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(Self, NULL);
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 simply_set_owner_from_BasicLock(Self, cur); // Convert from BasicLock* to Thread*.
267 return;
268 }
269
270 if (AsyncDeflateIdleMonitors &&
271 try_set_owner_from(Self, DEFLATER_MARKER) == 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 // Prevent deflation. See ObjectSynchronizer::deflate_monitor(),
310 // ObjectSynchronizer::deflate_monitor_using_JT() and is_busy().
311 // Ensure the object <-> monitor relationship remains stable while
312 // there's contention.
313 Atomic::add(&_contentions, 1);
314
315 JFR_ONLY(JfrConditionalFlushWithStacktrace<EventJavaMonitorEnter> flush(jt);)
316 EventJavaMonitorEnter event;
317 if (event.should_commit()) {
318 event.set_monitorClass(((oop)this->object())->klass());
319 event.set_address((uintptr_t)(this->object_addr()));
320 }
321
322 { // Change java thread status to indicate blocked on monitor enter.
323 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
324
325 Self->set_current_pending_monitor(this);
326
327 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
328 if (JvmtiExport::should_post_monitor_contended_enter()) {
329 JvmtiExport::post_monitor_contended_enter(jt, this);
330
331 // The current thread does not yet own the monitor and does not
332 // yet appear on any queues that would get it made the successor.
333 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
334 // handler cannot accidentally consume an unpark() meant for the
335 // ParkEvent associated with this ObjectMonitor.
336 }
337
338 OSThreadContendState osts(Self->osthread());
339 ThreadBlockInVM tbivm(jt);
340
341 // TODO-FIXME: change the following for(;;) loop to straight-line code.
342 for (;;) {
343 jt->set_suspend_equivalent();
344 // cleared by handle_special_suspend_equivalent_condition()
345 // or java_suspend_self()
346
347 EnterI(THREAD);
348
349 if (!ExitSuspendEquivalent(jt)) break;
350
351 // We have acquired the contended monitor, but while we were
352 // waiting another thread suspended us. We don't want to enter
353 // the monitor while suspended because that would surprise the
354 // thread that suspended us.
355 //
356 _recursions = 0;
357 _succ = NULL;
358 exit(false, Self);
359
360 jt->java_suspend_self();
361 }
362 Self->set_current_pending_monitor(NULL);
363
364 // We cleared the pending monitor info since we've just gotten past
365 // the enter-check-for-suspend dance and we now own the monitor free
366 // and clear, i.e., it is no longer pending. The ThreadBlockInVM
367 // destructor can go to a safepoint at the end of this block. If we
368 // do a thread dump during that safepoint, then this thread will show
369 // as having "-locked" the monitor, but the OS and java.lang.Thread
370 // states will still report that the thread is blocked trying to
371 // acquire it.
372 }
373
374 Atomic::dec(&_contentions);
375 assert(_contentions >= 0, "must not be negative: contentions=%d", _contentions);
376 Self->_Stalled = 0;
377
378 // Must either set _recursions = 0 or ASSERT _recursions == 0.
379 assert(_recursions == 0, "invariant");
380 assert(_owner == Self, "invariant");
381 assert(_succ != Self, "invariant");
382 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant");
383
384 // The thread -- now the owner -- is back in vm mode.
385 // Report the glorious news via TI,DTrace and jvmstat.
386 // The probe effect is non-trivial. All the reportage occurs
387 // while we hold the monitor, increasing the length of the critical
388 // section. Amdahl's parallel speedup law comes vividly into play.
389 //
390 // Another option might be to aggregate the events (thread local or
391 // per-monitor aggregation) and defer reporting until a more opportune
392 // time -- such as next time some thread encounters contention but has
393 // yet to acquire the lock. While spinning that thread could
394 // spinning we could increment JVMStat counters, etc.
395
396 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
397 if (JvmtiExport::should_post_monitor_contended_entered()) {
398 JvmtiExport::post_monitor_contended_entered(jt, this);
399
400 // The current thread already owns the monitor and is not going to
401 // call park() for the remainder of the monitor enter protocol. So
402 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
403 // event handler consumed an unpark() issued by the thread that
404 // just exited the monitor.
405 }
406 if (event.should_commit()) {
407 event.set_previousOwner((uintptr_t)_previous_owner_tid);
408 event.commit();
409 }
410 OM_PERFDATA_OP(ContendedLockAttempts, inc());
411 }
412
413 // Caveat: TryLock() is not necessarily serializing if it returns failure.
414 // Callers must compensate as needed.
415
416 int ObjectMonitor::TryLock(Thread * Self) {
417 void * own = _owner;
418 if (own != NULL) return 0;
419 if (try_set_owner_from(Self, NULL) == NULL) {
420 assert(_recursions == 0, "invariant");
421 return 1;
422 }
423 // The lock had been free momentarily, but we lost the race to the lock.
424 // Interference -- the CAS failed.
425 // We can either return -1 or retry.
426 // Retry doesn't make as much sense because the lock was just acquired.
427 return -1;
428 }
429
430 // Install the displaced mark word (dmw) of a deflating ObjectMonitor
431 // into the header of the object associated with the monitor. This
432 // idempotent method is called by a thread that is deflating a
433 // monitor and by other threads that have detected a race with the
434 // deflation process.
435 void ObjectMonitor::install_displaced_markword_in_object(const oop obj) {
436 // This function must only be called when (owner == DEFLATER_MARKER
437 // && ref_count <= 0), but we can't guarantee that here because
438 // those values could change when the ObjectMonitor gets moved from
439 // the global free list to a per-thread free list.
440
441 guarantee(obj != NULL, "must be non-NULL");
442 if (object() != obj) {
443 // ObjectMonitor's object ref no longer refers to the target object
444 // so the object's header has already been restored.
445 return;
446 }
447
448 markWord dmw = header();
449 if (dmw.value() == 0) {
450 // ObjectMonitor's header/dmw has been cleared so the ObjectMonitor
451 // has been deflated and taken off the global free list.
452 return;
453 }
454
455 // A non-NULL dmw has to be either neutral (not locked and not marked)
456 // or is already participating in this restoration protocol.
457 assert(dmw.is_neutral() || (dmw.is_marked() && dmw.hash() == 0),
458 "failed precondition: dmw=" INTPTR_FORMAT, dmw.value());
459
460 markWord marked_dmw = markWord::zero();
461 if (!dmw.is_marked() && dmw.hash() == 0) {
462 // This dmw has not yet started the restoration protocol so we
463 // mark a copy of the dmw to begin the protocol.
464 // Note: A dmw with a hashcode does not take this code path.
465 marked_dmw = dmw.set_marked();
466
467 // All of the callers to this function can be racing with each
468 // other trying to update the _header field.
469 dmw = (markWord) Atomic::cmpxchg(&_header, dmw, marked_dmw);
470 if (dmw.value() == 0) {
471 // ObjectMonitor's header/dmw has been cleared so the object's
472 // header has already been restored.
473 return;
474 }
475 // The _header field is now marked. The winner's 'dmw' variable
476 // contains the original, unmarked header/dmw value and any
477 // losers have a marked header/dmw value that will be cleaned
478 // up below.
479 }
480
481 if (dmw.is_marked()) {
482 // Clear the mark from the header/dmw copy in preparation for
483 // possible restoration from this thread.
484 assert(dmw.hash() == 0, "hashcode must be 0: dmw=" INTPTR_FORMAT,
485 dmw.value());
486 dmw = dmw.set_unmarked();
487 }
488 assert(dmw.is_neutral(), "must be neutral: dmw=" INTPTR_FORMAT, dmw.value());
489
490 // Install displaced mark word if the object's header still points
491 // to this ObjectMonitor. All racing callers to this function will
492 // reach this point, but only one can win.
493 obj->cas_set_mark(dmw, markWord::encode(this));
494
495 // Note: It does not matter which thread restored the header/dmw
496 // into the object's header. The thread deflating the monitor just
497 // wanted the object's header restored and it is. The threads that
498 // detected a race with the deflation process also wanted the
499 // object's header restored before they retry their operation and
500 // because it is restored they will only retry once.
501 }
502
503 // Convert the fields used by is_busy() to a string that can be
504 // used for diagnostic output.
505 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) {
506 ss->print("is_busy: contentions=%d, waiters=%d, ", _contentions, _waiters);
507 if (!AsyncDeflateIdleMonitors) {
508 ss->print("owner=" INTPTR_FORMAT, p2i(_owner));
509 } else if (_owner != DEFLATER_MARKER) {
510 ss->print("owner=" INTPTR_FORMAT, p2i(_owner));
511 } else {
512 // We report NULL instead of DEFLATER_MARKER here because is_busy()
513 // ignores DEFLATER_MARKER values.
514 ss->print("owner=" INTPTR_FORMAT, NULL);
515 }
516 ss->print(", cxq=" INTPTR_FORMAT ", EntryList=" INTPTR_FORMAT, p2i(_cxq),
517 p2i(_EntryList));
518 return ss->base();
519 }
520
521 #define MAX_RECHECK_INTERVAL 1000
522
523 void ObjectMonitor::EnterI(TRAPS) {
524 jint l_ref_count = ref_count();
525 ADIM_guarantee(l_ref_count > 0, "must be positive: l_ref_count=%d, ref_count=%d", l_ref_count, ref_count());
526
527 Thread * const Self = THREAD;
528 assert(Self->is_Java_thread(), "invariant");
529 assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant");
530
531 // Try the lock - TATAS
532 if (TryLock (Self) > 0) {
533 assert(_succ != Self, "invariant");
534 assert(_owner == Self, "invariant");
535 assert(_Responsible != Self, "invariant");
536 return;
537 }
538
539 if (AsyncDeflateIdleMonitors &&
540 try_set_owner_from(Self, DEFLATER_MARKER) == DEFLATER_MARKER) {
541 // The deflation protocol finished the first part (setting owner),
542 // but it failed the second part (making ref_count negative) and
543 // bailed. Or the ObjectMonitor was async deflated and reused.
