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 "utilities/dtrace.hpp"
46 #include "utilities/events.hpp"
47 #include "utilities/macros.hpp"
48 #include "utilities/preserveException.hpp"
49 #if INCLUDE_TRACE
50 #include "trace/tracing.hpp"
51 #endif
52
53 // The "core" versions of monitor enter and exit reside in this file.
54 // The interpreter and compilers contain specialized transliterated
55 // variants of the enter-exit fast-path operations. See i486.ad fast_lock(),
56 // for instance. If you make changes here, make sure to modify the
57 // interpreter, and both C1 and C2 fast-path inline locking code emission.
58 //
59 // -----------------------------------------------------------------------------
60
61 #ifdef DTRACE_ENABLED
62
63 // Only bother with this argument setup if dtrace is available
64 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
65
66 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
67 char* bytes = NULL; \
68 int len = 0; \
69 jlong jtid = SharedRuntime::get_java_tid(thread); \
70 Symbol* klassname = ((oop)(obj))->klass()->name(); \
71 if (klassname != NULL) { \
72 bytes = (char*)klassname->bytes(); \
73 len = klassname->utf8_length(); \
74 }
75
76 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
77 { \
78 if (DTraceMonitorProbes) { \
79 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
80 HOTSPOT_MONITOR_WAIT(jtid, \
81 (uintptr_t)(monitor), bytes, len, (millis)); \
82 } \
83 }
84
85 #define HOTSPOT_MONITOR_PROBE_notify HOTSPOT_MONITOR_NOTIFY
86 #define HOTSPOT_MONITOR_PROBE_notifyAll HOTSPOT_MONITOR_NOTIFYALL
87 #define HOTSPOT_MONITOR_PROBE_waited HOTSPOT_MONITOR_WAITED
88
89 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
90 { \
91 if (DTraceMonitorProbes) { \
92 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
93 HOTSPOT_MONITOR_PROBE_##probe(jtid, /* probe = waited */ \
94 (uintptr_t)(monitor), bytes, len); \
95 } \
96 }
97
98 #else // ndef DTRACE_ENABLED
99
100 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
101 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
102
103 #endif // ndef DTRACE_ENABLED
104
105 // This exists only as a workaround of dtrace bug 6254741
106 int dtrace_waited_probe(ObjectMonitor* monitor, Handle obj, Thread* thr) {
107 DTRACE_MONITOR_PROBE(waited, monitor, obj(), thr);
108 return 0;
109 }
110
111 #define NINFLATIONLOCKS 256
112 static volatile intptr_t gInflationLocks[NINFLATIONLOCKS];
113
114 // global list of blocks of monitors
115 // gBlockList is really PaddedEnd<ObjectMonitor> *, but we don't
116 // want to expose the PaddedEnd template more than necessary.
117 ObjectMonitor * volatile ObjectSynchronizer::gBlockList = NULL;
118 // global monitor free list
119 ObjectMonitor * volatile ObjectSynchronizer::gFreeList = NULL;
120 // global monitor in-use list, for moribund threads,
121 // monitors they inflated need to be scanned for deflation
122 ObjectMonitor * volatile ObjectSynchronizer::gOmInUseList = NULL;
123 // count of entries in gOmInUseList
124 int ObjectSynchronizer::gOmInUseCount = 0;
125
126 static volatile intptr_t gListLock = 0; // protects global monitor lists
127 static volatile int gMonitorFreeCount = 0; // # on gFreeList
128 static volatile int gMonitorPopulation = 0; // # Extant -- in circulation
129
130 #define CHAINMARKER (cast_to_oop<intptr_t>(-1))
131
132
133 // =====================> Quick functions
134
135 // The quick_* forms are special fast-path variants used to improve
136 // performance. In the simplest case, a "quick_*" implementation could
137 // simply return false, in which case the caller will perform the necessary
138 // state transitions and call the slow-path form.
139 // The fast-path is designed to handle frequently arising cases in an efficient
140 // manner and is just a degenerate "optimistic" variant of the slow-path.
141 // returns true -- to indicate the call was satisfied.
142 // returns false -- to indicate the call needs the services of the slow-path.
