1 /*
2 * Copyright (c) 1998, 2024, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 #include "precompiled.hpp"
26 #include "classfile/vmSymbols.hpp"
27 #include "gc/shared/oopStorage.hpp"
28 #include "gc/shared/oopStorageSet.hpp"
29 #include "jfr/jfrEvents.hpp"
30 #include "jfr/support/jfrThreadId.hpp"
31 #include "logging/log.hpp"
32 #include "logging/logStream.hpp"
33 #include "memory/allocation.inline.hpp"
34 #include "memory/resourceArea.hpp"
35 #include "oops/markWord.hpp"
36 #include "oops/oop.inline.hpp"
37 #include "oops/oopHandle.inline.hpp"
38 #include "oops/weakHandle.inline.hpp"
39 #include "prims/jvmtiDeferredUpdates.hpp"
40 #include "prims/jvmtiExport.hpp"
41 #include "runtime/atomic.hpp"
42 #include "runtime/globals.hpp"
43 #include "runtime/handles.inline.hpp"
44 #include "runtime/interfaceSupport.inline.hpp"
45 #include "runtime/mutexLocker.hpp"
46 #include "runtime/objectMonitor.hpp"
47 #include "runtime/objectMonitor.inline.hpp"
48 #include "runtime/orderAccess.hpp"
49 #include "runtime/osThread.hpp"
50 #include "runtime/perfData.hpp"
51 #include "runtime/safefetch.hpp"
52 #include "runtime/safepointMechanism.inline.hpp"
53 #include "runtime/sharedRuntime.hpp"
54 #include "runtime/thread.inline.hpp"
55 #include "services/threadService.hpp"
56 #include "utilities/dtrace.hpp"
57 #include "utilities/globalDefinitions.hpp"
58 #include "utilities/macros.hpp"
59 #include "utilities/preserveException.hpp"
60 #if INCLUDE_JFR
61 #include "jfr/support/jfrFlush.hpp"
62 #endif
63
64 #ifdef DTRACE_ENABLED
65
66 // Only bother with this argument setup if dtrace is available
67 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
68
69
70 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
71 char* bytes = NULL; \
72 int len = 0; \
73 jlong jtid = SharedRuntime::get_java_tid(thread); \
74 Symbol* klassname = obj->klass()->name(); \
75 if (klassname != NULL) { \
76 bytes = (char*)klassname->bytes(); \
77 len = klassname->utf8_length(); \
78 }
79
80 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
81 { \
82 if (DTraceMonitorProbes) { \
83 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
84 HOTSPOT_MONITOR_WAIT(jtid, \
85 (monitor), bytes, len, (millis)); \
86 } \
87 }
88
89 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
90 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
91 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
92 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
93 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
94
95 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
96 { \
97 if (DTraceMonitorProbes) { \
98 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
99 HOTSPOT_MONITOR_##probe(jtid, \
100 (uintptr_t)(monitor), bytes, len); \
101 } \
102 }
103
104 #else // ndef DTRACE_ENABLED
105
106 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
107 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
108
109 #endif // ndef DTRACE_ENABLED
110
111 // Tunables ...
112 // The knob* variables are effectively final. Once set they should
113 // never be modified hence. Consider using __read_mostly with GCC.
114
115 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool -
116
117 static int Knob_Bonus = 100; // spin success bonus
118 static int Knob_BonusB = 100; // spin success bonus
119 static int Knob_Penalty = 200; // spin failure penalty
120 static int Knob_Poverty = 1000;
121 static int Knob_FixedSpin = 0;
122 static int Knob_PreSpin = 10; // 20-100 likely better
123
124 DEBUG_ONLY(static volatile bool InitDone = false;)
125
126 OopStorage* ObjectMonitor::_oop_storage = NULL;
127
128 // -----------------------------------------------------------------------------
129 // Theory of operations -- Monitors lists, thread residency, etc:
130 //
131 // * A thread acquires ownership of a monitor by successfully
132 // CAS()ing the _owner field from null to non-null.
133 //
134 // * Invariant: A thread appears on at most one monitor list --
135 // cxq, EntryList or WaitSet -- at any one time.
136 //
137 // * Contending threads "push" themselves onto the cxq with CAS
138 // and then spin/park.
139 //
140 // * After a contending thread eventually acquires the lock it must
141 // dequeue itself from either the EntryList or the cxq.
142 //
143 // * The exiting thread identifies and unparks an "heir presumptive"
144 // tentative successor thread on the EntryList. Critically, the
145 // exiting thread doesn't unlink the successor thread from the EntryList.
146 // After having been unparked, the wakee will recontend for ownership of
147 // the monitor. The successor (wakee) will either acquire the lock or
148 // re-park itself.
149 //
150 // Succession is provided for by a policy of competitive handoff.
151 // The exiting thread does _not_ grant or pass ownership to the
152 // successor thread. (This is also referred to as "handoff" succession").
153 // Instead the exiting thread releases ownership and possibly wakes
154 // a successor, so the successor can (re)compete for ownership of the lock.
155 // If the EntryList is empty but the cxq is populated the exiting
156 // thread will drain the cxq into the EntryList. It does so by
157 // by detaching the cxq (installing null with CAS) and folding
158 // the threads from the cxq into the EntryList. The EntryList is
159 // doubly linked, while the cxq is singly linked because of the
160 // CAS-based "push" used to enqueue recently arrived threads (RATs).
161 //
162 // * Concurrency invariants:
163 //
164 // -- only the monitor owner may access or mutate the EntryList.
165 // The mutex property of the monitor itself protects the EntryList
166 // from concurrent interference.
167 // -- Only the monitor owner may detach the cxq.
168 //
169 // * The monitor entry list operations avoid locks, but strictly speaking
170 // they're not lock-free. Enter is lock-free, exit is not.
171 // For a description of 'Methods and apparatus providing non-blocking access
172 // to a resource,' see U.S. Pat. No. 7844973.
173 //
174 // * The cxq can have multiple concurrent "pushers" but only one concurrent
175 // detaching thread. This mechanism is immune from the ABA corruption.
176 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
177 //
178 // * Taken together, the cxq and the EntryList constitute or form a
179 // single logical queue of threads stalled trying to acquire the lock.
180 // We use two distinct lists to improve the odds of a constant-time
181 // dequeue operation after acquisition (in the ::enter() epilogue) and
182 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
183 // A key desideratum is to minimize queue & monitor metadata manipulation
184 // that occurs while holding the monitor lock -- that is, we want to
185 // minimize monitor lock holds times. Note that even a small amount of
186 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
187 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
188 // locks and monitor metadata.
189 //
190 // Cxq points to the set of Recently Arrived Threads attempting entry.
191 // Because we push threads onto _cxq with CAS, the RATs must take the form of
192 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
193 // the unlocking thread notices that EntryList is null but _cxq is != null.
194 //
195 // The EntryList is ordered by the prevailing queue discipline and
196 // can be organized in any convenient fashion, such as a doubly-linked list or
197 // a circular doubly-linked list. Critically, we want insert and delete operations
198 // to operate in constant-time. If we need a priority queue then something akin
199 // to Solaris' sleepq would work nicely. Viz.,
200 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
201 // Queue discipline is enforced at ::exit() time, when the unlocking thread
202 // drains the cxq into the EntryList, and orders or reorders the threads on the
203 // EntryList accordingly.
204 //
205 // Barring "lock barging", this mechanism provides fair cyclic ordering,
206 // somewhat similar to an elevator-scan.
207 //
208 // * The monitor synchronization subsystem avoids the use of native
209 // synchronization primitives except for the narrow platform-specific
210 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
211 // the semantics of park-unpark. Put another way, this monitor implementation
212 // depends only on atomic operations and park-unpark. The monitor subsystem
213 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
214 // underlying OS manages the READY<->RUN transitions.
215 //
216 // * Waiting threads reside on the WaitSet list -- wait() puts
217 // the caller onto the WaitSet.
218 //
219 // * notify() or notifyAll() simply transfers threads from the WaitSet to
220 // either the EntryList or cxq. Subsequent exit() operations will
221 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
222 // it's likely the notifyee would simply impale itself on the lock held
223 // by the notifier.
224 //
225 // * An interesting alternative is to encode cxq as (List,LockByte) where
226 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
227 // variable, like _recursions, in the scheme. The threads or Events that form
228 // the list would have to be aligned in 256-byte addresses. A thread would
229 // try to acquire the lock or enqueue itself with CAS, but exiting threads
230 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
231 // Note that is is *not* word-tearing, but it does presume that full-word
232 // CAS operations are coherent with intermix with STB operations. That's true
233 // on most common processors.
234 //
235 // * See also http://blogs.sun.com/dave
236
237
238 // Check that object() and set_object() are called from the right context:
239 static void check_object_context() {
240 #ifdef ASSERT
241 Thread* self = Thread::current();
242 if (self->is_Java_thread()) {
243 // Mostly called from JavaThreads so sanity check the thread state.
244 JavaThread* jt = self->as_Java_thread();
245 switch (jt->thread_state()) {
246 case _thread_in_vm: // the usual case
247 case _thread_in_Java: // during deopt
248 break;
249 default:
250 fatal("called from an unsafe thread state");
251 }
252 assert(jt->is_active_Java_thread(), "must be active JavaThread");
253 } else {
254 // However, ThreadService::get_current_contended_monitor()
255 // can call here via the VMThread so sanity check it.
256 assert(self->is_VM_thread(), "must be");
257 }
258 #endif // ASSERT
259 }
260
261 ObjectMonitor::ObjectMonitor(oop object) :
262 _header(markWord::zero()),
263 _object(_oop_storage, object),
264 _owner(NULL),
265 _previous_owner_tid(0),
266 _next_om(NULL),
267 _recursions(0),
268 _EntryList(NULL),
269 _cxq(NULL),
270 _succ(NULL),
271 _Responsible(NULL),
272 _Spinner(0),
273 _SpinDuration(ObjectMonitor::Knob_SpinLimit),
274 _contentions(0),
275 _WaitSet(NULL),
276 _waiters(0),
277 _WaitSetLock(0)
278 { }
279
280 ObjectMonitor::~ObjectMonitor() {
281 _object.release(_oop_storage);
282 }
283
284 oop ObjectMonitor::object() const {
285 check_object_context();
286 if (_object.is_null()) {
287 return NULL;
288 }
289 return _object.resolve();
290 }
291
292 oop ObjectMonitor::object_peek() const {
293 if (_object.is_null()) {
294 return NULL;
295 }
296 return _object.peek();
297 }
298
299 void ObjectMonitor::ExitOnSuspend::operator()(JavaThread* current) {
300 if (current->is_suspended()) {
301 _om->_recursions = 0;
302 _om->_succ = NULL;
303 // Don't need a full fence after clearing successor here because of the call to exit().
304 _om->exit(current, false /* not_suspended */);
305 _om_exited = true;
306
307 current->set_current_pending_monitor(_om);
308 }
309 }
310
311 void ObjectMonitor::ClearSuccOnSuspend::operator()(JavaThread* current) {
312 if (current->is_suspended()) {
313 if (_om->_succ == current) {
314 _om->_succ = NULL;
315 OrderAccess::fence(); // always do a full fence when successor is cleared
316 }
317 }
318 }
319
320 // -----------------------------------------------------------------------------
321 // Enter support
322
323 bool ObjectMonitor::enter_for(JavaThread* locking_thread) {
324 // Used by ObjectSynchronizer::enter_for to enter for another thread.
325 // The monitor is private to or already owned by locking_thread which must be suspended.
326 // So this code may only contend with deflation.