544 // Acquired the monitor.
545 assert(_succ != Self, "invariant");
546 assert(_Responsible != Self, "invariant");
547 return;
548 }
549
550 assert(InitDone, "Unexpectedly not initialized");
551
552 // We try one round of spinning *before* enqueueing Self.
553 //
554 // If the _owner is ready but OFFPROC we could use a YieldTo()
555 // operation to donate the remainder of this thread's quantum
556 // to the owner. This has subtle but beneficial affinity
557 // effects.
558
559 if (TrySpin(Self) > 0) {
560 assert(_owner == Self, "invariant");
561 assert(_succ != Self, "invariant");
562 assert(_Responsible != Self, "invariant");
563 return;
564 }
565
566 // The Spin failed -- Enqueue and park the thread ...
567 assert(_succ != Self, "invariant");
568 assert(_owner != Self, "invariant");
569 assert(_Responsible != Self, "invariant");
570
571 // Enqueue "Self" on ObjectMonitor's _cxq.
572 //
573 // Node acts as a proxy for Self.
574 // As an aside, if were to ever rewrite the synchronization code mostly
575 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
576 // Java objects. This would avoid awkward lifecycle and liveness issues,
577 // as well as eliminate a subset of ABA issues.
578 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
579
580 ObjectWaiter node(Self);
581 Self->_ParkEvent->reset();
582 node._prev = (ObjectWaiter *) 0xBAD;
583 node.TState = ObjectWaiter::TS_CXQ;
584
585 // Push "Self" onto the front of the _cxq.
586 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
587 // Note that spinning tends to reduce the rate at which threads
588 // enqueue and dequeue on EntryList|cxq.
589 ObjectWaiter * nxt;
590 for (;;) {
591 node._next = nxt = _cxq;
592 if (Atomic::cmpxchg(&_cxq, nxt, &node) == nxt) break;
593
594 // Interference - the CAS failed because _cxq changed. Just retry.
595 // As an optional optimization we retry the lock.
596 if (TryLock (Self) > 0) {
597 assert(_succ != Self, "invariant");
598 assert(_owner == Self, "invariant");
599 assert(_Responsible != Self, "invariant");
600 return;
601 }
602 }
603
604 // Check for cxq|EntryList edge transition to non-null. This indicates
605 // the onset of contention. While contention persists exiting threads
606 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
607 // operations revert to the faster 1-0 mode. This enter operation may interleave
608 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
609 // arrange for one of the contending thread to use a timed park() operations
610 // to detect and recover from the race. (Stranding is form of progress failure
611 // where the monitor is unlocked but all the contending threads remain parked).
612 // That is, at least one of the contended threads will periodically poll _owner.
613 // One of the contending threads will become the designated "Responsible" thread.
614 // The Responsible thread uses a timed park instead of a normal indefinite park
615 // operation -- it periodically wakes and checks for and recovers from potential
616 // strandings admitted by 1-0 exit operations. We need at most one Responsible
617 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
618 // be responsible for a monitor.
619 //
620 // Currently, one of the contended threads takes on the added role of "Responsible".
621 // A viable alternative would be to use a dedicated "stranding checker" thread
622 // that periodically iterated over all the threads (or active monitors) and unparked
623 // successors where there was risk of stranding. This would help eliminate the
624 // timer scalability issues we see on some platforms as we'd only have one thread
625 // -- the checker -- parked on a timer.
626
627 if (nxt == NULL && _EntryList == NULL) {
628 // Try to assume the role of responsible thread for the monitor.
629 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
630 Atomic::replace_if_null(&_Responsible, Self);
631 }
632
633 // The lock might have been released while this thread was occupied queueing
634 // itself onto _cxq. To close the race and avoid "stranding" and
635 // progress-liveness failure we must resample-retry _owner before parking.
636 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
637 // In this case the ST-MEMBAR is accomplished with CAS().
638 //
639 // TODO: Defer all thread state transitions until park-time.
640 // Since state transitions are heavy and inefficient we'd like
641 // to defer the state transitions until absolutely necessary,
642 // and in doing so avoid some transitions ...
643
644 int nWakeups = 0;
645 int recheckInterval = 1;
646
647 for (;;) {
648
649 if (TryLock(Self) > 0) break;
650 assert(_owner != Self, "invariant");
651
652 // park self
653 if (_Responsible == Self) {
654 Self->_ParkEvent->park((jlong) recheckInterval);
655 // Increase the recheckInterval, but clamp the value.
656 recheckInterval *= 8;
657 if (recheckInterval > MAX_RECHECK_INTERVAL) {
658 recheckInterval = MAX_RECHECK_INTERVAL;
659 }
660 } else {
661 Self->_ParkEvent->park();
662 }
663
664 if (TryLock(Self) > 0) break;
665
666 if (AsyncDeflateIdleMonitors &&
667 try_set_owner_from(Self, DEFLATER_MARKER) == DEFLATER_MARKER) {
668 // The deflation protocol finished the first part (setting owner),
669 // but it failed the second part (making ref_count negative) and
670 // bailed. Or the ObjectMonitor was async deflated and reused.
671 // Acquired the monitor.
672 break;
673 }
674
675 // The lock is still contested.
676 // Keep a tally of the # of futile wakeups.
677 // Note that the counter is not protected by a lock or updated by atomics.
678 // That is by design - we trade "lossy" counters which are exposed to
679 // races during updates for a lower probe effect.
680
681 // This PerfData object can be used in parallel with a safepoint.
682 // See the work around in PerfDataManager::destroy().
683 OM_PERFDATA_OP(FutileWakeups, inc());
684 ++nWakeups;
685
686 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
687 // We can defer clearing _succ until after the spin completes
688 // TrySpin() must tolerate being called with _succ == Self.
689 // Try yet another round of adaptive spinning.
690 if (TrySpin(Self) > 0) break;
691
692 // We can find that we were unpark()ed and redesignated _succ while
693 // we were spinning. That's harmless. If we iterate and call park(),
694 // park() will consume the event and return immediately and we'll
695 // just spin again. This pattern can repeat, leaving _succ to simply
696 // spin on a CPU.
697
698 if (_succ == Self) _succ = NULL;
699
700 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
701 OrderAccess::fence();
702 }
703
704 // Egress :
705 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
706 // Normally we'll find Self on the EntryList .
707 // From the perspective of the lock owner (this thread), the
708 // EntryList is stable and cxq is prepend-only.
709 // The head of cxq is volatile but the interior is stable.
710 // In addition, Self.TState is stable.
711
712 assert(_owner == Self, "invariant");
713 assert(object() != NULL, "invariant");
714 // I'd like to write:
715 // guarantee (((oop)(object()))->mark() == markWord::encode(this), "invariant") ;
716 // but as we're at a safepoint that's not safe.
717
718 UnlinkAfterAcquire(Self, &node);
719 if (_succ == Self) _succ = NULL;
720
721 assert(_succ != Self, "invariant");
722 if (_Responsible == Self) {
723 _Responsible = NULL;
724 OrderAccess::fence(); // Dekker pivot-point
725
726 // We may leave threads on cxq|EntryList without a designated
727 // "Responsible" thread. This is benign. When this thread subsequently
728 // exits the monitor it can "see" such preexisting "old" threads --
729 // threads that arrived on the cxq|EntryList before the fence, above --
730 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
731 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
732 // non-null and elect a new "Responsible" timer thread.
733 //
734 // This thread executes:
735 // ST Responsible=null; MEMBAR (in enter epilogue - here)
736 // LD cxq|EntryList (in subsequent exit)
737 //
738 // Entering threads in the slow/contended path execute:
739 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
740 // The (ST cxq; MEMBAR) is accomplished with CAS().
741 //
742 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
743 // exit operation from floating above the ST Responsible=null.
744 }
745
746 // We've acquired ownership with CAS().
747 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
748 // But since the CAS() this thread may have also stored into _succ,
749 // EntryList, cxq or Responsible. These meta-data updates must be
750 // visible __before this thread subsequently drops the lock.
751 // Consider what could occur if we didn't enforce this constraint --
752 // STs to monitor meta-data and user-data could reorder with (become
753 // visible after) the ST in exit that drops ownership of the lock.
754 // Some other thread could then acquire the lock, but observe inconsistent
755 // or old monitor meta-data and heap data. That violates the JMM.
756 // To that end, the 1-0 exit() operation must have at least STST|LDST
757 // "release" barrier semantics. Specifically, there must be at least a
758 // STST|LDST barrier in exit() before the ST of null into _owner that drops
759 // the lock. The barrier ensures that changes to monitor meta-data and data
760 // protected by the lock will be visible before we release the lock, and
761 // therefore before some other thread (CPU) has a chance to acquire the lock.
762 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
763 //
764 // Critically, any prior STs to _succ or EntryList must be visible before
765 // the ST of null into _owner in the *subsequent* (following) corresponding
766 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
767 // execute a serializing instruction.
768
769 return;
770 }
771
772 // ReenterI() is a specialized inline form of the latter half of the
773 // contended slow-path from EnterI(). We use ReenterI() only for
774 // monitor reentry in wait().
775 //
776 // In the future we should reconcile EnterI() and ReenterI().
777
778 void ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) {
779 jint l_ref_count = ref_count();
780 ADIM_guarantee(l_ref_count > 0, "must be positive: l_ref_count=%d, ref_count=%d", l_ref_count, ref_count());
781
782 assert(Self != NULL, "invariant");
783 assert(SelfNode != NULL, "invariant");
784 assert(SelfNode->_thread == Self, "invariant");
785 assert(_waiters > 0, "invariant");
786 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant");
787 assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
788 JavaThread * jt = (JavaThread *) Self;
789
790 int nWakeups = 0;
791 for (;;) {
792 ObjectWaiter::TStates v = SelfNode->TState;
793 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
794 assert(_owner != Self, "invariant");
795
796 if (TryLock(Self) > 0) break;
797 if (TrySpin(Self) > 0) break;
798
799 if (AsyncDeflateIdleMonitors &&
800 try_set_owner_from(Self, DEFLATER_MARKER) == DEFLATER_MARKER) {
801 // The deflation protocol finished the first part (setting owner),
802 // but it failed the second part (making ref_count negative) and
803 // bailed. Or the ObjectMonitor was async deflated and reused.
