1 /*
2 * Copyright (c) 1998, 2019, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 #include "precompiled.hpp"
26 #include "classfile/vmSymbols.hpp"
27 #include "jfr/jfrEvents.hpp"
28 #include "jfr/support/jfrThreadId.hpp"
29 #include "logging/log.hpp"
30 #include "logging/logStream.hpp"
31 #include "memory/allocation.inline.hpp"
32 #include "memory/resourceArea.hpp"
33 #include "oops/markWord.hpp"
34 #include "oops/oop.inline.hpp"
35 #include "runtime/atomic.hpp"
36 #include "runtime/handles.inline.hpp"
37 #include "runtime/interfaceSupport.inline.hpp"
38 #include "runtime/mutexLocker.hpp"
39 #include "runtime/objectMonitor.hpp"
40 #include "runtime/objectMonitor.inline.hpp"
41 #include "runtime/orderAccess.hpp"
42 #include "runtime/osThread.hpp"
43 #include "runtime/safepointMechanism.inline.hpp"
44 #include "runtime/sharedRuntime.hpp"
45 #include "runtime/stubRoutines.hpp"
46 #include "runtime/thread.inline.hpp"
47 #include "services/threadService.hpp"
48 #include "utilities/dtrace.hpp"
49 #include "utilities/macros.hpp"
50 #include "utilities/preserveException.hpp"
51 #if INCLUDE_JFR
52 #include "jfr/support/jfrFlush.hpp"
53 #endif
54
55 #ifdef DTRACE_ENABLED
56
57 // Only bother with this argument setup if dtrace is available
58 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
59
60
61 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
62 char* bytes = NULL; \
63 int len = 0; \
64 jlong jtid = SharedRuntime::get_java_tid(thread); \
65 Symbol* klassname = ((oop)obj)->klass()->name(); \
66 if (klassname != NULL) { \
67 bytes = (char*)klassname->bytes(); \
68 len = klassname->utf8_length(); \
69 }
70
71 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
72 { \
73 if (DTraceMonitorProbes) { \
74 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
75 HOTSPOT_MONITOR_WAIT(jtid, \
76 (monitor), bytes, len, (millis)); \
77 } \
78 }
79
80 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
81 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
82 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
83 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
84 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
85
86 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
87 { \
88 if (DTraceMonitorProbes) { \
89 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
90 HOTSPOT_MONITOR_##probe(jtid, \
91 (uintptr_t)(monitor), bytes, len); \
92 } \
93 }
94
95 #else // ndef DTRACE_ENABLED
96
97 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
98 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
99
100 #endif // ndef DTRACE_ENABLED
101
102 // Tunables ...
103 // The knob* variables are effectively final. Once set they should
104 // never be modified hence. Consider using __read_mostly with GCC.
105
106 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool -
107
108 static int Knob_Bonus = 100; // spin success bonus
109 static int Knob_BonusB = 100; // spin success bonus
110 static int Knob_Penalty = 200; // spin failure penalty
111 static int Knob_Poverty = 1000;
112 static int Knob_FixedSpin = 0;
113 static int Knob_PreSpin = 10; // 20-100 likely better
114
115 DEBUG_ONLY(static volatile bool InitDone = false;)
116
117 // -----------------------------------------------------------------------------
118 // Theory of operations -- Monitors lists, thread residency, etc:
119 //
120 // * A thread acquires ownership of a monitor by successfully
121 // CAS()ing the _owner field from null to non-null.
122 //
123 // * Invariant: A thread appears on at most one monitor list --
124 // cxq, EntryList or WaitSet -- at any one time.
125 //
126 // * Contending threads "push" themselves onto the cxq with CAS
127 // and then spin/park.
128 //
129 // * After a contending thread eventually acquires the lock it must
130 // dequeue itself from either the EntryList or the cxq.
131 //
132 // * The exiting thread identifies and unparks an "heir presumptive"
133 // tentative successor thread on the EntryList. Critically, the
134 // exiting thread doesn't unlink the successor thread from the EntryList.
135 // After having been unparked, the wakee will recontend for ownership of
136 // the monitor. The successor (wakee) will either acquire the lock or
137 // re-park itself.
138 //
139 // Succession is provided for by a policy of competitive handoff.
140 // The exiting thread does _not_ grant or pass ownership to the
141 // successor thread. (This is also referred to as "handoff" succession").
142 // Instead the exiting thread releases ownership and possibly wakes
143 // a successor, so the successor can (re)compete for ownership of the lock.
144 // If the EntryList is empty but the cxq is populated the exiting
145 // thread will drain the cxq into the EntryList. It does so by
146 // by detaching the cxq (installing null with CAS) and folding
147 // the threads from the cxq into the EntryList. The EntryList is
148 // doubly linked, while the cxq is singly linked because of the
149 // CAS-based "push" used to enqueue recently arrived threads (RATs).
150 //
151 // * Concurrency invariants:
152 //
153 // -- only the monitor owner may access or mutate the EntryList.
154 // The mutex property of the monitor itself protects the EntryList
155 // from concurrent interference.
156 // -- Only the monitor owner may detach the cxq.
157 //
158 // * The monitor entry list operations avoid locks, but strictly speaking
159 // they're not lock-free. Enter is lock-free, exit is not.
160 // For a description of 'Methods and apparatus providing non-blocking access
161 // to a resource,' see U.S. Pat. No. 7844973.
162 //
163 // * The cxq can have multiple concurrent "pushers" but only one concurrent
164 // detaching thread. This mechanism is immune from the ABA corruption.
165 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
166 //
167 // * Taken together, the cxq and the EntryList constitute or form a
168 // single logical queue of threads stalled trying to acquire the lock.
169 // We use two distinct lists to improve the odds of a constant-time
170 // dequeue operation after acquisition (in the ::enter() epilogue) and
171 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
172 // A key desideratum is to minimize queue & monitor metadata manipulation
173 // that occurs while holding the monitor lock -- that is, we want to
174 // minimize monitor lock holds times. Note that even a small amount of
175 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
176 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
177 // locks and monitor metadata.
178 //
179 // Cxq points to the set of Recently Arrived Threads attempting entry.
180 // Because we push threads onto _cxq with CAS, the RATs must take the form of
181 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
182 // the unlocking thread notices that EntryList is null but _cxq is != null.
183 //
184 // The EntryList is ordered by the prevailing queue discipline and
185 // can be organized in any convenient fashion, such as a doubly-linked list or
186 // a circular doubly-linked list. Critically, we want insert and delete operations
187 // to operate in constant-time. If we need a priority queue then something akin
188 // to Solaris' sleepq would work nicely. Viz.,
189 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
190 // Queue discipline is enforced at ::exit() time, when the unlocking thread
191 // drains the cxq into the EntryList, and orders or reorders the threads on the
192 // EntryList accordingly.
193 //
194 // Barring "lock barging", this mechanism provides fair cyclic ordering,
195 // somewhat similar to an elevator-scan.
196 //
197 // * The monitor synchronization subsystem avoids the use of native
198 // synchronization primitives except for the narrow platform-specific
199 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
200 // the semantics of park-unpark. Put another way, this monitor implementation
201 // depends only on atomic operations and park-unpark. The monitor subsystem
202 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
203 // underlying OS manages the READY<->RUN transitions.
204 //
205 // * Waiting threads reside on the WaitSet list -- wait() puts
206 // the caller onto the WaitSet.
207 //
208 // * notify() or notifyAll() simply transfers threads from the WaitSet to
209 // either the EntryList or cxq. Subsequent exit() operations will
210 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
211 // it's likely the notifyee would simply impale itself on the lock held
212 // by the notifier.
213 //
214 // * An interesting alternative is to encode cxq as (List,LockByte) where
215 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
216 // variable, like _recursions, in the scheme. The threads or Events that form
217 // the list would have to be aligned in 256-byte addresses. A thread would
218 // try to acquire the lock or enqueue itself with CAS, but exiting threads
219 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
220 // Note that is is *not* word-tearing, but it does presume that full-word
221 // CAS operations are coherent with intermix with STB operations. That's true
222 // on most common processors.
223 //
224 // * See also http://blogs.sun.com/dave
225
226
227 void* ObjectMonitor::operator new (size_t size) throw() {
228 return AllocateHeap(size, mtInternal);
229 }
230 void* ObjectMonitor::operator new[] (size_t size) throw() {
231 return operator new (size);
232 }
233 void ObjectMonitor::operator delete(void* p) {
234 FreeHeap(p);
235 }
236 void ObjectMonitor::operator delete[] (void *p) {
237 operator delete(p);
238 }
239
240 // -----------------------------------------------------------------------------
241 // Enter support
242
243 void ObjectMonitor::enter(TRAPS) {
244 jint l_ref_count = ref_count();
245 ADIM_guarantee(l_ref_count > 0, "must be positive: l_ref_count=%d, ref_count=%d", l_ref_count, ref_count());
246
247 // The following code is ordered to check the most common cases first
248 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
249 Thread * const Self = THREAD;
250
251 void* cur = try_set_owner_from(NULL, Self);
252 if (cur == NULL) {
253 assert(_recursions == 0, "invariant");
254 return;
255 }
256
257 if (cur == Self) {
258 // TODO-FIXME: check for integer overflow! BUGID 6557169.
259 _recursions++;
260 return;
261 }
262
263 if (Self->is_lock_owned((address)cur)) {
264 assert(_recursions == 0, "internal state error");
265 _recursions = 1;
266 set_owner_from_BasicLock(cur, Self); // Convert from BasicLock* to Thread*.
267 return;
268 }
269
270 if (AsyncDeflateIdleMonitors &&
271 try_set_owner_from(DEFLATER_MARKER, Self) == DEFLATER_MARKER) {
272 // The deflation protocol finished the first part (setting owner),
273 // but it failed the second part (making ref_count negative) and
274 // bailed. Or the ObjectMonitor was async deflated and reused.
275 // Acquired the monitor.
276 assert(_recursions == 0, "invariant");
277 return;
278 }
279
280 // We've encountered genuine contention.
281 assert(Self->_Stalled == 0, "invariant");
282 Self->_Stalled = intptr_t(this);
283
284 // Try one round of spinning *before* enqueueing Self
285 // and before going through the awkward and expensive state
286 // transitions. The following spin is strictly optional ...
287 // Note that if we acquire the monitor from an initial spin
288 // we forgo posting JVMTI events and firing DTRACE probes.
289 if (TrySpin(Self) > 0) {
290 assert(_owner == Self, "must be Self: owner=" INTPTR_FORMAT, p2i(_owner));
291 assert(_recursions == 0, "must be 0: recursions=" INTX_FORMAT, _recursions);
292 assert(((oop)object())->mark() == markWord::encode(this),
293 "object mark must match encoded this: mark=" INTPTR_FORMAT
294 ", encoded this=" INTPTR_FORMAT, ((oop)object())->mark().value(),
295 markWord::encode(this).value());
296 Self->_Stalled = 0;
297 return;
298 }
299
300 assert(_owner != Self, "invariant");
301 assert(_succ != Self, "invariant");
302 assert(Self->is_Java_thread(), "invariant");
303 JavaThread * jt = (JavaThread *) Self;
304 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
305 assert(jt->thread_state() != _thread_blocked, "invariant");
306 assert(this->object() != NULL, "invariant");
307 assert(_contentions >= 0, "must not be negative: contentions=%d", _contentions);
308
309 // Keep track of contention for JVM/TI and M&M queries.