327 assert(locking_thread == Thread::current() || locking_thread->is_obj_deopt_suspend(), "must be");
328
329 // Block out deflation as soon as possible.
330 add_to_contentions(1);
331
332 bool success = false;
333 if (!is_being_async_deflated()) {
334 void* prev_owner = try_set_owner_from(nullptr, locking_thread);
335
336 if (prev_owner == nullptr) {
337 assert(_recursions == 0, "invariant");
338 success = true;
339 } else if (prev_owner == locking_thread) {
340 _recursions++;
341 success = true;
342 } else if (prev_owner == DEFLATER_MARKER) {
343 // Racing with deflation.
344 prev_owner = try_set_owner_from(DEFLATER_MARKER, locking_thread);
345 if (prev_owner == DEFLATER_MARKER) {
346 // Cancelled deflation. Increment contentions as part of the deflation protocol.
347 add_to_contentions(1);
348 success = true;
349 } else if (prev_owner == nullptr) {
350 // At this point we cannot race with deflation as we have both incremented
351 // contentions, seen contention > 0 and seen a DEFLATER_MARKER.
352 // success will only be false if this races with something other than
353 // deflation.
354 prev_owner = try_set_owner_from(nullptr, locking_thread);
355 success = prev_owner == nullptr;
356 }
357 } else if (LockingMode == LM_LEGACY && locking_thread->is_lock_owned((address)prev_owner)) {
358 assert(_recursions == 0, "must be");
359 _recursions = 1;
360 set_owner_from_BasicLock(prev_owner, locking_thread);
361 success = true;
362 }
363 assert(success, "Failed to enter_for: locking_thread=" INTPTR_FORMAT
364 ", this=" INTPTR_FORMAT "{owner=" INTPTR_FORMAT "}, observed owner: " INTPTR_FORMAT,
365 p2i(locking_thread), p2i(this), p2i(owner_raw()), p2i(prev_owner));
366 } else {
367 // Async deflation is in progress and our contentions increment
368 // above lost the race to async deflation. Undo the work and
369 // force the caller to retry.
370 const oop l_object = object();
371 if (l_object != nullptr) {
372 // Attempt to restore the header/dmw to the object's header so that
373 // we only retry once if the deflater thread happens to be slow.
374 install_displaced_markword_in_object(l_object);
375 }
376 }
377
378 add_to_contentions(-1);
379
380 assert(!success || owner_raw() == locking_thread, "must be");
381
382 return success;
383 }
384
385 bool ObjectMonitor::enter(JavaThread* current) {
386 assert(current == JavaThread::current(), "must be");
387 // The following code is ordered to check the most common cases first
388 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
389
390 void* cur = try_set_owner_from(NULL, current);
391 if (cur == NULL) {
392 assert(_recursions == 0, "invariant");
393 return true;
394 }
395
396 if (cur == current) {
397 // TODO-FIXME: check for integer overflow! BUGID 6557169.
398 _recursions++;
399 return true;
400 }
401
402 if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) {
403 assert(_recursions == 0, "internal state error");
404 _recursions = 1;
405 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*.
406 return true;
407 }
408
409 // We've encountered genuine contention.
410 assert(current->_Stalled == 0, "invariant");
411 current->_Stalled = intptr_t(this);
412
413 // Try one round of spinning *before* enqueueing current
414 // and before going through the awkward and expensive state
415 // transitions. The following spin is strictly optional ...
416 // Note that if we acquire the monitor from an initial spin
417 // we forgo posting JVMTI events and firing DTRACE probes.
418 if (TrySpin(current) > 0) {
419 assert(owner_raw() == current, "must be current: owner=" INTPTR_FORMAT, p2i(owner_raw()));
420 assert(_recursions == 0, "must be 0: recursions=" INTX_FORMAT, _recursions);
421 assert(object()->mark() == markWord::encode(this),
422 "object mark must match encoded this: mark=" INTPTR_FORMAT
423 ", encoded this=" INTPTR_FORMAT, object()->mark().value(),
424 markWord::encode(this).value());
425 current->_Stalled = 0;
426 return true;
427 }
428
429 assert(owner_raw() != current, "invariant");
430 assert(_succ != current, "invariant");
431 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
432 assert(current->thread_state() != _thread_blocked, "invariant");
433
434 // Keep track of contention for JVM/TI and M&M queries.
435 add_to_contentions(1);
436 if (is_being_async_deflated()) {
437 // Async deflation is in progress and our contentions increment
438 // above lost the race to async deflation. Undo the work and
439 // force the caller to retry.
440 const oop l_object = object();
441 if (l_object != NULL) {
442 // Attempt to restore the header/dmw to the object's header so that
443 // we only retry once if the deflater thread happens to be slow.
444 install_displaced_markword_in_object(l_object);
445 }
446 current->_Stalled = 0;
447 add_to_contentions(-1);
448 return false;
449 }
450
451 JFR_ONLY(JfrConditionalFlushWithStacktrace<EventJavaMonitorEnter> flush(current);)
452 EventJavaMonitorEnter event;
453 if (event.is_started()) {
454 event.set_monitorClass(object()->klass());
455 // Set an address that is 'unique enough', such that events close in
456 // time and with the same address are likely (but not guaranteed) to
457 // belong to the same object.
458 event.set_address((uintptr_t)this);
459 }
460
461 { // Change java thread status to indicate blocked on monitor enter.
462 JavaThreadBlockedOnMonitorEnterState jtbmes(current, this);
463
464 assert(current->current_pending_monitor() == NULL, "invariant");
465 current->set_current_pending_monitor(this);
466
467 DTRACE_MONITOR_PROBE(contended__enter, this, object(), current);
468 if (JvmtiExport::should_post_monitor_contended_enter()) {
469 JvmtiExport::post_monitor_contended_enter(current, this);
470
471 // The current thread does not yet own the monitor and does not
472 // yet appear on any queues that would get it made the successor.
473 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
474 // handler cannot accidentally consume an unpark() meant for the
475 // ParkEvent associated with this ObjectMonitor.
476 }
477
478 OSThreadContendState osts(current->osthread());
479
480 assert(current->thread_state() == _thread_in_vm, "invariant");
481
482 for (;;) {
483 ExitOnSuspend eos(this);
484 {
485 ThreadBlockInVMPreprocess<ExitOnSuspend> tbivs(current, eos);
486 EnterI(current);
487 current->set_current_pending_monitor(NULL);
488 // We can go to a safepoint at the end of this block. If we
489 // do a thread dump during that safepoint, then this thread will show
490 // as having "-locked" the monitor, but the OS and java.lang.Thread
491 // states will still report that the thread is blocked trying to
492 // acquire it.
493 // If there is a suspend request, ExitOnSuspend will exit the OM
494 // and set the OM as pending.
495 }
496 if (!eos.exited()) {
497 // ExitOnSuspend did not exit the OM
498 assert(owner_raw() == current, "invariant");
499 break;
500 }
501 }
502
503 // We've just gotten past the enter-check-for-suspend dance and we now own
504 // the monitor free and clear.
505 }
506
507 add_to_contentions(-1);
508 assert(contentions() >= 0, "must not be negative: contentions=%d", contentions());
509 current->_Stalled = 0;
510
511 // Must either set _recursions = 0 or ASSERT _recursions == 0.
512 assert(_recursions == 0, "invariant");
513 assert(owner_raw() == current, "invariant");
514 assert(_succ != current, "invariant");
515 assert(object()->mark() == markWord::encode(this), "invariant");
516
517 // The thread -- now the owner -- is back in vm mode.
518 // Report the glorious news via TI,DTrace and jvmstat.
519 // The probe effect is non-trivial. All the reportage occurs
520 // while we hold the monitor, increasing the length of the critical
521 // section. Amdahl's parallel speedup law comes vividly into play.
522 //
523 // Another option might be to aggregate the events (thread local or
524 // per-monitor aggregation) and defer reporting until a more opportune
525 // time -- such as next time some thread encounters contention but has
526 // yet to acquire the lock. While spinning that thread could
527 // spinning we could increment JVMStat counters, etc.
528
529 DTRACE_MONITOR_PROBE(contended__entered, this, object(), current);
530 if (JvmtiExport::should_post_monitor_contended_entered()) {
531 JvmtiExport::post_monitor_contended_entered(current, this);
532
533 // The current thread already owns the monitor and is not going to
534 // call park() for the remainder of the monitor enter protocol. So
535 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
536 // event handler consumed an unpark() issued by the thread that
537 // just exited the monitor.
538 }
539 if (event.should_commit()) {
540 event.set_previousOwner((uintptr_t)_previous_owner_tid);
541 event.commit();
542 }
543 OM_PERFDATA_OP(ContendedLockAttempts, inc());
544 return true;
545 }
546
547 // Caveat: TryLock() is not necessarily serializing if it returns failure.
548 // Callers must compensate as needed.
549
550 int ObjectMonitor::TryLock(JavaThread* current) {
551 void* own = owner_raw();
552 if (own != NULL) return 0;
553 if (try_set_owner_from(NULL, current) == NULL) {
554 assert(_recursions == 0, "invariant");
555 return 1;
556 }
557 // The lock had been free momentarily, but we lost the race to the lock.
558 // Interference -- the CAS failed.
559 // We can either return -1 or retry.
560 // Retry doesn't make as much sense because the lock was just acquired.
561 return -1;
562 }
563
564 // Deflate the specified ObjectMonitor if not in-use. Returns true if it
565 // was deflated and false otherwise.
566 //
567 // The async deflation protocol sets owner to DEFLATER_MARKER and
568 // makes contentions negative as signals to contending threads that
569 // an async deflation is in progress. There are a number of checks
570 // as part of the protocol to make sure that the calling thread has
571 // not lost the race to a contending thread.
572 //
573 // The ObjectMonitor has been successfully async deflated when:
574 // (contentions < 0)
575 // Contending threads that see that condition know to retry their operation.
576 //
577 bool ObjectMonitor::deflate_monitor() {
578 if (is_busy()) {
579 // Easy checks are first - the ObjectMonitor is busy so no deflation.
580 return false;
581 }
582
583 if (ObjectSynchronizer::is_final_audit() && owner_is_DEFLATER_MARKER()) {
584 // The final audit can see an already deflated ObjectMonitor on the
585 // in-use list because MonitorList::unlink_deflated() might have
586 // blocked for the final safepoint before unlinking all the deflated
587 // monitors.
588 assert(contentions() < 0, "must be negative: contentions=%d", contentions());
589 // Already returned 'true' when it was originally deflated.
590 return false;
591 }
592
593 const oop obj = object_peek();
594
595 if (obj == NULL) {
596 // If the object died, we can recycle the monitor without racing with
597 // Java threads. The GC already broke the association with the object.
598 set_owner_from(NULL, DEFLATER_MARKER);
599 assert(contentions() >= 0, "must be non-negative: contentions=%d", contentions());
600 _contentions = -max_jint;
601 } else {
602 // Attempt async deflation protocol.
603
604 // Set a NULL owner to DEFLATER_MARKER to force any contending thread
605 // through the slow path. This is just the first part of the async
606 // deflation dance.
607 if (try_set_owner_from(NULL, DEFLATER_MARKER) != NULL) {
608 // The owner field is no longer NULL so we lost the race since the
609 // ObjectMonitor is now busy.
610 return false;
611 }
612
613 if (contentions() > 0 || _waiters != 0) {
614 // Another thread has raced to enter the ObjectMonitor after
615 // is_busy() above or has already entered and waited on
616 // it which makes it busy so no deflation. Restore owner to
617 // NULL if it is still DEFLATER_MARKER.
