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 "runtime/vframe.hpp"
44 #include "utilities/dtrace.hpp"
45 #include "utilities/events.hpp"
46 #include "utilities/preserveException.hpp"
47
48 #if defined(__GNUC__) && !defined(PPC64)
49 // Need to inhibit inlining for older versions of GCC to avoid build-time failures
50 #define NOINLINE __attribute__((noinline))
51 #else
52 #define NOINLINE
53 #endif
54
55 PRAGMA_FORMAT_MUTE_WARNINGS_FOR_GCC
56
57 // The "core" versions of monitor enter and exit reside in this file.
58 // The interpreter and compilers contain specialized transliterated
59 // variants of the enter-exit fast-path operations. See i486.ad fast_lock(),
60 // for instance. If you make changes here, make sure to modify the
61 // interpreter, and both C1 and C2 fast-path inline locking code emission.
62 //
63 // -----------------------------------------------------------------------------
64
65 #ifdef DTRACE_ENABLED
66
67 // Only bother with this argument setup if dtrace is available
68 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
69
70 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
71 char* bytes = NULL; \
72 int len = 0; \
73 jlong jtid = SharedRuntime::get_java_tid(thread); \
74 Symbol* klassname = ((oop)(obj))->klass()->name(); \
75 if (klassname != NULL) { \
76 bytes = (char*)klassname->bytes(); \
77 len = klassname->utf8_length(); \
78 }
79
80 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
81 { \
82 if (DTraceMonitorProbes) { \
83 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
84 HOTSPOT_MONITOR_WAIT(jtid, \
85 (uintptr_t)(monitor), bytes, len, (millis)); \
86 } \
87 }
88
89 #define HOTSPOT_MONITOR_PROBE_notify HOTSPOT_MONITOR_NOTIFY
90 #define HOTSPOT_MONITOR_PROBE_notifyAll HOTSPOT_MONITOR_NOTIFYALL
91 #define HOTSPOT_MONITOR_PROBE_waited HOTSPOT_MONITOR_WAITED
92
93 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
94 { \
95 if (DTraceMonitorProbes) { \
96 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
97 HOTSPOT_MONITOR_PROBE_##probe(jtid, /* probe = waited */ \
98 (uintptr_t)(monitor), bytes, len); \
99 } \
100 }
101
102 #else // ndef DTRACE_ENABLED
103
104 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
105 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
106
107 #endif // ndef DTRACE_ENABLED
108
109 // This exists only as a workaround of dtrace bug 6254741
110 int dtrace_waited_probe(ObjectMonitor* monitor, Handle obj, Thread* thr) {
111 DTRACE_MONITOR_PROBE(waited, monitor, obj(), thr);
112 return 0;
113 }
114
115 #define NINFLATIONLOCKS 256
116 static volatile intptr_t gInflationLocks[NINFLATIONLOCKS];
117
118 // global list of blocks of monitors
119 // gBlockList is really PaddedEnd<ObjectMonitor> *, but we don't
120 // want to expose the PaddedEnd template more than necessary.
121 ObjectMonitor * ObjectSynchronizer::gBlockList = NULL;
122 // global monitor free list
123 ObjectMonitor * volatile ObjectSynchronizer::gFreeList = NULL;
124 // global monitor in-use list, for moribund threads,
125 // monitors they inflated need to be scanned for deflation
126 ObjectMonitor * volatile ObjectSynchronizer::gOmInUseList = NULL;
127 // count of entries in gOmInUseList
128 int ObjectSynchronizer::gOmInUseCount = 0;
129
130 static volatile intptr_t gListLock = 0; // protects global monitor lists
131 static volatile int gMonitorFreeCount = 0; // # on gFreeList
132 static volatile int gMonitorPopulation = 0; // # Extant -- in circulation
133
134 #define CHAINMARKER (cast_to_oop<intptr_t>(-1))
135
136
137 // =====================> Quick functions
138
139 // The quick_* forms are special fast-path variants used to improve
140 // performance. In the simplest case, a "quick_*" implementation could
141 // simply return false, in which case the caller will perform the necessary
142 // state transitions and call the slow-path form.
143 // The fast-path is designed to handle frequently arising cases in an efficient
144 // manner and is just a degenerate "optimistic" variant of the slow-path.
145 // returns true -- to indicate the call was satisfied.
146 // returns false -- to indicate the call needs the services of the slow-path.
147 // A no-loitering ordinance is in effect for code in the quick_* family
148 // operators: safepoints or indefinite blocking (blocking that might span a
149 // safepoint) are forbidden. Generally the thread_state() is _in_Java upon
150 // entry.
151 //
152 // Consider: An interesting optimization is to have the JIT recognize the
153 // following common idiom:
154 // synchronized (someobj) { .... ; notify(); }
155 // That is, we find a notify() or notifyAll() call that immediately precedes
156 // the monitorexit operation. In that case the JIT could fuse the operations
157 // into a single notifyAndExit() runtime primitive.
158
159 bool ObjectSynchronizer::quick_notify(oopDesc * obj, Thread * self, bool all) {
160 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
161 assert(self->is_Java_thread(), "invariant");
162 assert(((JavaThread *) self)->thread_state() == _thread_in_Java, "invariant");
163 No_Safepoint_Verifier nsv;
164 if (obj == NULL) return false; // slow-path for invalid obj
165 const markOop mark = obj->mark();
166
167 if (mark->has_locker() && self->is_lock_owned((address)mark->locker())) {
168 // Degenerate notify
169 // stack-locked by caller so by definition the implied waitset is empty.
170 return true;
171 }
172
173 if (mark->has_monitor()) {
174 ObjectMonitor * const mon = mark->monitor();
175 assert(mon->object() == obj, "invariant");
176 if (mon->owner() != self) return false; // slow-path for IMS exception
177
178 if (mon->first_waiter() != NULL) {
179 // We have one or more waiters. Since this is an inflated monitor
180 // that we own, we can transfer one or more threads from the waitset
181 // to the entrylist here and now, avoiding the slow-path.
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 do {
189 mon->INotify(self);
190 ++tally;
191 } while (mon->first_waiter() != NULL && all);
192 OM_PERFDATA_OP(Notifications, inc(tally));
193 }
194 return true;
195 }
196
197 // biased locking and any other IMS exception states take the slow-path
198 return false;
199 }
200
201
202 // The LockNode emitted directly at the synchronization site would have
203 // been too big if it were to have included support for the cases of inflated
204 // recursive enter and exit, so they go here instead.
205 // Note that we can't safely call AsyncPrintJavaStack() from within
206 // quick_enter() as our thread state remains _in_Java.
207
208 bool ObjectSynchronizer::quick_enter(oop obj, Thread * Self,
209 BasicLock * Lock) {
210 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
211 assert(Self->is_Java_thread(), "invariant");
212 assert(((JavaThread *) Self)->thread_state() == _thread_in_Java, "invariant");
213 No_Safepoint_Verifier nsv;
214 if (obj == NULL) return false; // Need to throw NPE
215 const markOop mark = obj->mark();
216
217 if (mark->has_monitor()) {
218 ObjectMonitor * const m = mark->monitor();
219 assert(m->object() == obj, "invariant");
220 Thread * const owner = (Thread *) m->_owner;
221
222 // Lock contention and Transactional Lock Elision (TLE) diagnostics
223 // and observability
224 // Case: light contention possibly amenable to TLE
225 // Case: TLE inimical operations such as nested/recursive synchronization
226
227 if (owner == Self) {
228 m->_recursions++;
229 return true;
230 }
231
232 if (owner == NULL &&
233 Atomic::cmpxchg_ptr(Self, &(m->_owner), NULL) == NULL) {
234 assert(m->_recursions == 0, "invariant");
235 assert(m->_owner == Self, "invariant");
236 return true;
237 }
238 }
239
240 // Note that we could inflate in quick_enter.
241 // This is likely a useful optimization
242 // Critically, in quick_enter() we must not:
243 // -- perform bias revocation, or
244 // -- block indefinitely, or
245 // -- reach a safepoint
246
247 return false; // revert to slow-path
248 }
249
250 // -----------------------------------------------------------------------------
251 // Fast Monitor Enter/Exit
252 // This the fast monitor enter. The interpreter and compiler use
253 // some assembly copies of this code. Make sure update those code
254 // if the following function is changed. The implementation is
255 // extremely sensitive to race condition. Be careful.
256
257 void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock,
258 bool attempt_rebias, TRAPS) {
259 if (UseBiasedLocking) {
260 if (!SafepointSynchronize::is_at_safepoint()) {
261 BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD);
262 if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) {
263 return;
264 }
265 } else {
266 assert(!attempt_rebias, "can not rebias toward VM thread");
267 BiasedLocking::revoke_at_safepoint(obj);
268 }
269 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
270 }
271
272 slow_enter(obj, lock, THREAD);
273 }
274
275 void ObjectSynchronizer::fast_exit(oop object, BasicLock* lock, TRAPS) {
276 assert(!object->mark()->has_bias_pattern(), "should not see bias pattern here");
277 // if displaced header is null, the previous enter is recursive enter, no-op
278 markOop dhw = lock->displaced_header();
279 markOop mark;
280 if (dhw == NULL) {
281 // Recursive stack-lock.
282 // Diagnostics -- Could be: stack-locked, inflating, inflated.
