1 /*
2 * Copyright (c) 1998, 2015, 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 "memory/padded.hpp"
28 #include "memory/resourceArea.hpp"
29 #include "oops/markOop.hpp"
30 #include "oops/oop.inline.hpp"
31 #include "runtime/atomic.inline.hpp"
32 #include "runtime/biasedLocking.hpp"
33 #include "runtime/handles.inline.hpp"
34 #include "runtime/interfaceSupport.hpp"
35 #include "runtime/mutexLocker.hpp"
36 #include "runtime/objectMonitor.hpp"
37 #include "runtime/objectMonitor.inline.hpp"
38 #include "runtime/osThread.hpp"
39 #include "runtime/stubRoutines.hpp"
40 #include "runtime/synchronizer.hpp"
41 #include "runtime/thread.inline.hpp"
42 #include "utilities/dtrace.hpp"
43 #include "utilities/events.hpp"
44 #include "utilities/preserveException.hpp"
45
46 #if defined(__GNUC__) && !defined(PPC64)
47 // Need to inhibit inlining for older versions of GCC to avoid build-time failures
48 #define NOINLINE __attribute__((noinline))
49 #else
50 #define NOINLINE
51 #endif
52
53 PRAGMA_FORMAT_MUTE_WARNINGS_FOR_GCC
54
55 // The "core" versions of monitor enter and exit reside in this file.
56 // The interpreter and compilers contain specialized transliterated
57 // variants of the enter-exit fast-path operations. See i486.ad fast_lock(),
58 // for instance. If you make changes here, make sure to modify the
59 // interpreter, and both C1 and C2 fast-path inline locking code emission.
60 //
61 // -----------------------------------------------------------------------------
62
63 #ifdef DTRACE_ENABLED
64
65 // Only bother with this argument setup if dtrace is available
66 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
67
68 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
69 char* bytes = NULL; \
70 int len = 0; \
71 jlong jtid = SharedRuntime::get_java_tid(thread); \
72 Symbol* klassname = ((oop)(obj))->klass()->name(); \
73 if (klassname != NULL) { \
74 bytes = (char*)klassname->bytes(); \
75 len = klassname->utf8_length(); \
76 }
77
78 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
79 { \
80 if (DTraceMonitorProbes) { \
81 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
82 HOTSPOT_MONITOR_WAIT(jtid, \
83 (uintptr_t)(monitor), bytes, len, (millis)); \
84 } \
85 }
86
87 #define HOTSPOT_MONITOR_PROBE_notify HOTSPOT_MONITOR_NOTIFY
88 #define HOTSPOT_MONITOR_PROBE_notifyAll HOTSPOT_MONITOR_NOTIFYALL
89 #define HOTSPOT_MONITOR_PROBE_waited HOTSPOT_MONITOR_WAITED
90
91 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
92 { \
93 if (DTraceMonitorProbes) { \
94 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
95 HOTSPOT_MONITOR_PROBE_##probe(jtid, /* probe = waited */ \
96 (uintptr_t)(monitor), bytes, len); \
97 } \
98 }
99
100 #else // ndef DTRACE_ENABLED
101
102 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
103 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
104
105 #endif // ndef DTRACE_ENABLED
106
107 // This exists only as a workaround of dtrace bug 6254741
108 int dtrace_waited_probe(ObjectMonitor* monitor, Handle obj, Thread* thr) {
109 DTRACE_MONITOR_PROBE(waited, monitor, obj(), thr);
110 return 0;
111 }
112
113 #define NINFLATIONLOCKS 256
114 static volatile intptr_t gInflationLocks[NINFLATIONLOCKS];
115
116 // global list of blocks of monitors
117 // gBlockList is really PaddedEnd<ObjectMonitor> *, but we don't
118 // want to expose the PaddedEnd template more than necessary.
119 ObjectMonitor * ObjectSynchronizer::gBlockList = NULL;
120 // global monitor free list
121 ObjectMonitor * volatile ObjectSynchronizer::gFreeList = NULL;
122 // global monitor in-use list, for moribund threads,
123 // monitors they inflated need to be scanned for deflation
124 ObjectMonitor * volatile ObjectSynchronizer::gOmInUseList = NULL;
125 // count of entries in gOmInUseList
126 int ObjectSynchronizer::gOmInUseCount = 0;
127
128 static volatile intptr_t gListLock = 0; // protects global monitor lists
129 static volatile int gMonitorFreeCount = 0; // # on gFreeList
130 static volatile int gMonitorPopulation = 0; // # Extant -- in circulation
131
132 #define CHAINMARKER (cast_to_oop<intptr_t>(-1))
133
134
135 // =====================> Quick functions
136
137 // The quick_* forms are special fast-path variants used to improve
138 // performance. In the simplest case, a "quick_*" implementation could
139 // simply return false, in which case the caller will perform the necessary
140 // state transitions and call the slow-path form.
141 // The fast-path is designed to handle frequently arising cases in an efficient
142 // manner and is just a degenerate "optimistic" variant of the slow-path.
143 // returns true -- to indicate the call was satisfied.
144 // returns false -- to indicate the call needs the services of the slow-path.
145 // A no-loitering ordinance is in effect for code in the quick_* family
146 // operators: safepoints or indefinite blocking (blocking that might span a
147 // safepoint) are forbidden. Generally the thread_state() is _in_Java upon
148 // entry.
149 //
150 // An interesting optimization is to have the JIT recognize the following
151 // common idiom:
152 // synchronized (someobj) { .... ; notify(); }
153 // That is, we find a notify() or notifyAll() call that immediately precedes
154 // the monitorexit operation. In that case the JIT could fuse the operations
155 // into a single notifyAndExit() runtime primitive.
156
157 bool ObjectSynchronizer::quick_notify(oopDesc * obj, Thread * Self, bool All) {
158 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
159 assert(Self->is_Java_thread(), "invariant");
160 assert(((JavaThread *) Self)->thread_state() == _thread_in_Java, "invariant");
161 No_Safepoint_Verifier nsv;
162 if (obj == NULL) return false;
163 const markOop mark = obj->mark();
164
165 if (mark->has_locker() && Self->is_lock_owned((address)mark->locker())) {
166 // Degenerate notify
167 // stack-locked by caller so by definition the implied waitset is empty.
168 return true;
169 }
170
171 if (mark->has_monitor()) {
172 ObjectMonitor * const mon = mark->monitor();
173 assert(mon->object() == obj, "invariant");
174 if (mon->owner() != Self) return false;
175
176 // As long as the object is unbiased and doesn't require safepoint revocation
177 // and is owned by the caller we can transfer a thread or threads from
178 // the waitset to the entrylist here and now, avoiding the slow-path.
179 // That is, the only case where the slow-path is mandatory is
180 // when the object is biased or we need to throw IMSX exceptions.
181 if (mon->first_waiter() != NULL) {
182 if (All) {
183 DTRACE_MONITOR_PROBE(notifyAll, mon, obj, Self);
184 } else {
185 DTRACE_MONITOR_PROBE(notify, mon, obj, Self);
186 }
187 int tally = 0;
188 for (;;) {
189 if (mon->first_waiter() == NULL) break;
190 mon->INotify(Self);
191 ++tally;
192 if (!All) break;
193 }
194 if (ObjectMonitor::_sync_Notifications != NULL) {
195 ObjectMonitor::_sync_Notifications->inc(tally);
196 }
197 }
198 return true;
199 }
200
201 return false; // revert to slow-path
202 }
203
204
205 // The LockNode emitted directly at the synchronization site would have
206 // been too big if it were to have included support for the cases of inflated
207 // recursive enter and exit, so they go here instead.
208 // Note that we can't safely call AsyncPrintJavaStack() from within
209 // quick_enter() as our thread state remains _in_Java.
210
211 bool ObjectSynchronizer::quick_enter(oop obj, Thread * Self,
212 BasicLock * Lock) {
213 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
214 assert(Self->is_Java_thread(), "invariant");
215 assert(((JavaThread *) Self)->thread_state() == _thread_in_Java, "invariant");
216 No_Safepoint_Verifier nsv;
217 if (obj == NULL) return false; // Need to throw NPE
218 const markOop mark = obj->mark();
219
220 if (mark->has_monitor()) {
221 ObjectMonitor * const m = mark->monitor();
222 assert(m->object() == obj, "invariant");
223 Thread * const owner = (Thread *) m->_owner;
224
225 // Lock contention and Transactional Lock Elision (TLE) diagnostics
226 // and observability
227 // Case: light contention possibly amenable to TLE
228 // Case: TLE inimical operations such as nested/recursive synchronization
229
230 if (owner == Self) {
231 m->_recursions++;
232 return true;
233 }
234
235 if (owner == NULL &&
236 Atomic::cmpxchg_ptr(Self, &(m->_owner), NULL) == NULL) {
237 assert(m->_recursions == 0, "invariant");
238 assert(m->_owner == Self, "invariant");
239 return true;
240 }
241 }
242
243 // Note that we could inflate in quick_enter.
244 // This is likely a useful optimization
245 // Critically, in quick_enter() we must not:
246 // -- perform bias revocation, or
247 // -- block indefinitely, or
248 // -- reach a safepoint
249
250 return false; // revert to slow-path
251 }
252
253 // -----------------------------------------------------------------------------
254 // Fast Monitor Enter/Exit
255 // This the fast monitor enter. The interpreter and compiler use
256 // some assembly copies of this code. Make sure update those code
257 // if the following function is changed. The implementation is
258 // extremely sensitive to race condition. Be careful.
