1 /*
2 * Copyright (c) 1998, 2017, 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 "logging/log.hpp"
28 #include "memory/metaspaceShared.hpp"
29 #include "memory/padded.hpp"
30 #include "memory/resourceArea.hpp"
31 #include "oops/markOop.hpp"
32 #include "oops/oop.inline.hpp"
33 #include "runtime/atomic.hpp"
34 #include "runtime/biasedLocking.hpp"
35 #include "runtime/handles.inline.hpp"
36 #include "runtime/interfaceSupport.hpp"
37 #include "runtime/mutexLocker.hpp"
38 #include "runtime/objectMonitor.hpp"
39 #include "runtime/objectMonitor.inline.hpp"
40 #include "runtime/osThread.hpp"
41 #include "runtime/stubRoutines.hpp"
42 #include "runtime/synchronizer.hpp"
43 #include "runtime/thread.inline.hpp"
44 #include "runtime/vframe.hpp"
45 #include "trace/traceMacros.hpp"
46 #include "trace/tracing.hpp"
47 #include "utilities/dtrace.hpp"
48 #include "utilities/events.hpp"
49 #include "utilities/preserveException.hpp"
50
51 // The "core" versions of monitor enter and exit reside in this file.
52 // The interpreter and compilers contain specialized transliterated
53 // variants of the enter-exit fast-path operations. See i486.ad fast_lock(),
54 // for instance. If you make changes here, make sure to modify the
55 // interpreter, and both C1 and C2 fast-path inline locking code emission.
56 //
57 // -----------------------------------------------------------------------------
58
59 #ifdef DTRACE_ENABLED
60
61 // Only bother with this argument setup if dtrace is available
62 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
63
64 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
65 char* bytes = NULL; \
66 int len = 0; \
67 jlong jtid = SharedRuntime::get_java_tid(thread); \
68 Symbol* klassname = ((oop)(obj))->klass()->name(); \
69 if (klassname != NULL) { \
70 bytes = (char*)klassname->bytes(); \
71 len = klassname->utf8_length(); \
72 }
73
74 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
75 { \
76 if (DTraceMonitorProbes) { \
77 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
78 HOTSPOT_MONITOR_WAIT(jtid, \
79 (uintptr_t)(monitor), bytes, len, (millis)); \
80 } \
81 }
82
83 #define HOTSPOT_MONITOR_PROBE_notify HOTSPOT_MONITOR_NOTIFY
84 #define HOTSPOT_MONITOR_PROBE_notifyAll HOTSPOT_MONITOR_NOTIFYALL
85 #define HOTSPOT_MONITOR_PROBE_waited HOTSPOT_MONITOR_WAITED
86
87 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
88 { \
89 if (DTraceMonitorProbes) { \
90 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
91 HOTSPOT_MONITOR_PROBE_##probe(jtid, /* probe = waited */ \
92 (uintptr_t)(monitor), bytes, len); \
93 } \
94 }
95
96 #else // ndef DTRACE_ENABLED
97
98 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
99 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
100
101 #endif // ndef DTRACE_ENABLED
102
103 // This exists only as a workaround of dtrace bug 6254741
104 int dtrace_waited_probe(ObjectMonitor* monitor, Handle obj, Thread* thr) {
105 DTRACE_MONITOR_PROBE(waited, monitor, obj(), thr);
106 return 0;
107 }
108
109 #define NINFLATIONLOCKS 256
110 static volatile intptr_t gInflationLocks[NINFLATIONLOCKS];
111
112 // global list of blocks of monitors
113 // gBlockList is really PaddedEnd<ObjectMonitor> *, but we don't
114 // want to expose the PaddedEnd template more than necessary.
115 ObjectMonitor * volatile ObjectSynchronizer::gBlockList = NULL;
116 // global monitor free list
117 ObjectMonitor * volatile ObjectSynchronizer::gFreeList = NULL;
118 // global monitor in-use list, for moribund threads,
119 // monitors they inflated need to be scanned for deflation
120 ObjectMonitor * volatile ObjectSynchronizer::gOmInUseList = NULL;
121 // count of entries in gOmInUseList
122 int ObjectSynchronizer::gOmInUseCount = 0;
123
124 static volatile intptr_t gListLock = 0; // protects global monitor lists
125 static volatile int gMonitorFreeCount = 0; // # on gFreeList
126 static volatile int gMonitorPopulation = 0; // # Extant -- in circulation
127
128 static void post_monitor_inflate_event(EventJavaMonitorInflate&,
129 const oop,
130 const ObjectSynchronizer::InflateCause);
131
132 #define CHAINMARKER (cast_to_oop<intptr_t>(-1))
133
134
135 // =====================> Quick functions
136
137 // The quick_* forms are special fast-path variants used to improve
138 // performance. In the simplest case, a "quick_*" implementation could
139 // simply return false, in which case the caller will perform the necessary
140 // state transitions and call the slow-path form.
141 // The fast-path is designed to handle frequently arising cases in an efficient
142 // manner and is just a degenerate "optimistic" variant of the slow-path.
143 // returns true -- to indicate the call was satisfied.
144 // returns false -- to indicate the call needs the services of the slow-path.
145 // A no-loitering ordinance is in effect for code in the quick_* family
146 // operators: safepoints or indefinite blocking (blocking that might span a
147 // safepoint) are forbidden. Generally the thread_state() is _in_Java upon
148 // entry.
149 //
150 // Consider: An interesting optimization is to have the JIT recognize the
151 // following common idiom:
152 // synchronized (someobj) { .... ; notify(); }
153 // That is, we find a notify() or notifyAll() call that immediately precedes
154 // the monitorexit operation. In that case the JIT could fuse the operations
155 // into a single notifyAndExit() runtime primitive.
156
157 bool ObjectSynchronizer::quick_notify(oopDesc * obj, Thread * self, bool all) {
158 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
159 assert(self->is_Java_thread(), "invariant");
160 assert(((JavaThread *) self)->thread_state() == _thread_in_Java, "invariant");
161 NoSafepointVerifier nsv;
162 if (obj == NULL) return false; // slow-path for invalid obj
163 const markOop mark = obj->mark();
164
165 if (mark->has_locker() && self->is_lock_owned((address)mark->locker())) {
166 // Degenerate notify
167 // stack-locked by caller so by definition the implied waitset is empty.
168 return true;
169 }
170
171 if (mark->has_monitor()) {
172 ObjectMonitor * const mon = mark->monitor();
173 assert(mon->object() == obj, "invariant");
174 if (mon->owner() != self) return false; // slow-path for IMS exception
175
176 if (mon->first_waiter() != NULL) {
177 // We have one or more waiters. Since this is an inflated monitor
178 // that we own, we can transfer one or more threads from the waitset
179 // to the entrylist here and now, avoiding the slow-path.
180 if (all) {
181 DTRACE_MONITOR_PROBE(notifyAll, mon, obj, self);
182 } else {
183 DTRACE_MONITOR_PROBE(notify, mon, obj, self);
184 }
185 int tally = 0;
186 do {
187 mon->INotify(self);
188 ++tally;
189 } while (mon->first_waiter() != NULL && all);
190 OM_PERFDATA_OP(Notifications, inc(tally));
191 }
192 return true;
193 }
194
195 // biased locking and any other IMS exception states take the slow-path
196 return false;
197 }
198
199
200 // The LockNode emitted directly at the synchronization site would have
201 // been too big if it were to have included support for the cases of inflated
202 // recursive enter and exit, so they go here instead.
203 // Note that we can't safely call AsyncPrintJavaStack() from within
204 // quick_enter() as our thread state remains _in_Java.
205
206 bool ObjectSynchronizer::quick_enter(oop obj, Thread * Self,
207 BasicLock * lock) {
208 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
209 assert(Self->is_Java_thread(), "invariant");
210 assert(((JavaThread *) Self)->thread_state() == _thread_in_Java, "invariant");
211 NoSafepointVerifier nsv;
212 if (obj == NULL) return false; // Need to throw NPE
213 const markOop mark = obj->mark();
214
215 if (mark->has_monitor()) {
216 ObjectMonitor * const m = mark->monitor();
217 assert(m->object() == obj, "invariant");
218 Thread * const owner = (Thread *) m->_owner;
219
220 // Lock contention and Transactional Lock Elision (TLE) diagnostics
221 // and observability
222 // Case: light contention possibly amenable to TLE
223 // Case: TLE inimical operations such as nested/recursive synchronization
224
225 if (owner == Self) {
226 m->_recursions++;
227 return true;
228 }
229
230 // This Java Monitor is inflated so obj's header will never be
231 // displaced to this thread's BasicLock. Make the displaced header
232 // non-NULL so this BasicLock is not seen as recursive nor as
233 // being locked. We do this unconditionally so that this thread's
234 // BasicLock cannot be mis-interpreted by any stack walkers. For
235 // performance reasons, stack walkers generally first check for
236 // Biased Locking in the object's header, the second check is for
237 // stack-locking in the object's header, the third check is for
238 // recursive stack-locking in the displaced header in the BasicLock,
239 // and last are the inflated Java Monitor (ObjectMonitor) checks.
240 lock->set_displaced_header(markOopDesc::unused_mark());
241
242 if (owner == NULL &&
243 Atomic::cmpxchg_ptr(Self, &(m->_owner), NULL) == NULL) {
244 assert(m->_recursions == 0, "invariant");
245 assert(m->_owner == Self, "invariant");
246 return true;
247 }
248 }
249
250 // Note that we could inflate in quick_enter.
251 // This is likely a useful optimization
252 // Critically, in quick_enter() we must not:
253 // -- perform bias revocation, or
254 // -- block indefinitely, or
255 // -- reach a safepoint
256
257 return false; // revert to slow-path
258 }
259
260 // -----------------------------------------------------------------------------
261 // Fast Monitor Enter/Exit
262 // This the fast monitor enter. The interpreter and compiler use
263 // some assembly copies of this code. Make sure update those code
264 // if the following function is changed. The implementation is
265 // extremely sensitive to race condition. Be careful.
266
267 void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock,
268 bool attempt_rebias, TRAPS) {
269 if (UseBiasedLocking) {
270 if (!SafepointSynchronize::is_at_safepoint()) {
271 BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD);
272 if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) {
273 return;
274 }
275 } else {
276 assert(!attempt_rebias, "can not rebias toward VM thread");
277 BiasedLocking::revoke_at_safepoint(obj);
278 }
279 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
280 }
281
282 slow_enter(obj, lock, THREAD);
283 }
284
285 void ObjectSynchronizer::fast_exit(oop object, BasicLock* lock, TRAPS) {
286 markOop mark = object->mark();
287 // We cannot check for Biased Locking if we are racing an inflation.