143 // A no-loitering ordinance is in effect for code in the quick_* family
144 // operators: safepoints or indefinite blocking (blocking that might span a
145 // safepoint) are forbidden. Generally the thread_state() is _in_Java upon
146 // entry.
147 //
148 // Consider: An interesting optimization is to have the JIT recognize the
149 // following common idiom:
150 // synchronized (someobj) { .... ; notify(); }
151 // That is, we find a notify() or notifyAll() call that immediately precedes
152 // the monitorexit operation. In that case the JIT could fuse the operations
153 // into a single notifyAndExit() runtime primitive.
154
155 bool ObjectSynchronizer::quick_notify(oopDesc * obj, Thread * self, bool all) {
156 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
157 assert(self->is_Java_thread(), "invariant");
158 assert(((JavaThread *) self)->thread_state() == _thread_in_Java, "invariant");
159 NoSafepointVerifier nsv;
160 if (obj == NULL) return false; // slow-path for invalid obj
161 const markOop mark = obj->mark();
162
163 if (mark->has_locker() && self->is_lock_owned((address)mark->locker())) {
164 // Degenerate notify
165 // stack-locked by caller so by definition the implied waitset is empty.
166 return true;
167 }
168
169 if (mark->has_monitor()) {
170 ObjectMonitor * const mon = mark->monitor();
171 assert(mon->object() == obj, "invariant");
172 if (mon->owner() != self) return false; // slow-path for IMS exception
173
174 if (mon->first_waiter() != NULL) {
175 // We have one or more waiters. Since this is an inflated monitor
176 // that we own, we can transfer one or more threads from the waitset
177 // to the entrylist here and now, avoiding the slow-path.
178 if (all) {
179 DTRACE_MONITOR_PROBE(notifyAll, mon, obj, self);
180 } else {
181 DTRACE_MONITOR_PROBE(notify, mon, obj, self);
182 }
183 int tally = 0;
184 do {
185 mon->INotify(self);
186 ++tally;
187 } while (mon->first_waiter() != NULL && all);
188 OM_PERFDATA_OP(Notifications, inc(tally));
189 }
190 return true;
191 }
192
193 // biased locking and any other IMS exception states take the slow-path
194 return false;
195 }
196
197
198 // The LockNode emitted directly at the synchronization site would have
199 // been too big if it were to have included support for the cases of inflated
200 // recursive enter and exit, so they go here instead.
201 // Note that we can't safely call AsyncPrintJavaStack() from within
202 // quick_enter() as our thread state remains _in_Java.
203
204 bool ObjectSynchronizer::quick_enter(oop obj, Thread * Self,
205 BasicLock * lock) {
206 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
207 assert(Self->is_Java_thread(), "invariant");
208 assert(((JavaThread *) Self)->thread_state() == _thread_in_Java, "invariant");
209 NoSafepointVerifier nsv;
210 if (obj == NULL) return false; // Need to throw NPE
211 const markOop mark = obj->mark();
212
213 if (mark->has_monitor()) {
214 ObjectMonitor * const m = mark->monitor();
215 assert(m->object() == obj, "invariant");
216 Thread * const owner = (Thread *) m->_owner;
217
218 // Lock contention and Transactional Lock Elision (TLE) diagnostics
219 // and observability
220 // Case: light contention possibly amenable to TLE
221 // Case: TLE inimical operations such as nested/recursive synchronization
222
223 if (owner == Self) {
224 m->_recursions++;
225 return true;
226 }
227
228 // This Java Monitor is inflated so obj's header will never be
229 // displaced to this thread's BasicLock. Make the displaced header
230 // non-NULL so this BasicLock is not seen as recursive nor as
231 // being locked. We do this unconditionally so that this thread's
232 // BasicLock cannot be mis-interpreted by any stack walkers. For
233 // performance reasons, stack walkers generally first check for
234 // Biased Locking in the object's header, the second check is for
235 // stack-locking in the object's header, the third check is for
236 // recursive stack-locking in the displaced header in the BasicLock,
237 // and last are the inflated Java Monitor (ObjectMonitor) checks.
