1 /*
2 * Copyright (c) 1998, 2022, 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/handles.inline.hpp"
43 #include "runtime/interfaceSupport.inline.hpp"
44 #include "runtime/mutexLocker.hpp"
45 #include "runtime/objectMonitor.hpp"
46 #include "runtime/objectMonitor.inline.hpp"
47 #include "runtime/orderAccess.hpp"
48 #include "runtime/osThread.hpp"
49 #include "runtime/perfData.hpp"
50 #include "runtime/safefetch.hpp"
51 #include "runtime/safepointMechanism.inline.hpp"
52 #include "runtime/sharedRuntime.hpp"
53 #include "runtime/thread.inline.hpp"
54 #include "services/threadService.hpp"
55 #include "utilities/dtrace.hpp"
56 #include "utilities/macros.hpp"
57 #include "utilities/preserveException.hpp"
58 #if INCLUDE_JFR
59 #include "jfr/support/jfrFlush.hpp"
60 #endif
61
62 #ifdef DTRACE_ENABLED
63
64 // Only bother with this argument setup if dtrace is available
65 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
66
67
68 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
69 char* bytes = NULL; \
70 int len = 0; \
71 jlong jtid = SharedRuntime::get_java_tid(thread); \
72 Symbol* klassname = obj->klass()->name(); \
73 if (klassname != NULL) { \
74 bytes = (char*)klassname->bytes(); \
75 len = klassname->utf8_length(); \
76 }
77
78 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
79 { \
80 if (DTraceMonitorProbes) { \
81 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
82 HOTSPOT_MONITOR_WAIT(jtid, \
83 (monitor), bytes, len, (millis)); \
84 } \
85 }
86
87 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
88 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
89 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
90 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
91 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
92
93 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
94 { \
95 if (DTraceMonitorProbes) { \
96 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
97 HOTSPOT_MONITOR_##probe(jtid, \
98 (uintptr_t)(monitor), bytes, len); \
99 } \
100 }
101
102 #else // ndef DTRACE_ENABLED
103
104 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
105 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
106
107 #endif // ndef DTRACE_ENABLED
108
109 // Tunables ...
110 // The knob* variables are effectively final. Once set they should
111 // never be modified hence. Consider using __read_mostly with GCC.
112
113 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool -
114
115 static int Knob_Bonus = 100; // spin success bonus
116 static int Knob_BonusB = 100; // spin success bonus
117 static int Knob_Penalty = 200; // spin failure penalty
118 static int Knob_Poverty = 1000;
119 static int Knob_FixedSpin = 0;
120 static int Knob_PreSpin = 10; // 20-100 likely better
121
122 DEBUG_ONLY(static volatile bool InitDone = false;)
123
124 OopStorage* ObjectMonitor::_oop_storage = NULL;
125
126 // -----------------------------------------------------------------------------
127 // Theory of operations -- Monitors lists, thread residency, etc:
128 //
129 // * A thread acquires ownership of a monitor by successfully
130 // CAS()ing the _owner field from null to non-null.
131 //
132 // * Invariant: A thread appears on at most one monitor list --
133 // cxq, EntryList or WaitSet -- at any one time.
134 //
135 // * Contending threads "push" themselves onto the cxq with CAS
136 // and then spin/park.
137 //
138 // * After a contending thread eventually acquires the lock it must
139 // dequeue itself from either the EntryList or the cxq.
140 //
141 // * The exiting thread identifies and unparks an "heir presumptive"
142 // tentative successor thread on the EntryList. Critically, the
143 // exiting thread doesn't unlink the successor thread from the EntryList.
144 // After having been unparked, the wakee will recontend for ownership of
145 // the monitor. The successor (wakee) will either acquire the lock or
146 // re-park itself.
147 //
148 // Succession is provided for by a policy of competitive handoff.
149 // The exiting thread does _not_ grant or pass ownership to the
150 // successor thread. (This is also referred to as "handoff" succession").
151 // Instead the exiting thread releases ownership and possibly wakes
152 // a successor, so the successor can (re)compete for ownership of the lock.
153 // If the EntryList is empty but the cxq is populated the exiting
154 // thread will drain the cxq into the EntryList. It does so by
155 // by detaching the cxq (installing null with CAS) and folding
156 // the threads from the cxq into the EntryList. The EntryList is
157 // doubly linked, while the cxq is singly linked because of the
158 // CAS-based "push" used to enqueue recently arrived threads (RATs).
159 //
160 // * Concurrency invariants:
161 //
162 // -- only the monitor owner may access or mutate the EntryList.
163 // The mutex property of the monitor itself protects the EntryList
164 // from concurrent interference.
165 // -- Only the monitor owner may detach the cxq.
166 //
167 // * The monitor entry list operations avoid locks, but strictly speaking
168 // they're not lock-free. Enter is lock-free, exit is not.
169 // For a description of 'Methods and apparatus providing non-blocking access
170 // to a resource,' see U.S. Pat. No. 7844973.
171 //
172 // * The cxq can have multiple concurrent "pushers" but only one concurrent
173 // detaching thread. This mechanism is immune from the ABA corruption.
174 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
175 //
176 // * Taken together, the cxq and the EntryList constitute or form a
177 // single logical queue of threads stalled trying to acquire the lock.
178 // We use two distinct lists to improve the odds of a constant-time
179 // dequeue operation after acquisition (in the ::enter() epilogue) and
180 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
181 // A key desideratum is to minimize queue & monitor metadata manipulation
182 // that occurs while holding the monitor lock -- that is, we want to
183 // minimize monitor lock holds times. Note that even a small amount of
184 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
185 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
186 // locks and monitor metadata.
187 //
188 // Cxq points to the set of Recently Arrived Threads attempting entry.
189 // Because we push threads onto _cxq with CAS, the RATs must take the form of
190 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
191 // the unlocking thread notices that EntryList is null but _cxq is != null.
192 //
193 // The EntryList is ordered by the prevailing queue discipline and
194 // can be organized in any convenient fashion, such as a doubly-linked list or
195 // a circular doubly-linked list. Critically, we want insert and delete operations
196 // to operate in constant-time. If we need a priority queue then something akin
197 // to Solaris' sleepq would work nicely. Viz.,
198 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
199 // Queue discipline is enforced at ::exit() time, when the unlocking thread
200 // drains the cxq into the EntryList, and orders or reorders the threads on the
201 // EntryList accordingly.
202 //
203 // Barring "lock barging", this mechanism provides fair cyclic ordering,
204 // somewhat similar to an elevator-scan.
205 //
206 // * The monitor synchronization subsystem avoids the use of native
207 // synchronization primitives except for the narrow platform-specific
208 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
209 // the semantics of park-unpark. Put another way, this monitor implementation
210 // depends only on atomic operations and park-unpark. The monitor subsystem
211 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
212 // underlying OS manages the READY<->RUN transitions.
213 //
214 // * Waiting threads reside on the WaitSet list -- wait() puts
215 // the caller onto the WaitSet.
216 //
217 // * notify() or notifyAll() simply transfers threads from the WaitSet to
218 // either the EntryList or cxq. Subsequent exit() operations will
219 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
220 // it's likely the notifyee would simply impale itself on the lock held
221 // by the notifier.
222 //
223 // * An interesting alternative is to encode cxq as (List,LockByte) where
224 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
225 // variable, like _recursions, in the scheme. The threads or Events that form
226 // the list would have to be aligned in 256-byte addresses. A thread would
227 // try to acquire the lock or enqueue itself with CAS, but exiting threads
228 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
229 // Note that is is *not* word-tearing, but it does presume that full-word
230 // CAS operations are coherent with intermix with STB operations. That's true
231 // on most common processors.
232 //
233 // * See also http://blogs.sun.com/dave
234
235
236 // Check that object() and set_object() are called from the right context:
237 static void check_object_context() {
238 #ifdef ASSERT
239 Thread* self = Thread::current();
240 if (self->is_Java_thread()) {
241 // Mostly called from JavaThreads so sanity check the thread state.
242 JavaThread* jt = self->as_Java_thread();
243 switch (jt->thread_state()) {
244 case _thread_in_vm: // the usual case
245 case _thread_in_Java: // during deopt
246 break;
247 default:
248 fatal("called from an unsafe thread state");
249 }
250 assert(jt->is_active_Java_thread(), "must be active JavaThread");
251 } else {
252 // However, ThreadService::get_current_contended_monitor()
253 // can call here via the VMThread so sanity check it.
254 assert(self->is_VM_thread(), "must be");
255 }
256 #endif // ASSERT
257 }
258
259 ObjectMonitor::ObjectMonitor(oop object) :
260 _header(markWord::zero()),
261 _object(_oop_storage, object),
262 _owner(NULL),
263 _previous_owner_tid(0),
264 _next_om(NULL),
265 _recursions(0),
266 _EntryList(NULL),
267 _cxq(NULL),
268 _succ(NULL),
269 _Responsible(NULL),
270 _Spinner(0),
271 _SpinDuration(ObjectMonitor::Knob_SpinLimit),
272 _contentions(0),
273 _WaitSet(NULL),
274 _waiters(0),
275 _WaitSetLock(0)
276 { }
277
278 ObjectMonitor::~ObjectMonitor() {
279 _object.release(_oop_storage);
280 }
281
282 oop ObjectMonitor::object() const {
283 check_object_context();
284 if (_object.is_null()) {
285 return NULL;
286 }
287 return _object.resolve();
288 }
289
290 oop ObjectMonitor::object_peek() const {
291 if (_object.is_null()) {
292 return NULL;
293 }
294 return _object.peek();
295 }
296
297 void ObjectMonitor::ExitOnSuspend::operator()(JavaThread* current) {
298 if (current->is_suspended()) {
299 _om->_recursions = 0;
300 _om->_succ = NULL;
301 // Don't need a full fence after clearing successor here because of the call to exit().
302 _om->exit(current, false /* not_suspended */);
303 _om_exited = true;
304
305 current->set_current_pending_monitor(_om);
306 }
307 }
308
309 void ObjectMonitor::ClearSuccOnSuspend::operator()(JavaThread* current) {
310 if (current->is_suspended()) {
311 if (_om->_succ == current) {
312 _om->_succ = NULL;
313 OrderAccess::fence(); // always do a full fence when successor is cleared
314 }
315 }
316 }
317
318 // -----------------------------------------------------------------------------
319 // Enter support
320
321 bool ObjectMonitor::enter(JavaThread* current) {
322 // The following code is ordered to check the most common cases first
323 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
324
325 void* cur = try_set_owner_from(NULL, current);
326 if (cur == NULL) {
327 assert(_recursions == 0, "invariant");
328 return true;
329 }
330
331 if (cur == current) {
332 // TODO-FIXME: check for integer overflow! BUGID 6557169.
333 _recursions++;
334 return true;
335 }
336
337 if (current->is_lock_owned((address)cur)) {
338 assert(_recursions == 0, "internal state error");
339 _recursions = 1;
340 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*.
341 return true;
342 }
343
344 // We've encountered genuine contention.