288 assert(mark == markOopDesc::INFLATING() ||
289 !mark->has_bias_pattern(), "should not see bias pattern here");
290
291 markOop dhw = lock->displaced_header();
292 if (dhw == NULL) {
293 // If the displaced header is NULL, then this exit matches up with
294 // a recursive enter. No real work to do here except for diagnostics.
295 #ifndef PRODUCT
296 if (mark != markOopDesc::INFLATING()) {
297 // Only do diagnostics if we are not racing an inflation. Simply
298 // exiting a recursive enter of a Java Monitor that is being
299 // inflated is safe; see the has_monitor() comment below.
300 assert(!mark->is_neutral(), "invariant");
301 assert(!mark->has_locker() ||
302 THREAD->is_lock_owned((address)mark->locker()), "invariant");
303 if (mark->has_monitor()) {
304 // The BasicLock's displaced_header is marked as a recursive
305 // enter and we have an inflated Java Monitor (ObjectMonitor).
306 // This is a special case where the Java Monitor was inflated
307 // after this thread entered the stack-lock recursively. When a
308 // Java Monitor is inflated, we cannot safely walk the Java
309 // Monitor owner's stack and update the BasicLocks because a
310 // Java Monitor can be asynchronously inflated by a thread that
311 // does not own the Java Monitor.
312 ObjectMonitor * m = mark->monitor();
313 assert(((oop)(m->object()))->mark() == mark, "invariant");
314 assert(m->is_entered(THREAD), "invariant");
315 }
316 }
317 #endif
318 return;
319 }
320
321 if (mark == (markOop) lock) {
322 // If the object is stack-locked by the current thread, try to
323 // swing the displaced header from the BasicLock back to the mark.
324 assert(dhw->is_neutral(), "invariant");
325 if ((markOop) Atomic::cmpxchg_ptr(dhw, object->mark_addr(), mark) == mark) {
326 TEVENT(fast_exit: release stack-lock);
327 return;
328 }
329 }
330
331 // We have to take the slow-path of possible inflation and then exit.
332 ObjectSynchronizer::inflate(THREAD,
333 object,
334 inflate_cause_vm_internal)->exit(true, THREAD);
335 }
336
337 // -----------------------------------------------------------------------------
338 // Interpreter/Compiler Slow Case
339 // This routine is used to handle interpreter/compiler slow case
340 // We don't need to use fast path here, because it must have been
341 // failed in the interpreter/compiler code.
342 void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) {
343 markOop mark = obj->mark();
344 assert(!mark->has_bias_pattern(), "should not see bias pattern here");
345
346 if (mark->is_neutral()) {
347 // Anticipate successful CAS -- the ST of the displaced mark must
348 // be visible <= the ST performed by the CAS.
349 lock->set_displaced_header(mark);
350 if (mark == (markOop) Atomic::cmpxchg_ptr(lock, obj()->mark_addr(), mark)) {
351 TEVENT(slow_enter: release stacklock);
352 return;
353 }
354 // Fall through to inflate() ...
355 } else if (mark->has_locker() &&
356 THREAD->is_lock_owned((address)mark->locker())) {
357 assert(lock != mark->locker(), "must not re-lock the same lock");
358 assert(lock != (BasicLock*)obj->mark(), "don't relock with same BasicLock");
359 lock->set_displaced_header(NULL);
360 return;
361 }
362
363 // The object header will never be displaced to this lock,
364 // so it does not matter what the value is, except that it
365 // must be non-zero to avoid looking like a re-entrant lock,
366 // and must not look locked either.
367 lock->set_displaced_header(markOopDesc::unused_mark());
368 ObjectSynchronizer::inflate(THREAD,
369 obj(),
370 inflate_cause_monitor_enter)->enter(THREAD);
371 }
372
373 // This routine is used to handle interpreter/compiler slow case
374 // We don't need to use fast path here, because it must have
375 // failed in the interpreter/compiler code. Simply use the heavy
376 // weight monitor should be ok, unless someone find otherwise.
377 void ObjectSynchronizer::slow_exit(oop object, BasicLock* lock, TRAPS) {
378 fast_exit(object, lock, THREAD);
379 }
380
381 // -----------------------------------------------------------------------------
382 // Class Loader support to workaround deadlocks on the class loader lock objects
383 // Also used by GC
384 // complete_exit()/reenter() are used to wait on a nested lock
385 // i.e. to give up an outer lock completely and then re-enter
386 // Used when holding nested locks - lock acquisition order: lock1 then lock2
387 // 1) complete_exit lock1 - saving recursion count
388 // 2) wait on lock2
389 // 3) when notified on lock2, unlock lock2
390 // 4) reenter lock1 with original recursion count
391 // 5) lock lock2
392 // NOTE: must use heavy weight monitor to handle complete_exit/reenter()
393 intptr_t ObjectSynchronizer::complete_exit(Handle obj, TRAPS) {
394 TEVENT(complete_exit);
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
400 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD,
401 obj(),
402 inflate_cause_vm_internal);
403
404 return monitor->complete_exit(THREAD);
405 }
406
407 // NOTE: must use heavy weight monitor to handle complete_exit/reenter()
408 void ObjectSynchronizer::reenter(Handle obj, intptr_t recursion, TRAPS) {
409 TEVENT(reenter);
410 if (UseBiasedLocking) {
411 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
412 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
413 }
414
415 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD,
416 obj(),
417 inflate_cause_vm_internal);
418
419 monitor->reenter(recursion, THREAD);
420 }
421 // -----------------------------------------------------------------------------
422 // JNI locks on java objects
423 // NOTE: must use heavy weight monitor to handle jni monitor enter
424 void ObjectSynchronizer::jni_enter(Handle obj, TRAPS) {
425 // the current locking is from JNI instead of Java code
426 TEVENT(jni_enter);
427 if (UseBiasedLocking) {
428 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
429 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
430 }
431 THREAD->set_current_pending_monitor_is_from_java(false);
432 ObjectSynchronizer::inflate(THREAD, obj(), inflate_cause_jni_enter)->enter(THREAD);
433 THREAD->set_current_pending_monitor_is_from_java(true);
434 }
435
436 // NOTE: must use heavy weight monitor to handle jni monitor exit
437 void ObjectSynchronizer::jni_exit(oop obj, Thread* THREAD) {
438 TEVENT(jni_exit);
439 if (UseBiasedLocking) {
440 Handle h_obj(THREAD, obj);
441 BiasedLocking::revoke_and_rebias(h_obj, false, THREAD);
442 obj = h_obj();
443 }
444 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
445
446 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD,
447 obj,
448 inflate_cause_jni_exit);
449 // If this thread has locked the object, exit the monitor. Note: can't use
450 // monitor->check(CHECK); must exit even if an exception is pending.
451 if (monitor->check(THREAD)) {
452 monitor->exit(true, THREAD);
453 }
454 }
455
456 // -----------------------------------------------------------------------------
457 // Internal VM locks on java objects
458 // standard constructor, allows locking failures
459 ObjectLocker::ObjectLocker(Handle obj, Thread* thread, bool doLock) {
460 _dolock = doLock;
461 _thread = thread;
462 debug_only(if (StrictSafepointChecks) _thread->check_for_valid_safepoint_state(false);)
463 _obj = obj;
464
465 if (_dolock) {
466 TEVENT(ObjectLocker);
467
468 ObjectSynchronizer::fast_enter(_obj, &_lock, false, _thread);
469 }
470 }
471
472 ObjectLocker::~ObjectLocker() {
473 if (_dolock) {
474 ObjectSynchronizer::fast_exit(_obj(), &_lock, _thread);
475 }
476 }
477
478
479 // -----------------------------------------------------------------------------
480 // Wait/Notify/NotifyAll
481 // NOTE: must use heavy weight monitor to handle wait()
482 int ObjectSynchronizer::wait(Handle obj, jlong millis, TRAPS) {
483 if (UseBiasedLocking) {
484 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
485 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
486 }
487 if (millis < 0) {
488 TEVENT(wait - throw IAX);
489 THROW_MSG_0(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
490 }
491 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD,
492 obj(),
493 inflate_cause_wait);
494
495 DTRACE_MONITOR_WAIT_PROBE(monitor, obj(), THREAD, millis);
496 monitor->wait(millis, true, THREAD);
497
498 // This dummy call is in place to get around dtrace bug 6254741. Once
499 // that's fixed we can uncomment the following line, remove the call
500 // and change this function back into a "void" func.
501 // DTRACE_MONITOR_PROBE(waited, monitor, obj(), THREAD);
502 return dtrace_waited_probe(monitor, obj, THREAD);
503 }
504
505 void ObjectSynchronizer::waitUninterruptibly(Handle obj, jlong millis, TRAPS) {
506 if (UseBiasedLocking) {
507 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
508 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
509 }
510 if (millis < 0) {
511 TEVENT(wait - throw IAX);
512 THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
513 }
514 ObjectSynchronizer::inflate(THREAD,
515 obj(),
516 inflate_cause_wait)->wait(millis, false, THREAD);
517 }
518
519 void ObjectSynchronizer::notify(Handle obj, TRAPS) {
520 if (UseBiasedLocking) {
521 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
522 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
523 }
524
525 markOop mark = obj->mark();
526 if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
527 return;
528 }
529 ObjectSynchronizer::inflate(THREAD,
530 obj(),
531 inflate_cause_notify)->notify(THREAD);
532 }
533
534 // NOTE: see comment of notify()
535 void ObjectSynchronizer::notifyall(Handle obj, TRAPS) {
536 if (UseBiasedLocking) {
537 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
538 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
539 }
540
541 markOop mark = obj->mark();
542 if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
543 return;
544 }
545 ObjectSynchronizer::inflate(THREAD,
546 obj(),
547 inflate_cause_notify)->notifyAll(THREAD);
548 }
549
550 // -----------------------------------------------------------------------------
551 // Hash Code handling
552 //
553 // Performance concern:
554 // OrderAccess::storestore() calls release() which at one time stored 0
555 // into the global volatile OrderAccess::dummy variable. This store was
556 // unnecessary for correctness. Many threads storing into a common location
557 // causes considerable cache migration or "sloshing" on large SMP systems.