259
260 void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock,
261 bool attempt_rebias, TRAPS) {
262 if (UseBiasedLocking) {
263 if (!SafepointSynchronize::is_at_safepoint()) {
264 BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD);
265 if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) {
266 return;
267 }
268 } else {
269 assert(!attempt_rebias, "can not rebias toward VM thread");
270 BiasedLocking::revoke_at_safepoint(obj);
271 }
272 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
273 }
274
275 slow_enter(obj, lock, THREAD);
276 }
277
278 void ObjectSynchronizer::fast_exit(oop object, BasicLock* lock, TRAPS) {
279 assert(!object->mark()->has_bias_pattern(), "should not see bias pattern here");
280 // if displaced header is null, the previous enter is recursive enter, no-op
281 markOop dhw = lock->displaced_header();
282 markOop mark;
283 if (dhw == NULL) {
284 // Recursive stack-lock.
285 // Diagnostics -- Could be: stack-locked, inflating, inflated.
286 mark = object->mark();
287 assert(!mark->is_neutral(), "invariant");
288 if (mark->has_locker() && mark != markOopDesc::INFLATING()) {
289 assert(THREAD->is_lock_owned((address)mark->locker()), "invariant");
290 }
291 if (mark->has_monitor()) {
292 ObjectMonitor * m = mark->monitor();
293 assert(((oop)(m->object()))->mark() == mark, "invariant");
294 assert(m->is_entered(THREAD), "invariant");
295 }
296 return;
297 }
298
299 mark = object->mark();
300
301 // If the object is stack-locked by the current thread, try to
302 // swing the displaced header from the box back to the mark.
303 if (mark == (markOop) lock) {
304 assert(dhw->is_neutral(), "invariant");
305 if ((markOop) Atomic::cmpxchg_ptr (dhw, object->mark_addr(), mark) == mark) {
306 TEVENT(fast_exit: release stacklock);
307 return;
308 }
309 }
310
311 ObjectSynchronizer::inflate(THREAD, object)->exit(true, THREAD);
312 }
313
314 // -----------------------------------------------------------------------------
315 // Interpreter/Compiler Slow Case
316 // This routine is used to handle interpreter/compiler slow case
317 // We don't need to use fast path here, because it must have been
318 // failed in the interpreter/compiler code.
319 void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) {
320 markOop mark = obj->mark();
321 assert(!mark->has_bias_pattern(), "should not see bias pattern here");
322
323 if (mark->is_neutral()) {
324 // Anticipate successful CAS -- the ST of the displaced mark must
325 // be visible <= the ST performed by the CAS.
326 lock->set_displaced_header(mark);
327 if (mark == (markOop) Atomic::cmpxchg_ptr(lock, obj()->mark_addr(), mark)) {
328 TEVENT(slow_enter: release stacklock);
329 return;
330 }
331 // Fall through to inflate() ...
332 } else if (mark->has_locker() &&
333 THREAD->is_lock_owned((address)mark->locker())) {
334 assert(lock != mark->locker(), "must not re-lock the same lock");
335 assert(lock != (BasicLock*)obj->mark(), "don't relock with same BasicLock");
336 lock->set_displaced_header(NULL);
337 return;
338 }
339
340 // The object header will never be displaced to this lock,
341 // so it does not matter what the value is, except that it
342 // must be non-zero to avoid looking like a re-entrant lock,
343 // and must not look locked either.
344 lock->set_displaced_header(markOopDesc::unused_mark());
345 ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);
346 }
347
348 // This routine is used to handle interpreter/compiler slow case
349 // We don't need to use fast path here, because it must have
350 // failed in the interpreter/compiler code. Simply use the heavy
351 // weight monitor should be ok, unless someone find otherwise.
352 void ObjectSynchronizer::slow_exit(oop object, BasicLock* lock, TRAPS) {
353 fast_exit(object, lock, THREAD);
354 }
355
356 // -----------------------------------------------------------------------------
357 // Class Loader support to workaround deadlocks on the class loader lock objects
358 // Also used by GC
359 // complete_exit()/reenter() are used to wait on a nested lock
360 // i.e. to give up an outer lock completely and then re-enter
361 // Used when holding nested locks - lock acquisition order: lock1 then lock2
362 // 1) complete_exit lock1 - saving recursion count
363 // 2) wait on lock2
364 // 3) when notified on lock2, unlock lock2
365 // 4) reenter lock1 with original recursion count
366 // 5) lock lock2
367 // NOTE: must use heavy weight monitor to handle complete_exit/reenter()
368 intptr_t ObjectSynchronizer::complete_exit(Handle obj, TRAPS) {
369 TEVENT(complete_exit);
370 if (UseBiasedLocking) {
371 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
372 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
373 }
374
375 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
376
377 return monitor->complete_exit(THREAD);
378 }
379
380 // NOTE: must use heavy weight monitor to handle complete_exit/reenter()
381 void ObjectSynchronizer::reenter(Handle obj, intptr_t recursion, TRAPS) {
382 TEVENT(reenter);
383 if (UseBiasedLocking) {
384 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
385 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
386 }
387
388 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
389
390 monitor->reenter(recursion, THREAD);
391 }
392 // -----------------------------------------------------------------------------
393 // JNI locks on java objects
394 // NOTE: must use heavy weight monitor to handle jni monitor enter
395 void ObjectSynchronizer::jni_enter(Handle obj, TRAPS) {
396 // the current locking is from JNI instead of Java code
397 TEVENT(jni_enter);
398 if (UseBiasedLocking) {
399 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
400 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
401 }
402 THREAD->set_current_pending_monitor_is_from_java(false);
403 ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);
404 THREAD->set_current_pending_monitor_is_from_java(true);
405 }
406
407 // NOTE: must use heavy weight monitor to handle jni monitor exit
408 void ObjectSynchronizer::jni_exit(oop obj, Thread* THREAD) {
409 TEVENT(jni_exit);
410 if (UseBiasedLocking) {
411 Handle h_obj(THREAD, obj);
412 BiasedLocking::revoke_and_rebias(h_obj, false, THREAD);
413 obj = h_obj();
414 }
415 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
416
417 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj);
418 // If this thread has locked the object, exit the monitor. Note: can't use
419 // monitor->check(CHECK); must exit even if an exception is pending.
420 if (monitor->check(THREAD)) {
421 monitor->exit(true, THREAD);
422 }
423 }
424
425 // -----------------------------------------------------------------------------
426 // Internal VM locks on java objects
427 // standard constructor, allows locking failures
428 ObjectLocker::ObjectLocker(Handle obj, Thread* thread, bool doLock) {
429 _dolock = doLock;
430 _thread = thread;
431 debug_only(if (StrictSafepointChecks) _thread->check_for_valid_safepoint_state(false);)
432 _obj = obj;
433
434 if (_dolock) {
435 TEVENT(ObjectLocker);
436
437 ObjectSynchronizer::fast_enter(_obj, &_lock, false, _thread);
438 }
439 }
440
441 ObjectLocker::~ObjectLocker() {
442 if (_dolock) {
443 ObjectSynchronizer::fast_exit(_obj(), &_lock, _thread);
444 }
445 }
446
447
448 // -----------------------------------------------------------------------------
449 // Wait/Notify/NotifyAll
450 // NOTE: must use heavy weight monitor to handle wait()
451 int ObjectSynchronizer::wait(Handle obj, jlong millis, TRAPS) {
452 if (UseBiasedLocking) {
453 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
454 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
455 }
456 if (millis < 0) {
457 TEVENT(wait - throw IAX);
458 THROW_MSG_0(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
459 }
460 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
461 DTRACE_MONITOR_WAIT_PROBE(monitor, obj(), THREAD, millis);
462 monitor->wait(millis, true, THREAD);
463
464 // This dummy call is in place to get around dtrace bug 6254741. Once
465 // that's fixed we can uncomment the following line, remove the call
466 // and change this function back into a "void" func.
467 // DTRACE_MONITOR_PROBE(waited, monitor, obj(), THREAD);
468 return dtrace_waited_probe(monitor, obj, THREAD);
469 }
470
471 void ObjectSynchronizer::waitUninterruptibly(Handle obj, jlong millis, TRAPS) {
472 if (UseBiasedLocking) {
473 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
474 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
475 }
476 if (millis < 0) {
477 TEVENT(wait - throw IAX);
478 THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
479 }
480 ObjectSynchronizer::inflate(THREAD, obj()) -> wait(millis, false, THREAD);
481 }
482
483 void ObjectSynchronizer::notify(Handle obj, TRAPS) {
484 if (UseBiasedLocking) {
485 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
486 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
487 }
488
489 markOop mark = obj->mark();
490 if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
491 return;
492 }
493 ObjectSynchronizer::inflate(THREAD, obj())->notify(THREAD);
494 }
495
496 // NOTE: see comment of notify()
497 void ObjectSynchronizer::notifyall(Handle obj, TRAPS) {
498 if (UseBiasedLocking) {
499 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
500 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
501 }
502
503 markOop mark = obj->mark();
504 if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
505 return;
506 }
507 ObjectSynchronizer::inflate(THREAD, obj())->notifyAll(THREAD);
508 }
509
510 // -----------------------------------------------------------------------------
511 // Hash Code handling
512 //
513 // Performance concern:
514 // OrderAccess::storestore() calls release() which at one time stored 0
515 // into the global volatile OrderAccess::dummy variable. This store was
516 // unnecessary for correctness. Many threads storing into a common location
517 // causes considerable cache migration or "sloshing" on large SMP systems.
518 // As such, I avoided using OrderAccess::storestore(). In some cases
519 // OrderAccess::fence() -- which incurs local latency on the executing
520 // processor -- is a better choice as it scales on SMP systems.
521 //
522 // See http://blogs.oracle.com/dave/entry/biased_locking_in_hotspot for
523 // a discussion of coherency costs. Note that all our current reference
524 // platforms provide strong ST-ST order, so the issue is moot on IA32,
525 // x64, and SPARC.