310 Atomic::inc(&_contentions);
311
312 JFR_ONLY(JfrConditionalFlushWithStacktrace<EventJavaMonitorEnter> flush(jt);)
313 EventJavaMonitorEnter event;
314 if (event.should_commit()) {
315 event.set_monitorClass(((oop)this->object())->klass());
316 event.set_address((uintptr_t)(this->object_addr()));
317 }
318
319 { // Change java thread status to indicate blocked on monitor enter.
320 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
321
322 Self->set_current_pending_monitor(this);
323
324 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
325 if (JvmtiExport::should_post_monitor_contended_enter()) {
326 JvmtiExport::post_monitor_contended_enter(jt, this);
327
328 // The current thread does not yet own the monitor and does not
329 // yet appear on any queues that would get it made the successor.
330 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
331 // handler cannot accidentally consume an unpark() meant for the
332 // ParkEvent associated with this ObjectMonitor.
333 }
334
335 OSThreadContendState osts(Self->osthread());
336 ThreadBlockInVM tbivm(jt);
337
338 // TODO-FIXME: change the following for(;;) loop to straight-line code.
339 for (;;) {
340 jt->set_suspend_equivalent();
341 // cleared by handle_special_suspend_equivalent_condition()
342 // or java_suspend_self()
343
344 EnterI(THREAD);
345
346 if (!ExitSuspendEquivalent(jt)) break;
347
348 // We have acquired the contended monitor, but while we were
349 // waiting another thread suspended us. We don't want to enter
350 // the monitor while suspended because that would surprise the
351 // thread that suspended us.
352 //
353 _recursions = 0;
354 _succ = NULL;
355 exit(false, Self);
356
357 jt->java_suspend_self();
358 }
359 Self->set_current_pending_monitor(NULL);
360
361 // We cleared the pending monitor info since we've just gotten past
362 // the enter-check-for-suspend dance and we now own the monitor free
363 // and clear, i.e., it is no longer pending. The ThreadBlockInVM
364 // destructor can go to a safepoint at the end of this block. If we
365 // do a thread dump during that safepoint, then this thread will show
366 // as having "-locked" the monitor, but the OS and java.lang.Thread
367 // states will still report that the thread is blocked trying to
368 // acquire it.
369 }
370
371 Atomic::dec(&_contentions);
372 assert(_contentions >= 0, "must not be negative: contentions=%d", _contentions);
373 Self->_Stalled = 0;
374
375 // Must either set _recursions = 0 or ASSERT _recursions == 0.
376 assert(_recursions == 0, "invariant");
377 assert(_owner == Self, "invariant");
378 assert(_succ != Self, "invariant");
379 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant");
380
381 // The thread -- now the owner -- is back in vm mode.
382 // Report the glorious news via TI,DTrace and jvmstat.
383 // The probe effect is non-trivial. All the reportage occurs
384 // while we hold the monitor, increasing the length of the critical
385 // section. Amdahl's parallel speedup law comes vividly into play.
386 //
387 // Another option might be to aggregate the events (thread local or
388 // per-monitor aggregation) and defer reporting until a more opportune
389 // time -- such as next time some thread encounters contention but has
390 // yet to acquire the lock. While spinning that thread could
391 // spinning we could increment JVMStat counters, etc.
392
393 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
394 if (JvmtiExport::should_post_monitor_contended_entered()) {
395 JvmtiExport::post_monitor_contended_entered(jt, this);
396
397 // The current thread already owns the monitor and is not going to
398 // call park() for the remainder of the monitor enter protocol. So
399 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
400 // event handler consumed an unpark() issued by the thread that
401 // just exited the monitor.
402 }
403 if (event.should_commit()) {
404 event.set_previousOwner((uintptr_t)_previous_owner_tid);
405 event.commit();
406 }
407 OM_PERFDATA_OP(ContendedLockAttempts, inc());
408 }
409
410 // Caveat: TryLock() is not necessarily serializing if it returns failure.
411 // Callers must compensate as needed.
412
413 int ObjectMonitor::TryLock(Thread * Self) {
414 void * own = _owner;
415 if (own != NULL) return 0;
416 if (try_set_owner_from(NULL, Self) == NULL) {
417 assert(_recursions == 0, "invariant");
418 return 1;
419 }
420 // The lock had been free momentarily, but we lost the race to the lock.
421 // Interference -- the CAS failed.
422 // We can either return -1 or retry.
423 // Retry doesn't make as much sense because the lock was just acquired.
424 return -1;
425 }
426
427 // Install the displaced mark word (dmw) of a deflating ObjectMonitor
428 // into the header of the object associated with the monitor. This
429 // idempotent method is called by a thread that is deflating a
430 // monitor and by other threads that have detected a race with the
431 // deflation process.
432 void ObjectMonitor::install_displaced_markword_in_object(const oop obj) {
433 // This function must only be called when (owner == DEFLATER_MARKER
434 // && ref_count <= 0), but we can't guarantee that here because
435 // those values could change when the ObjectMonitor gets moved from
436 // the global free list to a per-thread free list.
437
438 guarantee(obj != NULL, "must be non-NULL");
439 if (object() != obj) {
440 // ObjectMonitor's object ref no longer refers to the target object
441 // so the object's header has already been restored.
442 return;
443 }
444
445 markWord dmw = header();
446 if (dmw.value() == 0) {
447 // ObjectMonitor's header/dmw has been cleared so the ObjectMonitor
448 // has been deflated and taken off the global free list.
449 return;
450 }
451
452 // A non-NULL dmw has to be either neutral (not locked and not marked)
453 // or is already participating in this restoration protocol.
454 assert(dmw.is_neutral() || (dmw.is_marked() && dmw.hash() == 0),
455 "failed precondition: dmw=" INTPTR_FORMAT, dmw.value());
456
457 markWord marked_dmw = markWord::zero();
458 if (!dmw.is_marked() && dmw.hash() == 0) {
459 // This dmw has not yet started the restoration protocol so we
460 // mark a copy of the dmw to begin the protocol.
461 // Note: A dmw with a hashcode does not take this code path.
462 marked_dmw = dmw.set_marked();
463
464 // All of the callers to this function can be racing with each
465 // other trying to update the _header field.
466 dmw = (markWord) Atomic::cmpxchg(&_header, dmw, marked_dmw);
467 if (dmw.value() == 0) {
468 // ObjectMonitor's header/dmw has been cleared so the object's
469 // header has already been restored.
470 return;
471 }
472 // The _header field is now marked. The winner's 'dmw' variable
473 // contains the original, unmarked header/dmw value and any
474 // losers have a marked header/dmw value that will be cleaned
475 // up below.
476 }
477
478 if (dmw.is_marked()) {
479 // Clear the mark from the header/dmw copy in preparation for
480 // possible restoration from this thread.
481 assert(dmw.hash() == 0, "hashcode must be 0: dmw=" INTPTR_FORMAT,
482 dmw.value());
483 dmw = dmw.set_unmarked();
484 }
485 assert(dmw.is_neutral(), "must be neutral: dmw=" INTPTR_FORMAT, dmw.value());
486
487 // Install displaced mark word if the object's header still points
488 // to this ObjectMonitor. All racing callers to this function will
489 // reach this point, but only one can win.
490 obj->cas_set_mark(dmw, markWord::encode(this));
491
492 // Note: It does not matter which thread restored the header/dmw
493 // into the object's header. The thread deflating the monitor just
494 // wanted the object's header restored and it is. The threads that
495 // detected a race with the deflation process also wanted the
496 // object's header restored before they retry their operation and
497 // because it is restored they will only retry once.
498 }
499
500 // Convert the fields used by is_busy() to a string that can be
501 // used for diagnostic output.
502 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) {
503 ss->print("is_busy: contentions=%d, waiters=%d, ", _contentions, _waiters);
504 if (!AsyncDeflateIdleMonitors) {
505 ss->print("owner=" INTPTR_FORMAT, p2i(_owner));
506 } else if (_owner != DEFLATER_MARKER) {
507 ss->print("owner=" INTPTR_FORMAT, p2i(_owner));
508 } else {
509 // We report NULL instead of DEFLATER_MARKER here because is_busy()
510 // ignores DEFLATER_MARKER values.
511 ss->print("owner=" INTPTR_FORMAT, NULL);
512 }
513 ss->print(", cxq=" INTPTR_FORMAT ", EntryList=" INTPTR_FORMAT, p2i(_cxq),
514 p2i(_EntryList));
515 return ss->base();
516 }
517
518 #define MAX_RECHECK_INTERVAL 1000
519
520 void ObjectMonitor::EnterI(TRAPS) {
521 jint l_ref_count = ref_count();
522 ADIM_guarantee(l_ref_count > 0, "must be positive: l_ref_count=%d, ref_count=%d", l_ref_count, ref_count());
523
524 Thread * const Self = THREAD;
525 assert(Self->is_Java_thread(), "invariant");
526 assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant");
527
528 // Try the lock - TATAS
529 if (TryLock (Self) > 0) {
530 assert(_succ != Self, "invariant");
531 assert(_owner == Self, "invariant");
532 assert(_Responsible != Self, "invariant");
533 return;
534 }
535
536 if (AsyncDeflateIdleMonitors &&
537 try_set_owner_from(DEFLATER_MARKER, Self) == DEFLATER_MARKER) {
538 // The deflation protocol finished the first part (setting owner),
539 // but it failed the second part (making ref_count negative) and
540 // bailed. Or the ObjectMonitor was async deflated and reused.
541 // Acquired the monitor.
542 assert(_succ != Self, "invariant");
543 assert(_Responsible != Self, "invariant");
544 return;
545 }
546
547 assert(InitDone, "Unexpectedly not initialized");
548
549 // We try one round of spinning *before* enqueueing Self.
550 //
551 // If the _owner is ready but OFFPROC we could use a YieldTo()
552 // operation to donate the remainder of this thread's quantum
553 // to the owner. This has subtle but beneficial affinity
554 // effects.
555
556 if (TrySpin(Self) > 0) {
557 assert(_owner == Self, "invariant");
558 assert(_succ != Self, "invariant");
559 assert(_Responsible != Self, "invariant");
560 return;
561 }
562
563 // The Spin failed -- Enqueue and park the thread ...
564 assert(_succ != Self, "invariant");
565 assert(_owner != Self, "invariant");
566 assert(_Responsible != Self, "invariant");
567
568 // Enqueue "Self" on ObjectMonitor's _cxq.
569 //
570 // Node acts as a proxy for Self.
571 // As an aside, if were to ever rewrite the synchronization code mostly
572 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
573 // Java objects. This would avoid awkward lifecycle and liveness issues,
574 // as well as eliminate a subset of ABA issues.
575 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
576
577 ObjectWaiter node(Self);
578 Self->_ParkEvent->reset();
579 node._prev = (ObjectWaiter *) 0xBAD;
580 node.TState = ObjectWaiter::TS_CXQ;
581
582 // Push "Self" onto the front of the _cxq.
583 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
584 // Note that spinning tends to reduce the rate at which threads
585 // enqueue and dequeue on EntryList|cxq.