558 // As such, I avoided using OrderAccess::storestore(). In some cases
559 // OrderAccess::fence() -- which incurs local latency on the executing
560 // processor -- is a better choice as it scales on SMP systems.
561 //
562 // See http://blogs.oracle.com/dave/entry/biased_locking_in_hotspot for
563 // a discussion of coherency costs. Note that all our current reference
564 // platforms provide strong ST-ST order, so the issue is moot on IA32,
565 // x64, and SPARC.
566 //
567 // As a general policy we use "volatile" to control compiler-based reordering
568 // and explicit fences (barriers) to control for architectural reordering
569 // performed by the CPU(s) or platform.
570
571 struct SharedGlobals {
572 char _pad_prefix[DEFAULT_CACHE_LINE_SIZE];
573 // These are highly shared mostly-read variables.
574 // To avoid false-sharing they need to be the sole occupants of a cache line.
575 volatile int stwRandom;
576 volatile int stwCycle;
577 DEFINE_PAD_MINUS_SIZE(1, DEFAULT_CACHE_LINE_SIZE, sizeof(volatile int) * 2);
578 // Hot RW variable -- Sequester to avoid false-sharing
579 volatile int hcSequence;
580 DEFINE_PAD_MINUS_SIZE(2, DEFAULT_CACHE_LINE_SIZE, sizeof(volatile int));
581 };
582
583 static SharedGlobals GVars;
584 static int MonitorScavengeThreshold = 1000000;
585 static volatile int ForceMonitorScavenge = 0; // Scavenge required and pending
586
587 static markOop ReadStableMark(oop obj) {
588 markOop mark = obj->mark();
589 if (!mark->is_being_inflated()) {
590 return mark; // normal fast-path return
591 }
592
593 int its = 0;
594 for (;;) {
595 markOop mark = obj->mark();
596 if (!mark->is_being_inflated()) {
597 return mark; // normal fast-path return
598 }
599
600 // The object is being inflated by some other thread.
601 // The caller of ReadStableMark() must wait for inflation to complete.
602 // Avoid live-lock
603 // TODO: consider calling SafepointSynchronize::do_call_back() while
604 // spinning to see if there's a safepoint pending. If so, immediately
605 // yielding or blocking would be appropriate. Avoid spinning while
606 // there is a safepoint pending.
607 // TODO: add inflation contention performance counters.
608 // TODO: restrict the aggregate number of spinners.
609
610 ++its;
611 if (its > 10000 || !os::is_MP()) {
612 if (its & 1) {
613 os::naked_yield();
614 TEVENT(Inflate: INFLATING - yield);
615 } else {
616 // Note that the following code attenuates the livelock problem but is not
617 // a complete remedy. A more complete solution would require that the inflating
618 // thread hold the associated inflation lock. The following code simply restricts
619 // the number of spinners to at most one. We'll have N-2 threads blocked
620 // on the inflationlock, 1 thread holding the inflation lock and using
621 // a yield/park strategy, and 1 thread in the midst of inflation.
622 // A more refined approach would be to change the encoding of INFLATING
623 // to allow encapsulation of a native thread pointer. Threads waiting for
624 // inflation to complete would use CAS to push themselves onto a singly linked
625 // list rooted at the markword. Once enqueued, they'd loop, checking a per-thread flag
626 // and calling park(). When inflation was complete the thread that accomplished inflation
627 // would detach the list and set the markword to inflated with a single CAS and
628 // then for each thread on the list, set the flag and unpark() the thread.
629 // This is conceptually similar to muxAcquire-muxRelease, except that muxRelease
630 // wakes at most one thread whereas we need to wake the entire list.
631 int ix = (cast_from_oop<intptr_t>(obj) >> 5) & (NINFLATIONLOCKS-1);
632 int YieldThenBlock = 0;
633 assert(ix >= 0 && ix < NINFLATIONLOCKS, "invariant");
634 assert((NINFLATIONLOCKS & (NINFLATIONLOCKS-1)) == 0, "invariant");
635 Thread::muxAcquire(gInflationLocks + ix, "gInflationLock");
636 while (obj->mark() == markOopDesc::INFLATING()) {
637 // Beware: NakedYield() is advisory and has almost no effect on some platforms
638 // so we periodically call Self->_ParkEvent->park(1).
639 // We use a mixed spin/yield/block mechanism.
640 if ((YieldThenBlock++) >= 16) {
641 Thread::current()->_ParkEvent->park(1);
642 } else {
643 os::naked_yield();
644 }
645 }
646 Thread::muxRelease(gInflationLocks + ix);
647 TEVENT(Inflate: INFLATING - yield/park);
648 }
649 } else {
650 SpinPause(); // SMP-polite spinning
651 }
652 }
653 }
654
655 // hashCode() generation :
656 //
657 // Possibilities:
658 // * MD5Digest of {obj,stwRandom}
659 // * CRC32 of {obj,stwRandom} or any linear-feedback shift register function.
660 // * A DES- or AES-style SBox[] mechanism
661 // * One of the Phi-based schemes, such as:
662 // 2654435761 = 2^32 * Phi (golden ratio)
663 // HashCodeValue = ((uintptr_t(obj) >> 3) * 2654435761) ^ GVars.stwRandom ;
664 // * A variation of Marsaglia's shift-xor RNG scheme.
665 // * (obj ^ stwRandom) is appealing, but can result
666 // in undesirable regularity in the hashCode values of adjacent objects
667 // (objects allocated back-to-back, in particular). This could potentially
668 // result in hashtable collisions and reduced hashtable efficiency.
669 // There are simple ways to "diffuse" the middle address bits over the
670 // generated hashCode values:
671
672 static inline intptr_t get_next_hash(Thread * Self, oop obj) {
673 intptr_t value = 0;
674 if (hashCode == 0) {
675 // This form uses an unguarded global Park-Miller RNG,
676 // so it's possible for two threads to race and generate the same RNG.
677 // On MP system we'll have lots of RW access to a global, so the
678 // mechanism induces lots of coherency traffic.
679 value = os::random();
680 } else if (hashCode == 1) {
681 // This variation has the property of being stable (idempotent)
682 // between STW operations. This can be useful in some of the 1-0
683 // synchronization schemes.
684 intptr_t addrBits = cast_from_oop<intptr_t>(obj) >> 3;
685 value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom;
686 } else if (hashCode == 2) {
687 value = 1; // for sensitivity testing
688 } else if (hashCode == 3) {
689 value = ++GVars.hcSequence;
690 } else if (hashCode == 4) {
691 value = cast_from_oop<intptr_t>(obj);
692 } else {
693 // Marsaglia's xor-shift scheme with thread-specific state
694 // This is probably the best overall implementation -- we'll
695 // likely make this the default in future releases.
696 unsigned t = Self->_hashStateX;
697 t ^= (t << 11);
698 Self->_hashStateX = Self->_hashStateY;
699 Self->_hashStateY = Self->_hashStateZ;
700 Self->_hashStateZ = Self->_hashStateW;
701 unsigned v = Self->_hashStateW;
702 v = (v ^ (v >> 19)) ^ (t ^ (t >> 8));
703 Self->_hashStateW = v;
704 value = v;
705 }
706
707 value &= markOopDesc::hash_mask;
708 if (value == 0) value = 0xBAD;
709 assert(value != markOopDesc::no_hash, "invariant");
710 TEVENT(hashCode: GENERATE);
711 return value;
712 }
713
714 intptr_t ObjectSynchronizer::FastHashCode(Thread * Self, oop obj) {
715 if (UseBiasedLocking) {
716 // NOTE: many places throughout the JVM do not expect a safepoint
717 // to be taken here, in particular most operations on perm gen
718 // objects. However, we only ever bias Java instances and all of
719 // the call sites of identity_hash that might revoke biases have
720 // been checked to make sure they can handle a safepoint. The
721 // added check of the bias pattern is to avoid useless calls to
722 // thread-local storage.
723 if (obj->mark()->has_bias_pattern()) {
724 // Handle for oop obj in case of STW safepoint
725 Handle hobj(Self, obj);
726 // Relaxing assertion for bug 6320749.
727 assert(Universe::verify_in_progress() ||
728 !SafepointSynchronize::is_at_safepoint(),
729 "biases should not be seen by VM thread here");
730 BiasedLocking::revoke_and_rebias(hobj, false, JavaThread::current());
731 obj = hobj();
732 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
733 }
734 }
735
736 // hashCode() is a heap mutator ...
737 // Relaxing assertion for bug 6320749.
738 assert(Universe::verify_in_progress() || DumpSharedSpaces ||
739 !SafepointSynchronize::is_at_safepoint(), "invariant");
740 assert(Universe::verify_in_progress() || DumpSharedSpaces ||
741 Self->is_Java_thread() , "invariant");
742 assert(Universe::verify_in_progress() || DumpSharedSpaces ||
743 ((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
744
745 ObjectMonitor* monitor = NULL;
746 markOop temp, test;
747 intptr_t hash;
748 markOop mark = ReadStableMark(obj);
749
750 // object should remain ineligible for biased locking
751 assert(!mark->has_bias_pattern(), "invariant");
752
753 if (mark->is_neutral()) {
754 hash = mark->hash(); // this is a normal header
755 if (hash) { // if it has hash, just return it
756 return hash;
757 }
758 hash = get_next_hash(Self, obj); // allocate a new hash code
759 temp = mark->copy_set_hash(hash); // merge the hash code into header
760 // use (machine word version) atomic operation to install the hash
761 test = (markOop) Atomic::cmpxchg_ptr(temp, obj->mark_addr(), mark);
762 if (test == mark) {
763 return hash;
764 }
765 // If atomic operation failed, we must inflate the header
766 // into heavy weight monitor. We could add more code here
767 // for fast path, but it does not worth the complexity.
768 } else if (mark->has_monitor()) {
769 monitor = mark->monitor();
770 temp = monitor->header();
771 assert(temp->is_neutral(), "invariant");
772 hash = temp->hash();
773 if (hash) {
774 return hash;
775 }
776 // Skip to the following code to reduce code size
777 } else if (Self->is_lock_owned((address)mark->locker())) {
778 temp = mark->displaced_mark_helper(); // this is a lightweight monitor owned
779 assert(temp->is_neutral(), "invariant");
780 hash = temp->hash(); // by current thread, check if the displaced
781 if (hash) { // header contains hash code
782 return hash;
783 }
784 // WARNING:
785 // The displaced header is strictly immutable.