526 //
527 // As a general policy we use "volatile" to control compiler-based reordering
528 // and explicit fences (barriers) to control for architectural reordering
529 // performed by the CPU(s) or platform.
530
531 struct SharedGlobals {
532 char _pad_prefix[DEFAULT_CACHE_LINE_SIZE];
533 // These are highly shared mostly-read variables.
534 // To avoid false-sharing they need to be the sole occupants of a cache line.
535 volatile int stwRandom;
536 volatile int stwCycle;
537 DEFINE_PAD_MINUS_SIZE(1, DEFAULT_CACHE_LINE_SIZE, sizeof(volatile int) * 2);
538 // Hot RW variable -- Sequester to avoid false-sharing
539 volatile int hcSequence;
540 DEFINE_PAD_MINUS_SIZE(2, DEFAULT_CACHE_LINE_SIZE, sizeof(volatile int));
541 };
542
543 static SharedGlobals GVars;
544 static int MonitorScavengeThreshold = 1000000;
545 static volatile int ForceMonitorScavenge = 0; // Scavenge required and pending
546
547 static markOop ReadStableMark(oop obj) {
548 markOop mark = obj->mark();
549 if (!mark->is_being_inflated()) {
550 return mark; // normal fast-path return
551 }
552
553 int its = 0;
554 for (;;) {
555 markOop mark = obj->mark();
556 if (!mark->is_being_inflated()) {
557 return mark; // normal fast-path return
558 }
559
560 // The object is being inflated by some other thread.
561 // The caller of ReadStableMark() must wait for inflation to complete.
562 // Avoid live-lock
563 // TODO: consider calling SafepointSynchronize::do_call_back() while
564 // spinning to see if there's a safepoint pending. If so, immediately
565 // yielding or blocking would be appropriate. Avoid spinning while
566 // there is a safepoint pending.
567 // TODO: add inflation contention performance counters.
568 // TODO: restrict the aggregate number of spinners.
569
570 ++its;
571 if (its > 10000 || !os::is_MP()) {
572 if (its & 1) {
573 os::naked_yield();
574 TEVENT(Inflate: INFLATING - yield);
575 } else {
576 // Note that the following code attenuates the livelock problem but is not
577 // a complete remedy. A more complete solution would require that the inflating
578 // thread hold the associated inflation lock. The following code simply restricts
579 // the number of spinners to at most one. We'll have N-2 threads blocked
580 // on the inflationlock, 1 thread holding the inflation lock and using
581 // a yield/park strategy, and 1 thread in the midst of inflation.
582 // A more refined approach would be to change the encoding of INFLATING
583 // to allow encapsulation of a native thread pointer. Threads waiting for
584 // inflation to complete would use CAS to push themselves onto a singly linked
585 // list rooted at the markword. Once enqueued, they'd loop, checking a per-thread flag
586 // and calling park(). When inflation was complete the thread that accomplished inflation
587 // would detach the list and set the markword to inflated with a single CAS and
588 // then for each thread on the list, set the flag and unpark() the thread.
589 // This is conceptually similar to muxAcquire-muxRelease, except that muxRelease
590 // wakes at most one thread whereas we need to wake the entire list.
591 int ix = (cast_from_oop<intptr_t>(obj) >> 5) & (NINFLATIONLOCKS-1);
592 int YieldThenBlock = 0;
593 assert(ix >= 0 && ix < NINFLATIONLOCKS, "invariant");
594 assert((NINFLATIONLOCKS & (NINFLATIONLOCKS-1)) == 0, "invariant");
595 Thread::muxAcquire(gInflationLocks + ix, "gInflationLock");
596 while (obj->mark() == markOopDesc::INFLATING()) {
597 // Beware: NakedYield() is advisory and has almost no effect on some platforms
598 // so we periodically call Self->_ParkEvent->park(1).
599 // We use a mixed spin/yield/block mechanism.
600 if ((YieldThenBlock++) >= 16) {
601 Thread::current()->_ParkEvent->park(1);
602 } else {
603 os::naked_yield();
604 }
605 }
606 Thread::muxRelease(gInflationLocks + ix);
607 TEVENT(Inflate: INFLATING - yield/park);
608 }
609 } else {
610 SpinPause(); // SMP-polite spinning
611 }
612 }
613 }
614
615 // hashCode() generation :
616 //
617 // Possibilities:
618 // * MD5Digest of {obj,stwRandom}
619 // * CRC32 of {obj,stwRandom} or any linear-feedback shift register function.
620 // * A DES- or AES-style SBox[] mechanism
621 // * One of the Phi-based schemes, such as:
622 // 2654435761 = 2^32 * Phi (golden ratio)
623 // HashCodeValue = ((uintptr_t(obj) >> 3) * 2654435761) ^ GVars.stwRandom ;
624 // * A variation of Marsaglia's shift-xor RNG scheme.
625 // * (obj ^ stwRandom) is appealing, but can result
626 // in undesirable regularity in the hashCode values of adjacent objects
627 // (objects allocated back-to-back, in particular). This could potentially
628 // result in hashtable collisions and reduced hashtable efficiency.
629 // There are simple ways to "diffuse" the middle address bits over the
630 // generated hashCode values:
631
632 static inline intptr_t get_next_hash(Thread * Self, oop obj) {
633 intptr_t value = 0;
634 if (hashCode == 0) {
635 // This form uses an unguarded global Park-Miller RNG,
636 // so it's possible for two threads to race and generate the same RNG.
637 // On MP system we'll have lots of RW access to a global, so the
638 // mechanism induces lots of coherency traffic.
639 value = os::random();
640 } else if (hashCode == 1) {
641 // This variation has the property of being stable (idempotent)
642 // between STW operations. This can be useful in some of the 1-0
643 // synchronization schemes.
644 intptr_t addrBits = cast_from_oop<intptr_t>(obj) >> 3;
645 value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom;
646 } else if (hashCode == 2) {
647 value = 1; // for sensitivity testing
648 } else if (hashCode == 3) {
649 value = ++GVars.hcSequence;
650 } else if (hashCode == 4) {
651 value = cast_from_oop<intptr_t>(obj);
652 } else {
653 // Marsaglia's xor-shift scheme with thread-specific state
654 // This is probably the best overall implementation -- we'll
655 // likely make this the default in future releases.
656 unsigned t = Self->_hashStateX;
657 t ^= (t << 11);
658 Self->_hashStateX = Self->_hashStateY;
659 Self->_hashStateY = Self->_hashStateZ;
660 Self->_hashStateZ = Self->_hashStateW;
661 unsigned v = Self->_hashStateW;
662 v = (v ^ (v >> 19)) ^ (t ^ (t >> 8));
663 Self->_hashStateW = v;
664 value = v;
665 }
666
667 value &= markOopDesc::hash_mask;
668 if (value == 0) value = 0xBAD;
669 assert(value != markOopDesc::no_hash, "invariant");
670 TEVENT(hashCode: GENERATE);
671 return value;
672 }
673
674 intptr_t ObjectSynchronizer::FastHashCode(Thread * Self, oop obj) {
675 if (UseBiasedLocking) {
676 // NOTE: many places throughout the JVM do not expect a safepoint
677 // to be taken here, in particular most operations on perm gen
678 // objects. However, we only ever bias Java instances and all of
679 // the call sites of identity_hash that might revoke biases have
680 // been checked to make sure they can handle a safepoint. The
681 // added check of the bias pattern is to avoid useless calls to
682 // thread-local storage.
683 if (obj->mark()->has_bias_pattern()) {
684 // Handle for oop obj in case of STW safepoint
685 Handle hobj(Self, obj);
686 // Relaxing assertion for bug 6320749.
687 assert(Universe::verify_in_progress() ||
688 !SafepointSynchronize::is_at_safepoint(),
689 "biases should not be seen by VM thread here");
690 BiasedLocking::revoke_and_rebias(hobj, false, JavaThread::current());
691 obj = hobj();
692 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
693 }
694 }
695
696 // hashCode() is a heap mutator ...
697 // Relaxing assertion for bug 6320749.
698 assert(Universe::verify_in_progress() ||
699 !SafepointSynchronize::is_at_safepoint(), "invariant");
700 assert(Universe::verify_in_progress() ||
701 Self->is_Java_thread() , "invariant");
702 assert(Universe::verify_in_progress() ||
703 ((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
704
705 ObjectMonitor* monitor = NULL;
706 markOop temp, test;
707 intptr_t hash;
708 markOop mark = ReadStableMark(obj);
709
710 // object should remain ineligible for biased locking
711 assert(!mark->has_bias_pattern(), "invariant");
712
713 if (mark->is_neutral()) {
714 hash = mark->hash(); // this is a normal header
715 if (hash) { // if it has hash, just return it
716 return hash;
717 }
718 hash = get_next_hash(Self, obj); // allocate a new hash code
719 temp = mark->copy_set_hash(hash); // merge the hash code into header
720 // use (machine word version) atomic operation to install the hash
721 test = (markOop) Atomic::cmpxchg_ptr(temp, obj->mark_addr(), mark);
722 if (test == mark) {
723 return hash;
724 }
725 // If atomic operation failed, we must inflate the header
726 // into heavy weight monitor. We could add more code here
727 // for fast path, but it does not worth the complexity.
728 } else if (mark->has_monitor()) {
729 monitor = mark->monitor();
730 temp = monitor->header();
731 assert(temp->is_neutral(), "invariant");
732 hash = temp->hash();
733 if (hash) {
734 return hash;
735 }
736 // Skip to the following code to reduce code size
737 } else if (Self->is_lock_owned((address)mark->locker())) {
738 temp = mark->displaced_mark_helper(); // this is a lightweight monitor owned
739 assert(temp->is_neutral(), "invariant");
740 hash = temp->hash(); // by current thread, check if the displaced
741 if (hash) { // header contains hash code
742 return hash;
743 }
744 // WARNING:
745 // The displaced header is strictly immutable.
