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