786 // It can NOT be changed in ANY cases. So we have
787 // to inflate the header into heavyweight monitor
788 // even the current thread owns the lock. The reason
789 // is the BasicLock (stack slot) will be asynchronously
790 // read by other threads during the inflate() function.
791 // Any change to stack may not propagate to other threads
792 // correctly.
793 }
794
795 // Inflate the monitor to set hash code
796 monitor = ObjectSynchronizer::inflate(Self, obj, inflate_cause_hash_code);
797 // Load displaced header and check it has hash code
798 mark = monitor->header();
799 assert(mark->is_neutral(), "invariant");
800 hash = mark->hash();
801 if (hash == 0) {
802 hash = get_next_hash(Self, obj);
803 temp = mark->copy_set_hash(hash); // merge hash code into header
804 assert(temp->is_neutral(), "invariant");
805 test = (markOop) Atomic::cmpxchg_ptr(temp, monitor, mark);
806 if (test != mark) {
807 // The only update to the header in the monitor (outside GC)
808 // is install the hash code. If someone add new usage of
809 // displaced header, please update this code
810 hash = test->hash();
811 assert(test->is_neutral(), "invariant");
812 assert(hash != 0, "Trivial unexpected object/monitor header usage.");
813 }
814 }
815 // We finally get the hash
816 return hash;
817 }
818
819 // Deprecated -- use FastHashCode() instead.
820
821 intptr_t ObjectSynchronizer::identity_hash_value_for(Handle obj) {
822 return FastHashCode(Thread::current(), obj());
823 }
824
825
826 bool ObjectSynchronizer::current_thread_holds_lock(JavaThread* thread,
827 Handle h_obj) {
828 if (UseBiasedLocking) {
829 BiasedLocking::revoke_and_rebias(h_obj, false, thread);
830 assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
831 }
832
833 assert(thread == JavaThread::current(), "Can only be called on current thread");
834 oop obj = h_obj();
835
836 markOop mark = ReadStableMark(obj);
837
838 // Uncontended case, header points to stack
839 if (mark->has_locker()) {
840 return thread->is_lock_owned((address)mark->locker());
841 }
842 // Contended case, header points to ObjectMonitor (tagged pointer)
843 if (mark->has_monitor()) {
844 ObjectMonitor* monitor = mark->monitor();
845 return monitor->is_entered(thread) != 0;
846 }
847 // Unlocked case, header in place
848 assert(mark->is_neutral(), "sanity check");
849 return false;
850 }
851
852 // Be aware of this method could revoke bias of the lock object.
853 // This method queries the ownership of the lock handle specified by 'h_obj'.
854 // If the current thread owns the lock, it returns owner_self. If no
855 // thread owns the lock, it returns owner_none. Otherwise, it will return
856 // owner_other.
857 ObjectSynchronizer::LockOwnership ObjectSynchronizer::query_lock_ownership
858 (JavaThread *self, Handle h_obj) {
859 // The caller must beware this method can revoke bias, and
860 // revocation can result in a safepoint.
861 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
862 assert(self->thread_state() != _thread_blocked, "invariant");
863
864 // Possible mark states: neutral, biased, stack-locked, inflated
865
866 if (UseBiasedLocking && h_obj()->mark()->has_bias_pattern()) {
867 // CASE: biased
868 BiasedLocking::revoke_and_rebias(h_obj, false, self);
869 assert(!h_obj->mark()->has_bias_pattern(),
870 "biases should be revoked by now");
871 }
872
873 assert(self == JavaThread::current(), "Can only be called on current thread");
874 oop obj = h_obj();
875 markOop mark = ReadStableMark(obj);
876
877 // CASE: stack-locked. Mark points to a BasicLock on the owner's stack.
878 if (mark->has_locker()) {
879 return self->is_lock_owned((address)mark->locker()) ?
880 owner_self : owner_other;
881 }
882
883 // CASE: inflated. Mark (tagged pointer) points to an objectMonitor.
884 // The Object:ObjectMonitor relationship is stable as long as we're
885 // not at a safepoint.
886 if (mark->has_monitor()) {
887 void * owner = mark->monitor()->_owner;
888 if (owner == NULL) return owner_none;
889 return (owner == self ||
890 self->is_lock_owned((address)owner)) ? owner_self : owner_other;
891 }
892
893 // CASE: neutral
894 assert(mark->is_neutral(), "sanity check");
895 return owner_none; // it's unlocked
896 }
897
898 // FIXME: jvmti should call this
899 JavaThread* ObjectSynchronizer::get_lock_owner(Handle h_obj, bool doLock) {
900 if (UseBiasedLocking) {
901 if (SafepointSynchronize::is_at_safepoint()) {
902 BiasedLocking::revoke_at_safepoint(h_obj);
903 } else {
904 BiasedLocking::revoke_and_rebias(h_obj, false, JavaThread::current());
905 }
906 assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
907 }
908
909 oop obj = h_obj();
910 address owner = NULL;
911
912 markOop mark = ReadStableMark(obj);
913
914 // Uncontended case, header points to stack
915 if (mark->has_locker()) {
916 owner = (address) mark->locker();
917 }
918
919 // Contended case, header points to ObjectMonitor (tagged pointer)
920 if (mark->has_monitor()) {
921 ObjectMonitor* monitor = mark->monitor();
922 assert(monitor != NULL, "monitor should be non-null");
923 owner = (address) monitor->owner();
924 }
925
926 if (owner != NULL) {
927 // owning_thread_from_monitor_owner() may also return NULL here
928 return Threads::owning_thread_from_monitor_owner(owner, doLock);
929 }
930
931 // Unlocked case, header in place
932 // Cannot have assertion since this object may have been
933 // locked by another thread when reaching here.
934 // assert(mark->is_neutral(), "sanity check");
935
936 return NULL;
937 }
938
939 // Visitors ...
940
941 void ObjectSynchronizer::monitors_iterate(MonitorClosure* closure) {
942 PaddedEnd<ObjectMonitor> * block =
943 (PaddedEnd<ObjectMonitor> *)OrderAccess::load_ptr_acquire(&gBlockList);
944 while (block != NULL) {
945 assert(block->object() == CHAINMARKER, "must be a block header");
946 for (int i = _BLOCKSIZE - 1; i > 0; i--) {
947 ObjectMonitor* mid = (ObjectMonitor *)(block + i);
948 oop object = (oop)mid->object();
949 if (object != NULL) {
950 closure->do_monitor(mid);
951 }
952 }
953 block = (PaddedEnd<ObjectMonitor> *)block->FreeNext;
954 }
955 }
956
957 // Get the next block in the block list.
958 static inline ObjectMonitor* next(ObjectMonitor* block) {
959 assert(block->object() == CHAINMARKER, "must be a block header");
960 block = block->FreeNext;
961 assert(block == NULL || block->object() == CHAINMARKER, "must be a block header");
962 return block;
963 }
964
965 bool ObjectSynchronizer::is_cleanup_needed() {
966 int monitors_used = gMonitorPopulation - gMonitorFreeCount;
967 return MonitorUsedDeflationThreshold > 0 && (monitors_used * 100LL) / gMonitorPopulation > MonitorUsedDeflationThreshold;
968 }
969
970 void ObjectSynchronizer::oops_do(OopClosure* f) {
971 if (MonitorInUseLists) {
972 // When using thread local monitor lists, we only scan the
973 // global used list here (for moribund threads), and
974 // the thread-local monitors in Thread::oops_do().
975 global_used_oops_do(f);
976 } else {
977 global_oops_do(f);
978 }
979 }
980
981 void ObjectSynchronizer::global_oops_do(OopClosure* f) {
982 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
983 PaddedEnd<ObjectMonitor> * block =
984 (PaddedEnd<ObjectMonitor> *)OrderAccess::load_ptr_acquire(&gBlockList);
985 for (; block != NULL; block = (PaddedEnd<ObjectMonitor> *)next(block)) {
986 assert(block->object() == CHAINMARKER, "must be a block header");
987 for (int i = 1; i < _BLOCKSIZE; i++) {
988 ObjectMonitor* mid = (ObjectMonitor *)&block[i];
989 if (mid->object() != NULL) {
990 f->do_oop((oop*)mid->object_addr());
991 }
992 }
993 }
994 }
995
996 void ObjectSynchronizer::global_used_oops_do(OopClosure* f) {
997 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
998 list_oops_do(gOmInUseList, f);
999 }
1000
1001 void ObjectSynchronizer::thread_local_used_oops_do(Thread* thread, OopClosure* f) {
1002 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1003 list_oops_do(thread->omInUseList, f);
1004 }
1005
1006 void ObjectSynchronizer::list_oops_do(ObjectMonitor* list, OopClosure* f) {
1007 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1008 ObjectMonitor* mid;
1009 for (mid = list; mid != NULL; mid = mid->FreeNext) {
1010 if (mid->object() != NULL) {
1011 f->do_oop((oop*)mid->object_addr());
1012 }
1013 }
1014 }
1015
1016
1017 // -----------------------------------------------------------------------------
1018 // ObjectMonitor Lifecycle
1019 // -----------------------
1020 // Inflation unlinks monitors from the global gFreeList and
1021 // associates them with objects. Deflation -- which occurs at
1022 // STW-time -- disassociates idle monitors from objects. Such
1023 // scavenged monitors are returned to the gFreeList.
1024 //
1025 // The global list is protected by gListLock. All the critical sections
1026 // are short and operate in constant-time.
1027 //
1028 // ObjectMonitors reside in type-stable memory (TSM) and are immortal.
1029 //
1030 // Lifecycle:
1031 // -- unassigned and on the global free list
1032 // -- unassigned and on a thread's private omFreeList
1033 // -- assigned to an object. The object is inflated and the mark refers
1034 // to the objectmonitor.
1035
1036
1037 // Constraining monitor pool growth via MonitorBound ...
1038 //
1039 // The monitor pool is grow-only. We scavenge at STW safepoint-time, but the
1040 // the rate of scavenging is driven primarily by GC. As such, we can find
1041 // an inordinate number of monitors in circulation.
1042 // To avoid that scenario we can artificially induce a STW safepoint
1043 // if the pool appears to be growing past some reasonable bound.
1044 // Generally we favor time in space-time tradeoffs, but as there's no
1045 // natural back-pressure on the # of extant monitors we need to impose some
1046 // type of limit. Beware that if MonitorBound is set to too low a value
1047 // we could just loop. In addition, if MonitorBound is set to a low value
1048 // we'll incur more safepoints, which are harmful to performance.
