)) {
1230       assert(_recursions == 0, "internal state error");
1231       _owner = THREAD;   // Convert from basiclock addr to Thread addr
1232       _recursions = 0;
1233     }
1234   }
1235 
1236   guarantee(Self == _owner, "complete_exit not owner");
1237   intptr_t save = _recursions; // record the old recursion count
1238   _recursions = 0;        // set the recursion level to be 0
1239   exit(true, Self);           // exit the monitor
1240   guarantee(_owner != Self, "invariant");
1241   return save;
1242 }
1243 
1244 // reenter() enters a lock and sets recursion count
1245 // complete_exit/reenter operate as a wait without waiting
1246 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1247   Thread * const Self = THREAD;
1248   assert(Self->is_Java_thread(), "Must be Java thread!");
1249   JavaThread *jt = (JavaThread *)THREAD;
1250 
1251   guarantee(_owner != Self, "reenter already owner");
1252   enter(THREAD);
1253   // Entered the monitor.
1254   guarantee(_recursions == 0, "reenter recursion");
1255   _recursions = recursions;

1256 }
1257 
1258 
1259 // -----------------------------------------------------------------------------
1260 // A macro is used below because there may already be a pending
1261 // exception which should not abort the execution of the routines
1262 // which use this (which is why we don't put this into check_slow and
1263 // call it with a CHECK argument).
1264 
1265 #define CHECK_OWNER()                                                       \
1266   do {                                                                      \
1267     if (THREAD != _owner) {                                                 \
1268       if (THREAD->is_lock_owned((address) _owner)) {                        \
1269         _owner = THREAD;  /* Convert from basiclock addr to Thread addr */  \
1270         _recursions = 0;                                                    \
1271       } else {                                                              \
1272         THROW(vmSymbols::java_lang_IllegalMonitorStateException());         \
1273       }                                                                     \
1274     }                                                                       \
1275   } while (false)
1276 
1277 // check_slow() is a misnomer.  It's called to simply to throw an IMSX exception.
1278 // TODO-FIXME: remove check_slow() -- it's likely dead.
1279 
1280 void ObjectMonitor::check_slow(TRAPS) {
1281   assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1282   THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1283 }
1284 
1285 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1286                                     ObjectMonitor* monitor,
1287                                     jlong notifier_tid,
1288                                     jlong timeout,
1289                                     bool timedout) {
1290   assert(event != NULL, "invariant");
1291   assert(monitor != NULL, "invariant");
1292   event->set_monitorClass(((oop)monitor->object())->klass());
1293   event->set_timeout(timeout);
1294   event->set_address((uintptr_t)monitor->object_addr());
1295   event->set_notifier(notifier_tid);
1296   event->set_timedOut(timedout);
1297   event->commit();
1298 }
1299 
1300 // -----------------------------------------------------------------------------
1301 // Wait/Notify/NotifyAll
1302 //
1303 // Note: a subset of changes to ObjectMonitor::wait()
1304 // will need to be replicated in complete_exit
1305 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1306   Thread * const Self = THREAD;
1307   assert(Self->is_Java_thread(), "Must be Java thread!");
1308   JavaThread *jt = (JavaThread *)THREAD;
1309 
1310   assert(InitDone, "Unexpectedly not initialized");
1311 
1312   // Throw IMSX or IEX.
1313   CHECK_OWNER();
1314 
1315   EventJavaMonitorWait event;
1316 
1317   // check for a pending interrupt
1318   if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1319     // post monitor waited event.  Note that this is past-tense, we are done waiting.
1320     if (JvmtiExport::should_post_monitor_waited()) {
1321       // Note: 'false' parameter is passed here because the
1322       // wait was not timed out due to thread interrupt.
1323       JvmtiExport::post_monitor_waited(jt, this, false);
1324 
1325       // In this short circuit of the monitor wait protocol, the
1326       // current thread never drops ownership of the monitor and
1327       // never gets added to the wait queue so the current thread
1328       // cannot be made the successor. This means that the
1329       // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1330       // consume an unpark() meant for the ParkEvent associated with
1331       // this ObjectMonitor.
1332     }
1333     if (event.should_commit()) {
1334       post_monitor_wait_event(&event, this, 0, millis, false);
1335     }
1336     THROW(vmSymbols::java_lang_InterruptedException());
1337     return;
1338   }
1339 
1340   assert(Self->_Stalled == 0, "invariant");
1341   Self->_Stalled = intptr_t(this);
1342   jt->set_current_waiting_monitor(this);
1343 
1344   // create a node to be put into the queue
1345   // Critically, after we reset() the event but prior to park(), we must check
1346   // for a pending interrupt.