345 assert(current->_Stalled == 0, "invariant");
346 current->_Stalled = intptr_t(this);
347
348 // Try one round of spinning *before* enqueueing current
349 // and before going through the awkward and expensive state
350 // transitions. The following spin is strictly optional ...
351 // Note that if we acquire the monitor from an initial spin
352 // we forgo posting JVMTI events and firing DTRACE probes.
353 if (TrySpin(current) > 0) {
354 assert(owner_raw() == current, "must be current: owner=" INTPTR_FORMAT, p2i(owner_raw()));
355 assert(_recursions == 0, "must be 0: recursions=" INTX_FORMAT, _recursions);
356 assert(object()->mark() == markWord::encode(this),
357 "object mark must match encoded this: mark=" INTPTR_FORMAT
358 ", encoded this=" INTPTR_FORMAT, object()->mark().value(),
359 markWord::encode(this).value());
360 current->_Stalled = 0;
361 return true;
362 }
363
364 assert(owner_raw() != current, "invariant");
365 assert(_succ != current, "invariant");
366 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
367 assert(current->thread_state() != _thread_blocked, "invariant");
368
369 // Keep track of contention for JVM/TI and M&M queries.
370 add_to_contentions(1);
371 if (is_being_async_deflated()) {
372 // Async deflation is in progress and our contentions increment
373 // above lost the race to async deflation. Undo the work and
374 // force the caller to retry.
375 const oop l_object = object();
376 if (l_object != NULL) {
377 // Attempt to restore the header/dmw to the object's header so that
378 // we only retry once if the deflater thread happens to be slow.
379 install_displaced_markword_in_object(l_object);
380 }
381 current->_Stalled = 0;
382 add_to_contentions(-1);
383 return false;
384 }
385
386 JFR_ONLY(JfrConditionalFlushWithStacktrace<EventJavaMonitorEnter> flush(current);)
387 EventJavaMonitorEnter event;
388 if (event.is_started()) {
389 event.set_monitorClass(object()->klass());
390 // Set an address that is 'unique enough', such that events close in
391 // time and with the same address are likely (but not guaranteed) to
392 // belong to the same object.
393 event.set_address((uintptr_t)this);
394 }
395
396 { // Change java thread status to indicate blocked on monitor enter.
397 JavaThreadBlockedOnMonitorEnterState jtbmes(current, this);
398
399 assert(current->current_pending_monitor() == NULL, "invariant");
400 current->set_current_pending_monitor(this);
401
402 DTRACE_MONITOR_PROBE(contended__enter, this, object(), current);
403 if (JvmtiExport::should_post_monitor_contended_enter()) {
404 JvmtiExport::post_monitor_contended_enter(current, this);
405
406 // The current thread does not yet own the monitor and does not
407 // yet appear on any queues that would get it made the successor.
408 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
409 // handler cannot accidentally consume an unpark() meant for the
410 // ParkEvent associated with this ObjectMonitor.
411 }
412
413 OSThreadContendState osts(current->osthread());
414
415 assert(current->thread_state() == _thread_in_vm, "invariant");
416
417 for (;;) {
418 ExitOnSuspend eos(this);
419 {
420 ThreadBlockInVMPreprocess<ExitOnSuspend> tbivs(current, eos);
421 EnterI(current);
422 current->set_current_pending_monitor(NULL);
423 // We can go to a safepoint at the end of this block. If we
424 // do a thread dump during that safepoint, then this thread will show
425 // as having "-locked" the monitor, but the OS and java.lang.Thread
426 // states will still report that the thread is blocked trying to
427 // acquire it.
428 // If there is a suspend request, ExitOnSuspend will exit the OM
429 // and set the OM as pending.
430 }
431 if (!eos.exited()) {
432 // ExitOnSuspend did not exit the OM
433 assert(owner_raw() == current, "invariant");
434 break;
435 }
436 }
437
438 // We've just gotten past the enter-check-for-suspend dance and we now own
439 // the monitor free and clear.
440 }
441
442 add_to_contentions(-1);
443 assert(contentions() >= 0, "must not be negative: contentions=%d", contentions());
444 current->_Stalled = 0;
445
446 // Must either set _recursions = 0 or ASSERT _recursions == 0.
447 assert(_recursions == 0, "invariant");
448 assert(owner_raw() == current, "invariant");
449 assert(_succ != current, "invariant");
450 assert(object()->mark() == markWord::encode(this), "invariant");
451
452 // The thread -- now the owner -- is back in vm mode.
453 // Report the glorious news via TI,DTrace and jvmstat.
454 // The probe effect is non-trivial. All the reportage occurs
455 // while we hold the monitor, increasing the length of the critical
456 // section. Amdahl's parallel speedup law comes vividly into play.
457 //
458 // Another option might be to aggregate the events (thread local or
459 // per-monitor aggregation) and defer reporting until a more opportune
460 // time -- such as next time some thread encounters contention but has
461 // yet to acquire the lock. While spinning that thread could
462 // spinning we could increment JVMStat counters, etc.
463
464 DTRACE_MONITOR_PROBE(contended__entered, this, object(), current);
465 if (JvmtiExport::should_post_monitor_contended_entered()) {
466 JvmtiExport::post_monitor_contended_entered(current, this);
467
468 // The current thread already owns the monitor and is not going to
469 // call park() for the remainder of the monitor enter protocol. So
470 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
471 // event handler consumed an unpark() issued by the thread that
472 // just exited the monitor.
473 }
474 if (event.should_commit()) {
475 event.set_previousOwner((uintptr_t)_previous_owner_tid);
476 event.commit();
477 }
478 OM_PERFDATA_OP(ContendedLockAttempts, inc());
479 return true;
480 }
481
482 // Caveat: TryLock() is not necessarily serializing if it returns failure.
483 // Callers must compensate as needed.
484
485 int ObjectMonitor::TryLock(JavaThread* current) {
486 void* own = owner_raw();
487 if (own != NULL) return 0;
488 if (try_set_owner_from(NULL, current) == NULL) {
489 assert(_recursions == 0, "invariant");
490 return 1;
491 }
492 // The lock had been free momentarily, but we lost the race to the lock.
493 // Interference -- the CAS failed.
494 // We can either return -1 or retry.
495 // Retry doesn't make as much sense because the lock was just acquired.
496 return -1;
497 }
498
499 // Deflate the specified ObjectMonitor if not in-use. Returns true if it
500 // was deflated and false otherwise.
501 //
502 // The async deflation protocol sets owner to DEFLATER_MARKER and
503 // makes contentions negative as signals to contending threads that
504 // an async deflation is in progress. There are a number of checks
505 // as part of the protocol to make sure that the calling thread has
506 // not lost the race to a contending thread.
507 //
508 // The ObjectMonitor has been successfully async deflated when:
509 // (contentions < 0)
510 // Contending threads that see that condition know to retry their operation.
511 //
512 bool ObjectMonitor::deflate_monitor() {
513 if (is_busy()) {
514 // Easy checks are first - the ObjectMonitor is busy so no deflation.
515 return false;
516 }
517
518 if (ObjectSynchronizer::is_final_audit() && owner_is_DEFLATER_MARKER()) {
519 // The final audit can see an already deflated ObjectMonitor on the
520 // in-use list because MonitorList::unlink_deflated() might have
521 // blocked for the final safepoint before unlinking all the deflated
522 // monitors.
523 assert(contentions() < 0, "must be negative: contentions=%d", contentions());
524 // Already returned 'true' when it was originally deflated.
525 return false;
526 }
527
528 const oop obj = object_peek();
529
530 if (obj == NULL) {
531 // If the object died, we can recycle the monitor without racing with
532 // Java threads. The GC already broke the association with the object.
533 set_owner_from(NULL, DEFLATER_MARKER);
534 assert(contentions() >= 0, "must be non-negative: contentions=%d", contentions());
535 _contentions = -max_jint;
536 } else {
537 // Attempt async deflation protocol.
538
539 // Set a NULL owner to DEFLATER_MARKER to force any contending thread
540 // through the slow path. This is just the first part of the async
541 // deflation dance.
542 if (try_set_owner_from(NULL, DEFLATER_MARKER) != NULL) {
543 // The owner field is no longer NULL so we lost the race since the
544 // ObjectMonitor is now busy.
545 return false;
546 }
547
548 if (contentions() > 0 || _waiters != 0) {
549 // Another thread has raced to enter the ObjectMonitor after
550 // is_busy() above or has already entered and waited on
551 // it which makes it busy so no deflation. Restore owner to
552 // NULL if it is still DEFLATER_MARKER.
553 if (try_set_owner_from(DEFLATER_MARKER, NULL) != DEFLATER_MARKER) {
554 // Deferred decrement for the JT EnterI() that cancelled the async deflation.
555 add_to_contentions(-1);
556 }
557 return false;
558 }
559
560 // Make a zero contentions field negative to force any contending threads
561 // to retry. This is the second part of the async deflation dance.
562 if (Atomic::cmpxchg(&_contentions, (jint)0, -max_jint) != 0) {
563 // Contentions was no longer 0 so we lost the race since the
564 // ObjectMonitor is now busy. Restore owner to NULL if it is
565 // still DEFLATER_MARKER:
566 if (try_set_owner_from(DEFLATER_MARKER, NULL) != DEFLATER_MARKER) {
567 // Deferred decrement for the JT EnterI() that cancelled the async deflation.
568 add_to_contentions(-1);
569 }
570 return false;
571 }
572 }
573
574 // Sanity checks for the races:
575 guarantee(owner_is_DEFLATER_MARKER(), "must be deflater marker");
576 guarantee(contentions() < 0, "must be negative: contentions=%d",
577 contentions());
578 guarantee(_waiters == 0, "must be 0: waiters=%d", _waiters);
579 guarantee(_cxq == NULL, "must be no contending threads: cxq="
580 INTPTR_FORMAT, p2i(_cxq));
581 guarantee(_EntryList == NULL,
582 "must be no entering threads: EntryList=" INTPTR_FORMAT,
583 p2i(_EntryList));
584
585 if (obj != NULL) {
586 if (log_is_enabled(Trace, monitorinflation)) {
587 ResourceMark rm;
588 log_trace(monitorinflation)("deflate_monitor: object=" INTPTR_FORMAT
589 ", mark=" INTPTR_FORMAT ", type='%s'",
590 p2i(obj), obj->mark().value(),
591 obj->klass()->external_name());
592 }
593
594 // Install the old mark word if nobody else has already done it.
595 install_displaced_markword_in_object(obj);
596 }
597
598 // We leave owner == DEFLATER_MARKER and contentions < 0
599 // to force any racing threads to retry.
600 return true; // Success, ObjectMonitor has been deflated.
601 }
602
603 // Install the displaced mark word (dmw) of a deflating ObjectMonitor
604 // into the header of the object associated with the monitor. This
605 // idempotent method is called by a thread that is deflating a
606 // monitor and by other threads that have detected a race with the
607 // deflation process.