804 // Acquired the monitor.
805 break;
806 }
807
808 // State transition wrappers around park() ...
809 // ReenterI() wisely defers state transitions until
810 // it's clear we must park the thread.
811 {
812 OSThreadContendState osts(Self->osthread());
813 ThreadBlockInVM tbivm(jt);
814
815 // cleared by handle_special_suspend_equivalent_condition()
816 // or java_suspend_self()
817 jt->set_suspend_equivalent();
818 Self->_ParkEvent->park();
819
820 // were we externally suspended while we were waiting?
821 for (;;) {
822 if (!ExitSuspendEquivalent(jt)) break;
823 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
824 jt->java_suspend_self();
825 jt->set_suspend_equivalent();
826 }
827 }
828
829 // Try again, but just so we distinguish between futile wakeups and
830 // successful wakeups. The following test isn't algorithmically
831 // necessary, but it helps us maintain sensible statistics.
832 if (TryLock(Self) > 0) break;
833
834 // The lock is still contested.
835 // Keep a tally of the # of futile wakeups.
836 // Note that the counter is not protected by a lock or updated by atomics.
837 // That is by design - we trade "lossy" counters which are exposed to
838 // races during updates for a lower probe effect.
839 ++nWakeups;
840
841 // Assuming this is not a spurious wakeup we'll normally
842 // find that _succ == Self.
843 if (_succ == Self) _succ = NULL;
844
845 // Invariant: after clearing _succ a contending thread
846 // *must* retry _owner before parking.
847 OrderAccess::fence();
848
849 // This PerfData object can be used in parallel with a safepoint.
850 // See the work around in PerfDataManager::destroy().
851 OM_PERFDATA_OP(FutileWakeups, inc());
852 }
853
854 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
855 // Normally we'll find Self on the EntryList.
856 // Unlinking from the EntryList is constant-time and atomic-free.
857 // From the perspective of the lock owner (this thread), the
858 // EntryList is stable and cxq is prepend-only.
859 // The head of cxq is volatile but the interior is stable.
860 // In addition, Self.TState is stable.
861
862 assert(_owner == Self, "invariant");
863 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant");
864 UnlinkAfterAcquire(Self, SelfNode);
865 if (_succ == Self) _succ = NULL;
866 assert(_succ != Self, "invariant");
867 SelfNode->TState = ObjectWaiter::TS_RUN;
868 OrderAccess::fence(); // see comments at the end of EnterI()
869 }
870
871 // By convention we unlink a contending thread from EntryList|cxq immediately
872 // after the thread acquires the lock in ::enter(). Equally, we could defer
873 // unlinking the thread until ::exit()-time.
874
875 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) {
876 assert(_owner == Self, "invariant");
877 assert(SelfNode->_thread == Self, "invariant");
878
879 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
880 // Normal case: remove Self from the DLL EntryList .
881 // This is a constant-time operation.
882 ObjectWaiter * nxt = SelfNode->_next;
883 ObjectWaiter * prv = SelfNode->_prev;
884 if (nxt != NULL) nxt->_prev = prv;
885 if (prv != NULL) prv->_next = nxt;
886 if (SelfNode == _EntryList) _EntryList = nxt;
887 assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
888 assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
889 } else {
890 assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant");
891 // Inopportune interleaving -- Self is still on the cxq.
892 // This usually means the enqueue of self raced an exiting thread.
893 // Normally we'll find Self near the front of the cxq, so
894 // dequeueing is typically fast. If needbe we can accelerate
895 // this with some MCS/CHL-like bidirectional list hints and advisory
896 // back-links so dequeueing from the interior will normally operate
897 // in constant-time.
898 // Dequeue Self from either the head (with CAS) or from the interior
899 // with a linear-time scan and normal non-atomic memory operations.
900 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
901 // and then unlink Self from EntryList. We have to drain eventually,
902 // so it might as well be now.
903
904 ObjectWaiter * v = _cxq;
905 assert(v != NULL, "invariant");
906 if (v != SelfNode || Atomic::cmpxchg(&_cxq, v, SelfNode->_next) != v) {
907 // The CAS above can fail from interference IFF a "RAT" arrived.
908 // In that case Self must be in the interior and can no longer be
909 // at the head of cxq.
910 if (v == SelfNode) {
911 assert(_cxq != v, "invariant");
912 v = _cxq; // CAS above failed - start scan at head of list
913 }
914 ObjectWaiter * p;
915 ObjectWaiter * q = NULL;
916 for (p = v; p != NULL && p != SelfNode; p = p->_next) {
917 q = p;
918 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
919 }
920 assert(v != SelfNode, "invariant");
921 assert(p == SelfNode, "Node not found on cxq");
922 assert(p != _cxq, "invariant");
923 assert(q != NULL, "invariant");
924 assert(q->_next == p, "invariant");
925 q->_next = p->_next;
926 }
927 }
928
929 #ifdef ASSERT
930 // Diagnostic hygiene ...
931 SelfNode->_prev = (ObjectWaiter *) 0xBAD;
932 SelfNode->_next = (ObjectWaiter *) 0xBAD;
933 SelfNode->TState = ObjectWaiter::TS_RUN;
934 #endif
935 }
936
937 // -----------------------------------------------------------------------------
938 // Exit support
939 //
940 // exit()
941 // ~~~~~~
942 // Note that the collector can't reclaim the objectMonitor or deflate
943 // the object out from underneath the thread calling ::exit() as the
944 // thread calling ::exit() never transitions to a stable state.
945 // This inhibits GC, which in turn inhibits asynchronous (and
946 // inopportune) reclamation of "this".
947 //
948 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
949 // There's one exception to the claim above, however. EnterI() can call
950 // exit() to drop a lock if the acquirer has been externally suspended.
951 // In that case exit() is called with _thread_state as _thread_blocked,
952 // but the monitor's _contentions field is > 0, which inhibits reclamation.
953 //
954 // 1-0 exit
955 // ~~~~~~~~
956 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
957 // the fast-path operators have been optimized so the common ::exit()
958 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
959 // The code emitted by fast_unlock() elides the usual MEMBAR. This
960 // greatly improves latency -- MEMBAR and CAS having considerable local
961 // latency on modern processors -- but at the cost of "stranding". Absent the
962 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
963 // ::enter() path, resulting in the entering thread being stranding
964 // and a progress-liveness failure. Stranding is extremely rare.
965 // We use timers (timed park operations) & periodic polling to detect
966 // and recover from stranding. Potentially stranded threads periodically
967 // wake up and poll the lock. See the usage of the _Responsible variable.
968 //
969 // The CAS() in enter provides for safety and exclusion, while the CAS or
970 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
971 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
972 // We detect and recover from stranding with timers.
973 //
974 // If a thread transiently strands it'll park until (a) another
975 // thread acquires the lock and then drops the lock, at which time the
976 // exiting thread will notice and unpark the stranded thread, or, (b)
977 // the timer expires. If the lock is high traffic then the stranding latency
978 // will be low due to (a). If the lock is low traffic then the odds of
979 // stranding are lower, although the worst-case stranding latency
980 // is longer. Critically, we don't want to put excessive load in the
981 // platform's timer subsystem. We want to minimize both the timer injection
982 // rate (timers created/sec) as well as the number of timers active at
983 // any one time. (more precisely, we want to minimize timer-seconds, which is
984 // the integral of the # of active timers at any instant over time).
985 // Both impinge on OS scalability. Given that, at most one thread parked on
986 // a monitor will use a timer.
987 //
988 // There is also the risk of a futile wake-up. If we drop the lock
989 // another thread can reacquire the lock immediately, and we can
990 // then wake a thread unnecessarily. This is benign, and we've
991 // structured the code so the windows are short and the frequency
992 // of such futile wakups is low.
993
994 void ObjectMonitor::exit(bool not_suspended, TRAPS) {
995 Thread * const Self = THREAD;
996 if (THREAD != _owner) {
997 void* cur = _owner;
998 if (THREAD->is_lock_owned((address)cur)) {
999 assert(_recursions == 0, "invariant");
1000 simply_set_owner_from_BasicLock(Self, cur); // Convert from BasicLock* to Thread*.
1001 _recursions = 0;
1002 } else {
1003 // Apparent unbalanced locking ...
1004 // Naively we'd like to throw IllegalMonitorStateException.
1005 // As a practical matter we can neither allocate nor throw an
1006 // exception as ::exit() can be called from leaf routines.
1007 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
1008 // Upon deeper reflection, however, in a properly run JVM the only
1009 // way we should encounter this situation is in the presence of
1010 // unbalanced JNI locking. TODO: CheckJNICalls.
1011 // See also: CR4414101
1012 //#ifdef ASSERT
1013 LogStreamHandle(Error, monitorinflation) lsh;
1014 lsh.print_cr("ERROR: ObjectMonitor::exit(): thread=" INTPTR_FORMAT
1015 " is exiting an ObjectMonitor it does not own.", p2i(THREAD));
1016 lsh.print_cr("The imbalance is possibly caused by JNI locking.");
1017 print_debug_style_on(&lsh);
1018 //#endif
1019 // Changing this from an assert() to ADIM_guarantee() may run
1020 // afoul of any test that is inducing non-balanced JNI locking.
1021 ADIM_guarantee(false, "Non-balanced monitor enter/exit!");
1022 return;
1023 }
1024 }
1025
1026 if (_recursions != 0) {
1027 _recursions--; // this is simple recursive enter
1028 return;
1029 }
1030
1031 // Invariant: after setting Responsible=null an thread must execute
1032 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
1033 _Responsible = NULL;
1034
1035 #if INCLUDE_JFR
1036 // get the owner's thread id for the MonitorEnter event
1037 // if it is enabled and the thread isn't suspended
1038 if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
1039 _previous_owner_tid = JFR_THREAD_ID(Self);
1040 }
1041 #endif
1042
1043 for (;;) {
1044 assert(THREAD == _owner, "invariant");
1045
1046 // release semantics: prior loads and stores from within the critical section
1047 // must not float (reorder) past the following store that drops the lock.