1347   ObjectWaiter node(Self);
1348   node.TState = ObjectWaiter::TS_WAIT;
1349   Self->_ParkEvent->reset();
1350   OrderAccess::fence();          // ST into Event; membar ; LD interrupted-flag
1351 
1352   // Enter the waiting queue, which is a circular doubly linked list in this case
1353   // but it could be a priority queue or any data structure.
1354   // _WaitSetLock protects the wait queue.  Normally the wait queue is accessed only
1355   // by the the owner of the monitor *except* in the case where park()
1356   // returns because of a timeout of interrupt.  Contention is exceptionally rare
1357   // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1358 
1359   Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1360   AddWaiter(&node);
1361   Thread::SpinRelease(&_WaitSetLock);
1362 
1363   _Responsible = NULL;
1364 
1365   intptr_t save = _recursions; // record the old recursion count
1366   _waiters++;                  // increment the number of waiters
1367   _recursions = 0;             // set the recursion level to be 1
1368   exit(true, Self);                    // exit the monitor
1369   guarantee(_owner != Self, "invariant");
1370 
1371   // The thread is on the WaitSet list - now park() it.
1372   // On MP systems it's conceivable that a brief spin before we park
1373   // could be profitable.
1374   //
1375   // TODO-FIXME: change the following logic to a loop of the form
1376   //   while (!timeout && !interrupted && _notified == 0) park()
1377 
1378   int ret = OS_OK;
1379   int WasNotified = 0;
1380   { // State transition wrappers
1381     OSThread* osthread = Self->osthread();
1382     OSThreadWaitState osts(osthread, true);
1383     {
1384       ThreadBlockInVM tbivm(jt);
1385       // Thread is in thread_blocked state and oop access is unsafe.
1386       jt->set_suspend_equivalent();
1387 
1388       if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1389         // Intentionally empty
1390       } else if (node._notified == 0) {
1391         if (millis <= 0) {
1392           Self->_ParkEvent->park();
1393         } else {
1394           ret = Self->_ParkEvent->park(millis);
1395         }
1396       }
1397 
1398       // were we externally suspended while we were waiting?
1399       if (ExitSuspendEquivalent (jt)) {
1400         // TODO-FIXME: add -- if succ == Self then succ = null.
1401         jt->java_suspend_self();
1402       }
1403 
1404     } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1405 
1406     // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1407     // from the WaitSet to the EntryList.
1408     // See if we need to remove Node from the WaitSet.
1409     // We use double-checked locking to avoid grabbing _WaitSetLock
1410     // if the thread is not on the wait queue.
1411     //
1412     // Note that we don't need a fence before the fetch of TState.
1413     // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1414     // written by the is thread. (perhaps the fetch might even be satisfied
1415     // by a look-aside into the processor's own store buffer, although given
1416     // the length of the code path between the prior ST and this load that's
1417     // highly unlikely).  If the following LD fetches a stale TS_WAIT value
1418     // then we'll acquire the lock and then re-fetch a fresh TState value.
1419     // That is, we fail toward safety.
1420 
1421     if (node.TState == ObjectWaiter::TS_WAIT) {
1422       Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1423       if (node.TState == ObjectWaiter::TS_WAIT) {
1424         DequeueSpecificWaiter(&node);       // unlink from WaitSet
1425         assert(node._notified == 0, "invariant");
1426         node.TState = ObjectWaiter::TS_RUN;
1427       }
1428       Thread::SpinRelease(&_WaitSetLock);
1429     }
1430 
1431     // The thread is now either on off-list (TS_RUN),
1432     // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1433     // The Node's TState variable is stable from the perspective of this thread.
1434     // No other threads will asynchronously modify TState.
1435     guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1436     OrderAccess::loadload();
1437     if (_succ == Self) _succ = NULL;
1438     WasNotified = node._notified;
1439 
1440     // Reentry phase -- reacquire the monitor.
1441     // re-enter contended monitor after object.wait().
1442     // retain OBJECT_WAIT state until re-enter successfully completes
1443     // Thread state is thread_in_vm and oop access is again safe,
1444     // although the raw address of the object may have changed.
1445     // (Don't cache naked oops over safepoints, of course).
1446 
1447     // post monitor waited event. Note that this is past-tense, we are done waiting.
1448     if (JvmtiExport::should_post_monitor_waited()) {
1449       JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1450 
1451       if (node._notified != 0 && _succ == Self) {
1452         // In this part of the monitor wait-notify-reenter protocol it
1453         // is possible (and normal) for another thread to do a fastpath
1454         // monitor enter-exit while this thread is still trying to get
1455         // to the reenter portion of the protocol.
