1 /*
2 * Copyright (c) 1998, 2024, 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 "gc/shared/oopStorage.hpp"
28 #include "gc/shared/oopStorageSet.hpp"
29 #include "jfr/jfrEvents.hpp"
30 #include "jfr/support/jfrThreadId.hpp"
31 #include "logging/log.hpp"
32 #include "logging/logStream.hpp"
33 #include "memory/allocation.inline.hpp"
34 #include "memory/resourceArea.hpp"
35 #include "oops/markWord.hpp"
36 #include "oops/oop.inline.hpp"
37 #include "oops/oopHandle.inline.hpp"
38 #include "oops/weakHandle.inline.hpp"
39 #include "prims/jvmtiDeferredUpdates.hpp"
40 #include "prims/jvmtiExport.hpp"
41 #include "runtime/atomic.hpp"
42 #include "runtime/globals.hpp"
43 #include "runtime/handles.inline.hpp"
44 #include "runtime/interfaceSupport.inline.hpp"
45 #include "runtime/javaThread.inline.hpp"
46 #include "runtime/lightweightSynchronizer.hpp"
47 #include "runtime/mutexLocker.hpp"
48 #include "runtime/objectMonitor.hpp"
49 #include "runtime/objectMonitor.inline.hpp"
50 #include "runtime/orderAccess.hpp"
51 #include "runtime/osThread.hpp"
52 #include "runtime/perfData.hpp"
53 #include "runtime/safefetch.hpp"
54 #include "runtime/safepointMechanism.inline.hpp"
55 #include "runtime/sharedRuntime.hpp"
56 #include "services/threadService.hpp"
57 #include "utilities/debug.hpp"
58 #include "utilities/dtrace.hpp"
59 #include "utilities/globalDefinitions.hpp"
60 #include "utilities/macros.hpp"
61 #include "utilities/preserveException.hpp"
62 #if INCLUDE_JFR
63 #include "jfr/support/jfrFlush.hpp"
64 #endif
65
66 #ifdef DTRACE_ENABLED
67
68 // Only bother with this argument setup if dtrace is available
69 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
70
71
72 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
73 char* bytes = nullptr; \
74 int len = 0; \
75 jlong jtid = SharedRuntime::get_java_tid(thread); \
76 Symbol* klassname = obj->klass()->name(); \
77 if (klassname != nullptr) { \
78 bytes = (char*)klassname->bytes(); \
79 len = klassname->utf8_length(); \
80 }
81
82 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
83 { \
84 if (DTraceMonitorProbes) { \
85 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
86 HOTSPOT_MONITOR_WAIT(jtid, \
87 (monitor), bytes, len, (millis)); \
88 } \
89 }
90
91 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
92 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
93 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
94 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
95 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
96
97 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
98 { \
99 if (DTraceMonitorProbes) { \
100 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
101 HOTSPOT_MONITOR_##probe(jtid, \
102 (uintptr_t)(monitor), bytes, len); \
103 } \
104 }
105
106 #else // ndef DTRACE_ENABLED
107
108 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
109 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
110
111 #endif // ndef DTRACE_ENABLED
112
113 DEBUG_ONLY(static volatile bool InitDone = false;)
114
115 OopStorage* ObjectMonitor::_oop_storage = nullptr;
116
117 // -----------------------------------------------------------------------------
118 // Theory of operations -- Monitors lists, thread residency, etc:
119 //
120 // * A thread acquires ownership of a monitor by successfully
121 // CAS()ing the _owner field from null to non-null.
122 //
123 // * Invariant: A thread appears on at most one monitor list --
124 // cxq, EntryList or WaitSet -- at any one time.
125 //
126 // * Contending threads "push" themselves onto the cxq with CAS
127 // and then spin/park.
128 //
129 // * After a contending thread eventually acquires the lock it must
130 // dequeue itself from either the EntryList or the cxq.
131 //
132 // * The exiting thread identifies and unparks an "heir presumptive"
133 // tentative successor thread on the EntryList. Critically, the
134 // exiting thread doesn't unlink the successor thread from the EntryList.
135 // After having been unparked, the wakee will recontend for ownership of
136 // the monitor. The successor (wakee) will either acquire the lock or
137 // re-park itself.
138 //
139 // Succession is provided for by a policy of competitive handoff.
140 // The exiting thread does _not_ grant or pass ownership to the
141 // successor thread. (This is also referred to as "handoff" succession").
142 // Instead the exiting thread releases ownership and possibly wakes
143 // a successor, so the successor can (re)compete for ownership of the lock.
144 // If the EntryList is empty but the cxq is populated the exiting
145 // thread will drain the cxq into the EntryList. It does so by
146 // by detaching the cxq (installing null with CAS) and folding
147 // the threads from the cxq into the EntryList. The EntryList is
148 // doubly linked, while the cxq is singly linked because of the
149 // CAS-based "push" used to enqueue recently arrived threads (RATs).
150 //
151 // * Concurrency invariants:
152 //
153 // -- only the monitor owner may access or mutate the EntryList.
154 // The mutex property of the monitor itself protects the EntryList
155 // from concurrent interference.
156 // -- Only the monitor owner may detach the cxq.
157 //
158 // * The monitor entry list operations avoid locks, but strictly speaking
159 // they're not lock-free. Enter is lock-free, exit is not.
160 // For a description of 'Methods and apparatus providing non-blocking access
161 // to a resource,' see U.S. Pat. No. 7844973.
162 //
163 // * The cxq can have multiple concurrent "pushers" but only one concurrent
164 // detaching thread. This mechanism is immune from the ABA corruption.
165 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
166 //
167 // * Taken together, the cxq and the EntryList constitute or form a
168 // single logical queue of threads stalled trying to acquire the lock.
169 // We use two distinct lists to improve the odds of a constant-time
170 // dequeue operation after acquisition (in the ::enter() epilogue) and
171 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
172 // A key desideratum is to minimize queue & monitor metadata manipulation
173 // that occurs while holding the monitor lock -- that is, we want to
174 // minimize monitor lock holds times. Note that even a small amount of
175 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
176 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
177 // locks and monitor metadata.
178 //
179 // Cxq points to the set of Recently Arrived Threads attempting entry.
180 // Because we push threads onto _cxq with CAS, the RATs must take the form of
181 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
182 // the unlocking thread notices that EntryList is null but _cxq is != null.
183 //
184 // The EntryList is ordered by the prevailing queue discipline and
185 // can be organized in any convenient fashion, such as a doubly-linked list or
186 // a circular doubly-linked list. Critically, we want insert and delete operations
187 // to operate in constant-time. If we need a priority queue then something akin
188 // to Solaris' sleepq would work nicely. Viz.,
189 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
190 // Queue discipline is enforced at ::exit() time, when the unlocking thread
191 // drains the cxq into the EntryList, and orders or reorders the threads on the
192 // EntryList accordingly.
193 //
194 // Barring "lock barging", this mechanism provides fair cyclic ordering,
195 // somewhat similar to an elevator-scan.
196 //
197 // * The monitor synchronization subsystem avoids the use of native
198 // synchronization primitives except for the narrow platform-specific
199 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
200 // the semantics of park-unpark. Put another way, this monitor implementation
201 // depends only on atomic operations and park-unpark. The monitor subsystem
202 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
203 // underlying OS manages the READY<->RUN transitions.
204 //
205 // * Waiting threads reside on the WaitSet list -- wait() puts
206 // the caller onto the WaitSet.
207 //
208 // * notify() or notifyAll() simply transfers threads from the WaitSet to
209 // either the EntryList or cxq. Subsequent exit() operations will
210 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
211 // it's likely the notifyee would simply impale itself on the lock held
212 // by the notifier.
213 //
214 // * An interesting alternative is to encode cxq as (List,LockByte) where
215 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
216 // variable, like _recursions, in the scheme. The threads or Events that form
217 // the list would have to be aligned in 256-byte addresses. A thread would
218 // try to acquire the lock or enqueue itself with CAS, but exiting threads
219 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
220 // Note that is is *not* word-tearing, but it does presume that full-word
221 // CAS operations are coherent with intermix with STB operations. That's true
222 // on most common processors.
223 //
224 // * See also http://blogs.sun.com/dave
225
226
227 // Check that object() and set_object() are called from the right context:
228 static void check_object_context() {
229 #ifdef ASSERT
230 Thread* self = Thread::current();
231 if (self->is_Java_thread()) {
232 // Mostly called from JavaThreads so sanity check the thread state.
233 JavaThread* jt = JavaThread::cast(self);
234 switch (jt->thread_state()) {
235 case _thread_in_vm: // the usual case
236 case _thread_in_Java: // during deopt
237 break;
238 default:
239 fatal("called from an unsafe thread state");
240 }
241 assert(jt->is_active_Java_thread(), "must be active JavaThread");
242 } else {
243 // However, ThreadService::get_current_contended_monitor()
244 // can call here via the VMThread so sanity check it.
245 assert(self->is_VM_thread(), "must be");
246 }
247 #endif // ASSERT
248 }
249
250 ObjectMonitor::ObjectMonitor(oop object) :
251 _metadata(0),
252 _object(_oop_storage, object),
253 _owner(nullptr),
254 _previous_owner_tid(0),
255 _next_om(nullptr),
256 _recursions(0),
257 _EntryList(nullptr),
258 _cxq(nullptr),
259 _succ(nullptr),
260 _Responsible(nullptr),
261 _SpinDuration(ObjectMonitor::Knob_SpinLimit),
262 _contentions(0),
263 _WaitSet(nullptr),
264 _waiters(0),
265 _WaitSetLock(0)
266 { }
267
268 ObjectMonitor::~ObjectMonitor() {
269 _object.release(_oop_storage);
270 }
271
272 oop ObjectMonitor::object() const {
273 check_object_context();
274 return _object.resolve();
275 }
276
277 void ObjectMonitor::ExitOnSuspend::operator()(JavaThread* current) {
278 if (current->is_suspended()) {
279 _om->_recursions = 0;
280 _om->_succ = nullptr;
281 // Don't need a full fence after clearing successor here because of the call to exit().
282 _om->exit(current, false /* not_suspended */);
283 _om_exited = true;
284
285 current->set_current_pending_monitor(_om);
286 }
287 }
288
289 void ObjectMonitor::ClearSuccOnSuspend::operator()(JavaThread* current) {
290 if (current->is_suspended()) {
291 if (_om->_succ == current) {
292 _om->_succ = nullptr;
293 OrderAccess::fence(); // always do a full fence when successor is cleared
294 }
295 }
296 }
297
298 #define assert_mark_word_consistency() \
299 assert(UseObjectMonitorTable || object()->mark() == markWord::encode(this), \
300 "object mark must match encoded this: mark=" INTPTR_FORMAT \
301 ", encoded this=" INTPTR_FORMAT, object()->mark().value(), \
302 markWord::encode(this).value());
303
304 // -----------------------------------------------------------------------------
305 // Enter support
306
307 bool ObjectMonitor::enter_is_async_deflating() {
308 if (is_being_async_deflated()) {
309 if (!UseObjectMonitorTable) {
310 const oop l_object = object();
311 if (l_object != nullptr) {
312 // Attempt to restore the header/dmw to the object's header so that
313 // we only retry once if the deflater thread happens to be slow.