618 if (try_set_owner_from(DEFLATER_MARKER, NULL) != DEFLATER_MARKER) {
619 // Deferred decrement for the JT EnterI() that cancelled the async deflation.
620 add_to_contentions(-1);
621 }
622 return false;
623 }
624
625 // Make a zero contentions field negative to force any contending threads
626 // to retry. This is the second part of the async deflation dance.
627 if (Atomic::cmpxchg(&_contentions, (jint)0, -max_jint) != 0) {
628 // Contentions was no longer 0 so we lost the race since the
629 // ObjectMonitor is now busy. Restore owner to NULL if it is
630 // still DEFLATER_MARKER:
631 if (try_set_owner_from(DEFLATER_MARKER, NULL) != DEFLATER_MARKER) {
632 // Deferred decrement for the JT EnterI() that cancelled the async deflation.
633 add_to_contentions(-1);
634 }
635 return false;
636 }
637 }
638
639 // Sanity checks for the races:
640 guarantee(owner_is_DEFLATER_MARKER(), "must be deflater marker");
641 guarantee(contentions() < 0, "must be negative: contentions=%d",
642 contentions());
643 guarantee(_waiters == 0, "must be 0: waiters=%d", _waiters);
644 guarantee(_cxq == NULL, "must be no contending threads: cxq="
645 INTPTR_FORMAT, p2i(_cxq));
646 guarantee(_EntryList == NULL,
647 "must be no entering threads: EntryList=" INTPTR_FORMAT,
648 p2i(_EntryList));
649
650 if (obj != NULL) {
651 if (log_is_enabled(Trace, monitorinflation)) {
652 ResourceMark rm;
653 log_trace(monitorinflation)("deflate_monitor: object=" INTPTR_FORMAT
654 ", mark=" INTPTR_FORMAT ", type='%s'",
655 p2i(obj), obj->mark().value(),
656 obj->klass()->external_name());
657 }
658
659 // Install the old mark word if nobody else has already done it.
660 install_displaced_markword_in_object(obj);
661 }
662
663 // We leave owner == DEFLATER_MARKER and contentions < 0
664 // to force any racing threads to retry.
665 return true; // Success, ObjectMonitor has been deflated.
666 }
667
668 // We might access the dead object headers for parsable heap walk, make sure
669 // headers are in correct shape, e.g. monitors deflated.
670 void ObjectMonitor::maybe_deflate_dead(oop* p) {
671 oop obj = *p;
672 assert(obj != NULL, "must not yet been cleared");
673 markWord mark = obj->mark();
674 if (mark.has_monitor()) {
675 ObjectMonitor* monitor = mark.monitor();
676 if (p == monitor->_object.ptr_raw()) {
677 assert(monitor->object_peek() == obj, "lock object must match");
678 markWord dmw = monitor->header();
679 obj->set_mark(dmw);
680 }
681 }
682 }
683
684 // Install the displaced mark word (dmw) of a deflating ObjectMonitor
685 // into the header of the object associated with the monitor. This
686 // idempotent method is called by a thread that is deflating a
687 // monitor and by other threads that have detected a race with the
688 // deflation process.
689 void ObjectMonitor::install_displaced_markword_in_object(const oop obj) {
690 // This function must only be called when (owner == DEFLATER_MARKER
691 // && contentions <= 0), but we can't guarantee that here because
692 // those values could change when the ObjectMonitor gets moved from
693 // the global free list to a per-thread free list.
694
695 guarantee(obj != NULL, "must be non-NULL");
696
697 // Separate loads in is_being_async_deflated(), which is almost always
698 // called before this function, from the load of dmw/header below.
699
700 // _contentions and dmw/header may get written by different threads.
701 // Make sure to observe them in the same order when having several observers.
702 OrderAccess::loadload_for_IRIW();
703
704 const oop l_object = object_peek();
705 if (l_object == NULL) {
706 // ObjectMonitor's object ref has already been cleared by async
707 // deflation or GC so we're done here.
708 return;
709 }
710 assert(l_object == obj, "object=" INTPTR_FORMAT " must equal obj="
711 INTPTR_FORMAT, p2i(l_object), p2i(obj));
712
713 markWord dmw = header();
714 // The dmw has to be neutral (not NULL, not locked and not marked).
715 assert(dmw.is_neutral(), "must be neutral: dmw=" INTPTR_FORMAT, dmw.value());
716
717 // Install displaced mark word if the object's header still points
718 // to this ObjectMonitor. More than one racing caller to this function
719 // can rarely reach this point, but only one can win.
720 markWord res = obj->cas_set_mark(dmw, markWord::encode(this));
721 if (res != markWord::encode(this)) {
722 // This should be rare so log at the Info level when it happens.
723 log_info(monitorinflation)("install_displaced_markword_in_object: "
724 "failed cas_set_mark: new_mark=" INTPTR_FORMAT
725 ", old_mark=" INTPTR_FORMAT ", res=" INTPTR_FORMAT,
726 dmw.value(), markWord::encode(this).value(),
727 res.value());
728 }
729
730 // Note: It does not matter which thread restored the header/dmw
731 // into the object's header. The thread deflating the monitor just
732 // wanted the object's header restored and it is. The threads that
733 // detected a race with the deflation process also wanted the
734 // object's header restored before they retry their operation and
735 // because it is restored they will only retry once.
736 }
737
738 // Convert the fields used by is_busy() to a string that can be
739 // used for diagnostic output.
740 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) {
741 ss->print("is_busy: waiters=%d, ", _waiters);
742 if (contentions() > 0) {
743 ss->print("contentions=%d, ", contentions());
744 } else {
745 ss->print("contentions=0");
746 }
747 if (!owner_is_DEFLATER_MARKER()) {
748 ss->print("owner=" INTPTR_FORMAT, p2i(owner_raw()));
749 } else {
750 // We report NULL instead of DEFLATER_MARKER here because is_busy()
751 // ignores DEFLATER_MARKER values.
752 ss->print("owner=" INTPTR_FORMAT, NULL_WORD);
753 }
754 ss->print(", cxq=" INTPTR_FORMAT ", EntryList=" INTPTR_FORMAT, p2i(_cxq),
755 p2i(_EntryList));
756 return ss->base();
757 }
758
759 #define MAX_RECHECK_INTERVAL 1000
760
761 void ObjectMonitor::EnterI(JavaThread* current) {
762 assert(current->thread_state() == _thread_blocked, "invariant");
763
764 // Try the lock - TATAS
765 if (TryLock (current) > 0) {
766 assert(_succ != current, "invariant");
767 assert(owner_raw() == current, "invariant");
768 assert(_Responsible != current, "invariant");
769 return;
770 }
771
772 if (try_set_owner_from(DEFLATER_MARKER, current) == DEFLATER_MARKER) {
773 // Cancelled the in-progress async deflation by changing owner from
774 // DEFLATER_MARKER to current. As part of the contended enter protocol,
775 // contentions was incremented to a positive value before EnterI()
776 // was called and that prevents the deflater thread from winning the
777 // last part of the 2-part async deflation protocol. After EnterI()
778 // returns to enter(), contentions is decremented because the caller
779 // now owns the monitor. We bump contentions an extra time here to
780 // prevent the deflater thread from winning the last part of the
781 // 2-part async deflation protocol after the regular decrement
782 // occurs in enter(). The deflater thread will decrement contentions
783 // after it recognizes that the async deflation was cancelled.
784 add_to_contentions(1);
785 assert(_succ != current, "invariant");
786 assert(_Responsible != current, "invariant");
787 return;
788 }
789
790 assert(InitDone, "Unexpectedly not initialized");
791
792 // We try one round of spinning *before* enqueueing current.
793 //
794 // If the _owner is ready but OFFPROC we could use a YieldTo()
795 // operation to donate the remainder of this thread's quantum
796 // to the owner. This has subtle but beneficial affinity
797 // effects.
798
799 if (TrySpin(current) > 0) {
800 assert(owner_raw() == current, "invariant");
801 assert(_succ != current, "invariant");
802 assert(_Responsible != current, "invariant");
803 return;
804 }
805
806 // The Spin failed -- Enqueue and park the thread ...
807 assert(_succ != current, "invariant");
808 assert(owner_raw() != current, "invariant");
809 assert(_Responsible != current, "invariant");
810
811 // Enqueue "current" on ObjectMonitor's _cxq.
812 //
813 // Node acts as a proxy for current.
814 // As an aside, if were to ever rewrite the synchronization code mostly
815 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
816 // Java objects. This would avoid awkward lifecycle and liveness issues,
817 // as well as eliminate a subset of ABA issues.
818 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
819
820 ObjectWaiter node(current);
821 current->_ParkEvent->reset();
822 node._prev = (ObjectWaiter*) 0xBAD;
823 node.TState = ObjectWaiter::TS_CXQ;
824
825 // Push "current" onto the front of the _cxq.
826 // Once on cxq/EntryList, current stays on-queue until it acquires the lock.
827 // Note that spinning tends to reduce the rate at which threads
828 // enqueue and dequeue on EntryList|cxq.
829 ObjectWaiter* nxt;
830 for (;;) {
831 node._next = nxt = _cxq;
832 if (Atomic::cmpxchg(&_cxq, nxt, &node) == nxt) break;
833
834 // Interference - the CAS failed because _cxq changed. Just retry.
835 // As an optional optimization we retry the lock.
836 if (TryLock (current) > 0) {
837 assert(_succ != current, "invariant");
838 assert(owner_raw() == current, "invariant");
839 assert(_Responsible != current, "invariant");
840 return;
841 }
842 }
843
844 // Check for cxq|EntryList edge transition to non-null. This indicates
845 // the onset of contention. While contention persists exiting threads
846 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
847 // operations revert to the faster 1-0 mode. This enter operation may interleave
848 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
849 // arrange for one of the contending thread to use a timed park() operations
850 // to detect and recover from the race. (Stranding is form of progress failure
851 // where the monitor is unlocked but all the contending threads remain parked).
852 // That is, at least one of the contended threads will periodically poll _owner.
853 // One of the contending threads will become the designated "Responsible" thread.
854 // The Responsible thread uses a timed park instead of a normal indefinite park
855 // operation -- it periodically wakes and checks for and recovers from potential
856 // strandings admitted by 1-0 exit operations. We need at most one Responsible
857 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
858 // be responsible for a monitor.
859 //
860 // Currently, one of the contended threads takes on the added role of "Responsible".
861 // A viable alternative would be to use a dedicated "stranding checker" thread
862 // that periodically iterated over all the threads (or active monitors) and unparked
863 // successors where there was risk of stranding. This would help eliminate the
864 // timer scalability issues we see on some platforms as we'd only have one thread
865 // -- the checker -- parked on a timer.
866
867 if (nxt == NULL && _EntryList == NULL) {
868 // Try to assume the role of responsible thread for the monitor.
869 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=current }
870 Atomic::replace_if_null(&_Responsible, current);
871 }
872
873 // The lock might have been released while this thread was occupied queueing
874 // itself onto _cxq. To close the race and avoid "stranding" and
875 // progress-liveness failure we must resample-retry _owner before parking.
876 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
877 // In this case the ST-MEMBAR is accomplished with CAS().
878 //
879 // TODO: Defer all thread state transitions until park-time.
880 // Since state transitions are heavy and inefficient we'd like
881 // to defer the state transitions until absolutely necessary,
882 // and in doing so avoid some transitions ...