283 mark = object->mark();
284 assert(!mark->is_neutral(), "invariant");
285 if (mark->has_locker() && mark != markOopDesc::INFLATING()) {
286 assert(THREAD->is_lock_owned((address)mark->locker()), "invariant");
287 }
288 if (mark->has_monitor()) {
289 ObjectMonitor * m = mark->monitor();
290 assert(((oop)(m->object()))->mark() == mark, "invariant");
291 assert(m->is_entered(THREAD), "invariant");
292 }
293 return;
294 }
295
296 mark = object->mark();
297
298 // If the object is stack-locked by the current thread, try to
299 // swing the displaced header from the box back to the mark.
300 if (mark == (markOop) lock) {
301 assert(dhw->is_neutral(), "invariant");
302 if ((markOop) Atomic::cmpxchg_ptr (dhw, object->mark_addr(), mark) == mark) {
303 TEVENT(fast_exit: release stacklock);
304 return;
305 }
306 }
307
308 ObjectSynchronizer::inflate(THREAD, object)->exit(true, THREAD);
309 }
310
311 // -----------------------------------------------------------------------------
312 // Interpreter/Compiler Slow Case
313 // This routine is used to handle interpreter/compiler slow case
314 // We don't need to use fast path here, because it must have been
315 // failed in the interpreter/compiler code.
316 void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) {
317 markOop mark = obj->mark();
318 assert(!mark->has_bias_pattern(), "should not see bias pattern here");
319
320 if (mark->is_neutral()) {
321 // Anticipate successful CAS -- the ST of the displaced mark must
322 // be visible <= the ST performed by the CAS.
323 lock->set_displaced_header(mark);
324 if (mark == (markOop) Atomic::cmpxchg_ptr(lock, obj()->mark_addr(), mark)) {
325 TEVENT(slow_enter: release stacklock);
326 return;
327 }
328 // Fall through to inflate() ...
329 } else if (mark->has_locker() &&
330 THREAD->is_lock_owned((address)mark->locker())) {
331 assert(lock != mark->locker(), "must not re-lock the same lock");
332 assert(lock != (BasicLock*)obj->mark(), "don't relock with same BasicLock");
333 lock->set_displaced_header(NULL);
334 return;
335 }
336
337 // The object header will never be displaced to this lock,
338 // so it does not matter what the value is, except that it
339 // must be non-zero to avoid looking like a re-entrant lock,
340 // and must not look locked either.
341 lock->set_displaced_header(markOopDesc::unused_mark());
342 ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);
343 }
344
345 // This routine is used to handle interpreter/compiler slow case
346 // We don't need to use fast path here, because it must have
347 // failed in the interpreter/compiler code. Simply use the heavy
348 // weight monitor should be ok, unless someone find otherwise.
349 void ObjectSynchronizer::slow_exit(oop object, BasicLock* lock, TRAPS) {
350 fast_exit(object, lock, THREAD);
351 }
352
353 // -----------------------------------------------------------------------------
354 // Class Loader support to workaround deadlocks on the class loader lock objects
355 // Also used by GC
356 // complete_exit()/reenter() are used to wait on a nested lock
357 // i.e. to give up an outer lock completely and then re-enter
358 // Used when holding nested locks - lock acquisition order: lock1 then lock2
359 // 1) complete_exit lock1 - saving recursion count
360 // 2) wait on lock2
361 // 3) when notified on lock2, unlock lock2
362 // 4) reenter lock1 with original recursion count
363 // 5) lock lock2
364 // NOTE: must use heavy weight monitor to handle complete_exit/reenter()
365 intptr_t ObjectSynchronizer::complete_exit(Handle obj, TRAPS) {
366 TEVENT(complete_exit);
367 if (UseBiasedLocking) {
368 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
369 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
370 }
371
372 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
373
374 return monitor->complete_exit(THREAD);
375 }
376
377 // NOTE: must use heavy weight monitor to handle complete_exit/reenter()
378 void ObjectSynchronizer::reenter(Handle obj, intptr_t recursion, TRAPS) {
379 TEVENT(reenter);
380 if (UseBiasedLocking) {
381 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
382 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
383 }
384
385 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
386
387 monitor->reenter(recursion, THREAD);
388 }
389 // -----------------------------------------------------------------------------
390 // JNI locks on java objects
391 // NOTE: must use heavy weight monitor to handle jni monitor enter
392 void ObjectSynchronizer::jni_enter(Handle obj, TRAPS) {
393 // the current locking is from JNI instead of Java code
394 TEVENT(jni_enter);
395 if (UseBiasedLocking) {
396 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
397 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
398 }
399 THREAD->set_current_pending_monitor_is_from_java(false);
400 ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);
401 THREAD->set_current_pending_monitor_is_from_java(true);
402 }
403
404 // NOTE: must use heavy weight monitor to handle jni monitor exit
405 void ObjectSynchronizer::jni_exit(oop obj, Thread* THREAD) {
406 TEVENT(jni_exit);
407 if (UseBiasedLocking) {
408 Handle h_obj(THREAD, obj);
409 BiasedLocking::revoke_and_rebias(h_obj, false, THREAD);
410 obj = h_obj();
411 }
412 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
413
414 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj);
415 // If this thread has locked the object, exit the monitor. Note: can't use
416 // monitor->check(CHECK); must exit even if an exception is pending.
417 if (monitor->check(THREAD)) {
418 monitor->exit(true, THREAD);
419 }
420 }
421
422 // -----------------------------------------------------------------------------
423 // Internal VM locks on java objects
424 // standard constructor, allows locking failures
425 ObjectLocker::ObjectLocker(Handle obj, Thread* thread, bool doLock) {
426 _dolock = doLock;
427 _thread = thread;
428 debug_only(if (StrictSafepointChecks) _thread->check_for_valid_safepoint_state(false);)
429 _obj = obj;
430
431 if (_dolock) {
432 TEVENT(ObjectLocker);
433
434 ObjectSynchronizer::fast_enter(_obj, &_lock, false, _thread);
435 }
436 }
437
438 ObjectLocker::~ObjectLocker() {
439 if (_dolock) {
440 ObjectSynchronizer::fast_exit(_obj(), &_lock, _thread);
441 }
442 }
443
444
445 // -----------------------------------------------------------------------------
446 // Wait/Notify/NotifyAll
447 // NOTE: must use heavy weight monitor to handle wait()
448 int ObjectSynchronizer::wait(Handle obj, jlong millis, TRAPS) {
449 if (UseBiasedLocking) {
450 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
451 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
452 }
453 if (millis < 0) {
454 TEVENT(wait - throw IAX);
455 THROW_MSG_0(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
456 }
457 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
458 DTRACE_MONITOR_WAIT_PROBE(monitor, obj(), THREAD, millis);
459 monitor->wait(millis, true, THREAD);
460
461 // This dummy call is in place to get around dtrace bug 6254741. Once
462 // that's fixed we can uncomment the following line, remove the call
463 // and change this function back into a "void" func.
464 // DTRACE_MONITOR_PROBE(waited, monitor, obj(), THREAD);
465 return dtrace_waited_probe(monitor, obj, THREAD);
466 }
467
468 void ObjectSynchronizer::waitUninterruptibly(Handle obj, jlong millis, TRAPS) {
469 if (UseBiasedLocking) {
470 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
471 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
472 }
473 if (millis < 0) {
474 TEVENT(wait - throw IAX);
475 THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
476 }
477 ObjectSynchronizer::inflate(THREAD, obj()) -> wait(millis, false, THREAD);
478 }
479
480 void ObjectSynchronizer::notify(Handle obj, TRAPS) {
481 if (UseBiasedLocking) {
482 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
483 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
484 }
485
486 markOop mark = obj->mark();
487 if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
488 return;
489 }
490 ObjectSynchronizer::inflate(THREAD, obj())->notify(THREAD);
491 }
492
493 // NOTE: see comment of notify()
494 void ObjectSynchronizer::notifyall(Handle obj, TRAPS) {
495 if (UseBiasedLocking) {
496 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
497 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
498 }
499
500 markOop mark = obj->mark();
501 if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
502 return;
503 }
504 ObjectSynchronizer::inflate(THREAD, obj())->notifyAll(THREAD);
505 }
506
507 // -----------------------------------------------------------------------------
508 // Hash Code handling
509 //
510 // Performance concern:
511 // OrderAccess::storestore() calls release() which at one time stored 0
512 // into the global volatile OrderAccess::dummy variable. This store was
513 // unnecessary for correctness. Many threads storing into a common location
514 // causes considerable cache migration or "sloshing" on large SMP systems.
515 // As such, I avoided using OrderAccess::storestore(). In some cases
516 // OrderAccess::fence() -- which incurs local latency on the executing
517 // processor -- is a better choice as it scales on SMP systems.
518 //
519 // See http://blogs.oracle.com/dave/entry/biased_locking_in_hotspot for
520 // a discussion of coherency costs. Note that all our current reference
521 // platforms provide strong ST-ST order, so the issue is moot on IA32,
522 // x64, and SPARC.
523 //
524 // As a general policy we use "volatile" to control compiler-based reordering
525 // and explicit fences (barriers) to control for architectural reordering
526 // performed by the CPU(s) or platform.
527
528 struct SharedGlobals {
529 char _pad_prefix[DEFAULT_CACHE_LINE_SIZE];
530 // These are highly shared mostly-read variables.
531 // To avoid false-sharing they need to be the sole occupants of a cache line.