314 install_displaced_markword_in_object(l_object);
315 }
316 }
317 return true;
318 }
319
320 return false;
321 }
322
323 void ObjectMonitor::enter_for_with_contention_mark(JavaThread* locking_thread, ObjectMonitorContentionMark& contention_mark) {
324 // Used by ObjectSynchronizer::enter_for to enter for another thread.
325 // The monitor is private to or already owned by locking_thread which must be suspended.
326 // So this code may only contend with deflation.
327 assert(locking_thread == Thread::current() || locking_thread->is_obj_deopt_suspend(), "must be");
328 assert(contention_mark._monitor == this, "must be");
329 assert(!is_being_async_deflated(), "must be");
330
331
332 void* prev_owner = try_set_owner_from(nullptr, locking_thread);
333
334 bool success = false;
335
336 if (prev_owner == nullptr) {
337 assert(_recursions == 0, "invariant");
338 success = true;
339 } else if (prev_owner == locking_thread) {
340 _recursions++;
341 success = true;
342 } else if (prev_owner == DEFLATER_MARKER) {
343 // Racing with deflation.
344 prev_owner = try_set_owner_from(DEFLATER_MARKER, locking_thread);
345 if (prev_owner == DEFLATER_MARKER) {
346 // Cancelled deflation. Increment contentions as part of the deflation protocol.
347 add_to_contentions(1);
348 success = true;
349 } else if (prev_owner == nullptr) {
350 // At this point we cannot race with deflation as we have both incremented
351 // contentions, seen contention > 0 and seen a DEFLATER_MARKER.
352 // success will only be false if this races with something other than
353 // deflation.
354 prev_owner = try_set_owner_from(nullptr, locking_thread);
355 success = prev_owner == nullptr;
356 }
357 } else if (LockingMode == LM_LEGACY && locking_thread->is_lock_owned((address)prev_owner)) { 358 assert(_recursions == 0, "must be"); 359 _recursions = 1; 360 set_owner_from_BasicLock(prev_owner, locking_thread); 361 success = true;
362 }
363 assert(success, "Failed to enter_for: locking_thread=" INTPTR_FORMAT
364 ", this=" INTPTR_FORMAT "{owner=" INTPTR_FORMAT "}, observed owner: " INTPTR_FORMAT,
365 p2i(locking_thread), p2i(this), p2i(owner_raw()), p2i(prev_owner));
366 }
367
368 bool ObjectMonitor::enter_for(JavaThread* locking_thread) {
369
370 // Block out deflation as soon as possible.
371 ObjectMonitorContentionMark contention_mark(this);
372
373 // Check for deflation.
374 if (enter_is_async_deflating()) {
375 return false;
376 }
377
378 enter_for_with_contention_mark(locking_thread, contention_mark);
379 assert(owner_raw() == locking_thread, "must be");
380 return true;
381 }
382
383 bool ObjectMonitor::try_enter(JavaThread* current) {
384 // TryLock avoids the CAS
385 TryLockResult r = TryLock(current);
386 if (r == TryLockResult::Success) {
387 assert(_recursions == 0, "invariant");
388 return true;
389 }
390
391 if (r == TryLockResult::HasOwner && owner() == current) {
392 _recursions++;
393 return true;
394 }
395
396 void* cur = owner_raw(); 397 if (LockingMode == LM_LEGACY && current->is_lock_owned((address)cur)) { 398 assert(_recursions == 0, "internal state error"); 399 _recursions = 1; 400 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*. 401 return true; 402 } 403
404 return false;
405 }
406
407 bool ObjectMonitor::spin_enter(JavaThread* current) {
408 assert(current == JavaThread::current(), "must be");
409
410 // Check for recursion.
411 if (try_enter(current)) {
412 return true;
413 }
414
415 // Check for deflation.
416 if (enter_is_async_deflating()) {
417 return false;
418 }
419
420 // We've encountered genuine contention.
421
422 // Do one round of spinning.
423 // Note that if we acquire the monitor from an initial spin
424 // we forgo posting JVMTI events and firing DTRACE probes.
425 if (TrySpin(current)) {
426 assert(owner_raw() == current, "must be current: owner=" INTPTR_FORMAT, p2i(owner_raw()));
427 assert(_recursions == 0, "must be 0: recursions=" INTX_FORMAT, _recursions);
428 assert_mark_word_consistency();
429 return true;
430 }
431
432 return false;
433 }
434
435 bool ObjectMonitor::enter(JavaThread* current) {
436 assert(current == JavaThread::current(), "must be");
437
438 if (spin_enter(current)) {
439 return true;
440 }
441
442 assert(owner_raw() != current, "invariant");
443 assert(_succ != current, "invariant");
444 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
445 assert(current->thread_state() != _thread_blocked, "invariant");
446
447 // Keep is_being_async_deflated stable across the rest of enter
448 ObjectMonitorContentionMark contention_mark(this);
449
450 // Check for deflation.
451 if (enter_is_async_deflating()) {
452 return false;
453 }
454
455 // At this point this ObjectMonitor cannot be deflated, finish contended enter
456 enter_with_contention_mark(current, contention_mark);
457 return true;
458 }
459
460 void ObjectMonitor::enter_with_contention_mark(JavaThread *current, ObjectMonitorContentionMark &cm) {
461 assert(current == JavaThread::current(), "must be");
462 assert(owner_raw() != current, "must be");
463 assert(cm._monitor == this, "must be");
464 assert(!is_being_async_deflated(), "must be");
465
466 JFR_ONLY(JfrConditionalFlush<EventJavaMonitorEnter> flush(current);)
467 EventJavaMonitorEnter event;
468 if (event.is_started()) {
469 event.set_monitorClass(object()->klass());
470 // Set an address that is 'unique enough', such that events close in
471 // time and with the same address are likely (but not guaranteed) to
472 // belong to the same object.
473 event.set_address((uintptr_t)this);
474 }
475
476 { // Change java thread status to indicate blocked on monitor enter.
477 JavaThreadBlockedOnMonitorEnterState jtbmes(current, this);
478
479 assert(current->current_pending_monitor() == nullptr, "invariant");
480 current->set_current_pending_monitor(this);
481
482 DTRACE_MONITOR_PROBE(contended__enter, this, object(), current);
483 if (JvmtiExport::should_post_monitor_contended_enter()) {
484 JvmtiExport::post_monitor_contended_enter(current, this);
485
486 // The current thread does not yet own the monitor and does not
487 // yet appear on any queues that would get it made the successor.
488 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
489 // handler cannot accidentally consume an unpark() meant for the
490 // ParkEvent associated with this ObjectMonitor.
491 }
492
493 OSThreadContendState osts(current->osthread());
494
495 assert(current->thread_state() == _thread_in_vm, "invariant");
496
497 for (;;) {
498 ExitOnSuspend eos(this);
499 {
500 ThreadBlockInVMPreprocess<ExitOnSuspend> tbivs(current, eos, true /* allow_suspend */);
501 EnterI(current);
502 current->set_current_pending_monitor(nullptr);
503 // We can go to a safepoint at the end of this block. If we
504 // do a thread dump during that safepoint, then this thread will show
505 // as having "-locked" the monitor, but the OS and java.lang.Thread
506 // states will still report that the thread is blocked trying to
507 // acquire it.
508 // If there is a suspend request, ExitOnSuspend will exit the OM
509 // and set the OM as pending.
510 }
511 if (!eos.exited()) {
512 // ExitOnSuspend did not exit the OM
513 assert(owner_raw() == current, "invariant");
514 break;
515 }
516 }
517
518 // We've just gotten past the enter-check-for-suspend dance and we now own
519 // the monitor free and clear.
520 }
521
522 assert(contentions() >= 0, "must not be negative: contentions=%d", contentions());
523
524 // Must either set _recursions = 0 or ASSERT _recursions == 0.
525 assert(_recursions == 0, "invariant");
526 assert(owner_raw() == current, "invariant");
527 assert(_succ != current, "invariant");
528 assert_mark_word_consistency();
529
530 // The thread -- now the owner -- is back in vm mode.
531 // Report the glorious news via TI,DTrace and jvmstat.
532 // The probe effect is non-trivial. All the reportage occurs
533 // while we hold the monitor, increasing the length of the critical
534 // section. Amdahl's parallel speedup law comes vividly into play.
535 //
536 // Another option might be to aggregate the events (thread local or
537 // per-monitor aggregation) and defer reporting until a more opportune
538 // time -- such as next time some thread encounters contention but has
539 // yet to acquire the lock. While spinning that thread could
540 // spinning we could increment JVMStat counters, etc.
541
542 DTRACE_MONITOR_PROBE(contended__entered, this, object(), current);
543 if (JvmtiExport::should_post_monitor_contended_entered()) {
544 JvmtiExport::post_monitor_contended_entered(current, this);
545
546 // The current thread already owns the monitor and is not going to
547 // call park() for the remainder of the monitor enter protocol. So
548 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
549 // event handler consumed an unpark() issued by the thread that
550 // just exited the monitor.
551 }
552 if (event.should_commit()) {
553 event.set_previousOwner(_previous_owner_tid);
554 event.commit();
555 }
556 OM_PERFDATA_OP(ContendedLockAttempts, inc());
557 }
558
559 // Caveat: TryLock() is not necessarily serializing if it returns failure.
560 // Callers must compensate as needed.
561
562 ObjectMonitor::TryLockResult ObjectMonitor::TryLock(JavaThread* current) {
563 void* own = owner_raw();
564 if (own != nullptr) return TryLockResult::HasOwner;
565 if (try_set_owner_from(nullptr, current) == nullptr) {
566 assert(_recursions == 0, "invariant");
567 return TryLockResult::Success;
568 }
569 // The lock had been free momentarily, but we lost the race to the lock.
570 // Interference -- the CAS failed.
571 // We can either return -1 or retry.
572 // Retry doesn't make as much sense because the lock was just acquired.
573 return TryLockResult::Interference;
574 }
575
576 // Deflate the specified ObjectMonitor if not in-use. Returns true if it
577 // was deflated and false otherwise.
578 //
579 // The async deflation protocol sets owner to DEFLATER_MARKER and
580 // makes contentions negative as signals to contending threads that
581 // an async deflation is in progress. There are a number of checks
582 // as part of the protocol to make sure that the calling thread has
583 // not lost the race to a contending thread.
584 //
585 // The ObjectMonitor has been successfully async deflated when:
586 // (contentions < 0)
587 // Contending threads that see that condition know to retry their operation.
588 //
589 bool ObjectMonitor::deflate_monitor(Thread* current) {
590 if (is_busy()) {
591 // Easy checks are first - the ObjectMonitor is busy so no deflation.
592 return false;
593 }
594
595 const oop obj = object_peek();
596
597 if (obj == nullptr) {
598 // If the object died, we can recycle the monitor without racing with
599 // Java threads. The GC already broke the association with the object.
600 set_owner_from(nullptr, DEFLATER_MARKER);
601 assert(contentions() >= 0, "must be non-negative: contentions=%d", contentions());
602 _contentions = INT_MIN; // minimum negative int
603 } else {
604 // Attempt async deflation protocol.
605
606 // Set a null owner to DEFLATER_MARKER to force any contending thread
607 // through the slow path. This is just the first part of the async
608 // deflation dance.