238 lock->set_displaced_header(markOopDesc::unused_mark());
239
240 if (owner == NULL &&
241 Atomic::cmpxchg_ptr(Self, &(m->_owner), NULL) == NULL) {
242 assert(m->_recursions == 0, "invariant");
243 assert(m->_owner == Self, "invariant");
244 return true;
245 }
246 }
247
248 // Note that we could inflate in quick_enter.
249 // This is likely a useful optimization
250 // Critically, in quick_enter() we must not:
251 // -- perform bias revocation, or
252 // -- block indefinitely, or
253 // -- reach a safepoint
254
255 return false; // revert to slow-path
256 }
257
258 // -----------------------------------------------------------------------------
259 // Fast Monitor Enter/Exit
260 // This the fast monitor enter. The interpreter and compiler use
261 // some assembly copies of this code. Make sure update those code
262 // if the following function is changed. The implementation is
263 // extremely sensitive to race condition. Be careful.
264
265 void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock,
266 bool attempt_rebias, TRAPS) {
267 if (UseBiasedLocking) {
268 if (!SafepointSynchronize::is_at_safepoint()) {
269 BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD);
270 if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) {
271 return;
272 }
273 } else {
274 assert(!attempt_rebias, "can not rebias toward VM thread");
275 BiasedLocking::revoke_at_safepoint(obj);
276 }
277 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
278 }
279
280 slow_enter(obj, lock, THREAD);
281 }
282
283 void ObjectSynchronizer::fast_exit(oop object, BasicLock* lock, TRAPS) {
284 markOop mark = object->mark();
285 // We cannot check for Biased Locking if we are racing an inflation.
286 assert(mark == markOopDesc::INFLATING() ||
287 !mark->has_bias_pattern(), "should not see bias pattern here");
288
289 markOop dhw = lock->displaced_header();
290 if (dhw == NULL) {
291 // If the displaced header is NULL, then this exit matches up with
292 // a recursive enter. No real work to do here except for diagnostics.
293 #ifndef PRODUCT
294 if (mark != markOopDesc::INFLATING()) {
295 // Only do diagnostics if we are not racing an inflation. Simply
296 // exiting a recursive enter of a Java Monitor that is being
297 // inflated is safe; see the has_monitor() comment below.
298 assert(!mark->is_neutral(), "invariant");
299 assert(!mark->has_locker() ||
300 THREAD->is_lock_owned((address)mark->locker()), "invariant");
301 if (mark->has_monitor()) {
302 // The BasicLock's displaced_header is marked as a recursive
303 // enter and we have an inflated Java Monitor (ObjectMonitor).
304 // This is a special case where the Java Monitor was inflated
305 // after this thread entered the stack-lock recursively. When a
306 // Java Monitor is inflated, we cannot safely walk the Java
307 // Monitor owner's stack and update the BasicLocks because a
308 // Java Monitor can be asynchronously inflated by a thread that
309 // does not own the Java Monitor.
310 ObjectMonitor * m = mark->monitor();
311 assert(((oop)(m->object()))->mark() == mark, "invariant");
312 assert(m->is_entered(THREAD), "invariant");
313 }
314 }
315 #endif
316 return;
317 }
318
319 if (mark == (markOop) lock) {
320 // If the object is stack-locked by the current thread, try to
321 // swing the displaced header from the BasicLock back to the mark.
322 assert(dhw->is_neutral(), "invariant");
323 if ((markOop) Atomic::cmpxchg_ptr(dhw, object->mark_addr(), mark) == mark) {
324 TEVENT(fast_exit: release stack-lock);
325 return;
326 }
327 }
328
329 // We have to take the slow-path of possible inflation and then exit.
330 ObjectSynchronizer::inflate(THREAD,
331 object,
332 inflate_cause_vm_internal)->exit(true, THREAD);
333 }
334
335 // -----------------------------------------------------------------------------
336 // Interpreter/Compiler Slow Case
337 // This routine is used to handle interpreter/compiler slow case
338 // We don't need to use fast path here, because it must have been
339 // failed in the interpreter/compiler code.