608 void ObjectMonitor::install_displaced_markword_in_object(const oop obj) {
609 // This function must only be called when (owner == DEFLATER_MARKER
610 // && contentions <= 0), but we can't guarantee that here because
611 // those values could change when the ObjectMonitor gets moved from
612 // the global free list to a per-thread free list.
613
614 guarantee(obj != NULL, "must be non-NULL");
615
616 // Separate loads in is_being_async_deflated(), which is almost always
617 // called before this function, from the load of dmw/header below.
618
619 // _contentions and dmw/header may get written by different threads.
620 // Make sure to observe them in the same order when having several observers.
621 OrderAccess::loadload_for_IRIW();
622
623 const oop l_object = object_peek();
624 if (l_object == NULL) {
625 // ObjectMonitor's object ref has already been cleared by async
626 // deflation or GC so we're done here.
627 return;
628 }
629 assert(l_object == obj, "object=" INTPTR_FORMAT " must equal obj="
630 INTPTR_FORMAT, p2i(l_object), p2i(obj));
631
632 markWord dmw = header();
633 // The dmw has to be neutral (not NULL, not locked and not marked).
634 assert(dmw.is_neutral(), "must be neutral: dmw=" INTPTR_FORMAT, dmw.value());
635
636 // Install displaced mark word if the object's header still points
637 // to this ObjectMonitor. More than one racing caller to this function
638 // can rarely reach this point, but only one can win.
639 markWord res = obj->cas_set_mark(dmw, markWord::encode(this));
640 if (res != markWord::encode(this)) {
641 // This should be rare so log at the Info level when it happens.
642 log_info(monitorinflation)("install_displaced_markword_in_object: "
643 "failed cas_set_mark: new_mark=" INTPTR_FORMAT
644 ", old_mark=" INTPTR_FORMAT ", res=" INTPTR_FORMAT,
645 dmw.value(), markWord::encode(this).value(),
646 res.value());
647 }
648
649 // Note: It does not matter which thread restored the header/dmw
650 // into the object's header. The thread deflating the monitor just
651 // wanted the object's header restored and it is. The threads that
652 // detected a race with the deflation process also wanted the
653 // object's header restored before they retry their operation and
654 // because it is restored they will only retry once.
655 }
656
657 // Convert the fields used by is_busy() to a string that can be
658 // used for diagnostic output.
659 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) {
660 ss->print("is_busy: waiters=%d, ", _waiters);
661 if (contentions() > 0) {
662 ss->print("contentions=%d, ", contentions());
663 } else {
664 ss->print("contentions=0");
665 }
666 if (!owner_is_DEFLATER_MARKER()) {
667 ss->print("owner=" INTPTR_FORMAT, p2i(owner_raw()));
668 } else {
669 // We report NULL instead of DEFLATER_MARKER here because is_busy()
670 // ignores DEFLATER_MARKER values.
671 ss->print("owner=" INTPTR_FORMAT, NULL_WORD);
672 }
673 ss->print(", cxq=" INTPTR_FORMAT ", EntryList=" INTPTR_FORMAT, p2i(_cxq),
674 p2i(_EntryList));
675 return ss->base();
676 }
677
678 #define MAX_RECHECK_INTERVAL 1000
679
680 void ObjectMonitor::EnterI(JavaThread* current) {
681 assert(current->thread_state() == _thread_blocked, "invariant");
682
683 // Try the lock - TATAS
684 if (TryLock (current) > 0) {
685 assert(_succ != current, "invariant");
686 assert(owner_raw() == current, "invariant");
687 assert(_Responsible != current, "invariant");
688 return;
689 }
690
691 if (try_set_owner_from(DEFLATER_MARKER, current) == DEFLATER_MARKER) {
692 // Cancelled the in-progress async deflation by changing owner from
693 // DEFLATER_MARKER to current. As part of the contended enter protocol,
694 // contentions was incremented to a positive value before EnterI()
695 // was called and that prevents the deflater thread from winning the
696 // last part of the 2-part async deflation protocol. After EnterI()
697 // returns to enter(), contentions is decremented because the caller
698 // now owns the monitor. We bump contentions an extra time here to
699 // prevent the deflater thread from winning the last part of the
700 // 2-part async deflation protocol after the regular decrement
701 // occurs in enter(). The deflater thread will decrement contentions
702 // after it recognizes that the async deflation was cancelled.
703 add_to_contentions(1);
704 assert(_succ != current, "invariant");
705 assert(_Responsible != current, "invariant");
706 return;
707 }
708
709 assert(InitDone, "Unexpectedly not initialized");
710
711 // We try one round of spinning *before* enqueueing current.
712 //
713 // If the _owner is ready but OFFPROC we could use a YieldTo()
714 // operation to donate the remainder of this thread's quantum
715 // to the owner. This has subtle but beneficial affinity
716 // effects.
717
718 if (TrySpin(current) > 0) {
719 assert(owner_raw() == current, "invariant");
720 assert(_succ != current, "invariant");
721 assert(_Responsible != current, "invariant");
722 return;
723 }
724
725 // The Spin failed -- Enqueue and park the thread ...
726 assert(_succ != current, "invariant");
727 assert(owner_raw() != current, "invariant");
728 assert(_Responsible != current, "invariant");
729
730 // Enqueue "current" on ObjectMonitor's _cxq.
731 //
732 // Node acts as a proxy for current.
733 // As an aside, if were to ever rewrite the synchronization code mostly
734 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
735 // Java objects. This would avoid awkward lifecycle and liveness issues,
736 // as well as eliminate a subset of ABA issues.
737 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
738
739 ObjectWaiter node(current);
740 current->_ParkEvent->reset();
741 node._prev = (ObjectWaiter*) 0xBAD;
742 node.TState = ObjectWaiter::TS_CXQ;
743
744 // Push "current" onto the front of the _cxq.
745 // Once on cxq/EntryList, current stays on-queue until it acquires the lock.
746 // Note that spinning tends to reduce the rate at which threads
747 // enqueue and dequeue on EntryList|cxq.
748 ObjectWaiter* nxt;
749 for (;;) {
750 node._next = nxt = _cxq;
751 if (Atomic::cmpxchg(&_cxq, nxt, &node) == nxt) break;
752
753 // Interference - the CAS failed because _cxq changed. Just retry.
754 // As an optional optimization we retry the lock.
755 if (TryLock (current) > 0) {
756 assert(_succ != current, "invariant");
757 assert(owner_raw() == current, "invariant");
758 assert(_Responsible != current, "invariant");
759 return;
760 }
761 }
762
763 // Check for cxq|EntryList edge transition to non-null. This indicates
764 // the onset of contention. While contention persists exiting threads
765 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
766 // operations revert to the faster 1-0 mode. This enter operation may interleave
767 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
768 // arrange for one of the contending thread to use a timed park() operations
769 // to detect and recover from the race. (Stranding is form of progress failure
770 // where the monitor is unlocked but all the contending threads remain parked).
771 // That is, at least one of the contended threads will periodically poll _owner.
772 // One of the contending threads will become the designated "Responsible" thread.
773 // The Responsible thread uses a timed park instead of a normal indefinite park
774 // operation -- it periodically wakes and checks for and recovers from potential
775 // strandings admitted by 1-0 exit operations. We need at most one Responsible
776 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
777 // be responsible for a monitor.
778 //
779 // Currently, one of the contended threads takes on the added role of "Responsible".
780 // A viable alternative would be to use a dedicated "stranding checker" thread
781 // that periodically iterated over all the threads (or active monitors) and unparked
782 // successors where there was risk of stranding. This would help eliminate the
783 // timer scalability issues we see on some platforms as we'd only have one thread
784 // -- the checker -- parked on a timer.
785
786 if (nxt == NULL && _EntryList == NULL) {
787 // Try to assume the role of responsible thread for the monitor.
788 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=current }
789 Atomic::replace_if_null(&_Responsible, current);
790 }
791
792 // The lock might have been released while this thread was occupied queueing
793 // itself onto _cxq. To close the race and avoid "stranding" and
794 // progress-liveness failure we must resample-retry _owner before parking.
795 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
796 // In this case the ST-MEMBAR is accomplished with CAS().
797 //
798 // TODO: Defer all thread state transitions until park-time.
799 // Since state transitions are heavy and inefficient we'd like
800 // to defer the state transitions until absolutely necessary,
801 // and in doing so avoid some transitions ...
802
803 int nWakeups = 0;
804 int recheckInterval = 1;
805
806 for (;;) {
807
808 if (TryLock(current) > 0) break;
809 assert(owner_raw() != current, "invariant");
810
811 // park self
812 if (_Responsible == current) {
813 current->_ParkEvent->park((jlong) recheckInterval);
814 // Increase the recheckInterval, but clamp the value.
815 recheckInterval *= 8;
816 if (recheckInterval > MAX_RECHECK_INTERVAL) {
817 recheckInterval = MAX_RECHECK_INTERVAL;
818 }
819 } else {
820 current->_ParkEvent->park();
821 }
822
823 if (TryLock(current) > 0) break;
824
825 if (try_set_owner_from(DEFLATER_MARKER, current) == DEFLATER_MARKER) {
826 // Cancelled the in-progress async deflation by changing owner from
827 // DEFLATER_MARKER to current. As part of the contended enter protocol,
828 // contentions was incremented to a positive value before EnterI()
829 // was called and that prevents the deflater thread from winning the
830 // last part of the 2-part async deflation protocol. After EnterI()
831 // returns to enter(), contentions is decremented because the caller
832 // now owns the monitor. We bump contentions an extra time here to
833 // prevent the deflater thread from winning the last part of the
834 // 2-part async deflation protocol after the regular decrement
835 // occurs in enter(). The deflater thread will decrement contentions
836 // after it recognizes that the async deflation was cancelled.
837 add_to_contentions(1);
838 break;
839 }
840
841 // The lock is still contested.
842 // Keep a tally of the # of futile wakeups.
843 // Note that the counter is not protected by a lock or updated by atomics.
844 // That is by design - we trade "lossy" counters which are exposed to
845 // races during updates for a lower probe effect.
846
847 // This PerfData object can be used in parallel with a safepoint.
848 // See the work around in PerfDataManager::destroy().
849 OM_PERFDATA_OP(FutileWakeups, inc());
850 ++nWakeups;
851
852 // Assuming this is not a spurious wakeup we'll normally find _succ == current.
853 // We can defer clearing _succ until after the spin completes
854 // TrySpin() must tolerate being called with _succ == current.
855 // Try yet another round of adaptive spinning.
856 if (TrySpin(current) > 0) break;
857
858 // We can find that we were unpark()ed and redesignated _succ while
859 // we were spinning. That's harmless. If we iterate and call park(),
860 // park() will consume the event and return immediately and we'll
861 // just spin again. This pattern can repeat, leaving _succ to simply
862 // spin on a CPU.
863
864 if (_succ == current) _succ = NULL;
865
866 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
867 OrderAccess::fence();
868 }
869
870 // Egress :
871 // current has acquired the lock -- Unlink current from the cxq or EntryList.
872 // Normally we'll find current on the EntryList .
873 // From the perspective of the lock owner (this thread), the
874 // EntryList is stable and cxq is prepend-only.