609 if (try_set_owner_from(nullptr, DEFLATER_MARKER) != nullptr) {
610 // The owner field is no longer null so we lost the race since the
611 // ObjectMonitor is now busy.
612 return false;
613 }
614
615 if (contentions() > 0 || _waiters != 0) {
616 // Another thread has raced to enter the ObjectMonitor after
617 // is_busy() above or has already entered and waited on
618 // it which makes it busy so no deflation. Restore owner to
619 // null if it is still DEFLATER_MARKER.
620 if (try_set_owner_from(DEFLATER_MARKER, nullptr) != DEFLATER_MARKER) {
621 // Deferred decrement for the JT EnterI() that cancelled the async deflation.
622 add_to_contentions(-1);
623 }
624 return false;
625 }
626
627 // Make a zero contentions field negative to force any contending threads
628 // to retry. This is the second part of the async deflation dance.
629 if (Atomic::cmpxchg(&_contentions, 0, INT_MIN) != 0) {
630 // Contentions was no longer 0 so we lost the race since the
631 // ObjectMonitor is now busy. Restore owner to null if it is
632 // still DEFLATER_MARKER:
633 if (try_set_owner_from(DEFLATER_MARKER, nullptr) != DEFLATER_MARKER) {
634 // Deferred decrement for the JT EnterI() that cancelled the async deflation.
635 add_to_contentions(-1);
636 }
637 return false;
638 }
639 }
640
641 // Sanity checks for the races:
642 guarantee(owner_is_DEFLATER_MARKER(), "must be deflater marker");
643 guarantee(contentions() < 0, "must be negative: contentions=%d",
644 contentions());
645 guarantee(_waiters == 0, "must be 0: waiters=%d", _waiters);
646 guarantee(_cxq == nullptr, "must be no contending threads: cxq="
647 INTPTR_FORMAT, p2i(_cxq));
648 guarantee(_EntryList == nullptr,
649 "must be no entering threads: EntryList=" INTPTR_FORMAT,
650 p2i(_EntryList));
651
652 if (obj != nullptr) {
653 if (log_is_enabled(Trace, monitorinflation)) {
654 ResourceMark rm;
655 log_trace(monitorinflation)("deflate_monitor: object=" INTPTR_FORMAT
656 ", mark=" INTPTR_FORMAT ", type='%s'",
657 p2i(obj), obj->mark().value(),
658 obj->klass()->external_name());
659 }
660 }
661
662 if (UseObjectMonitorTable) {
663 LightweightSynchronizer::deflate_monitor(current, obj, this);
664 } else if (obj != nullptr) {
665 // Install the old mark word if nobody else has already done it.
666 install_displaced_markword_in_object(obj);
667 }
668
669 // We leave owner == DEFLATER_MARKER and contentions < 0
670 // to force any racing threads to retry.
671 return true; // Success, ObjectMonitor has been deflated.
672 }
673
674 // Install the displaced mark word (dmw) of a deflating ObjectMonitor
675 // into the header of the object associated with the monitor. This
676 // idempotent method is called by a thread that is deflating a
677 // monitor and by other threads that have detected a race with the
678 // deflation process.
679 void ObjectMonitor::install_displaced_markword_in_object(const oop obj) {
680 assert(!UseObjectMonitorTable, "ObjectMonitorTable has no dmw");
681 // This function must only be called when (owner == DEFLATER_MARKER
682 // && contentions <= 0), but we can't guarantee that here because
683 // those values could change when the ObjectMonitor gets moved from
684 // the global free list to a per-thread free list.
685
686 guarantee(obj != nullptr, "must be non-null");
687
688 // Separate loads in is_being_async_deflated(), which is almost always
689 // called before this function, from the load of dmw/header below.
690
691 // _contentions and dmw/header may get written by different threads.
692 // Make sure to observe them in the same order when having several observers.
693 OrderAccess::loadload_for_IRIW();
694
695 const oop l_object = object_peek();
696 if (l_object == nullptr) {
697 // ObjectMonitor's object ref has already been cleared by async
698 // deflation or GC so we're done here.
699 return;
700 }
701 assert(l_object == obj, "object=" INTPTR_FORMAT " must equal obj="
702 INTPTR_FORMAT, p2i(l_object), p2i(obj));
703
704 markWord dmw = header();
705 // The dmw has to be neutral (not null, not locked and not marked).
706 assert(dmw.is_neutral(), "must be neutral: dmw=" INTPTR_FORMAT, dmw.value());
707
708 // Install displaced mark word if the object's header still points
709 // to this ObjectMonitor. More than one racing caller to this function
710 // can rarely reach this point, but only one can win.
711 markWord res = obj->cas_set_mark(dmw, markWord::encode(this));
712 if (res != markWord::encode(this)) {
713 // This should be rare so log at the Info level when it happens.
714 log_info(monitorinflation)("install_displaced_markword_in_object: "
715 "failed cas_set_mark: new_mark=" INTPTR_FORMAT
716 ", old_mark=" INTPTR_FORMAT ", res=" INTPTR_FORMAT,
717 dmw.value(), markWord::encode(this).value(),
718 res.value());
719 }
720
721 // Note: It does not matter which thread restored the header/dmw
722 // into the object's header. The thread deflating the monitor just
723 // wanted the object's header restored and it is. The threads that
724 // detected a race with the deflation process also wanted the
725 // object's header restored before they retry their operation and
726 // because it is restored they will only retry once.
727 }
728
729 // Convert the fields used by is_busy() to a string that can be
730 // used for diagnostic output.
731 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) {
732 ss->print("is_busy: waiters=%d"
733 ", contentions=%d"
734 ", owner=" PTR_FORMAT
735 ", cxq=" PTR_FORMAT
736 ", EntryList=" PTR_FORMAT,
737 _waiters,
738 (contentions() > 0 ? contentions() : 0),
739 owner_is_DEFLATER_MARKER()
740 // We report null instead of DEFLATER_MARKER here because is_busy()
741 // ignores DEFLATER_MARKER values.
742 ? p2i(nullptr)
743 : p2i(owner_raw()),
744 p2i(_cxq),
745 p2i(_EntryList));
746 return ss->base();
747 }
748
749 #define MAX_RECHECK_INTERVAL 1000
750
751 void ObjectMonitor::EnterI(JavaThread* current) {
752 assert(current->thread_state() == _thread_blocked, "invariant");
753
754 // Try the lock - TATAS
755 if (TryLock(current) == TryLockResult::Success) {
756 assert(_succ != current, "invariant");
757 assert(owner_raw() == current, "invariant");
758 assert(_Responsible != current, "invariant");
759 return;
760 }
761
762 if (try_set_owner_from(DEFLATER_MARKER, current) == DEFLATER_MARKER) {
763 // Cancelled the in-progress async deflation by changing owner from
764 // DEFLATER_MARKER to current. As part of the contended enter protocol,
765 // contentions was incremented to a positive value before EnterI()
766 // was called and that prevents the deflater thread from winning the
767 // last part of the 2-part async deflation protocol. After EnterI()
768 // returns to enter(), contentions is decremented because the caller
769 // now owns the monitor. We bump contentions an extra time here to
770 // prevent the deflater thread from winning the last part of the
771 // 2-part async deflation protocol after the regular decrement
772 // occurs in enter(). The deflater thread will decrement contentions
773 // after it recognizes that the async deflation was cancelled.
774 add_to_contentions(1);
775 assert(_succ != current, "invariant");
776 assert(_Responsible != current, "invariant");
777 return;
778 }
779
780 assert(InitDone, "Unexpectedly not initialized");
781
782 // We try one round of spinning *before* enqueueing current.
783 //
784 // If the _owner is ready but OFFPROC we could use a YieldTo()
785 // operation to donate the remainder of this thread's quantum
786 // to the owner. This has subtle but beneficial affinity
787 // effects.
788
789 if (TrySpin(current)) {
790 assert(owner_raw() == current, "invariant");
791 assert(_succ != current, "invariant");
792 assert(_Responsible != current, "invariant");
793 return;
794 }
795
796 // The Spin failed -- Enqueue and park the thread ...
797 assert(_succ != current, "invariant");
798 assert(owner_raw() != current, "invariant");
799 assert(_Responsible != current, "invariant");
800
801 // Enqueue "current" on ObjectMonitor's _cxq.
802 //
803 // Node acts as a proxy for current.
804 // As an aside, if were to ever rewrite the synchronization code mostly
805 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
806 // Java objects. This would avoid awkward lifecycle and liveness issues,
807 // as well as eliminate a subset of ABA issues.
808 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
809
810 ObjectWaiter node(current);
811 current->_ParkEvent->reset();
812 node._prev = (ObjectWaiter*) 0xBAD;
813 node.TState = ObjectWaiter::TS_CXQ;
814
815 // Push "current" onto the front of the _cxq.
816 // Once on cxq/EntryList, current stays on-queue until it acquires the lock.
817 // Note that spinning tends to reduce the rate at which threads
818 // enqueue and dequeue on EntryList|cxq.
819 ObjectWaiter* nxt;
820 for (;;) {
821 node._next = nxt = _cxq;
822 if (Atomic::cmpxchg(&_cxq, nxt, &node) == nxt) break;
823
824 // Interference - the CAS failed because _cxq changed. Just retry.
825 // As an optional optimization we retry the lock.
826 if (TryLock(current) == TryLockResult::Success) {
827 assert(_succ != current, "invariant");
828 assert(owner_raw() == current, "invariant");
829 assert(_Responsible != current, "invariant");
830 return;
831 }
832 }
833
834 // Check for cxq|EntryList edge transition to non-null. This indicates
835 // the onset of contention. While contention persists exiting threads
836 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
837 // operations revert to the faster 1-0 mode. This enter operation may interleave
838 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
839 // arrange for one of the contending thread to use a timed park() operations
840 // to detect and recover from the race. (Stranding is form of progress failure
841 // where the monitor is unlocked but all the contending threads remain parked).
842 // That is, at least one of the contended threads will periodically poll _owner.
843 // One of the contending threads will become the designated "Responsible" thread.
844 // The Responsible thread uses a timed park instead of a normal indefinite park
845 // operation -- it periodically wakes and checks for and recovers from potential
846 // strandings admitted by 1-0 exit operations. We need at most one Responsible
847 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
848 // be responsible for a monitor.
849 //
850 // Currently, one of the contended threads takes on the added role of "Responsible".
851 // A viable alternative would be to use a dedicated "stranding checker" thread
852 // that periodically iterated over all the threads (or active monitors) and unparked
853 // successors where there was risk of stranding. This would help eliminate the
854 // timer scalability issues we see on some platforms as we'd only have one thread
855 // -- the checker -- parked on a timer.
856
857 if (nxt == nullptr && _EntryList == nullptr) {
858 // Try to assume the role of responsible thread for the monitor.
859 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=current }
860 Atomic::replace_if_null(&_Responsible, current);
861 }
862
863 // The lock might have been released while this thread was occupied queueing
864 // itself onto _cxq. To close the race and avoid "stranding" and
865 // progress-liveness failure we must resample-retry _owner before parking.
866 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
867 // In this case the ST-MEMBAR is accomplished with CAS().
868 //
869 // TODO: Defer all thread state transitions until park-time.
870 // Since state transitions are heavy and inefficient we'd like
871 // to defer the state transitions until absolutely necessary,
872 // and in doing so avoid some transitions ...