340 void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) {
341 markOop mark = obj->mark();
342 assert(!mark->has_bias_pattern(), "should not see bias pattern here");
343
344 if (mark->is_neutral()) {
345 // Anticipate successful CAS -- the ST of the displaced mark must
346 // be visible <= the ST performed by the CAS.
347 lock->set_displaced_header(mark);
348 if (mark == (markOop) Atomic::cmpxchg_ptr(lock, obj()->mark_addr(), mark)) {
349 TEVENT(slow_enter: release stacklock);
350 return;
351 }
352 // Fall through to inflate() ...
353 } else if (mark->has_locker() &&
354 THREAD->is_lock_owned((address)mark->locker())) {
355 assert(lock != mark->locker(), "must not re-lock the same lock");
356 assert(lock != (BasicLock*)obj->mark(), "don't relock with same BasicLock");
357 lock->set_displaced_header(NULL);
358 return;
359 }
360
361 // The object header will never be displaced to this lock,
362 // so it does not matter what the value is, except that it
363 // must be non-zero to avoid looking like a re-entrant lock,
364 // and must not look locked either.
365 lock->set_displaced_header(markOopDesc::unused_mark());
366 ObjectSynchronizer::inflate(THREAD,
367 obj(),
368 inflate_cause_monitor_enter)->enter(THREAD);
369 }
370
371 // This routine is used to handle interpreter/compiler slow case
372 // We don't need to use fast path here, because it must have
373 // failed in the interpreter/compiler code. Simply use the heavy
374 // weight monitor should be ok, unless someone find otherwise.
375 void ObjectSynchronizer::slow_exit(oop object, BasicLock* lock, TRAPS) {
376 fast_exit(object, lock, THREAD);
377 }
378
379 // -----------------------------------------------------------------------------
380 // Class Loader support to workaround deadlocks on the class loader lock objects
381 // Also used by GC
382 // complete_exit()/reenter() are used to wait on a nested lock
383 // i.e. to give up an outer lock completely and then re-enter
384 // Used when holding nested locks - lock acquisition order: lock1 then lock2
385 // 1) complete_exit lock1 - saving recursion count
386 // 2) wait on lock2
387 // 3) when notified on lock2, unlock lock2
388 // 4) reenter lock1 with original recursion count
389 // 5) lock lock2
390 // NOTE: must use heavy weight monitor to handle complete_exit/reenter()
391 intptr_t ObjectSynchronizer::complete_exit(Handle obj, TRAPS) {
392 TEVENT(complete_exit);
393 if (UseBiasedLocking) {
394 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
395 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
396 }
397
398 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD,
399 obj(),
400 inflate_cause_vm_internal);
401
402 return monitor->complete_exit(THREAD);
403 }
404
405 // NOTE: must use heavy weight monitor to handle complete_exit/reenter()
406 void ObjectSynchronizer::reenter(Handle obj, intptr_t recursion, TRAPS) {
407 TEVENT(reenter);
408 if (UseBiasedLocking) {
409 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
410 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
411 }
412
413 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD,
414 obj(),
415 inflate_cause_vm_internal);
416
417 monitor->reenter(recursion, THREAD);
418 }
419 // -----------------------------------------------------------------------------
420 // JNI locks on java objects
421 // NOTE: must use heavy weight monitor to handle jni monitor enter
422 void ObjectSynchronizer::jni_enter(Handle obj, TRAPS) {
423 // the current locking is from JNI instead of Java code
424 TEVENT(jni_enter);
425 if (UseBiasedLocking) {
426 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
427 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
428 }
429 THREAD->set_current_pending_monitor_is_from_java(false);
430 ObjectSynchronizer::inflate(THREAD, obj(), inflate_cause_jni_enter)->enter(THREAD);
431 THREAD->set_current_pending_monitor_is_from_java(true);
432 }
433
434 // NOTE: must use heavy weight monitor to handle jni monitor exit
435 void ObjectSynchronizer::jni_exit(oop obj, Thread* THREAD) {
436 TEVENT(jni_exit);
437 if (UseBiasedLocking) {
438 Handle h_obj(THREAD, obj);
439 BiasedLocking::revoke_and_rebias(h_obj, false, THREAD);
440 obj = h_obj();
441 }
442 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
443
444 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD,
445 obj,
446 inflate_cause_jni_exit);
447 // If this thread has locked the object, exit the monitor. Note: can't use
448 // monitor->check(CHECK); must exit even if an exception is pending.