883
884 int nWakeups = 0;
885 int recheckInterval = 1;
886
887 for (;;) {
888
889 if (TryLock(current) > 0) break;
890 assert(owner_raw() != current, "invariant");
891
892 // park self
893 if (_Responsible == current) {
894 current->_ParkEvent->park((jlong) recheckInterval);
895 // Increase the recheckInterval, but clamp the value.
896 recheckInterval *= 8;
897 if (recheckInterval > MAX_RECHECK_INTERVAL) {
898 recheckInterval = MAX_RECHECK_INTERVAL;
899 }
900 } else {
901 current->_ParkEvent->park();
902 }
903
904 if (TryLock(current) > 0) break;
905
906 if (try_set_owner_from(DEFLATER_MARKER, current) == DEFLATER_MARKER) {
907 // Cancelled the in-progress async deflation by changing owner from
908 // DEFLATER_MARKER to current. As part of the contended enter protocol,
909 // contentions was incremented to a positive value before EnterI()
910 // was called and that prevents the deflater thread from winning the
911 // last part of the 2-part async deflation protocol. After EnterI()
912 // returns to enter(), contentions is decremented because the caller
913 // now owns the monitor. We bump contentions an extra time here to
914 // prevent the deflater thread from winning the last part of the
915 // 2-part async deflation protocol after the regular decrement
916 // occurs in enter(). The deflater thread will decrement contentions
917 // after it recognizes that the async deflation was cancelled.
918 add_to_contentions(1);
919 break;
920 }
921
922 // The lock is still contested.
923 // Keep a tally of the # of futile wakeups.
924 // Note that the counter is not protected by a lock or updated by atomics.
925 // That is by design - we trade "lossy" counters which are exposed to
926 // races during updates for a lower probe effect.
927
928 // This PerfData object can be used in parallel with a safepoint.
929 // See the work around in PerfDataManager::destroy().
930 OM_PERFDATA_OP(FutileWakeups, inc());
931 ++nWakeups;
932
933 // Assuming this is not a spurious wakeup we'll normally find _succ == current.
934 // We can defer clearing _succ until after the spin completes
935 // TrySpin() must tolerate being called with _succ == current.
936 // Try yet another round of adaptive spinning.
937 if (TrySpin(current) > 0) break;
938
939 // We can find that we were unpark()ed and redesignated _succ while
940 // we were spinning. That's harmless. If we iterate and call park(),
941 // park() will consume the event and return immediately and we'll
942 // just spin again. This pattern can repeat, leaving _succ to simply
943 // spin on a CPU.
944
945 if (_succ == current) _succ = NULL;
946
947 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
948 OrderAccess::fence();
949 }
950
951 // Egress :
952 // current has acquired the lock -- Unlink current from the cxq or EntryList.
953 // Normally we'll find current on the EntryList .
954 // From the perspective of the lock owner (this thread), the
955 // EntryList is stable and cxq is prepend-only.
956 // The head of cxq is volatile but the interior is stable.
957 // In addition, current.TState is stable.
958
959 assert(owner_raw() == current, "invariant");
960
961 UnlinkAfterAcquire(current, &node);
962 if (_succ == current) _succ = NULL;
963
964 assert(_succ != current, "invariant");
965 if (_Responsible == current) {
966 _Responsible = NULL;
967 OrderAccess::fence(); // Dekker pivot-point
968
969 // We may leave threads on cxq|EntryList without a designated
970 // "Responsible" thread. This is benign. When this thread subsequently
971 // exits the monitor it can "see" such preexisting "old" threads --
972 // threads that arrived on the cxq|EntryList before the fence, above --
973 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
974 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
975 // non-null and elect a new "Responsible" timer thread.
976 //
977 // This thread executes:
978 // ST Responsible=null; MEMBAR (in enter epilogue - here)
979 // LD cxq|EntryList (in subsequent exit)
980 //
981 // Entering threads in the slow/contended path execute:
982 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
983 // The (ST cxq; MEMBAR) is accomplished with CAS().
984 //
985 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
986 // exit operation from floating above the ST Responsible=null.
987 }
988
989 // We've acquired ownership with CAS().
990 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
991 // But since the CAS() this thread may have also stored into _succ,
992 // EntryList, cxq or Responsible. These meta-data updates must be
993 // visible __before this thread subsequently drops the lock.
994 // Consider what could occur if we didn't enforce this constraint --
995 // STs to monitor meta-data and user-data could reorder with (become
996 // visible after) the ST in exit that drops ownership of the lock.
997 // Some other thread could then acquire the lock, but observe inconsistent
998 // or old monitor meta-data and heap data. That violates the JMM.
999 // To that end, the 1-0 exit() operation must have at least STST|LDST
1000 // "release" barrier semantics. Specifically, there must be at least a
1001 // STST|LDST barrier in exit() before the ST of null into _owner that drops
1002 // the lock. The barrier ensures that changes to monitor meta-data and data
1003 // protected by the lock will be visible before we release the lock, and
1004 // therefore before some other thread (CPU) has a chance to acquire the lock.
1005 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
1006 //
1007 // Critically, any prior STs to _succ or EntryList must be visible before
1008 // the ST of null into _owner in the *subsequent* (following) corresponding
1009 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
1010 // execute a serializing instruction.
1011
1012 return;
1013 }
1014
1015 // ReenterI() is a specialized inline form of the latter half of the
1016 // contended slow-path from EnterI(). We use ReenterI() only for
1017 // monitor reentry in wait().
1018 //
1019 // In the future we should reconcile EnterI() and ReenterI().
1020
1021 void ObjectMonitor::ReenterI(JavaThread* current, ObjectWaiter* currentNode) {
1022 assert(current != NULL, "invariant");
1023 assert(currentNode != NULL, "invariant");
1024 assert(currentNode->_thread == current, "invariant");
1025 assert(_waiters > 0, "invariant");
1026 assert(object()->mark() == markWord::encode(this), "invariant");
1027
1028 assert(current->thread_state() != _thread_blocked, "invariant");
1029
1030 int nWakeups = 0;
1031 for (;;) {
1032 ObjectWaiter::TStates v = currentNode->TState;
1033 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1034 assert(owner_raw() != current, "invariant");
1035
1036 if (TryLock(current) > 0) break;
1037 if (TrySpin(current) > 0) break;
1038
1039 {
1040 OSThreadContendState osts(current->osthread());
1041
1042 assert(current->thread_state() == _thread_in_vm, "invariant");
1043
1044 {
1045 ClearSuccOnSuspend csos(this);
1046 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos);
1047 current->_ParkEvent->park();
1048 }
1049 }
1050
1051 // Try again, but just so we distinguish between futile wakeups and
1052 // successful wakeups. The following test isn't algorithmically
1053 // necessary, but it helps us maintain sensible statistics.
1054 if (TryLock(current) > 0) break;
1055
1056 // The lock is still contested.
1057 // Keep a tally of the # of futile wakeups.
1058 // Note that the counter is not protected by a lock or updated by atomics.
1059 // That is by design - we trade "lossy" counters which are exposed to
1060 // races during updates for a lower probe effect.
1061 ++nWakeups;
1062
1063 // Assuming this is not a spurious wakeup we'll normally
1064 // find that _succ == current.
1065 if (_succ == current) _succ = NULL;
1066
1067 // Invariant: after clearing _succ a contending thread
1068 // *must* retry _owner before parking.
1069 OrderAccess::fence();
1070
1071 // This PerfData object can be used in parallel with a safepoint.
1072 // See the work around in PerfDataManager::destroy().
1073 OM_PERFDATA_OP(FutileWakeups, inc());
1074 }
1075
1076 // current has acquired the lock -- Unlink current from the cxq or EntryList .
1077 // Normally we'll find current on the EntryList.
1078 // Unlinking from the EntryList is constant-time and atomic-free.
1079 // From the perspective of the lock owner (this thread), the
1080 // EntryList is stable and cxq is prepend-only.
1081 // The head of cxq is volatile but the interior is stable.
1082 // In addition, current.TState is stable.
1083
1084 assert(owner_raw() == current, "invariant");
1085 assert(object()->mark() == markWord::encode(this), "invariant");
1086 UnlinkAfterAcquire(current, currentNode);
1087 if (_succ == current) _succ = NULL;
1088 assert(_succ != current, "invariant");
1089 currentNode->TState = ObjectWaiter::TS_RUN;
1090 OrderAccess::fence(); // see comments at the end of EnterI()
1091 }
1092
1093 // By convention we unlink a contending thread from EntryList|cxq immediately
1094 // after the thread acquires the lock in ::enter(). Equally, we could defer
1095 // unlinking the thread until ::exit()-time.
1096
1097 void ObjectMonitor::UnlinkAfterAcquire(JavaThread* current, ObjectWaiter* currentNode) {
1098 assert(owner_raw() == current, "invariant");
1099 assert(currentNode->_thread == current, "invariant");
1100
1101 if (currentNode->TState == ObjectWaiter::TS_ENTER) {
1102 // Normal case: remove current from the DLL EntryList .
1103 // This is a constant-time operation.
1104 ObjectWaiter* nxt = currentNode->_next;
1105 ObjectWaiter* prv = currentNode->_prev;
1106 if (nxt != NULL) nxt->_prev = prv;
1107 if (prv != NULL) prv->_next = nxt;
1108 if (currentNode == _EntryList) _EntryList = nxt;
1109 assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
1110 assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
1111 } else {
1112 assert(currentNode->TState == ObjectWaiter::TS_CXQ, "invariant");
1113 // Inopportune interleaving -- current is still on the cxq.
1114 // This usually means the enqueue of self raced an exiting thread.
1115 // Normally we'll find current near the front of the cxq, so
1116 // dequeueing is typically fast. If needbe we can accelerate
1117 // this with some MCS/CHL-like bidirectional list hints and advisory
1118 // back-links so dequeueing from the interior will normally operate
1119 // in constant-time.
1120 // Dequeue current from either the head (with CAS) or from the interior
1121 // with a linear-time scan and normal non-atomic memory operations.
1122 // CONSIDER: if current is on the cxq then simply drain cxq into EntryList
1123 // and then unlink current from EntryList. We have to drain eventually,
1124 // so it might as well be now.
1125
1126 ObjectWaiter* v = _cxq;
1127 assert(v != NULL, "invariant");
1128 if (v != currentNode || Atomic::cmpxchg(&_cxq, v, currentNode->_next) != v) {
1129 // The CAS above can fail from interference IFF a "RAT" arrived.
1130 // In that case current must be in the interior and can no longer be
1131 // at the head of cxq.
1132 if (v == currentNode) {
1133 assert(_cxq != v, "invariant");
1134 v = _cxq; // CAS above failed - start scan at head of list
1135 }
1136 ObjectWaiter* p;
1137 ObjectWaiter* q = NULL;
1138 for (p = v; p != NULL && p != currentNode; p = p->_next) {
1139 q = p;
1140 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
1141 }
1142 assert(v != currentNode, "invariant");
1143 assert(p == currentNode, "Node not found on cxq");
1144 assert(p != _cxq, "invariant");
1145 assert(q != NULL, "invariant");
1146 assert(q->_next == p, "invariant");
1147 q->_next = p->_next;
1148 }
1149 }
1150
1151 #ifdef ASSERT
1152 // Diagnostic hygiene ...
1153 currentNode->_prev = (ObjectWaiter*) 0xBAD;
1154 currentNode->_next = (ObjectWaiter*) 0xBAD;
1155 currentNode->TState = ObjectWaiter::TS_RUN;
1156 #endif
1157 }
1158
1159 // -----------------------------------------------------------------------------
1160 // Exit support
1161 //
1162 // exit()
1163 // ~~~~~~
1164 // Note that the collector can't reclaim the objectMonitor or deflate
1165 // the object out from underneath the thread calling ::exit() as the
1166 // thread calling ::exit() never transitions to a stable state.