873
874 int recheckInterval = 1;
875
876 for (;;) {
877
878 if (TryLock(current) == TryLockResult::Success) {
879 break;
880 }
881 assert(owner_raw() != current, "invariant");
882
883 // park self
884 if (_Responsible == current) {
885 current->_ParkEvent->park((jlong) recheckInterval);
886 // Increase the recheckInterval, but clamp the value.
887 recheckInterval *= 8;
888 if (recheckInterval > MAX_RECHECK_INTERVAL) {
889 recheckInterval = MAX_RECHECK_INTERVAL;
890 }
891 } else {
892 current->_ParkEvent->park();
893 }
894
895 if (TryLock(current) == TryLockResult::Success) {
896 break;
897 }
898
899 if (try_set_owner_from(DEFLATER_MARKER, current) == DEFLATER_MARKER) {
900 // Cancelled the in-progress async deflation by changing owner from
901 // DEFLATER_MARKER to current. As part of the contended enter protocol,
902 // contentions was incremented to a positive value before EnterI()
903 // was called and that prevents the deflater thread from winning the
904 // last part of the 2-part async deflation protocol. After EnterI()
905 // returns to enter(), contentions is decremented because the caller
906 // now owns the monitor. We bump contentions an extra time here to
907 // prevent the deflater thread from winning the last part of the
908 // 2-part async deflation protocol after the regular decrement
909 // occurs in enter(). The deflater thread will decrement contentions
910 // after it recognizes that the async deflation was cancelled.
911 add_to_contentions(1);
912 break;
913 }
914
915 // The lock is still contested.
916
917 // Keep a tally of the # of futile wakeups.
918 // Note that the counter is not protected by a lock or updated by atomics.
919 // That is by design - we trade "lossy" counters which are exposed to
920 // races during updates for a lower probe effect.
921 // This PerfData object can be used in parallel with a safepoint.
922 // See the work around in PerfDataManager::destroy().
923 OM_PERFDATA_OP(FutileWakeups, inc());
924
925 // Assuming this is not a spurious wakeup we'll normally find _succ == current.
926 // We can defer clearing _succ until after the spin completes
927 // TrySpin() must tolerate being called with _succ == current.
928 // Try yet another round of adaptive spinning.
929 if (TrySpin(current)) {
930 break;
931 }
932
933 // We can find that we were unpark()ed and redesignated _succ while
934 // we were spinning. That's harmless. If we iterate and call park(),
935 // park() will consume the event and return immediately and we'll
936 // just spin again. This pattern can repeat, leaving _succ to simply
937 // spin on a CPU.
938
939 if (_succ == current) _succ = nullptr;
940
941 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
942 OrderAccess::fence();
943 }
944
945 // Egress :
946 // current has acquired the lock -- Unlink current from the cxq or EntryList.
947 // Normally we'll find current on the EntryList .
948 // From the perspective of the lock owner (this thread), the
949 // EntryList is stable and cxq is prepend-only.
950 // The head of cxq is volatile but the interior is stable.
951 // In addition, current.TState is stable.
952
953 assert(owner_raw() == current, "invariant");
954
955 UnlinkAfterAcquire(current, &node);
956 if (_succ == current) _succ = nullptr;
957
958 assert(_succ != current, "invariant");
959 if (_Responsible == current) {
960 _Responsible = nullptr;
961 OrderAccess::fence(); // Dekker pivot-point
962
963 // We may leave threads on cxq|EntryList without a designated
964 // "Responsible" thread. This is benign. When this thread subsequently
965 // exits the monitor it can "see" such preexisting "old" threads --
966 // threads that arrived on the cxq|EntryList before the fence, above --
967 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
968 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
969 // non-null and elect a new "Responsible" timer thread.
970 //
971 // This thread executes:
972 // ST Responsible=null; MEMBAR (in enter epilogue - here)
973 // LD cxq|EntryList (in subsequent exit)
974 //
975 // Entering threads in the slow/contended path execute:
976 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
977 // The (ST cxq; MEMBAR) is accomplished with CAS().
978 //
979 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
980 // exit operation from floating above the ST Responsible=null.
981 }
982
983 // We've acquired ownership with CAS().
984 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
985 // But since the CAS() this thread may have also stored into _succ,
986 // EntryList, cxq or Responsible. These meta-data updates must be
987 // visible __before this thread subsequently drops the lock.
988 // Consider what could occur if we didn't enforce this constraint --
989 // STs to monitor meta-data and user-data could reorder with (become
990 // visible after) the ST in exit that drops ownership of the lock.
991 // Some other thread could then acquire the lock, but observe inconsistent
992 // or old monitor meta-data and heap data. That violates the JMM.
993 // To that end, the 1-0 exit() operation must have at least STST|LDST
994 // "release" barrier semantics. Specifically, there must be at least a
995 // STST|LDST barrier in exit() before the ST of null into _owner that drops
996 // the lock. The barrier ensures that changes to monitor meta-data and data
997 // protected by the lock will be visible before we release the lock, and
998 // therefore before some other thread (CPU) has a chance to acquire the lock.
999 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
1000 //
1001 // Critically, any prior STs to _succ or EntryList must be visible before
1002 // the ST of null into _owner in the *subsequent* (following) corresponding
1003 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
1004 // execute a serializing instruction.
1005
1006 return;
1007 }
1008
1009 // ReenterI() is a specialized inline form of the latter half of the
1010 // contended slow-path from EnterI(). We use ReenterI() only for
1011 // monitor reentry in wait().
1012 //
1013 // In the future we should reconcile EnterI() and ReenterI().
1014
1015 void ObjectMonitor::ReenterI(JavaThread* current, ObjectWaiter* currentNode) {
1016 assert(current != nullptr, "invariant");
1017 assert(current->thread_state() != _thread_blocked, "invariant");
1018 assert(currentNode != nullptr, "invariant");
1019 assert(currentNode->_thread == current, "invariant");
1020 assert(_waiters > 0, "invariant");
1021 assert_mark_word_consistency();
1022
1023 for (;;) {
1024 ObjectWaiter::TStates v = currentNode->TState;
1025 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1026 assert(owner_raw() != current, "invariant");
1027
1028 // This thread has been notified so try to reacquire the lock.
1029 if (TryLock(current) == TryLockResult::Success) {
1030 break;
1031 }
1032
1033 // If that fails, spin again. Note that spin count may be zero so the above TryLock
1034 // is necessary.
1035 if (TrySpin(current)) {
1036 break;
1037 }
1038
1039 {
1040 OSThreadContendState osts(current->osthread());
1041
1042 assert(current->thread_state() == _thread_in_vm, "invariant");
1043
1044 {
1045 ClearSuccOnSuspend csos(this);
1046 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
1047 current->_ParkEvent->park();
1048 }
1049 }
1050
1051 // Try again, but just so we distinguish between futile wakeups and
1052 // successful wakeups. The following test isn't algorithmically
1053 // necessary, but it helps us maintain sensible statistics.
1054 if (TryLock(current) == TryLockResult::Success) {
1055 break;
1056 }
1057
1058 // The lock is still contested.
1059
1060 // Assuming this is not a spurious wakeup we'll normally
1061 // find that _succ == current.
1062 if (_succ == current) _succ = nullptr;
1063
1064 // Invariant: after clearing _succ a contending thread
1065 // *must* retry _owner before parking.
1066 OrderAccess::fence();
1067
1068 // Keep a tally of the # of futile wakeups.
1069 // Note that the counter is not protected by a lock or updated by atomics.
1070 // That is by design - we trade "lossy" counters which are exposed to
1071 // races during updates for a lower probe effect.
1072 // This PerfData object can be used in parallel with a safepoint.
1073 // See the work around in PerfDataManager::destroy().
1074 OM_PERFDATA_OP(FutileWakeups, inc());
1075 }
1076
1077 // current has acquired the lock -- Unlink current from the cxq or EntryList .
1078 // Normally we'll find current on the EntryList.
1079 // Unlinking from the EntryList is constant-time and atomic-free.
1080 // From the perspective of the lock owner (this thread), the
1081 // EntryList is stable and cxq is prepend-only.
1082 // The head of cxq is volatile but the interior is stable.
1083 // In addition, current.TState is stable.
1084
1085 assert(owner_raw() == current, "invariant");
1086 assert_mark_word_consistency();
1087 UnlinkAfterAcquire(current, currentNode);
1088 if (_succ == current) _succ = nullptr;
1089 assert(_succ != current, "invariant");
1090 currentNode->TState = ObjectWaiter::TS_RUN;
1091 OrderAccess::fence(); // see comments at the end of EnterI()
1092 }
1093
1094 // By convention we unlink a contending thread from EntryList|cxq immediately
1095 // after the thread acquires the lock in ::enter(). Equally, we could defer
1096 // unlinking the thread until ::exit()-time.
1097
1098 void ObjectMonitor::UnlinkAfterAcquire(JavaThread* current, ObjectWaiter* currentNode) {
1099 assert(owner_raw() == current, "invariant");1100 assert(currentNode->_thread == current, "invariant");
1101
1102 if (currentNode->TState == ObjectWaiter::TS_ENTER) {
1103 // Normal case: remove current from the DLL EntryList .
1104 // This is a constant-time operation.
1105 ObjectWaiter* nxt = currentNode->_next;
1106 ObjectWaiter* prv = currentNode->_prev;
1107 if (nxt != nullptr) nxt->_prev = prv;
1108 if (prv != nullptr) prv->_next = nxt;
1109 if (currentNode == _EntryList) _EntryList = nxt;
1110 assert(nxt == nullptr || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
1111 assert(prv == nullptr || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
1112 } else {
1113 assert(currentNode->TState == ObjectWaiter::TS_CXQ, "invariant");
1114 // Inopportune interleaving -- current is still on the cxq.
1115 // This usually means the enqueue of self raced an exiting thread.
1116 // Normally we'll find current near the front of the cxq, so
1117 // dequeueing is typically fast. If needbe we can accelerate
1118 // this with some MCS/CHL-like bidirectional list hints and advisory
1119 // back-links so dequeueing from the interior will normally operate
1120 // in constant-time.
1121 // Dequeue current from either the head (with CAS) or from the interior
1122 // with a linear-time scan and normal non-atomic memory operations.
1123 // CONSIDER: if current is on the cxq then simply drain cxq into EntryList
1124 // and then unlink current from EntryList. We have to drain eventually,
1125 // so it might as well be now.
1126
1127 ObjectWaiter* v = _cxq;
1128 assert(v != nullptr, "invariant");
1129 if (v != currentNode || Atomic::cmpxchg(&_cxq, v, currentNode->_next) != v) {
1130 // The CAS above can fail from interference IFF a "RAT" arrived.
1131 // In that case current must be in the interior and can no longer be
1132 // at the head of cxq.
1133 if (v == currentNode) {
1134 assert(_cxq != v, "invariant");
1135 v = _cxq; // CAS above failed - start scan at head of list
1136 }
1137 ObjectWaiter* p;
1138 ObjectWaiter* q = nullptr;
1139 for (p = v; p != nullptr && p != currentNode; p = p->_next) {
1140 q = p;
1141 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
1142 }
1143 assert(v != currentNode, "invariant");
1144 assert(p == currentNode, "Node not found on cxq");
1145 assert(p != _cxq, "invariant");
1146 assert(q != nullptr, "invariant");
1147 assert(q->_next == p, "invariant");
1148 q->_next = p->_next;
1149 }
1150 }
1151
1152 #ifdef ASSERT
1153 // Diagnostic hygiene ...