746 // It can NOT be changed in ANY cases. So we have
747 // to inflate the header into heavyweight monitor
748 // even the current thread owns the lock. The reason
749 // is the BasicLock (stack slot) will be asynchronously
750 // read by other threads during the inflate() function.
751 // Any change to stack may not propagate to other threads
752 // correctly.
753 }
754
755 // Inflate the monitor to set hash code
756 monitor = ObjectSynchronizer::inflate(Self, obj);
757 // Load displaced header and check it has hash code
758 mark = monitor->header();
759 assert(mark->is_neutral(), "invariant");
760 hash = mark->hash();
761 if (hash == 0) {
762 hash = get_next_hash(Self, obj);
763 temp = mark->copy_set_hash(hash); // merge hash code into header
764 assert(temp->is_neutral(), "invariant");
765 test = (markOop) Atomic::cmpxchg_ptr(temp, monitor, mark);
766 if (test != mark) {
767 // The only update to the header in the monitor (outside GC)
768 // is install the hash code. If someone add new usage of
769 // displaced header, please update this code
770 hash = test->hash();
771 assert(test->is_neutral(), "invariant");
772 assert(hash != 0, "Trivial unexpected object/monitor header usage.");
773 }
774 }
775 // We finally get the hash
776 return hash;
777 }
778
779 // Deprecated -- use FastHashCode() instead.
780
781 intptr_t ObjectSynchronizer::identity_hash_value_for(Handle obj) {
782 return FastHashCode(Thread::current(), obj());
783 }
784
785
786 bool ObjectSynchronizer::current_thread_holds_lock(JavaThread* thread,
787 Handle h_obj) {
788 if (UseBiasedLocking) {
789 BiasedLocking::revoke_and_rebias(h_obj, false, thread);
790 assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
791 }
792
793 assert(thread == JavaThread::current(), "Can only be called on current thread");
794 oop obj = h_obj();
795
796 markOop mark = ReadStableMark(obj);
797
798 // Uncontended case, header points to stack
799 if (mark->has_locker()) {
800 return thread->is_lock_owned((address)mark->locker());
801 }
802 // Contended case, header points to ObjectMonitor (tagged pointer)
803 if (mark->has_monitor()) {
804 ObjectMonitor* monitor = mark->monitor();
805 return monitor->is_entered(thread) != 0;
806 }
807 // Unlocked case, header in place
808 assert(mark->is_neutral(), "sanity check");
809 return false;
810 }
811
812 // Be aware of this method could revoke bias of the lock object.
813 // This method queries the ownership of the lock handle specified by 'h_obj'.
814 // If the current thread owns the lock, it returns owner_self. If no
815 // thread owns the lock, it returns owner_none. Otherwise, it will return
816 // owner_other.
817 ObjectSynchronizer::LockOwnership ObjectSynchronizer::query_lock_ownership
818 (JavaThread *self, Handle h_obj) {
819 // The caller must beware this method can revoke bias, and
820 // revocation can result in a safepoint.
821 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
822 assert(self->thread_state() != _thread_blocked, "invariant");
823
824 // Possible mark states: neutral, biased, stack-locked, inflated
825
826 if (UseBiasedLocking && h_obj()->mark()->has_bias_pattern()) {
827 // CASE: biased
828 BiasedLocking::revoke_and_rebias(h_obj, false, self);
829 assert(!h_obj->mark()->has_bias_pattern(),
830 "biases should be revoked by now");
831 }
832
833 assert(self == JavaThread::current(), "Can only be called on current thread");
834 oop obj = h_obj();
835 markOop mark = ReadStableMark(obj);
836
837 // CASE: stack-locked. Mark points to a BasicLock on the owner's stack.
838 if (mark->has_locker()) {
839 return self->is_lock_owned((address)mark->locker()) ?
840 owner_self : owner_other;
841 }
842
843 // CASE: inflated. Mark (tagged pointer) points to an objectMonitor.
844 // The Object:ObjectMonitor relationship is stable as long as we're
845 // not at a safepoint.
846 if (mark->has_monitor()) {
847 void * owner = mark->monitor()->_owner;
848 if (owner == NULL) return owner_none;
849 return (owner == self ||
850 self->is_lock_owned((address)owner)) ? owner_self : owner_other;
851 }
852
853 // CASE: neutral
854 assert(mark->is_neutral(), "sanity check");
855 return owner_none; // it's unlocked
856 }
857
858 // FIXME: jvmti should call this
859 JavaThread* ObjectSynchronizer::get_lock_owner(Handle h_obj, bool doLock) {
860 if (UseBiasedLocking) {
861 if (SafepointSynchronize::is_at_safepoint()) {
862 BiasedLocking::revoke_at_safepoint(h_obj);
863 } else {
864 BiasedLocking::revoke_and_rebias(h_obj, false, JavaThread::current());
865 }
866 assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
867 }
868
869 oop obj = h_obj();
870 address owner = NULL;
871
872 markOop mark = ReadStableMark(obj);
873
874 // Uncontended case, header points to stack
875 if (mark->has_locker()) {
876 owner = (address) mark->locker();
877 }
878
879 // Contended case, header points to ObjectMonitor (tagged pointer)
880 if (mark->has_monitor()) {
881 ObjectMonitor* monitor = mark->monitor();
882 assert(monitor != NULL, "monitor should be non-null");
883 owner = (address) monitor->owner();
884 }
885
886 if (owner != NULL) {
887 // owning_thread_from_monitor_owner() may also return NULL here
888 return Threads::owning_thread_from_monitor_owner(owner, doLock);
889 }
890
891 // Unlocked case, header in place
892 // Cannot have assertion since this object may have been
893 // locked by another thread when reaching here.
894 // assert(mark->is_neutral(), "sanity check");
895
896 return NULL;
897 }
898 // Visitors ...
899
900 void ObjectSynchronizer::monitors_iterate(MonitorClosure* closure) {
901 PaddedEnd<ObjectMonitor> * block = (PaddedEnd<ObjectMonitor> *)gBlockList;
902 ObjectMonitor* mid;
903 while (block) {
904 assert(block->object() == CHAINMARKER, "must be a block header");
905 for (int i = _BLOCKSIZE - 1; i > 0; i--) {
906 mid = (ObjectMonitor *)(block + i);
907 oop object = (oop) mid->object();
908 if (object != NULL) {
909 closure->do_monitor(mid);
910 }
911 }
912 block = (PaddedEnd<ObjectMonitor> *) block->FreeNext;
913 }
914 }
915
916 // Get the next block in the block list.
917 static inline ObjectMonitor* next(ObjectMonitor* block) {
918 assert(block->object() == CHAINMARKER, "must be a block header");
919 block = block->FreeNext;
920 assert(block == NULL || block->object() == CHAINMARKER, "must be a block header");
921 return block;
922 }
923
924
925 void ObjectSynchronizer::oops_do(OopClosure* f) {
926 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
927 for (PaddedEnd<ObjectMonitor> * block =
928 (PaddedEnd<ObjectMonitor> *)gBlockList; block != NULL;
929 block = (PaddedEnd<ObjectMonitor> *)next(block)) {
930 assert(block->object() == CHAINMARKER, "must be a block header");
931 for (int i = 1; i < _BLOCKSIZE; i++) {
932 ObjectMonitor* mid = (ObjectMonitor *)&block[i];
933 if (mid->object() != NULL) {
934 f->do_oop((oop*)mid->object_addr());
935 }
936 }
937 }
938 }
939
940
941 // -----------------------------------------------------------------------------
942 // ObjectMonitor Lifecycle
943 // -----------------------
944 // Inflation unlinks monitors from the global gFreeList and
945 // associates them with objects. Deflation -- which occurs at
946 // STW-time -- disassociates idle monitors from objects. Such
947 // scavenged monitors are returned to the gFreeList.
948 //
949 // The global list is protected by gListLock. All the critical sections
950 // are short and operate in constant-time.
951 //
952 // ObjectMonitors reside in type-stable memory (TSM) and are immortal.
953 //
954 // Lifecycle:
955 // -- unassigned and on the global free list
956 // -- unassigned and on a thread's private omFreeList
957 // -- assigned to an object. The object is inflated and the mark refers
958 // to the objectmonitor.
959
960
961 // Constraining monitor pool growth via MonitorBound ...
962 //
963 // The monitor pool is grow-only. We scavenge at STW safepoint-time, but the
964 // the rate of scavenging is driven primarily by GC. As such, we can find
965 // an inordinate number of monitors in circulation.
966 // To avoid that scenario we can artificially induce a STW safepoint
967 // if the pool appears to be growing past some reasonable bound.
968 // Generally we favor time in space-time tradeoffs, but as there's no
969 // natural back-pressure on the # of extant monitors we need to impose some
970 // type of limit. Beware that if MonitorBound is set to too low a value
971 // we could just loop. In addition, if MonitorBound is set to a low value
972 // we'll incur more safepoints, which are harmful to performance.
973 // See also: GuaranteedSafepointInterval
974 //
975 // The current implementation uses asynchronous VM operations.
976
977 static void InduceScavenge(Thread * Self, const char * Whence) {
978 // Induce STW safepoint to trim monitors
979 // Ultimately, this results in a call to deflate_idle_monitors() in the near future.
980 // More precisely, trigger an asynchronous STW safepoint as the number
981 // of active monitors passes the specified threshold.