875 // The head of cxq is volatile but the interior is stable.
876 // In addition, current.TState is stable.
877
878 assert(owner_raw() == current, "invariant");
879
880 UnlinkAfterAcquire(current, &node);
881 if (_succ == current) _succ = NULL;
882
883 assert(_succ != current, "invariant");
884 if (_Responsible == current) {
885 _Responsible = NULL;
886 OrderAccess::fence(); // Dekker pivot-point
887
888 // We may leave threads on cxq|EntryList without a designated
889 // "Responsible" thread. This is benign. When this thread subsequently
890 // exits the monitor it can "see" such preexisting "old" threads --
891 // threads that arrived on the cxq|EntryList before the fence, above --
892 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
893 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
894 // non-null and elect a new "Responsible" timer thread.
895 //
896 // This thread executes:
897 // ST Responsible=null; MEMBAR (in enter epilogue - here)
898 // LD cxq|EntryList (in subsequent exit)
899 //
900 // Entering threads in the slow/contended path execute:
901 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
902 // The (ST cxq; MEMBAR) is accomplished with CAS().
903 //
904 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
905 // exit operation from floating above the ST Responsible=null.
906 }
907
908 // We've acquired ownership with CAS().
909 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
910 // But since the CAS() this thread may have also stored into _succ,
911 // EntryList, cxq or Responsible. These meta-data updates must be
912 // visible __before this thread subsequently drops the lock.
913 // Consider what could occur if we didn't enforce this constraint --
914 // STs to monitor meta-data and user-data could reorder with (become
915 // visible after) the ST in exit that drops ownership of the lock.
916 // Some other thread could then acquire the lock, but observe inconsistent
917 // or old monitor meta-data and heap data. That violates the JMM.
918 // To that end, the 1-0 exit() operation must have at least STST|LDST
919 // "release" barrier semantics. Specifically, there must be at least a
920 // STST|LDST barrier in exit() before the ST of null into _owner that drops
921 // the lock. The barrier ensures that changes to monitor meta-data and data
922 // protected by the lock will be visible before we release the lock, and
923 // therefore before some other thread (CPU) has a chance to acquire the lock.
924 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
925 //
926 // Critically, any prior STs to _succ or EntryList must be visible before
927 // the ST of null into _owner in the *subsequent* (following) corresponding
928 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
929 // execute a serializing instruction.
930
931 return;
932 }
933
934 // ReenterI() is a specialized inline form of the latter half of the
935 // contended slow-path from EnterI(). We use ReenterI() only for
936 // monitor reentry in wait().
937 //
938 // In the future we should reconcile EnterI() and ReenterI().
939
940 void ObjectMonitor::ReenterI(JavaThread* current, ObjectWaiter* currentNode) {
941 assert(current != NULL, "invariant");
942 assert(currentNode != NULL, "invariant");
943 assert(currentNode->_thread == current, "invariant");
944 assert(_waiters > 0, "invariant");
945 assert(object()->mark() == markWord::encode(this), "invariant");
946
947 assert(current->thread_state() != _thread_blocked, "invariant");
948
949 int nWakeups = 0;
950 for (;;) {
951 ObjectWaiter::TStates v = currentNode->TState;
952 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
953 assert(owner_raw() != current, "invariant");
954
955 if (TryLock(current) > 0) break;
956 if (TrySpin(current) > 0) break;
957
958 {
959 OSThreadContendState osts(current->osthread());
960
961 assert(current->thread_state() == _thread_in_vm, "invariant");
962
963 {
964 ClearSuccOnSuspend csos(this);
965 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos);
966 current->_ParkEvent->park();
967 }
968 }
969
970 // Try again, but just so we distinguish between futile wakeups and
971 // successful wakeups. The following test isn't algorithmically
972 // necessary, but it helps us maintain sensible statistics.
973 if (TryLock(current) > 0) break;
974
975 // The lock is still contested.
976 // Keep a tally of the # of futile wakeups.
977 // Note that the counter is not protected by a lock or updated by atomics.
978 // That is by design - we trade "lossy" counters which are exposed to
979 // races during updates for a lower probe effect.
980 ++nWakeups;
981
982 // Assuming this is not a spurious wakeup we'll normally
983 // find that _succ == current.
984 if (_succ == current) _succ = NULL;
985
986 // Invariant: after clearing _succ a contending thread
987 // *must* retry _owner before parking.
988 OrderAccess::fence();
989
990 // This PerfData object can be used in parallel with a safepoint.
991 // See the work around in PerfDataManager::destroy().
992 OM_PERFDATA_OP(FutileWakeups, inc());
993 }
994
995 // current has acquired the lock -- Unlink current from the cxq or EntryList .
996 // Normally we'll find current on the EntryList.
997 // Unlinking from the EntryList is constant-time and atomic-free.
998 // From the perspective of the lock owner (this thread), the
999 // EntryList is stable and cxq is prepend-only.
1000 // The head of cxq is volatile but the interior is stable.
1001 // In addition, current.TState is stable.
1002
1003 assert(owner_raw() == current, "invariant");
1004 assert(object()->mark() == markWord::encode(this), "invariant");
1005 UnlinkAfterAcquire(current, currentNode);
1006 if (_succ == current) _succ = NULL;
1007 assert(_succ != current, "invariant");
1008 currentNode->TState = ObjectWaiter::TS_RUN;
1009 OrderAccess::fence(); // see comments at the end of EnterI()
1010 }
1011
1012 // By convention we unlink a contending thread from EntryList|cxq immediately
1013 // after the thread acquires the lock in ::enter(). Equally, we could defer
1014 // unlinking the thread until ::exit()-time.
1015
1016 void ObjectMonitor::UnlinkAfterAcquire(JavaThread* current, ObjectWaiter* currentNode) {
1017 assert(owner_raw() == current, "invariant");
1018 assert(currentNode->_thread == current, "invariant");
1019
1020 if (currentNode->TState == ObjectWaiter::TS_ENTER) {
1021 // Normal case: remove current from the DLL EntryList .
1022 // This is a constant-time operation.
1023 ObjectWaiter* nxt = currentNode->_next;
1024 ObjectWaiter* prv = currentNode->_prev;
1025 if (nxt != NULL) nxt->_prev = prv;
1026 if (prv != NULL) prv->_next = nxt;
1027 if (currentNode == _EntryList) _EntryList = nxt;
1028 assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
1029 assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
1030 } else {
1031 assert(currentNode->TState == ObjectWaiter::TS_CXQ, "invariant");
1032 // Inopportune interleaving -- current is still on the cxq.
1033 // This usually means the enqueue of self raced an exiting thread.
1034 // Normally we'll find current near the front of the cxq, so
1035 // dequeueing is typically fast. If needbe we can accelerate
1036 // this with some MCS/CHL-like bidirectional list hints and advisory
1037 // back-links so dequeueing from the interior will normally operate
1038 // in constant-time.
1039 // Dequeue current from either the head (with CAS) or from the interior
1040 // with a linear-time scan and normal non-atomic memory operations.
1041 // CONSIDER: if current is on the cxq then simply drain cxq into EntryList
1042 // and then unlink current from EntryList. We have to drain eventually,
1043 // so it might as well be now.
1044
1045 ObjectWaiter* v = _cxq;
1046 assert(v != NULL, "invariant");
1047 if (v != currentNode || Atomic::cmpxchg(&_cxq, v, currentNode->_next) != v) {
1048 // The CAS above can fail from interference IFF a "RAT" arrived.
1049 // In that case current must be in the interior and can no longer be
1050 // at the head of cxq.
1051 if (v == currentNode) {
1052 assert(_cxq != v, "invariant");
1053 v = _cxq; // CAS above failed - start scan at head of list
1054 }
1055 ObjectWaiter* p;
1056 ObjectWaiter* q = NULL;
1057 for (p = v; p != NULL && p != currentNode; p = p->_next) {
1058 q = p;
1059 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
1060 }
1061 assert(v != currentNode, "invariant");
1062 assert(p == currentNode, "Node not found on cxq");
1063 assert(p != _cxq, "invariant");
1064 assert(q != NULL, "invariant");
1065 assert(q->_next == p, "invariant");
1066 q->_next = p->_next;
1067 }
1068 }
1069
1070 #ifdef ASSERT
1071 // Diagnostic hygiene ...
1072 currentNode->_prev = (ObjectWaiter*) 0xBAD;
1073 currentNode->_next = (ObjectWaiter*) 0xBAD;
1074 currentNode->TState = ObjectWaiter::TS_RUN;
1075 #endif
1076 }
1077
1078 // -----------------------------------------------------------------------------
1079 // Exit support
1080 //
1081 // exit()
1082 // ~~~~~~
1083 // Note that the collector can't reclaim the objectMonitor or deflate
1084 // the object out from underneath the thread calling ::exit() as the
1085 // thread calling ::exit() never transitions to a stable state.
1086 // This inhibits GC, which in turn inhibits asynchronous (and
1087 // inopportune) reclamation of "this".
1088 //
1089 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
1090 // There's one exception to the claim above, however. EnterI() can call
1091 // exit() to drop a lock if the acquirer has been externally suspended.
1092 // In that case exit() is called with _thread_state == _thread_blocked,
1093 // but the monitor's _contentions field is > 0, which inhibits reclamation.
1094 //
1095 // 1-0 exit
1096 // ~~~~~~~~
1097 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
1098 // the fast-path operators have been optimized so the common ::exit()
1099 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
1100 // The code emitted by fast_unlock() elides the usual MEMBAR. This
1101 // greatly improves latency -- MEMBAR and CAS having considerable local
1102 // latency on modern processors -- but at the cost of "stranding". Absent the
1103 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
1104 // ::enter() path, resulting in the entering thread being stranding
1105 // and a progress-liveness failure. Stranding is extremely rare.
1106 // We use timers (timed park operations) & periodic polling to detect
1107 // and recover from stranding. Potentially stranded threads periodically
1108 // wake up and poll the lock. See the usage of the _Responsible variable.
1109 //
1110 // The CAS() in enter provides for safety and exclusion, while the CAS or
1111 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
1112 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
1113 // We detect and recover from stranding with timers.
1114 //
1115 // If a thread transiently strands it'll park until (a) another
1116 // thread acquires the lock and then drops the lock, at which time the
1117 // exiting thread will notice and unpark the stranded thread, or, (b)
1118 // the timer expires. If the lock is high traffic then the stranding latency
1119 // will be low due to (a). If the lock is low traffic then the odds of
1120 // stranding are lower, although the worst-case stranding latency
1121 // is longer. Critically, we don't want to put excessive load in the
1122 // platform's timer subsystem. We want to minimize both the timer injection
1123 // rate (timers created/sec) as well as the number of timers active at
1124 // any one time. (more precisely, we want to minimize timer-seconds, which is
1125 // the integral of the # of active timers at any instant over time).
1126 // Both impinge on OS scalability. Given that, at most one thread parked on
1127 // a monitor will use a timer.