1049 // See also: GuaranteedSafepointInterval
1050 //
1051 // The current implementation uses asynchronous VM operations.
1052
1053 static void InduceScavenge(Thread * Self, const char * Whence) {
1054 // Induce STW safepoint to trim monitors
1055 // Ultimately, this results in a call to deflate_idle_monitors() in the near future.
1056 // More precisely, trigger an asynchronous STW safepoint as the number
1057 // of active monitors passes the specified threshold.
1058 // TODO: assert thread state is reasonable
1059
1060 if (ForceMonitorScavenge == 0 && Atomic::xchg (1, &ForceMonitorScavenge) == 0) {
1061 if (ObjectMonitor::Knob_Verbose) {
1062 tty->print_cr("INFO: Monitor scavenge - Induced STW @%s (%d)",
1063 Whence, ForceMonitorScavenge) ;
1064 tty->flush();
1065 }
1066 // Induce a 'null' safepoint to scavenge monitors
1067 // Must VM_Operation instance be heap allocated as the op will be enqueue and posted
1068 // to the VMthread and have a lifespan longer than that of this activation record.
1069 // The VMThread will delete the op when completed.
1070 VMThread::execute(new VM_ScavengeMonitors());
1071
1072 if (ObjectMonitor::Knob_Verbose) {
1073 tty->print_cr("INFO: Monitor scavenge - STW posted @%s (%d)",
1074 Whence, ForceMonitorScavenge) ;
1075 tty->flush();
1076 }
1077 }
1078 }
1079
1080 void ObjectSynchronizer::verifyInUse(Thread *Self) {
1081 ObjectMonitor* mid;
1082 int in_use_tally = 0;
1083 for (mid = Self->omInUseList; mid != NULL; mid = mid->FreeNext) {
1084 in_use_tally++;
1085 }
1086 assert(in_use_tally == Self->omInUseCount, "in-use count off");
1087
1088 int free_tally = 0;
1089 for (mid = Self->omFreeList; mid != NULL; mid = mid->FreeNext) {
1090 free_tally++;
1091 }
1092 assert(free_tally == Self->omFreeCount, "free count off");
1093 }
1094
1095 ObjectMonitor* ObjectSynchronizer::omAlloc(Thread * Self) {
1096 // A large MAXPRIVATE value reduces both list lock contention
1097 // and list coherency traffic, but also tends to increase the
1098 // number of objectMonitors in circulation as well as the STW
1099 // scavenge costs. As usual, we lean toward time in space-time
1100 // tradeoffs.
1101 const int MAXPRIVATE = 1024;
1102 for (;;) {
1103 ObjectMonitor * m;
1104
1105 // 1: try to allocate from the thread's local omFreeList.
1106 // Threads will attempt to allocate first from their local list, then
1107 // from the global list, and only after those attempts fail will the thread
1108 // attempt to instantiate new monitors. Thread-local free lists take
1109 // heat off the gListLock and improve allocation latency, as well as reducing
1110 // coherency traffic on the shared global list.
1111 m = Self->omFreeList;
1112 if (m != NULL) {
1113 Self->omFreeList = m->FreeNext;
1114 Self->omFreeCount--;
1115 // CONSIDER: set m->FreeNext = BAD -- diagnostic hygiene
1116 guarantee(m->object() == NULL, "invariant");
1117 if (MonitorInUseLists) {
1118 m->FreeNext = Self->omInUseList;
1119 Self->omInUseList = m;
1120 Self->omInUseCount++;
1121 if (ObjectMonitor::Knob_VerifyInUse) {
1122 verifyInUse(Self);
1123 }
1124 } else {
1125 m->FreeNext = NULL;
1126 }
1127 return m;
1128 }
1129
1130 // 2: try to allocate from the global gFreeList
1131 // CONSIDER: use muxTry() instead of muxAcquire().
1132 // If the muxTry() fails then drop immediately into case 3.
1133 // If we're using thread-local free lists then try
1134 // to reprovision the caller's free list.
1135 if (gFreeList != NULL) {
1136 // Reprovision the thread's omFreeList.
1137 // Use bulk transfers to reduce the allocation rate and heat
1138 // on various locks.
1139 Thread::muxAcquire(&gListLock, "omAlloc");
1140 for (int i = Self->omFreeProvision; --i >= 0 && gFreeList != NULL;) {
1141 gMonitorFreeCount--;
1142 ObjectMonitor * take = gFreeList;
1143 gFreeList = take->FreeNext;
1144 guarantee(take->object() == NULL, "invariant");
1145 guarantee(!take->is_busy(), "invariant");
1146 take->Recycle();
1147 omRelease(Self, take, false);
1148 }
1149 Thread::muxRelease(&gListLock);
1150 Self->omFreeProvision += 1 + (Self->omFreeProvision/2);
1151 if (Self->omFreeProvision > MAXPRIVATE) Self->omFreeProvision = MAXPRIVATE;
1152 TEVENT(omFirst - reprovision);
1153
1154 const int mx = MonitorBound;
1155 if (mx > 0 && (gMonitorPopulation-gMonitorFreeCount) > mx) {
1156 // We can't safely induce a STW safepoint from omAlloc() as our thread
1157 // state may not be appropriate for such activities and callers may hold
1158 // naked oops, so instead we defer the action.
1159 InduceScavenge(Self, "omAlloc");
1160 }
1161 continue;
1162 }
1163
1164 // 3: allocate a block of new ObjectMonitors
1165 // Both the local and global free lists are empty -- resort to malloc().
1166 // In the current implementation objectMonitors are TSM - immortal.
1167 // Ideally, we'd write "new ObjectMonitor[_BLOCKSIZE], but we want
1168 // each ObjectMonitor to start at the beginning of a cache line,
1169 // so we use align_size_up().
1170 // A better solution would be to use C++ placement-new.
1171 // BEWARE: As it stands currently, we don't run the ctors!
1172 assert(_BLOCKSIZE > 1, "invariant");
1173 size_t neededsize = sizeof(PaddedEnd<ObjectMonitor>) * _BLOCKSIZE;
1174 PaddedEnd<ObjectMonitor> * temp;
1175 size_t aligned_size = neededsize + (DEFAULT_CACHE_LINE_SIZE - 1);
1176 void* real_malloc_addr = (void *)NEW_C_HEAP_ARRAY(char, aligned_size,
1177 mtInternal);
1178 temp = (PaddedEnd<ObjectMonitor> *)
1179 align_size_up((intptr_t)real_malloc_addr,
1180 DEFAULT_CACHE_LINE_SIZE);
1181
1182 // NOTE: (almost) no way to recover if allocation failed.
1183 // We might be able to induce a STW safepoint and scavenge enough
1184 // objectMonitors to permit progress.
1185 if (temp == NULL) {
1186 vm_exit_out_of_memory(neededsize, OOM_MALLOC_ERROR,
1187 "Allocate ObjectMonitors");
1188 }
1189 (void)memset((void *) temp, 0, neededsize);
1190
1191 // Format the block.
1192 // initialize the linked list, each monitor points to its next
1193 // forming the single linked free list, the very first monitor
1194 // will points to next block, which forms the block list.
1195 // The trick of using the 1st element in the block as gBlockList
1196 // linkage should be reconsidered. A better implementation would
1197 // look like: class Block { Block * next; int N; ObjectMonitor Body [N] ; }
1198
1199 for (int i = 1; i < _BLOCKSIZE; i++) {
1200 temp[i].FreeNext = (ObjectMonitor *)&temp[i+1];
1201 }
1202
1203 // terminate the last monitor as the end of list
1204 temp[_BLOCKSIZE - 1].FreeNext = NULL;
1205
1206 // Element [0] is reserved for global list linkage
1207 temp[0].set_object(CHAINMARKER);
1208
1209 // Consider carving out this thread's current request from the
1210 // block in hand. This avoids some lock traffic and redundant
1211 // list activity.
1212
1213 // Acquire the gListLock to manipulate gBlockList and gFreeList.
1214 // An Oyama-Taura-Yonezawa scheme might be more efficient.
1215 Thread::muxAcquire(&gListLock, "omAlloc [2]");
1216 gMonitorPopulation += _BLOCKSIZE-1;
1217 gMonitorFreeCount += _BLOCKSIZE-1;
1218
1219 // Add the new block to the list of extant blocks (gBlockList).
1220 // The very first objectMonitor in a block is reserved and dedicated.
1221 // It serves as blocklist "next" linkage.
1222 temp[0].FreeNext = gBlockList;
1223 // There are lock-free uses of gBlockList so make sure that
1224 // the previous stores happen before we update gBlockList.
1225 OrderAccess::release_store_ptr(&gBlockList, temp);
1226
1227 // Add the new string of objectMonitors to the global free list
1228 temp[_BLOCKSIZE - 1].FreeNext = gFreeList;
1229 gFreeList = temp + 1;
1230 Thread::muxRelease(&gListLock);
1231 TEVENT(Allocate block of monitors);
1232 }
1233 }
1234
1235 // Place "m" on the caller's private per-thread omFreeList.
1236 // In practice there's no need to clamp or limit the number of
1237 // monitors on a thread's omFreeList as the only time we'll call
1238 // omRelease is to return a monitor to the free list after a CAS
1239 // attempt failed. This doesn't allow unbounded #s of monitors to
1240 // accumulate on a thread's free list.
1241 //
1242 // Key constraint: all ObjectMonitors on a thread's free list and the global
1243 // free list must have their object field set to null. This prevents the
1244 // scavenger -- deflate_idle_monitors -- from reclaiming them.