586 ObjectWaiter * nxt;
587 for (;;) {
588 node._next = nxt = _cxq;
589 if (Atomic::cmpxchg(&_cxq, nxt, &node) == nxt) break;
590
591 // Interference - the CAS failed because _cxq changed. Just retry.
592 // As an optional optimization we retry the lock.
593 if (TryLock (Self) > 0) {
594 assert(_succ != Self, "invariant");
595 assert(_owner == Self, "invariant");
596 assert(_Responsible != Self, "invariant");
597 return;
598 }
599 }
600
601 // Check for cxq|EntryList edge transition to non-null. This indicates
602 // the onset of contention. While contention persists exiting threads
603 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
604 // operations revert to the faster 1-0 mode. This enter operation may interleave
605 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
606 // arrange for one of the contending thread to use a timed park() operations
607 // to detect and recover from the race. (Stranding is form of progress failure
608 // where the monitor is unlocked but all the contending threads remain parked).
609 // That is, at least one of the contended threads will periodically poll _owner.
610 // One of the contending threads will become the designated "Responsible" thread.
611 // The Responsible thread uses a timed park instead of a normal indefinite park
612 // operation -- it periodically wakes and checks for and recovers from potential
613 // strandings admitted by 1-0 exit operations. We need at most one Responsible
614 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
615 // be responsible for a monitor.
616 //
617 // Currently, one of the contended threads takes on the added role of "Responsible".
618 // A viable alternative would be to use a dedicated "stranding checker" thread
619 // that periodically iterated over all the threads (or active monitors) and unparked
620 // successors where there was risk of stranding. This would help eliminate the
621 // timer scalability issues we see on some platforms as we'd only have one thread
622 // -- the checker -- parked on a timer.
623
624 if (nxt == NULL && _EntryList == NULL) {
625 // Try to assume the role of responsible thread for the monitor.
626 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
627 Atomic::replace_if_null(&_Responsible, Self);
628 }
629
630 // The lock might have been released while this thread was occupied queueing
631 // itself onto _cxq. To close the race and avoid "stranding" and
632 // progress-liveness failure we must resample-retry _owner before parking.
633 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
634 // In this case the ST-MEMBAR is accomplished with CAS().
635 //
636 // TODO: Defer all thread state transitions until park-time.
637 // Since state transitions are heavy and inefficient we'd like
638 // to defer the state transitions until absolutely necessary,
639 // and in doing so avoid some transitions ...
640
641 int nWakeups = 0;
642 int recheckInterval = 1;
643
644 for (;;) {
645
646 if (TryLock(Self) > 0) break;
647 assert(_owner != Self, "invariant");
648
649 // park self
650 if (_Responsible == Self) {
651 Self->_ParkEvent->park((jlong) recheckInterval);
652 // Increase the recheckInterval, but clamp the value.
653 recheckInterval *= 8;
654 if (recheckInterval > MAX_RECHECK_INTERVAL) {
655 recheckInterval = MAX_RECHECK_INTERVAL;
656 }
657 } else {
658 Self->_ParkEvent->park();
659 }
660
661 if (TryLock(Self) > 0) break;
662
663 if (AsyncDeflateIdleMonitors &&
664 try_set_owner_from(DEFLATER_MARKER, Self) == DEFLATER_MARKER) {
665 // The deflation protocol finished the first part (setting owner),
666 // but it failed the second part (making ref_count negative) and
667 // bailed. Or the ObjectMonitor was async deflated and reused.
668 // Acquired the monitor.
669 break;
670 }
671
672 // The lock is still contested.
673 // Keep a tally of the # of futile wakeups.
674 // Note that the counter is not protected by a lock or updated by atomics.
675 // That is by design - we trade "lossy" counters which are exposed to
676 // races during updates for a lower probe effect.
677
678 // This PerfData object can be used in parallel with a safepoint.
679 // See the work around in PerfDataManager::destroy().
680 OM_PERFDATA_OP(FutileWakeups, inc());
681 ++nWakeups;
682
683 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
684 // We can defer clearing _succ until after the spin completes
685 // TrySpin() must tolerate being called with _succ == Self.
686 // Try yet another round of adaptive spinning.
687 if (TrySpin(Self) > 0) break;
688
689 // We can find that we were unpark()ed and redesignated _succ while
690 // we were spinning. That's harmless. If we iterate and call park(),
691 // park() will consume the event and return immediately and we'll
692 // just spin again. This pattern can repeat, leaving _succ to simply
693 // spin on a CPU.
694
695 if (_succ == Self) _succ = NULL;
696
697 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
698 OrderAccess::fence();
699 }
700
701 // Egress :
702 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
703 // Normally we'll find Self on the EntryList .
704 // From the perspective of the lock owner (this thread), the
705 // EntryList is stable and cxq is prepend-only.
706 // The head of cxq is volatile but the interior is stable.
707 // In addition, Self.TState is stable.
708
709 assert(_owner == Self, "invariant");
710 assert(object() != NULL, "invariant");
711 // I'd like to write:
712 // guarantee (((oop)(object()))->mark() == markWord::encode(this), "invariant") ;
713 // but as we're at a safepoint that's not safe.
714
715 UnlinkAfterAcquire(Self, &node);
716 if (_succ == Self) _succ = NULL;
717
718 assert(_succ != Self, "invariant");
719 if (_Responsible == Self) {
720 _Responsible = NULL;
721 OrderAccess::fence(); // Dekker pivot-point
722
723 // We may leave threads on cxq|EntryList without a designated
724 // "Responsible" thread. This is benign. When this thread subsequently
725 // exits the monitor it can "see" such preexisting "old" threads --
726 // threads that arrived on the cxq|EntryList before the fence, above --
727 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
728 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
729 // non-null and elect a new "Responsible" timer thread.
730 //
731 // This thread executes:
732 // ST Responsible=null; MEMBAR (in enter epilogue - here)
733 // LD cxq|EntryList (in subsequent exit)
734 //
735 // Entering threads in the slow/contended path execute:
736 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
737 // The (ST cxq; MEMBAR) is accomplished with CAS().
738 //
739 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
740 // exit operation from floating above the ST Responsible=null.
741 }
742
743 // We've acquired ownership with CAS().
744 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
745 // But since the CAS() this thread may have also stored into _succ,
746 // EntryList, cxq or Responsible. These meta-data updates must be
747 // visible __before this thread subsequently drops the lock.
748 // Consider what could occur if we didn't enforce this constraint --
749 // STs to monitor meta-data and user-data could reorder with (become
750 // visible after) the ST in exit that drops ownership of the lock.
751 // Some other thread could then acquire the lock, but observe inconsistent
752 // or old monitor meta-data and heap data. That violates the JMM.
753 // To that end, the 1-0 exit() operation must have at least STST|LDST
754 // "release" barrier semantics. Specifically, there must be at least a
755 // STST|LDST barrier in exit() before the ST of null into _owner that drops
756 // the lock. The barrier ensures that changes to monitor meta-data and data
757 // protected by the lock will be visible before we release the lock, and
758 // therefore before some other thread (CPU) has a chance to acquire the lock.
759 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
760 //
761 // Critically, any prior STs to _succ or EntryList must be visible before
762 // the ST of null into _owner in the *subsequent* (following) corresponding
763 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
764 // execute a serializing instruction.
765
766 return;
767 }
768
769 // ReenterI() is a specialized inline form of the latter half of the
770 // contended slow-path from EnterI(). We use ReenterI() only for
771 // monitor reentry in wait().
772 //
773 // In the future we should reconcile EnterI() and ReenterI().
774
775 void ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) {
776 jint l_ref_count = ref_count();
777 ADIM_guarantee(l_ref_count > 0, "must be positive: l_ref_count=%d, ref_count=%d", l_ref_count, ref_count());
778
779 assert(Self != NULL, "invariant");
780 assert(SelfNode != NULL, "invariant");
781 assert(SelfNode->_thread == Self, "invariant");
782 assert(_waiters > 0, "invariant");
783 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant");
784 assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
785 JavaThread * jt = (JavaThread *) Self;
786
787 int nWakeups = 0;
788 for (;;) {
789 ObjectWaiter::TStates v = SelfNode->TState;
790 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
791 assert(_owner != Self, "invariant");
792
793 if (TryLock(Self) > 0) break;
794 if (TrySpin(Self) > 0) break;
795
796 if (AsyncDeflateIdleMonitors &&
797 try_set_owner_from(DEFLATER_MARKER, Self) == DEFLATER_MARKER) {
798 // The deflation protocol finished the first part (setting owner),
799 // but it failed the second part (making ref_count negative) and
800 // bailed. Or the ObjectMonitor was async deflated and reused.
801 // Acquired the monitor.
802 break;
803 }
804
805 // State transition wrappers around park() ...
806 // ReenterI() wisely defers state transitions until
807 // it's clear we must park the thread.
808 {
809 OSThreadContendState osts(Self->osthread());
810 ThreadBlockInVM tbivm(jt);
811
812 // cleared by handle_special_suspend_equivalent_condition()
813 // or java_suspend_self()
814 jt->set_suspend_equivalent();
815 Self->_ParkEvent->park();
816
817 // were we externally suspended while we were waiting?
818 for (;;) {
819 if (!ExitSuspendEquivalent(jt)) break;
820 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
821 jt->java_suspend_self();
822 jt->set_suspend_equivalent();
823 }
824 }
825
826 // Try again, but just so we distinguish between futile wakeups and
827 // successful wakeups. The following test isn't algorithmically
828 // necessary, but it helps us maintain sensible statistics.
829 if (TryLock(Self) > 0) break;
830
831 // The lock is still contested.
832 // Keep a tally of the # of futile wakeups.
833 // Note that the counter is not protected by a lock or updated by atomics.
834 // That is by design - we trade "lossy" counters which are exposed to
835 // races during updates for a lower probe effect.
836 ++nWakeups;
837
838 // Assuming this is not a spurious wakeup we'll normally
839 // find that _succ == Self.
840 if (_succ == Self) _succ = NULL;
841
842 // Invariant: after clearing _succ a contending thread
843 // *must* retry _owner before parking.
844 OrderAccess::fence();
845
846 // This PerfData object can be used in parallel with a safepoint.
847 // See the work around in PerfDataManager::destroy().
848 OM_PERFDATA_OP(FutileWakeups, inc());
849 }
850
851 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
852 // Normally we'll find Self on the EntryList.
853 // Unlinking from the EntryList is constant-time and atomic-free.
854 // From the perspective of the lock owner (this thread), the
855 // EntryList is stable and cxq is prepend-only.
856 // The head of cxq is volatile but the interior is stable.
857 // In addition, Self.TState is stable.
858
859 assert(_owner == Self, "invariant");
860 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant");
861 UnlinkAfterAcquire(Self, SelfNode);
862 if (_succ == Self) _succ = NULL;
863 assert(_succ != Self, "invariant");
864 SelfNode->TState = ObjectWaiter::TS_RUN;
865 OrderAccess::fence(); // see comments at the end of EnterI()
866 }
867
868 // By convention we unlink a contending thread from EntryList|cxq immediately
869 // after the thread acquires the lock in ::enter(). Equally, we could defer
870 // unlinking the thread until ::exit()-time.
871
872 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) {
873 assert(_owner == Self, "invariant");
874 assert(SelfNode->_thread == Self, "invariant");
875
876 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
877 // Normal case: remove Self from the DLL EntryList .