1456         //
1457         // The ObjectMonitor was notified and the current thread is
1458         // the successor which also means that an unpark() has already
1459         // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1460         // consume the unpark() that was done when the successor was
1461         // set because the same ParkEvent is shared between Java
1462         // monitors and JVM/TI RawMonitors (for now).
1463         //
1464         // We redo the unpark() to ensure forward progress, i.e., we
1465         // don't want all pending threads hanging (parked) with none
1466         // entering the unlocked monitor.
1467         node._event->unpark();
1468       }
1469     }
1470 
1471     if (event.should_commit()) {
1472       post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1473     }
1474 
1475     OrderAccess::fence();
1476 
1477     assert(Self->_Stalled != 0, "invariant");
1478     Self->_Stalled = 0;
1479 
1480     assert(_owner != Self, "invariant");
1481     ObjectWaiter::TStates v = node.TState;
1482     if (v == ObjectWaiter::TS_RUN) {
1483       enter(Self);
1484     } else {
1485       guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1486       ReenterI(Self, &node);
1487       node.wait_reenter_end(this);
1488     }
1489 
1490     // Self has reacquired the lock.
1491     // Lifecycle - the node representing Self must not appear on any queues.
1492     // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1493     // want residual elements associated with this thread left on any lists.
1494     guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1495     assert(_owner == Self, "invariant");
1496     assert(_succ != Self, "invariant");
1497   } // OSThreadWaitState()
1498 
1499   jt->set_current_waiting_monitor(NULL);
1500 
1501   guarantee(_recursions == 0, "invariant");
1502   _recursions = save;     // restore the old recursion count
1503   _waiters--;             // decrement the number of waiters
1504 
1505   // Verify a few postconditions
1506   assert(_owner == Self, "invariant");
1507   assert(_succ != Self, "invariant");
1508   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
1509 
1510   // check if the notification happened
1511   if (!WasNotified) {
1512     // no, it could be timeout or Thread.interrupt() or both
1513     // check for interrupt event, otherwise it is timeout
1514     if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1515       THROW(vmSymbols::java_lang_InterruptedException());
1516     }
1517   }
1518 
1519   // NOTE: Spurious wake up will be consider as timeout.
1520   // Monitor notify has precedence over thread interrupt.
1521 }
1522 
1523 
1524 // Consider:
1525 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1526 // then instead of transferring a thread from the WaitSet to the EntryList
1527 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1528 
1529 void ObjectMonitor::INotify(Thread * Self) {
1530   Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1531   ObjectWaiter * iterator = DequeueWaiter();
1532   if (iterator != NULL) {
1533     guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1534     guarantee(iterator->_notified == 0, "invariant");
1535     // Disposition - what might we do with iterator ?
1536     // a.  add it directly to the EntryList - either tail (policy == 1)
1537     //     or head (policy == 0).
1538     // b.  push it onto the front of the _cxq (policy == 2).
1539     // For now we use (b).
1540 
1541     iterator->TState = ObjectWaiter::TS_ENTER;
1542 
1543     iterator->_notified = 1;
1544     iterator->_notifier_tid = JFR_THREAD_ID(Self);
1545 
1546     ObjectWaiter * list = _EntryList;
1547     if (list != NULL) {
1548       assert(list->_prev == NULL, "invariant");
1549       assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1550       assert(list != iterator, "invariant");
1551     }
1552 
1553     // prepend to cxq
1554     if (list == NULL) {
1555       iterator->_next = iterator->_prev = NULL;
1556       _EntryList = iterator;
1557     } else {
1558       iterator->TState = ObjectWaiter::TS_CXQ;
1559       for (;;) {
1560         ObjectWaiter * front = _cxq;
1561         iterator->_next = front;
1562         if (Atomic::cmpxchg(iterator, &_cxq, front) == front) {
1563           break;
1564         }
1565       }
1566     }
1567 
1568     // _WaitSetLock protects the wait queue, not the EntryList.  We could
1569     // move the add-to-EntryList operation, above, outside the critical section
1570     // protected by _WaitSetLock.  In practice that's not useful.  With the
1571     // exception of  wait() timeouts and interrupts the monitor owner
1572     // is the only thread that grabs _WaitSetLock.  There's almost no contention
1573     // on _WaitSetLock so it's not profitable to reduce the length of the
1574     // critical section.