1245
1246 void ObjectSynchronizer::omRelease(Thread * Self, ObjectMonitor * m,
1247 bool fromPerThreadAlloc) {
1248 guarantee(m->object() == NULL, "invariant");
1249 guarantee(((m->is_busy()|m->_recursions) == 0), "freeing in-use monitor");
1250 // Remove from omInUseList
1251 if (MonitorInUseLists && fromPerThreadAlloc) {
1252 ObjectMonitor* cur_mid_in_use = NULL;
1253 bool extracted = false;
1254 for (ObjectMonitor* mid = Self->omInUseList; mid != NULL; cur_mid_in_use = mid, mid = mid->FreeNext) {
1255 if (m == mid) {
1256 // extract from per-thread in-use list
1257 if (mid == Self->omInUseList) {
1258 Self->omInUseList = mid->FreeNext;
1259 } else if (cur_mid_in_use != NULL) {
1260 cur_mid_in_use->FreeNext = mid->FreeNext; // maintain the current thread in-use list
1261 }
1262 extracted = true;
1263 Self->omInUseCount--;
1264 if (ObjectMonitor::Knob_VerifyInUse) {
1265 verifyInUse(Self);
1266 }
1267 break;
1268 }
1269 }
1270 assert(extracted, "Should have extracted from in-use list");
1271 }
1272
1273 // FreeNext is used for both omInUseList and omFreeList, so clear old before setting new
1274 m->FreeNext = Self->omFreeList;
1275 Self->omFreeList = m;
1276 Self->omFreeCount++;
1277 }
1278
1279 // Return the monitors of a moribund thread's local free list to
1280 // the global free list. Typically a thread calls omFlush() when
1281 // it's dying. We could also consider having the VM thread steal
1282 // monitors from threads that have not run java code over a few
1283 // consecutive STW safepoints. Relatedly, we might decay
1284 // omFreeProvision at STW safepoints.
1285 //
1286 // Also return the monitors of a moribund thread's omInUseList to
1287 // a global gOmInUseList under the global list lock so these
1288 // will continue to be scanned.
1289 //
1290 // We currently call omFlush() from Threads::remove() _before the thread
1291 // has been excised from the thread list and is no longer a mutator.
1292 // This means that omFlush() can not run concurrently with a safepoint and
1293 // interleave with the scavenge operator. In particular, this ensures that
1294 // the thread's monitors are scanned by a GC safepoint, either via
1295 // Thread::oops_do() (if safepoint happens before omFlush()) or via
1296 // ObjectSynchronizer::oops_do() (if it happens after omFlush() and the thread's
1297 // monitors have been transferred to the global in-use list).
1298
1299 void ObjectSynchronizer::omFlush(Thread * Self) {
1300 ObjectMonitor * list = Self->omFreeList; // Null-terminated SLL
1301 Self->omFreeList = NULL;
1302 ObjectMonitor * tail = NULL;
1303 int tally = 0;
1304 if (list != NULL) {
1305 ObjectMonitor * s;
1306 // The thread is going away, the per-thread free monitors
1307 // are freed via set_owner(NULL)
1308 // Link them to tail, which will be linked into the global free list
1309 // gFreeList below, under the gListLock
1310 for (s = list; s != NULL; s = s->FreeNext) {
1311 tally++;
1312 tail = s;
1313 guarantee(s->object() == NULL, "invariant");
1314 guarantee(!s->is_busy(), "invariant");
1315 s->set_owner(NULL); // redundant but good hygiene
1316 TEVENT(omFlush - Move one);
1317 }
1318 guarantee(tail != NULL && list != NULL, "invariant");
1319 }
1320
1321 ObjectMonitor * inUseList = Self->omInUseList;
1322 ObjectMonitor * inUseTail = NULL;
1323 int inUseTally = 0;
1324 if (inUseList != NULL) {
1325 Self->omInUseList = NULL;
1326 ObjectMonitor *cur_om;
1327 // The thread is going away, however the omInUseList inflated
1328 // monitors may still be in-use by other threads.
1329 // Link them to inUseTail, which will be linked into the global in-use list
1330 // gOmInUseList below, under the gListLock
1331 for (cur_om = inUseList; cur_om != NULL; cur_om = cur_om->FreeNext) {
1332 inUseTail = cur_om;
1333 inUseTally++;
1334 }
1335 assert(Self->omInUseCount == inUseTally, "in-use count off");
1336 Self->omInUseCount = 0;
1337 guarantee(inUseTail != NULL && inUseList != NULL, "invariant");
1338 }
1339
1340 Thread::muxAcquire(&gListLock, "omFlush");
1341 if (tail != NULL) {
1342 tail->FreeNext = gFreeList;
1343 gFreeList = list;
1344 gMonitorFreeCount += tally;
1345 assert(Self->omFreeCount == tally, "free-count off");
1346 Self->omFreeCount = 0;
1347 }
1348
1349 if (inUseTail != NULL) {
1350 inUseTail->FreeNext = gOmInUseList;
1351 gOmInUseList = inUseList;
1352 gOmInUseCount += inUseTally;
1353 }
1354
1355 Thread::muxRelease(&gListLock);
1356 TEVENT(omFlush);
1357 }
1358
1359 // Fast path code shared by multiple functions
1360 ObjectMonitor* ObjectSynchronizer::inflate_helper(oop obj) {
1361 markOop mark = obj->mark();
1362 if (mark->has_monitor()) {
1363 assert(ObjectSynchronizer::verify_objmon_isinpool(mark->monitor()), "monitor is invalid");
1364 assert(mark->monitor()->header()->is_neutral(), "monitor must record a good object header");
1365 return mark->monitor();
1366 }
1367 return ObjectSynchronizer::inflate(Thread::current(),
1368 obj,
1369 inflate_cause_vm_internal);
1370 }
1371
1372 ObjectMonitor* ObjectSynchronizer::inflate(Thread * Self,
1373 oop object,
1374 const InflateCause cause) {
1375
1376 // Inflate mutates the heap ...
1377 // Relaxing assertion for bug 6320749.
1378 assert(Universe::verify_in_progress() ||
1379 !SafepointSynchronize::is_at_safepoint(), "invariant");
1380
1381 EventJavaMonitorInflate event;
1382
1383 for (;;) {
1384 const markOop mark = object->mark();
1385 assert(!mark->has_bias_pattern(), "invariant");
1386
1387 // The mark can be in one of the following states:
1388 // * Inflated - just return
1389 // * Stack-locked - coerce it to inflated
1390 // * INFLATING - busy wait for conversion to complete
1391 // * Neutral - aggressively inflate the object.
1392 // * BIASED - Illegal. We should never see this
1393
1394 // CASE: inflated
1395 if (mark->has_monitor()) {
1396 ObjectMonitor * inf = mark->monitor();
1397 assert(inf->header()->is_neutral(), "invariant");
1398 assert(inf->object() == object, "invariant");
1399 assert(ObjectSynchronizer::verify_objmon_isinpool(inf), "monitor is invalid");
1400 event.cancel(); // let's not post an inflation event, unless we did the deed ourselves
1401 return inf;
1402 }
1403
1404 // CASE: inflation in progress - inflating over a stack-lock.
1405 // Some other thread is converting from stack-locked to inflated.
1406 // Only that thread can complete inflation -- other threads must wait.
1407 // The INFLATING value is transient.
1408 // Currently, we spin/yield/park and poll the markword, waiting for inflation to finish.
1409 // We could always eliminate polling by parking the thread on some auxiliary list.
1410 if (mark == markOopDesc::INFLATING()) {
1411 TEVENT(Inflate: spin while INFLATING);
1412 ReadStableMark(object);
1413 continue;
1414 }
1415
1416 // CASE: stack-locked
1417 // Could be stack-locked either by this thread or by some other thread.
1418 //
1419 // Note that we allocate the objectmonitor speculatively, _before_ attempting
1420 // to install INFLATING into the mark word. We originally installed INFLATING,
1421 // allocated the objectmonitor, and then finally STed the address of the
1422 // objectmonitor into the mark. This was correct, but artificially lengthened
1423 // the interval in which INFLATED appeared in the mark, thus increasing
1424 // the odds of inflation contention.
1425 //
1426 // We now use per-thread private objectmonitor free lists.
1427 // These list are reprovisioned from the global free list outside the
1428 // critical INFLATING...ST interval. A thread can transfer
1429 // multiple objectmonitors en-mass from the global free list to its local free list.
1430 // This reduces coherency traffic and lock contention on the global free list.
1431 // Using such local free lists, it doesn't matter if the omAlloc() call appears
1432 // before or after the CAS(INFLATING) operation.
1433 // See the comments in omAlloc().
1434
1435 if (mark->has_locker()) {
1436 ObjectMonitor * m = omAlloc(Self);
1437 // Optimistically prepare the objectmonitor - anticipate successful CAS
1438 // We do this before the CAS in order to minimize the length of time
1439 // in which INFLATING appears in the mark.
1440 m->Recycle();
1441 m->_Responsible = NULL;
1442 m->_recursions = 0;
1443 m->_SpinDuration = ObjectMonitor::Knob_SpinLimit; // Consider: maintain by type/class
1444
1445 markOop cmp = (markOop) Atomic::cmpxchg_ptr(markOopDesc::INFLATING(), object->mark_addr(), mark);
1446 if (cmp != mark) {
1447 omRelease(Self, m, true);
1448 continue; // Interference -- just retry
1449 }
1450
1451 // We've successfully installed INFLATING (0) into the mark-word.
1452 // This is the only case where 0 will appear in a mark-word.
1453 // Only the singular thread that successfully swings the mark-word
1454 // to 0 can perform (or more precisely, complete) inflation.
1455 //
1456 // Why do we CAS a 0 into the mark-word instead of just CASing the
1457 // mark-word from the stack-locked value directly to the new inflated state?
1458 // Consider what happens when a thread unlocks a stack-locked object.
1459 // It attempts to use CAS to swing the displaced header value from the
1460 // on-stack basiclock back into the object header. Recall also that the
1461 // header value (hashcode, etc) can reside in (a) the object header, or
1462 // (b) a displaced header associated with the stack-lock, or (c) a displaced
1463 // header in an objectMonitor. The inflate() routine must copy the header
1464 // value from the basiclock on the owner's stack to the objectMonitor, all
1465 // the while preserving the hashCode stability invariants. If the owner
1466 // decides to release the lock while the value is 0, the unlock will fail
1467 // and control will eventually pass from slow_exit() to inflate. The owner
1468 // will then spin, waiting for the 0 value to disappear. Put another way,
1469 // the 0 causes the owner to stall if the owner happens to try to
1470 // drop the lock (restoring the header from the basiclock to the object)
1471 // while inflation is in-progress. This protocol avoids races that might
1472 // would otherwise permit hashCode values to change or "flicker" for an object.
1473 // Critically, while object->mark is 0 mark->displaced_mark_helper() is stable.
1474 // 0 serves as a "BUSY" inflate-in-progress indicator.
1475
1476
1477 // fetch the displaced mark from the owner's stack.
1478 // The owner can't die or unwind past the lock while our INFLATING
1479 // object is in the mark. Furthermore the owner can't complete
1480 // an unlock on the object, either.