532 volatile int stwRandom;
533 volatile int stwCycle;
534 DEFINE_PAD_MINUS_SIZE(1, DEFAULT_CACHE_LINE_SIZE, sizeof(volatile int) * 2);
535 // Hot RW variable -- Sequester to avoid false-sharing
536 volatile int hcSequence;
537 DEFINE_PAD_MINUS_SIZE(2, DEFAULT_CACHE_LINE_SIZE, sizeof(volatile int));
538 };
539
540 static SharedGlobals GVars;
541 static int MonitorScavengeThreshold = 1000000;
542 static volatile int ForceMonitorScavenge = 0; // Scavenge required and pending
543
544 static markOop ReadStableMark(oop obj) {
545 markOop mark = obj->mark();
546 if (!mark->is_being_inflated()) {
547 return mark; // normal fast-path return
548 }
549
550 int its = 0;
551 for (;;) {
552 markOop mark = obj->mark();
553 if (!mark->is_being_inflated()) {
554 return mark; // normal fast-path return
555 }
556
557 // The object is being inflated by some other thread.
558 // The caller of ReadStableMark() must wait for inflation to complete.
559 // Avoid live-lock
560 // TODO: consider calling SafepointSynchronize::do_call_back() while
561 // spinning to see if there's a safepoint pending. If so, immediately
562 // yielding or blocking would be appropriate. Avoid spinning while
563 // there is a safepoint pending.
564 // TODO: add inflation contention performance counters.
565 // TODO: restrict the aggregate number of spinners.
566
567 ++its;
568 if (its > 10000 || !os::is_MP()) {
569 if (its & 1) {
570 os::naked_yield();
571 TEVENT(Inflate: INFLATING - yield);
572 } else {
573 // Note that the following code attenuates the livelock problem but is not
574 // a complete remedy. A more complete solution would require that the inflating
575 // thread hold the associated inflation lock. The following code simply restricts
576 // the number of spinners to at most one. We'll have N-2 threads blocked
577 // on the inflationlock, 1 thread holding the inflation lock and using
578 // a yield/park strategy, and 1 thread in the midst of inflation.
579 // A more refined approach would be to change the encoding of INFLATING
580 // to allow encapsulation of a native thread pointer. Threads waiting for
581 // inflation to complete would use CAS to push themselves onto a singly linked
582 // list rooted at the markword. Once enqueued, they'd loop, checking a per-thread flag
583 // and calling park(). When inflation was complete the thread that accomplished inflation
584 // would detach the list and set the markword to inflated with a single CAS and
585 // then for each thread on the list, set the flag and unpark() the thread.
586 // This is conceptually similar to muxAcquire-muxRelease, except that muxRelease
587 // wakes at most one thread whereas we need to wake the entire list.
588 int ix = (cast_from_oop<intptr_t>(obj) >> 5) & (NINFLATIONLOCKS-1);
589 int YieldThenBlock = 0;
590 assert(ix >= 0 && ix < NINFLATIONLOCKS, "invariant");
591 assert((NINFLATIONLOCKS & (NINFLATIONLOCKS-1)) == 0, "invariant");
592 Thread::muxAcquire(gInflationLocks + ix, "gInflationLock");
593 while (obj->mark() == markOopDesc::INFLATING()) {
594 // Beware: NakedYield() is advisory and has almost no effect on some platforms
595 // so we periodically call Self->_ParkEvent->park(1).
596 // We use a mixed spin/yield/block mechanism.
597 if ((YieldThenBlock++) >= 16) {
598 Thread::current()->_ParkEvent->park(1);
599 } else {
600 os::naked_yield();
601 }
602 }
603 Thread::muxRelease(gInflationLocks + ix);
604 TEVENT(Inflate: INFLATING - yield/park);
605 }
606 } else {
607 SpinPause(); // SMP-polite spinning
608 }
609 }
610 }
611
612 // hashCode() generation :
613 //
614 // Possibilities:
615 // * MD5Digest of {obj,stwRandom}
616 // * CRC32 of {obj,stwRandom} or any linear-feedback shift register function.
617 // * A DES- or AES-style SBox[] mechanism
618 // * One of the Phi-based schemes, such as:
619 // 2654435761 = 2^32 * Phi (golden ratio)
620 // HashCodeValue = ((uintptr_t(obj) >> 3) * 2654435761) ^ GVars.stwRandom ;
621 // * A variation of Marsaglia's shift-xor RNG scheme.
622 // * (obj ^ stwRandom) is appealing, but can result
623 // in undesirable regularity in the hashCode values of adjacent objects
624 // (objects allocated back-to-back, in particular). This could potentially
625 // result in hashtable collisions and reduced hashtable efficiency.
626 // There are simple ways to "diffuse" the middle address bits over the
627 // generated hashCode values:
628
629 static inline intptr_t get_next_hash(Thread * Self, oop obj) {
630 intptr_t value = 0;
631 if (hashCode == 0) {
632 // This form uses an unguarded global Park-Miller RNG,
633 // so it's possible for two threads to race and generate the same RNG.
634 // On MP system we'll have lots of RW access to a global, so the
635 // mechanism induces lots of coherency traffic.
636 value = os::random();
637 } else if (hashCode == 1) {
638 // This variation has the property of being stable (idempotent)
639 // between STW operations. This can be useful in some of the 1-0
640 // synchronization schemes.
641 intptr_t addrBits = cast_from_oop<intptr_t>(obj) >> 3;
642 value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom;
643 } else if (hashCode == 2) {
644 value = 1; // for sensitivity testing
645 } else if (hashCode == 3) {
646 value = ++GVars.hcSequence;
647 } else if (hashCode == 4) {
648 value = cast_from_oop<intptr_t>(obj);
649 } else {
650 // Marsaglia's xor-shift scheme with thread-specific state
651 // This is probably the best overall implementation -- we'll
652 // likely make this the default in future releases.
653 unsigned t = Self->_hashStateX;
654 t ^= (t << 11);
655 Self->_hashStateX = Self->_hashStateY;
656 Self->_hashStateY = Self->_hashStateZ;
657 Self->_hashStateZ = Self->_hashStateW;
658 unsigned v = Self->_hashStateW;
659 v = (v ^ (v >> 19)) ^ (t ^ (t >> 8));
660 Self->_hashStateW = v;
661 value = v;
662 }
663
664 value &= markOopDesc::hash_mask;
665 if (value == 0) value = 0xBAD;
666 assert(value != markOopDesc::no_hash, "invariant");
667 TEVENT(hashCode: GENERATE);
668 return value;
669 }
670
671 intptr_t ObjectSynchronizer::FastHashCode(Thread * Self, oop obj) {
672 if (UseBiasedLocking) {
673 // NOTE: many places throughout the JVM do not expect a safepoint
674 // to be taken here, in particular most operations on perm gen
675 // objects. However, we only ever bias Java instances and all of
676 // the call sites of identity_hash that might revoke biases have
677 // been checked to make sure they can handle a safepoint. The
678 // added check of the bias pattern is to avoid useless calls to
679 // thread-local storage.
680 if (obj->mark()->has_bias_pattern()) {
681 // Handle for oop obj in case of STW safepoint
682 Handle hobj(Self, obj);
683 // Relaxing assertion for bug 6320749.
684 assert(Universe::verify_in_progress() ||
685 !SafepointSynchronize::is_at_safepoint(),
686 "biases should not be seen by VM thread here");
687 BiasedLocking::revoke_and_rebias(hobj, false, JavaThread::current());
688 obj = hobj();
689 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
690 }
691 }
692
693 // hashCode() is a heap mutator ...
694 // Relaxing assertion for bug 6320749.
695 assert(Universe::verify_in_progress() || DumpSharedSpaces ||
696 !SafepointSynchronize::is_at_safepoint(), "invariant");
697 assert(Universe::verify_in_progress() || DumpSharedSpaces ||
698 Self->is_Java_thread() , "invariant");
699 assert(Universe::verify_in_progress() || DumpSharedSpaces ||
700 ((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
701
702 ObjectMonitor* monitor = NULL;
703 markOop temp, test;
704 intptr_t hash;
705 markOop mark = ReadStableMark(obj);
706
707 // object should remain ineligible for biased locking
708 assert(!mark->has_bias_pattern(), "invariant");
709
710 if (mark->is_neutral()) {
711 hash = mark->hash(); // this is a normal header
712 if (hash) { // if it has hash, just return it
713 return hash;
714 }
715 hash = get_next_hash(Self, obj); // allocate a new hash code
716 temp = mark->copy_set_hash(hash); // merge the hash code into header
717 // use (machine word version) atomic operation to install the hash
718 test = (markOop) Atomic::cmpxchg_ptr(temp, obj->mark_addr(), mark);
719 if (test == mark) {
720 return hash;
721 }
722 // If atomic operation failed, we must inflate the header
723 // into heavy weight monitor. We could add more code here
724 // for fast path, but it does not worth the complexity.
725 } else if (mark->has_monitor()) {
726 monitor = mark->monitor();
727 temp = monitor->header();
728 assert(temp->is_neutral(), "invariant");
729 hash = temp->hash();
730 if (hash) {
731 return hash;
732 }
733 // Skip to the following code to reduce code size
734 } else if (Self->is_lock_owned((address)mark->locker())) {
735 temp = mark->displaced_mark_helper(); // this is a lightweight monitor owned
736 assert(temp->is_neutral(), "invariant");
737 hash = temp->hash(); // by current thread, check if the displaced
738 if (hash) { // header contains hash code
739 return hash;
740 }
741 // WARNING:
742 // The displaced header is strictly immutable.
743 // It can NOT be changed in ANY cases. So we have
744 // to inflate the header into heavyweight monitor
745 // even the current thread owns the lock. The reason
746 // is the BasicLock (stack slot) will be asynchronously
747 // read by other threads during the inflate() function.
748 // Any change to stack may not propagate to other threads
749 // correctly.