982 // TODO: assert thread state is reasonable
983
984 if (ForceMonitorScavenge == 0 && Atomic::xchg (1, &ForceMonitorScavenge) == 0) {
985 if (ObjectMonitor::Knob_Verbose) {
986 ::printf ("Monitor scavenge - Induced STW @%s (%d)\n", Whence, ForceMonitorScavenge) ;
987 ::fflush(stdout);
988 }
989 // Induce a 'null' safepoint to scavenge monitors
990 // Must VM_Operation instance be heap allocated as the op will be enqueue and posted
991 // to the VMthread and have a lifespan longer than that of this activation record.
992 // The VMThread will delete the op when completed.
993 VMThread::execute(new VM_ForceAsyncSafepoint());
994
995 if (ObjectMonitor::Knob_Verbose) {
996 ::printf ("Monitor scavenge - STW posted @%s (%d)\n", Whence, ForceMonitorScavenge) ;
997 ::fflush(stdout);
998 }
999 }
1000 }
1001
1002 void ObjectSynchronizer::verifyInUse(Thread *Self) {
1003 ObjectMonitor* mid;
1004 int in_use_tally = 0;
1005 for (mid = Self->omInUseList; mid != NULL; mid = mid->FreeNext) {
1006 in_use_tally++;
1007 }
1008 assert(in_use_tally == Self->omInUseCount, "in-use count off");
1009
1010 int free_tally = 0;
1011 for (mid = Self->omFreeList; mid != NULL; mid = mid->FreeNext) {
1012 free_tally++;
1013 }
1014 assert(free_tally == Self->omFreeCount, "free count off");
1015 }
1016
1017 ObjectMonitor * NOINLINE ObjectSynchronizer::omAlloc(Thread * Self) {
1018 // A large MAXPRIVATE value reduces both list lock contention
1019 // and list coherency traffic, but also tends to increase the
1020 // number of objectMonitors in circulation as well as the STW
1021 // scavenge costs. As usual, we lean toward time in space-time
1022 // tradeoffs.
1023 const int MAXPRIVATE = 1024;
1024 for (;;) {
1025 ObjectMonitor * m;
1026
1027 // 1: try to allocate from the thread's local omFreeList.
1028 // Threads will attempt to allocate first from their local list, then
1029 // from the global list, and only after those attempts fail will the thread
1030 // attempt to instantiate new monitors. Thread-local free lists take
1031 // heat off the gListLock and improve allocation latency, as well as reducing
1032 // coherency traffic on the shared global list.
1033 m = Self->omFreeList;
1034 if (m != NULL) {
1035 Self->omFreeList = m->FreeNext;
1036 Self->omFreeCount--;
1037 // CONSIDER: set m->FreeNext = BAD -- diagnostic hygiene
1038 guarantee(m->object() == NULL, "invariant");
1039 if (MonitorInUseLists) {
1040 m->FreeNext = Self->omInUseList;
1041 Self->omInUseList = m;
1042 Self->omInUseCount++;
1043 if (ObjectMonitor::Knob_VerifyInUse) {
1044 verifyInUse(Self);
1045 }
1046 } else {
1047 m->FreeNext = NULL;
1048 }
1049 return m;
1050 }
1051
1052 // 2: try to allocate from the global gFreeList
1053 // CONSIDER: use muxTry() instead of muxAcquire().
1054 // If the muxTry() fails then drop immediately into case 3.
1055 // If we're using thread-local free lists then try
1056 // to reprovision the caller's free list.
1057 if (gFreeList != NULL) {
1058 // Reprovision the thread's omFreeList.
1059 // Use bulk transfers to reduce the allocation rate and heat
1060 // on various locks.
1061 Thread::muxAcquire(&gListLock, "omAlloc");
1062 for (int i = Self->omFreeProvision; --i >= 0 && gFreeList != NULL;) {
1063 gMonitorFreeCount--;
1064 ObjectMonitor * take = gFreeList;
1065 gFreeList = take->FreeNext;
1066 guarantee(take->object() == NULL, "invariant");
1067 guarantee(!take->is_busy(), "invariant");
1068 take->Recycle();
1069 omRelease(Self, take, false);
1070 }
1071 Thread::muxRelease(&gListLock);
1072 Self->omFreeProvision += 1 + (Self->omFreeProvision/2);
1073 if (Self->omFreeProvision > MAXPRIVATE) Self->omFreeProvision = MAXPRIVATE;
1074 TEVENT(omFirst - reprovision);
1075
1076 const int mx = MonitorBound;
1077 if (mx > 0 && (gMonitorPopulation-gMonitorFreeCount) > mx) {
1078 // We can't safely induce a STW safepoint from omAlloc() as our thread
1079 // state may not be appropriate for such activities and callers may hold
1080 // naked oops, so instead we defer the action.
1081 InduceScavenge(Self, "omAlloc");
1082 }
1083 continue;
1084 }
1085
1086 // 3: allocate a block of new ObjectMonitors
1087 // Both the local and global free lists are empty -- resort to malloc().
1088 // In the current implementation objectMonitors are TSM - immortal.
1089 // Ideally, we'd write "new ObjectMonitor[_BLOCKSIZE], but we want
1090 // each ObjectMonitor to start at the beginning of a cache line,
1091 // so we use align_size_up().
1092 // A better solution would be to use C++ placement-new.
1093 // BEWARE: As it stands currently, we don't run the ctors!
1094 assert(_BLOCKSIZE > 1, "invariant");
1095 size_t neededsize = sizeof(PaddedEnd<ObjectMonitor>) * _BLOCKSIZE;
1096 PaddedEnd<ObjectMonitor> * temp;
1097 size_t aligned_size = neededsize + (DEFAULT_CACHE_LINE_SIZE - 1);
1098 void* real_malloc_addr = (void *)NEW_C_HEAP_ARRAY(char, aligned_size,
1099 mtInternal);
1100 temp = (PaddedEnd<ObjectMonitor> *)
1101 align_size_up((intptr_t)real_malloc_addr,
1102 DEFAULT_CACHE_LINE_SIZE);
1103
1104 // NOTE: (almost) no way to recover if allocation failed.
1105 // We might be able to induce a STW safepoint and scavenge enough
1106 // objectMonitors to permit progress.
1107 if (temp == NULL) {
1108 vm_exit_out_of_memory(neededsize, OOM_MALLOC_ERROR,
1109 "Allocate ObjectMonitors");
1110 }
1111 (void)memset((void *) temp, 0, neededsize);
1112
1113 // Format the block.
1114 // initialize the linked list, each monitor points to its next
1115 // forming the single linked free list, the very first monitor
1116 // will points to next block, which forms the block list.
1117 // The trick of using the 1st element in the block as gBlockList
1118 // linkage should be reconsidered. A better implementation would
1119 // look like: class Block { Block * next; int N; ObjectMonitor Body [N] ; }
1120
1121 for (int i = 1; i < _BLOCKSIZE; i++) {
1122 temp[i].FreeNext = (ObjectMonitor *)&temp[i+1];
1123 }
1124
1125 // terminate the last monitor as the end of list
1126 temp[_BLOCKSIZE - 1].FreeNext = NULL;
1127
1128 // Element [0] is reserved for global list linkage
1129 temp[0].set_object(CHAINMARKER);
1130
1131 // Consider carving out this thread's current request from the
1132 // block in hand. This avoids some lock traffic and redundant
1133 // list activity.
1134
1135 // Acquire the gListLock to manipulate gBlockList and gFreeList.
1136 // An Oyama-Taura-Yonezawa scheme might be more efficient.
1137 Thread::muxAcquire(&gListLock, "omAlloc [2]");
1138 gMonitorPopulation += _BLOCKSIZE-1;
1139 gMonitorFreeCount += _BLOCKSIZE-1;
1140
1141 // Add the new block to the list of extant blocks (gBlockList).
1142 // The very first objectMonitor in a block is reserved and dedicated.
1143 // It serves as blocklist "next" linkage.
1144 temp[0].FreeNext = gBlockList;
1145 gBlockList = temp;
1146
1147 // Add the new string of objectMonitors to the global free list
1148 temp[_BLOCKSIZE - 1].FreeNext = gFreeList;
1149 gFreeList = temp + 1;
1150 Thread::muxRelease(&gListLock);
1151 TEVENT(Allocate block of monitors);
1152 }
1153 }
1154
1155 // Place "m" on the caller's private per-thread omFreeList.
1156 // In practice there's no need to clamp or limit the number of
1157 // monitors on a thread's omFreeList as the only time we'll call
1158 // omRelease is to return a monitor to the free list after a CAS
1159 // attempt failed. This doesn't allow unbounded #s of monitors to
1160 // accumulate on a thread's free list.
1161 //
1162 // Key constraint: all ObjectMonitors on a thread's free list and the global
1163 // free list must have their object field set to null. This prevents the
1164 // scavenger -- deflate_idle_monitors -- from reclaiming them.
1165
1166 void ObjectSynchronizer::omRelease(Thread * Self, ObjectMonitor * m,
1167 bool fromPerThreadAlloc) {
1168 guarantee(m->object() == NULL, "invariant");
1169 guarantee(((m->is_busy()|m->_recursions) == 0), "freeing in-use monitor");
1170 // Remove from omInUseList
1171 if (MonitorInUseLists && fromPerThreadAlloc) {
1172 ObjectMonitor* cur_mid_in_use = NULL;
1173 bool extracted = false;
1174 for (ObjectMonitor* mid = Self->omInUseList; mid != NULL; cur_mid_in_use = mid, mid = mid->FreeNext) {
1175 if (m == mid) {
1176 // extract from per-thread in-use list
1177 if (mid == Self->omInUseList) {
1178 Self->omInUseList = mid->FreeNext;
1179 } else if (cur_mid_in_use != NULL) {
1180 cur_mid_in_use->FreeNext = mid->FreeNext; // maintain the current thread in-use list
1181 }
1182 extracted = true;
1183 Self->omInUseCount--;
1184 if (ObjectMonitor::Knob_VerifyInUse) {
1185 verifyInUse(Self);
1186 }
1187 break;
1188 }
1189 }
1190 assert(extracted, "Should have extracted from in-use list");
1191 }
1192
1193 // FreeNext is used for both omInUseList and omFreeList, so clear old before setting new
1194 m->FreeNext = Self->omFreeList;
1195 Self->omFreeList = m;
1196 Self->omFreeCount++;
1197 }
1198
1199 // Return the monitors of a moribund thread's local free list to
1200 // the global free list. Typically a thread calls omFlush() when
1201 // it's dying. We could also consider having the VM thread steal
1202 // monitors from threads that have not run java code over a few
1203 // consecutive STW safepoints. Relatedly, we might decay
1204 // omFreeProvision at STW safepoints.