1481 markOop dmw = mark->displaced_mark_helper();
1482 assert(dmw->is_neutral(), "invariant");
1483
1484 // Setup monitor fields to proper values -- prepare the monitor
1485 m->set_header(dmw);
1486
1487 // Optimization: if the mark->locker stack address is associated
1488 // with this thread we could simply set m->_owner = Self.
1489 // Note that a thread can inflate an object
1490 // that it has stack-locked -- as might happen in wait() -- directly
1491 // with CAS. That is, we can avoid the xchg-NULL .... ST idiom.
1492 m->set_owner(mark->locker());
1493 m->set_object(object);
1494 // TODO-FIXME: assert BasicLock->dhw != 0.
1495
1496 // Must preserve store ordering. The monitor state must
1497 // be stable at the time of publishing the monitor address.
1498 guarantee(object->mark() == markOopDesc::INFLATING(), "invariant");
1499 object->release_set_mark(markOopDesc::encode(m));
1500
1501 // Hopefully the performance counters are allocated on distinct cache lines
1502 // to avoid false sharing on MP systems ...
1503 OM_PERFDATA_OP(Inflations, inc());
1504 TEVENT(Inflate: overwrite stacklock);
1505 if (log_is_enabled(Debug, monitorinflation)) {
1506 if (object->is_instance()) {
1507 ResourceMark rm;
1508 log_debug(monitorinflation)("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1509 p2i(object), p2i(object->mark()),
1510 object->klass()->external_name());
1511 }
1512 }
1513 if (event.should_commit()) {
1514 post_monitor_inflate_event(event, object, cause);
1515 }
1516 return m;
1517 }
1518
1519 // CASE: neutral
1520 // TODO-FIXME: for entry we currently inflate and then try to CAS _owner.
1521 // If we know we're inflating for entry it's better to inflate by swinging a
1522 // pre-locked objectMonitor pointer into the object header. A successful
1523 // CAS inflates the object *and* confers ownership to the inflating thread.
1524 // In the current implementation we use a 2-step mechanism where we CAS()
1525 // to inflate and then CAS() again to try to swing _owner from NULL to Self.
1526 // An inflateTry() method that we could call from fast_enter() and slow_enter()
1527 // would be useful.
1528
1529 assert(mark->is_neutral(), "invariant");
1530 ObjectMonitor * m = omAlloc(Self);
1531 // prepare m for installation - set monitor to initial state
1532 m->Recycle();
1533 m->set_header(mark);
1534 m->set_owner(NULL);
1535 m->set_object(object);
1536 m->_recursions = 0;
1537 m->_Responsible = NULL;
1538 m->_SpinDuration = ObjectMonitor::Knob_SpinLimit; // consider: keep metastats by type/class
1539
1540 if (Atomic::cmpxchg_ptr (markOopDesc::encode(m), object->mark_addr(), mark) != mark) {
1541 m->set_object(NULL);
1542 m->set_owner(NULL);
1543 m->Recycle();
1544 omRelease(Self, m, true);
1545 m = NULL;
1546 continue;
1547 // interference - the markword changed - just retry.
1548 // The state-transitions are one-way, so there's no chance of
1549 // live-lock -- "Inflated" is an absorbing state.
1550 }
1551
1552 // Hopefully the performance counters are allocated on distinct
1553 // cache lines to avoid false sharing on MP systems ...
1554 OM_PERFDATA_OP(Inflations, inc());
1555 TEVENT(Inflate: overwrite neutral);
1556 if (log_is_enabled(Debug, monitorinflation)) {
1557 if (object->is_instance()) {
1558 ResourceMark rm;
1559 log_debug(monitorinflation)("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1560 p2i(object), p2i(object->mark()),
1561 object->klass()->external_name());
1562 }
1563 }
1564 if (event.should_commit()) {
1565 post_monitor_inflate_event(event, object, cause);
1566 }
1567 return m;
1568 }
1569 }
1570
1571
1572 // Deflate_idle_monitors() is called at all safepoints, immediately
1573 // after all mutators are stopped, but before any objects have moved.
1574 // It traverses the list of known monitors, deflating where possible.
1575 // The scavenged monitor are returned to the monitor free list.
1576 //
1577 // Beware that we scavenge at *every* stop-the-world point.
1578 // Having a large number of monitors in-circulation negatively
1579 // impacts the performance of some applications (e.g., PointBase).
1580 // Broadly, we want to minimize the # of monitors in circulation.
1581 //
1582 // We have added a flag, MonitorInUseLists, which creates a list
1583 // of active monitors for each thread. deflate_idle_monitors()
1584 // only scans the per-thread in-use lists. omAlloc() puts all
1585 // assigned monitors on the per-thread list. deflate_idle_monitors()
1586 // returns the non-busy monitors to the global free list.
1587 // When a thread dies, omFlush() adds the list of active monitors for
1588 // that thread to a global gOmInUseList acquiring the
1589 // global list lock. deflate_idle_monitors() acquires the global
1590 // list lock to scan for non-busy monitors to the global free list.
1591 // An alternative could have used a single global in-use list. The
1592 // downside would have been the additional cost of acquiring the global list lock
1593 // for every omAlloc().
1594 //
1595 // Perversely, the heap size -- and thus the STW safepoint rate --
1596 // typically drives the scavenge rate. Large heaps can mean infrequent GC,
1597 // which in turn can mean large(r) numbers of objectmonitors in circulation.
1598 // This is an unfortunate aspect of this design.
1599
1600 enum ManifestConstants {
1601 ClearResponsibleAtSTW = 0
1602 };
1603
1604 // Deflate a single monitor if not in-use
1605 // Return true if deflated, false if in-use
1606 bool ObjectSynchronizer::deflate_monitor(ObjectMonitor* mid, oop obj,
1607 ObjectMonitor** freeHeadp,
1608 ObjectMonitor** freeTailp) {
1609 bool deflated;
1610 // Normal case ... The monitor is associated with obj.
1611 guarantee(obj->mark() == markOopDesc::encode(mid), "invariant");
1612 guarantee(mid == obj->mark()->monitor(), "invariant");
1613 guarantee(mid->header()->is_neutral(), "invariant");
1614
1615 if (mid->is_busy()) {
1616 if (ClearResponsibleAtSTW) mid->_Responsible = NULL;
1617 deflated = false;
1618 } else {
1619 // Deflate the monitor if it is no longer being used
1620 // It's idle - scavenge and return to the global free list
1621 // plain old deflation ...
1622 TEVENT(deflate_idle_monitors - scavenge1);
1623 if (log_is_enabled(Debug, monitorinflation)) {
1624 if (obj->is_instance()) {
1625 ResourceMark rm;
1626 log_debug(monitorinflation)("Deflating object " INTPTR_FORMAT " , "
1627 "mark " INTPTR_FORMAT " , type %s",
1628 p2i(obj), p2i(obj->mark()),
1629 obj->klass()->external_name());
1630 }
1631 }
1632
1633 // Restore the header back to obj
1634 obj->release_set_mark(mid->header());
1635 mid->clear();
1636
1637 assert(mid->object() == NULL, "invariant");
1638
1639 // Move the object to the working free list defined by freeHeadp, freeTailp
1640 if (*freeHeadp == NULL) *freeHeadp = mid;
1641 if (*freeTailp != NULL) {
1642 ObjectMonitor * prevtail = *freeTailp;
1643 assert(prevtail->FreeNext == NULL, "cleaned up deflated?");
1644 prevtail->FreeNext = mid;
1645 }
1646 *freeTailp = mid;
1647 deflated = true;
1648 }
1649 return deflated;
1650 }
1651
1652 // Walk a given monitor list, and deflate idle monitors
1653 // The given list could be a per-thread list or a global list
1654 // Caller acquires gListLock
1655 int ObjectSynchronizer::deflate_monitor_list(ObjectMonitor** listHeadp,
1656 ObjectMonitor** freeHeadp,
1657 ObjectMonitor** freeTailp) {
1658 ObjectMonitor* mid;
1659 ObjectMonitor* next;
1660 ObjectMonitor* cur_mid_in_use = NULL;
1661 int deflated_count = 0;
1662
1663 for (mid = *listHeadp; mid != NULL;) {
1664 oop obj = (oop) mid->object();
1665 if (obj != NULL && deflate_monitor(mid, obj, freeHeadp, freeTailp)) {
1666 // if deflate_monitor succeeded,
1667 // extract from per-thread in-use list
1668 if (mid == *listHeadp) {
1669 *listHeadp = mid->FreeNext;
1670 } else if (cur_mid_in_use != NULL) {
1671 cur_mid_in_use->FreeNext = mid->FreeNext; // maintain the current thread in-use list
1672 }
1673 next = mid->FreeNext;
1674 mid->FreeNext = NULL; // This mid is current tail in the freeHeadp list
1675 mid = next;
1676 deflated_count++;
1677 } else {
1678 cur_mid_in_use = mid;
1679 mid = mid->FreeNext;
1680 }
1681 }
1682 return deflated_count;
1683 }
1684
1685 void ObjectSynchronizer::deflate_idle_monitors() {
1686 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1687 int nInuse = 0; // currently associated with objects
1688 int nInCirculation = 0; // extant
1689 int nScavenged = 0; // reclaimed
1690 bool deflated = false;
1691
1692 ObjectMonitor * freeHeadp = NULL; // Local SLL of scavenged monitors
1693 ObjectMonitor * freeTailp = NULL;
1694
1695 TEVENT(deflate_idle_monitors);
1696 // Prevent omFlush from changing mids in Thread dtor's during deflation
1697 // And in case the vm thread is acquiring a lock during a safepoint
1698 // See e.g. 6320749
1699 Thread::muxAcquire(&gListLock, "scavenge - return");
1700
1701 if (MonitorInUseLists) {
1702 int inUse = 0;
1703 for (JavaThread* cur = Threads::first(); cur != NULL; cur = cur->next()) {
1704 nInCirculation+= cur->omInUseCount;
1705 int deflated_count = deflate_monitor_list(cur->omInUseList_addr(), &freeHeadp, &freeTailp);
1706 cur->omInUseCount-= deflated_count;
1707 if (ObjectMonitor::Knob_VerifyInUse) {
1708 verifyInUse(cur);
1709 }
1710 nScavenged += deflated_count;
1711 nInuse += cur->omInUseCount;
1712 }
1713
1714 // For moribund threads, scan gOmInUseList
1715 if (gOmInUseList) {
1716 nInCirculation += gOmInUseCount;
1717 int deflated_count = deflate_monitor_list((ObjectMonitor **)&gOmInUseList, &freeHeadp, &freeTailp);
1718 gOmInUseCount-= deflated_count;
1719 nScavenged += deflated_count;
1720 nInuse += gOmInUseCount;
1721 }
1722
1723 } else {
1724 PaddedEnd<ObjectMonitor> * block =
1725 (PaddedEnd<ObjectMonitor> *)OrderAccess::load_ptr_acquire(&gBlockList);
1726 for (; block != NULL; block = (PaddedEnd<ObjectMonitor> *)next(block)) {
1727 // Iterate over all extant monitors - Scavenge all idle monitors.