1154 currentNode->_prev = (ObjectWaiter*) 0xBAD;
1155 currentNode->_next = (ObjectWaiter*) 0xBAD;
1156 currentNode->TState = ObjectWaiter::TS_RUN;
1157 #endif
1158 }
1159
1160 // -----------------------------------------------------------------------------
1161 // Exit support
1162 //
1163 // exit()
1164 // ~~~~~~
1165 // Note that the collector can't reclaim the objectMonitor or deflate
1166 // the object out from underneath the thread calling ::exit() as the
1167 // thread calling ::exit() never transitions to a stable state.
1168 // This inhibits GC, which in turn inhibits asynchronous (and
1169 // inopportune) reclamation of "this".
1170 //
1171 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
1172 // There's one exception to the claim above, however. EnterI() can call
1173 // exit() to drop a lock if the acquirer has been externally suspended.
1174 // In that case exit() is called with _thread_state == _thread_blocked,
1175 // but the monitor's _contentions field is > 0, which inhibits reclamation.
1176 //
1177 // 1-0 exit
1178 // ~~~~~~~~
1179 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
1180 // the fast-path operators have been optimized so the common ::exit()
1181 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
1182 // The code emitted by fast_unlock() elides the usual MEMBAR. This
1183 // greatly improves latency -- MEMBAR and CAS having considerable local
1184 // latency on modern processors -- but at the cost of "stranding". Absent the
1185 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
1186 // ::enter() path, resulting in the entering thread being stranding
1187 // and a progress-liveness failure. Stranding is extremely rare.
1188 // We use timers (timed park operations) & periodic polling to detect
1189 // and recover from stranding. Potentially stranded threads periodically
1190 // wake up and poll the lock. See the usage of the _Responsible variable.
1191 //
1192 // The CAS() in enter provides for safety and exclusion, while the CAS or
1193 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
1194 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
1195 // We detect and recover from stranding with timers.
1196 //
1197 // If a thread transiently strands it'll park until (a) another
1198 // thread acquires the lock and then drops the lock, at which time the
1199 // exiting thread will notice and unpark the stranded thread, or, (b)
1200 // the timer expires. If the lock is high traffic then the stranding latency
1201 // will be low due to (a). If the lock is low traffic then the odds of
1202 // stranding are lower, although the worst-case stranding latency
1203 // is longer. Critically, we don't want to put excessive load in the
1204 // platform's timer subsystem. We want to minimize both the timer injection
1205 // rate (timers created/sec) as well as the number of timers active at
1206 // any one time. (more precisely, we want to minimize timer-seconds, which is
1207 // the integral of the # of active timers at any instant over time).
1208 // Both impinge on OS scalability. Given that, at most one thread parked on
1209 // a monitor will use a timer.
1210 //
1211 // There is also the risk of a futile wake-up. If we drop the lock
1212 // another thread can reacquire the lock immediately, and we can
1213 // then wake a thread unnecessarily. This is benign, and we've
1214 // structured the code so the windows are short and the frequency
1215 // of such futile wakups is low.
1216
1217 void ObjectMonitor::exit(JavaThread* current, bool not_suspended) {
1218 void* cur = owner_raw();
1219 if (current != cur) {1220 if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) {1221 assert(_recursions == 0, "invariant");1222 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*.1223 _recursions = 0;1224 } else {1225 // Apparent unbalanced locking ...1226 // Naively we'd like to throw IllegalMonitorStateException.1227 // As a practical matter we can neither allocate nor throw an1228 // exception as ::exit() can be called from leaf routines.1229 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.1230 // Upon deeper reflection, however, in a properly run JVM the only1231 // way we should encounter this situation is in the presence of1232 // unbalanced JNI locking. TODO: CheckJNICalls.1233 // See also: CR4414101
1234 #ifdef ASSERT
1235 LogStreamHandle(Error, monitorinflation) lsh;1236 lsh.print_cr("ERROR: ObjectMonitor::exit(): thread=" INTPTR_FORMAT1237 " is exiting an ObjectMonitor it does not own.", p2i(current));1238 lsh.print_cr("The imbalance is possibly caused by JNI locking.");1239 print_debug_style_on(&lsh);1240 assert(false, "Non-balanced monitor enter/exit!");
1241 #endif
1242 return;1243 }
1244 }
1245
1246 if (_recursions != 0) {
1247 _recursions--; // this is simple recursive enter
1248 return;
1249 }
1250
1251 // Invariant: after setting Responsible=null an thread must execute
1252 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
1253 _Responsible = nullptr;
1254
1255 #if INCLUDE_JFR
1256 // get the owner's thread id for the MonitorEnter event
1257 // if it is enabled and the thread isn't suspended
1258 if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
1259 _previous_owner_tid = JFR_THREAD_ID(current);
1260 }
1261 #endif
1262
1263 for (;;) {
1264 assert(current == owner_raw(), "invariant");
1265
1266 // Drop the lock.
1267 // release semantics: prior loads and stores from within the critical section
1268 // must not float (reorder) past the following store that drops the lock.
1269 // Uses a storeload to separate release_store(owner) from the
1270 // successor check. The try_set_owner() below uses cmpxchg() so
1271 // we get the fence down there.
1272 release_clear_owner(current);
1273 OrderAccess::storeload();
1274
1275 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != nullptr) {
1276 return;
1277 }
1278 // Other threads are blocked trying to acquire the lock.
1279
1280 // Normally the exiting thread is responsible for ensuring succession,
1281 // but if other successors are ready or other entering threads are spinning
1282 // then this thread can simply store null into _owner and exit without
1283 // waking a successor. The existence of spinners or ready successors
1284 // guarantees proper succession (liveness). Responsibility passes to the
1285 // ready or running successors. The exiting thread delegates the duty.
1286 // More precisely, if a successor already exists this thread is absolved
1287 // of the responsibility of waking (unparking) one.
1288 //
1289 // The _succ variable is critical to reducing futile wakeup frequency.
1290 // _succ identifies the "heir presumptive" thread that has been made
1291 // ready (unparked) but that has not yet run. We need only one such
1292 // successor thread to guarantee progress.
1293 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1294 // section 3.3 "Futile Wakeup Throttling" for details.
1295 //
1296 // Note that spinners in Enter() also set _succ non-null.
1297 // In the current implementation spinners opportunistically set
1298 // _succ so that exiting threads might avoid waking a successor.
1299 // Another less appealing alternative would be for the exiting thread
1300 // to drop the lock and then spin briefly to see if a spinner managed
1301 // to acquire the lock. If so, the exiting thread could exit
1302 // immediately without waking a successor, otherwise the exiting
1303 // thread would need to dequeue and wake a successor.
1304 // (Note that we'd need to make the post-drop spin short, but no
1305 // shorter than the worst-case round-trip cache-line migration time.
1306 // The dropped lock needs to become visible to the spinner, and then
1307 // the acquisition of the lock by the spinner must become visible to
1308 // the exiting thread).
1309
1310 // It appears that an heir-presumptive (successor) must be made ready.
1311 // Only the current lock owner can manipulate the EntryList or
1312 // drain _cxq, so we need to reacquire the lock. If we fail
1313 // to reacquire the lock the responsibility for ensuring succession
1314 // falls to the new owner.
1315 //
1316 if (try_set_owner_from(nullptr, current) != nullptr) {
1317 return;
1318 }
1319
1320 guarantee(owner_raw() == current, "invariant");
1321
1322 ObjectWaiter* w = nullptr;
1323
1324 w = _EntryList;
1325 if (w != nullptr) {
1326 // I'd like to write: guarantee (w->_thread != current).
1327 // But in practice an exiting thread may find itself on the EntryList.
1328 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1329 // then calls exit(). Exit release the lock by setting O._owner to null.
1330 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1331 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1332 // release the lock "O". T2 resumes immediately after the ST of null into
1333 // _owner, above. T2 notices that the EntryList is populated, so it
1334 // reacquires the lock and then finds itself on the EntryList.
1335 // Given all that, we have to tolerate the circumstance where "w" is
1336 // associated with current.
1337 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1338 ExitEpilog(current, w);
1339 return;
1340 }
1341
1342 // If we find that both _cxq and EntryList are null then just
1343 // re-run the exit protocol from the top.
1344 w = _cxq;
1345 if (w == nullptr) continue;
1346
1347 // Drain _cxq into EntryList - bulk transfer.
1348 // First, detach _cxq.
1349 // The following loop is tantamount to: w = swap(&cxq, nullptr)
1350 for (;;) {
1351 assert(w != nullptr, "Invariant");
1352 ObjectWaiter* u = Atomic::cmpxchg(&_cxq, w, (ObjectWaiter*)nullptr);
1353 if (u == w) break;
1354 w = u;
1355 }
1356
1357 assert(w != nullptr, "invariant");
1358 assert(_EntryList == nullptr, "invariant");
1359
1360 // Convert the LIFO SLL anchored by _cxq into a DLL.
1361 // The list reorganization step operates in O(LENGTH(w)) time.
1362 // It's critical that this step operate quickly as
1363 // "current" still holds the outer-lock, restricting parallelism
1364 // and effectively lengthening the critical section.
1365 // Invariant: s chases t chases u.
1366 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1367 // we have faster access to the tail.
1368
1369 _EntryList = w;
1370 ObjectWaiter* q = nullptr;
1371 ObjectWaiter* p;
1372 for (p = w; p != nullptr; p = p->_next) {
1373 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1374 p->TState = ObjectWaiter::TS_ENTER;
1375 p->_prev = q;
1376 q = p;
1377 }
1378
1379 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = nullptr
1380 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1381
1382 // See if we can abdicate to a spinner instead of waking a thread.
1383 // A primary goal of the implementation is to reduce the
1384 // context-switch rate.
1385 if (_succ != nullptr) continue;
1386
1387 w = _EntryList;
1388 if (w != nullptr) {
1389 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1390 ExitEpilog(current, w);
1391 return;
1392 }
1393 }
1394 }
1395
1396 void ObjectMonitor::ExitEpilog(JavaThread* current, ObjectWaiter* Wakee) {
1397 assert(owner_raw() == current, "invariant");
1398
1399 // Exit protocol:
1400 // 1. ST _succ = wakee
1401 // 2. membar #loadstore|#storestore;
1402 // 2. ST _owner = nullptr
1403 // 3. unpark(wakee)
1404
1405 _succ = Wakee->_thread;1406 ParkEvent * Trigger = Wakee->_event;
1407
1408 // Hygiene -- once we've set _owner = nullptr we can't safely dereference Wakee again.
1409 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1410 // out-of-scope (non-extant).
1411 Wakee = nullptr;
1412
1413 // Drop the lock.
1414 // Uses a fence to separate release_store(owner) from the LD in unpark().
1415 release_clear_owner(current);
1416 OrderAccess::fence();
1417
1418 DTRACE_MONITOR_PROBE(contended__exit, this, object(), current);
1419 Trigger->unpark();
1420
1421 // Maintain stats and report events to JVMTI
1422 OM_PERFDATA_OP(Parks, inc());
1423 }
1424
1425 // complete_exit exits a lock returning recursion count
1426 // complete_exit requires an inflated monitor
1427 // The _owner field is not always the Thread addr even with an
1428 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1429 // thread due to contention.