449 if (monitor->check(THREAD)) {
450 monitor->exit(true, THREAD);
451 }
452 }
453
454 // -----------------------------------------------------------------------------
455 // Internal VM locks on java objects
456 // standard constructor, allows locking failures
457 ObjectLocker::ObjectLocker(Handle obj, Thread* thread, bool doLock) {
458 _dolock = doLock;
459 _thread = thread;
460 debug_only(if (StrictSafepointChecks) _thread->check_for_valid_safepoint_state(false);)
461 _obj = obj;
462
463 if (_dolock) {
464 TEVENT(ObjectLocker);
465
466 ObjectSynchronizer::fast_enter(_obj, &_lock, false, _thread);
467 }
468 }
469
470 ObjectLocker::~ObjectLocker() {
471 if (_dolock) {
472 ObjectSynchronizer::fast_exit(_obj(), &_lock, _thread);
473 }
474 }
475
476
477 // -----------------------------------------------------------------------------
478 // Wait/Notify/NotifyAll
479 // NOTE: must use heavy weight monitor to handle wait()
480 int ObjectSynchronizer::wait(Handle obj, jlong millis, 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 if (millis < 0) {
486 TEVENT(wait - throw IAX);
487 THROW_MSG_0(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
488 }
489 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD,
490 obj(),
491 inflate_cause_wait);
492
493 DTRACE_MONITOR_WAIT_PROBE(monitor, obj(), THREAD, millis);
494 monitor->wait(millis, true, THREAD);
495
496 // This dummy call is in place to get around dtrace bug 6254741. Once
497 // that's fixed we can uncomment the following line, remove the call
498 // and change this function back into a "void" func.
499 // DTRACE_MONITOR_PROBE(waited, monitor, obj(), THREAD);
500 return dtrace_waited_probe(monitor, obj, THREAD);
501 }
502
503 void ObjectSynchronizer::waitUninterruptibly(Handle obj, jlong millis, TRAPS) {
504 if (UseBiasedLocking) {
505 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
506 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
507 }
508 if (millis < 0) {
509 TEVENT(wait - throw IAX);
510 THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
511 }
512 ObjectSynchronizer::inflate(THREAD,
513 obj(),
514 inflate_cause_wait)->wait(millis, false, THREAD);
515 }
516
517 void ObjectSynchronizer::notify(Handle obj, TRAPS) {
518 if (UseBiasedLocking) {
519 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
520 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
521 }
522
523 markOop mark = obj->mark();
524 if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
525 return;
526 }
527 ObjectSynchronizer::inflate(THREAD,
528 obj(),
529 inflate_cause_notify)->notify(THREAD);
530 }
531
532 // NOTE: see comment of notify()
533 void ObjectSynchronizer::notifyall(Handle obj, TRAPS) {
534 if (UseBiasedLocking) {
535 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
536 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
537 }
538
539 markOop mark = obj->mark();
540 if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
541 return;
542 }
543 ObjectSynchronizer::inflate(THREAD,
544 obj(),
545 inflate_cause_notify)->notifyAll(THREAD);
546 }
547
548 // -----------------------------------------------------------------------------
549 // Hash Code handling
550 //
551 // Performance concern:
552 // OrderAccess::storestore() calls release() which at one time stored 0
553 // into the global volatile OrderAccess::dummy variable. This store was
554 // unnecessary for correctness. Many threads storing into a common location
555 // causes considerable cache migration or "sloshing" on large SMP systems.
556 // As such, I avoided using OrderAccess::storestore(). In some cases
557 // OrderAccess::fence() -- which incurs local latency on the executing
558 // processor -- is a better choice as it scales on SMP systems.
559 //
560 // See http://blogs.oracle.com/dave/entry/biased_locking_in_hotspot for
561 // a discussion of coherency costs. Note that all our current reference
562 // platforms provide strong ST-ST order, so the issue is moot on IA32,
563 // x64, and SPARC.