750 }
751
752 // Inflate the monitor to set hash code
753 monitor = ObjectSynchronizer::inflate(Self, obj);
754 // Load displaced header and check it has hash code
755 mark = monitor->header();
756 assert(mark->is_neutral(), "invariant");
757 hash = mark->hash();
758 if (hash == 0) {
759 hash = get_next_hash(Self, obj);
760 temp = mark->copy_set_hash(hash); // merge hash code into header
761 assert(temp->is_neutral(), "invariant");
762 test = (markOop) Atomic::cmpxchg_ptr(temp, monitor, mark);
763 if (test != mark) {
764 // The only update to the header in the monitor (outside GC)
765 // is install the hash code. If someone add new usage of
766 // displaced header, please update this code
767 hash = test->hash();
768 assert(test->is_neutral(), "invariant");
769 assert(hash != 0, "Trivial unexpected object/monitor header usage.");
770 }
771 }
772 // We finally get the hash
773 return hash;
774 }
775
776 // Deprecated -- use FastHashCode() instead.
777
778 intptr_t ObjectSynchronizer::identity_hash_value_for(Handle obj) {
779 return FastHashCode(Thread::current(), obj());
780 }
781
782
783 bool ObjectSynchronizer::current_thread_holds_lock(JavaThread* thread,
784 Handle h_obj) {
785 if (UseBiasedLocking) {
786 BiasedLocking::revoke_and_rebias(h_obj, false, thread);
787 assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
788 }
789
790 assert(thread == JavaThread::current(), "Can only be called on current thread");
791 oop obj = h_obj();
792
793 markOop mark = ReadStableMark(obj);
794
795 // Uncontended case, header points to stack
796 if (mark->has_locker()) {
797 return thread->is_lock_owned((address)mark->locker());
798 }
799 // Contended case, header points to ObjectMonitor (tagged pointer)
800 if (mark->has_monitor()) {
801 ObjectMonitor* monitor = mark->monitor();
802 return monitor->is_entered(thread) != 0;
803 }
804 // Unlocked case, header in place
805 assert(mark->is_neutral(), "sanity check");
806 return false;
807 }
808
809 // Be aware of this method could revoke bias of the lock object.
810 // This method queries the ownership of the lock handle specified by 'h_obj'.
811 // If the current thread owns the lock, it returns owner_self. If no
812 // thread owns the lock, it returns owner_none. Otherwise, it will return
813 // owner_other.
814 ObjectSynchronizer::LockOwnership ObjectSynchronizer::query_lock_ownership
815 (JavaThread *self, Handle h_obj) {
816 // The caller must beware this method can revoke bias, and
817 // revocation can result in a safepoint.
818 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
819 assert(self->thread_state() != _thread_blocked, "invariant");
820
821 // Possible mark states: neutral, biased, stack-locked, inflated
822
823 if (UseBiasedLocking && h_obj()->mark()->has_bias_pattern()) {
824 // CASE: biased
825 BiasedLocking::revoke_and_rebias(h_obj, false, self);
826 assert(!h_obj->mark()->has_bias_pattern(),
827 "biases should be revoked by now");
828 }
829
830 assert(self == JavaThread::current(), "Can only be called on current thread");
831 oop obj = h_obj();
832 markOop mark = ReadStableMark(obj);
833
834 // CASE: stack-locked. Mark points to a BasicLock on the owner's stack.
835 if (mark->has_locker()) {
836 return self->is_lock_owned((address)mark->locker()) ?
837 owner_self : owner_other;
838 }
839
840 // CASE: inflated. Mark (tagged pointer) points to an objectMonitor.
841 // The Object:ObjectMonitor relationship is stable as long as we're
842 // not at a safepoint.
843 if (mark->has_monitor()) {
844 void * owner = mark->monitor()->_owner;
845 if (owner == NULL) return owner_none;
846 return (owner == self ||
847 self->is_lock_owned((address)owner)) ? owner_self : owner_other;
848 }
849
850 // CASE: neutral
851 assert(mark->is_neutral(), "sanity check");
852 return owner_none; // it's unlocked
853 }
854
855 // FIXME: jvmti should call this
856 JavaThread* ObjectSynchronizer::get_lock_owner(Handle h_obj, bool doLock) {
857 if (UseBiasedLocking) {
858 if (SafepointSynchronize::is_at_safepoint()) {
859 BiasedLocking::revoke_at_safepoint(h_obj);
860 } else {
861 BiasedLocking::revoke_and_rebias(h_obj, false, JavaThread::current());
862 }
863 assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
864 }
865
866 oop obj = h_obj();
867 address owner = NULL;
868
869 markOop mark = ReadStableMark(obj);
870
871 // Uncontended case, header points to stack
872 if (mark->has_locker()) {
873 owner = (address) mark->locker();
874 }
875
876 // Contended case, header points to ObjectMonitor (tagged pointer)
877 if (mark->has_monitor()) {
878 ObjectMonitor* monitor = mark->monitor();
879 assert(monitor != NULL, "monitor should be non-null");
880 owner = (address) monitor->owner();
881 }
882
883 if (owner != NULL) {
884 // owning_thread_from_monitor_owner() may also return NULL here
885 return Threads::owning_thread_from_monitor_owner(owner, doLock);
886 }
887
888 // Unlocked case, header in place
889 // Cannot have assertion since this object may have been
890 // locked by another thread when reaching here.
891 // assert(mark->is_neutral(), "sanity check");
892
893 return NULL;
894 }
895 // Visitors ...
896
897 void ObjectSynchronizer::monitors_iterate(MonitorClosure* closure) {
898 PaddedEnd<ObjectMonitor> * block = (PaddedEnd<ObjectMonitor> *)gBlockList;
899 ObjectMonitor* mid;
900 while (block) {
901 assert(block->object() == CHAINMARKER, "must be a block header");
902 for (int i = _BLOCKSIZE - 1; i > 0; i--) {
903 mid = (ObjectMonitor *)(block + i);
904 oop object = (oop) mid->object();
905 if (object != NULL) {
906 closure->do_monitor(mid);
907 }
908 }
909 block = (PaddedEnd<ObjectMonitor> *) block->FreeNext;
910 }
911 }
912
913 // Get the next block in the block list.
914 static inline ObjectMonitor* next(ObjectMonitor* block) {
915 assert(block->object() == CHAINMARKER, "must be a block header");
916 block = block->FreeNext;
917 assert(block == NULL || block->object() == CHAINMARKER, "must be a block header");
918 return block;
919 }
920
921
922 void ObjectSynchronizer::oops_do(OopClosure* f) {
923 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
924 for (PaddedEnd<ObjectMonitor> * block =
925 (PaddedEnd<ObjectMonitor> *)gBlockList; block != NULL;
926 block = (PaddedEnd<ObjectMonitor> *)next(block)) {
927 assert(block->object() == CHAINMARKER, "must be a block header");
928 for (int i = 1; i < _BLOCKSIZE; i++) {
929 ObjectMonitor* mid = (ObjectMonitor *)&block[i];
930 if (mid->object() != NULL) {
931 f->do_oop((oop*)mid->object_addr());
932 }
933 }
934 }
935 }
936
937
938 // -----------------------------------------------------------------------------
939 // ObjectMonitor Lifecycle
940 // -----------------------
941 // Inflation unlinks monitors from the global gFreeList and
942 // associates them with objects. Deflation -- which occurs at
943 // STW-time -- disassociates idle monitors from objects. Such
944 // scavenged monitors are returned to the gFreeList.
945 //
946 // The global list is protected by gListLock. All the critical sections
947 // are short and operate in constant-time.
948 //
949 // ObjectMonitors reside in type-stable memory (TSM) and are immortal.
950 //
951 // Lifecycle:
952 // -- unassigned and on the global free list
953 // -- unassigned and on a thread's private omFreeList
954 // -- assigned to an object. The object is inflated and the mark refers
955 // to the objectmonitor.
956
957
958 // Constraining monitor pool growth via MonitorBound ...
959 //
960 // The monitor pool is grow-only. We scavenge at STW safepoint-time, but the
961 // the rate of scavenging is driven primarily by GC. As such, we can find
962 // an inordinate number of monitors in circulation.
963 // To avoid that scenario we can artificially induce a STW safepoint
964 // if the pool appears to be growing past some reasonable bound.
965 // Generally we favor time in space-time tradeoffs, but as there's no
966 // natural back-pressure on the # of extant monitors we need to impose some
967 // type of limit. Beware that if MonitorBound is set to too low a value
968 // we could just loop. In addition, if MonitorBound is set to a low value
969 // we'll incur more safepoints, which are harmful to performance.
970 // See also: GuaranteedSafepointInterval
971 //
972 // The current implementation uses asynchronous VM operations.
973
974 static void InduceScavenge(Thread * Self, const char * Whence) {
975 // Induce STW safepoint to trim monitors
976 // Ultimately, this results in a call to deflate_idle_monitors() in the near future.
977 // More precisely, trigger an asynchronous STW safepoint as the number
978 // of active monitors passes the specified threshold.