1430 intx ObjectMonitor::complete_exit(JavaThread* current) {
1431 assert(InitDone, "Unexpectedly not initialized");
1432
1433 void* cur = owner_raw();
1434 if (current != cur) {1435 if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) {
1436 assert(_recursions == 0, "internal state error");
1437 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*.
1438 _recursions = 0;
1439 }
1440 }
1441
1442 guarantee(current == owner_raw(), "complete_exit not owner");
1443 intx save = _recursions; // record the old recursion count
1444 _recursions = 0; // set the recursion level to be 0
1445 exit(current); // exit the monitor
1446 guarantee(owner_raw() != current, "invariant");
1447 return save;
1448 }
1449
1450 // Checks that the current THREAD owns this monitor and causes an
1451 // immediate return if it doesn't. We don't use the CHECK macro
1452 // because we want the IMSE to be the only exception that is thrown
1453 // from the call site when false is returned. Any other pending
1454 // exception is ignored.
1455 #define CHECK_OWNER() \
1456 do { \
1457 if (!check_owner(THREAD)) { \
1458 assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \
1459 return; \
1460 } \
1461 } while (false)
1462
1463 // Returns true if the specified thread owns the ObjectMonitor.
1464 // Otherwise returns false and throws IllegalMonitorStateException
1465 // (IMSE). If there is a pending exception and the specified thread
1466 // is not the owner, that exception will be replaced by the IMSE.
1467 bool ObjectMonitor::check_owner(TRAPS) {
1468 JavaThread* current = THREAD;
1469 void* cur = owner_raw();
1470 assert(cur != anon_owner_ptr(), "no anon owner here");1471 if (cur == current) {1472 return true;1473 }1474 if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) {1475 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*.1476 _recursions = 0;
1477 return true;
1478 }
1479 THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(),
1480 "current thread is not owner", false);
1481 }
1482
1483 static inline bool is_excluded(const Klass* monitor_klass) {
1484 assert(monitor_klass != nullptr, "invariant");
1485 NOT_JFR_RETURN_(false);
1486 JFR_ONLY(return vmSymbols::jdk_jfr_internal_management_HiddenWait() == monitor_klass->name();)
1487 }
1488
1489 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1490 ObjectMonitor* monitor,
1491 uint64_t notifier_tid,
1492 jlong timeout,
1493 bool timedout) {
1494 assert(event != nullptr, "invariant");
1495 assert(monitor != nullptr, "invariant");
1496 const Klass* monitor_klass = monitor->object()->klass();
1497 if (is_excluded(monitor_klass)) {
1498 return;
1499 }
1500 event->set_monitorClass(monitor_klass);
1501 event->set_timeout(timeout);
1502 // Set an address that is 'unique enough', such that events close in
1503 // time and with the same address are likely (but not guaranteed) to
1504 // belong to the same object.
1505 event->set_address((uintptr_t)monitor);
1506 event->set_notifier(notifier_tid);
1507 event->set_timedOut(timedout);
1508 event->commit();
1509 }
1510
1511 // -----------------------------------------------------------------------------
1512 // Wait/Notify/NotifyAll
1513 //
1514 // Note: a subset of changes to ObjectMonitor::wait()
1515 // will need to be replicated in complete_exit
1516 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1517 JavaThread* current = THREAD;
1518
1519 assert(InitDone, "Unexpectedly not initialized");
1520
1521 CHECK_OWNER(); // Throws IMSE if not owner.
1522
1523 EventJavaMonitorWait event;
1524
1525 // check for a pending interrupt
1526 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1527 // post monitor waited event. Note that this is past-tense, we are done waiting.
1528 if (JvmtiExport::should_post_monitor_waited()) {
1529 // Note: 'false' parameter is passed here because the
1530 // wait was not timed out due to thread interrupt.
1531 JvmtiExport::post_monitor_waited(current, this, false);
1532
1533 // In this short circuit of the monitor wait protocol, the
1534 // current thread never drops ownership of the monitor and
1535 // never gets added to the wait queue so the current thread
1536 // cannot be made the successor. This means that the
1537 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1538 // consume an unpark() meant for the ParkEvent associated with
1539 // this ObjectMonitor.
1540 }
1541 if (event.should_commit()) {
1542 post_monitor_wait_event(&event, this, 0, millis, false);
1543 }
1544 THROW(vmSymbols::java_lang_InterruptedException());
1545 return;
1546 }
1547
1548 current->set_current_waiting_monitor(this);
1549
1550 // create a node to be put into the queue
1551 // Critically, after we reset() the event but prior to park(), we must check
1552 // for a pending interrupt.
1553 ObjectWaiter node(current);
1554 node.TState = ObjectWaiter::TS_WAIT;
1555 current->_ParkEvent->reset();
1556 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1557
1558 // Enter the waiting queue, which is a circular doubly linked list in this case
1559 // but it could be a priority queue or any data structure.
1560 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1561 // by the owner of the monitor *except* in the case where park()
1562 // returns because of a timeout of interrupt. Contention is exceptionally rare
1563 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1564
1565 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1566 AddWaiter(&node);
1567 Thread::SpinRelease(&_WaitSetLock);
1568
1569 _Responsible = nullptr;
1570
1571 intx save = _recursions; // record the old recursion count
1572 _waiters++; // increment the number of waiters
1573 _recursions = 0; // set the recursion level to be 1
1574 exit(current); // exit the monitor
1575 guarantee(owner_raw() != current, "invariant");
1576
1577 // The thread is on the WaitSet list - now park() it.
1578 // On MP systems it's conceivable that a brief spin before we park
1579 // could be profitable.
1580 //
1581 // TODO-FIXME: change the following logic to a loop of the form
1582 // while (!timeout && !interrupted && _notified == 0) park()
1583
1584 int ret = OS_OK;
1585 int WasNotified = 0;
1586
1587 // Need to check interrupt state whilst still _thread_in_vm
1588 bool interrupted = interruptible && current->is_interrupted(false);
1589
1590 { // State transition wrappers
1591 OSThread* osthread = current->osthread();
1592 OSThreadWaitState osts(osthread, true);
1593
1594 assert(current->thread_state() == _thread_in_vm, "invariant");
1595
1596 {
1597 ClearSuccOnSuspend csos(this);
1598 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
1599 if (interrupted || HAS_PENDING_EXCEPTION) {
1600 // Intentionally empty
1601 } else if (node._notified == 0) {
1602 if (millis <= 0) {
1603 current->_ParkEvent->park();
1604 } else {
1605 ret = current->_ParkEvent->park(millis);
1606 }
1607 }
1608 }
1609
1610 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1611 // from the WaitSet to the EntryList.
1612 // See if we need to remove Node from the WaitSet.
1613 // We use double-checked locking to avoid grabbing _WaitSetLock
1614 // if the thread is not on the wait queue.
1615 //
1616 // Note that we don't need a fence before the fetch of TState.
1617 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1618 // written by the is thread. (perhaps the fetch might even be satisfied
1619 // by a look-aside into the processor's own store buffer, although given
1620 // the length of the code path between the prior ST and this load that's
1621 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1622 // then we'll acquire the lock and then re-fetch a fresh TState value.
1623 // That is, we fail toward safety.
1624
1625 if (node.TState == ObjectWaiter::TS_WAIT) {
1626 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1627 if (node.TState == ObjectWaiter::TS_WAIT) {
1628 DequeueSpecificWaiter(&node); // unlink from WaitSet
1629 assert(node._notified == 0, "invariant");
1630 node.TState = ObjectWaiter::TS_RUN;
1631 }
1632 Thread::SpinRelease(&_WaitSetLock);
1633 }
1634
1635 // The thread is now either on off-list (TS_RUN),
1636 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1637 // The Node's TState variable is stable from the perspective of this thread.
1638 // No other threads will asynchronously modify TState.
1639 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1640 OrderAccess::loadload();
1641 if (_succ == current) _succ = nullptr;
1642 WasNotified = node._notified;
1643
1644 // Reentry phase -- reacquire the monitor.
1645 // re-enter contended monitor after object.wait().
1646 // retain OBJECT_WAIT state until re-enter successfully completes
1647 // Thread state is thread_in_vm and oop access is again safe,
1648 // although the raw address of the object may have changed.
1649 // (Don't cache naked oops over safepoints, of course).
1650
1651 // post monitor waited event. Note that this is past-tense, we are done waiting.
1652 if (JvmtiExport::should_post_monitor_waited()) {
1653 JvmtiExport::post_monitor_waited(current, this, ret == OS_TIMEOUT);
1654
1655 if (node._notified != 0 && _succ == current) {
1656 // In this part of the monitor wait-notify-reenter protocol it
1657 // is possible (and normal) for another thread to do a fastpath
1658 // monitor enter-exit while this thread is still trying to get
1659 // to the reenter portion of the protocol.
1660 //
1661 // The ObjectMonitor was notified and the current thread is
1662 // the successor which also means that an unpark() has already
1663 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1664 // consume the unpark() that was done when the successor was
1665 // set because the same ParkEvent is shared between Java
1666 // monitors and JVM/TI RawMonitors (for now).
1667 //
1668 // We redo the unpark() to ensure forward progress, i.e., we
1669 // don't want all pending threads hanging (parked) with none
1670 // entering the unlocked monitor.
1671 node._event->unpark();
1672 }
1673 }
1674
1675 if (event.should_commit()) {
1676 post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1677 }
1678
1679 OrderAccess::fence();
1680
1681 assert(owner_raw() != current, "invariant");
1682 ObjectWaiter::TStates v = node.TState;
1683 if (v == ObjectWaiter::TS_RUN) {
1684 enter(current);
1685 } else {
1686 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1687 ReenterI(current, &node);
1688 node.wait_reenter_end(this);
1689 }
1690
1691 // current has reacquired the lock.
1692 // Lifecycle - the node representing current must not appear on any queues.
1693 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1694 // want residual elements associated with this thread left on any lists.
1695 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1696 assert(owner_raw() == current, "invariant");
1697 assert(_succ != current, "invariant");
1698 } // OSThreadWaitState()
1699
1700 current->set_current_waiting_monitor(nullptr);
1701
1702 guarantee(_recursions == 0, "invariant");
1703 int relock_count = JvmtiDeferredUpdates::get_and_reset_relock_count_after_wait(current);
1704 _recursions = save // restore the old recursion count
1705 + relock_count; // increased by the deferred relock count
1706 current->inc_held_monitor_count(relock_count); // Deopt never entered these counts.
1707 _waiters--; // decrement the number of waiters
1708
1709 // Verify a few postconditions
1710 assert(owner_raw() == current, "invariant");
1711 assert(_succ != current, "invariant");
1712 assert_mark_word_consistency();
1713
1714 // check if the notification happened
1715 if (!WasNotified) {
1716 // no, it could be timeout or Thread.interrupt() or both
1717 // check for interrupt event, otherwise it is timeout
1718 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1719 THROW(vmSymbols::java_lang_InterruptedException());
1720 }
1721 }
1722
1723 // NOTE: Spurious wake up will be consider as timeout.