1205 //
1206 // Also return the monitors of a moribund thread's omInUseList to
1207 // a global gOmInUseList under the global list lock so these
1208 // will continue to be scanned.
1209 //
1210 // We currently call omFlush() from the Thread:: dtor _after the thread
1211 // has been excised from the thread list and is no longer a mutator.
1212 // That means that omFlush() can run concurrently with a safepoint and
1213 // the scavenge operator. Calling omFlush() from JavaThread::exit() might
1214 // be a better choice as we could safely reason that that the JVM is
1215 // not at a safepoint at the time of the call, and thus there could
1216 // be not inopportune interleavings between omFlush() and the scavenge
1217 // operator.
1218
1219 void ObjectSynchronizer::omFlush(Thread * Self) {
1220 ObjectMonitor * list = Self->omFreeList; // Null-terminated SLL
1221 Self->omFreeList = NULL;
1222 ObjectMonitor * tail = NULL;
1223 int tally = 0;
1224 if (list != NULL) {
1225 ObjectMonitor * s;
1226 // The thread is going away, the per-thread free monitors
1227 // are freed via set_owner(NULL)
1228 // Link them to tail, which will be linked into the global free list
1229 // gFreeList below, under the gListLock
1230 for (s = list; s != NULL; s = s->FreeNext) {
1231 tally++;
1232 tail = s;
1233 guarantee(s->object() == NULL, "invariant");
1234 guarantee(!s->is_busy(), "invariant");
1235 s->set_owner(NULL); // redundant but good hygiene
1236 TEVENT(omFlush - Move one);
1237 }
1238 guarantee(tail != NULL && list != NULL, "invariant");
1239 }
1240
1241 ObjectMonitor * inUseList = Self->omInUseList;
1242 ObjectMonitor * inUseTail = NULL;
1243 int inUseTally = 0;
1244 if (inUseList != NULL) {
1245 Self->omInUseList = NULL;
1246 ObjectMonitor *cur_om;
1247 // The thread is going away, however the omInUseList inflated
1248 // monitors may still be in-use by other threads.
1249 // Link them to inUseTail, which will be linked into the global in-use list
1250 // gOmInUseList below, under the gListLock
1251 for (cur_om = inUseList; cur_om != NULL; cur_om = cur_om->FreeNext) {
1252 inUseTail = cur_om;
1253 inUseTally++;
1254 }
1255 assert(Self->omInUseCount == inUseTally, "in-use count off");
1256 Self->omInUseCount = 0;
1257 guarantee(inUseTail != NULL && inUseList != NULL, "invariant");
1258 }
1259
1260 Thread::muxAcquire(&gListLock, "omFlush");
1261 if (tail != NULL) {
1262 tail->FreeNext = gFreeList;
1263 gFreeList = list;
1264 gMonitorFreeCount += tally;
1265 }
1266
1267 if (inUseTail != NULL) {
1268 inUseTail->FreeNext = gOmInUseList;
1269 gOmInUseList = inUseList;
1270 gOmInUseCount += inUseTally;
1271 }
1272
1273 Thread::muxRelease(&gListLock);
1274 TEVENT(omFlush);
1275 }
1276
1277 // Fast path code shared by multiple functions
1278 ObjectMonitor* ObjectSynchronizer::inflate_helper(oop obj) {
1279 markOop mark = obj->mark();
1280 if (mark->has_monitor()) {
1281 assert(ObjectSynchronizer::verify_objmon_isinpool(mark->monitor()), "monitor is invalid");
1282 assert(mark->monitor()->header()->is_neutral(), "monitor must record a good object header");
1283 return mark->monitor();
1284 }
1285 return ObjectSynchronizer::inflate(Thread::current(), obj);
1286 }
1287
1288
1289 ObjectMonitor * NOINLINE ObjectSynchronizer::inflate(Thread * Self,
1290 oop object) {
1291 // Inflate mutates the heap ...
1292 // Relaxing assertion for bug 6320749.
1293 assert(Universe::verify_in_progress() ||
1294 !SafepointSynchronize::is_at_safepoint(), "invariant");
1295
1296 for (;;) {
1297 const markOop mark = object->mark();
1298 assert(!mark->has_bias_pattern(), "invariant");
1299
1300 // The mark can be in one of the following states:
1301 // * Inflated - just return
1302 // * Stack-locked - coerce it to inflated
1303 // * INFLATING - busy wait for conversion to complete
1304 // * Neutral - aggressively inflate the object.
1305 // * BIASED - Illegal. We should never see this
1306
1307 // CASE: inflated
1308 if (mark->has_monitor()) {
1309 ObjectMonitor * inf = mark->monitor();
1310 assert(inf->header()->is_neutral(), "invariant");
1311 assert(inf->object() == object, "invariant");
1312 assert(ObjectSynchronizer::verify_objmon_isinpool(inf), "monitor is invalid");
1313 return inf;
1314 }
1315
1316 // CASE: inflation in progress - inflating over a stack-lock.
1317 // Some other thread is converting from stack-locked to inflated.
1318 // Only that thread can complete inflation -- other threads must wait.
1319 // The INFLATING value is transient.
1320 // Currently, we spin/yield/park and poll the markword, waiting for inflation to finish.
1321 // We could always eliminate polling by parking the thread on some auxiliary list.
1322 if (mark == markOopDesc::INFLATING()) {
1323 TEVENT(Inflate: spin while INFLATING);
1324 ReadStableMark(object);
1325 continue;
1326 }
1327
1328 // CASE: stack-locked
1329 // Could be stack-locked either by this thread or by some other thread.
1330 //
1331 // Note that we allocate the objectmonitor speculatively, _before_ attempting
1332 // to install INFLATING into the mark word. We originally installed INFLATING,
1333 // allocated the objectmonitor, and then finally STed the address of the
1334 // objectmonitor into the mark. This was correct, but artificially lengthened
1335 // the interval in which INFLATED appeared in the mark, thus increasing
1336 // the odds of inflation contention.
1337 //
1338 // We now use per-thread private objectmonitor free lists.
1339 // These list are reprovisioned from the global free list outside the
1340 // critical INFLATING...ST interval. A thread can transfer
1341 // multiple objectmonitors en-mass from the global free list to its local free list.
1342 // This reduces coherency traffic and lock contention on the global free list.
1343 // Using such local free lists, it doesn't matter if the omAlloc() call appears
1344 // before or after the CAS(INFLATING) operation.
1345 // See the comments in omAlloc().
1346
1347 if (mark->has_locker()) {
1348 ObjectMonitor * m = omAlloc(Self);
1349 // Optimistically prepare the objectmonitor - anticipate successful CAS
1350 // We do this before the CAS in order to minimize the length of time
1351 // in which INFLATING appears in the mark.
1352 m->Recycle();
1353 m->_Responsible = NULL;
1354 m->_recursions = 0;
1355 m->_SpinDuration = ObjectMonitor::Knob_SpinLimit; // Consider: maintain by type/class
1356
1357 markOop cmp = (markOop) Atomic::cmpxchg_ptr(markOopDesc::INFLATING(), object->mark_addr(), mark);
1358 if (cmp != mark) {
1359 omRelease(Self, m, true);
1360 continue; // Interference -- just retry
1361 }
1362
1363 // We've successfully installed INFLATING (0) into the mark-word.
1364 // This is the only case where 0 will appear in a mark-work.
1365 // Only the singular thread that successfully swings the mark-word
1366 // to 0 can perform (or more precisely, complete) inflation.
1367 //
1368 // Why do we CAS a 0 into the mark-word instead of just CASing the
1369 // mark-word from the stack-locked value directly to the new inflated state?
1370 // Consider what happens when a thread unlocks a stack-locked object.
1371 // It attempts to use CAS to swing the displaced header value from the
1372 // on-stack basiclock back into the object header. Recall also that the
1373 // header value (hashcode, etc) can reside in (a) the object header, or
1374 // (b) a displaced header associated with the stack-lock, or (c) a displaced
1375 // header in an objectMonitor. The inflate() routine must copy the header
1376 // value from the basiclock on the owner's stack to the objectMonitor, all
1377 // the while preserving the hashCode stability invariants. If the owner
1378 // decides to release the lock while the value is 0, the unlock will fail
1379 // and control will eventually pass from slow_exit() to inflate. The owner
1380 // will then spin, waiting for the 0 value to disappear. Put another way,
1381 // the 0 causes the owner to stall if the owner happens to try to
1382 // drop the lock (restoring the header from the basiclock to the object)
1383 // while inflation is in-progress. This protocol avoids races that might
1384 // would otherwise permit hashCode values to change or "flicker" for an object.
1385 // Critically, while object->mark is 0 mark->displaced_mark_helper() is stable.
1386 // 0 serves as a "BUSY" inflate-in-progress indicator.
1387
1388
1389 // fetch the displaced mark from the owner's stack.
1390 // The owner can't die or unwind past the lock while our INFLATING
1391 // object is in the mark. Furthermore the owner can't complete
1392 // an unlock on the object, either.