878 // This is a constant-time operation.
879 ObjectWaiter * nxt = SelfNode->_next;
880 ObjectWaiter * prv = SelfNode->_prev;
881 if (nxt != NULL) nxt->_prev = prv;
882 if (prv != NULL) prv->_next = nxt;
883 if (SelfNode == _EntryList) _EntryList = nxt;
884 assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
885 assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
886 } else {
887 assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant");
888 // Inopportune interleaving -- Self is still on the cxq.
889 // This usually means the enqueue of self raced an exiting thread.
890 // Normally we'll find Self near the front of the cxq, so
891 // dequeueing is typically fast. If needbe we can accelerate
892 // this with some MCS/CHL-like bidirectional list hints and advisory
893 // back-links so dequeueing from the interior will normally operate
894 // in constant-time.
895 // Dequeue Self from either the head (with CAS) or from the interior
896 // with a linear-time scan and normal non-atomic memory operations.
897 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
898 // and then unlink Self from EntryList. We have to drain eventually,
899 // so it might as well be now.
900
901 ObjectWaiter * v = _cxq;
902 assert(v != NULL, "invariant");
903 if (v != SelfNode || Atomic::cmpxchg(&_cxq, v, SelfNode->_next) != v) {
904 // The CAS above can fail from interference IFF a "RAT" arrived.
905 // In that case Self must be in the interior and can no longer be
906 // at the head of cxq.
907 if (v == SelfNode) {
908 assert(_cxq != v, "invariant");
909 v = _cxq; // CAS above failed - start scan at head of list
910 }
911 ObjectWaiter * p;
912 ObjectWaiter * q = NULL;
913 for (p = v; p != NULL && p != SelfNode; p = p->_next) {
914 q = p;
915 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
916 }
917 assert(v != SelfNode, "invariant");
918 assert(p == SelfNode, "Node not found on cxq");
919 assert(p != _cxq, "invariant");
920 assert(q != NULL, "invariant");
921 assert(q->_next == p, "invariant");
922 q->_next = p->_next;
923 }
924 }
925
926 #ifdef ASSERT
927 // Diagnostic hygiene ...
928 SelfNode->_prev = (ObjectWaiter *) 0xBAD;
929 SelfNode->_next = (ObjectWaiter *) 0xBAD;
930 SelfNode->TState = ObjectWaiter::TS_RUN;
931 #endif
932 }
933
934 // -----------------------------------------------------------------------------
935 // Exit support
936 //
937 // exit()
938 // ~~~~~~
939 // Note that the collector can't reclaim the objectMonitor or deflate
940 // the object out from underneath the thread calling ::exit() as the
941 // thread calling ::exit() never transitions to a stable state.
942 // This inhibits GC, which in turn inhibits asynchronous (and
943 // inopportune) reclamation of "this".
944 //
945 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
946 // There's one exception to the claim above, however. EnterI() can call
947 // exit() to drop a lock if the acquirer has been externally suspended.
948 // In that case exit() is called with _thread_state == _thread_blocked,
949 // but the monitor's ref_count is > 0, which inhibits reclamation.
950 //
951 // 1-0 exit
952 // ~~~~~~~~
953 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
954 // the fast-path operators have been optimized so the common ::exit()
955 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
956 // The code emitted by fast_unlock() elides the usual MEMBAR. This
957 // greatly improves latency -- MEMBAR and CAS having considerable local
958 // latency on modern processors -- but at the cost of "stranding". Absent the
959 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
960 // ::enter() path, resulting in the entering thread being stranding
961 // and a progress-liveness failure. Stranding is extremely rare.
962 // We use timers (timed park operations) & periodic polling to detect
963 // and recover from stranding. Potentially stranded threads periodically
964 // wake up and poll the lock. See the usage of the _Responsible variable.
965 //
966 // The CAS() in enter provides for safety and exclusion, while the CAS or
967 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
968 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
969 // We detect and recover from stranding with timers.
970 //
971 // If a thread transiently strands it'll park until (a) another
972 // thread acquires the lock and then drops the lock, at which time the
973 // exiting thread will notice and unpark the stranded thread, or, (b)
974 // the timer expires. If the lock is high traffic then the stranding latency
975 // will be low due to (a). If the lock is low traffic then the odds of
976 // stranding are lower, although the worst-case stranding latency
977 // is longer. Critically, we don't want to put excessive load in the
978 // platform's timer subsystem. We want to minimize both the timer injection
979 // rate (timers created/sec) as well as the number of timers active at
980 // any one time. (more precisely, we want to minimize timer-seconds, which is
981 // the integral of the # of active timers at any instant over time).
982 // Both impinge on OS scalability. Given that, at most one thread parked on
983 // a monitor will use a timer.
984 //
985 // There is also the risk of a futile wake-up. If we drop the lock
986 // another thread can reacquire the lock immediately, and we can
987 // then wake a thread unnecessarily. This is benign, and we've
988 // structured the code so the windows are short and the frequency
989 // of such futile wakups is low.
990
991 void ObjectMonitor::exit(bool not_suspended, TRAPS) {
992 Thread* const Self = THREAD;
993 void* cur = Atomic::load(&_owner);
994 if (THREAD != cur) {
995 if (THREAD->is_lock_owned((address)cur)) {
996 assert(_recursions == 0, "invariant");
997 set_owner_from_BasicLock(cur, Self); // Convert from BasicLock* to Thread*.
998 _recursions = 0;
999 } else {
1000 // Apparent unbalanced locking ...
1001 // Naively we'd like to throw IllegalMonitorStateException.
1002 // As a practical matter we can neither allocate nor throw an
1003 // exception as ::exit() can be called from leaf routines.
1004 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
1005 // Upon deeper reflection, however, in a properly run JVM the only
1006 // way we should encounter this situation is in the presence of
1007 // unbalanced JNI locking. TODO: CheckJNICalls.
1008 // See also: CR4414101
1009 #ifdef ASSERT
1010 LogStreamHandle(Error, monitorinflation) lsh;
1011 lsh.print_cr("ERROR: ObjectMonitor::exit(): thread=" INTPTR_FORMAT
1012 " is exiting an ObjectMonitor it does not own.", p2i(THREAD));
1013 lsh.print_cr("The imbalance is possibly caused by JNI locking.");
1014 print_debug_style_on(&lsh);
1015 #endif
1016 assert(false, "Non-balanced monitor enter/exit!");
1017 return;
1018 }
1019 }
1020
1021 if (_recursions != 0) {
1022 _recursions--; // this is simple recursive enter
1023 return;
1024 }
1025
1026 // Invariant: after setting Responsible=null an thread must execute
1027 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
1028 _Responsible = NULL;
1029
1030 #if INCLUDE_JFR
1031 // get the owner's thread id for the MonitorEnter event
1032 // if it is enabled and the thread isn't suspended
1033 if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
1034 _previous_owner_tid = JFR_THREAD_ID(Self);
1035 }
1036 #endif
1037
1038 for (;;) {
1039 assert(THREAD == _owner, "invariant");
1040
1041 // Drop the lock.
1042 // release semantics: prior loads and stores from within the critical section
1043 // must not float (reorder) past the following store that drops the lock.
1044 // Uses a storeload to separate release_store(owner) from the
1045 // successor check. The try_set_owner() below uses cmpxchg() so
1046 // we get the fence down there.
1047 release_clear_owner(Self);
1048 OrderAccess::storeload();
1049
1050 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1051 return;
1052 }
1053 // Other threads are blocked trying to acquire the lock.
1054
1055 // Normally the exiting thread is responsible for ensuring succession,
1056 // but if other successors are ready or other entering threads are spinning
1057 // then this thread can simply store NULL into _owner and exit without
1058 // waking a successor. The existence of spinners or ready successors
1059 // guarantees proper succession (liveness). Responsibility passes to the
1060 // ready or running successors. The exiting thread delegates the duty.
1061 // More precisely, if a successor already exists this thread is absolved
1062 // of the responsibility of waking (unparking) one.
1063 //
1064 // The _succ variable is critical to reducing futile wakeup frequency.
1065 // _succ identifies the "heir presumptive" thread that has been made
1066 // ready (unparked) but that has not yet run. We need only one such
1067 // successor thread to guarantee progress.
1068 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1069 // section 3.3 "Futile Wakeup Throttling" for details.
1070 //
1071 // Note that spinners in Enter() also set _succ non-null.
1072 // In the current implementation spinners opportunistically set
1073 // _succ so that exiting threads might avoid waking a successor.
1074 // Another less appealing alternative would be for the exiting thread
1075 // to drop the lock and then spin briefly to see if a spinner managed
1076 // to acquire the lock. If so, the exiting thread could exit
1077 // immediately without waking a successor, otherwise the exiting
1078 // thread would need to dequeue and wake a successor.
1079 // (Note that we'd need to make the post-drop spin short, but no
1080 // shorter than the worst-case round-trip cache-line migration time.
1081 // The dropped lock needs to become visible to the spinner, and then
1082 // the acquisition of the lock by the spinner must become visible to
1083 // the exiting thread).
1084
1085 // It appears that an heir-presumptive (successor) must be made ready.
1086 // Only the current lock owner can manipulate the EntryList or
1087 // drain _cxq, so we need to reacquire the lock. If we fail
1088 // to reacquire the lock the responsibility for ensuring succession
1089 // falls to the new owner.
1090 //
1091 if (try_set_owner_from(NULL, Self) != NULL) {
1092 return;
1093 }
1094
1095 guarantee(_owner == THREAD, "invariant");
1096
1097 ObjectWaiter * w = NULL;
1098
1099 w = _EntryList;
1100 if (w != NULL) {
1101 // I'd like to write: guarantee (w->_thread != Self).
1102 // But in practice an exiting thread may find itself on the EntryList.
1103 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1104 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1105 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1106 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1107 // release the lock "O". T2 resumes immediately after the ST of null into
1108 // _owner, above. T2 notices that the EntryList is populated, so it
1109 // reacquires the lock and then finds itself on the EntryList.
1110 // Given all that, we have to tolerate the circumstance where "w" is
1111 // associated with Self.
1112 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1113 ExitEpilog(Self, w);
1114 return;
1115 }
1116
1117 // If we find that both _cxq and EntryList are null then just
1118 // re-run the exit protocol from the top.
1119 w = _cxq;
1120 if (w == NULL) continue;
1121
1122 // Drain _cxq into EntryList - bulk transfer.
1123 // First, detach _cxq.
1124 // The following loop is tantamount to: w = swap(&cxq, NULL)
1125 for (;;) {
1126 assert(w != NULL, "Invariant");
1127 ObjectWaiter * u = Atomic::cmpxchg(&_cxq, w, (ObjectWaiter*)NULL);
1128 if (u == w) break;
1129 w = u;
1130 }
1131
1132 assert(w != NULL, "invariant");
1133 assert(_EntryList == NULL, "invariant");
1134
1135 // Convert the LIFO SLL anchored by _cxq into a DLL.
1136 // The list reorganization step operates in O(LENGTH(w)) time.
1137 // It's critical that this step operate quickly as
1138 // "Self" still holds the outer-lock, restricting parallelism
1139 // and effectively lengthening the critical section.
1140 // Invariant: s chases t chases u.
1141 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1142 // we have faster access to the tail.