1724 // Monitor notify has precedence over thread interrupt.
1725 }
1726
1727
1728 // Consider:
1729 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1730 // then instead of transferring a thread from the WaitSet to the EntryList
1731 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1732
1733 void ObjectMonitor::INotify(JavaThread* current) {
1734 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1735 ObjectWaiter* iterator = DequeueWaiter();
1736 if (iterator != nullptr) {
1737 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1738 guarantee(iterator->_notified == 0, "invariant");
1739 // Disposition - what might we do with iterator ?
1740 // a. add it directly to the EntryList - either tail (policy == 1)
1741 // or head (policy == 0).
1742 // b. push it onto the front of the _cxq (policy == 2).
1743 // For now we use (b).
1744
1745 iterator->TState = ObjectWaiter::TS_ENTER;
1746
1747 iterator->_notified = 1;
1748 iterator->_notifier_tid = JFR_THREAD_ID(current);
1749
1750 ObjectWaiter* list = _EntryList;
1751 if (list != nullptr) {
1752 assert(list->_prev == nullptr, "invariant");
1753 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1754 assert(list != iterator, "invariant");
1755 }
1756
1757 // prepend to cxq
1758 if (list == nullptr) {
1759 iterator->_next = iterator->_prev = nullptr;
1760 _EntryList = iterator;
1761 } else {
1762 iterator->TState = ObjectWaiter::TS_CXQ;
1763 for (;;) {
1764 ObjectWaiter* front = _cxq;
1765 iterator->_next = front;
1766 if (Atomic::cmpxchg(&_cxq, front, iterator) == front) {
1767 break;
1768 }
1769 }
1770 }
1771
1772 // _WaitSetLock protects the wait queue, not the EntryList. We could
1773 // move the add-to-EntryList operation, above, outside the critical section
1774 // protected by _WaitSetLock. In practice that's not useful. With the
1775 // exception of wait() timeouts and interrupts the monitor owner
1776 // is the only thread that grabs _WaitSetLock. There's almost no contention
1777 // on _WaitSetLock so it's not profitable to reduce the length of the
1778 // critical section.
1779 1780 iterator->wait_reenter_begin(this);
1781 }
1782 Thread::SpinRelease(&_WaitSetLock);
1783 }
1784
1785 // Consider: a not-uncommon synchronization bug is to use notify() when
1786 // notifyAll() is more appropriate, potentially resulting in stranded
1787 // threads; this is one example of a lost wakeup. A useful diagnostic
1788 // option is to force all notify() operations to behave as notifyAll().
1789 //
1790 // Note: We can also detect many such problems with a "minimum wait".
1791 // When the "minimum wait" is set to a small non-zero timeout value
1792 // and the program does not hang whereas it did absent "minimum wait",
1793 // that suggests a lost wakeup bug.
1794
1795 void ObjectMonitor::notify(TRAPS) {
1796 JavaThread* current = THREAD;
1797 CHECK_OWNER(); // Throws IMSE if not owner.
1798 if (_WaitSet == nullptr) {
1799 return;
1800 }
1801 DTRACE_MONITOR_PROBE(notify, this, object(), current);
1802 INotify(current);
1803 OM_PERFDATA_OP(Notifications, inc(1));
1804 }
1805
1806
1807 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1808 // from the waitset to the EntryList. This could be done more efficiently with a
1809 // single bulk transfer but in practice it's not time-critical. Beware too,
1810 // that in prepend-mode we invert the order of the waiters. Let's say that the
1811 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1812 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1813
1814 void ObjectMonitor::notifyAll(TRAPS) {
1815 JavaThread* current = THREAD;
1816 CHECK_OWNER(); // Throws IMSE if not owner.
1817 if (_WaitSet == nullptr) {
1818 return;
1819 }
1820
1821 DTRACE_MONITOR_PROBE(notifyAll, this, object(), current);
1822 int tally = 0;
1823 while (_WaitSet != nullptr) {
1824 tally++;
1825 INotify(current);
1826 }
1827
1828 OM_PERFDATA_OP(Notifications, inc(tally));
1829 }
1830
1831 // -----------------------------------------------------------------------------
1832 // Adaptive Spinning Support
1833 //
1834 // Adaptive spin-then-block - rational spinning
1835 //
1836 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1837 // algorithm. On high order SMP systems it would be better to start with
1838 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1839 // a contending thread could enqueue itself on the cxq and then spin locally
1840 // on a thread-specific variable such as its ParkEvent._Event flag.
1841 // That's left as an exercise for the reader. Note that global spinning is
1842 // not problematic on Niagara, as the L2 cache serves the interconnect and
1843 // has both low latency and massive bandwidth.
1844 //
1845 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1846 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1847 // (duration) or we can fix the count at approximately the duration of
1848 // a context switch and vary the frequency. Of course we could also
1849 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1850 // For a description of 'Adaptive spin-then-block mutual exclusion in
1851 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1852 //
1853 // This implementation varies the duration "D", where D varies with
1854 // the success rate of recent spin attempts. (D is capped at approximately
1855 // length of a round-trip context switch). The success rate for recent
1856 // spin attempts is a good predictor of the success rate of future spin
1857 // attempts. The mechanism adapts automatically to varying critical
1858 // section length (lock modality), system load and degree of parallelism.
1859 // D is maintained per-monitor in _SpinDuration and is initialized
1860 // optimistically. Spin frequency is fixed at 100%.
1861 //
1862 // Note that _SpinDuration is volatile, but we update it without locks
1863 // or atomics. The code is designed so that _SpinDuration stays within
1864 // a reasonable range even in the presence of races. The arithmetic
1865 // operations on _SpinDuration are closed over the domain of legal values,
1866 // so at worst a race will install and older but still legal value.
1867 // At the very worst this introduces some apparent non-determinism.
1868 // We might spin when we shouldn't or vice-versa, but since the spin
1869 // count are relatively short, even in the worst case, the effect is harmless.
1870 //
1871 // Care must be taken that a low "D" value does not become an
1872 // an absorbing state. Transient spinning failures -- when spinning
1873 // is overall profitable -- should not cause the system to converge
1874 // on low "D" values. We want spinning to be stable and predictable
1875 // and fairly responsive to change and at the same time we don't want
1876 // it to oscillate, become metastable, be "too" non-deterministic,
1877 // or converge on or enter undesirable stable absorbing states.
1878 //
1879 // We implement a feedback-based control system -- using past behavior
1880 // to predict future behavior. We face two issues: (a) if the
1881 // input signal is random then the spin predictor won't provide optimal
1882 // results, and (b) if the signal frequency is too high then the control
1883 // system, which has some natural response lag, will "chase" the signal.
1884 // (b) can arise from multimodal lock hold times. Transient preemption
1885 // can also result in apparent bimodal lock hold times.
1886 // Although sub-optimal, neither condition is particularly harmful, as
1887 // in the worst-case we'll spin when we shouldn't or vice-versa.
1888 // The maximum spin duration is rather short so the failure modes aren't bad.
1889 // To be conservative, I've tuned the gain in system to bias toward
1890 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1891 // "rings" or oscillates between spinning and not spinning. This happens
1892 // when spinning is just on the cusp of profitability, however, so the
1893 // situation is not dire. The state is benign -- there's no need to add
1894 // hysteresis control to damp the transition rate between spinning and
1895 // not spinning.
1896
1897 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool
1898
1899 static int Knob_Bonus = 100; // spin success bonus
1900 static int Knob_Penalty = 200; // spin failure penalty
1901 static int Knob_Poverty = 1000;
1902 static int Knob_FixedSpin = 0;
1903 static int Knob_PreSpin = 10; // 20-100 likely better, but it's not better in my testing.
1904
1905 inline static int adjust_up(int spin_duration) {
1906 int x = spin_duration;
1907 if (x < ObjectMonitor::Knob_SpinLimit) {
1908 if (x < Knob_Poverty) {
1909 x = Knob_Poverty;
1910 }
1911 return x + Knob_Bonus;
1912 } else {
1913 return spin_duration;
1914 }
1915 }
1916
1917 inline static int adjust_down(int spin_duration) {
1918 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
1919 // AIMD is globally stable.
1920 int x = spin_duration;
1921 if (x > 0) {
1922 // Consider an AIMD scheme like: x -= (x >> 3) + 100
1923 // This is globally sample and tends to damp the response.
1924 x -= Knob_Penalty;
1925 if (x < 0) { x = 0; }
1926 return x;
1927 } else {
1928 return spin_duration;
1929 }
1930 }
1931
1932 bool ObjectMonitor::short_fixed_spin(JavaThread* current, int spin_count, bool adapt) {
1933 for (int ctr = 0; ctr < spin_count; ctr++) {
1934 TryLockResult status = TryLock(current);
1935 if (status == TryLockResult::Success) {
1936 if (adapt) {
1937 _SpinDuration = adjust_up(_SpinDuration);
1938 }
1939 return true;
1940 } else if (status == TryLockResult::Interference) {
1941 break;
1942 }
1943 SpinPause();
1944 }
1945 return false;
1946 }
1947
1948 // Spinning: Fixed frequency (100%), vary duration
1949 bool ObjectMonitor::TrySpin(JavaThread* current) {
1950
1951 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1952 int knob_fixed_spin = Knob_FixedSpin; // 0 (don't spin: default), 2000 good test
1953 if (knob_fixed_spin > 0) {
1954 return short_fixed_spin(current, knob_fixed_spin, false);
1955 }
1956
1957 // Admission control - verify preconditions for spinning
1958 //
1959 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1960 // becoming an absorbing state. Put another way, we spin briefly to
1961 // sample, just in case the system load, parallelism, contention, or lock
1962 // modality changed.
1963
1964 int knob_pre_spin = Knob_PreSpin; // 10 (default), 100, 1000 or 2000
1965 if (short_fixed_spin(current, knob_pre_spin, true)) {
1966 return true;
1967 }
1968
1969 //
1970 // Consider the following alternative:
1971 // Periodically set _SpinDuration = _SpinLimit and try a long/full
1972 // spin attempt. "Periodically" might mean after a tally of
1973 // the # of failed spin attempts (or iterations) reaches some threshold.
1974 // This takes us into the realm of 1-out-of-N spinning, where we
1975 // hold the duration constant but vary the frequency.
1976
1977 int ctr = _SpinDuration;
1978 if (ctr <= 0) return false;
1979
1980 // We're good to spin ... spin ingress.
1981 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1982 // when preparing to LD...CAS _owner, etc and the CAS is likely
1983 // to succeed.
1984 if (_succ == nullptr) {
1985 _succ = current;
1986 }
1987 Thread* prv = nullptr;
1988
1989 // There are three ways to exit the following loop:
1990 // 1. A successful spin where this thread has acquired the lock.
1991 // 2. Spin failure with prejudice
1992 // 3. Spin failure without prejudice
1993
1994 while (--ctr >= 0) {
1995
1996 // Periodic polling -- Check for pending GC
1997 // Threads may spin while they're unsafe.
1998 // We don't want spinning threads to delay the JVM from reaching
1999 // a stop-the-world safepoint or to steal cycles from GC.
2000 // If we detect a pending safepoint we abort in order that
2001 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
2002 // this thread, if safe, doesn't steal cycles from GC.
2003 // This is in keeping with the "no loitering in runtime" rule.