979 // TODO: assert thread state is reasonable
980
981 if (ForceMonitorScavenge == 0 && Atomic::xchg (1, &ForceMonitorScavenge) == 0) {
982 if (ObjectMonitor::Knob_Verbose) {
983 tty->print_cr("INFO: Monitor scavenge - Induced STW @%s (%d)",
984 Whence, ForceMonitorScavenge) ;
985 tty->flush();
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 tty->print_cr("INFO: Monitor scavenge - STW posted @%s (%d)",
995 Whence, ForceMonitorScavenge) ;
996 tty->flush();
997 }
998 }
999 }
1000
1001 void ObjectSynchronizer::verifyInUse(Thread *Self) {
1002 ObjectMonitor* mid;
1003 int in_use_tally = 0;
1004 for (mid = Self->omInUseList; mid != NULL; mid = mid->FreeNext) {
1005 in_use_tally++;
1006 }
1007 assert(in_use_tally == Self->omInUseCount, "in-use count off");
1008
1009 int free_tally = 0;
1010 for (mid = Self->omFreeList; mid != NULL; mid = mid->FreeNext) {
1011 free_tally++;
1012 }
1013 assert(free_tally == Self->omFreeCount, "free count off");
1014 }
1015
1016 ObjectMonitor * NOINLINE ObjectSynchronizer::omAlloc(Thread * Self) {
1017 // A large MAXPRIVATE value reduces both list lock contention
1018 // and list coherency traffic, but also tends to increase the
1019 // number of objectMonitors in circulation as well as the STW
1020 // scavenge costs. As usual, we lean toward time in space-time
1021 // tradeoffs.
1022 const int MAXPRIVATE = 1024;
1023 for (;;) {
1024 ObjectMonitor * m;
1025
1026 // 1: try to allocate from the thread's local omFreeList.
1027 // Threads will attempt to allocate first from their local list, then
1028 // from the global list, and only after those attempts fail will the thread
1029 // attempt to instantiate new monitors. Thread-local free lists take
1030 // heat off the gListLock and improve allocation latency, as well as reducing
1031 // coherency traffic on the shared global list.
1032 m = Self->omFreeList;
1033 if (m != NULL) {
1034 Self->omFreeList = m->FreeNext;
1035 Self->omFreeCount--;
1036 // CONSIDER: set m->FreeNext = BAD -- diagnostic hygiene
1037 guarantee(m->object() == NULL, "invariant");
1038 if (MonitorInUseLists) {
1039 m->FreeNext = Self->omInUseList;
1040 Self->omInUseList = m;
1041 Self->omInUseCount++;
1042 if (ObjectMonitor::Knob_VerifyInUse) {
1043 verifyInUse(Self);
1044 }
1045 } else {
1046 m->FreeNext = NULL;
1047 }
1048 return m;
1049 }
1050
1051 // 2: try to allocate from the global gFreeList
1052 // CONSIDER: use muxTry() instead of muxAcquire().
1053 // If the muxTry() fails then drop immediately into case 3.
1054 // If we're using thread-local free lists then try
1055 // to reprovision the caller's free list.
1056 if (gFreeList != NULL) {
1057 // Reprovision the thread's omFreeList.
1058 // Use bulk transfers to reduce the allocation rate and heat
1059 // on various locks.
1060 Thread::muxAcquire(&gListLock, "omAlloc");
1061 for (int i = Self->omFreeProvision; --i >= 0 && gFreeList != NULL;) {
1062 gMonitorFreeCount--;
1063 ObjectMonitor * take = gFreeList;
1064 gFreeList = take->FreeNext;
1065 guarantee(take->object() == NULL, "invariant");
1066 guarantee(!take->is_busy(), "invariant");
1067 take->Recycle();
1068 omRelease(Self, take, false);
1069 }
1070 Thread::muxRelease(&gListLock);
1071 Self->omFreeProvision += 1 + (Self->omFreeProvision/2);
1072 if (Self->omFreeProvision > MAXPRIVATE) Self->omFreeProvision = MAXPRIVATE;
1073 TEVENT(omFirst - reprovision);
1074
1075 const int mx = MonitorBound;
1076 if (mx > 0 && (gMonitorPopulation-gMonitorFreeCount) > mx) {
1077 // We can't safely induce a STW safepoint from omAlloc() as our thread
1078 // state may not be appropriate for such activities and callers may hold
1079 // naked oops, so instead we defer the action.
1080 InduceScavenge(Self, "omAlloc");
1081 }
1082 continue;
1083 }
1084
1085 // 3: allocate a block of new ObjectMonitors
1086 // Both the local and global free lists are empty -- resort to malloc().
1087 // In the current implementation objectMonitors are TSM - immortal.
1088 // Ideally, we'd write "new ObjectMonitor[_BLOCKSIZE], but we want
1089 // each ObjectMonitor to start at the beginning of a cache line,
1090 // so we use align_size_up().
1091 // A better solution would be to use C++ placement-new.
1092 // BEWARE: As it stands currently, we don't run the ctors!
1093 assert(_BLOCKSIZE > 1, "invariant");
1094 size_t neededsize = sizeof(PaddedEnd<ObjectMonitor>) * _BLOCKSIZE;
1095 PaddedEnd<ObjectMonitor> * temp;
1096 size_t aligned_size = neededsize + (DEFAULT_CACHE_LINE_SIZE - 1);
1097 void* real_malloc_addr = (void *)NEW_C_HEAP_ARRAY(char, aligned_size,
1098 mtInternal);
1099 temp = (PaddedEnd<ObjectMonitor> *)
1100 align_size_up((intptr_t)real_malloc_addr,
1101 DEFAULT_CACHE_LINE_SIZE);
1102
1103 // NOTE: (almost) no way to recover if allocation failed.
1104 // We might be able to induce a STW safepoint and scavenge enough
1105 // objectMonitors to permit progress.
1106 if (temp == NULL) {
1107 vm_exit_out_of_memory(neededsize, OOM_MALLOC_ERROR,
1108 "Allocate ObjectMonitors");
1109 }
1110 (void)memset((void *) temp, 0, neededsize);
1111
1112 // Format the block.
1113 // initialize the linked list, each monitor points to its next
1114 // forming the single linked free list, the very first monitor
1115 // will points to next block, which forms the block list.
1116 // The trick of using the 1st element in the block as gBlockList
1117 // linkage should be reconsidered. A better implementation would
1118 // look like: class Block { Block * next; int N; ObjectMonitor Body [N] ; }
1119
1120 for (int i = 1; i < _BLOCKSIZE; i++) {
1121 temp[i].FreeNext = (ObjectMonitor *)&temp[i+1];
1122 }
1123
1124 // terminate the last monitor as the end of list
1125 temp[_BLOCKSIZE - 1].FreeNext = NULL;
1126
1127 // Element [0] is reserved for global list linkage
1128 temp[0].set_object(CHAINMARKER);
1129
1130 // Consider carving out this thread's current request from the
1131 // block in hand. This avoids some lock traffic and redundant
1132 // list activity.
1133
1134 // Acquire the gListLock to manipulate gBlockList and gFreeList.
1135 // An Oyama-Taura-Yonezawa scheme might be more efficient.
1136 Thread::muxAcquire(&gListLock, "omAlloc [2]");
1137 gMonitorPopulation += _BLOCKSIZE-1;
1138 gMonitorFreeCount += _BLOCKSIZE-1;
1139
1140 // Add the new block to the list of extant blocks (gBlockList).
1141 // The very first objectMonitor in a block is reserved and dedicated.
1142 // It serves as blocklist "next" linkage.
1143 temp[0].FreeNext = gBlockList;
1144 gBlockList = temp;
1145
1146 // Add the new string of objectMonitors to the global free list
1147 temp[_BLOCKSIZE - 1].FreeNext = gFreeList;
1148 gFreeList = temp + 1;
1149 Thread::muxRelease(&gListLock);
1150 TEVENT(Allocate block of monitors);
1151 }
1152 }
1153
1154 // Place "m" on the caller's private per-thread omFreeList.
1155 // In practice there's no need to clamp or limit the number of
1156 // monitors on a thread's omFreeList as the only time we'll call
1157 // omRelease is to return a monitor to the free list after a CAS
1158 // attempt failed. This doesn't allow unbounded #s of monitors to
1159 // accumulate on a thread's free list.
1160 //
1161 // Key constraint: all ObjectMonitors on a thread's free list and the global
1162 // free list must have their object field set to null. This prevents the
1163 // scavenger -- deflate_idle_monitors -- from reclaiming them.
1164
1165 void ObjectSynchronizer::omRelease(Thread * Self, ObjectMonitor * m,
1166 bool fromPerThreadAlloc) {
1167 guarantee(m->object() == NULL, "invariant");
1168 guarantee(((m->is_busy()|m->_recursions) == 0), "freeing in-use monitor");
1169 // Remove from omInUseList
1170 if (MonitorInUseLists && fromPerThreadAlloc) {
1171 ObjectMonitor* cur_mid_in_use = NULL;
1172 bool extracted = false;
1173 for (ObjectMonitor* mid = Self->omInUseList; mid != NULL; cur_mid_in_use = mid, mid = mid->FreeNext) {
1174 if (m == mid) {
1175 // extract from per-thread in-use list
1176 if (mid == Self->omInUseList) {
1177 Self->omInUseList = mid->FreeNext;
1178 } else if (cur_mid_in_use != NULL) {
1179 cur_mid_in_use->FreeNext = mid->FreeNext; // maintain the current thread in-use list
1180 }
1181 extracted = true;
1182 Self->omInUseCount--;
1183 if (ObjectMonitor::Knob_VerifyInUse) {
1184 verifyInUse(Self);
1185 }
1186 break;
1187 }
1188 }
1189 assert(extracted, "Should have extracted from in-use list");
1190 }
1191
1192 // FreeNext is used for both omInUseList and omFreeList, so clear old before setting new
1193 m->FreeNext = Self->omFreeList;
1194 Self->omFreeList = m;
1195 Self->omFreeCount++;
1196 }
1197
1198 // Return the monitors of a moribund thread's local free list to
1199 // the global free list. Typically a thread calls omFlush() when
1200 // it's dying. We could also consider having the VM thread steal
1201 // monitors from threads that have not run java code over a few
1202 // consecutive STW safepoints. Relatedly, we might decay
1203 // omFreeProvision at STW safepoints.