564 //
565 // As a general policy we use "volatile" to control compiler-based reordering
566 // and explicit fences (barriers) to control for architectural reordering
567 // performed by the CPU(s) or platform.
568
569 struct SharedGlobals {
570 char _pad_prefix[DEFAULT_CACHE_LINE_SIZE];
571 // These are highly shared mostly-read variables.
572 // To avoid false-sharing they need to be the sole occupants of a cache line.
573 volatile int stwRandom;
574 volatile int stwCycle;
575 DEFINE_PAD_MINUS_SIZE(1, DEFAULT_CACHE_LINE_SIZE, sizeof(volatile int) * 2);
576 // Hot RW variable -- Sequester to avoid false-sharing
577 volatile int hcSequence;
578 DEFINE_PAD_MINUS_SIZE(2, DEFAULT_CACHE_LINE_SIZE, sizeof(volatile int));
579 };
580
581 static SharedGlobals GVars;
582 static int MonitorScavengeThreshold = 1000000;
583 static volatile int ForceMonitorScavenge = 0; // Scavenge required and pending
584
585 static markOop ReadStableMark(oop obj) {
586 markOop mark = obj->mark();
587 if (!mark->is_being_inflated()) {
588 return mark; // normal fast-path return
589 }
590
591 int its = 0;
592 for (;;) {
593 markOop mark = obj->mark();
594 if (!mark->is_being_inflated()) {
595 return mark; // normal fast-path return
596 }
597
598 // The object is being inflated by some other thread.
599 // The caller of ReadStableMark() must wait for inflation to complete.
600 // Avoid live-lock
601 // TODO: consider calling SafepointSynchronize::do_call_back() while
602 // spinning to see if there's a safepoint pending. If so, immediately
603 // yielding or blocking would be appropriate. Avoid spinning while
604 // there is a safepoint pending.
605 // TODO: add inflation contention performance counters.
606 // TODO: restrict the aggregate number of spinners.
607
608 ++its;
609 if (its > 10000 || !os::is_MP()) {
610 if (its & 1) {
611 os::naked_yield();
612 TEVENT(Inflate: INFLATING - yield);
613 } else {
614 // Note that the following code attenuates the livelock problem but is not
615 // a complete remedy. A more complete solution would require that the inflating
616 // thread hold the associated inflation lock. The following code simply restricts
617 // the number of spinners to at most one. We'll have N-2 threads blocked
618 // on the inflationlock, 1 thread holding the inflation lock and using
619 // a yield/park strategy, and 1 thread in the midst of inflation.
620 // A more refined approach would be to change the encoding of INFLATING
621 // to allow encapsulation of a native thread pointer. Threads waiting for
622 // inflation to complete would use CAS to push themselves onto a singly linked
623 // list rooted at the markword. Once enqueued, they'd loop, checking a per-thread flag
624 // and calling park(). When inflation was complete the thread that accomplished inflation
625 // would detach the list and set the markword to inflated with a single CAS and
626 // then for each thread on the list, set the flag and unpark() the thread.
627 // This is conceptually similar to muxAcquire-muxRelease, except that muxRelease
628 // wakes at most one thread whereas we need to wake the entire list.
629 int ix = (cast_from_oop<intptr_t>(obj) >> 5) & (NINFLATIONLOCKS-1);
630 int YieldThenBlock = 0;
631 assert(ix >= 0 && ix < NINFLATIONLOCKS, "invariant");
632 assert((NINFLATIONLOCKS & (NINFLATIONLOCKS-1)) == 0, "invariant");
633 Thread::muxAcquire(gInflationLocks + ix, "gInflationLock");
634 while (obj->mark() == markOopDesc::INFLATING()) {
635 // Beware: NakedYield() is advisory and has almost no effect on some platforms
636 // so we periodically call Self->_ParkEvent->park(1).
637 // We use a mixed spin/yield/block mechanism.