1048 if (AsyncDeflateIdleMonitors) {
1049 set_owner_from(NULL, Self);
1050 } else {
1051 Atomic::release_store(&_owner, (void*)NULL); // drop the lock
1052 OrderAccess::storeload(); // See if we need to wake a successor
1053 }
1054 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1055 return;
1056 }
1057 // Other threads are blocked trying to acquire the lock.
1058
1059 // Normally the exiting thread is responsible for ensuring succession,
1060 // but if other successors are ready or other entering threads are spinning
1061 // then this thread can simply store NULL into _owner and exit without
1062 // waking a successor. The existence of spinners or ready successors
1063 // guarantees proper succession (liveness). Responsibility passes to the
1064 // ready or running successors. The exiting thread delegates the duty.
1065 // More precisely, if a successor already exists this thread is absolved
1066 // of the responsibility of waking (unparking) one.
1067 //
1068 // The _succ variable is critical to reducing futile wakeup frequency.
1069 // _succ identifies the "heir presumptive" thread that has been made
1070 // ready (unparked) but that has not yet run. We need only one such
1071 // successor thread to guarantee progress.
1072 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1073 // section 3.3 "Futile Wakeup Throttling" for details.
1074 //
1075 // Note that spinners in Enter() also set _succ non-null.
1076 // In the current implementation spinners opportunistically set
1077 // _succ so that exiting threads might avoid waking a successor.
1078 // Another less appealing alternative would be for the exiting thread
1079 // to drop the lock and then spin briefly to see if a spinner managed
1080 // to acquire the lock. If so, the exiting thread could exit
1081 // immediately without waking a successor, otherwise the exiting
1082 // thread would need to dequeue and wake a successor.
1083 // (Note that we'd need to make the post-drop spin short, but no
1084 // shorter than the worst-case round-trip cache-line migration time.
1085 // The dropped lock needs to become visible to the spinner, and then
1086 // the acquisition of the lock by the spinner must become visible to
1087 // the exiting thread).
1088
1089 // It appears that an heir-presumptive (successor) must be made ready.
1090 // Only the current lock owner can manipulate the EntryList or
1091 // drain _cxq, so we need to reacquire the lock. If we fail
1092 // to reacquire the lock the responsibility for ensuring succession
1093 // falls to the new owner.
1094 //
1095 if (try_set_owner_from(Self, NULL) != NULL) {
1096 return;
1097 }
1098
1099 guarantee(_owner == THREAD, "invariant");
1100
1101 ObjectWaiter * w = NULL;
1102
1103 w = _EntryList;
1104 if (w != NULL) {
1105 // I'd like to write: guarantee (w->_thread != Self).
1106 // But in practice an exiting thread may find itself on the EntryList.
1107 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1108 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1109 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1110 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1111 // release the lock "O". T2 resumes immediately after the ST of null into
1112 // _owner, above. T2 notices that the EntryList is populated, so it
1113 // reacquires the lock and then finds itself on the EntryList.
1114 // Given all that, we have to tolerate the circumstance where "w" is
1115 // associated with Self.
1116 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1117 ExitEpilog(Self, w);
1118 return;
1119 }
1120
1121 // If we find that both _cxq and EntryList are null then just
1122 // re-run the exit protocol from the top.
1123 w = _cxq;
1124 if (w == NULL) continue;
1125
1126 // Drain _cxq into EntryList - bulk transfer.
1127 // First, detach _cxq.
1128 // The following loop is tantamount to: w = swap(&cxq, NULL)
1129 for (;;) {
1130 assert(w != NULL, "Invariant");
1131 ObjectWaiter * u = Atomic::cmpxchg(&_cxq, w, (ObjectWaiter*)NULL);
1132 if (u == w) break;
1133 w = u;
1134 }
1135
1136 assert(w != NULL, "invariant");
1137 assert(_EntryList == NULL, "invariant");
1138
1139 // Convert the LIFO SLL anchored by _cxq into a DLL.
1140 // The list reorganization step operates in O(LENGTH(w)) time.
1141 // It's critical that this step operate quickly as
1142 // "Self" still holds the outer-lock, restricting parallelism
1143 // and effectively lengthening the critical section.
1144 // Invariant: s chases t chases u.
1145 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1146 // we have faster access to the tail.
1147
1148 _EntryList = w;
1149 ObjectWaiter * q = NULL;
1150 ObjectWaiter * p;
1151 for (p = w; p != NULL; p = p->_next) {
1152 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1153 p->TState = ObjectWaiter::TS_ENTER;
1154 p->_prev = q;
1155 q = p;
1156 }
1157
1158 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1159 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1160
1161 // See if we can abdicate to a spinner instead of waking a thread.
1162 // A primary goal of the implementation is to reduce the
1163 // context-switch rate.
1164 if (_succ != NULL) continue;
1165
1166 w = _EntryList;
1167 if (w != NULL) {
1168 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1169 ExitEpilog(Self, w);
1170 return;
1171 }
1172 }
1173 }
1174
1175 // ExitSuspendEquivalent:
1176 // A faster alternate to handle_special_suspend_equivalent_condition()
1177 //
1178 // handle_special_suspend_equivalent_condition() unconditionally
1179 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1180 // operations have high latency. Note that in ::enter() we call HSSEC
1181 // while holding the monitor, so we effectively lengthen the critical sections.
1182 //
1183 // There are a number of possible solutions:
1184 //
1185 // A. To ameliorate the problem we might also defer state transitions
1186 // to as late as possible -- just prior to parking.
1187 // Given that, we'd call HSSEC after having returned from park(),
1188 // but before attempting to acquire the monitor. This is only a
1189 // partial solution. It avoids calling HSSEC while holding the
1190 // monitor (good), but it still increases successor reacquisition latency --
1191 // the interval between unparking a successor and the time the successor
1192 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1193 // If we use this technique we can also avoid EnterI()-exit() loop
1194 // in ::enter() where we iteratively drop the lock and then attempt
1195 // to reacquire it after suspending.
1196 //
1197 // B. In the future we might fold all the suspend bits into a
1198 // composite per-thread suspend flag and then update it with CAS().
1199 // Alternately, a Dekker-like mechanism with multiple variables
1200 // would suffice:
1201 // ST Self->_suspend_equivalent = false
1202 // MEMBAR
1203 // LD Self_>_suspend_flags
1204
1205 bool ObjectMonitor::ExitSuspendEquivalent(JavaThread * jSelf) {
1206 return jSelf->handle_special_suspend_equivalent_condition();
1207 }
1208
1209
1210 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) {
1211 assert(_owner == Self, "invariant");
1212
1213 // Exit protocol:
1214 // 1. ST _succ = wakee
1215 // 2. membar #loadstore|#storestore;
1216 // 2. ST _owner = NULL
1217 // 3. unpark(wakee)
1218
1219 _succ = Wakee->_thread;
1220 ParkEvent * Trigger = Wakee->_event;
1221
1222 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1223 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1224 // out-of-scope (non-extant).
1225 Wakee = NULL;
1226
1227 // Drop the lock
1228 if (AsyncDeflateIdleMonitors) {
1229 set_owner_from(NULL, Self);
1230 } else {
1231 Atomic::release_store(&_owner, (void*)NULL);
1232 OrderAccess::fence(); // ST _owner vs LD in unpark()
1233 }
1234
1235 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1236 Trigger->unpark();
1237
1238 // Maintain stats and report events to JVMTI
1239 OM_PERFDATA_OP(Parks, inc());
1240 }
1241
1242
1243 // -----------------------------------------------------------------------------
1244 // Class Loader deadlock handling.
1245 //
1246 // complete_exit exits a lock returning recursion count
1247 // complete_exit/reenter operate as a wait without waiting
1248 // complete_exit requires an inflated monitor
1249 // The _owner field is not always the Thread addr even with an
1250 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1251 // thread due to contention.
1252 intx ObjectMonitor::complete_exit(TRAPS) {
1253 Thread * const Self = THREAD;
1254 assert(Self->is_Java_thread(), "Must be Java thread!");
1255 JavaThread *jt = (JavaThread *)THREAD;
1256
1257 assert(InitDone, "Unexpectedly not initialized");
1258
1259 if (THREAD != _owner) {
1260 void* cur = _owner;
1261 if (THREAD->is_lock_owned((address)cur)) {
1262 assert(_recursions == 0, "internal state error");
1263 simply_set_owner_from_BasicLock(Self, cur); // Convert from BasicLock* to Thread*.
1264 _recursions = 0;
1265 }
1266 }
1267
1268 guarantee(Self == _owner, "complete_exit not owner");
1269 intx save = _recursions; // record the old recursion count
1270 _recursions = 0; // set the recursion level to be 0
1271 exit(true, Self); // exit the monitor
1272 guarantee(_owner != Self, "invariant");
1273 return save;
1274 }
1275
1276 // reenter() enters a lock and sets recursion count
1277 // complete_exit/reenter operate as a wait without waiting
1278 void ObjectMonitor::reenter(intx recursions, TRAPS) {
1279 Thread * const Self = THREAD;
1280 assert(Self->is_Java_thread(), "Must be Java thread!");
1281 JavaThread *jt = (JavaThread *)THREAD;
1282
1283 guarantee(_owner != Self, "reenter already owner");
1284 enter(THREAD);
1285 // Entered the monitor.
1286 guarantee(_recursions == 0, "reenter recursion");
1287 _recursions = recursions;
1288 }
1289
1290 // Checks that the current THREAD owns this monitor and causes an
1291 // immediate return if it doesn't. We don't use the CHECK macro
1292 // because we want the IMSE to be the only exception that is thrown
1293 // from the call site when false is returned. Any other pending
1294 // exception is ignored.