1204 //
1205 // Also return the monitors of a moribund thread's omInUseList to
1206 // a global gOmInUseList under the global list lock so these
1207 // will continue to be scanned.
1208 //
1209 // We currently call omFlush() from the Thread:: dtor _after the thread
1210 // has been excised from the thread list and is no longer a mutator.
1211 // That means that omFlush() can run concurrently with a safepoint and
1212 // the scavenge operator. Calling omFlush() from JavaThread::exit() might
1213 // be a better choice as we could safely reason that that the JVM is
1214 // not at a safepoint at the time of the call, and thus there could
1215 // be not inopportune interleavings between omFlush() and the scavenge
1216 // operator.
1217
1218 void ObjectSynchronizer::omFlush(Thread * Self) {
1219 ObjectMonitor * list = Self->omFreeList; // Null-terminated SLL
1220 Self->omFreeList = NULL;
1221 ObjectMonitor * tail = NULL;
1222 int tally = 0;
1223 if (list != NULL) {
1224 ObjectMonitor * s;
1225 // The thread is going away, the per-thread free monitors
1226 // are freed via set_owner(NULL)
1227 // Link them to tail, which will be linked into the global free list
1228 // gFreeList below, under the gListLock
1229 for (s = list; s != NULL; s = s->FreeNext) {
1230 tally++;
1231 tail = s;
1232 guarantee(s->object() == NULL, "invariant");
1233 guarantee(!s->is_busy(), "invariant");
1234 s->set_owner(NULL); // redundant but good hygiene
1235 TEVENT(omFlush - Move one);
1236 }
1237 guarantee(tail != NULL && list != NULL, "invariant");
1238 }
1239
1240 ObjectMonitor * inUseList = Self->omInUseList;
1241 ObjectMonitor * inUseTail = NULL;
1242 int inUseTally = 0;
1243 if (inUseList != NULL) {
1244 Self->omInUseList = NULL;
1245 ObjectMonitor *cur_om;
1246 // The thread is going away, however the omInUseList inflated
1247 // monitors may still be in-use by other threads.
1248 // Link them to inUseTail, which will be linked into the global in-use list
1249 // gOmInUseList below, under the gListLock
1250 for (cur_om = inUseList; cur_om != NULL; cur_om = cur_om->FreeNext) {
1251 inUseTail = cur_om;
1252 inUseTally++;
1253 }
1254 assert(Self->omInUseCount == inUseTally, "in-use count off");
1255 Self->omInUseCount = 0;
1256 guarantee(inUseTail != NULL && inUseList != NULL, "invariant");
1257 }
1258
1259 Thread::muxAcquire(&gListLock, "omFlush");
1260 if (tail != NULL) {
1261 tail->FreeNext = gFreeList;
1262 gFreeList = list;
1263 gMonitorFreeCount += tally;
1264 }
1265
1266 if (inUseTail != NULL) {
1267 inUseTail->FreeNext = gOmInUseList;
1268 gOmInUseList = inUseList;
1269 gOmInUseCount += inUseTally;
1270 }
1271
1272 Thread::muxRelease(&gListLock);
1273 TEVENT(omFlush);
1274 }
1275
1276 // Fast path code shared by multiple functions
1277 ObjectMonitor* ObjectSynchronizer::inflate_helper(oop obj) {
1278 markOop mark = obj->mark();
1279 if (mark->has_monitor()) {
1280 assert(ObjectSynchronizer::verify_objmon_isinpool(mark->monitor()), "monitor is invalid");
1281 assert(mark->monitor()->header()->is_neutral(), "monitor must record a good object header");
1282 return mark->monitor();
1283 }
1284 return ObjectSynchronizer::inflate(Thread::current(), obj);
1285 }
1286
1287
1288 ObjectMonitor * NOINLINE ObjectSynchronizer::inflate(Thread * Self,
1289 oop object) {
1290 // Inflate mutates the heap ...
1291 // Relaxing assertion for bug 6320749.
1292 assert(Universe::verify_in_progress() ||
1293 !SafepointSynchronize::is_at_safepoint(), "invariant");
1294
1295 for (;;) {
1296 const markOop mark = object->mark();
1297 assert(!mark->has_bias_pattern(), "invariant");
1298
1299 // The mark can be in one of the following states:
1300 // * Inflated - just return
1301 // * Stack-locked - coerce it to inflated
1302 // * INFLATING - busy wait for conversion to complete
1303 // * Neutral - aggressively inflate the object.
1304 // * BIASED - Illegal. We should never see this
1305
1306 // CASE: inflated
1307 if (mark->has_monitor()) {
1308 ObjectMonitor * inf = mark->monitor();
1309 assert(inf->header()->is_neutral(), "invariant");
1310 assert(inf->object() == object, "invariant");
1311 assert(ObjectSynchronizer::verify_objmon_isinpool(inf), "monitor is invalid");
1312 return inf;
1313 }
1314
1315 // CASE: inflation in progress - inflating over a stack-lock.
1316 // Some other thread is converting from stack-locked to inflated.
1317 // Only that thread can complete inflation -- other threads must wait.
1318 // The INFLATING value is transient.
1319 // Currently, we spin/yield/park and poll the markword, waiting for inflation to finish.
1320 // We could always eliminate polling by parking the thread on some auxiliary list.
1321 if (mark == markOopDesc::INFLATING()) {
1322 TEVENT(Inflate: spin while INFLATING);
1323 ReadStableMark(object);
1324 continue;
1325 }
1326
1327 // CASE: stack-locked
1328 // Could be stack-locked either by this thread or by some other thread.
1329 //
1330 // Note that we allocate the objectmonitor speculatively, _before_ attempting
1331 // to install INFLATING into the mark word. We originally installed INFLATING,
1332 // allocated the objectmonitor, and then finally STed the address of the
1333 // objectmonitor into the mark. This was correct, but artificially lengthened
1334 // the interval in which INFLATED appeared in the mark, thus increasing
1335 // the odds of inflation contention.
1336 //
1337 // We now use per-thread private objectmonitor free lists.
1338 // These list are reprovisioned from the global free list outside the
1339 // critical INFLATING...ST interval. A thread can transfer
1340 // multiple objectmonitors en-mass from the global free list to its local free list.
1341 // This reduces coherency traffic and lock contention on the global free list.
1342 // Using such local free lists, it doesn't matter if the omAlloc() call appears
1343 // before or after the CAS(INFLATING) operation.
1344 // See the comments in omAlloc().
1345
1346 if (mark->has_locker()) {
1347 ObjectMonitor * m = omAlloc(Self);
1348 // Optimistically prepare the objectmonitor - anticipate successful CAS
1349 // We do this before the CAS in order to minimize the length of time
1350 // in which INFLATING appears in the mark.
1351 m->Recycle();
1352 m->_Responsible = NULL;
1353 m->_recursions = 0;
1354 m->_SpinDuration = ObjectMonitor::Knob_SpinLimit; // Consider: maintain by type/class
1355
1356 markOop cmp = (markOop) Atomic::cmpxchg_ptr(markOopDesc::INFLATING(), object->mark_addr(), mark);
1357 if (cmp != mark) {
1358 omRelease(Self, m, true);
1359 continue; // Interference -- just retry
1360 }
1361
1362 // We've successfully installed INFLATING (0) into the mark-word.
1363 // This is the only case where 0 will appear in a mark-word.
1364 // Only the singular thread that successfully swings the mark-word
1365 // to 0 can perform (or more precisely, complete) inflation.
1366 //
1367 // Why do we CAS a 0 into the mark-word instead of just CASing the
1368 // mark-word from the stack-locked value directly to the new inflated state?
1369 // Consider what happens when a thread unlocks a stack-locked object.
1370 // It attempts to use CAS to swing the displaced header value from the
1371 // on-stack basiclock back into the object header. Recall also that the
1372 // header value (hashcode, etc) can reside in (a) the object header, or
1373 // (b) a displaced header associated with the stack-lock, or (c) a displaced
1374 // header in an objectMonitor. The inflate() routine must copy the header
1375 // value from the basiclock on the owner's stack to the objectMonitor, all
1376 // the while preserving the hashCode stability invariants. If the owner
1377 // decides to release the lock while the value is 0, the unlock will fail
1378 // and control will eventually pass from slow_exit() to inflate. The owner
1379 // will then spin, waiting for the 0 value to disappear. Put another way,
1380 // the 0 causes the owner to stall if the owner happens to try to
1381 // drop the lock (restoring the header from the basiclock to the object)
1382 // while inflation is in-progress. This protocol avoids races that might
1383 // would otherwise permit hashCode values to change or "flicker" for an object.
1384 // Critically, while object->mark is 0 mark->displaced_mark_helper() is stable.
1385 // 0 serves as a "BUSY" inflate-in-progress indicator.
1386
1387
1388 // fetch the displaced mark from the owner's stack.
1389 // The owner can't die or unwind past the lock while our INFLATING
1390 // object is in the mark. Furthermore the owner can't complete
1391 // an unlock on the object, either.
1392 markOop dmw = mark->displaced_mark_helper();
1393 assert(dmw->is_neutral(), "invariant");
1394
1395 // Setup monitor fields to proper values -- prepare the monitor
1396 m->set_header(dmw);
1397
1398 // Optimization: if the mark->locker stack address is associated
1399 // with this thread we could simply set m->_owner = Self.
1400 // Note that a thread can inflate an object
1401 // that it has stack-locked -- as might happen in wait() -- directly
1402 // with CAS. That is, we can avoid the xchg-NULL .... ST idiom.