1128 //
1129 // There is also the risk of a futile wake-up. If we drop the lock
1130 // another thread can reacquire the lock immediately, and we can
1131 // then wake a thread unnecessarily. This is benign, and we've
1132 // structured the code so the windows are short and the frequency
1133 // of such futile wakups is low.
1134
1135 void ObjectMonitor::exit(JavaThread* current, bool not_suspended) {
1136 void* cur = owner_raw();
1137 if (current != cur) {
1138 if (current->is_lock_owned((address)cur)) {
1139 assert(_recursions == 0, "invariant");
1140 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*.
1141 _recursions = 0;
1142 } else {
1143 // Apparent unbalanced locking ...
1144 // Naively we'd like to throw IllegalMonitorStateException.
1145 // As a practical matter we can neither allocate nor throw an
1146 // exception as ::exit() can be called from leaf routines.
1147 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
1148 // Upon deeper reflection, however, in a properly run JVM the only
1149 // way we should encounter this situation is in the presence of
1150 // unbalanced JNI locking. TODO: CheckJNICalls.
1151 // See also: CR4414101
1152 #ifdef ASSERT
1153 LogStreamHandle(Error, monitorinflation) lsh;
1154 lsh.print_cr("ERROR: ObjectMonitor::exit(): thread=" INTPTR_FORMAT
1155 " is exiting an ObjectMonitor it does not own.", p2i(current));
1156 lsh.print_cr("The imbalance is possibly caused by JNI locking.");
1157 print_debug_style_on(&lsh);
1158 assert(false, "Non-balanced monitor enter/exit!");
1159 #endif
1160 return;
1161 }
1162 }
1163
1164 if (_recursions != 0) {
1165 _recursions--; // this is simple recursive enter
1166 return;
1167 }
1168
1169 // Invariant: after setting Responsible=null an thread must execute
1170 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
1171 _Responsible = NULL;
1172
1173 #if INCLUDE_JFR
1174 // get the owner's thread id for the MonitorEnter event
1175 // if it is enabled and the thread isn't suspended
1176 if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
1177 _previous_owner_tid = JFR_THREAD_ID(current);
1178 }
1179 #endif
1180
1181 for (;;) {
1182 assert(current == owner_raw(), "invariant");
1183
1184 // Drop the lock.
1185 // release semantics: prior loads and stores from within the critical section
1186 // must not float (reorder) past the following store that drops the lock.
1187 // Uses a storeload to separate release_store(owner) from the
1188 // successor check. The try_set_owner() below uses cmpxchg() so
1189 // we get the fence down there.
1190 release_clear_owner(current);
1191 OrderAccess::storeload();
1192
1193 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1194 return;
1195 }
1196 // Other threads are blocked trying to acquire the lock.
1197
1198 // Normally the exiting thread is responsible for ensuring succession,
1199 // but if other successors are ready or other entering threads are spinning
1200 // then this thread can simply store NULL into _owner and exit without
1201 // waking a successor. The existence of spinners or ready successors
1202 // guarantees proper succession (liveness). Responsibility passes to the
1203 // ready or running successors. The exiting thread delegates the duty.
1204 // More precisely, if a successor already exists this thread is absolved
1205 // of the responsibility of waking (unparking) one.
1206 //
1207 // The _succ variable is critical to reducing futile wakeup frequency.
1208 // _succ identifies the "heir presumptive" thread that has been made
1209 // ready (unparked) but that has not yet run. We need only one such
1210 // successor thread to guarantee progress.
1211 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1212 // section 3.3 "Futile Wakeup Throttling" for details.
1213 //
1214 // Note that spinners in Enter() also set _succ non-null.
1215 // In the current implementation spinners opportunistically set
1216 // _succ so that exiting threads might avoid waking a successor.
1217 // Another less appealing alternative would be for the exiting thread
1218 // to drop the lock and then spin briefly to see if a spinner managed
1219 // to acquire the lock. If so, the exiting thread could exit
1220 // immediately without waking a successor, otherwise the exiting
1221 // thread would need to dequeue and wake a successor.
1222 // (Note that we'd need to make the post-drop spin short, but no
1223 // shorter than the worst-case round-trip cache-line migration time.
1224 // The dropped lock needs to become visible to the spinner, and then
1225 // the acquisition of the lock by the spinner must become visible to
1226 // the exiting thread).
1227
1228 // It appears that an heir-presumptive (successor) must be made ready.
1229 // Only the current lock owner can manipulate the EntryList or
1230 // drain _cxq, so we need to reacquire the lock. If we fail
1231 // to reacquire the lock the responsibility for ensuring succession
1232 // falls to the new owner.
1233 //
1234 if (try_set_owner_from(NULL, current) != NULL) {
1235 return;
1236 }
1237
1238 guarantee(owner_raw() == current, "invariant");
1239
1240 ObjectWaiter* w = NULL;
1241
1242 w = _EntryList;
1243 if (w != NULL) {
1244 // I'd like to write: guarantee (w->_thread != current).
1245 // But in practice an exiting thread may find itself on the EntryList.
1246 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1247 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1248 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1249 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1250 // release the lock "O". T2 resumes immediately after the ST of null into
1251 // _owner, above. T2 notices that the EntryList is populated, so it
1252 // reacquires the lock and then finds itself on the EntryList.
1253 // Given all that, we have to tolerate the circumstance where "w" is
1254 // associated with current.
1255 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1256 ExitEpilog(current, w);
1257 return;
1258 }
1259
1260 // If we find that both _cxq and EntryList are null then just
1261 // re-run the exit protocol from the top.
1262 w = _cxq;
1263 if (w == NULL) continue;
1264
1265 // Drain _cxq into EntryList - bulk transfer.
1266 // First, detach _cxq.
1267 // The following loop is tantamount to: w = swap(&cxq, NULL)
1268 for (;;) {
1269 assert(w != NULL, "Invariant");
1270 ObjectWaiter* u = Atomic::cmpxchg(&_cxq, w, (ObjectWaiter*)NULL);
1271 if (u == w) break;
1272 w = u;
1273 }
1274
1275 assert(w != NULL, "invariant");
1276 assert(_EntryList == NULL, "invariant");
1277
1278 // Convert the LIFO SLL anchored by _cxq into a DLL.
1279 // The list reorganization step operates in O(LENGTH(w)) time.
1280 // It's critical that this step operate quickly as
1281 // "current" still holds the outer-lock, restricting parallelism
1282 // and effectively lengthening the critical section.
1283 // Invariant: s chases t chases u.
1284 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1285 // we have faster access to the tail.
1286
1287 _EntryList = w;
1288 ObjectWaiter* q = NULL;
1289 ObjectWaiter* p;
1290 for (p = w; p != NULL; p = p->_next) {
1291 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1292 p->TState = ObjectWaiter::TS_ENTER;
1293 p->_prev = q;
1294 q = p;
1295 }
1296
1297 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1298 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1299
1300 // See if we can abdicate to a spinner instead of waking a thread.
1301 // A primary goal of the implementation is to reduce the
1302 // context-switch rate.
1303 if (_succ != NULL) continue;
1304
1305 w = _EntryList;
1306 if (w != NULL) {
1307 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1308 ExitEpilog(current, w);
1309 return;
1310 }
1311 }
1312 }
1313
1314 void ObjectMonitor::ExitEpilog(JavaThread* current, ObjectWaiter* Wakee) {
1315 assert(owner_raw() == current, "invariant");
1316
1317 // Exit protocol:
1318 // 1. ST _succ = wakee
1319 // 2. membar #loadstore|#storestore;
1320 // 2. ST _owner = NULL
1321 // 3. unpark(wakee)
1322
1323 _succ = Wakee->_thread;
1324 ParkEvent * Trigger = Wakee->_event;
1325
1326 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1327 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1328 // out-of-scope (non-extant).
1329 Wakee = NULL;
1330
1331 // Drop the lock.
1332 // Uses a fence to separate release_store(owner) from the LD in unpark().
1333 release_clear_owner(current);
1334 OrderAccess::fence();
1335
1336 DTRACE_MONITOR_PROBE(contended__exit, this, object(), current);
1337 Trigger->unpark();
1338
1339 // Maintain stats and report events to JVMTI
1340 OM_PERFDATA_OP(Parks, inc());
1341 }
1342
1343
1344 // -----------------------------------------------------------------------------
1345 // Class Loader deadlock handling.
1346 //
1347 // complete_exit exits a lock returning recursion count
1348 // complete_exit/reenter operate as a wait without waiting
1349 // complete_exit requires an inflated monitor
1350 // The _owner field is not always the Thread addr even with an
1351 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1352 // thread due to contention.
1353 intx ObjectMonitor::complete_exit(JavaThread* current) {
1354 assert(InitDone, "Unexpectedly not initialized");
1355
1356 void* cur = owner_raw();
1357 if (current != cur) {
1358 if (current->is_lock_owned((address)cur)) {
1359 assert(_recursions == 0, "internal state error");
1360 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*.
1361 _recursions = 0;
1362 }
1363 }
1364
1365 guarantee(current == owner_raw(), "complete_exit not owner");
1366 intx save = _recursions; // record the old recursion count
1367 _recursions = 0; // set the recursion level to be 0
1368 exit(current); // exit the monitor
1369 guarantee(owner_raw() != current, "invariant");
1370 return save;
1371 }
1372
1373 // reenter() enters a lock and sets recursion count
1374 // complete_exit/reenter operate as a wait without waiting
1375 bool ObjectMonitor::reenter(intx recursions, JavaThread* current) {
1376
1377 guarantee(owner_raw() != current, "reenter already owner");
1378 if (!enter(current)) {
1379 return false;
1380 }
1381 // Entered the monitor.
1382 guarantee(_recursions == 0, "reenter recursion");
1383 _recursions = recursions;
1384 return true;
1385 }
1386
1387 // Checks that the current THREAD owns this monitor and causes an
1388 // immediate return if it doesn't. We don't use the CHECK macro
1389 // because we want the IMSE to be the only exception that is thrown
1390 // from the call site when false is returned. Any other pending
1391 // exception is ignored.
1392 #define CHECK_OWNER() \
1393 do { \
1394 if (!check_owner(THREAD)) { \
1395 assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \
1396 return; \
1397 } \
1398 } while (false)
1399
1400 // Returns true if the specified thread owns the ObjectMonitor.
1401 // Otherwise returns false and throws IllegalMonitorStateException
1402 // (IMSE). If there is a pending exception and the specified thread
1403 // is not the owner, that exception will be replaced by the IMSE.
1404 bool ObjectMonitor::check_owner(TRAPS) {
1405 JavaThread* current = THREAD;
1406 void* cur = owner_raw();
1407 if (cur == current) {
1408 return true;
1409 }
1410 if (current->is_lock_owned((address)cur)) {
1411 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*.
