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