1403 m->set_owner(mark->locker());
1404 m->set_object(object);
1405 // TODO-FIXME: assert BasicLock->dhw != 0.
1406
1407 // Must preserve store ordering. The monitor state must
1408 // be stable at the time of publishing the monitor address.
1409 guarantee(object->mark() == markOopDesc::INFLATING(), "invariant");
1410 object->release_set_mark(markOopDesc::encode(m));
1411
1412 // Hopefully the performance counters are allocated on distinct cache lines
1413 // to avoid false sharing on MP systems ...
1414 OM_PERFDATA_OP(Inflations, inc());
1415 TEVENT(Inflate: overwrite stacklock);
1416 if (TraceMonitorInflation) {
1417 if (object->is_instance()) {
1418 ResourceMark rm;
1419 tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1420 (void *) object, (intptr_t) object->mark(),
1421 object->klass()->external_name());
1422 }
1423 }
1424 return m;
1425 }
1426
1427 // CASE: neutral
1428 // TODO-FIXME: for entry we currently inflate and then try to CAS _owner.
1429 // If we know we're inflating for entry it's better to inflate by swinging a
1430 // pre-locked objectMonitor pointer into the object header. A successful
1431 // CAS inflates the object *and* confers ownership to the inflating thread.
1432 // In the current implementation we use a 2-step mechanism where we CAS()
1433 // to inflate and then CAS() again to try to swing _owner from NULL to Self.
1434 // An inflateTry() method that we could call from fast_enter() and slow_enter()
1435 // would be useful.
1436
1437 assert(mark->is_neutral(), "invariant");
1438 ObjectMonitor * m = omAlloc(Self);
1439 // prepare m for installation - set monitor to initial state
1440 m->Recycle();
1441 m->set_header(mark);
1442 m->set_owner(NULL);
1443 m->set_object(object);
1444 m->_recursions = 0;
1445 m->_Responsible = NULL;
1446 m->_SpinDuration = ObjectMonitor::Knob_SpinLimit; // consider: keep metastats by type/class
1447
1448 if (Atomic::cmpxchg_ptr (markOopDesc::encode(m), object->mark_addr(), mark) != mark) {
1449 m->set_object(NULL);
1450 m->set_owner(NULL);
1451 m->Recycle();
1452 omRelease(Self, m, true);
1453 m = NULL;
1454 continue;
1455 // interference - the markword changed - just retry.
1456 // The state-transitions are one-way, so there's no chance of
1457 // live-lock -- "Inflated" is an absorbing state.
1458 }
1459
1460 // Hopefully the performance counters are allocated on distinct
1461 // cache lines to avoid false sharing on MP systems ...
1462 OM_PERFDATA_OP(Inflations, inc());
1463 TEVENT(Inflate: overwrite neutral);
1464 if (TraceMonitorInflation) {
1465 if (object->is_instance()) {
1466 ResourceMark rm;
1467 tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1468 (void *) object, (intptr_t) object->mark(),
1469 object->klass()->external_name());
1470 }
1471 }
1472 return m;
1473 }
1474 }
1475
1476
1477 // Deflate_idle_monitors() is called at all safepoints, immediately
1478 // after all mutators are stopped, but before any objects have moved.
1479 // It traverses the list of known monitors, deflating where possible.
1480 // The scavenged monitor are returned to the monitor free list.
1481 //
1482 // Beware that we scavenge at *every* stop-the-world point.
1483 // Having a large number of monitors in-circulation negatively
1484 // impacts the performance of some applications (e.g., PointBase).
1485 // Broadly, we want to minimize the # of monitors in circulation.
1486 //
1487 // We have added a flag, MonitorInUseLists, which creates a list
1488 // of active monitors for each thread. deflate_idle_monitors()
1489 // only scans the per-thread in-use lists. omAlloc() puts all
1490 // assigned monitors on the per-thread list. deflate_idle_monitors()
1491 // returns the non-busy monitors to the global free list.
1492 // When a thread dies, omFlush() adds the list of active monitors for
1493 // that thread to a global gOmInUseList acquiring the
1494 // global list lock. deflate_idle_monitors() acquires the global
1495 // list lock to scan for non-busy monitors to the global free list.
1496 // An alternative could have used a single global in-use list. The
1497 // downside would have been the additional cost of acquiring the global list lock
1498 // for every omAlloc().
1499 //
1500 // Perversely, the heap size -- and thus the STW safepoint rate --
1501 // typically drives the scavenge rate. Large heaps can mean infrequent GC,
1502 // which in turn can mean large(r) numbers of objectmonitors in circulation.
1503 // This is an unfortunate aspect of this design.
1504
1505 enum ManifestConstants {
1506 ClearResponsibleAtSTW = 0
1507 };
1508
1509 // Deflate a single monitor if not in-use
1510 // Return true if deflated, false if in-use
1511 bool ObjectSynchronizer::deflate_monitor(ObjectMonitor* mid, oop obj,
1512 ObjectMonitor** freeHeadp,
1513 ObjectMonitor** freeTailp) {
1514 bool deflated;
1515 // Normal case ... The monitor is associated with obj.
1516 guarantee(obj->mark() == markOopDesc::encode(mid), "invariant");
1517 guarantee(mid == obj->mark()->monitor(), "invariant");
1518 guarantee(mid->header()->is_neutral(), "invariant");
1519
1520 if (mid->is_busy()) {
1521 if (ClearResponsibleAtSTW) mid->_Responsible = NULL;
1522 deflated = false;
1523 } else {
1524 // Deflate the monitor if it is no longer being used
1525 // It's idle - scavenge and return to the global free list
1526 // plain old deflation ...
1527 TEVENT(deflate_idle_monitors - scavenge1);
1528 if (TraceMonitorInflation) {
1529 if (obj->is_instance()) {
1530 ResourceMark rm;
1531 tty->print_cr("Deflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1532 (void *) obj, (intptr_t) obj->mark(), obj->klass()->external_name());
1533 }
1534 }
1535
1536 // Restore the header back to obj
1537 obj->release_set_mark(mid->header());
1538 mid->clear();
1539
1540 assert(mid->object() == NULL, "invariant");
1541
1542 // Move the object to the working free list defined by freeHeadp, freeTailp
1543 if (*freeHeadp == NULL) *freeHeadp = mid;
1544 if (*freeTailp != NULL) {
1545 ObjectMonitor * prevtail = *freeTailp;
1546 assert(prevtail->FreeNext == NULL, "cleaned up deflated?");
1547 prevtail->FreeNext = mid;
1548 }
1549 *freeTailp = mid;
1550 deflated = true;
1551 }
1552 return deflated;
1553 }
1554
1555 // Walk a given monitor list, and deflate idle monitors
1556 // The given list could be a per-thread list or a global list
1557 // Caller acquires gListLock
1558 int ObjectSynchronizer::deflate_monitor_list(ObjectMonitor** listHeadp,
1559 ObjectMonitor** freeHeadp,
1560 ObjectMonitor** freeTailp) {
1561 ObjectMonitor* mid;
1562 ObjectMonitor* next;
1563 ObjectMonitor* cur_mid_in_use = NULL;
1564 int deflated_count = 0;
1565
1566 for (mid = *listHeadp; mid != NULL;) {
1567 oop obj = (oop) mid->object();
1568 if (obj != NULL && deflate_monitor(mid, obj, freeHeadp, freeTailp)) {
1569 // if deflate_monitor succeeded,
1570 // extract from per-thread in-use list
1571 if (mid == *listHeadp) {
1572 *listHeadp = mid->FreeNext;
1573 } else if (cur_mid_in_use != NULL) {
1574 cur_mid_in_use->FreeNext = mid->FreeNext; // maintain the current thread in-use list
1575 }
1576 next = mid->FreeNext;
1577 mid->FreeNext = NULL; // This mid is current tail in the freeHeadp list
1578 mid = next;
1579 deflated_count++;
1580 } else {
1581 cur_mid_in_use = mid;
1582 mid = mid->FreeNext;
1583 }
1584 }
1585 return deflated_count;
1586 }
1587
1588 void ObjectSynchronizer::deflate_idle_monitors() {
1589 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1590 int nInuse = 0; // currently associated with objects
1591 int nInCirculation = 0; // extant
1592 int nScavenged = 0; // reclaimed
1593 bool deflated = false;
1594
1595 ObjectMonitor * freeHeadp = NULL; // Local SLL of scavenged monitors
1596 ObjectMonitor * freeTailp = NULL;
1597
1598 TEVENT(deflate_idle_monitors);
1599 // Prevent omFlush from changing mids in Thread dtor's during deflation
1600 // And in case the vm thread is acquiring a lock during a safepoint
1601 // See e.g. 6320749
1602 Thread::muxAcquire(&gListLock, "scavenge - return");
1603
1604 if (MonitorInUseLists) {
1605 int inUse = 0;
1606 for (JavaThread* cur = Threads::first(); cur != NULL; cur = cur->next()) {
1607 nInCirculation+= cur->omInUseCount;
1608 int deflated_count = deflate_monitor_list(cur->omInUseList_addr(), &freeHeadp, &freeTailp);
1609 cur->omInUseCount-= deflated_count;
1610 if (ObjectMonitor::Knob_VerifyInUse) {
1611 verifyInUse(cur);
1612 }
1613 nScavenged += deflated_count;
1614 nInuse += cur->omInUseCount;
1615 }
1616
1617 // For moribund threads, scan gOmInUseList
1618 if (gOmInUseList) {
1619 nInCirculation += gOmInUseCount;
1620 int deflated_count = deflate_monitor_list((ObjectMonitor **)&gOmInUseList, &freeHeadp, &freeTailp);
1621 gOmInUseCount-= deflated_count;
1622 nScavenged += deflated_count;
1623 nInuse += gOmInUseCount;
1624 }
1625
1626 } else for (PaddedEnd<ObjectMonitor> * block =
1627 (PaddedEnd<ObjectMonitor> *)gBlockList; block != NULL;
1628 block = (PaddedEnd<ObjectMonitor> *)next(block)) {
1629 // Iterate over all extant monitors - Scavenge all idle monitors.