1167 // This inhibits GC, which in turn inhibits asynchronous (and
1168 // inopportune) reclamation of "this".
1169 //
1170 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
1171 // There's one exception to the claim above, however. EnterI() can call
1172 // exit() to drop a lock if the acquirer has been externally suspended.
1173 // In that case exit() is called with _thread_state == _thread_blocked,
1174 // but the monitor's _contentions field is > 0, which inhibits reclamation.
1175 //
1176 // 1-0 exit
1177 // ~~~~~~~~
1178 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
1179 // the fast-path operators have been optimized so the common ::exit()
1180 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
1181 // The code emitted by fast_unlock() elides the usual MEMBAR. This
1182 // greatly improves latency -- MEMBAR and CAS having considerable local
1183 // latency on modern processors -- but at the cost of "stranding". Absent the
1184 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
1185 // ::enter() path, resulting in the entering thread being stranding
1186 // and a progress-liveness failure. Stranding is extremely rare.
1187 // We use timers (timed park operations) & periodic polling to detect
1188 // and recover from stranding. Potentially stranded threads periodically
1189 // wake up and poll the lock. See the usage of the _Responsible variable.
1190 //
1191 // The CAS() in enter provides for safety and exclusion, while the CAS or
1192 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
1193 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
1194 // We detect and recover from stranding with timers.
1195 //
1196 // If a thread transiently strands it'll park until (a) another
1197 // thread acquires the lock and then drops the lock, at which time the
1198 // exiting thread will notice and unpark the stranded thread, or, (b)
1199 // the timer expires. If the lock is high traffic then the stranding latency
1200 // will be low due to (a). If the lock is low traffic then the odds of
1201 // stranding are lower, although the worst-case stranding latency
1202 // is longer. Critically, we don't want to put excessive load in the
1203 // platform's timer subsystem. We want to minimize both the timer injection
1204 // rate (timers created/sec) as well as the number of timers active at
1205 // any one time. (more precisely, we want to minimize timer-seconds, which is
1206 // the integral of the # of active timers at any instant over time).
1207 // Both impinge on OS scalability. Given that, at most one thread parked on
1208 // a monitor will use a timer.
1209 //
1210 // There is also the risk of a futile wake-up. If we drop the lock
1211 // another thread can reacquire the lock immediately, and we can
1212 // then wake a thread unnecessarily. This is benign, and we've
1213 // structured the code so the windows are short and the frequency
1214 // of such futile wakups is low.
1215
1216 void ObjectMonitor::exit(JavaThread* current, bool not_suspended) {
1217 void* cur = owner_raw();
1218 if (current != cur) {
1219 if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) {
1220 assert(_recursions == 0, "invariant");
1221 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*.
1222 _recursions = 0;
1223 } else {
1224 // Apparent unbalanced locking ...
1225 // Naively we'd like to throw IllegalMonitorStateException.
1226 // As a practical matter we can neither allocate nor throw an
1227 // exception as ::exit() can be called from leaf routines.
1228 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
1229 // Upon deeper reflection, however, in a properly run JVM the only
1230 // way we should encounter this situation is in the presence of
1231 // unbalanced JNI locking. TODO: CheckJNICalls.
1232 // See also: CR4414101
1233 #ifdef ASSERT
1234 LogStreamHandle(Error, monitorinflation) lsh;
1235 lsh.print_cr("ERROR: ObjectMonitor::exit(): thread=" INTPTR_FORMAT
1236 " is exiting an ObjectMonitor it does not own.", p2i(current));
1237 lsh.print_cr("The imbalance is possibly caused by JNI locking.");
1238 print_debug_style_on(&lsh);
1239 assert(false, "Non-balanced monitor enter/exit!");
1240 #endif
1241 return;
1242 }
1243 }
1244
1245 if (_recursions != 0) {
1246 _recursions--; // this is simple recursive enter
1247 return;
1248 }
1249
1250 // Invariant: after setting Responsible=null an thread must execute
1251 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
1252 _Responsible = NULL;
1253
1254 #if INCLUDE_JFR
1255 // get the owner's thread id for the MonitorEnter event
1256 // if it is enabled and the thread isn't suspended
1257 if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
1258 _previous_owner_tid = JFR_THREAD_ID(current);
1259 }
1260 #endif
1261
1262 for (;;) {
1263 assert(current == owner_raw(), "invariant");
1264
1265 // Drop the lock.
1266 // release semantics: prior loads and stores from within the critical section
1267 // must not float (reorder) past the following store that drops the lock.
1268 // Uses a storeload to separate release_store(owner) from the
1269 // successor check. The try_set_owner() below uses cmpxchg() so
1270 // we get the fence down there.
1271 release_clear_owner(current);
1272 OrderAccess::storeload();
1273
1274 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1275 return;
1276 }
1277 // Other threads are blocked trying to acquire the lock.
1278
1279 // Normally the exiting thread is responsible for ensuring succession,
1280 // but if other successors are ready or other entering threads are spinning
1281 // then this thread can simply store NULL into _owner and exit without
1282 // waking a successor. The existence of spinners or ready successors
1283 // guarantees proper succession (liveness). Responsibility passes to the
1284 // ready or running successors. The exiting thread delegates the duty.
1285 // More precisely, if a successor already exists this thread is absolved
1286 // of the responsibility of waking (unparking) one.
1287 //
1288 // The _succ variable is critical to reducing futile wakeup frequency.
1289 // _succ identifies the "heir presumptive" thread that has been made
1290 // ready (unparked) but that has not yet run. We need only one such
1291 // successor thread to guarantee progress.
1292 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1293 // section 3.3 "Futile Wakeup Throttling" for details.
1294 //
1295 // Note that spinners in Enter() also set _succ non-null.
1296 // In the current implementation spinners opportunistically set
1297 // _succ so that exiting threads might avoid waking a successor.
1298 // Another less appealing alternative would be for the exiting thread
1299 // to drop the lock and then spin briefly to see if a spinner managed
1300 // to acquire the lock. If so, the exiting thread could exit
1301 // immediately without waking a successor, otherwise the exiting
1302 // thread would need to dequeue and wake a successor.
1303 // (Note that we'd need to make the post-drop spin short, but no
1304 // shorter than the worst-case round-trip cache-line migration time.
1305 // The dropped lock needs to become visible to the spinner, and then
1306 // the acquisition of the lock by the spinner must become visible to
1307 // the exiting thread).
1308
1309 // It appears that an heir-presumptive (successor) must be made ready.
1310 // Only the current lock owner can manipulate the EntryList or
1311 // drain _cxq, so we need to reacquire the lock. If we fail
1312 // to reacquire the lock the responsibility for ensuring succession
1313 // falls to the new owner.
1314 //
1315 if (try_set_owner_from(NULL, current) != NULL) {
1316 return;
1317 }
1318
1319 guarantee(owner_raw() == current, "invariant");
1320
1321 ObjectWaiter* w = NULL;
1322
1323 w = _EntryList;
1324 if (w != NULL) {
1325 // I'd like to write: guarantee (w->_thread != current).
1326 // But in practice an exiting thread may find itself on the EntryList.
1327 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1328 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1329 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1330 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1331 // release the lock "O". T2 resumes immediately after the ST of null into
1332 // _owner, above. T2 notices that the EntryList is populated, so it
1333 // reacquires the lock and then finds itself on the EntryList.
1334 // Given all that, we have to tolerate the circumstance where "w" is
1335 // associated with current.
1336 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1337 ExitEpilog(current, w);
1338 return;
1339 }
1340
1341 // If we find that both _cxq and EntryList are null then just
1342 // re-run the exit protocol from the top.
1343 w = _cxq;
1344 if (w == NULL) continue;
1345
1346 // Drain _cxq into EntryList - bulk transfer.
1347 // First, detach _cxq.
1348 // The following loop is tantamount to: w = swap(&cxq, NULL)
1349 for (;;) {
1350 assert(w != NULL, "Invariant");
1351 ObjectWaiter* u = Atomic::cmpxchg(&_cxq, w, (ObjectWaiter*)NULL);
1352 if (u == w) break;
1353 w = u;
1354 }
1355
1356 assert(w != NULL, "invariant");
1357 assert(_EntryList == NULL, "invariant");
1358
1359 // Convert the LIFO SLL anchored by _cxq into a DLL.
1360 // The list reorganization step operates in O(LENGTH(w)) time.
1361 // It's critical that this step operate quickly as
1362 // "current" still holds the outer-lock, restricting parallelism
1363 // and effectively lengthening the critical section.
1364 // Invariant: s chases t chases u.
1365 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1366 // we have faster access to the tail.
1367
1368 _EntryList = w;
1369 ObjectWaiter* q = NULL;
1370 ObjectWaiter* p;
1371 for (p = w; p != NULL; p = p->_next) {
1372 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1373 p->TState = ObjectWaiter::TS_ENTER;
1374 p->_prev = q;
1375 q = p;
1376 }
1377
1378 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1379 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1380
1381 // See if we can abdicate to a spinner instead of waking a thread.
1382 // A primary goal of the implementation is to reduce the
1383 // context-switch rate.
1384 if (_succ != NULL) continue;
1385
1386 w = _EntryList;
1387 if (w != NULL) {
1388 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1389 ExitEpilog(current, w);
1390 return;
1391 }
1392 }
1393 }
1394
1395 void ObjectMonitor::ExitEpilog(JavaThread* current, ObjectWaiter* Wakee) {
1396 assert(owner_raw() == current, "invariant");
1397
1398 // Exit protocol:
1399 // 1. ST _succ = wakee
1400 // 2. membar #loadstore|#storestore;
1401 // 2. ST _owner = NULL
1402 // 3. unpark(wakee)
1403
1404 _succ = Wakee->_thread;
1405 ParkEvent * Trigger = Wakee->_event;
1406
1407 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1408 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1409 // out-of-scope (non-extant).
1410 Wakee = NULL;
1411
1412 // Drop the lock.
1413 // Uses a fence to separate release_store(owner) from the LD in unpark().
1414 release_clear_owner(current);
1415 OrderAccess::fence();
1416
1417 DTRACE_MONITOR_PROBE(contended__exit, this, object(), current);
1418 Trigger->unpark();
1419
1420 // Maintain stats and report events to JVMTI
1421 OM_PERFDATA_OP(Parks, inc());
1422 }
1423
1424
1425 // -----------------------------------------------------------------------------
1426 // Class Loader deadlock handling.
1427 //
1428 // complete_exit exits a lock returning recursion count
1429 // complete_exit/reenter operate as a wait without waiting
1430 // complete_exit requires an inflated monitor
1431 // The _owner field is not always the Thread addr even with an
1432 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1433 // thread due to contention.
1434 intx ObjectMonitor::complete_exit(JavaThread* current) {
1435 assert(InitDone, "Unexpectedly not initialized");
1436
1437 void* cur = owner_raw();
1438 if (current != cur) {
1439 if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) {
1440 assert(_recursions == 0, "internal state error");
1441 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*.
1442 _recursions = 0;
1443 }
1444 }
1445
1446 guarantee(current == owner_raw(), "complete_exit not owner");
1447 intx save = _recursions; // record the old recursion count
1448 _recursions = 0; // set the recursion level to be 0
1449 exit(current); // exit the monitor
1450 guarantee(owner_raw() != current, "invariant");
1451 return save;
1452 }
1453
1454 // reenter() enters a lock and sets recursion count
1455 // complete_exit/reenter operate as a wait without waiting
1456 bool ObjectMonitor::reenter(intx recursions, JavaThread* current) {
1457
1458 guarantee(owner_raw() != current, "reenter already owner");
1459 if (!enter(current)) {
1460 return false;
1461 }
1462 // Entered the monitor.