1295 #define CHECK_OWNER() \
1296 do { \
1297 if (!check_owner(THREAD)) { \
1298 assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \
1299 return; \
1300 } \
1301 } while (false)
1302
1303 // Returns true if the specified thread owns the ObjectMonitor.
1304 // Otherwise returns false and throws IllegalMonitorStateException
1305 // (IMSE). If there is a pending exception and the specified thread
1306 // is not the owner, that exception will be replaced by the IMSE.
1307 bool ObjectMonitor::check_owner(Thread* THREAD) {
1308 if (_owner == THREAD) {
1309 return true;
1310 }
1311 void* cur = _owner;
1312 if (THREAD->is_lock_owned((address)cur)) {
1313 simply_set_owner_from_BasicLock(THREAD, cur); // Convert from BasicLock* to Thread*.
1314 _recursions = 0;
1315 return true;
1316 }
1317 THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(),
1318 "current thread is not owner", false);
1319 }
1320
1321 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1322 ObjectMonitor* monitor,
1323 jlong notifier_tid,
1324 jlong timeout,
1325 bool timedout) {
1326 assert(event != NULL, "invariant");
1327 assert(monitor != NULL, "invariant");
1328 event->set_monitorClass(((oop)monitor->object())->klass());
1329 event->set_timeout(timeout);
1330 event->set_address((uintptr_t)monitor->object_addr());
1331 event->set_notifier(notifier_tid);
1332 event->set_timedOut(timedout);
1333 event->commit();
1334 }
1335
1336 // -----------------------------------------------------------------------------
1337 // Wait/Notify/NotifyAll
1338 //
1339 // Note: a subset of changes to ObjectMonitor::wait()
1340 // will need to be replicated in complete_exit
1341 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1342 Thread * const Self = THREAD;
1343 assert(Self->is_Java_thread(), "Must be Java thread!");
1344 JavaThread *jt = (JavaThread *)THREAD;
1345
1346 assert(InitDone, "Unexpectedly not initialized");
1347
1348 CHECK_OWNER(); // Throws IMSE if not owner.
1349
1350 EventJavaMonitorWait event;
1351
1352 // check for a pending interrupt
1353 if (interruptible && jt->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1354 // post monitor waited event. Note that this is past-tense, we are done waiting.
1355 if (JvmtiExport::should_post_monitor_waited()) {
1356 // Note: 'false' parameter is passed here because the
1357 // wait was not timed out due to thread interrupt.
1358 JvmtiExport::post_monitor_waited(jt, this, false);
1359
1360 // In this short circuit of the monitor wait protocol, the
1361 // current thread never drops ownership of the monitor and
1362 // never gets added to the wait queue so the current thread
1363 // cannot be made the successor. This means that the
1364 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1365 // consume an unpark() meant for the ParkEvent associated with
1366 // this ObjectMonitor.
1367 }
1368 if (event.should_commit()) {
1369 post_monitor_wait_event(&event, this, 0, millis, false);
1370 }
1371 THROW(vmSymbols::java_lang_InterruptedException());
1372 return;
1373 }
1374
1375 assert(Self->_Stalled == 0, "invariant");
1376 Self->_Stalled = intptr_t(this);
1377 jt->set_current_waiting_monitor(this);
1378
1379 // create a node to be put into the queue
1380 // Critically, after we reset() the event but prior to park(), we must check
1381 // for a pending interrupt.
1382 ObjectWaiter node(Self);
1383 node.TState = ObjectWaiter::TS_WAIT;
1384 Self->_ParkEvent->reset();
1385 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1386
1387 // Enter the waiting queue, which is a circular doubly linked list in this case
1388 // but it could be a priority queue or any data structure.
1389 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1390 // by the the owner of the monitor *except* in the case where park()
1391 // returns because of a timeout of interrupt. Contention is exceptionally rare
1392 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1393
1394 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1395 AddWaiter(&node);
1396 Thread::SpinRelease(&_WaitSetLock);
1397
1398 _Responsible = NULL;
1399
1400 intx save = _recursions; // record the old recursion count
1401 _waiters++; // increment the number of waiters
1402 _recursions = 0; // set the recursion level to be 1
1403 exit(true, Self); // exit the monitor
1404 guarantee(_owner != Self, "invariant");
1405
1406 // The thread is on the WaitSet list - now park() it.
1407 // On MP systems it's conceivable that a brief spin before we park
1408 // could be profitable.
1409 //
1410 // TODO-FIXME: change the following logic to a loop of the form
1411 // while (!timeout && !interrupted && _notified == 0) park()
1412
1413 int ret = OS_OK;
1414 int WasNotified = 0;
1415
1416 // Need to check interrupt state whilst still _thread_in_vm
1417 bool interrupted = interruptible && jt->is_interrupted(false);
1418
1419 { // State transition wrappers
1420 OSThread* osthread = Self->osthread();
1421 OSThreadWaitState osts(osthread, true);
1422 {
1423 ThreadBlockInVM tbivm(jt);
1424 // Thread is in thread_blocked state and oop access is unsafe.
1425 jt->set_suspend_equivalent();
1426
1427 if (interrupted || HAS_PENDING_EXCEPTION) {
1428 // Intentionally empty
1429 } else if (node._notified == 0) {
1430 if (millis <= 0) {
1431 Self->_ParkEvent->park();
1432 } else {
1433 ret = Self->_ParkEvent->park(millis);
1434 }
1435 }
1436
1437 // were we externally suspended while we were waiting?
1438 if (ExitSuspendEquivalent (jt)) {
1439 // TODO-FIXME: add -- if succ == Self then succ = null.
1440 jt->java_suspend_self();
1441 }
1442
1443 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1444
1445 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1446 // from the WaitSet to the EntryList.
1447 // See if we need to remove Node from the WaitSet.
1448 // We use double-checked locking to avoid grabbing _WaitSetLock
1449 // if the thread is not on the wait queue.
1450 //
1451 // Note that we don't need a fence before the fetch of TState.
1452 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1453 // written by the is thread. (perhaps the fetch might even be satisfied
1454 // by a look-aside into the processor's own store buffer, although given
1455 // the length of the code path between the prior ST and this load that's
1456 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1457 // then we'll acquire the lock and then re-fetch a fresh TState value.
1458 // That is, we fail toward safety.
1459
1460 if (node.TState == ObjectWaiter::TS_WAIT) {
1461 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1462 if (node.TState == ObjectWaiter::TS_WAIT) {
1463 DequeueSpecificWaiter(&node); // unlink from WaitSet
1464 assert(node._notified == 0, "invariant");
1465 node.TState = ObjectWaiter::TS_RUN;
1466 }
1467 Thread::SpinRelease(&_WaitSetLock);
1468 }
1469
1470 // The thread is now either on off-list (TS_RUN),
1471 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1472 // The Node's TState variable is stable from the perspective of this thread.
1473 // No other threads will asynchronously modify TState.
1474 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1475 OrderAccess::loadload();
1476 if (_succ == Self) _succ = NULL;
1477 WasNotified = node._notified;
1478
1479 // Reentry phase -- reacquire the monitor.
1480 // re-enter contended monitor after object.wait().
1481 // retain OBJECT_WAIT state until re-enter successfully completes
1482 // Thread state is thread_in_vm and oop access is again safe,
1483 // although the raw address of the object may have changed.
1484 // (Don't cache naked oops over safepoints, of course).
1485
1486 // post monitor waited event. Note that this is past-tense, we are done waiting.
1487 if (JvmtiExport::should_post_monitor_waited()) {
1488 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1489
1490 if (node._notified != 0 && _succ == Self) {
1491 // In this part of the monitor wait-notify-reenter protocol it
1492 // is possible (and normal) for another thread to do a fastpath
1493 // monitor enter-exit while this thread is still trying to get
1494 // to the reenter portion of the protocol.
1495 //
1496 // The ObjectMonitor was notified and the current thread is
1497 // the successor which also means that an unpark() has already
1498 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1499 // consume the unpark() that was done when the successor was
1500 // set because the same ParkEvent is shared between Java
1501 // monitors and JVM/TI RawMonitors (for now).
1502 //
1503 // We redo the unpark() to ensure forward progress, i.e., we
1504 // don't want all pending threads hanging (parked) with none
1505 // entering the unlocked monitor.
1506 node._event->unpark();
1507 }
1508 }
1509
1510 if (event.should_commit()) {
1511 post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1512 }
1513
1514 OrderAccess::fence();
1515
1516 assert(Self->_Stalled != 0, "invariant");
1517 Self->_Stalled = 0;
1518
1519 assert(_owner != Self, "invariant");
1520 ObjectWaiter::TStates v = node.TState;
1521 if (v == ObjectWaiter::TS_RUN) {
1522 enter(Self);
1523 } else {
1524 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1525 ReenterI(Self, &node);
1526 node.wait_reenter_end(this);
1527 }
1528
1529 // Self has reacquired the lock.
1530 // Lifecycle - the node representing Self must not appear on any queues.
1531 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1532 // want residual elements associated with this thread left on any lists.
1533 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1534 assert(_owner == Self, "invariant");
1535 assert(_succ != Self, "invariant");
1536 } // OSThreadWaitState()
1537
1538 jt->set_current_waiting_monitor(NULL);
1539
1540 guarantee(_recursions == 0, "invariant");
1541 _recursions = save; // restore the old recursion count
1542 _waiters--; // decrement the number of waiters
1543
1544 // Verify a few postconditions
1545 assert(_owner == Self, "invariant");
1546 assert(_succ != Self, "invariant");
1547 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant");
1548
1549 // check if the notification happened
1550 if (!WasNotified) {
1551 // no, it could be timeout or Thread.interrupt() or both
1552 // check for interrupt event, otherwise it is timeout
1553 if (interruptible && jt->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1554 THROW(vmSymbols::java_lang_InterruptedException());
1555 }
1556 }
1557
1558 // NOTE: Spurious wake up will be consider as timeout.