1412 _recursions = 0;
1413 return true;
1414 }
1415 THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(),
1416 "current thread is not owner", false);
1417 }
1418
1419 static inline bool is_excluded(const Klass* monitor_klass) {
1420 assert(monitor_klass != nullptr, "invariant");
1421 NOT_JFR_RETURN_(false);
1422 JFR_ONLY(return vmSymbols::jfr_chunk_rotation_monitor() == monitor_klass->name());
1423 }
1424
1425 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1426 ObjectMonitor* monitor,
1427 jlong notifier_tid,
1428 jlong timeout,
1429 bool timedout) {
1430 assert(event != NULL, "invariant");
1431 assert(monitor != NULL, "invariant");
1432 const Klass* monitor_klass = monitor->object()->klass();
1433 if (is_excluded(monitor_klass)) {
1434 return;
1435 }
1436 event->set_monitorClass(monitor_klass);
1437 event->set_timeout(timeout);
1438 // Set an address that is 'unique enough', such that events close in
1439 // time and with the same address are likely (but not guaranteed) to
1440 // belong to the same object.
1441 event->set_address((uintptr_t)monitor);
1442 event->set_notifier(notifier_tid);
1443 event->set_timedOut(timedout);
1444 event->commit();
1445 }
1446
1447 // -----------------------------------------------------------------------------
1448 // Wait/Notify/NotifyAll
1449 //
1450 // Note: a subset of changes to ObjectMonitor::wait()
1451 // will need to be replicated in complete_exit
1452 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1453 JavaThread* current = THREAD;
1454
1455 assert(InitDone, "Unexpectedly not initialized");
1456
1457 CHECK_OWNER(); // Throws IMSE if not owner.
1458
1459 EventJavaMonitorWait event;
1460
1461 // check for a pending interrupt
1462 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1463 // post monitor waited event. Note that this is past-tense, we are done waiting.
1464 if (JvmtiExport::should_post_monitor_waited()) {
1465 // Note: 'false' parameter is passed here because the
1466 // wait was not timed out due to thread interrupt.
1467 JvmtiExport::post_monitor_waited(current, this, false);
1468
1469 // In this short circuit of the monitor wait protocol, the
1470 // current thread never drops ownership of the monitor and
1471 // never gets added to the wait queue so the current thread
1472 // cannot be made the successor. This means that the
1473 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1474 // consume an unpark() meant for the ParkEvent associated with
1475 // this ObjectMonitor.
1476 }
1477 if (event.should_commit()) {
1478 post_monitor_wait_event(&event, this, 0, millis, false);
1479 }
1480 THROW(vmSymbols::java_lang_InterruptedException());
1481 return;
1482 }
1483
1484 assert(current->_Stalled == 0, "invariant");
1485 current->_Stalled = intptr_t(this);
1486 current->set_current_waiting_monitor(this);
1487
1488 // create a node to be put into the queue
1489 // Critically, after we reset() the event but prior to park(), we must check
1490 // for a pending interrupt.
1491 ObjectWaiter node(current);
1492 node.TState = ObjectWaiter::TS_WAIT;
1493 current->_ParkEvent->reset();
1494 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1495
1496 // Enter the waiting queue, which is a circular doubly linked list in this case
1497 // but it could be a priority queue or any data structure.
1498 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1499 // by the the owner of the monitor *except* in the case where park()
1500 // returns because of a timeout of interrupt. Contention is exceptionally rare
1501 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1502
1503 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1504 AddWaiter(&node);
1505 Thread::SpinRelease(&_WaitSetLock);
1506
1507 _Responsible = NULL;
1508
1509 intx save = _recursions; // record the old recursion count
1510 _waiters++; // increment the number of waiters
1511 _recursions = 0; // set the recursion level to be 1
1512 exit(current); // exit the monitor
1513 guarantee(owner_raw() != current, "invariant");
1514
1515 // The thread is on the WaitSet list - now park() it.
1516 // On MP systems it's conceivable that a brief spin before we park
1517 // could be profitable.
1518 //
1519 // TODO-FIXME: change the following logic to a loop of the form
1520 // while (!timeout && !interrupted && _notified == 0) park()
1521
1522 int ret = OS_OK;
1523 int WasNotified = 0;
1524
1525 // Need to check interrupt state whilst still _thread_in_vm
1526 bool interrupted = interruptible && current->is_interrupted(false);
1527
1528 { // State transition wrappers
1529 OSThread* osthread = current->osthread();
1530 OSThreadWaitState osts(osthread, true);
1531
1532 assert(current->thread_state() == _thread_in_vm, "invariant");
1533
1534 {
1535 ClearSuccOnSuspend csos(this);
1536 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos);
1537 if (interrupted || HAS_PENDING_EXCEPTION) {
1538 // Intentionally empty
1539 } else if (node._notified == 0) {
1540 if (millis <= 0) {
1541 current->_ParkEvent->park();
1542 } else {
1543 ret = current->_ParkEvent->park(millis);
1544 }
1545 }
1546 }
1547
1548 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1549 // from the WaitSet to the EntryList.
1550 // See if we need to remove Node from the WaitSet.
1551 // We use double-checked locking to avoid grabbing _WaitSetLock
1552 // if the thread is not on the wait queue.
1553 //
1554 // Note that we don't need a fence before the fetch of TState.
1555 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1556 // written by the is thread. (perhaps the fetch might even be satisfied
1557 // by a look-aside into the processor's own store buffer, although given
1558 // the length of the code path between the prior ST and this load that's
1559 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1560 // then we'll acquire the lock and then re-fetch a fresh TState value.
1561 // That is, we fail toward safety.
1562
1563 if (node.TState == ObjectWaiter::TS_WAIT) {
1564 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1565 if (node.TState == ObjectWaiter::TS_WAIT) {
1566 DequeueSpecificWaiter(&node); // unlink from WaitSet
1567 assert(node._notified == 0, "invariant");
1568 node.TState = ObjectWaiter::TS_RUN;
1569 }
1570 Thread::SpinRelease(&_WaitSetLock);
1571 }
1572
1573 // The thread is now either on off-list (TS_RUN),
1574 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1575 // The Node's TState variable is stable from the perspective of this thread.
1576 // No other threads will asynchronously modify TState.
1577 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1578 OrderAccess::loadload();
1579 if (_succ == current) _succ = NULL;
1580 WasNotified = node._notified;
1581
1582 // Reentry phase -- reacquire the monitor.
1583 // re-enter contended monitor after object.wait().
1584 // retain OBJECT_WAIT state until re-enter successfully completes
1585 // Thread state is thread_in_vm and oop access is again safe,
1586 // although the raw address of the object may have changed.
1587 // (Don't cache naked oops over safepoints, of course).
1588
1589 // post monitor waited event. Note that this is past-tense, we are done waiting.
1590 if (JvmtiExport::should_post_monitor_waited()) {
1591 JvmtiExport::post_monitor_waited(current, this, ret == OS_TIMEOUT);
1592
1593 if (node._notified != 0 && _succ == current) {
1594 // In this part of the monitor wait-notify-reenter protocol it
1595 // is possible (and normal) for another thread to do a fastpath
1596 // monitor enter-exit while this thread is still trying to get
1597 // to the reenter portion of the protocol.
1598 //
1599 // The ObjectMonitor was notified and the current thread is
1600 // the successor which also means that an unpark() has already
1601 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1602 // consume the unpark() that was done when the successor was
1603 // set because the same ParkEvent is shared between Java
1604 // monitors and JVM/TI RawMonitors (for now).
1605 //
1606 // We redo the unpark() to ensure forward progress, i.e., we
1607 // don't want all pending threads hanging (parked) with none
1608 // entering the unlocked monitor.
1609 node._event->unpark();
1610 }
1611 }
1612
1613 if (event.should_commit()) {
1614 post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1615 }
1616
1617 OrderAccess::fence();
1618
1619 assert(current->_Stalled != 0, "invariant");
1620 current->_Stalled = 0;
1621
1622 assert(owner_raw() != current, "invariant");
1623 ObjectWaiter::TStates v = node.TState;
1624 if (v == ObjectWaiter::TS_RUN) {
1625 enter(current);
1626 } else {
1627 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1628 ReenterI(current, &node);
1629 node.wait_reenter_end(this);
1630 }
1631
1632 // current has reacquired the lock.
1633 // Lifecycle - the node representing current must not appear on any queues.
1634 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1635 // want residual elements associated with this thread left on any lists.
1636 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1637 assert(owner_raw() == current, "invariant");
1638 assert(_succ != current, "invariant");
1639 } // OSThreadWaitState()
1640
1641 current->set_current_waiting_monitor(NULL);
1642
1643 guarantee(_recursions == 0, "invariant");
1644 _recursions = save // restore the old recursion count
1645 + JvmtiDeferredUpdates::get_and_reset_relock_count_after_wait(current); // increased by the deferred relock count
1646 _waiters--; // decrement the number of waiters
1647
1648 // Verify a few postconditions
1649 assert(owner_raw() == current, "invariant");
1650 assert(_succ != current, "invariant");
1651 assert(object()->mark() == markWord::encode(this), "invariant");
1652
1653 // check if the notification happened
1654 if (!WasNotified) {
1655 // no, it could be timeout or Thread.interrupt() or both
1656 // check for interrupt event, otherwise it is timeout
1657 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1658 THROW(vmSymbols::java_lang_InterruptedException());
1659 }
1660 }
1661
1662 // NOTE: Spurious wake up will be consider as timeout.
1663 // Monitor notify has precedence over thread interrupt.
1664 }
1665
1666
1667 // Consider:
1668 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1669 // then instead of transferring a thread from the WaitSet to the EntryList
1670 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1671
1672 void ObjectMonitor::INotify(JavaThread* current) {
1673 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1674 ObjectWaiter* iterator = DequeueWaiter();
1675 if (iterator != NULL) {
1676 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1677 guarantee(iterator->_notified == 0, "invariant");
1678 // Disposition - what might we do with iterator ?
1679 // a. add it directly to the EntryList - either tail (policy == 1)
1680 // or head (policy == 0).
1681 // b. push it onto the front of the _cxq (policy == 2).
1682 // For now we use (b).
1683
1684 iterator->TState = ObjectWaiter::TS_ENTER;
1685
1686 iterator->_notified = 1;
1687 iterator->_notifier_tid = JFR_THREAD_ID(current);
1688
1689 ObjectWaiter* list = _EntryList;
1690 if (list != NULL) {
1691 assert(list->_prev == NULL, "invariant");
1692 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1693 assert(list != iterator, "invariant");
1694 }
1695
1696 // prepend to cxq
1697 if (list == NULL) {
1698 iterator->_next = iterator->_prev = NULL;
1699 _EntryList = iterator;
1700 } else {
1701 iterator->TState = ObjectWaiter::TS_CXQ;
1702 for (;;) {
1703 ObjectWaiter* front = _cxq;
1704 iterator->_next = front;
1705 if (Atomic::cmpxchg(&_cxq, front, iterator) == front) {
1706 break;
1707 }
1708 }
1709 }
1710
1711 // _WaitSetLock protects the wait queue, not the EntryList. We could
1712 // move the add-to-EntryList operation, above, outside the critical section
1713 // protected by _WaitSetLock. In practice that's not useful. With the
1714 // exception of wait() timeouts and interrupts the monitor owner
1715 // is the only thread that grabs _WaitSetLock. There's almost no contention
1716 // on _WaitSetLock so it's not profitable to reduce the length of the
1717 // critical section.