1143
1144 _EntryList = w;
1145 ObjectWaiter * q = NULL;
1146 ObjectWaiter * p;
1147 for (p = w; p != NULL; p = p->_next) {
1148 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1149 p->TState = ObjectWaiter::TS_ENTER;
1150 p->_prev = q;
1151 q = p;
1152 }
1153
1154 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1155 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1156
1157 // See if we can abdicate to a spinner instead of waking a thread.
1158 // A primary goal of the implementation is to reduce the
1159 // context-switch rate.
1160 if (_succ != NULL) continue;
1161
1162 w = _EntryList;
1163 if (w != NULL) {
1164 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1165 ExitEpilog(Self, w);
1166 return;
1167 }
1168 }
1169 }
1170
1171 // ExitSuspendEquivalent:
1172 // A faster alternate to handle_special_suspend_equivalent_condition()
1173 //
1174 // handle_special_suspend_equivalent_condition() unconditionally
1175 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1176 // operations have high latency. Note that in ::enter() we call HSSEC
1177 // while holding the monitor, so we effectively lengthen the critical sections.
1178 //
1179 // There are a number of possible solutions:
1180 //
1181 // A. To ameliorate the problem we might also defer state transitions
1182 // to as late as possible -- just prior to parking.
1183 // Given that, we'd call HSSEC after having returned from park(),
1184 // but before attempting to acquire the monitor. This is only a
1185 // partial solution. It avoids calling HSSEC while holding the
1186 // monitor (good), but it still increases successor reacquisition latency --
1187 // the interval between unparking a successor and the time the successor
1188 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1189 // If we use this technique we can also avoid EnterI()-exit() loop
1190 // in ::enter() where we iteratively drop the lock and then attempt
1191 // to reacquire it after suspending.
1192 //
1193 // B. In the future we might fold all the suspend bits into a
1194 // composite per-thread suspend flag and then update it with CAS().
1195 // Alternately, a Dekker-like mechanism with multiple variables
1196 // would suffice:
1197 // ST Self->_suspend_equivalent = false
1198 // MEMBAR
1199 // LD Self_>_suspend_flags
1200
1201 bool ObjectMonitor::ExitSuspendEquivalent(JavaThread * jSelf) {
1202 return jSelf->handle_special_suspend_equivalent_condition();
1203 }
1204
1205
1206 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) {
1207 assert(_owner == Self, "invariant");
1208
1209 // Exit protocol:
1210 // 1. ST _succ = wakee
1211 // 2. membar #loadstore|#storestore;
1212 // 2. ST _owner = NULL
1213 // 3. unpark(wakee)
1214
1215 _succ = Wakee->_thread;
1216 ParkEvent * Trigger = Wakee->_event;
1217
1218 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1219 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1220 // out-of-scope (non-extant).
1221 Wakee = NULL;
1222
1223 // Drop the lock.
1224 // Uses a fence to separate release_store(owner) from the LD in unpark().
1225 release_clear_owner(Self);
1226 OrderAccess::fence();
1227
1228 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1229 Trigger->unpark();
1230
1231 // Maintain stats and report events to JVMTI
1232 OM_PERFDATA_OP(Parks, inc());
1233 }
1234
1235
1236 // -----------------------------------------------------------------------------
1237 // Class Loader deadlock handling.
1238 //
1239 // complete_exit exits a lock returning recursion count
1240 // complete_exit/reenter operate as a wait without waiting
1241 // complete_exit requires an inflated monitor
1242 // The _owner field is not always the Thread addr even with an
1243 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1244 // thread due to contention.
1245 intx ObjectMonitor::complete_exit(TRAPS) {
1246 Thread * const Self = THREAD;
1247 assert(Self->is_Java_thread(), "Must be Java thread!");
1248 JavaThread *jt = (JavaThread *)THREAD;
1249
1250 assert(InitDone, "Unexpectedly not initialized");
1251
1252 void* cur = Atomic::load(&_owner);
1253 if (THREAD != cur) {
1254 if (THREAD->is_lock_owned((address)cur)) {
1255 assert(_recursions == 0, "internal state error");
1256 set_owner_from_BasicLock(cur, Self); // Convert from BasicLock* to Thread*.
1257 _recursions = 0;
1258 }
1259 }
1260
1261 guarantee(Self == _owner, "complete_exit not owner");
1262 intx save = _recursions; // record the old recursion count
1263 _recursions = 0; // set the recursion level to be 0
1264 exit(true, Self); // exit the monitor
1265 guarantee(_owner != Self, "invariant");
1266 return save;
1267 }
1268
1269 // reenter() enters a lock and sets recursion count
1270 // complete_exit/reenter operate as a wait without waiting
1271 void ObjectMonitor::reenter(intx recursions, TRAPS) {
1272 Thread * const Self = THREAD;
1273 assert(Self->is_Java_thread(), "Must be Java thread!");
1274 JavaThread *jt = (JavaThread *)THREAD;
1275
1276 guarantee(_owner != Self, "reenter already owner");
1277 enter(THREAD);
1278 // Entered the monitor.
1279 guarantee(_recursions == 0, "reenter recursion");
1280 _recursions = recursions;
1281 }
1282
1283 // Checks that the current THREAD owns this monitor and causes an
1284 // immediate return if it doesn't. We don't use the CHECK macro
1285 // because we want the IMSE to be the only exception that is thrown
1286 // from the call site when false is returned. Any other pending
1287 // exception is ignored.
1288 #define CHECK_OWNER() \
1289 do { \
1290 if (!check_owner(THREAD)) { \
1291 assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \
1292 return; \
1293 } \
1294 } while (false)
1295
1296 // Returns true if the specified thread owns the ObjectMonitor.
1297 // Otherwise returns false and throws IllegalMonitorStateException
1298 // (IMSE). If there is a pending exception and the specified thread
1299 // is not the owner, that exception will be replaced by the IMSE.
1300 bool ObjectMonitor::check_owner(Thread* THREAD) {
1301 void* cur = Atomic::load(&_owner);
1302 if (cur == THREAD) {
1303 return true;
1304 }
1305 if (THREAD->is_lock_owned((address)cur)) {
1306 set_owner_from_BasicLock(cur, THREAD); // Convert from BasicLock* to Thread*.
1307 _recursions = 0;
1308 return true;
1309 }
1310 THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(),
1311 "current thread is not owner", false);
1312 }
1313
1314 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1315 ObjectMonitor* monitor,
1316 jlong notifier_tid,
1317 jlong timeout,
1318 bool timedout) {
1319 assert(event != NULL, "invariant");
1320 assert(monitor != NULL, "invariant");
1321 event->set_monitorClass(((oop)monitor->object())->klass());
1322 event->set_timeout(timeout);
1323 event->set_address((uintptr_t)monitor->object_addr());
1324 event->set_notifier(notifier_tid);
1325 event->set_timedOut(timedout);
1326 event->commit();
1327 }
1328
1329 // -----------------------------------------------------------------------------
1330 // Wait/Notify/NotifyAll
1331 //
1332 // Note: a subset of changes to ObjectMonitor::wait()
1333 // will need to be replicated in complete_exit
1334 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1335 Thread * const Self = THREAD;
1336 assert(Self->is_Java_thread(), "Must be Java thread!");
1337 JavaThread *jt = (JavaThread *)THREAD;
1338
1339 assert(InitDone, "Unexpectedly not initialized");
1340
1341 CHECK_OWNER(); // Throws IMSE if not owner.
1342
1343 EventJavaMonitorWait event;
1344
1345 // check for a pending interrupt
1346 if (interruptible && jt->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1347 // post monitor waited event. Note that this is past-tense, we are done waiting.
1348 if (JvmtiExport::should_post_monitor_waited()) {
1349 // Note: 'false' parameter is passed here because the
1350 // wait was not timed out due to thread interrupt.
1351 JvmtiExport::post_monitor_waited(jt, this, false);
1352
1353 // In this short circuit of the monitor wait protocol, the
1354 // current thread never drops ownership of the monitor and
1355 // never gets added to the wait queue so the current thread
1356 // cannot be made the successor. This means that the
1357 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1358 // consume an unpark() meant for the ParkEvent associated with
1359 // this ObjectMonitor.
1360 }
1361 if (event.should_commit()) {
1362 post_monitor_wait_event(&event, this, 0, millis, false);
1363 }
1364 THROW(vmSymbols::java_lang_InterruptedException());
1365 return;
1366 }
1367
1368 assert(Self->_Stalled == 0, "invariant");
1369 Self->_Stalled = intptr_t(this);
1370 jt->set_current_waiting_monitor(this);
1371
1372 // create a node to be put into the queue
1373 // Critically, after we reset() the event but prior to park(), we must check
1374 // for a pending interrupt.
1375 ObjectWaiter node(Self);
1376 node.TState = ObjectWaiter::TS_WAIT;
1377 Self->_ParkEvent->reset();
1378 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1379
1380 // Enter the waiting queue, which is a circular doubly linked list in this case
1381 // but it could be a priority queue or any data structure.
1382 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1383 // by the the owner of the monitor *except* in the case where park()
1384 // returns because of a timeout of interrupt. Contention is exceptionally rare
1385 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1386
1387 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1388 AddWaiter(&node);
1389 Thread::SpinRelease(&_WaitSetLock);
1390
1391 _Responsible = NULL;
1392
1393 intx save = _recursions; // record the old recursion count
1394 _waiters++; // increment the number of waiters
1395 _recursions = 0; // set the recursion level to be 1
1396 exit(true, Self); // exit the monitor
1397 guarantee(_owner != Self, "invariant");
1398
1399 // The thread is on the WaitSet list - now park() it.
1400 // On MP systems it's conceivable that a brief spin before we park
1401 // could be profitable.
1402 //
1403 // TODO-FIXME: change the following logic to a loop of the form
1404 // while (!timeout && !interrupted && _notified == 0) park()
1405
1406 int ret = OS_OK;
1407 int WasNotified = 0;
1408
1409 // Need to check interrupt state whilst still _thread_in_vm
1410 bool interrupted = interruptible && jt->is_interrupted(false);
1411
1412 { // State transition wrappers
1413 OSThread* osthread = Self->osthread();
1414 OSThreadWaitState osts(osthread, true);
1415 {
1416 ThreadBlockInVM tbivm(jt);
1417 // Thread is in thread_blocked state and oop access is unsafe.
1418 jt->set_suspend_equivalent();
1419
1420 if (interrupted || HAS_PENDING_EXCEPTION) {
1421 // Intentionally empty
1422 } else if (node._notified == 0) {
1423 if (millis <= 0) {
1424 Self->_ParkEvent->park();
1425 } else {
1426 ret = Self->_ParkEvent->park(millis);
1427 }
1428 }
1429
1430 // were we externally suspended while we were waiting?
1431 if (ExitSuspendEquivalent (jt)) {
1432 // TODO-FIXME: add -- if succ == Self then succ = null.
1433 jt->java_suspend_self();
1434 }
1435
1436 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1437
1438 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1439 // from the WaitSet to the EntryList.
1440 // See if we need to remove Node from the WaitSet.
1441 // We use double-checked locking to avoid grabbing _WaitSetLock
1442 // if the thread is not on the wait queue.
1443 //
1444 // Note that we don't need a fence before the fetch of TState.