1393 markOop dmw = mark->displaced_mark_helper();
1394 assert(dmw->is_neutral(), "invariant");
1395
1396 // Setup monitor fields to proper values -- prepare the monitor
1397 m->set_header(dmw);
1398
1399 // Optimization: if the mark->locker stack address is associated
1400 // with this thread we could simply set m->_owner = Self.
1401 // Note that a thread can inflate an object
1402 // that it has stack-locked -- as might happen in wait() -- directly
1403 // with CAS. That is, we can avoid the xchg-NULL .... ST idiom.
1404 m->set_owner(mark->locker());
1405 m->set_object(object);
1406 // TODO-FIXME: assert BasicLock->dhw != 0.
1407
1408 // Must preserve store ordering. The monitor state must
1409 // be stable at the time of publishing the monitor address.
1410 guarantee(object->mark() == markOopDesc::INFLATING(), "invariant");
1411 object->release_set_mark(markOopDesc::encode(m));
1412
1413 // Hopefully the performance counters are allocated on distinct cache lines
1414 // to avoid false sharing on MP systems ...
1415 if (ObjectMonitor::_sync_Inflations != NULL) ObjectMonitor::_sync_Inflations->inc();
1416 TEVENT(Inflate: overwrite stacklock);
1417 if (TraceMonitorInflation) {
1418 if (object->is_instance()) {
1419 ResourceMark rm;
1420 tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1421 (void *) object, (intptr_t) object->mark(),
1422 object->klass()->external_name());
1423 }
1424 }
1425 return m;
1426 }
1427
1428 // CASE: neutral
1429 // TODO-FIXME: for entry we currently inflate and then try to CAS _owner.
1430 // If we know we're inflating for entry it's better to inflate by swinging a
1431 // pre-locked objectMonitor pointer into the object header. A successful
1432 // CAS inflates the object *and* confers ownership to the inflating thread.
1433 // In the current implementation we use a 2-step mechanism where we CAS()
1434 // to inflate and then CAS() again to try to swing _owner from NULL to Self.
1435 // An inflateTry() method that we could call from fast_enter() and slow_enter()
1436 // would be useful.
1437
1438 assert(mark->is_neutral(), "invariant");
1439 ObjectMonitor * m = omAlloc(Self);
1440 // prepare m for installation - set monitor to initial state
1441 m->Recycle();
1442 m->set_header(mark);
1443 m->set_owner(NULL);
1444 m->set_object(object);
1445 m->_recursions = 0;
1446 m->_Responsible = NULL;
1447 m->_SpinDuration = ObjectMonitor::Knob_SpinLimit; // consider: keep metastats by type/class
1448
1449 if (Atomic::cmpxchg_ptr (markOopDesc::encode(m), object->mark_addr(), mark) != mark) {
1450 m->set_object(NULL);
1451 m->set_owner(NULL);
1452 m->Recycle();
1453 omRelease(Self, m, true);
1454 m = NULL;
1455 continue;
1456 // interference - the markword changed - just retry.
1457 // The state-transitions are one-way, so there's no chance of
1458 // live-lock -- "Inflated" is an absorbing state.
1459 }
1460
1461 // Hopefully the performance counters are allocated on distinct
1462 // cache lines to avoid false sharing on MP systems ...
1463 if (ObjectMonitor::_sync_Inflations != NULL) ObjectMonitor::_sync_Inflations->inc();
1464 TEVENT(Inflate: overwrite neutral);
1465 if (TraceMonitorInflation) {
1466 if (object->is_instance()) {
1467 ResourceMark rm;
1468 tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1469 (void *) object, (intptr_t) object->mark(),
1470 object->klass()->external_name());
1471 }
1472 }
1473 return m;
1474 }
1475 }
1476
1477
1478 // Deflate_idle_monitors() is called at all safepoints, immediately
1479 // after all mutators are stopped, but before any objects have moved.
1480 // It traverses the list of known monitors, deflating where possible.
1481 // The scavenged monitor are returned to the monitor free list.
1482 //
1483 // Beware that we scavenge at *every* stop-the-world point.
1484 // Having a large number of monitors in-circulation negatively
1485 // impacts the performance of some applications (e.g., PointBase).
1486 // Broadly, we want to minimize the # of monitors in circulation.
1487 //
1488 // We have added a flag, MonitorInUseLists, which creates a list
1489 // of active monitors for each thread. deflate_idle_monitors()
1490 // only scans the per-thread in-use lists. omAlloc() puts all
1491 // assigned monitors on the per-thread list. deflate_idle_monitors()
1492 // returns the non-busy monitors to the global free list.
1493 // When a thread dies, omFlush() adds the list of active monitors for
1494 // that thread to a global gOmInUseList acquiring the
1495 // global list lock. deflate_idle_monitors() acquires the global
1496 // list lock to scan for non-busy monitors to the global free list.
1497 // An alternative could have used a single global in-use list. The
1498 // downside would have been the additional cost of acquiring the global list lock
1499 // for every omAlloc().
1500 //
1501 // Perversely, the heap size -- and thus the STW safepoint rate --
1502 // typically drives the scavenge rate. Large heaps can mean infrequent GC,
1503 // which in turn can mean large(r) numbers of objectmonitors in circulation.
1504 // This is an unfortunate aspect of this design.
1505
1506 enum ManifestConstants {
1507 ClearResponsibleAtSTW = 0,
1508 MaximumRecheckInterval = 1000
1509 };
1510
1511 // Deflate a single monitor if not in-use
1512 // Return true if deflated, false if in-use
1513 bool ObjectSynchronizer::deflate_monitor(ObjectMonitor* mid, oop obj,
1514 ObjectMonitor** freeHeadp,
1515 ObjectMonitor** freeTailp) {
1516 bool deflated;
1517 // Normal case ... The monitor is associated with obj.
1518 guarantee(obj->mark() == markOopDesc::encode(mid), "invariant");
1519 guarantee(mid == obj->mark()->monitor(), "invariant");
1520 guarantee(mid->header()->is_neutral(), "invariant");
1521
1522 if (mid->is_busy()) {
1523 if (ClearResponsibleAtSTW) mid->_Responsible = NULL;
1524 deflated = false;
1525 } else {
1526 // Deflate the monitor if it is no longer being used
1527 // It's idle - scavenge and return to the global free list
1528 // plain old deflation ...
1529 TEVENT(deflate_idle_monitors - scavenge1);
1530 if (TraceMonitorInflation) {
1531 if (obj->is_instance()) {
1532 ResourceMark rm;
1533 tty->print_cr("Deflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1534 (void *) obj, (intptr_t) obj->mark(), obj->klass()->external_name());
1535 }
1536 }
1537
1538 // Restore the header back to obj
1539 obj->release_set_mark(mid->header());
1540 mid->clear();
1541
1542 assert(mid->object() == NULL, "invariant");
1543
1544 // Move the object to the working free list defined by freeHeadp, freeTailp
1545 if (*freeHeadp == NULL) *freeHeadp = mid;
1546 if (*freeTailp != NULL) {
1547 ObjectMonitor * prevtail = *freeTailp;
1548 assert(prevtail->FreeNext == NULL, "cleaned up deflated?");
1549 prevtail->FreeNext = mid;
1550 }
1551 *freeTailp = mid;
1552 deflated = true;
1553 }
1554 return deflated;
1555 }
1556
1557 // Walk a given monitor list, and deflate idle monitors
1558 // The given list could be a per-thread list or a global list
1559 // Caller acquires gListLock
1560 int ObjectSynchronizer::deflate_monitor_list(ObjectMonitor** listHeadp,
1561 ObjectMonitor** freeHeadp,
1562 ObjectMonitor** freeTailp) {
1563 ObjectMonitor* mid;
1564 ObjectMonitor* next;
1565 ObjectMonitor* cur_mid_in_use = NULL;
1566 int deflated_count = 0;
1567
1568 for (mid = *listHeadp; mid != NULL;) {
1569 oop obj = (oop) mid->object();
1570 if (obj != NULL && deflate_monitor(mid, obj, freeHeadp, freeTailp)) {
1571 // if deflate_monitor succeeded,
1572 // extract from per-thread in-use list
1573 if (mid == *listHeadp) {
1574 *listHeadp = mid->FreeNext;
1575 } else if (cur_mid_in_use != NULL) {
1576 cur_mid_in_use->FreeNext = mid->FreeNext; // maintain the current thread in-use list
1577 }
1578 next = mid->FreeNext;
1579 mid->FreeNext = NULL; // This mid is current tail in the freeHeadp list
1580 mid = next;
1581 deflated_count++;
1582 } else {
1583 cur_mid_in_use = mid;
1584 mid = mid->FreeNext;
1585 }
1586 }
1587 return deflated_count;
1588 }
1589
1590 void ObjectSynchronizer::deflate_idle_monitors() {
1591 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1592 int nInuse = 0; // currently associated with objects
1593 int nInCirculation = 0; // extant
1594 int nScavenged = 0; // reclaimed
1595 bool deflated = false;
1596
1597 ObjectMonitor * freeHeadp = NULL; // Local SLL of scavenged monitors
1598 ObjectMonitor * freeTailp = NULL;
1599
1600 TEVENT(deflate_idle_monitors);
1601 // Prevent omFlush from changing mids in Thread dtor's during deflation
1602 // And in case the vm thread is acquiring a lock during a safepoint
1603 // See e.g. 6320749
1604 Thread::muxAcquire(&gListLock, "scavenge - return");
1605
1606 if (MonitorInUseLists) {
1607 int inUse = 0;
1608 for (JavaThread* cur = Threads::first(); cur != NULL; cur = cur->next()) {
1609 nInCirculation+= cur->omInUseCount;
1610 int deflated_count = deflate_monitor_list(cur->omInUseList_addr(), &freeHeadp, &freeTailp);
1611 cur->omInUseCount-= deflated_count;
1612 if (ObjectMonitor::Knob_VerifyInUse) {
1613 verifyInUse(cur);
1614 }
1615 nScavenged += deflated_count;
1616 nInuse += cur->omInUseCount;
1617 }
1618
1619 // For moribund threads, scan gOmInUseList
1620 if (gOmInUseList) {
1621 nInCirculation += gOmInUseCount;
1622 int deflated_count = deflate_monitor_list((ObjectMonitor **)&gOmInUseList, &freeHeadp, &freeTailp);
1623 gOmInUseCount-= deflated_count;
1624 nScavenged += deflated_count;
1625 nInuse += gOmInUseCount;
1626 }
1627
1628 } else for (PaddedEnd<ObjectMonitor> * block =
1629 (PaddedEnd<ObjectMonitor> *)gBlockList; block != NULL;
1630 block = (PaddedEnd<ObjectMonitor> *)next(block)) {
1631 // Iterate over all extant monitors - Scavenge all idle monitors.