1575 
1576     iterator->wait_reenter_begin(this);
1577   }
1578   Thread::SpinRelease(&_WaitSetLock);
1579 }
1580 
1581 // Consider: a not-uncommon synchronization bug is to use notify() when
1582 // notifyAll() is more appropriate, potentially resulting in stranded
1583 // threads; this is one example of a lost wakeup. A useful diagnostic
1584 // option is to force all notify() operations to behave as notifyAll().
1585 //
1586 // Note: We can also detect many such problems with a "minimum wait".
1587 // When the "minimum wait" is set to a small non-zero timeout value
1588 // and the program does not hang whereas it did absent "minimum wait",
1589 // that suggests a lost wakeup bug.
1590 
1591 void ObjectMonitor::notify(TRAPS) {
1592   CHECK_OWNER();
1593   if (_WaitSet == NULL) {
1594     return;
1595   }
1596   DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1597   INotify(THREAD);
1598   OM_PERFDATA_OP(Notifications, inc(1));
1599 }
1600 
1601 
1602 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1603 // from the waitset to the EntryList. This could be done more efficiently with a
1604 // single bulk transfer but in practice it's not time-critical. Beware too,
1605 // that in prepend-mode we invert the order of the waiters. Let's say that the
1606 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1607 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1608 
1609 void ObjectMonitor::notifyAll(TRAPS) {
1610   CHECK_OWNER();
1611   if (_WaitSet == NULL) {
1612     return;
1613   }
1614 
1615   DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1616   int tally = 0;
1617   while (_WaitSet != NULL) {
1618     tally++;
1619     INotify(THREAD);
1620   }
1621 
1622   OM_PERFDATA_OP(Notifications, inc(tally));
1623 }
1624 
1625 // -----------------------------------------------------------------------------
1626 // Adaptive Spinning Support
1627 //
1628 // Adaptive spin-then-block - rational spinning
1629 //
1630 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1631 // algorithm.  On high order SMP systems it would be better to start with
1632 // a brief global spin and then revert to spinning locally.  In the spirit of MCS/CLH,
1633 // a contending thread could enqueue itself on the cxq and then spin locally
1634 // on a thread-specific variable such as its ParkEvent._Event flag.
1635 // That's left as an exercise for the reader.  Note that global spinning is
1636 // not problematic on Niagara, as the L2 cache serves the interconnect and
1637 // has both low latency and massive bandwidth.
1638 //
1639 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1640 // acquisition attempts where we opt to spin --  at 100% and vary the spin count
1641 // (duration) or we can fix the count at approximately the duration of
1642 // a context switch and vary the frequency.   Of course we could also
1643 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1644 // For a description of 'Adaptive spin-then-block mutual exclusion in
1645 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1646 //
1647 // This implementation varies the duration "D", where D varies with
1648 // the success rate of recent spin attempts. (D is capped at approximately
1649 // length of a round-trip context switch).  The success rate for recent
1650 // spin attempts is a good predictor of the success rate of future spin
1651 // attempts.  The mechanism adapts automatically to varying critical
1652 // section length (lock modality), system load and degree of parallelism.
1653 // D is maintained per-monitor in _SpinDuration and is initialized
1654 // optimistically.  Spin frequency is fixed at 100%.
1655 //
1656 // Note that _SpinDuration is volatile, but we update it without locks
1657 // or atomics.  The code is designed so that _SpinDuration stays within
1658 // a reasonable range even in the presence of races.  The arithmetic
1659 // operations on _SpinDuration are closed over the domain of legal values,
1660 // so at worst a race will install and older but still legal value.
1661 // At the very worst this introduces some apparent non-determinism.
1662 // We might spin when we shouldn't or vice-versa, but since the spin
1663 // count are relatively short, even in the worst case, the effect is harmless.
1664 //
1665 // Care must be taken that a low "D" value does not become an
1666 // an absorbing state.  Transient spinning failures -- when spinning
1667 // is overall profitable -- should not cause the system to converge
1668 // on low "D" values.  We want spinning to be stable and predictable
1669 // and fairly responsive to change and at the same time we don't want
1670 // it to oscillate, become metastable, be "too" non-deterministic,
1671 // or converge on or enter undesirable stable absorbing states.
1672 //
1673 // We implement a feedback-based control system -- using past behavior
1674 // to predict future behavior.  We face two issues: (a) if the
1675 // input signal is random then the spin predictor won't provide optimal
1676 // results, and (b) if the signal frequency is too high then the control
1677 // system, which has some natural response lag, will "chase" the signal.