1445 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1446 // written by the is thread. (perhaps the fetch might even be satisfied
1447 // by a look-aside into the processor's own store buffer, although given
1448 // the length of the code path between the prior ST and this load that's
1449 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1450 // then we'll acquire the lock and then re-fetch a fresh TState value.
1451 // That is, we fail toward safety.
1452
1453 if (node.TState == ObjectWaiter::TS_WAIT) {
1454 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1455 if (node.TState == ObjectWaiter::TS_WAIT) {
1456 DequeueSpecificWaiter(&node); // unlink from WaitSet
1457 assert(node._notified == 0, "invariant");
1458 node.TState = ObjectWaiter::TS_RUN;
1459 }
1460 Thread::SpinRelease(&_WaitSetLock);
1461 }
1462
1463 // The thread is now either on off-list (TS_RUN),
1464 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1465 // The Node's TState variable is stable from the perspective of this thread.
1466 // No other threads will asynchronously modify TState.
1467 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1468 OrderAccess::loadload();
1469 if (_succ == Self) _succ = NULL;
1470 WasNotified = node._notified;
1471
1472 // Reentry phase -- reacquire the monitor.
1473 // re-enter contended monitor after object.wait().
1474 // retain OBJECT_WAIT state until re-enter successfully completes
1475 // Thread state is thread_in_vm and oop access is again safe,
1476 // although the raw address of the object may have changed.
1477 // (Don't cache naked oops over safepoints, of course).
1478
1479 // post monitor waited event. Note that this is past-tense, we are done waiting.
1480 if (JvmtiExport::should_post_monitor_waited()) {
1481 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1482
1483 if (node._notified != 0 && _succ == Self) {
1484 // In this part of the monitor wait-notify-reenter protocol it
1485 // is possible (and normal) for another thread to do a fastpath
1486 // monitor enter-exit while this thread is still trying to get
1487 // to the reenter portion of the protocol.
1488 //
1489 // The ObjectMonitor was notified and the current thread is
1490 // the successor which also means that an unpark() has already
1491 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1492 // consume the unpark() that was done when the successor was
1493 // set because the same ParkEvent is shared between Java
1494 // monitors and JVM/TI RawMonitors (for now).
1495 //
1496 // We redo the unpark() to ensure forward progress, i.e., we
1497 // don't want all pending threads hanging (parked) with none
1498 // entering the unlocked monitor.
1499 node._event->unpark();
1500 }
1501 }
1502
1503 if (event.should_commit()) {
1504 post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1505 }
1506
1507 OrderAccess::fence();
1508
1509 assert(Self->_Stalled != 0, "invariant");
1510 Self->_Stalled = 0;
1511
1512 assert(_owner != Self, "invariant");
1513 ObjectWaiter::TStates v = node.TState;
1514 if (v == ObjectWaiter::TS_RUN) {
1515 enter(Self);
1516 } else {
1517 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1518 ReenterI(Self, &node);
1519 node.wait_reenter_end(this);
1520 }
1521
1522 // Self has reacquired the lock.
1523 // Lifecycle - the node representing Self must not appear on any queues.
1524 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1525 // want residual elements associated with this thread left on any lists.
1526 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1527 assert(_owner == Self, "invariant");
1528 assert(_succ != Self, "invariant");
1529 } // OSThreadWaitState()
1530
1531 jt->set_current_waiting_monitor(NULL);
1532
1533 guarantee(_recursions == 0, "invariant");
1534 _recursions = save; // restore the old recursion count
1535 _waiters--; // decrement the number of waiters
1536
1537 // Verify a few postconditions
1538 assert(_owner == Self, "invariant");
1539 assert(_succ != Self, "invariant");
1540 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant");
1541
1542 // check if the notification happened
1543 if (!WasNotified) {
1544 // no, it could be timeout or Thread.interrupt() or both
1545 // check for interrupt event, otherwise it is timeout
1546 if (interruptible && jt->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1547 THROW(vmSymbols::java_lang_InterruptedException());
1548 }
1549 }
1550
1551 // NOTE: Spurious wake up will be consider as timeout.
1552 // Monitor notify has precedence over thread interrupt.
1553 }
1554
1555
1556 // Consider:
1557 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1558 // then instead of transferring a thread from the WaitSet to the EntryList
1559 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1560
1561 void ObjectMonitor::INotify(Thread * Self) {
1562 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1563 ObjectWaiter * iterator = DequeueWaiter();
1564 if (iterator != NULL) {
1565 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1566 guarantee(iterator->_notified == 0, "invariant");
1567 // Disposition - what might we do with iterator ?
1568 // a. add it directly to the EntryList - either tail (policy == 1)
1569 // or head (policy == 0).
1570 // b. push it onto the front of the _cxq (policy == 2).
1571 // For now we use (b).
1572
1573 iterator->TState = ObjectWaiter::TS_ENTER;
1574
1575 iterator->_notified = 1;
1576 iterator->_notifier_tid = JFR_THREAD_ID(Self);
1577
1578 ObjectWaiter * list = _EntryList;
1579 if (list != NULL) {
1580 assert(list->_prev == NULL, "invariant");
1581 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1582 assert(list != iterator, "invariant");
1583 }
1584
1585 // prepend to cxq
1586 if (list == NULL) {
1587 iterator->_next = iterator->_prev = NULL;
1588 _EntryList = iterator;
1589 } else {
1590 iterator->TState = ObjectWaiter::TS_CXQ;
1591 for (;;) {
1592 ObjectWaiter * front = _cxq;
1593 iterator->_next = front;
1594 if (Atomic::cmpxchg(&_cxq, front, iterator) == front) {
1595 break;
1596 }
1597 }
1598 }
1599
1600 // _WaitSetLock protects the wait queue, not the EntryList. We could
1601 // move the add-to-EntryList operation, above, outside the critical section
1602 // protected by _WaitSetLock. In practice that's not useful. With the
1603 // exception of wait() timeouts and interrupts the monitor owner
1604 // is the only thread that grabs _WaitSetLock. There's almost no contention
1605 // on _WaitSetLock so it's not profitable to reduce the length of the
1606 // critical section.
1607
1608 iterator->wait_reenter_begin(this);
1609 }
1610 Thread::SpinRelease(&_WaitSetLock);
1611 }
1612
1613 // Consider: a not-uncommon synchronization bug is to use notify() when
1614 // notifyAll() is more appropriate, potentially resulting in stranded
1615 // threads; this is one example of a lost wakeup. A useful diagnostic
1616 // option is to force all notify() operations to behave as notifyAll().
1617 //
1618 // Note: We can also detect many such problems with a "minimum wait".
1619 // When the "minimum wait" is set to a small non-zero timeout value
1620 // and the program does not hang whereas it did absent "minimum wait",
1621 // that suggests a lost wakeup bug.
1622
1623 void ObjectMonitor::notify(TRAPS) {
1624 CHECK_OWNER(); // Throws IMSE if not owner.
1625 if (_WaitSet == NULL) {
1626 return;
1627 }
1628 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1629 INotify(THREAD);
1630 OM_PERFDATA_OP(Notifications, inc(1));
1631 }
1632
1633
1634 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1635 // from the waitset to the EntryList. This could be done more efficiently with a
1636 // single bulk transfer but in practice it's not time-critical. Beware too,
1637 // that in prepend-mode we invert the order of the waiters. Let's say that the
1638 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1639 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1640
1641 void ObjectMonitor::notifyAll(TRAPS) {
1642 CHECK_OWNER(); // Throws IMSE if not owner.
1643 if (_WaitSet == NULL) {
1644 return;
1645 }
1646
1647 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1648 int tally = 0;
1649 while (_WaitSet != NULL) {
1650 tally++;
1651 INotify(THREAD);
1652 }
1653
1654 OM_PERFDATA_OP(Notifications, inc(tally));
1655 }
1656
1657 // -----------------------------------------------------------------------------
1658 // Adaptive Spinning Support
1659 //
1660 // Adaptive spin-then-block - rational spinning
1661 //
1662 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1663 // algorithm. On high order SMP systems it would be better to start with
1664 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1665 // a contending thread could enqueue itself on the cxq and then spin locally
1666 // on a thread-specific variable such as its ParkEvent._Event flag.
1667 // That's left as an exercise for the reader. Note that global spinning is
1668 // not problematic on Niagara, as the L2 cache serves the interconnect and
1669 // has both low latency and massive bandwidth.
1670 //
1671 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1672 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1673 // (duration) or we can fix the count at approximately the duration of
1674 // a context switch and vary the frequency. Of course we could also
1675 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1676 // For a description of 'Adaptive spin-then-block mutual exclusion in
1677 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1678 //
1679 // This implementation varies the duration "D", where D varies with
1680 // the success rate of recent spin attempts. (D is capped at approximately
1681 // length of a round-trip context switch). The success rate for recent
1682 // spin attempts is a good predictor of the success rate of future spin
1683 // attempts. The mechanism adapts automatically to varying critical
1684 // section length (lock modality), system load and degree of parallelism.
1685 // D is maintained per-monitor in _SpinDuration and is initialized
1686 // optimistically. Spin frequency is fixed at 100%.
1687 //
1688 // Note that _SpinDuration is volatile, but we update it without locks
1689 // or atomics. The code is designed so that _SpinDuration stays within
1690 // a reasonable range even in the presence of races. The arithmetic
1691 // operations on _SpinDuration are closed over the domain of legal values,
1692 // so at worst a race will install and older but still legal value.
1693 // At the very worst this introduces some apparent non-determinism.
1694 // We might spin when we shouldn't or vice-versa, but since the spin
1695 // count are relatively short, even in the worst case, the effect is harmless.
1696 //
1697 // Care must be taken that a low "D" value does not become an
1698 // an absorbing state. Transient spinning failures -- when spinning
1699 // is overall profitable -- should not cause the system to converge
1700 // on low "D" values. We want spinning to be stable and predictable
1701 // and fairly responsive to change and at the same time we don't want
1702 // it to oscillate, become metastable, be "too" non-deterministic,
1703 // or converge on or enter undesirable stable absorbing states.
1704 //
1705 // We implement a feedback-based control system -- using past behavior
1706 // to predict future behavior. We face two issues: (a) if the
1707 // input signal is random then the spin predictor won't provide optimal
1708 // results, and (b) if the signal frequency is too high then the control
1709 // system, which has some natural response lag, will "chase" the signal.
1710 // (b) can arise from multimodal lock hold times. Transient preemption
1711 // can also result in apparent bimodal lock hold times.
1712 // Although sub-optimal, neither condition is particularly harmful, as
1713 // in the worst-case we'll spin when we shouldn't or vice-versa.
1714 // The maximum spin duration is rather short so the failure modes aren't bad.
1715 // To be conservative, I've tuned the gain in system to bias toward
1716 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1717 // "rings" or oscillates between spinning and not spinning. This happens
1718 // when spinning is just on the cusp of profitability, however, so the
1719 // situation is not dire. The state is benign -- there's no need to add
1720 // hysteresis control to damp the transition rate between spinning and
1721 // not spinning.
1722
1723 // Spinning: Fixed frequency (100%), vary duration
1724 int ObjectMonitor::TrySpin(Thread * Self) {
1725 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1726 int ctr = Knob_FixedSpin;
1727 if (ctr != 0) {
1728 while (--ctr >= 0) {
1729 if (TryLock(Self) > 0) return 1;
1730 SpinPause();
1731 }
1732 return 0;
1733 }
1734
1735 for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1736 if (TryLock(Self) > 0) {
1737 // Increase _SpinDuration ...