1728 assert(block->object() == CHAINMARKER, "must be a block header");
1729 nInCirculation += _BLOCKSIZE;
1730 for (int i = 1; i < _BLOCKSIZE; i++) {
1731 ObjectMonitor* mid = (ObjectMonitor*)&block[i];
1732 oop obj = (oop)mid->object();
1733
1734 if (obj == NULL) {
1735 // The monitor is not associated with an object.
1736 // The monitor should either be a thread-specific private
1737 // free list or the global free list.
1738 // obj == NULL IMPLIES mid->is_busy() == 0
1739 guarantee(!mid->is_busy(), "invariant");
1740 continue;
1741 }
1742 deflated = deflate_monitor(mid, obj, &freeHeadp, &freeTailp);
1743
1744 if (deflated) {
1745 mid->FreeNext = NULL;
1746 nScavenged++;
1747 } else {
1748 nInuse++;
1749 }
1750 }
1751 }
1752 }
1753
1754 gMonitorFreeCount += nScavenged;
1755
1756 // Consider: audit gFreeList to ensure that gMonitorFreeCount and list agree.
1757
1758 if (ObjectMonitor::Knob_Verbose) {
1759 tty->print_cr("INFO: Deflate: InCirc=%d InUse=%d Scavenged=%d "
1760 "ForceMonitorScavenge=%d : pop=%d free=%d",
1761 nInCirculation, nInuse, nScavenged, ForceMonitorScavenge,
1762 gMonitorPopulation, gMonitorFreeCount);
1763 tty->flush();
1764 }
1765
1766 ForceMonitorScavenge = 0; // Reset
1767
1768 // Move the scavenged monitors back to the global free list.
1769 if (freeHeadp != NULL) {
1770 guarantee(freeTailp != NULL && nScavenged > 0, "invariant");
1771 assert(freeTailp->FreeNext == NULL, "invariant");
1772 // constant-time list splice - prepend scavenged segment to gFreeList
1773 freeTailp->FreeNext = gFreeList;
1774 gFreeList = freeHeadp;
1775 }
1776 Thread::muxRelease(&gListLock);
1777
1778 OM_PERFDATA_OP(Deflations, inc(nScavenged));
1779 OM_PERFDATA_OP(MonExtant, set_value(nInCirculation));
1780
1781 // TODO: Add objectMonitor leak detection.
1782 // Audit/inventory the objectMonitors -- make sure they're all accounted for.
1783 GVars.stwRandom = os::random();
1784 GVars.stwCycle++;
1785 }
1786
1787 // Monitor cleanup on JavaThread::exit
1788
1789 // Iterate through monitor cache and attempt to release thread's monitors
1790 // Gives up on a particular monitor if an exception occurs, but continues
1791 // the overall iteration, swallowing the exception.
1792 class ReleaseJavaMonitorsClosure: public MonitorClosure {
1793 private:
1794 TRAPS;
1795
1796 public:
1797 ReleaseJavaMonitorsClosure(Thread* thread) : THREAD(thread) {}
1798 void do_monitor(ObjectMonitor* mid) {
1799 if (mid->owner() == THREAD) {
1800 if (ObjectMonitor::Knob_VerifyMatch != 0) {
1801 ResourceMark rm;
1802 Handle obj(THREAD, (oop) mid->object());
1803 tty->print("INFO: unexpected locked object:");
1804 javaVFrame::print_locked_object_class_name(tty, obj, "locked");
1805 fatal("exiting JavaThread=" INTPTR_FORMAT
1806 " unexpectedly owns ObjectMonitor=" INTPTR_FORMAT,
1807 p2i(THREAD), p2i(mid));
1808 }
1809 (void)mid->complete_exit(CHECK);
1810 }
1811 }
1812 };
1813
1814 // Release all inflated monitors owned by THREAD. Lightweight monitors are
1815 // ignored. This is meant to be called during JNI thread detach which assumes
1816 // all remaining monitors are heavyweight. All exceptions are swallowed.
1817 // Scanning the extant monitor list can be time consuming.
1818 // A simple optimization is to add a per-thread flag that indicates a thread
1819 // called jni_monitorenter() during its lifetime.
1820 //
1821 // Instead of No_Savepoint_Verifier it might be cheaper to
1822 // use an idiom of the form:
1823 // auto int tmp = SafepointSynchronize::_safepoint_counter ;
1824 // <code that must not run at safepoint>
1825 // guarantee (((tmp ^ _safepoint_counter) | (tmp & 1)) == 0) ;
1826 // Since the tests are extremely cheap we could leave them enabled
1827 // for normal product builds.
1828
1829 void ObjectSynchronizer::release_monitors_owned_by_thread(TRAPS) {
1830 assert(THREAD == JavaThread::current(), "must be current Java thread");
1831 NoSafepointVerifier nsv;
1832 ReleaseJavaMonitorsClosure rjmc(THREAD);
1833 Thread::muxAcquire(&gListLock, "release_monitors_owned_by_thread");
1834 ObjectSynchronizer::monitors_iterate(&rjmc);
1835 Thread::muxRelease(&gListLock);
1836 THREAD->clear_pending_exception();
1837 }
1838
1839 const char* ObjectSynchronizer::inflate_cause_name(const InflateCause cause) {
1840 switch (cause) {
1841 case inflate_cause_vm_internal: return "VM Internal";
1842 case inflate_cause_monitor_enter: return "Monitor Enter";
1843 case inflate_cause_wait: return "Monitor Wait";
1844 case inflate_cause_notify: return "Monitor Notify";
1845 case inflate_cause_hash_code: return "Monitor Hash Code";
1846 case inflate_cause_jni_enter: return "JNI Monitor Enter";
1847 case inflate_cause_jni_exit: return "JNI Monitor Exit";
1848 default:
1849 ShouldNotReachHere();
1850 }
1851 return "Unknown";
1852 }
1853
1854 static void post_monitor_inflate_event(EventJavaMonitorInflate& event,
1855 const oop obj,
1856 const ObjectSynchronizer::InflateCause cause) {
1857 #if INCLUDE_TRACE
1858 assert(event.should_commit(), "check outside");
1859 event.set_monitorClass(obj->klass());
1860 event.set_address((TYPE_ADDRESS)(uintptr_t)(void*)obj);
1861 event.set_cause((u1)cause);
1862 event.commit();
1863 #endif
1864 }
1865
1866 //------------------------------------------------------------------------------
1867 // Debugging code
1868
1869 void ObjectSynchronizer::sanity_checks(const bool verbose,
1870 const uint cache_line_size,
1871 int *error_cnt_ptr,
1872 int *warning_cnt_ptr) {
1873 u_char *addr_begin = (u_char*)&GVars;
1874 u_char *addr_stwRandom = (u_char*)&GVars.stwRandom;
1875 u_char *addr_hcSequence = (u_char*)&GVars.hcSequence;
1876
1877 if (verbose) {
1878 tty->print_cr("INFO: sizeof(SharedGlobals)=" SIZE_FORMAT,
1879 sizeof(SharedGlobals));
1880 }
1881
1882 uint offset_stwRandom = (uint)(addr_stwRandom - addr_begin);
1883 if (verbose) tty->print_cr("INFO: offset(stwRandom)=%u", offset_stwRandom);
1884
1885 uint offset_hcSequence = (uint)(addr_hcSequence - addr_begin);
1886 if (verbose) {
1887 tty->print_cr("INFO: offset(_hcSequence)=%u", offset_hcSequence);
1888 }
1889
1890 if (cache_line_size != 0) {
1891 // We were able to determine the L1 data cache line size so
1892 // do some cache line specific sanity checks
1893
1894 if (offset_stwRandom < cache_line_size) {
1895 tty->print_cr("WARNING: the SharedGlobals.stwRandom field is closer "
1896 "to the struct beginning than a cache line which permits "
1897 "false sharing.");
1898 (*warning_cnt_ptr)++;
1899 }
1900
1901 if ((offset_hcSequence - offset_stwRandom) < cache_line_size) {
1902 tty->print_cr("WARNING: the SharedGlobals.stwRandom and "
1903 "SharedGlobals.hcSequence fields are closer than a cache "
1904 "line which permits false sharing.");
1905 (*warning_cnt_ptr)++;
1906 }
1907
1908 if ((sizeof(SharedGlobals) - offset_hcSequence) < cache_line_size) {
1909 tty->print_cr("WARNING: the SharedGlobals.hcSequence field is closer "
1910 "to the struct end than a cache line which permits false "
1911 "sharing.");
1912 (*warning_cnt_ptr)++;
1913 }
1914 }
1915 }
1916
1917 #ifndef PRODUCT
1918
1919 // Check if monitor belongs to the monitor cache
1920 // The list is grow-only so it's *relatively* safe to traverse
1921 // the list of extant blocks without taking a lock.
1922
1923 int ObjectSynchronizer::verify_objmon_isinpool(ObjectMonitor *monitor) {
1924 PaddedEnd<ObjectMonitor> * block =
1925 (PaddedEnd<ObjectMonitor> *)OrderAccess::load_ptr_acquire(&gBlockList);
1926 while (block != NULL) {
1927 assert(block->object() == CHAINMARKER, "must be a block header");
1928 if (monitor > (ObjectMonitor *)&block[0] &&
1929 monitor < (ObjectMonitor *)&block[_BLOCKSIZE]) {
1930 address mon = (address)monitor;
1931 address blk = (address)block;
1932 size_t diff = mon - blk;
1933 assert((diff % sizeof(PaddedEnd<ObjectMonitor>)) == 0, "must be aligned");
1934 return 1;
1935 }
1936 block = (PaddedEnd<ObjectMonitor> *)block->FreeNext;
1937 }
1938 return 0;
1939 }
1940
1941 #endif