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