1559 // Monitor notify has precedence over thread interrupt.
1560 }
1561
1562
1563 // Consider:
1564 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1565 // then instead of transferring a thread from the WaitSet to the EntryList
1566 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1567
1568 void ObjectMonitor::INotify(Thread * Self) {
1569 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1570 ObjectWaiter * iterator = DequeueWaiter();
1571 if (iterator != NULL) {
1572 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1573 guarantee(iterator->_notified == 0, "invariant");
1574 // Disposition - what might we do with iterator ?
1575 // a. add it directly to the EntryList - either tail (policy == 1)
1576 // or head (policy == 0).
1577 // b. push it onto the front of the _cxq (policy == 2).
1578 // For now we use (b).
1579
1580 iterator->TState = ObjectWaiter::TS_ENTER;
1581
1582 iterator->_notified = 1;
1583 iterator->_notifier_tid = JFR_THREAD_ID(Self);
1584
1585 ObjectWaiter * list = _EntryList;
1586 if (list != NULL) {
1587 assert(list->_prev == NULL, "invariant");
1588 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1589 assert(list != iterator, "invariant");
1590 }
1591
1592 // prepend to cxq
1593 if (list == NULL) {
1594 iterator->_next = iterator->_prev = NULL;
1595 _EntryList = iterator;
1596 } else {
1597 iterator->TState = ObjectWaiter::TS_CXQ;
1598 for (;;) {
1599 ObjectWaiter * front = _cxq;
1600 iterator->_next = front;
1601 if (Atomic::cmpxchg(&_cxq, front, iterator) == front) {
1602 break;
1603 }
1604 }
1605 }
1606
1607 // _WaitSetLock protects the wait queue, not the EntryList. We could
1608 // move the add-to-EntryList operation, above, outside the critical section
1609 // protected by _WaitSetLock. In practice that's not useful. With the
1610 // exception of wait() timeouts and interrupts the monitor owner
1611 // is the only thread that grabs _WaitSetLock. There's almost no contention
1612 // on _WaitSetLock so it's not profitable to reduce the length of the
1613 // critical section.
1614
1615 iterator->wait_reenter_begin(this);
1616 }
1617 Thread::SpinRelease(&_WaitSetLock);
1618 }
1619
1620 // Consider: a not-uncommon synchronization bug is to use notify() when
1621 // notifyAll() is more appropriate, potentially resulting in stranded
1622 // threads; this is one example of a lost wakeup. A useful diagnostic
1623 // option is to force all notify() operations to behave as notifyAll().
1624 //
1625 // Note: We can also detect many such problems with a "minimum wait".
1626 // When the "minimum wait" is set to a small non-zero timeout value
1627 // and the program does not hang whereas it did absent "minimum wait",
1628 // that suggests a lost wakeup bug.
1629
1630 void ObjectMonitor::notify(TRAPS) {
1631 CHECK_OWNER(); // Throws IMSE if not owner.
1632 if (_WaitSet == NULL) {
1633 return;
1634 }
1635 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1636 INotify(THREAD);
1637 OM_PERFDATA_OP(Notifications, inc(1));
1638 }
1639
1640
1641 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1642 // from the waitset to the EntryList. This could be done more efficiently with a
1643 // single bulk transfer but in practice it's not time-critical. Beware too,
1644 // that in prepend-mode we invert the order of the waiters. Let's say that the
1645 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1646 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1647
1648 void ObjectMonitor::notifyAll(TRAPS) {
1649 CHECK_OWNER(); // Throws IMSE if not owner.
1650 if (_WaitSet == NULL) {
1651 return;
1652 }
1653
1654 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1655 int tally = 0;
1656 while (_WaitSet != NULL) {
1657 tally++;
1658 INotify(THREAD);
1659 }
1660
1661 OM_PERFDATA_OP(Notifications, inc(tally));
1662 }
1663
1664 // -----------------------------------------------------------------------------
1665 // Adaptive Spinning Support
1666 //
1667 // Adaptive spin-then-block - rational spinning
1668 //
1669 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1670 // algorithm. On high order SMP systems it would be better to start with
1671 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1672 // a contending thread could enqueue itself on the cxq and then spin locally
1673 // on a thread-specific variable such as its ParkEvent._Event flag.
1674 // That's left as an exercise for the reader. Note that global spinning is
1675 // not problematic on Niagara, as the L2 cache serves the interconnect and
1676 // has both low latency and massive bandwidth.
1677 //
1678 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1679 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1680 // (duration) or we can fix the count at approximately the duration of
1681 // a context switch and vary the frequency. Of course we could also
1682 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1683 // For a description of 'Adaptive spin-then-block mutual exclusion in
1684 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1685 //
1686 // This implementation varies the duration "D", where D varies with
1687 // the success rate of recent spin attempts. (D is capped at approximately
1688 // length of a round-trip context switch). The success rate for recent
1689 // spin attempts is a good predictor of the success rate of future spin
1690 // attempts. The mechanism adapts automatically to varying critical
1691 // section length (lock modality), system load and degree of parallelism.
1692 // D is maintained per-monitor in _SpinDuration and is initialized
1693 // optimistically. Spin frequency is fixed at 100%.
1694 //
1695 // Note that _SpinDuration is volatile, but we update it without locks
1696 // or atomics. The code is designed so that _SpinDuration stays within
1697 // a reasonable range even in the presence of races. The arithmetic
1698 // operations on _SpinDuration are closed over the domain of legal values,
1699 // so at worst a race will install and older but still legal value.
1700 // At the very worst this introduces some apparent non-determinism.
1701 // We might spin when we shouldn't or vice-versa, but since the spin
1702 // count are relatively short, even in the worst case, the effect is harmless.
1703 //
1704 // Care must be taken that a low "D" value does not become an
1705 // an absorbing state. Transient spinning failures -- when spinning
1706 // is overall profitable -- should not cause the system to converge
1707 // on low "D" values. We want spinning to be stable and predictable
1708 // and fairly responsive to change and at the same time we don't want
1709 // it to oscillate, become metastable, be "too" non-deterministic,
1710 // or converge on or enter undesirable stable absorbing states.
1711 //
1712 // We implement a feedback-based control system -- using past behavior
1713 // to predict future behavior. We face two issues: (a) if the
1714 // input signal is random then the spin predictor won't provide optimal
1715 // results, and (b) if the signal frequency is too high then the control
1716 // system, which has some natural response lag, will "chase" the signal.
1717 // (b) can arise from multimodal lock hold times. Transient preemption
1718 // can also result in apparent bimodal lock hold times.
1719 // Although sub-optimal, neither condition is particularly harmful, as
1720 // in the worst-case we'll spin when we shouldn't or vice-versa.
1721 // The maximum spin duration is rather short so the failure modes aren't bad.
1722 // To be conservative, I've tuned the gain in system to bias toward
1723 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1724 // "rings" or oscillates between spinning and not spinning. This happens
1725 // when spinning is just on the cusp of profitability, however, so the
1726 // situation is not dire. The state is benign -- there's no need to add
1727 // hysteresis control to damp the transition rate between spinning and
1728 // not spinning.
1729
1730 // Spinning: Fixed frequency (100%), vary duration
1731 int ObjectMonitor::TrySpin(Thread * Self) {
1732 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1733 int ctr = Knob_FixedSpin;
1734 if (ctr != 0) {
1735 while (--ctr >= 0) {
1736 if (TryLock(Self) > 0) return 1;
1737 SpinPause();
1738 }
1739 return 0;
1740 }
1741
1742 for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1743 if (TryLock(Self) > 0) {
1744 // Increase _SpinDuration ...
1745 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1746 // Raising _SpurDuration to the poverty line is key.
1747 int x = _SpinDuration;
1748 if (x < Knob_SpinLimit) {
1749 if (x < Knob_Poverty) x = Knob_Poverty;
1750 _SpinDuration = x + Knob_BonusB;
1751 }
1752 return 1;
1753 }
1754 SpinPause();
1755 }
1756
1757 // Admission control - verify preconditions for spinning
1758 //
1759 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1760 // becoming an absorbing state. Put another way, we spin briefly to
1761 // sample, just in case the system load, parallelism, contention, or lock
1762 // modality changed.
1763 //
1764 // Consider the following alternative:
1765 // Periodically set _SpinDuration = _SpinLimit and try a long/full
1766 // spin attempt. "Periodically" might mean after a tally of
1767 // the # of failed spin attempts (or iterations) reaches some threshold.
1768 // This takes us into the realm of 1-out-of-N spinning, where we
1769 // hold the duration constant but vary the frequency.
1770
1771 ctr = _SpinDuration;
1772 if (ctr <= 0) return 0;
1773
1774 if (NotRunnable(Self, (Thread *) _owner)) {
1775 return 0;
1776 }
1777
1778 // We're good to spin ... spin ingress.
1779 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1780 // when preparing to LD...CAS _owner, etc and the CAS is likely
1781 // to succeed.
1782 if (_succ == NULL) {
1783 _succ = Self;
1784 }
1785 Thread * prv = NULL;
1786
1787 // There are three ways to exit the following loop:
1788 // 1. A successful spin where this thread has acquired the lock.
1789 // 2. Spin failure with prejudice
1790 // 3. Spin failure without prejudice
1791
1792 while (--ctr >= 0) {
1793
1794 // Periodic polling -- Check for pending GC
1795 // Threads may spin while they're unsafe.
1796 // We don't want spinning threads to delay the JVM from reaching
1797 // a stop-the-world safepoint or to steal cycles from GC.
1798 // If we detect a pending safepoint we abort in order that
1799 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1800 // this thread, if safe, doesn't steal cycles from GC.
1801 // This is in keeping with the "no loitering in runtime" rule.
1802 // We periodically check to see if there's a safepoint pending.