1678 // (b) can arise from multimodal lock hold times.  Transient preemption
1679 // can also result in apparent bimodal lock hold times.
1680 // Although sub-optimal, neither condition is particularly harmful, as
1681 // in the worst-case we'll spin when we shouldn't or vice-versa.
1682 // The maximum spin duration is rather short so the failure modes aren't bad.
1683 // To be conservative, I've tuned the gain in system to bias toward
1684 // _not spinning.  Relatedly, the system can sometimes enter a mode where it
1685 // "rings" or oscillates between spinning and not spinning.  This happens
1686 // when spinning is just on the cusp of profitability, however, so the
1687 // situation is not dire.  The state is benign -- there's no need to add
1688 // hysteresis control to damp the transition rate between spinning and
1689 // not spinning.
1690 
1691 // Spinning: Fixed frequency (100%), vary duration
1692 int ObjectMonitor::TrySpin(Thread * Self) {
1693   // Dumb, brutal spin.  Good for comparative measurements against adaptive spinning.
1694   int ctr = Knob_FixedSpin;
1695   if (ctr != 0) {
1696     while (--ctr >= 0) {
1697       if (TryLock(Self) > 0) return 1;
1698       SpinPause();
1699     }
1700     return 0;
1701   }
1702 
1703   for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1704     if (TryLock(Self) > 0) {
1705       // Increase _SpinDuration ...
1706       // Note that we don't clamp SpinDuration precisely at SpinLimit.
1707       // Raising _SpurDuration to the poverty line is key.
1708       int x = _SpinDuration;
1709       if (x < Knob_SpinLimit) {
1710         if (x < Knob_Poverty) x = Knob_Poverty;
1711         _SpinDuration = x + Knob_BonusB;
1712       }
1713       return 1;
1714     }
1715     SpinPause();
1716   }
1717 
1718   // Admission control - verify preconditions for spinning
1719   //
1720   // We always spin a little bit, just to prevent _SpinDuration == 0 from
1721   // becoming an absorbing state.  Put another way, we spin briefly to
1722   // sample, just in case the system load, parallelism, contention, or lock
1723   // modality changed.
1724   //
1725   // Consider the following alternative:
1726   // Periodically set _SpinDuration = _SpinLimit and try a long/full
1727   // spin attempt.  "Periodically" might mean after a tally of
1728   // the # of failed spin attempts (or iterations) reaches some threshold.
1729   // This takes us into the realm of 1-out-of-N spinning, where we
1730   // hold the duration constant but vary the frequency.
1731 
1732   ctr = _SpinDuration;
1733   if (ctr <= 0) return 0;
1734 
1735   if (NotRunnable(Self, (Thread *) _owner)) {
1736     return 0;
1737   }
1738 
1739   // We're good to spin ... spin ingress.
1740   // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1741   // when preparing to LD...CAS _owner, etc and the CAS is likely
1742   // to succeed.
1743   if (_succ == NULL) {
1744     _succ = Self;
1745   }
1746   Thread * prv = NULL;
1747 
1748   // There are three ways to exit the following loop:
1749   // 1.  A successful spin where this thread has acquired the lock.
1750   // 2.  Spin failure with prejudice
1751   // 3.  Spin failure without prejudice
1752 
1753   while (--ctr >= 0) {
1754 
1755     // Periodic polling -- Check for pending GC
1756     // Threads may spin while they're unsafe.
1757     // We don't want spinning threads to delay the JVM from reaching
1758     // a stop-the-world safepoint or to steal cycles from GC.
1759     // If we detect a pending safepoint we abort in order that
1760     // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1761     // this thread, if safe, doesn't steal cycles from GC.
1762     // This is in keeping with the "no loitering in runtime" rule.
1763     // We periodically check to see if there's a safepoint pending.
1764     if ((ctr & 0xFF) == 0) {
1765       if (SafepointMechanism::should_block(Self)) {
1766         goto Abort;           // abrupt spin egress
1767       }
1768       SpinPause();
1769     }
1770 
1771     // Probe _owner with TATAS
1772     // If this thread observes the monitor transition or flicker
1773     // from locked to unlocked to locked, then the odds that this
1774     // thread will acquire the lock in this spin attempt go down
1775     // considerably.  The same argument applies if the CAS fails
1776     // or if we observe _owner change from one non-null value to
1777     // another non-null value.   In such cases we might abort
1778     // the spin without prejudice or apply a "penalty" to the
1779     // spin count-down variable "ctr", reducing it by 100, say.