1718
1719 iterator->wait_reenter_begin(this);
1720 }
1721 Thread::SpinRelease(&_WaitSetLock);
1722 }
1723
1724 // Consider: a not-uncommon synchronization bug is to use notify() when
1725 // notifyAll() is more appropriate, potentially resulting in stranded
1726 // threads; this is one example of a lost wakeup. A useful diagnostic
1727 // option is to force all notify() operations to behave as notifyAll().
1728 //
1729 // Note: We can also detect many such problems with a "minimum wait".
1730 // When the "minimum wait" is set to a small non-zero timeout value
1731 // and the program does not hang whereas it did absent "minimum wait",
1732 // that suggests a lost wakeup bug.
1733
1734 void ObjectMonitor::notify(TRAPS) {
1735 JavaThread* current = THREAD;
1736 CHECK_OWNER(); // Throws IMSE if not owner.
1737 if (_WaitSet == NULL) {
1738 return;
1739 }
1740 DTRACE_MONITOR_PROBE(notify, this, object(), current);
1741 INotify(current);
1742 OM_PERFDATA_OP(Notifications, inc(1));
1743 }
1744
1745
1746 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1747 // from the waitset to the EntryList. This could be done more efficiently with a
1748 // single bulk transfer but in practice it's not time-critical. Beware too,
1749 // that in prepend-mode we invert the order of the waiters. Let's say that the
1750 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1751 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1752
1753 void ObjectMonitor::notifyAll(TRAPS) {
1754 JavaThread* current = THREAD;
1755 CHECK_OWNER(); // Throws IMSE if not owner.
1756 if (_WaitSet == NULL) {
1757 return;
1758 }
1759
1760 DTRACE_MONITOR_PROBE(notifyAll, this, object(), current);
1761 int tally = 0;
1762 while (_WaitSet != NULL) {
1763 tally++;
1764 INotify(current);
1765 }
1766
1767 OM_PERFDATA_OP(Notifications, inc(tally));
1768 }
1769
1770 // -----------------------------------------------------------------------------
1771 // Adaptive Spinning Support
1772 //
1773 // Adaptive spin-then-block - rational spinning
1774 //
1775 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1776 // algorithm. On high order SMP systems it would be better to start with
1777 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1778 // a contending thread could enqueue itself on the cxq and then spin locally
1779 // on a thread-specific variable such as its ParkEvent._Event flag.
1780 // That's left as an exercise for the reader. Note that global spinning is
1781 // not problematic on Niagara, as the L2 cache serves the interconnect and
1782 // has both low latency and massive bandwidth.
1783 //
1784 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1785 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1786 // (duration) or we can fix the count at approximately the duration of
1787 // a context switch and vary the frequency. Of course we could also
1788 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1789 // For a description of 'Adaptive spin-then-block mutual exclusion in
1790 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1791 //
1792 // This implementation varies the duration "D", where D varies with
1793 // the success rate of recent spin attempts. (D is capped at approximately
1794 // length of a round-trip context switch). The success rate for recent
1795 // spin attempts is a good predictor of the success rate of future spin
1796 // attempts. The mechanism adapts automatically to varying critical
1797 // section length (lock modality), system load and degree of parallelism.
1798 // D is maintained per-monitor in _SpinDuration and is initialized
1799 // optimistically. Spin frequency is fixed at 100%.
1800 //
1801 // Note that _SpinDuration is volatile, but we update it without locks
1802 // or atomics. The code is designed so that _SpinDuration stays within
1803 // a reasonable range even in the presence of races. The arithmetic
1804 // operations on _SpinDuration are closed over the domain of legal values,
1805 // so at worst a race will install and older but still legal value.
1806 // At the very worst this introduces some apparent non-determinism.
1807 // We might spin when we shouldn't or vice-versa, but since the spin
1808 // count are relatively short, even in the worst case, the effect is harmless.
1809 //
1810 // Care must be taken that a low "D" value does not become an
1811 // an absorbing state. Transient spinning failures -- when spinning
1812 // is overall profitable -- should not cause the system to converge
1813 // on low "D" values. We want spinning to be stable and predictable
1814 // and fairly responsive to change and at the same time we don't want
1815 // it to oscillate, become metastable, be "too" non-deterministic,
1816 // or converge on or enter undesirable stable absorbing states.
1817 //
1818 // We implement a feedback-based control system -- using past behavior
1819 // to predict future behavior. We face two issues: (a) if the
1820 // input signal is random then the spin predictor won't provide optimal
1821 // results, and (b) if the signal frequency is too high then the control
1822 // system, which has some natural response lag, will "chase" the signal.
1823 // (b) can arise from multimodal lock hold times. Transient preemption
1824 // can also result in apparent bimodal lock hold times.
1825 // Although sub-optimal, neither condition is particularly harmful, as
1826 // in the worst-case we'll spin when we shouldn't or vice-versa.
1827 // The maximum spin duration is rather short so the failure modes aren't bad.
1828 // To be conservative, I've tuned the gain in system to bias toward
1829 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1830 // "rings" or oscillates between spinning and not spinning. This happens
1831 // when spinning is just on the cusp of profitability, however, so the
1832 // situation is not dire. The state is benign -- there's no need to add
1833 // hysteresis control to damp the transition rate between spinning and
1834 // not spinning.
1835
1836 // Spinning: Fixed frequency (100%), vary duration
1837 int ObjectMonitor::TrySpin(JavaThread* current) {
1838 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1839 int ctr = Knob_FixedSpin;
1840 if (ctr != 0) {
1841 while (--ctr >= 0) {
1842 if (TryLock(current) > 0) return 1;
1843 SpinPause();
1844 }
1845 return 0;
1846 }
1847
1848 for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1849 if (TryLock(current) > 0) {
1850 // Increase _SpinDuration ...
1851 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1852 // Raising _SpurDuration to the poverty line is key.
1853 int x = _SpinDuration;
1854 if (x < Knob_SpinLimit) {
1855 if (x < Knob_Poverty) x = Knob_Poverty;
1856 _SpinDuration = x + Knob_BonusB;
1857 }
1858 return 1;
1859 }
1860 SpinPause();
1861 }
1862
1863 // Admission control - verify preconditions for spinning
1864 //
1865 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1866 // becoming an absorbing state. Put another way, we spin briefly to
1867 // sample, just in case the system load, parallelism, contention, or lock
1868 // modality changed.
1869 //
1870 // Consider the following alternative:
1871 // Periodically set _SpinDuration = _SpinLimit and try a long/full
1872 // spin attempt. "Periodically" might mean after a tally of
1873 // the # of failed spin attempts (or iterations) reaches some threshold.
1874 // This takes us into the realm of 1-out-of-N spinning, where we
1875 // hold the duration constant but vary the frequency.
1876
1877 ctr = _SpinDuration;
1878 if (ctr <= 0) return 0;
1879
1880 if (NotRunnable(current, (JavaThread*) owner_raw())) {
1881 return 0;
1882 }
1883
1884 // We're good to spin ... spin ingress.
1885 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1886 // when preparing to LD...CAS _owner, etc and the CAS is likely
1887 // to succeed.
1888 if (_succ == NULL) {
1889 _succ = current;
1890 }
1891 Thread* prv = NULL;
1892
1893 // There are three ways to exit the following loop:
1894 // 1. A successful spin where this thread has acquired the lock.
1895 // 2. Spin failure with prejudice
1896 // 3. Spin failure without prejudice
1897
1898 while (--ctr >= 0) {
1899
1900 // Periodic polling -- Check for pending GC
1901 // Threads may spin while they're unsafe.
1902 // We don't want spinning threads to delay the JVM from reaching
1903 // a stop-the-world safepoint or to steal cycles from GC.
1904 // If we detect a pending safepoint we abort in order that
1905 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1906 // this thread, if safe, doesn't steal cycles from GC.
1907 // This is in keeping with the "no loitering in runtime" rule.
1908 // We periodically check to see if there's a safepoint pending.
1909 if ((ctr & 0xFF) == 0) {
1910 if (SafepointMechanism::should_process(current)) {
1911 goto Abort; // abrupt spin egress
1912 }
1913 SpinPause();
1914 }
1915
1916 // Probe _owner with TATAS
1917 // If this thread observes the monitor transition or flicker
1918 // from locked to unlocked to locked, then the odds that this
1919 // thread will acquire the lock in this spin attempt go down
1920 // considerably. The same argument applies if the CAS fails
1921 // or if we observe _owner change from one non-null value to
1922 // another non-null value. In such cases we might abort
1923 // the spin without prejudice or apply a "penalty" to the
1924 // spin count-down variable "ctr", reducing it by 100, say.
1925
1926 JavaThread* ox = (JavaThread*) owner_raw();
1927 if (ox == NULL) {
1928 ox = (JavaThread*)try_set_owner_from(NULL, current);
1929 if (ox == NULL) {
1930 // The CAS succeeded -- this thread acquired ownership
1931 // Take care of some bookkeeping to exit spin state.
1932 if (_succ == current) {
1933 _succ = NULL;
1934 }
1935
1936 // Increase _SpinDuration :
1937 // The spin was successful (profitable) so we tend toward
1938 // longer spin attempts in the future.
1939 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
1940 // If we acquired the lock early in the spin cycle it
1941 // makes sense to increase _SpinDuration proportionally.
1942 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1943 int x = _SpinDuration;
1944 if (x < Knob_SpinLimit) {
1945 if (x < Knob_Poverty) x = Knob_Poverty;
1946 _SpinDuration = x + Knob_Bonus;
1947 }
1948 return 1;
1949 }
1950
1951 // The CAS failed ... we can take any of the following actions:
1952 // * penalize: ctr -= CASPenalty
1953 // * exit spin with prejudice -- goto Abort;
1954 // * exit spin without prejudice.
1955 // * Since CAS is high-latency, retry again immediately.
1956 prv = ox;
1957 goto Abort;
1958 }
1959
1960 // Did lock ownership change hands ?
1961 if (ox != prv && prv != NULL) {
1962 goto Abort;
1963 }
1964 prv = ox;
1965
1966 // Abort the spin if the owner is not executing.
1967 // The owner must be executing in order to drop the lock.
1968 // Spinning while the owner is OFFPROC is idiocy.
1969 // Consider: ctr -= RunnablePenalty ;
1970 if (NotRunnable(current, ox)) {
1971 goto Abort;
1972 }
1973 if (_succ == NULL) {
1974 _succ = current;
1975 }
1976 }
1977
1978 // Spin failed with prejudice -- reduce _SpinDuration.
1979 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
1980 // AIMD is globally stable.
1981 {
1982 int x = _SpinDuration;
1983 if (x > 0) {
1984 // Consider an AIMD scheme like: x -= (x >> 3) + 100
1985 // This is globally sample and tends to damp the response.