2004 // We periodically check to see if there's a safepoint pending.
2005 if ((ctr & 0xFF) == 0) {
2006 // Can't call SafepointMechanism::should_process() since that
2007 // might update the poll values and we could be in a thread_blocked
2008 // state here which is not allowed so just check the poll.
2009 if (SafepointMechanism::local_poll_armed(current)) {
2010 break;
2011 }
2012 SpinPause();
2013 }
2014
2015 // Probe _owner with TATAS
2016 // If this thread observes the monitor transition or flicker
2017 // from locked to unlocked to locked, then the odds that this
2018 // thread will acquire the lock in this spin attempt go down
2019 // considerably. The same argument applies if the CAS fails
2020 // or if we observe _owner change from one non-null value to
2021 // another non-null value. In such cases we might abort
2022 // the spin without prejudice or apply a "penalty" to the
2023 // spin count-down variable "ctr", reducing it by 100, say.
2024
2025 JavaThread* ox = static_cast<JavaThread*>(owner_raw());
2026 if (ox == nullptr) {
2027 ox = static_cast<JavaThread*>(try_set_owner_from(nullptr, current));
2028 if (ox == nullptr) {
2029 // The CAS succeeded -- this thread acquired ownership
2030 // Take care of some bookkeeping to exit spin state.
2031 if (_succ == current) {
2032 _succ = nullptr;
2033 }
2034
2035 // Increase _SpinDuration :
2036 // The spin was successful (profitable) so we tend toward
2037 // longer spin attempts in the future.
2038 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2039 // If we acquired the lock early in the spin cycle it
2040 // makes sense to increase _SpinDuration proportionally.
2041 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2042 _SpinDuration = adjust_up(_SpinDuration);
2043 return true;
2044 }
2045
2046 // The CAS failed ... we can take any of the following actions:
2047 // * penalize: ctr -= CASPenalty
2048 // * exit spin with prejudice -- abort without adapting spinner
2049 // * exit spin without prejudice.
2050 // * Since CAS is high-latency, retry again immediately.
2051 break;
2052 }
2053
2054 // Did lock ownership change hands ?
2055 if (ox != prv && prv != nullptr) {
2056 break;
2057 }
2058 prv = ox;
2059
2060 if (_succ == nullptr) {
2061 _succ = current;
2062 }
2063 }
2064
2065 // Spin failed with prejudice -- reduce _SpinDuration.
2066 if (ctr < 0) {
2067 _SpinDuration = adjust_down(_SpinDuration);
2068 }
2069
2070 if (_succ == current) {
2071 _succ = nullptr;
2072 // Invariant: after setting succ=null a contending thread
2073 // must recheck-retry _owner before parking. This usually happens
2074 // in the normal usage of TrySpin(), but it's safest
2075 // to make TrySpin() as foolproof as possible.
2076 OrderAccess::fence();
2077 if (TryLock(current) == TryLockResult::Success) {
2078 return true;
2079 }
2080 }
2081
2082 return false;
2083 }
2084
2085
2086 // -----------------------------------------------------------------------------
2087 // WaitSet management ...
2088
2089 ObjectWaiter::ObjectWaiter(JavaThread* current) {
2090 _next = nullptr;
2091 _prev = nullptr;
2092 _notified = 0;
2093 _notifier_tid = 0;
2094 TState = TS_RUN;
2095 _thread = current;2096 _event = _thread->_ParkEvent;
2097 _active = false;
2098 assert(_event != nullptr, "invariant");
2099 }
2100
2101 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2102 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(_thread, mon);
2103 }
2104
2105 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2106 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(_thread, _active);
2107 }
2108
2109 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2110 assert(node != nullptr, "should not add null node");
2111 assert(node->_prev == nullptr, "node already in list");
2112 assert(node->_next == nullptr, "node already in list");
2113 // put node at end of queue (circular doubly linked list)
2114 if (_WaitSet == nullptr) {
2115 _WaitSet = node;
2116 node->_prev = node;
2117 node->_next = node;
2118 } else {
2119 ObjectWaiter* head = _WaitSet;
2120 ObjectWaiter* tail = head->_prev;
2121 assert(tail->_next == head, "invariant check");
2122 tail->_next = node;
2123 head->_prev = node;
2124 node->_next = head;
2125 node->_prev = tail;
2126 }
2127 }
2128
2129 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2130 // dequeue the very first waiter
2131 ObjectWaiter* waiter = _WaitSet;
2132 if (waiter) {
2133 DequeueSpecificWaiter(waiter);
2134 }
2135 return waiter;
2136 }
2137
2138 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2139 assert(node != nullptr, "should not dequeue nullptr node");
2140 assert(node->_prev != nullptr, "node already removed from list");
2141 assert(node->_next != nullptr, "node already removed from list");
2142 // when the waiter has woken up because of interrupt,
2143 // timeout or other spurious wake-up, dequeue the
2144 // waiter from waiting list
2145 ObjectWaiter* next = node->_next;
2146 if (next == node) {
2147 assert(node->_prev == node, "invariant check");
2148 _WaitSet = nullptr;
2149 } else {
2150 ObjectWaiter* prev = node->_prev;
2151 assert(prev->_next == node, "invariant check");
2152 assert(next->_prev == node, "invariant check");
2153 next->_prev = prev;
2154 prev->_next = next;
2155 if (_WaitSet == node) {
2156 _WaitSet = next;
2157 }
2158 }
2159 node->_next = nullptr;
2160 node->_prev = nullptr;
2161 }
2162
2163 // -----------------------------------------------------------------------------
2164 // PerfData support
2165 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = nullptr;
2166 PerfCounter * ObjectMonitor::_sync_FutileWakeups = nullptr;
2167 PerfCounter * ObjectMonitor::_sync_Parks = nullptr;
2168 PerfCounter * ObjectMonitor::_sync_Notifications = nullptr;
2169 PerfCounter * ObjectMonitor::_sync_Inflations = nullptr;
2170 PerfCounter * ObjectMonitor::_sync_Deflations = nullptr;
2171 PerfLongVariable * ObjectMonitor::_sync_MonExtant = nullptr;
2172
2173 // One-shot global initialization for the sync subsystem.
2174 // We could also defer initialization and initialize on-demand
2175 // the first time we call ObjectSynchronizer::inflate().
2176 // Initialization would be protected - like so many things - by
2177 // the MonitorCache_lock.
2178
2179 void ObjectMonitor::Initialize() {
2180 assert(!InitDone, "invariant");
2181
2182 if (!os::is_MP()) {
2183 Knob_SpinLimit = 0;
2184 Knob_PreSpin = 0;
2185 Knob_FixedSpin = -1;
2186 }
2187
2188 if (UsePerfData) {
2189 EXCEPTION_MARK;
2190 #define NEWPERFCOUNTER(n) \
2191 { \
2192 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \
2193 CHECK); \
2194 }
2195 #define NEWPERFVARIABLE(n) \
2196 { \
2197 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \
2198 CHECK); \
2199 }
2200 NEWPERFCOUNTER(_sync_Inflations);
2201 NEWPERFCOUNTER(_sync_Deflations);
2202 NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2203 NEWPERFCOUNTER(_sync_FutileWakeups);
2204 NEWPERFCOUNTER(_sync_Parks);
2205 NEWPERFCOUNTER(_sync_Notifications);
2206 NEWPERFVARIABLE(_sync_MonExtant);
2207 #undef NEWPERFCOUNTER
2208 #undef NEWPERFVARIABLE
2209 }
2210
2211 _oop_storage = OopStorageSet::create_weak("ObjectSynchronizer Weak", mtSynchronizer);
2212
2213 DEBUG_ONLY(InitDone = true;)
2214 }
2215
2216 void ObjectMonitor::print_on(outputStream* st) const {
2217 // The minimal things to print for markWord printing, more can be added for debugging and logging.
2218 st->print("{contentions=0x%08x,waiters=0x%08x"
2219 ",recursions=" INTX_FORMAT ",owner=" INTPTR_FORMAT "}",
2220 contentions(), waiters(), recursions(),
2221 p2i(owner()));
2222 }
2223 void ObjectMonitor::print() const { print_on(tty); }
2224
2225 #ifdef ASSERT
2226 // Print the ObjectMonitor like a debugger would:
2227 //
2228 // (ObjectMonitor) 0x00007fdfb6012e40 = {
2229 // _metadata = 0x0000000000000001
2230 // _object = 0x000000070ff45fd0
2231 // _pad_buf0 = {
2232 // [0] = '\0'
2233 // ...
2234 // [43] = '\0'
2235 // }
2236 // _owner = 0x0000000000000000
2237 // _previous_owner_tid = 0
2238 // _pad_buf1 = {
2239 // [0] = '\0'
2240 // ...
2241 // [47] = '\0'
2242 // }
2243 // _next_om = 0x0000000000000000
2244 // _recursions = 0
2245 // _EntryList = 0x0000000000000000
2246 // _cxq = 0x0000000000000000
2247 // _succ = 0x0000000000000000
2248 // _Responsible = 0x0000000000000000
2249 // _SpinDuration = 5000
2250 // _contentions = 0
2251 // _WaitSet = 0x0000700009756248
2252 // _waiters = 1
2253 // _WaitSetLock = 0
2254 // }
2255 //
2256 void ObjectMonitor::print_debug_style_on(outputStream* st) const {
2257 st->print_cr("(ObjectMonitor*) " INTPTR_FORMAT " = {", p2i(this));
2258 st->print_cr(" _metadata = " INTPTR_FORMAT, _metadata);
2259 st->print_cr(" _object = " INTPTR_FORMAT, p2i(object_peek()));
2260 st->print_cr(" _pad_buf0 = {");
2261 st->print_cr(" [0] = '\\0'");
2262 st->print_cr(" ...");
2263 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf0) - 1);
2264 st->print_cr(" }");
2265 st->print_cr(" _owner = " INTPTR_FORMAT, p2i(owner_raw()));
2266 st->print_cr(" _previous_owner_tid = " UINT64_FORMAT, _previous_owner_tid);
2267 st->print_cr(" _pad_buf1 = {");
2268 st->print_cr(" [0] = '\\0'");
2269 st->print_cr(" ...");
2270 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf1) - 1);
2271 st->print_cr(" }");
2272 st->print_cr(" _next_om = " INTPTR_FORMAT, p2i(next_om()));
2273 st->print_cr(" _recursions = " INTX_FORMAT, _recursions);
2274 st->print_cr(" _EntryList = " INTPTR_FORMAT, p2i(_EntryList));
2275 st->print_cr(" _cxq = " INTPTR_FORMAT, p2i(_cxq));
2276 st->print_cr(" _succ = " INTPTR_FORMAT, p2i(_succ));
2277 st->print_cr(" _Responsible = " INTPTR_FORMAT, p2i(_Responsible));
2278 st->print_cr(" _SpinDuration = %d", _SpinDuration);
2279 st->print_cr(" _contentions = %d", contentions());
2280 st->print_cr(" _WaitSet = " INTPTR_FORMAT, p2i(_WaitSet));
2281 st->print_cr(" _waiters = %d", _waiters);
2282 st->print_cr(" _WaitSetLock = %d", _WaitSetLock);
2283 st->print_cr("}");
2284 }
2285 #endif
--- EOF ---