1630 assert(block->object() == CHAINMARKER, "must be a block header");
1631 nInCirculation += _BLOCKSIZE;
1632 for (int i = 1; i < _BLOCKSIZE; i++) {
1633 ObjectMonitor* mid = (ObjectMonitor*)&block[i];
1634 oop obj = (oop) mid->object();
1635
1636 if (obj == NULL) {
1637 // The monitor is not associated with an object.
1638 // The monitor should either be a thread-specific private
1639 // free list or the global free list.
1640 // obj == NULL IMPLIES mid->is_busy() == 0
1641 guarantee(!mid->is_busy(), "invariant");
1642 continue;
1643 }
1644 deflated = deflate_monitor(mid, obj, &freeHeadp, &freeTailp);
1645
1646 if (deflated) {
1647 mid->FreeNext = NULL;
1648 nScavenged++;
1649 } else {
1650 nInuse++;
1651 }
1652 }
1653 }
1654
1655 gMonitorFreeCount += nScavenged;
1656
1657 // Consider: audit gFreeList to ensure that gMonitorFreeCount and list agree.
1658
1659 if (ObjectMonitor::Knob_Verbose) {
1660 tty->print_cr("INFO: Deflate: InCirc=%d InUse=%d Scavenged=%d "
1661 "ForceMonitorScavenge=%d : pop=%d free=%d",
1662 nInCirculation, nInuse, nScavenged, ForceMonitorScavenge,
1663 gMonitorPopulation, gMonitorFreeCount);
1664 tty->flush();
1665 }
1666
1667 ForceMonitorScavenge = 0; // Reset
1668
1669 // Move the scavenged monitors back to the global free list.
1670 if (freeHeadp != NULL) {
1671 guarantee(freeTailp != NULL && nScavenged > 0, "invariant");
1672 assert(freeTailp->FreeNext == NULL, "invariant");
1673 // constant-time list splice - prepend scavenged segment to gFreeList
1674 freeTailp->FreeNext = gFreeList;
1675 gFreeList = freeHeadp;
1676 }
1677 Thread::muxRelease(&gListLock);
1678
1679 OM_PERFDATA_OP(Deflations, inc(nScavenged));
1680 OM_PERFDATA_OP(MonExtant, set_value(nInCirculation));
1681
1682 // TODO: Add objectMonitor leak detection.
1683 // Audit/inventory the objectMonitors -- make sure they're all accounted for.
1684 GVars.stwRandom = os::random();
1685 GVars.stwCycle++;
1686 }
1687
1688 // Monitor cleanup on JavaThread::exit
1689
1690 // Iterate through monitor cache and attempt to release thread's monitors
1691 // Gives up on a particular monitor if an exception occurs, but continues
1692 // the overall iteration, swallowing the exception.
1693 class ReleaseJavaMonitorsClosure: public MonitorClosure {
1694 private:
1695 TRAPS;
1696
1697 public:
1698 ReleaseJavaMonitorsClosure(Thread* thread) : THREAD(thread) {}
1699 void do_monitor(ObjectMonitor* mid) {
1700 if (mid->owner() == THREAD) {
1701 if (ObjectMonitor::Knob_VerifyMatch != 0) {
1702 Handle obj((oop) mid->object());
1703 tty->print("INFO: unexpected locked object:");
1704 javaVFrame::print_locked_object_class_name(tty, obj, "locked");
1705 fatal("exiting JavaThread=" INTPTR_FORMAT
1706 " unexpectedly owns ObjectMonitor=" INTPTR_FORMAT,
1707 THREAD, mid);
1708 }
1709 (void)mid->complete_exit(CHECK);
1710 }
1711 }
1712 };
1713
1714 // Release all inflated monitors owned by THREAD. Lightweight monitors are
1715 // ignored. This is meant to be called during JNI thread detach which assumes
1716 // all remaining monitors are heavyweight. All exceptions are swallowed.
1717 // Scanning the extant monitor list can be time consuming.
1718 // A simple optimization is to add a per-thread flag that indicates a thread
1719 // called jni_monitorenter() during its lifetime.
1720 //
1721 // Instead of No_Savepoint_Verifier it might be cheaper to
1722 // use an idiom of the form:
1723 // auto int tmp = SafepointSynchronize::_safepoint_counter ;
1724 // <code that must not run at safepoint>
1725 // guarantee (((tmp ^ _safepoint_counter) | (tmp & 1)) == 0) ;
1726 // Since the tests are extremely cheap we could leave them enabled
1727 // for normal product builds.
1728
1729 void ObjectSynchronizer::release_monitors_owned_by_thread(TRAPS) {
1730 assert(THREAD == JavaThread::current(), "must be current Java thread");
1731 No_Safepoint_Verifier nsv;
1732 ReleaseJavaMonitorsClosure rjmc(THREAD);
1733 Thread::muxAcquire(&gListLock, "release_monitors_owned_by_thread");
1734 ObjectSynchronizer::monitors_iterate(&rjmc);
1735 Thread::muxRelease(&gListLock);
1736 THREAD->clear_pending_exception();
1737 }
1738
1739 //------------------------------------------------------------------------------
1740 // Debugging code
1741
1742 void ObjectSynchronizer::sanity_checks(const bool verbose,
1743 const uint cache_line_size,
1744 int *error_cnt_ptr,
1745 int *warning_cnt_ptr) {
1746 u_char *addr_begin = (u_char*)&GVars;
1747 u_char *addr_stwRandom = (u_char*)&GVars.stwRandom;
1748 u_char *addr_hcSequence = (u_char*)&GVars.hcSequence;
1749
1750 if (verbose) {
1751 tty->print_cr("INFO: sizeof(SharedGlobals)=" SIZE_FORMAT,
1752 sizeof(SharedGlobals));
1753 }
1754
1755 uint offset_stwRandom = (uint)(addr_stwRandom - addr_begin);
1756 if (verbose) tty->print_cr("INFO: offset(stwRandom)=%u", offset_stwRandom);
1757
1758 uint offset_hcSequence = (uint)(addr_hcSequence - addr_begin);
1759 if (verbose) {
1760 tty->print_cr("INFO: offset(_hcSequence)=%u", offset_hcSequence);
1761 }
1762
1763 if (cache_line_size != 0) {
1764 // We were able to determine the L1 data cache line size so
1765 // do some cache line specific sanity checks
1766
1767 if (offset_stwRandom < cache_line_size) {
1768 tty->print_cr("WARNING: the SharedGlobals.stwRandom field is closer "
1769 "to the struct beginning than a cache line which permits "
1770 "false sharing.");
1771 (*warning_cnt_ptr)++;
1772 }
1773
1774 if ((offset_hcSequence - offset_stwRandom) < cache_line_size) {
1775 tty->print_cr("WARNING: the SharedGlobals.stwRandom and "
1776 "SharedGlobals.hcSequence fields are closer than a cache "
1777 "line which permits false sharing.");
1778 (*warning_cnt_ptr)++;
1779 }
1780
1781 if ((sizeof(SharedGlobals) - offset_hcSequence) < cache_line_size) {
1782 tty->print_cr("WARNING: the SharedGlobals.hcSequence field is closer "
1783 "to the struct end than a cache line which permits false "
1784 "sharing.");
1785 (*warning_cnt_ptr)++;
1786 }
1787 }
1788 }
1789
1790 #ifndef PRODUCT
1791
1792 // Verify all monitors in the monitor cache, the verification is weak.
1793 void ObjectSynchronizer::verify() {
1794 PaddedEnd<ObjectMonitor> * block = (PaddedEnd<ObjectMonitor> *)gBlockList;
1795 ObjectMonitor* mid;
1796 while (block) {
1797 assert(block->object() == CHAINMARKER, "must be a block header");
1798 for (int i = 1; i < _BLOCKSIZE; i++) {
1799 mid = (ObjectMonitor *)(block + i);
1800 oop object = (oop) mid->object();
1801 if (object != NULL) {
1802 mid->verify();
1803 }
1804 }
1805 block = (PaddedEnd<ObjectMonitor> *) block->FreeNext;
1806 }
1807 }
1808
1809 // Check if monitor belongs to the monitor cache
1810 // The list is grow-only so it's *relatively* safe to traverse
1811 // the list of extant blocks without taking a lock.
1812
1813 int ObjectSynchronizer::verify_objmon_isinpool(ObjectMonitor *monitor) {
1814 PaddedEnd<ObjectMonitor> * block = (PaddedEnd<ObjectMonitor> *)gBlockList;
1815
1816 while (block) {
1817 assert(block->object() == CHAINMARKER, "must be a block header");
1818 if (monitor > (ObjectMonitor *)&block[0] &&
1819 monitor < (ObjectMonitor *)&block[_BLOCKSIZE]) {
1820 address mon = (address) monitor;
1821 address blk = (address) block;
1822 size_t diff = mon - blk;
1823 assert((diff % sizeof(PaddedEnd<ObjectMonitor>)) == 0, "check");
1824 return 1;
1825 }
1826 block = (PaddedEnd<ObjectMonitor> *) block->FreeNext;
1827 }
1828 return 0;
1829 }
1830
1831 #endif