1780 
1781     Thread * ox = (Thread *) _owner;
1782     if (ox == NULL) {
1783       ox = (Thread*)Atomic::cmpxchg(Self, &_owner, (void*)NULL);
1784       if (ox == NULL) {
1785         // The CAS succeeded -- this thread acquired ownership
1786         // Take care of some bookkeeping to exit spin state.
1787         if (_succ == Self) {
1788           _succ = NULL;
1789         }
1790 
1791         // Increase _SpinDuration :
1792         // The spin was successful (profitable) so we tend toward
1793         // longer spin attempts in the future.
1794         // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
1795         // If we acquired the lock early in the spin cycle it
1796         // makes sense to increase _SpinDuration proportionally.
1797         // Note that we don't clamp SpinDuration precisely at SpinLimit.
1798         int x = _SpinDuration;
1799         if (x < Knob_SpinLimit) {
1800           if (x < Knob_Poverty) x = Knob_Poverty;
1801           _SpinDuration = x + Knob_Bonus;
1802         }
1803         return 1;
1804       }
1805 
1806       // The CAS failed ... we can take any of the following actions:
1807       // * penalize: ctr -= CASPenalty
1808       // * exit spin with prejudice -- goto Abort;
1809       // * exit spin without prejudice.
1810       // * Since CAS is high-latency, retry again immediately.
1811       prv = ox;
1812       goto Abort;
1813     }
1814 
1815     // Did lock ownership change hands ?
1816     if (ox != prv && prv != NULL) {
1817       goto Abort;
1818     }
1819     prv = ox;
1820 
1821     // Abort the spin if the owner is not executing.
1822     // The owner must be executing in order to drop the lock.
1823     // Spinning while the owner is OFFPROC is idiocy.
1824     // Consider: ctr -= RunnablePenalty ;
1825     if (NotRunnable(Self, ox)) {
1826       goto Abort;
1827     }
1828     if (_succ == NULL) {
1829       _succ = Self;
1830     }
1831   }
1832 
1833   // Spin failed with prejudice -- reduce _SpinDuration.
1834   // TODO: Use an AIMD-like policy to adjust _SpinDuration.
1835   // AIMD is globally stable.
1836   {
1837     int x = _SpinDuration;
1838     if (x > 0) {
1839       // Consider an AIMD scheme like: x -= (x >> 3) + 100
1840       // This is globally sample and tends to damp the response.
1841       x -= Knob_Penalty;
1842       if (x < 0) x = 0;
1843       _SpinDuration = x;
1844     }
1845   }
1846 
1847  Abort:
1848   if (_succ == Self) {
1849     _succ = NULL;
1850     // Invariant: after setting succ=null a contending thread
1851     // must recheck-retry _owner before parking.  This usually happens
1852     // in the normal usage of TrySpin(), but it's safest
1853     // to make TrySpin() as foolproof as possible.
1854     OrderAccess::fence();
1855     if (TryLock(Self) > 0) return 1;
1856   }
1857   return 0;
1858 }
1859 
1860 // NotRunnable() -- informed spinning
1861 //
1862 // Don't bother spinning if the owner is not eligible to drop the lock.
1863 // Spin only if the owner thread is _thread_in_Java or _thread_in_vm.
1864 // The thread must be runnable in order to drop the lock in timely fashion.
1865 // If the _owner is not runnable then spinning will not likely be
1866 // successful (profitable).
1867 //
1868 // Beware -- the thread referenced by _owner could have died
1869 // so a simply fetch from _owner->_thread_state might trap.
1870 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
1871 // Because of the lifecycle issues, the _thread_state values
1872 // observed by NotRunnable() might be garbage.  NotRunnable must
1873 // tolerate this and consider the observed _thread_state value
1874 // as advisory.
1875 //
1876 // Beware too, that _owner is sometimes a BasicLock address and sometimes
1877 // a thread pointer.
1878 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
1879 // with the LSB of _owner.  Another option would be to probabilistically probe
1880 // the putative _owner->TypeTag value.
1881 //
1882 // Checking _thread_state isn't perfect.  Even if the thread is
1883 // in_java it might be blocked on a page-fault or have been preempted
1884 // and sitting on a ready/dispatch queue.
1885 //
1886 // The return value from NotRunnable() is *advisory* -- the
1887 // result is based on sampling and is not necessarily coherent.
1888 // The caller must tolerate false-negative and false-positive errors.
1889 // Spinning, in general, is probabilistic anyway.
1890 
1891 
1892 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) {
1893   // Check ox->TypeTag == 2BAD.
1894   if (ox == NULL) return 0;
1895 
1896   // Avoid transitive spinning ...