1986 x -= Knob_Penalty;
1987 if (x < 0) x = 0;
1988 _SpinDuration = x;
1989 }
1990 }
1991
1992 Abort:
1993 if (_succ == current) {
1994 _succ = NULL;
1995 // Invariant: after setting succ=null a contending thread
1996 // must recheck-retry _owner before parking. This usually happens
1997 // in the normal usage of TrySpin(), but it's safest
1998 // to make TrySpin() as foolproof as possible.
1999 OrderAccess::fence();
2000 if (TryLock(current) > 0) return 1;
2001 }
2002 return 0;
2003 }
2004
2005 // NotRunnable() -- informed spinning
2006 //
2007 // Don't bother spinning if the owner is not eligible to drop the lock.
2008 // Spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2009 // The thread must be runnable in order to drop the lock in timely fashion.
2010 // If the _owner is not runnable then spinning will not likely be
2011 // successful (profitable).
2012 //
2013 // Beware -- the thread referenced by _owner could have died
2014 // so a simply fetch from _owner->_thread_state might trap.
2015 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2016 // Because of the lifecycle issues, the _thread_state values
2017 // observed by NotRunnable() might be garbage. NotRunnable must
2018 // tolerate this and consider the observed _thread_state value
2019 // as advisory.
2020 //
2021 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2022 // a thread pointer.
2023 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2024 // with the LSB of _owner. Another option would be to probabilistically probe
2025 // the putative _owner->TypeTag value.
2026 //
2027 // Checking _thread_state isn't perfect. Even if the thread is
2028 // in_java it might be blocked on a page-fault or have been preempted
2029 // and sitting on a ready/dispatch queue.
2030 //
2031 // The return value from NotRunnable() is *advisory* -- the
2032 // result is based on sampling and is not necessarily coherent.
2033 // The caller must tolerate false-negative and false-positive errors.
2034 // Spinning, in general, is probabilistic anyway.
2035
2036
2037 int ObjectMonitor::NotRunnable(JavaThread* current, JavaThread* ox) {
2038 // Check ox->TypeTag == 2BAD.
2039 if (ox == NULL) return 0;
2040
2041 // Avoid transitive spinning ...
2042 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2043 // Immediately after T1 acquires L it's possible that T2, also
2044 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2045 // This occurs transiently after T1 acquired L but before
2046 // T1 managed to clear T1.Stalled. T2 does not need to abort
2047 // its spin in this circumstance.
2048 intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
2049
2050 if (BlockedOn == 1) return 1;
2051 if (BlockedOn != 0) {
2052 return BlockedOn != intptr_t(this) && owner_raw() == ox;
2053 }
2054
2055 assert(sizeof(ox->_thread_state == sizeof(int)), "invariant");
2056 int jst = SafeFetch32((int *) &ox->_thread_state, -1);;
2057 // consider also: jst != _thread_in_Java -- but that's overspecific.
2058 return jst == _thread_blocked || jst == _thread_in_native;
2059 }
2060
2061
2062 // -----------------------------------------------------------------------------
2063 // WaitSet management ...
2064
2065 ObjectWaiter::ObjectWaiter(JavaThread* current) {
2066 _next = NULL;
2067 _prev = NULL;
2068 _notified = 0;
2069 _notifier_tid = 0;
2070 TState = TS_RUN;
2071 _thread = current;
2072 _event = _thread->_ParkEvent;
2073 _active = false;
2074 assert(_event != NULL, "invariant");
2075 }
2076
2077 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2078 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(_thread, mon);
2079 }
2080
2081 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2082 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(_thread, _active);
2083 }
2084
2085 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2086 assert(node != NULL, "should not add NULL node");
2087 assert(node->_prev == NULL, "node already in list");
2088 assert(node->_next == NULL, "node already in list");
2089 // put node at end of queue (circular doubly linked list)
2090 if (_WaitSet == NULL) {
2091 _WaitSet = node;
2092 node->_prev = node;
2093 node->_next = node;
2094 } else {
2095 ObjectWaiter* head = _WaitSet;
2096 ObjectWaiter* tail = head->_prev;
2097 assert(tail->_next == head, "invariant check");
2098 tail->_next = node;
2099 head->_prev = node;
2100 node->_next = head;
2101 node->_prev = tail;
2102 }
2103 }
2104
2105 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2106 // dequeue the very first waiter
2107 ObjectWaiter* waiter = _WaitSet;
2108 if (waiter) {
2109 DequeueSpecificWaiter(waiter);
2110 }
2111 return waiter;
2112 }
2113
2114 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2115 assert(node != NULL, "should not dequeue NULL node");
2116 assert(node->_prev != NULL, "node already removed from list");
2117 assert(node->_next != NULL, "node already removed from list");
2118 // when the waiter has woken up because of interrupt,
2119 // timeout or other spurious wake-up, dequeue the
2120 // waiter from waiting list
2121 ObjectWaiter* next = node->_next;
2122 if (next == node) {
2123 assert(node->_prev == node, "invariant check");
2124 _WaitSet = NULL;
2125 } else {
2126 ObjectWaiter* prev = node->_prev;
2127 assert(prev->_next == node, "invariant check");
2128 assert(next->_prev == node, "invariant check");
2129 next->_prev = prev;
2130 prev->_next = next;
2131 if (_WaitSet == node) {
2132 _WaitSet = next;
2133 }
2134 }
2135 node->_next = NULL;
2136 node->_prev = NULL;
2137 }
2138
2139 // -----------------------------------------------------------------------------
2140 // PerfData support
2141 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL;
2142 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL;
2143 PerfCounter * ObjectMonitor::_sync_Parks = NULL;
2144 PerfCounter * ObjectMonitor::_sync_Notifications = NULL;
2145 PerfCounter * ObjectMonitor::_sync_Inflations = NULL;
2146 PerfCounter * ObjectMonitor::_sync_Deflations = NULL;
2147 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL;
2148
2149 // One-shot global initialization for the sync subsystem.
2150 // We could also defer initialization and initialize on-demand
2151 // the first time we call ObjectSynchronizer::inflate().
2152 // Initialization would be protected - like so many things - by
2153 // the MonitorCache_lock.
2154
2155 void ObjectMonitor::Initialize() {
2156 assert(!InitDone, "invariant");
2157
2158 if (!os::is_MP()) {
2159 Knob_SpinLimit = 0;
2160 Knob_PreSpin = 0;
2161 Knob_FixedSpin = -1;
2162 }
2163
2164 if (UsePerfData) {
2165 EXCEPTION_MARK;
2166 #define NEWPERFCOUNTER(n) \
2167 { \
2168 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \
2169 CHECK); \
2170 }
2171 #define NEWPERFVARIABLE(n) \
2172 { \
2173 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \
2174 CHECK); \
2175 }
2176 NEWPERFCOUNTER(_sync_Inflations);
2177 NEWPERFCOUNTER(_sync_Deflations);
2178 NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2179 NEWPERFCOUNTER(_sync_FutileWakeups);
2180 NEWPERFCOUNTER(_sync_Parks);
2181 NEWPERFCOUNTER(_sync_Notifications);
2182 NEWPERFVARIABLE(_sync_MonExtant);
2183 #undef NEWPERFCOUNTER
2184 #undef NEWPERFVARIABLE
2185 }
2186
2187 _oop_storage = OopStorageSet::create_weak("ObjectSynchronizer Weak", mtSynchronizer);
2188
2189 DEBUG_ONLY(InitDone = true;)
2190 }
2191
2192 void ObjectMonitor::print_on(outputStream* st) const {
2193 // The minimal things to print for markWord printing, more can be added for debugging and logging.
2194 st->print("{contentions=0x%08x,waiters=0x%08x"
2195 ",recursions=" INTX_FORMAT ",owner=" INTPTR_FORMAT "}",
2196 contentions(), waiters(), recursions(),
2197 p2i(owner()));
2198 }
2199 void ObjectMonitor::print() const { print_on(tty); }
2200
2201 #ifdef ASSERT
2202 // Print the ObjectMonitor like a debugger would:
2203 //
2204 // (ObjectMonitor) 0x00007fdfb6012e40 = {
2205 // _header = 0x0000000000000001
2206 // _object = 0x000000070ff45fd0
2207 // _pad_buf0 = {
2208 // [0] = '\0'
2209 // ...
2210 // [43] = '\0'
2211 // }
2212 // _owner = 0x0000000000000000
2213 // _previous_owner_tid = 0
2214 // _pad_buf1 = {
2215 // [0] = '\0'
2216 // ...
2217 // [47] = '\0'
2218 // }
2219 // _next_om = 0x0000000000000000
2220 // _recursions = 0
2221 // _EntryList = 0x0000000000000000
2222 // _cxq = 0x0000000000000000
2223 // _succ = 0x0000000000000000
2224 // _Responsible = 0x0000000000000000
2225 // _Spinner = 0
2226 // _SpinDuration = 5000
2227 // _contentions = 0
2228 // _WaitSet = 0x0000700009756248
2229 // _waiters = 1
2230 // _WaitSetLock = 0
2231 // }
2232 //
2233 void ObjectMonitor::print_debug_style_on(outputStream* st) const {
2234 st->print_cr("(ObjectMonitor*) " INTPTR_FORMAT " = {", p2i(this));
2235 st->print_cr(" _header = " INTPTR_FORMAT, header().value());
2236 st->print_cr(" _object = " INTPTR_FORMAT, p2i(object_peek()));
2237 st->print_cr(" _pad_buf0 = {");
2238 st->print_cr(" [0] = '\\0'");
2239 st->print_cr(" ...");
2240 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf0) - 1);
2241 st->print_cr(" }");
2242 st->print_cr(" _owner = " INTPTR_FORMAT, p2i(owner_raw()));
2243 st->print_cr(" _previous_owner_tid = " JLONG_FORMAT, _previous_owner_tid);
2244 st->print_cr(" _pad_buf1 = {");
2245 st->print_cr(" [0] = '\\0'");
2246 st->print_cr(" ...");
2247 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf1) - 1);
2248 st->print_cr(" }");
2249 st->print_cr(" _next_om = " INTPTR_FORMAT, p2i(next_om()));
2250 st->print_cr(" _recursions = " INTX_FORMAT, _recursions);
2251 st->print_cr(" _EntryList = " INTPTR_FORMAT, p2i(_EntryList));
2252 st->print_cr(" _cxq = " INTPTR_FORMAT, p2i(_cxq));
2253 st->print_cr(" _succ = " INTPTR_FORMAT, p2i(_succ));
2254 st->print_cr(" _Responsible = " INTPTR_FORMAT, p2i(_Responsible));
2255 st->print_cr(" _Spinner = %d", _Spinner);
2256 st->print_cr(" _SpinDuration = %d", _SpinDuration);
2257 st->print_cr(" _contentions = %d", contentions());
2258 st->print_cr(" _WaitSet = " INTPTR_FORMAT, p2i(_WaitSet));
2259 st->print_cr(" _waiters = %d", _waiters);
2260 st->print_cr(" _WaitSetLock = %d", _WaitSetLock);
2261 st->print_cr("}");
2262 }
2263 #endif