1738 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1739 // Raising _SpurDuration to the poverty line is key.
1740 int x = _SpinDuration;
1741 if (x < Knob_SpinLimit) {
1742 if (x < Knob_Poverty) x = Knob_Poverty;
1743 _SpinDuration = x + Knob_BonusB;
1744 }
1745 return 1;
1746 }
1747 SpinPause();
1748 }
1749
1750 // Admission control - verify preconditions for spinning
1751 //
1752 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1753 // becoming an absorbing state. Put another way, we spin briefly to
1754 // sample, just in case the system load, parallelism, contention, or lock
1755 // modality changed.
1756 //
1757 // Consider the following alternative:
1758 // Periodically set _SpinDuration = _SpinLimit and try a long/full
1759 // spin attempt. "Periodically" might mean after a tally of
1760 // the # of failed spin attempts (or iterations) reaches some threshold.
1761 // This takes us into the realm of 1-out-of-N spinning, where we
1762 // hold the duration constant but vary the frequency.
1763
1764 ctr = _SpinDuration;
1765 if (ctr <= 0) return 0;
1766
1767 if (NotRunnable(Self, (Thread *) _owner)) {
1768 return 0;
1769 }
1770
1771 // We're good to spin ... spin ingress.
1772 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1773 // when preparing to LD...CAS _owner, etc and the CAS is likely
1774 // to succeed.
1775 if (_succ == NULL) {
1776 _succ = Self;
1777 }
1778 Thread * prv = NULL;
1779
1780 // There are three ways to exit the following loop:
1781 // 1. A successful spin where this thread has acquired the lock.
1782 // 2. Spin failure with prejudice
1783 // 3. Spin failure without prejudice
1784
1785 while (--ctr >= 0) {
1786
1787 // Periodic polling -- Check for pending GC
1788 // Threads may spin while they're unsafe.
1789 // We don't want spinning threads to delay the JVM from reaching
1790 // a stop-the-world safepoint or to steal cycles from GC.
1791 // If we detect a pending safepoint we abort in order that
1792 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1793 // this thread, if safe, doesn't steal cycles from GC.
1794 // This is in keeping with the "no loitering in runtime" rule.
1795 // We periodically check to see if there's a safepoint pending.
1796 if ((ctr & 0xFF) == 0) {
1797 if (SafepointMechanism::should_block(Self)) {
1798 goto Abort; // abrupt spin egress
1799 }
1800 SpinPause();
1801 }
1802
1803 // Probe _owner with TATAS
1804 // If this thread observes the monitor transition or flicker
1805 // from locked to unlocked to locked, then the odds that this
1806 // thread will acquire the lock in this spin attempt go down
1807 // considerably. The same argument applies if the CAS fails
1808 // or if we observe _owner change from one non-null value to
1809 // another non-null value. In such cases we might abort
1810 // the spin without prejudice or apply a "penalty" to the
1811 // spin count-down variable "ctr", reducing it by 100, say.
1812
1813 Thread * ox = (Thread *) _owner;
1814 if (ox == NULL) {
1815 ox = (Thread*)try_set_owner_from(NULL, Self);
1816 if (ox == NULL) {
1817 // The CAS succeeded -- this thread acquired ownership
1818 // Take care of some bookkeeping to exit spin state.
1819 if (_succ == Self) {
1820 _succ = NULL;
1821 }
1822
1823 // Increase _SpinDuration :
1824 // The spin was successful (profitable) so we tend toward
1825 // longer spin attempts in the future.
1826 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
1827 // If we acquired the lock early in the spin cycle it
1828 // makes sense to increase _SpinDuration proportionally.
1829 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1830 int x = _SpinDuration;
1831 if (x < Knob_SpinLimit) {
1832 if (x < Knob_Poverty) x = Knob_Poverty;
1833 _SpinDuration = x + Knob_Bonus;
1834 }
1835 return 1;
1836 }
1837
1838 // The CAS failed ... we can take any of the following actions:
1839 // * penalize: ctr -= CASPenalty
1840 // * exit spin with prejudice -- goto Abort;
1841 // * exit spin without prejudice.
1842 // * Since CAS is high-latency, retry again immediately.
1843 prv = ox;
1844 goto Abort;
1845 }
1846
1847 // Did lock ownership change hands ?
1848 if (ox != prv && prv != NULL) {
1849 goto Abort;
1850 }
1851 prv = ox;
1852
1853 // Abort the spin if the owner is not executing.
1854 // The owner must be executing in order to drop the lock.
1855 // Spinning while the owner is OFFPROC is idiocy.
1856 // Consider: ctr -= RunnablePenalty ;
1857 if (NotRunnable(Self, ox)) {
1858 goto Abort;
1859 }
1860 if (_succ == NULL) {
1861 _succ = Self;
1862 }
1863 }
1864
1865 // Spin failed with prejudice -- reduce _SpinDuration.
1866 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
1867 // AIMD is globally stable.
1868 {
1869 int x = _SpinDuration;
1870 if (x > 0) {
1871 // Consider an AIMD scheme like: x -= (x >> 3) + 100
1872 // This is globally sample and tends to damp the response.
1873 x -= Knob_Penalty;
1874 if (x < 0) x = 0;
1875 _SpinDuration = x;
1876 }
1877 }
1878
1879 Abort:
1880 if (_succ == Self) {
1881 _succ = NULL;
1882 // Invariant: after setting succ=null a contending thread
1883 // must recheck-retry _owner before parking. This usually happens
1884 // in the normal usage of TrySpin(), but it's safest
1885 // to make TrySpin() as foolproof as possible.
1886 OrderAccess::fence();
1887 if (TryLock(Self) > 0) return 1;
1888 }
1889 return 0;
1890 }
1891
1892 // NotRunnable() -- informed spinning
1893 //
1894 // Don't bother spinning if the owner is not eligible to drop the lock.
1895 // Spin only if the owner thread is _thread_in_Java or _thread_in_vm.
1896 // The thread must be runnable in order to drop the lock in timely fashion.
1897 // If the _owner is not runnable then spinning will not likely be
1898 // successful (profitable).
1899 //
1900 // Beware -- the thread referenced by _owner could have died
1901 // so a simply fetch from _owner->_thread_state might trap.
1902 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
1903 // Because of the lifecycle issues, the _thread_state values
1904 // observed by NotRunnable() might be garbage. NotRunnable must
1905 // tolerate this and consider the observed _thread_state value
1906 // as advisory.
1907 //
1908 // Beware too, that _owner is sometimes a BasicLock address and sometimes
1909 // a thread pointer.
1910 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
1911 // with the LSB of _owner. Another option would be to probabilistically probe
1912 // the putative _owner->TypeTag value.
1913 //
1914 // Checking _thread_state isn't perfect. Even if the thread is
1915 // in_java it might be blocked on a page-fault or have been preempted
1916 // and sitting on a ready/dispatch queue.
1917 //
1918 // The return value from NotRunnable() is *advisory* -- the
1919 // result is based on sampling and is not necessarily coherent.
1920 // The caller must tolerate false-negative and false-positive errors.
1921 // Spinning, in general, is probabilistic anyway.
1922
1923
1924 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) {
1925 // Check ox->TypeTag == 2BAD.
1926 if (ox == NULL) return 0;
1927
1928 // Avoid transitive spinning ...
1929 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
1930 // Immediately after T1 acquires L it's possible that T2, also
1931 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
1932 // This occurs transiently after T1 acquired L but before
1933 // T1 managed to clear T1.Stalled. T2 does not need to abort
1934 // its spin in this circumstance.
1935 intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
1936
1937 if (BlockedOn == 1) return 1;
1938 if (BlockedOn != 0) {
1939 return BlockedOn != intptr_t(this) && _owner == ox;
1940 }
1941
1942 assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant");
1943 int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);;
1944 // consider also: jst != _thread_in_Java -- but that's overspecific.
1945 return jst == _thread_blocked || jst == _thread_in_native;
1946 }
1947
1948
1949 // -----------------------------------------------------------------------------
1950 // WaitSet management ...
1951
1952 ObjectWaiter::ObjectWaiter(Thread* thread) {
1953 _next = NULL;
1954 _prev = NULL;
1955 _notified = 0;
1956 _notifier_tid = 0;
1957 TState = TS_RUN;
1958 _thread = thread;
1959 _event = thread->_ParkEvent;
1960 _active = false;
1961 assert(_event != NULL, "invariant");
1962 }
1963
1964 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
1965 JavaThread *jt = (JavaThread *)this->_thread;
1966 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
1967 }
1968
1969 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
1970 JavaThread *jt = (JavaThread *)this->_thread;
1971 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
1972 }
1973
1974 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
1975 assert(node != NULL, "should not add NULL node");
1976 assert(node->_prev == NULL, "node already in list");
1977 assert(node->_next == NULL, "node already in list");
1978 // put node at end of queue (circular doubly linked list)
1979 if (_WaitSet == NULL) {
1980 _WaitSet = node;
1981 node->_prev = node;
1982 node->_next = node;
1983 } else {
1984 ObjectWaiter* head = _WaitSet;
1985 ObjectWaiter* tail = head->_prev;
1986 assert(tail->_next == head, "invariant check");
1987 tail->_next = node;
1988 head->_prev = node;
1989 node->_next = head;
1990 node->_prev = tail;
1991 }
1992 }
1993
1994 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
1995 // dequeue the very first waiter
1996 ObjectWaiter* waiter = _WaitSet;
1997 if (waiter) {
1998 DequeueSpecificWaiter(waiter);
1999 }
2000 return waiter;
2001 }
2002
2003 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2004 assert(node != NULL, "should not dequeue NULL node");
2005 assert(node->_prev != NULL, "node already removed from list");
2006 assert(node->_next != NULL, "node already removed from list");
2007 // when the waiter has woken up because of interrupt,
2008 // timeout or other spurious wake-up, dequeue the
2009 // waiter from waiting list
2010 ObjectWaiter* next = node->_next;
2011 if (next == node) {
2012 assert(node->_prev == node, "invariant check");
2013 _WaitSet = NULL;
2014 } else {
2015 ObjectWaiter* prev = node->_prev;
2016 assert(prev->_next == node, "invariant check");
2017 assert(next->_prev == node, "invariant check");
2018 next->_prev = prev;
2019 prev->_next = next;
2020 if (_WaitSet == node) {
2021 _WaitSet = next;
2022 }
2023 }
2024 node->_next = NULL;
2025 node->_prev = NULL;
2026 }
2027
2028 // -----------------------------------------------------------------------------
2029 // PerfData support
2030 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL;
2031 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL;
2032 PerfCounter * ObjectMonitor::_sync_Parks = NULL;
2033 PerfCounter * ObjectMonitor::_sync_Notifications = NULL;
2034 PerfCounter * ObjectMonitor::_sync_Inflations = NULL;
2035 PerfCounter * ObjectMonitor::_sync_Deflations = NULL;
2036 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL;
2037
2038 // One-shot global initialization for the sync subsystem.
2039 // We could also defer initialization and initialize on-demand
2040 // the first time we call ObjectSynchronizer::inflate().
2041 // Initialization would be protected - like so many things - by
2042 // the MonitorCache_lock.