1897   // Say T1 spins or blocks trying to acquire L.  T1._Stalled is set to L.
1898   // Immediately after T1 acquires L it's possible that T2, also
1899   // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
1900   // This occurs transiently after T1 acquired L but before
1901   // T1 managed to clear T1.Stalled.  T2 does not need to abort
1902   // its spin in this circumstance.
1903   intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
1904 
1905   if (BlockedOn == 1) return 1;
1906   if (BlockedOn != 0) {
1907     return BlockedOn != intptr_t(this) && _owner == ox;
1908   }
1909 
1910   assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant");
1911   int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);;
1912   // consider also: jst != _thread_in_Java -- but that's overspecific.
1913   return jst == _thread_blocked || jst == _thread_in_native;
1914 }
1915 
1916 
1917 // -----------------------------------------------------------------------------
1918 // WaitSet management ...
1919 
1920 ObjectWaiter::ObjectWaiter(Thread* thread) {
1921   _next     = NULL;
1922   _prev     = NULL;
1923   _notified = 0;
1924   _notifier_tid = 0;
1925   TState    = TS_RUN;
1926   _thread   = thread;
1927   _event    = thread->_ParkEvent;
1928   _active   = false;
1929   assert(_event != NULL, "invariant");
1930 }
1931 
1932 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
1933   JavaThread *jt = (JavaThread *)this->_thread;
1934   _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
1935 }
1936 
1937 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
1938   JavaThread *jt = (JavaThread *)this->_thread;
1939   JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
1940 }
1941 
1942 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
1943   assert(node != NULL, "should not add NULL node");
1944   assert(node->_prev == NULL, "node already in list");
1945   assert(node->_next == NULL, "node already in list");
1946   // put node at end of queue (circular doubly linked list)
1947   if (_WaitSet == NULL) {
1948     _WaitSet = node;
1949     node->_prev = node;
1950     node->_next = node;
1951   } else {
1952     ObjectWaiter* head = _WaitSet;
1953     ObjectWaiter* tail = head->_prev;
1954     assert(tail->_next == head, "invariant check");
1955     tail->_next = node;
1956     head->_prev = node;
1957     node->_next = head;
1958     node->_prev = tail;
1959   }
1960 }
1961 
1962 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
1963   // dequeue the very first waiter
1964   ObjectWaiter* waiter = _WaitSet;
1965   if (waiter) {
1966     DequeueSpecificWaiter(waiter);
1967   }
1968   return waiter;
1969 }
1970 
1971 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
1972   assert(node != NULL, "should not dequeue NULL node");
1973   assert(node->_prev != NULL, "node already removed from list");
1974   assert(node->_next != NULL, "node already removed from list");
1975   // when the waiter has woken up because of interrupt,
1976   // timeout or other spurious wake-up, dequeue the
1977   // waiter from waiting list
1978   ObjectWaiter* next = node->_next;
1979   if (next == node) {
1980     assert(node->_prev == node, "invariant check");
1981     _WaitSet = NULL;
1982   } else {
1983     ObjectWaiter* prev = node->_prev;
1984     assert(prev->_next == node, "invariant check");
1985     assert(next->_prev == node, "invariant check");
1986     next->_prev = prev;
1987     prev->_next = next;
1988     if (_WaitSet == node) {
1989       _WaitSet = next;
1990     }
1991   }
1992   node->_next = NULL;
1993   node->_prev = NULL;
1994 }
1995 
1996 // -----------------------------------------------------------------------------
1997 // PerfData support
1998 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts       = NULL;
1999 PerfCounter * ObjectMonitor::_sync_FutileWakeups               = NULL;
2000 PerfCounter * ObjectMonitor::_sync_Parks                       = NULL;
2001 PerfCounter * ObjectMonitor::_sync_Notifications               = NULL;
2002 PerfCounter * ObjectMonitor::_sync_Inflations                  = NULL;
2003 PerfCounter * ObjectMonitor::_sync_Deflations                  = NULL;
2004 PerfLongVariable * ObjectMonitor::_sync_MonExtant              = NULL;
2005 
2006 // One-shot global initialization for the sync subsystem.
2007 // We could also defer initialization and initialize on-demand
2008 // the first time we call ObjectSynchronizer::inflate().
2009 // Initialization would be protected - like so many things - by
2010 // the MonitorCache_lock.