1463 guarantee(_recursions == 0, "reenter recursion");
1464 _recursions = recursions;
1465 return true;
1466 }
1467
1468 // Checks that the current THREAD owns this monitor and causes an
1469 // immediate return if it doesn't. We don't use the CHECK macro
1470 // because we want the IMSE to be the only exception that is thrown
1471 // from the call site when false is returned. Any other pending
1472 // exception is ignored.
1473 #define CHECK_OWNER() \
1474 do { \
1475 if (!check_owner(THREAD)) { \
1476 assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \
1477 return; \
1478 } \
1479 } while (false)
1480
1481 // Returns true if the specified thread owns the ObjectMonitor.
1482 // Otherwise returns false and throws IllegalMonitorStateException
1483 // (IMSE). If there is a pending exception and the specified thread
1484 // is not the owner, that exception will be replaced by the IMSE.
1485 bool ObjectMonitor::check_owner(TRAPS) {
1486 JavaThread* current = THREAD;
1487 void* cur = owner_raw();
1488 assert(cur != anon_owner_ptr(), "no anon owner here");
1489 if (cur == current) {
1490 return true;
1491 }
1492 if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) {
1493 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*.
1494 _recursions = 0;
1495 return true;
1496 }
1497 THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(),
1498 "current thread is not owner", false);
1499 }
1500
1501 static inline bool is_excluded(const Klass* monitor_klass) {
1502 assert(monitor_klass != nullptr, "invariant");
1503 NOT_JFR_RETURN_(false);
1504 JFR_ONLY(return vmSymbols::jfr_chunk_rotation_monitor() == monitor_klass->name());
1505 }
1506
1507 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1508 ObjectMonitor* monitor,
1509 jlong notifier_tid,
1510 jlong timeout,
1511 bool timedout) {
1512 assert(event != NULL, "invariant");
1513 assert(monitor != NULL, "invariant");
1514 const Klass* monitor_klass = monitor->object()->klass();
1515 if (is_excluded(monitor_klass)) {
1516 return;
1517 }
1518 event->set_monitorClass(monitor_klass);
1519 event->set_timeout(timeout);
1520 // Set an address that is 'unique enough', such that events close in
1521 // time and with the same address are likely (but not guaranteed) to
1522 // belong to the same object.
1523 event->set_address((uintptr_t)monitor);
1524 event->set_notifier(notifier_tid);
1525 event->set_timedOut(timedout);
1526 event->commit();
1527 }
1528
1529 // -----------------------------------------------------------------------------
1530 // Wait/Notify/NotifyAll
1531 //
1532 // Note: a subset of changes to ObjectMonitor::wait()
1533 // will need to be replicated in complete_exit
1534 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1535 JavaThread* current = THREAD;
1536
1537 assert(InitDone, "Unexpectedly not initialized");
1538
1539 CHECK_OWNER(); // Throws IMSE if not owner.
1540
1541 EventJavaMonitorWait event;
1542
1543 // check for a pending interrupt
1544 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1545 // post monitor waited event. Note that this is past-tense, we are done waiting.
1546 if (JvmtiExport::should_post_monitor_waited()) {
1547 // Note: 'false' parameter is passed here because the
1548 // wait was not timed out due to thread interrupt.
1549 JvmtiExport::post_monitor_waited(current, this, false);
1550
1551 // In this short circuit of the monitor wait protocol, the
1552 // current thread never drops ownership of the monitor and
1553 // never gets added to the wait queue so the current thread
1554 // cannot be made the successor. This means that the
1555 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1556 // consume an unpark() meant for the ParkEvent associated with
1557 // this ObjectMonitor.
1558 }
1559 if (event.should_commit()) {
1560 post_monitor_wait_event(&event, this, 0, millis, false);
1561 }
1562 THROW(vmSymbols::java_lang_InterruptedException());
1563 return;
1564 }
1565
1566 assert(current->_Stalled == 0, "invariant");
1567 current->_Stalled = intptr_t(this);
1568 current->set_current_waiting_monitor(this);
1569
1570 // create a node to be put into the queue
1571 // Critically, after we reset() the event but prior to park(), we must check
1572 // for a pending interrupt.
1573 ObjectWaiter node(current);
1574 node.TState = ObjectWaiter::TS_WAIT;
1575 current->_ParkEvent->reset();
1576 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1577
1578 // Enter the waiting queue, which is a circular doubly linked list in this case
1579 // but it could be a priority queue or any data structure.
1580 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1581 // by the the owner of the monitor *except* in the case where park()
1582 // returns because of a timeout of interrupt. Contention is exceptionally rare
1583 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1584
1585 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1586 AddWaiter(&node);
1587 Thread::SpinRelease(&_WaitSetLock);
1588
1589 _Responsible = NULL;
1590
1591 intx save = _recursions; // record the old recursion count
1592 _waiters++; // increment the number of waiters
1593 _recursions = 0; // set the recursion level to be 1
1594 exit(current); // exit the monitor
1595 guarantee(owner_raw() != current, "invariant");
1596
1597 // The thread is on the WaitSet list - now park() it.
1598 // On MP systems it's conceivable that a brief spin before we park
1599 // could be profitable.
1600 //
1601 // TODO-FIXME: change the following logic to a loop of the form
1602 // while (!timeout && !interrupted && _notified == 0) park()
1603
1604 int ret = OS_OK;
1605 int WasNotified = 0;
1606
1607 // Need to check interrupt state whilst still _thread_in_vm
1608 bool interrupted = interruptible && current->is_interrupted(false);
1609
1610 { // State transition wrappers
1611 OSThread* osthread = current->osthread();
1612 OSThreadWaitState osts(osthread, true);
1613
1614 assert(current->thread_state() == _thread_in_vm, "invariant");
1615
1616 {
1617 ClearSuccOnSuspend csos(this);
1618 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos);
1619 if (interrupted || HAS_PENDING_EXCEPTION) {
1620 // Intentionally empty
1621 } else if (node._notified == 0) {
1622 if (millis <= 0) {
1623 current->_ParkEvent->park();
1624 } else {
1625 ret = current->_ParkEvent->park(millis);
1626 }
1627 }
1628 }
1629
1630 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1631 // from the WaitSet to the EntryList.
1632 // See if we need to remove Node from the WaitSet.
1633 // We use double-checked locking to avoid grabbing _WaitSetLock
1634 // if the thread is not on the wait queue.
1635 //
1636 // Note that we don't need a fence before the fetch of TState.
1637 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1638 // written by the is thread. (perhaps the fetch might even be satisfied
1639 // by a look-aside into the processor's own store buffer, although given
1640 // the length of the code path between the prior ST and this load that's
1641 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1642 // then we'll acquire the lock and then re-fetch a fresh TState value.
1643 // That is, we fail toward safety.
1644
1645 if (node.TState == ObjectWaiter::TS_WAIT) {
1646 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1647 if (node.TState == ObjectWaiter::TS_WAIT) {
1648 DequeueSpecificWaiter(&node); // unlink from WaitSet
1649 assert(node._notified == 0, "invariant");
1650 node.TState = ObjectWaiter::TS_RUN;
1651 }
1652 Thread::SpinRelease(&_WaitSetLock);
1653 }
1654
1655 // The thread is now either on off-list (TS_RUN),
1656 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1657 // The Node's TState variable is stable from the perspective of this thread.
1658 // No other threads will asynchronously modify TState.
1659 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1660 OrderAccess::loadload();
1661 if (_succ == current) _succ = NULL;
1662 WasNotified = node._notified;
1663
1664 // Reentry phase -- reacquire the monitor.
1665 // re-enter contended monitor after object.wait().
1666 // retain OBJECT_WAIT state until re-enter successfully completes
1667 // Thread state is thread_in_vm and oop access is again safe,
1668 // although the raw address of the object may have changed.
1669 // (Don't cache naked oops over safepoints, of course).
1670
1671 // post monitor waited event. Note that this is past-tense, we are done waiting.
1672 if (JvmtiExport::should_post_monitor_waited()) {
1673 JvmtiExport::post_monitor_waited(current, this, ret == OS_TIMEOUT);
1674
1675 if (node._notified != 0 && _succ == current) {
1676 // In this part of the monitor wait-notify-reenter protocol it
1677 // is possible (and normal) for another thread to do a fastpath
1678 // monitor enter-exit while this thread is still trying to get
1679 // to the reenter portion of the protocol.
1680 //
1681 // The ObjectMonitor was notified and the current thread is
1682 // the successor which also means that an unpark() has already
1683 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1684 // consume the unpark() that was done when the successor was
1685 // set because the same ParkEvent is shared between Java
1686 // monitors and JVM/TI RawMonitors (for now).
1687 //
1688 // We redo the unpark() to ensure forward progress, i.e., we
1689 // don't want all pending threads hanging (parked) with none
1690 // entering the unlocked monitor.
1691 node._event->unpark();
1692 }
1693 }
1694
1695 if (event.should_commit()) {
1696 post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1697 }
1698
1699 OrderAccess::fence();
1700
1701 assert(current->_Stalled != 0, "invariant");
1702 current->_Stalled = 0;
1703
1704 assert(owner_raw() != current, "invariant");
1705 ObjectWaiter::TStates v = node.TState;
1706 if (v == ObjectWaiter::TS_RUN) {
1707 enter(current);
1708 } else {
1709 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1710 ReenterI(current, &node);
1711 node.wait_reenter_end(this);
1712 }
1713
1714 // current has reacquired the lock.
1715 // Lifecycle - the node representing current must not appear on any queues.
1716 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1717 // want residual elements associated with this thread left on any lists.
1718 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1719 assert(owner_raw() == current, "invariant");
1720 assert(_succ != current, "invariant");
1721 } // OSThreadWaitState()
1722
1723 current->set_current_waiting_monitor(NULL);
1724
1725 guarantee(_recursions == 0, "invariant");
1726 _recursions = save // restore the old recursion count
1727 + JvmtiDeferredUpdates::get_and_reset_relock_count_after_wait(current); // increased by the deferred relock count
1728 _waiters--; // decrement the number of waiters
1729
1730 // Verify a few postconditions
1731 assert(owner_raw() == current, "invariant");
1732 assert(_succ != current, "invariant");
1733 assert(object()->mark() == markWord::encode(this), "invariant");
1734
1735 // check if the notification happened
1736 if (!WasNotified) {
1737 // no, it could be timeout or Thread.interrupt() or both
1738 // check for interrupt event, otherwise it is timeout
1739 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1740 THROW(vmSymbols::java_lang_InterruptedException());
1741 }
1742 }
1743
1744 // NOTE: Spurious wake up will be consider as timeout.
1745 // Monitor notify has precedence over thread interrupt.
1746 }
1747
1748
1749 // Consider:
1750 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1751 // then instead of transferring a thread from the WaitSet to the EntryList
1752 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1753
1754 void ObjectMonitor::INotify(JavaThread* current) {
1755 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1756 ObjectWaiter* iterator = DequeueWaiter();
1757 if (iterator != NULL) {
1758 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1759 guarantee(iterator->_notified == 0, "invariant");
1760 // Disposition - what might we do with iterator ?
1761 // a. add it directly to the EntryList - either tail (policy == 1)
1762 // or head (policy == 0).
1763 // b. push it onto the front of the _cxq (policy == 2).
1764 // For now we use (b).