1632 assert(block->object() == CHAINMARKER, "must be a block header");
1633 nInCirculation += _BLOCKSIZE;
1634 for (int i = 1; i < _BLOCKSIZE; i++) {
1635 ObjectMonitor* mid = (ObjectMonitor*)&block[i];
1636 oop obj = (oop) mid->object();
1637
1638 if (obj == NULL) {
1639 // The monitor is not associated with an object.
1640 // The monitor should either be a thread-specific private
1641 // free list or the global free list.
1642 // obj == NULL IMPLIES mid->is_busy() == 0
1643 guarantee(!mid->is_busy(), "invariant");
1644 continue;
1645 }
1646 deflated = deflate_monitor(mid, obj, &freeHeadp, &freeTailp);
1647
1648 if (deflated) {
1649 mid->FreeNext = NULL;
1650 nScavenged++;
1651 } else {
1652 nInuse++;
1653 }
1654 }
1655 }
1656
1657 gMonitorFreeCount += nScavenged;
1658
1659 // Consider: audit gFreeList to ensure that gMonitorFreeCount and list agree.
1660
1661 if (ObjectMonitor::Knob_Verbose) {
1662 ::printf("Deflate: InCirc=%d InUse=%d Scavenged=%d ForceMonitorScavenge=%d : pop=%d free=%d\n",
1663 nInCirculation, nInuse, nScavenged, ForceMonitorScavenge,
1664 gMonitorPopulation, gMonitorFreeCount);
1665 ::fflush(stdout);
1666 }
1667
1668 ForceMonitorScavenge = 0; // Reset
1669
1670 // Move the scavenged monitors back to the global free list.
1671 if (freeHeadp != NULL) {
1672 guarantee(freeTailp != NULL && nScavenged > 0, "invariant");
1673 assert(freeTailp->FreeNext == NULL, "invariant");
1674 // constant-time list splice - prepend scavenged segment to gFreeList
1675 freeTailp->FreeNext = gFreeList;
1676 gFreeList = freeHeadp;
1677 }
1678 Thread::muxRelease(&gListLock);
1679
1680 if (ObjectMonitor::_sync_Deflations != NULL) ObjectMonitor::_sync_Deflations->inc(nScavenged);
1681 if (ObjectMonitor::_sync_MonExtant != NULL) ObjectMonitor::_sync_MonExtant ->set_value(nInCirculation);
1682
1683 // TODO: Add objectMonitor leak detection.
1684 // Audit/inventory the objectMonitors -- make sure they're all accounted for.
1685 GVars.stwRandom = os::random();
1686 GVars.stwCycle++;
1687 }
1688
1689 // Monitor cleanup on JavaThread::exit
1690
1691 // Iterate through monitor cache and attempt to release thread's monitors
1692 // Gives up on a particular monitor if an exception occurs, but continues
1693 // the overall iteration, swallowing the exception.
1694 class ReleaseJavaMonitorsClosure: public MonitorClosure {
1695 private:
1696 TRAPS;
1697
1698 public:
1699 ReleaseJavaMonitorsClosure(Thread* thread) : THREAD(thread) {}
1700 void do_monitor(ObjectMonitor* mid) {
1701 if (mid->owner() == THREAD) {
1702 (void)mid->complete_exit(CHECK);
1703 }
1704 }
1705 };
1706
1707 // Release all inflated monitors owned by THREAD. Lightweight monitors are
1708 // ignored. This is meant to be called during JNI thread detach which assumes
1709 // all remaining monitors are heavyweight. All exceptions are swallowed.
1710 // Scanning the extant monitor list can be time consuming.
1711 // A simple optimization is to add a per-thread flag that indicates a thread
1712 // called jni_monitorenter() during its lifetime.
1713 //
1714 // Instead of No_Savepoint_Verifier it might be cheaper to
1715 // use an idiom of the form:
1716 // auto int tmp = SafepointSynchronize::_safepoint_counter ;
1717 // <code that must not run at safepoint>
1718 // guarantee (((tmp ^ _safepoint_counter) | (tmp & 1)) == 0) ;
1719 // Since the tests are extremely cheap we could leave them enabled
1720 // for normal product builds.
1721
1722 void ObjectSynchronizer::release_monitors_owned_by_thread(TRAPS) {
1723 assert(THREAD == JavaThread::current(), "must be current Java thread");
1724 No_Safepoint_Verifier nsv;
1725 ReleaseJavaMonitorsClosure rjmc(THREAD);
1726 Thread::muxAcquire(&gListLock, "release_monitors_owned_by_thread");
1727 ObjectSynchronizer::monitors_iterate(&rjmc);
1728 Thread::muxRelease(&gListLock);
1729 THREAD->clear_pending_exception();
1730 }
1731
1732 //------------------------------------------------------------------------------
1733 // Debugging code
1734
1735 void ObjectSynchronizer::sanity_checks(const bool verbose,
1736 const uint cache_line_size,
1737 int *error_cnt_ptr,
1738 int *warning_cnt_ptr) {
1739 u_char *addr_begin = (u_char*)&GVars;
1740 u_char *addr_stwRandom = (u_char*)&GVars.stwRandom;
1741 u_char *addr_hcSequence = (u_char*)&GVars.hcSequence;
1742
1743 if (verbose) {
1744 tty->print_cr("INFO: sizeof(SharedGlobals)=" SIZE_FORMAT,
1745 sizeof(SharedGlobals));
1746 }
1747
1748 uint offset_stwRandom = (uint)(addr_stwRandom - addr_begin);
1749 if (verbose) tty->print_cr("INFO: offset(stwRandom)=%u", offset_stwRandom);
1750
1751 uint offset_hcSequence = (uint)(addr_hcSequence - addr_begin);
1752 if (verbose) {
1753 tty->print_cr("INFO: offset(_hcSequence)=%u", offset_hcSequence);
1754 }
1755
1756 if (cache_line_size != 0) {
1757 // We were able to determine the L1 data cache line size so
1758 // do some cache line specific sanity checks
1759
1760 if (offset_stwRandom < cache_line_size) {
1761 tty->print_cr("WARNING: the SharedGlobals.stwRandom field is closer "
1762 "to the struct beginning than a cache line which permits "
1763 "false sharing.");
1764 (*warning_cnt_ptr)++;
1765 }
1766
1767 if ((offset_hcSequence - offset_stwRandom) < cache_line_size) {
1768 tty->print_cr("WARNING: the SharedGlobals.stwRandom and "
1769 "SharedGlobals.hcSequence fields are closer than a cache "
1770 "line which permits false sharing.");
1771 (*warning_cnt_ptr)++;
1772 }
1773
1774 if ((sizeof(SharedGlobals) - offset_hcSequence) < cache_line_size) {
1775 tty->print_cr("WARNING: the SharedGlobals.hcSequence field is closer "
1776 "to the struct end than a cache line which permits false "
1777 "sharing.");
1778 (*warning_cnt_ptr)++;
1779 }
1780 }
1781 }
1782
1783 #ifndef PRODUCT
1784
1785 // Verify all monitors in the monitor cache, the verification is weak.
1786 void ObjectSynchronizer::verify() {
1787 PaddedEnd<ObjectMonitor> * block = (PaddedEnd<ObjectMonitor> *)gBlockList;
1788 ObjectMonitor* mid;
1789 while (block) {
1790 assert(block->object() == CHAINMARKER, "must be a block header");
1791 for (int i = 1; i < _BLOCKSIZE; i++) {
1792 mid = (ObjectMonitor *)(block + i);
1793 oop object = (oop) mid->object();
1794 if (object != NULL) {
1795 mid->verify();
1796 }
1797 }
1798 block = (PaddedEnd<ObjectMonitor> *) block->FreeNext;
1799 }
1800 }
1801
1802 // Check if monitor belongs to the monitor cache
1803 // The list is grow-only so it's *relatively* safe to traverse
1804 // the list of extant blocks without taking a lock.
1805
1806 int ObjectSynchronizer::verify_objmon_isinpool(ObjectMonitor *monitor) {
1807 PaddedEnd<ObjectMonitor> * block = (PaddedEnd<ObjectMonitor> *)gBlockList;
1808
1809 while (block) {
1810 assert(block->object() == CHAINMARKER, "must be a block header");
1811 if (monitor > (ObjectMonitor *)&block[0] &&
1812 monitor < (ObjectMonitor *)&block[_BLOCKSIZE]) {
1813 address mon = (address) monitor;
1814 address blk = (address) block;
1815 size_t diff = mon - blk;
1816 assert((diff % sizeof(PaddedEnd<ObjectMonitor>)) == 0, "check");
1817 return 1;
1818 }
1819 block = (PaddedEnd<ObjectMonitor> *) block->FreeNext;
1820 }
1821 return 0;
1822 }
1823
1824 #endif