2011 
2012 void ObjectMonitor::Initialize() {
2013   assert(!InitDone, "invariant");
2014 
2015   if (!os::is_MP()) {
2016     Knob_SpinLimit = 0;
2017     Knob_PreSpin   = 0;
2018     Knob_FixedSpin = -1;
2019   }
2020 
2021   if (UsePerfData) {
2022     EXCEPTION_MARK;
2023 #define NEWPERFCOUNTER(n)                                                \
2024   {                                                                      \
2025     n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,  \
2026                                         CHECK);                          \
2027   }
2028 #define NEWPERFVARIABLE(n)                                                \
2029   {                                                                       \
2030     n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,  \
2031                                          CHECK);                          \
2032   }
2033     NEWPERFCOUNTER(_sync_Inflations);
2034     NEWPERFCOUNTER(_sync_Deflations);
2035     NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2036     NEWPERFCOUNTER(_sync_FutileWakeups);
2037     NEWPERFCOUNTER(_sync_Parks);
2038     NEWPERFCOUNTER(_sync_Notifications);
2039     NEWPERFVARIABLE(_sync_MonExtant);
2040 #undef NEWPERFCOUNTER
2041 #undef NEWPERFVARIABLE
2042   }
2043 
2044   DEBUG_ONLY(InitDone = true;)
2045 }
2046 
2047 // For internal use by ObjectSynchronizer::monitors_iterate().
2048 ObjectMonitorHandle::ObjectMonitorHandle(ObjectMonitor * om_ptr) {
2049   om_ptr->inc_ref_count();
2050   _om_ptr = om_ptr;
2051 }
2052 
2053 ObjectMonitorHandle::~ObjectMonitorHandle() {
2054   if (_om_ptr != NULL) {
2055     _om_ptr->dec_ref_count();
2056     _om_ptr = NULL;
2057   }
2058 }
2059 
2060 // Save the ObjectMonitor* associated with the specified markOop and
2061 // increment the ref_count. This function should only be called if
2062 // the caller has verified mark->has_monitor() == true. The object
2063 // parameter is needed to verify that ObjectMonitor* has not been
2064 // deflated and reused for another object.
2065 //
2066 // This function returns true if the ObjectMonitor* has been safely
2067 // saved. This function returns false if we have lost a race with
2068 // async deflation; the caller should retry as appropriate.
2069 //
2070 bool ObjectMonitorHandle::save_om_ptr(oop object, markOop mark) {
2071   guarantee(mark->has_monitor(), "sanity check: mark=" INTPTR_FORMAT,
2072             p2i(mark));
2073 
2074   ObjectMonitor * om_ptr = mark->monitor();
2075   om_ptr->inc_ref_count();
2076 
2077   if (AsyncDeflateIdleMonitors) {
2078     // Race here if monitor is not owned! The above ref_count bump
2079     // will cause subsequent async deflation to skip it. However,
2080     // previous or concurrent async deflation is a race.
2081     if (om_ptr->_owner == DEFLATER_MARKER && om_ptr->ref_count() <= 0) {
2082       // Async deflation is in progress and our ref_count increment
2083       // above lost the race to async deflation. Attempt to restore
2084       // the header/dmw to the object's header so that we only retry
2085       // once if the deflater thread happens to be slow.
2086       om_ptr->install_displaced_markword_in_object(object);
2087       om_ptr->dec_ref_count();
2088       return false;
2089     }
2090     // The ObjectMonitor could have been deflated and reused for
2091     // another object before we bumped the ref_count so make sure
2092     // our object still refers to this ObjectMonitor.
2093     const markOop tmp = object->mark();
2094     if (!tmp->has_monitor() || tmp->monitor() != om_ptr) {
2095       // Async deflation and reuse won the race so we have to retry.
2096       // Skip object header restoration since that's already done.
2097       om_ptr->dec_ref_count();
2098       return false;
2099     }
2100   }
2101 
2102   ADIM_guarantee(_om_ptr == NULL, "sanity check: _om_ptr=" INTPTR_FORMAT,
2103                  p2i(_om_ptr));
2104   _om_ptr = om_ptr;
2105   return true;
2106 }
2107 
2108 // For internal use by ObjectSynchronizer::inflate().
2109 void ObjectMonitorHandle::set_om_ptr(ObjectMonitor * om_ptr) {
2110   if (_om_ptr == NULL) {
2111     ADIM_guarantee(om_ptr != NULL, "cannot clear an unset om_ptr");
2112     om_ptr->inc_ref_count();
2113     _om_ptr = om_ptr;
2114   } else {
2115     ADIM_guarantee(om_ptr == NULL, "can only clear a set om_ptr");
2116     _om_ptr->dec_ref_count();
2117     _om_ptr = NULL;
2118   }
2119 }
--- EOF ---