2043
2044 void ObjectMonitor::Initialize() {
2045 assert(!InitDone, "invariant");
2046
2047 if (!os::is_MP()) {
2048 Knob_SpinLimit = 0;
2049 Knob_PreSpin = 0;
2050 Knob_FixedSpin = -1;
2051 }
2052
2053 if (UsePerfData) {
2054 EXCEPTION_MARK;
2055 #define NEWPERFCOUNTER(n) \
2056 { \
2057 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \
2058 CHECK); \
2059 }
2060 #define NEWPERFVARIABLE(n) \
2061 { \
2062 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \
2063 CHECK); \
2064 }
2065 NEWPERFCOUNTER(_sync_Inflations);
2066 NEWPERFCOUNTER(_sync_Deflations);
2067 NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2068 NEWPERFCOUNTER(_sync_FutileWakeups);
2069 NEWPERFCOUNTER(_sync_Parks);
2070 NEWPERFCOUNTER(_sync_Notifications);
2071 NEWPERFVARIABLE(_sync_MonExtant);
2072 #undef NEWPERFCOUNTER
2073 #undef NEWPERFVARIABLE
2074 }
2075
2076 DEBUG_ONLY(InitDone = true;)
2077 }
2078
2079 ObjectMonitorHandle::~ObjectMonitorHandle() {
2080 if (_om_ptr != NULL) {
2081 _om_ptr->dec_ref_count();
2082 _om_ptr = NULL;
2083 }
2084 }
2085
2086 // Save the ObjectMonitor* associated with the specified markWord and
2087 // increment the ref_count. This function should only be called if
2088 // the caller has verified mark.has_monitor() == true. The object
2089 // parameter is needed to verify that ObjectMonitor* has not been
2090 // deflated and reused for another object.
2091 //
2092 // This function returns true if the ObjectMonitor* has been safely
2093 // saved. This function returns false if we have lost a race with
2094 // async deflation; the caller should retry as appropriate.
2095 //
2096 bool ObjectMonitorHandle::save_om_ptr(oop object, markWord mark) {
2097 // is_marked() is a superset of has_monitor() so make sure we
2098 // are called with the proper markWord value.
2099 guarantee(mark.has_monitor() && !mark.is_marked(), "sanity check: mark="
2100 INTPTR_FORMAT, mark.value());
2101
2102 ObjectMonitor* om_ptr = mark.monitor();
2103 om_ptr->inc_ref_count();
2104
2105 if (AsyncDeflateIdleMonitors) {
2106 // Race here if monitor is not owned! The above ref_count bump
2107 // will cause subsequent async deflation to skip it. However,
2108 // previous or concurrent async deflation is a race.
2109 if (om_ptr->owner_is_DEFLATER_MARKER() && om_ptr->ref_count() <= 0) {
2110 // Async deflation is in progress and our ref_count increment
2111 // above lost the race to async deflation. Attempt to restore
2112 // the header/dmw to the object's header so that we only retry
2113 // once if the deflater thread happens to be slow.
2114 om_ptr->install_displaced_markword_in_object(object);
2115 om_ptr->dec_ref_count();
2116 return false;
2117 }
2118 if (om_ptr->ref_count() <= 0) {
2119 // Async deflation is in the process of bailing out, but has not
2120 // yet restored the ref_count field so we return false to force
2121 // a retry. We want a positive ref_count value for a true return.
2122 om_ptr->dec_ref_count();
2123 return false;
2124 }
2125 // The ObjectMonitor could have been deflated and reused for
2126 // another object before we bumped the ref_count so make sure
2127 // our object still refers to this ObjectMonitor.
2128 const markWord tmp = object->mark();
2129 if (!tmp.has_monitor() || tmp.monitor() != om_ptr) {
2130 // Async deflation and reuse won the race so we have to retry.
2131 // Skip object header restoration since that's already done.
2132 om_ptr->dec_ref_count();
2133 return false;
2134 }
2135 }
2136
2137 ADIM_guarantee(_om_ptr == NULL, "sanity check: _om_ptr=" INTPTR_FORMAT,
2138 p2i(_om_ptr));
2139 _om_ptr = om_ptr;
2140 return true;
2141 }
2142
2143 // For internal use by ObjectSynchronizer::inflate().
2144 // This function is only used when we don't have to worry about async
2145 // deflation of the specified ObjectMonitor*.
2146 void ObjectMonitorHandle::set_om_ptr(ObjectMonitor* om_ptr) {
2147 if (_om_ptr == NULL) {
2148 ADIM_guarantee(om_ptr != NULL, "cannot clear an unset om_ptr");
2149 om_ptr->inc_ref_count();
2150 _om_ptr = om_ptr;
2151 } else {
2152 ADIM_guarantee(om_ptr == NULL, "can only clear a set om_ptr");
2153 _om_ptr->dec_ref_count();
2154 _om_ptr = NULL;
2155 }
2156 }
2157
2158 // Save the specified ObjectMonitor* if it is safe, i.e., not being
2159 // async deflated.
2160 //
2161 // This function returns true if the ObjectMonitor* has been safely
2162 // saved. This function returns false if the specified ObjectMonitor*
2163 // is NULL or if we have lost a race with async deflation; the caller
2164 // can retry as appropriate.
2165 bool ObjectMonitorHandle::set_om_ptr_if_safe(ObjectMonitor* om_ptr) {
2166 if (om_ptr == NULL) {
2167 return false; // Nothing to save if input is NULL
2168 }
2169
2170 om_ptr->inc_ref_count();
2171
2172 if (AsyncDeflateIdleMonitors) {
2173 if (om_ptr->owner_is_DEFLATER_MARKER() && om_ptr->ref_count() <= 0) {
2174 // Async deflation is in progress and our ref_count increment
2175 // above lost the race to async deflation.
2176 om_ptr->dec_ref_count();
2177 return false;
2178 }
2179 if (om_ptr->ref_count() <= 0) {
2180 // Async deflation is in the process of bailing out, but has not
2181 // yet restored the ref_count field so we return false to force
2182 // a retry. We want a positive ref_count value for a true return.
2183 om_ptr->dec_ref_count();
2184 return false;
2185 }
2186 // Unlike save_om_ptr(), we don't have context to determine if
2187 // the ObjectMonitor has been deflated and reused for another
2188 // object.
2189 }
2190
2191 ADIM_guarantee(_om_ptr == NULL, "sanity check: _om_ptr=" INTPTR_FORMAT,
2192 p2i(_om_ptr));
2193 _om_ptr = om_ptr;
2194 return true;
2195 }
2196
2197 // Unset the _om_ptr field and decrement the ref_count field.
2198 void ObjectMonitorHandle::unset_om_ptr() {
2199 ADIM_guarantee(_om_ptr != NULL, "_om_ptr must not be NULL");
2200 _om_ptr->dec_ref_count();
2201 _om_ptr = NULL;
2202 }
2203
2204 void ObjectMonitor::print_on(outputStream* st) const {
2205 // The minimal things to print for markWord printing, more can be added for debugging and logging.
2206 st->print("{contentions=0x%08x,waiters=0x%08x"
2207 ",recursions=" INTX_FORMAT ",owner=" INTPTR_FORMAT "}",
2208 contentions(), waiters(), recursions(),
2209 p2i(owner()));
2210 }
2211 void ObjectMonitor::print() const { print_on(tty); }
2212
2213 #ifdef ASSERT
2214 // Print the ObjectMonitor like a debugger would:
2215 //
2216 // (ObjectMonitor) 0x00007fdfb6012e40 = {
2217 // _header = 0x0000000000000001
2218 // _object = 0x000000070ff45fd0
2219 // _allocation_state = Old
2220 // _pad_buf0 = {
2221 // [0] = '\0'
2222 // ...
2223 // [43] = '\0'
2224 // }
2225 // _owner = 0x0000000000000000
2226 // _previous_owner_tid = 0
2227 // _pad_buf1 = {
2228 // [0] = '\0'
2229 // ...
2230 // [47] = '\0'
2231 // }
2232 // _ref_count = 1
2233 // _pad_buf2 = {
2234 // [0] = '\0'
2235 // ...
2236 // [47] = '\0'
2237 // }
2238 // _next_om = 0x0000000000000000
2239 // _recursions = 0
2240 // _EntryList = 0x0000000000000000
2241 // _cxq = 0x0000000000000000
2242 // _succ = 0x0000000000000000
2243 // _Responsible = 0x0000000000000000
2244 // _Spinner = 0
2245 // _SpinDuration = 5000
2246 // _contentions = 0
2247 // _WaitSet = 0x0000700009756248
2248 // _waiters = 1
2249 // _WaitSetLock = 0
2250 // }
2251 //
2252 void ObjectMonitor::print_debug_style_on(outputStream* st) const {
2253 st->print_cr("(ObjectMonitor*) " INTPTR_FORMAT " = {", p2i(this));
2254 st->print_cr(" _header = " INTPTR_FORMAT, header().value());
2255 st->print_cr(" _object = " INTPTR_FORMAT, p2i(_object));
2256 st->print(" _allocation_state = ");
2257 if (is_free()) {
2258 st->print("Free");
2259 } else if (is_old()) {
2260 st->print("Old");
2261 } else if (is_new()) {
2262 st->print("New");
2263 } else {
2264 st->print("unknown=%d", _allocation_state);
2265 }
2266 st->cr();
2267 st->print_cr(" _pad_buf0 = {");
2268 st->print_cr(" [0] = '\\0'");
2269 st->print_cr(" ...");
2270 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf0) - 1);
2271 st->print_cr(" }");
2272 st->print_cr(" _owner = " INTPTR_FORMAT, p2i(_owner));
2273 st->print_cr(" _previous_owner_tid = " JLONG_FORMAT, _previous_owner_tid);
2274 st->print_cr(" _pad_buf1 = {");
2275 st->print_cr(" [0] = '\\0'");
2276 st->print_cr(" ...");
2277 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf1) - 1);
2278 st->print_cr(" }");
2279 st->print_cr(" _ref_count = %d", ref_count());
2280 st->print_cr(" _pad_buf2 = {");
2281 st->print_cr(" [0] = '\\0'");
2282 st->print_cr(" ...");
2283 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf1) - 1);
2284 st->print_cr(" }");
2285 st->print_cr(" _next_om = " INTPTR_FORMAT, p2i(next_om()));
2286 st->print_cr(" _recursions = " INTX_FORMAT, _recursions);
2287 st->print_cr(" _EntryList = " INTPTR_FORMAT, p2i(_EntryList));
2288 st->print_cr(" _cxq = " INTPTR_FORMAT, p2i(_cxq));
2289 st->print_cr(" _succ = " INTPTR_FORMAT, p2i(_succ));
2290 st->print_cr(" _Responsible = " INTPTR_FORMAT, p2i(_Responsible));
2291 st->print_cr(" _Spinner = %d", _Spinner);
2292 st->print_cr(" _SpinDuration = %d", _SpinDuration);
2293 st->print_cr(" _contentions = %d", _contentions);
2294 st->print_cr(" _WaitSet = " INTPTR_FORMAT, p2i(_WaitSet));
2295 st->print_cr(" _waiters = %d", _waiters);
2296 st->print_cr(" _WaitSetLock = %d", _WaitSetLock);
2297 st->print_cr("}");
2298 }
2299 #endif