1803 if ((ctr & 0xFF) == 0) {
1804 if (SafepointMechanism::should_block(Self)) {
1805 goto Abort; // abrupt spin egress
1806 }
1807 SpinPause();
1808 }
1809
1810 // Probe _owner with TATAS
1811 // If this thread observes the monitor transition or flicker
1812 // from locked to unlocked to locked, then the odds that this
1813 // thread will acquire the lock in this spin attempt go down
1814 // considerably. The same argument applies if the CAS fails
1815 // or if we observe _owner change from one non-null value to
1816 // another non-null value. In such cases we might abort
1817 // the spin without prejudice or apply a "penalty" to the
1818 // spin count-down variable "ctr", reducing it by 100, say.
1819
1820 Thread * ox = (Thread *) _owner;
1821 if (ox == NULL) {
1822 ox = (Thread*)try_set_owner_from(Self, NULL);
1823 if (ox == NULL) {
1824 // The CAS succeeded -- this thread acquired ownership
1825 // Take care of some bookkeeping to exit spin state.
1826 if (_succ == Self) {
1827 _succ = NULL;
1828 }
1829
1830 // Increase _SpinDuration :
1831 // The spin was successful (profitable) so we tend toward
1832 // longer spin attempts in the future.
1833 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
1834 // If we acquired the lock early in the spin cycle it
1835 // makes sense to increase _SpinDuration proportionally.
1836 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1837 int x = _SpinDuration;
1838 if (x < Knob_SpinLimit) {
1839 if (x < Knob_Poverty) x = Knob_Poverty;
1840 _SpinDuration = x + Knob_Bonus;
1841 }
1842 return 1;
1843 }
1844
1845 // The CAS failed ... we can take any of the following actions:
1846 // * penalize: ctr -= CASPenalty
1847 // * exit spin with prejudice -- goto Abort;
1848 // * exit spin without prejudice.
1849 // * Since CAS is high-latency, retry again immediately.
1850 prv = ox;
1851 goto Abort;
1852 }
1853
1854 // Did lock ownership change hands ?
1855 if (ox != prv && prv != NULL) {
1856 goto Abort;
1857 }
1858 prv = ox;
1859
1860 // Abort the spin if the owner is not executing.
1861 // The owner must be executing in order to drop the lock.
1862 // Spinning while the owner is OFFPROC is idiocy.
1863 // Consider: ctr -= RunnablePenalty ;
1864 if (NotRunnable(Self, ox)) {
1865 goto Abort;
1866 }
1867 if (_succ == NULL) {
1868 _succ = Self;
1869 }
1870 }
1871
1872 // Spin failed with prejudice -- reduce _SpinDuration.
1873 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
1874 // AIMD is globally stable.
1875 {
1876 int x = _SpinDuration;
1877 if (x > 0) {
1878 // Consider an AIMD scheme like: x -= (x >> 3) + 100
1879 // This is globally sample and tends to damp the response.
1880 x -= Knob_Penalty;
1881 if (x < 0) x = 0;
1882 _SpinDuration = x;
1883 }
1884 }
1885
1886 Abort:
1887 if (_succ == Self) {
1888 _succ = NULL;
1889 // Invariant: after setting succ=null a contending thread
1890 // must recheck-retry _owner before parking. This usually happens
1891 // in the normal usage of TrySpin(), but it's safest
1892 // to make TrySpin() as foolproof as possible.
1893 OrderAccess::fence();
1894 if (TryLock(Self) > 0) return 1;
1895 }
1896 return 0;
1897 }
1898
1899 // NotRunnable() -- informed spinning
1900 //
1901 // Don't bother spinning if the owner is not eligible to drop the lock.
1902 // Spin only if the owner thread is _thread_in_Java or _thread_in_vm.
1903 // The thread must be runnable in order to drop the lock in timely fashion.
1904 // If the _owner is not runnable then spinning will not likely be
1905 // successful (profitable).
1906 //
1907 // Beware -- the thread referenced by _owner could have died
1908 // so a simply fetch from _owner->_thread_state might trap.
1909 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
1910 // Because of the lifecycle issues, the _thread_state values
1911 // observed by NotRunnable() might be garbage. NotRunnable must
1912 // tolerate this and consider the observed _thread_state value
1913 // as advisory.
1914 //
1915 // Beware too, that _owner is sometimes a BasicLock address and sometimes
1916 // a thread pointer.
1917 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
1918 // with the LSB of _owner. Another option would be to probabilistically probe
1919 // the putative _owner->TypeTag value.
1920 //
1921 // Checking _thread_state isn't perfect. Even if the thread is
1922 // in_java it might be blocked on a page-fault or have been preempted
1923 // and sitting on a ready/dispatch queue.
1924 //
1925 // The return value from NotRunnable() is *advisory* -- the
1926 // result is based on sampling and is not necessarily coherent.
1927 // The caller must tolerate false-negative and false-positive errors.
1928 // Spinning, in general, is probabilistic anyway.
1929
1930
1931 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) {
1932 // Check ox->TypeTag == 2BAD.
1933 if (ox == NULL) return 0;
1934
1935 // Avoid transitive spinning ...
1936 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
1937 // Immediately after T1 acquires L it's possible that T2, also
1938 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
1939 // This occurs transiently after T1 acquired L but before
1940 // T1 managed to clear T1.Stalled. T2 does not need to abort
1941 // its spin in this circumstance.
1942 intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
1943
1944 if (BlockedOn == 1) return 1;
1945 if (BlockedOn != 0) {
1946 return BlockedOn != intptr_t(this) && _owner == ox;
1947 }
1948
1949 assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant");
1950 int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);;
1951 // consider also: jst != _thread_in_Java -- but that's overspecific.
1952 return jst == _thread_blocked || jst == _thread_in_native;
1953 }
1954
1955
1956 // -----------------------------------------------------------------------------
1957 // WaitSet management ...
1958
1959 ObjectWaiter::ObjectWaiter(Thread* thread) {
1960 _next = NULL;
1961 _prev = NULL;
1962 _notified = 0;
1963 _notifier_tid = 0;
1964 TState = TS_RUN;
1965 _thread = thread;
1966 _event = thread->_ParkEvent;
1967 _active = false;
1968 assert(_event != NULL, "invariant");
1969 }
1970
1971 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
1972 JavaThread *jt = (JavaThread *)this->_thread;
1973 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
1974 }
1975
1976 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
1977 JavaThread *jt = (JavaThread *)this->_thread;
1978 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
1979 }
1980
1981 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
1982 assert(node != NULL, "should not add NULL node");
1983 assert(node->_prev == NULL, "node already in list");
1984 assert(node->_next == NULL, "node already in list");
1985 // put node at end of queue (circular doubly linked list)
1986 if (_WaitSet == NULL) {
1987 _WaitSet = node;
1988 node->_prev = node;
1989 node->_next = node;
1990 } else {
1991 ObjectWaiter* head = _WaitSet;
1992 ObjectWaiter* tail = head->_prev;
1993 assert(tail->_next == head, "invariant check");
1994 tail->_next = node;
1995 head->_prev = node;
1996 node->_next = head;
1997 node->_prev = tail;
1998 }
1999 }
2000
2001 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2002 // dequeue the very first waiter
2003 ObjectWaiter* waiter = _WaitSet;
2004 if (waiter) {
2005 DequeueSpecificWaiter(waiter);
2006 }
2007 return waiter;
2008 }
2009
2010 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2011 assert(node != NULL, "should not dequeue NULL node");
2012 assert(node->_prev != NULL, "node already removed from list");
2013 assert(node->_next != NULL, "node already removed from list");
2014 // when the waiter has woken up because of interrupt,
2015 // timeout or other spurious wake-up, dequeue the
2016 // waiter from waiting list
2017 ObjectWaiter* next = node->_next;
2018 if (next == node) {
2019 assert(node->_prev == node, "invariant check");
2020 _WaitSet = NULL;
2021 } else {
2022 ObjectWaiter* prev = node->_prev;
2023 assert(prev->_next == node, "invariant check");
2024 assert(next->_prev == node, "invariant check");
2025 next->_prev = prev;
2026 prev->_next = next;
2027 if (_WaitSet == node) {
2028 _WaitSet = next;
2029 }
2030 }
2031 node->_next = NULL;
2032 node->_prev = NULL;
2033 }
2034
2035 // -----------------------------------------------------------------------------
2036 // PerfData support
2037 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL;
2038 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL;
2039 PerfCounter * ObjectMonitor::_sync_Parks = NULL;
2040 PerfCounter * ObjectMonitor::_sync_Notifications = NULL;
2041 PerfCounter * ObjectMonitor::_sync_Inflations = NULL;
2042 PerfCounter * ObjectMonitor::_sync_Deflations = NULL;
2043 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL;
2044
2045 // One-shot global initialization for the sync subsystem.
2046 // We could also defer initialization and initialize on-demand
2047 // the first time we call ObjectSynchronizer::inflate().
2048 // Initialization would be protected - like so many things - by
2049 // the MonitorCache_lock.
2050
2051 void ObjectMonitor::Initialize() {
2052 assert(!InitDone, "invariant");
2053
2054 if (!os::is_MP()) {
2055 Knob_SpinLimit = 0;
2056 Knob_PreSpin = 0;
2057 Knob_FixedSpin = -1;
2058 }
2059
2060 if (UsePerfData) {
2061 EXCEPTION_MARK;
2062 #define NEWPERFCOUNTER(n) \
2063 { \
2064 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \
2065 CHECK); \
2066 }
2067 #define NEWPERFVARIABLE(n) \
2068 { \
2069 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \
2070 CHECK); \
2071 }
2072 NEWPERFCOUNTER(_sync_Inflations);
2073 NEWPERFCOUNTER(_sync_Deflations);
2074 NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2075 NEWPERFCOUNTER(_sync_FutileWakeups);
2076 NEWPERFCOUNTER(_sync_Parks);
2077 NEWPERFCOUNTER(_sync_Notifications);
2078 NEWPERFVARIABLE(_sync_MonExtant);
2079 #undef NEWPERFCOUNTER
2080 #undef NEWPERFVARIABLE
2081 }
2082
2083 DEBUG_ONLY(InitDone = true;)
2084 }
2085
2086 ObjectMonitorHandle::~ObjectMonitorHandle() {
2087 if (_om_ptr != NULL) {
2088 _om_ptr->dec_ref_count();
2089 _om_ptr = NULL;
2090 }
2091 }
2092
2093 // Save the ObjectMonitor* associated with the specified markWord and
2094 // increment the ref_count. This function should only be called if
2095 // the caller has verified mark.has_monitor() == true. The object
2096 // parameter is needed to verify that ObjectMonitor* has not been
2097 // deflated and reused for another object.