1765
1766 iterator->TState = ObjectWaiter::TS_ENTER;
1767
1768 iterator->_notified = 1;
1769 iterator->_notifier_tid = JFR_THREAD_ID(current);
1770
1771 ObjectWaiter* list = _EntryList;
1772 if (list != NULL) {
1773 assert(list->_prev == NULL, "invariant");
1774 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1775 assert(list != iterator, "invariant");
1776 }
1777
1778 // prepend to cxq
1779 if (list == NULL) {
1780 iterator->_next = iterator->_prev = NULL;
1781 _EntryList = iterator;
1782 } else {
1783 iterator->TState = ObjectWaiter::TS_CXQ;
1784 for (;;) {
1785 ObjectWaiter* front = _cxq;
1786 iterator->_next = front;
1787 if (Atomic::cmpxchg(&_cxq, front, iterator) == front) {
1788 break;
1789 }
1790 }
1791 }
1792
1793 // _WaitSetLock protects the wait queue, not the EntryList. We could
1794 // move the add-to-EntryList operation, above, outside the critical section
1795 // protected by _WaitSetLock. In practice that's not useful. With the
1796 // exception of wait() timeouts and interrupts the monitor owner
1797 // is the only thread that grabs _WaitSetLock. There's almost no contention
1798 // on _WaitSetLock so it's not profitable to reduce the length of the
1799 // critical section.
1800
1801 iterator->wait_reenter_begin(this);
1802 }
1803 Thread::SpinRelease(&_WaitSetLock);
1804 }
1805
1806 // Consider: a not-uncommon synchronization bug is to use notify() when
1807 // notifyAll() is more appropriate, potentially resulting in stranded
1808 // threads; this is one example of a lost wakeup. A useful diagnostic
1809 // option is to force all notify() operations to behave as notifyAll().
1810 //
1811 // Note: We can also detect many such problems with a "minimum wait".
1812 // When the "minimum wait" is set to a small non-zero timeout value
1813 // and the program does not hang whereas it did absent "minimum wait",
1814 // that suggests a lost wakeup bug.
1815
1816 void ObjectMonitor::notify(TRAPS) {
1817 JavaThread* current = THREAD;
1818 CHECK_OWNER(); // Throws IMSE if not owner.
1819 if (_WaitSet == NULL) {
1820 return;
1821 }
1822 DTRACE_MONITOR_PROBE(notify, this, object(), current);
1823 INotify(current);
1824 OM_PERFDATA_OP(Notifications, inc(1));
1825 }
1826
1827
1828 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1829 // from the waitset to the EntryList. This could be done more efficiently with a
1830 // single bulk transfer but in practice it's not time-critical. Beware too,
1831 // that in prepend-mode we invert the order of the waiters. Let's say that the
1832 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1833 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1834
1835 void ObjectMonitor::notifyAll(TRAPS) {
1836 JavaThread* current = THREAD;
1837 CHECK_OWNER(); // Throws IMSE if not owner.
1838 if (_WaitSet == NULL) {
1839 return;
1840 }
1841
1842 DTRACE_MONITOR_PROBE(notifyAll, this, object(), current);
1843 int tally = 0;
1844 while (_WaitSet != NULL) {
1845 tally++;
1846 INotify(current);
1847 }
1848
1849 OM_PERFDATA_OP(Notifications, inc(tally));
1850 }
1851
1852 // -----------------------------------------------------------------------------
1853 // Adaptive Spinning Support
1854 //
1855 // Adaptive spin-then-block - rational spinning
1856 //
1857 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1858 // algorithm. On high order SMP systems it would be better to start with
1859 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1860 // a contending thread could enqueue itself on the cxq and then spin locally
1861 // on a thread-specific variable such as its ParkEvent._Event flag.
1862 // That's left as an exercise for the reader. Note that global spinning is
1863 // not problematic on Niagara, as the L2 cache serves the interconnect and
1864 // has both low latency and massive bandwidth.
1865 //
1866 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1867 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1868 // (duration) or we can fix the count at approximately the duration of
1869 // a context switch and vary the frequency. Of course we could also
1870 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1871 // For a description of 'Adaptive spin-then-block mutual exclusion in
1872 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1873 //
1874 // This implementation varies the duration "D", where D varies with
1875 // the success rate of recent spin attempts. (D is capped at approximately
1876 // length of a round-trip context switch). The success rate for recent
1877 // spin attempts is a good predictor of the success rate of future spin
1878 // attempts. The mechanism adapts automatically to varying critical
1879 // section length (lock modality), system load and degree of parallelism.
1880 // D is maintained per-monitor in _SpinDuration and is initialized
1881 // optimistically. Spin frequency is fixed at 100%.
1882 //
1883 // Note that _SpinDuration is volatile, but we update it without locks
1884 // or atomics. The code is designed so that _SpinDuration stays within
1885 // a reasonable range even in the presence of races. The arithmetic
1886 // operations on _SpinDuration are closed over the domain of legal values,
1887 // so at worst a race will install and older but still legal value.
1888 // At the very worst this introduces some apparent non-determinism.
1889 // We might spin when we shouldn't or vice-versa, but since the spin
1890 // count are relatively short, even in the worst case, the effect is harmless.
1891 //
1892 // Care must be taken that a low "D" value does not become an
1893 // an absorbing state. Transient spinning failures -- when spinning
1894 // is overall profitable -- should not cause the system to converge
1895 // on low "D" values. We want spinning to be stable and predictable
1896 // and fairly responsive to change and at the same time we don't want
1897 // it to oscillate, become metastable, be "too" non-deterministic,
1898 // or converge on or enter undesirable stable absorbing states.
1899 //
1900 // We implement a feedback-based control system -- using past behavior
1901 // to predict future behavior. We face two issues: (a) if the
1902 // input signal is random then the spin predictor won't provide optimal
1903 // results, and (b) if the signal frequency is too high then the control
1904 // system, which has some natural response lag, will "chase" the signal.
1905 // (b) can arise from multimodal lock hold times. Transient preemption
1906 // can also result in apparent bimodal lock hold times.
1907 // Although sub-optimal, neither condition is particularly harmful, as
1908 // in the worst-case we'll spin when we shouldn't or vice-versa.
1909 // The maximum spin duration is rather short so the failure modes aren't bad.
1910 // To be conservative, I've tuned the gain in system to bias toward
1911 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1912 // "rings" or oscillates between spinning and not spinning. This happens
1913 // when spinning is just on the cusp of profitability, however, so the
1914 // situation is not dire. The state is benign -- there's no need to add
1915 // hysteresis control to damp the transition rate between spinning and
1916 // not spinning.
1917
1918 // Spinning: Fixed frequency (100%), vary duration
1919 int ObjectMonitor::TrySpin(JavaThread* current) {
1920 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1921 int ctr = Knob_FixedSpin;
1922 if (ctr != 0) {
1923 while (--ctr >= 0) {
1924 if (TryLock(current) > 0) return 1;
1925 SpinPause();
1926 }
1927 return 0;
1928 }
1929
1930 for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1931 if (TryLock(current) > 0) {
1932 // Increase _SpinDuration ...
1933 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1934 // Raising _SpurDuration to the poverty line is key.
1935 int x = _SpinDuration;
1936 if (x < Knob_SpinLimit) {
1937 if (x < Knob_Poverty) x = Knob_Poverty;
1938 _SpinDuration = x + Knob_BonusB;
1939 }
1940 return 1;
1941 }
1942 SpinPause();
1943 }
1944
1945 // Admission control - verify preconditions for spinning
1946 //
1947 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1948 // becoming an absorbing state. Put another way, we spin briefly to
1949 // sample, just in case the system load, parallelism, contention, or lock
1950 // modality changed.
1951 //
1952 // Consider the following alternative:
1953 // Periodically set _SpinDuration = _SpinLimit and try a long/full
1954 // spin attempt. "Periodically" might mean after a tally of
1955 // the # of failed spin attempts (or iterations) reaches some threshold.
1956 // This takes us into the realm of 1-out-of-N spinning, where we
1957 // hold the duration constant but vary the frequency.
1958
1959 ctr = _SpinDuration;
1960 if (ctr <= 0) return 0;
1961
1962 if (NotRunnable(current, (JavaThread*) owner_raw())) {
1963 return 0;
1964 }
1965
1966 // We're good to spin ... spin ingress.
1967 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1968 // when preparing to LD...CAS _owner, etc and the CAS is likely
1969 // to succeed.
1970 if (_succ == NULL) {
1971 _succ = current;
1972 }
1973 Thread* prv = NULL;
1974
1975 // There are three ways to exit the following loop:
1976 // 1. A successful spin where this thread has acquired the lock.
1977 // 2. Spin failure with prejudice
1978 // 3. Spin failure without prejudice
1979
1980 while (--ctr >= 0) {
1981
1982 // Periodic polling -- Check for pending GC
1983 // Threads may spin while they're unsafe.
1984 // We don't want spinning threads to delay the JVM from reaching
1985 // a stop-the-world safepoint or to steal cycles from GC.
1986 // If we detect a pending safepoint we abort in order that
1987 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1988 // this thread, if safe, doesn't steal cycles from GC.
1989 // This is in keeping with the "no loitering in runtime" rule.
1990 // We periodically check to see if there's a safepoint pending.
1991 if ((ctr & 0xFF) == 0) {
1992 if (SafepointMechanism::should_process(current)) {
1993 goto Abort; // abrupt spin egress
1994 }
1995 SpinPause();
1996 }
1997
1998 // Probe _owner with TATAS
1999 // If this thread observes the monitor transition or flicker
2000 // from locked to unlocked to locked, then the odds that this
2001 // thread will acquire the lock in this spin attempt go down
2002 // considerably. The same argument applies if the CAS fails
2003 // or if we observe _owner change from one non-null value to
2004 // another non-null value. In such cases we might abort
2005 // the spin without prejudice or apply a "penalty" to the
2006 // spin count-down variable "ctr", reducing it by 100, say.
2007
2008 JavaThread* ox = (JavaThread*) owner_raw();
2009 if (ox == NULL) {
2010 ox = (JavaThread*)try_set_owner_from(NULL, current);
2011 if (ox == NULL) {
2012 // The CAS succeeded -- this thread acquired ownership
2013 // Take care of some bookkeeping to exit spin state.
2014 if (_succ == current) {
2015 _succ = NULL;
2016 }
2017
2018 // Increase _SpinDuration :
2019 // The spin was successful (profitable) so we tend toward
2020 // longer spin attempts in the future.
2021 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2022 // If we acquired the lock early in the spin cycle it
2023 // makes sense to increase _SpinDuration proportionally.
2024 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2025 int x = _SpinDuration;
2026 if (x < Knob_SpinLimit) {
2027 if (x < Knob_Poverty) x = Knob_Poverty;
2028 _SpinDuration = x + Knob_Bonus;
2029 }
2030 return 1;
2031 }
2032
2033 // The CAS failed ... we can take any of the following actions:
2034 // * penalize: ctr -= CASPenalty
2035 // * exit spin with prejudice -- goto Abort;
2036 // * exit spin without prejudice.
2037 // * Since CAS is high-latency, retry again immediately.
2038 prv = ox;
2039 goto Abort;
2040 }
2041
2042 // Did lock ownership change hands ?
2043 if (ox != prv && prv != NULL) {
2044 goto Abort;
2045 }
2046 prv = ox;
2047
2048 // Abort the spin if the owner is not executing.
2049 // The owner must be executing in order to drop the lock.
2050 // Spinning while the owner is OFFPROC is idiocy.
2051 // Consider: ctr -= RunnablePenalty ;
2052 if (NotRunnable(current, ox)) {
2053 goto Abort;
2054 }
2055 if (_succ == NULL) {
2056 _succ = current;
2057 }
2058 }
2059
2060 // Spin failed with prejudice -- reduce _SpinDuration.
2061 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2062 // AIMD is globally stable.
2063 {
2064 int x = _SpinDuration;
2065 if (x > 0) {
2066 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2067 // This is globally sample and tends to damp the response.
2068 x -= Knob_Penalty;
2069 if (x < 0) x = 0;
2070 _SpinDuration = x;
2071 }
2072 }
2073
2074 Abort:
2075 if (_succ == current) {
2076 _succ = NULL;
2077 // Invariant: after setting succ=null a contending thread
2078 // must recheck-retry _owner before parking. This usually happens
2079 // in the normal usage of TrySpin(), but it's safest
2080 // to make TrySpin() as foolproof as possible.
2081 OrderAccess::fence();
2082 if (TryLock(current) > 0) return 1;
2083 }
2084 return 0;
2085 }
2086
2087 // NotRunnable() -- informed spinning
2088 //
2089 // Don't bother spinning if the owner is not eligible to drop the lock.
2090 // Spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2091 // The thread must be runnable in order to drop the lock in timely fashion.
2092 // If the _owner is not runnable then spinning will not likely be
2093 // successful (profitable).
2094 //
2095 // Beware -- the thread referenced by _owner could have died
2096 // so a simply fetch from _owner->_thread_state might trap.
2097 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2098 // Because of the lifecycle issues, the _thread_state values
2099 // observed by NotRunnable() might be garbage. NotRunnable must
2100 // tolerate this and consider the observed _thread_state value
2101 // as advisory.
2102 //
2103 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2104 // a thread pointer.
2105 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2106 // with the LSB of _owner. Another option would be to probabilistically probe
2107 // the putative _owner->TypeTag value.
2108 //
2109 // Checking _thread_state isn't perfect. Even if the thread is
2110 // in_java it might be blocked on a page-fault or have been preempted
2111 // and sitting on a ready/dispatch queue.
2112 //
2113 // The return value from NotRunnable() is *advisory* -- the
2114 // result is based on sampling and is not necessarily coherent.
2115 // The caller must tolerate false-negative and false-positive errors.
2116 // Spinning, in general, is probabilistic anyway.
2117
2118
2119 int ObjectMonitor::NotRunnable(JavaThread* current, JavaThread* ox) {
2120 // Check ox->TypeTag == 2BAD.
2121 if (ox == NULL) return 0;
2122
2123 // Avoid transitive spinning ...
2124 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2125 // Immediately after T1 acquires L it's possible that T2, also
2126 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2127 // This occurs transiently after T1 acquired L but before
2128 // T1 managed to clear T1.Stalled. T2 does not need to abort
2129 // its spin in this circumstance.
2130 intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
2131
2132 if (BlockedOn == 1) return 1;
2133 if (BlockedOn != 0) {
2134 return BlockedOn != intptr_t(this) && owner_raw() == ox;
2135 }
2136
2137 assert(sizeof(ox->_thread_state == sizeof(int)), "invariant");
2138 int jst = SafeFetch32((int *) &ox->_thread_state, -1);;
2139 // consider also: jst != _thread_in_Java -- but that's overspecific.
2140 return jst == _thread_blocked || jst == _thread_in_native;
2141 }
2142
2143
2144 // -----------------------------------------------------------------------------
2145 // WaitSet management ...
2146
2147 ObjectWaiter::ObjectWaiter(JavaThread* current) {
2148 _next = NULL;
2149 _prev = NULL;
2150 _notified = 0;
2151 _notifier_tid = 0;
2152 TState = TS_RUN;
2153 _thread = current;
2154 _event = _thread->_ParkEvent;
2155 _active = false;
2156 assert(_event != NULL, "invariant");
2157 }
2158
2159 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2160 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(_thread, mon);
2161 }
2162
2163 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2164 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(_thread, _active);
2165 }
2166
2167 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2168 assert(node != NULL, "should not add NULL node");
2169 assert(node->_prev == NULL, "node already in list");
2170 assert(node->_next == NULL, "node already in list");
2171 // put node at end of queue (circular doubly linked list)
2172 if (_WaitSet == NULL) {
2173 _WaitSet = node;
2174 node->_prev = node;
2175 node->_next = node;
2176 } else {
2177 ObjectWaiter* head = _WaitSet;
2178 ObjectWaiter* tail = head->_prev;
2179 assert(tail->_next == head, "invariant check");
2180 tail->_next = node;
2181 head->_prev = node;
2182 node->_next = head;
2183 node->_prev = tail;
2184 }
2185 }
2186
2187 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2188 // dequeue the very first waiter
2189 ObjectWaiter* waiter = _WaitSet;
2190 if (waiter) {
2191 DequeueSpecificWaiter(waiter);
2192 }
2193 return waiter;
2194 }
2195
2196 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2197 assert(node != NULL, "should not dequeue NULL node");
2198 assert(node->_prev != NULL, "node already removed from list");
2199 assert(node->_next != NULL, "node already removed from list");
2200 // when the waiter has woken up because of interrupt,
2201 // timeout or other spurious wake-up, dequeue the
2202 // waiter from waiting list
2203 ObjectWaiter* next = node->_next;
2204 if (next == node) {
2205 assert(node->_prev == node, "invariant check");
2206 _WaitSet = NULL;
2207 } else {
2208 ObjectWaiter* prev = node->_prev;
2209 assert(prev->_next == node, "invariant check");
2210 assert(next->_prev == node, "invariant check");
2211 next->_prev = prev;
2212 prev->_next = next;
2213 if (_WaitSet == node) {
2214 _WaitSet = next;
2215 }
2216 }
2217 node->_next = NULL;
2218 node->_prev = NULL;
2219 }
2220
2221 // -----------------------------------------------------------------------------
2222 // PerfData support
2223 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL;
2224 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL;
2225 PerfCounter * ObjectMonitor::_sync_Parks = NULL;
2226 PerfCounter * ObjectMonitor::_sync_Notifications = NULL;
2227 PerfCounter * ObjectMonitor::_sync_Inflations = NULL;
2228 PerfCounter * ObjectMonitor::_sync_Deflations = NULL;
2229 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL;
2230
2231 // One-shot global initialization for the sync subsystem.
2232 // We could also defer initialization and initialize on-demand
2233 // the first time we call ObjectSynchronizer::inflate().
2234 // Initialization would be protected - like so many things - by
2235 // the MonitorCache_lock.
2236
2237 void ObjectMonitor::Initialize() {
2238 assert(!InitDone, "invariant");
2239
2240 if (!os::is_MP()) {
2241 Knob_SpinLimit = 0;
2242 Knob_PreSpin = 0;
2243 Knob_FixedSpin = -1;
2244 }
2245
2246 if (UsePerfData) {
2247 EXCEPTION_MARK;
2248 #define NEWPERFCOUNTER(n) \
2249 { \
2250 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \
2251 CHECK); \
2252 }
2253 #define NEWPERFVARIABLE(n) \
2254 { \
2255 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \
2256 CHECK); \
2257 }
2258 NEWPERFCOUNTER(_sync_Inflations);
2259 NEWPERFCOUNTER(_sync_Deflations);
2260 NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2261 NEWPERFCOUNTER(_sync_FutileWakeups);
2262 NEWPERFCOUNTER(_sync_Parks);
2263 NEWPERFCOUNTER(_sync_Notifications);
2264 NEWPERFVARIABLE(_sync_MonExtant);
2265 #undef NEWPERFCOUNTER
2266 #undef NEWPERFVARIABLE
2267 }
2268
2269 _oop_storage = OopStorageSet::create_weak("ObjectSynchronizer Weak", mtSynchronizer);
2270
2271 DEBUG_ONLY(InitDone = true;)
2272 }
2273
2274 void ObjectMonitor::print_on(outputStream* st) const {
2275 // The minimal things to print for markWord printing, more can be added for debugging and logging.
2276 st->print("{contentions=0x%08x,waiters=0x%08x"
2277 ",recursions=" INTX_FORMAT ",owner=" INTPTR_FORMAT "}",
2278 contentions(), waiters(), recursions(),
2279 p2i(owner()));
2280 }
2281 void ObjectMonitor::print() const { print_on(tty); }
2282
2283 #ifdef ASSERT
2284 // Print the ObjectMonitor like a debugger would:
2285 //
2286 // (ObjectMonitor) 0x00007fdfb6012e40 = {
2287 // _header = 0x0000000000000001
2288 // _object = 0x000000070ff45fd0
2289 // _pad_buf0 = {
2290 // [0] = '\0'
2291 // ...
2292 // [43] = '\0'
2293 // }
2294 // _owner = 0x0000000000000000
2295 // _previous_owner_tid = 0
2296 // _pad_buf1 = {
2297 // [0] = '\0'
2298 // ...
2299 // [47] = '\0'
2300 // }
2301 // _next_om = 0x0000000000000000
2302 // _recursions = 0
2303 // _EntryList = 0x0000000000000000
2304 // _cxq = 0x0000000000000000
2305 // _succ = 0x0000000000000000
2306 // _Responsible = 0x0000000000000000
2307 // _Spinner = 0
2308 // _SpinDuration = 5000
2309 // _contentions = 0
2310 // _WaitSet = 0x0000700009756248
2311 // _waiters = 1
2312 // _WaitSetLock = 0
2313 // }
2314 //
2315 void ObjectMonitor::print_debug_style_on(outputStream* st) const {
2316 st->print_cr("(ObjectMonitor*) " INTPTR_FORMAT " = {", p2i(this));
2317 st->print_cr(" _header = " INTPTR_FORMAT, header().value());
2318 st->print_cr(" _object = " INTPTR_FORMAT, p2i(object_peek()));
2319 st->print_cr(" _pad_buf0 = {");
2320 st->print_cr(" [0] = '\\0'");
2321 st->print_cr(" ...");
2322 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf0) - 1);
2323 st->print_cr(" }");
2324 st->print_cr(" _owner = " INTPTR_FORMAT, p2i(owner_raw()));
2325 st->print_cr(" _previous_owner_tid = " JLONG_FORMAT, _previous_owner_tid);
2326 st->print_cr(" _pad_buf1 = {");
2327 st->print_cr(" [0] = '\\0'");
2328 st->print_cr(" ...");
2329 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf1) - 1);
2330 st->print_cr(" }");
2331 st->print_cr(" _next_om = " INTPTR_FORMAT, p2i(next_om()));
2332 st->print_cr(" _recursions = " INTX_FORMAT, _recursions);
2333 st->print_cr(" _EntryList = " INTPTR_FORMAT, p2i(_EntryList));
2334 st->print_cr(" _cxq = " INTPTR_FORMAT, p2i(_cxq));
2335 st->print_cr(" _succ = " INTPTR_FORMAT, p2i(_succ));
2336 st->print_cr(" _Responsible = " INTPTR_FORMAT, p2i(_Responsible));
2337 st->print_cr(" _Spinner = %d", _Spinner);
2338 st->print_cr(" _SpinDuration = %d", _SpinDuration);
2339 st->print_cr(" _contentions = %d", contentions());
2340 st->print_cr(" _WaitSet = " INTPTR_FORMAT, p2i(_WaitSet));
2341 st->print_cr(" _waiters = %d", _waiters);
2342 st->print_cr(" _WaitSetLock = %d", _WaitSetLock);
2343 st->print_cr("}");
2344 }
2345 #endif