2098 //
2099 // This function returns true if the ObjectMonitor* has been safely
2100 // saved. This function returns false if we have lost a race with
2101 // async deflation; the caller should retry as appropriate.
2102 //
2103 bool ObjectMonitorHandle::save_om_ptr(oop object, markWord mark) {
2104 guarantee(mark.has_monitor(), "sanity check: mark=" INTPTR_FORMAT,
2105 mark.value());
2106
2107 ObjectMonitor* om_ptr = mark.monitor();
2108 om_ptr->inc_ref_count();
2109
2110 if (AsyncDeflateIdleMonitors) {
2111 // Race here if monitor is not owned! The above ref_count bump
2112 // will cause subsequent async deflation to skip it. However,
2113 // previous or concurrent async deflation is a race.
2114 if (om_ptr->owner_is_DEFLATER_MARKER() && om_ptr->ref_count() <= 0) {
2115 // Async deflation is in progress and our ref_count increment
2116 // above lost the race to async deflation. Attempt to restore
2117 // the header/dmw to the object's header so that we only retry
2118 // once if the deflater thread happens to be slow.
2119 om_ptr->install_displaced_markword_in_object(object);
2120 om_ptr->dec_ref_count();
2121 return false;
2122 }
2123 if (om_ptr->ref_count() <= 0) {
2124 // Async deflation is in the process of bailing out, but has not
2125 // yet restored the ref_count field so we return false to force
2126 // a retry. We want a positive ref_count value for a true return.
2127 om_ptr->dec_ref_count();
2128 return false;
2129 }
2130 // The ObjectMonitor could have been deflated and reused for
2131 // another object before we bumped the ref_count so make sure
2132 // our object still refers to this ObjectMonitor.
2133 const markWord tmp = object->mark();
2134 if (!tmp.has_monitor() || tmp.monitor() != om_ptr) {
2135 // Async deflation and reuse won the race so we have to retry.
2136 // Skip object header restoration since that's already done.
2137 om_ptr->dec_ref_count();
2138 return false;
2139 }
2140 }
2141
2142 ADIM_guarantee(_om_ptr == NULL, "sanity check: _om_ptr=" INTPTR_FORMAT,
2143 p2i(_om_ptr));
2144 _om_ptr = om_ptr;
2145 return true;
2146 }
2147
2148 // For internal use by ObjectSynchronizer::inflate().
2149 // This function is only used when we don't have to worry about async
2150 // deflation of the specified ObjectMonitor*.
2151 void ObjectMonitorHandle::set_om_ptr(ObjectMonitor* om_ptr) {
2152 if (_om_ptr == NULL) {
2153 ADIM_guarantee(om_ptr != NULL, "cannot clear an unset om_ptr");
2154 om_ptr->inc_ref_count();
2155 _om_ptr = om_ptr;
2156 } else {
2157 ADIM_guarantee(om_ptr == NULL, "can only clear a set om_ptr");
2158 _om_ptr->dec_ref_count();
2159 _om_ptr = NULL;
2160 }
2161 }
2162
2163 // Save the specified ObjectMonitor* if it is safe, i.e., not being
2164 // async deflated.
2165 //
2166 // This function returns true if the ObjectMonitor* has been safely
2167 // saved. This function returns false if the specified ObjectMonitor*
2168 // is NULL or if we have lost a race with async deflation; the caller
2169 // can retry as appropriate.
2170 bool ObjectMonitorHandle::set_om_ptr_if_safe(ObjectMonitor* om_ptr) {
2171 if (om_ptr == NULL) {
2172 return false; // Nothing to save if input is NULL
2173 }
2174
2175 om_ptr->inc_ref_count();
2176
2177 if (AsyncDeflateIdleMonitors) {
2178 if (om_ptr->owner_is_DEFLATER_MARKER() && om_ptr->ref_count() <= 0) {
2179 // Async deflation is in progress and our ref_count increment
2180 // above lost the race to async deflation.
2181 om_ptr->dec_ref_count();
2182 return false;
2183 }
2184 if (om_ptr->ref_count() <= 0) {
2185 // Async deflation is in the process of bailing out, but has not
2186 // yet restored the ref_count field so we return false to force
2187 // a retry. We want a positive ref_count value for a true return.
2188 om_ptr->dec_ref_count();
2189 return false;
2190 }
2191 // Unlike save_om_ptr(), we don't have context to determine if
2192 // the ObjectMonitor has been deflated and reused for another
2193 // object.
2194 }
2195
2196 ADIM_guarantee(_om_ptr == NULL, "sanity check: _om_ptr=" INTPTR_FORMAT,
2197 p2i(_om_ptr));
2198 _om_ptr = om_ptr;
2199 return true;
2200 }
2201
2202 // Unset the _om_ptr field and decrement the ref_count field.
2203 void ObjectMonitorHandle::unset_om_ptr() {
2204 ADIM_guarantee(_om_ptr != NULL, "_om_ptr must not be NULL");
2205 _om_ptr->dec_ref_count();
2206 _om_ptr = NULL;
2207 }
2208
2209 void ObjectMonitor::print_on(outputStream* st) const {
2210 // The minimal things to print for markWord printing, more can be added for debugging and logging.
2211 st->print("{contentions=0x%08x,waiters=0x%08x"
2212 ",recursions=" INTX_FORMAT ",owner=" INTPTR_FORMAT "}",
2213 contentions(), waiters(), recursions(),
2214 p2i(owner()));
2215 }
2216 void ObjectMonitor::print() const { print_on(tty); }
2217
2218 //#ifdef ASSERT
2219 // Print the ObjectMonitor like a debugger would:
2220 //
2221 // (ObjectMonitor) 0x00007fdfb6012e40 = {
2222 // _header = 0x0000000000000001
2223 // _object = 0x000000070ff45fd0
2224 // _allocation_state = Old
2225 // _pad_buf0 = {
2226 // [0] = '\0'
2227 // ...
2228 // [43] = '\0'
2229 // }
2230 // _owner = 0x0000000000000000
2231 // _previous_owner_tid = 0
2232 // _pad_buf1 = {
2233 // [0] = '\0'
2234 // ...
2235 // [47] = '\0'
2236 // }
2237 // _ref_count = 1
2238 // _pad_buf2 = {
2239 // [0] = '\0'
2240 // ...
2241 // [47] = '\0'
2242 // }
2243 // _next_om = 0x0000000000000000
2244 // _recursions = 0
2245 // _EntryList = 0x0000000000000000
2246 // _cxq = 0x0000000000000000
2247 // _succ = 0x0000000000000000
2248 // _Responsible = 0x0000000000000000
2249 // _Spinner = 0
2250 // _SpinDuration = 5000
2251 // _contentions = 0
2252 // _WaitSet = 0x0000700009756248
2253 // _waiters = 1
2254 // _WaitSetLock = 0
2255 // }
2256 //
2257 void ObjectMonitor::print_debug_style_on(outputStream* st) const {
2258 st->print_cr("(ObjectMonitor*) " INTPTR_FORMAT " = {", p2i(this));
2259 st->print_cr(" _header = " INTPTR_FORMAT, header().value());
2260 st->print_cr(" _object = " INTPTR_FORMAT, p2i(_object));
2261 st->print(" _allocation_state = ");
2262 if (is_free()) {
2263 st->print("Free");
2264 } else if (is_old()) {
2265 st->print("Old");
2266 } else if (is_new()) {
2267 st->print("New");
2268 } else {
2269 st->print("unknown=%d", _allocation_state);
2270 }
2271 st->cr();
2272 st->print_cr(" _pad_buf0 = {");
2273 st->print_cr(" [0] = '\\0'");
2274 st->print_cr(" ...");
2275 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf0) - 1);
2276 st->print_cr(" }");
2277 st->print_cr(" _owner = " INTPTR_FORMAT, p2i(_owner));
2278 st->print_cr(" _previous_owner_tid = " JLONG_FORMAT, _previous_owner_tid);
2279 st->print_cr(" _pad_buf1 = {");
2280 st->print_cr(" [0] = '\\0'");
2281 st->print_cr(" ...");
2282 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf1) - 1);
2283 st->print_cr(" }");
2284 st->print_cr(" _ref_count = %d", ref_count());
2285 st->print_cr(" _pad_buf2 = {");
2286 st->print_cr(" [0] = '\\0'");
2287 st->print_cr(" ...");
2288 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf1) - 1);
2289 st->print_cr(" }");
2290 st->print_cr(" _next_om = " INTPTR_FORMAT, p2i(_next_om));
2291 st->print_cr(" _recursions = " INTX_FORMAT, _recursions);
2292 st->print_cr(" _EntryList = " INTPTR_FORMAT, p2i(_EntryList));
2293 st->print_cr(" _cxq = " INTPTR_FORMAT, p2i(_cxq));
2294 st->print_cr(" _succ = " INTPTR_FORMAT, p2i(_succ));
2295 st->print_cr(" _Responsible = " INTPTR_FORMAT, p2i(_Responsible));
2296 st->print_cr(" _Spinner = %d", _Spinner);
2297 st->print_cr(" _SpinDuration = %d", _SpinDuration);
2298 st->print_cr(" _contentions = %d", _contentions);
2299 st->print_cr(" _WaitSet = " INTPTR_FORMAT, p2i(_WaitSet));
2300 st->print_cr(" _waiters = %d", _waiters);
2301 st->print_cr(" _WaitSetLock = %d", _WaitSetLock);
2302 st->print_cr("}");
2303 }
2304 //#endif