638 if ((YieldThenBlock++) >= 16) {
639 Thread::current()->_ParkEvent->park(1);
640 } else {
641 os::naked_yield();
642 }
643 }
644 Thread::muxRelease(gInflationLocks + ix);
645 TEVENT(Inflate: INFLATING - yield/park);
646 }
647 } else {
648 SpinPause(); // SMP-polite spinning
649 }
650 }
651 }
652
653 // hashCode() generation :
654 //
655 // Possibilities:
656 // * MD5Digest of {obj,stwRandom}
657 // * CRC32 of {obj,stwRandom} or any linear-feedback shift register function.
658 // * A DES- or AES-style SBox[] mechanism
659 // * One of the Phi-based schemes, such as:
660 // 2654435761 = 2^32 * Phi (golden ratio)
661 // HashCodeValue = ((uintptr_t(obj) >> 3) * 2654435761) ^ GVars.stwRandom ;
662 // * A variation of Marsaglia's shift-xor RNG scheme.
663 // * (obj ^ stwRandom) is appealing, but can result
664 // in undesirable regularity in the hashCode values of adjacent objects
665 // (objects allocated back-to-back, in particular). This could potentially
666 // result in hashtable collisions and reduced hashtable efficiency.
667 // There are simple ways to "diffuse" the middle address bits over the
668 // generated hashCode values:
669
670 static inline intptr_t get_next_hash(Thread * Self, oop obj) {
671 intptr_t value = 0;
672 if (hashCode == 0) {
673 // This form uses an unguarded global Park-Miller RNG,
674 // so it's possible for two threads to race and generate the same RNG.
675 // On MP system we'll have lots of RW access to a global, so the
676 // mechanism induces lots of coherency traffic.
677 value = os::random();
678 } else if (hashCode == 1) {
679 // This variation has the property of being stable (idempotent)
680 // between STW operations. This can be useful in some of the 1-0
681 // synchronization schemes.
682 intptr_t addrBits = cast_from_oop<intptr_t>(obj) >> 3;
683 value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom;
684 } else if (hashCode == 2) {
685 value = 1; // for sensitivity testing
686 } else if (hashCode == 3) {
687 value = ++GVars.hcSequence;
688 } else if (hashCode == 4) {
689 value = cast_from_oop<intptr_t>(obj);
690 } else {
691 // Marsaglia's xor-shift scheme with thread-specific state
692 // This is probably the best overall implementation -- we'll
693 // likely make this the default in future releases.
694 unsigned t = Self->_hashStateX;
695 t ^= (t << 11);
696 Self->_hashStateX = Self->_hashStateY;
697 Self->_hashStateY = Self->_hashStateZ;
698 Self->_hashStateZ = Self->_hashStateW;
699 unsigned v = Self->_hashStateW;
700 v = (v ^ (v >> 19)) ^ (t ^ (t >> 8));
701 Self->_hashStateW = v;
702 value = v;
703 }
704
705 value &= markOopDesc::hash_mask;
706 if (value == 0) value = 0xBAD;
707 assert(value != markOopDesc::no_hash, "invariant");
708 TEVENT(hashCode: GENERATE);
709 return value;
710 }
711
712 intptr_t ObjectSynchronizer::FastHashCode(Thread * Self, oop obj) {
713 if (UseBiasedLocking) {
714 // NOTE: many places throughout the JVM do not expect a safepoint
715 // to be taken here, in particular most operations on perm gen
716 // objects. However, we only ever bias Java instances and all of
717 // the call sites of identity_hash that might revoke biases have
718 // been checked to make sure they can handle a safepoint. The
719 // added check of the bias pattern is to avoid useless calls to
720 // thread-local storage.
721 if (obj->mark()->has_bias_pattern()) {
722 // Handle for oop obj in case of STW safepoint
723 Handle hobj(Self, obj);
724 // Relaxing assertion for bug 6320749.
725 assert(Universe::verify_in_progress() ||
726 !SafepointSynchronize::is_at_safepoint(),
727 "biases should not be seen by VM thread here");
728 BiasedLocking::revoke_and_rebias(hobj, false, JavaThread::current());
729 obj = hobj();
730 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
731 }
732 }
733
734 // hashCode() is a heap mutator ...
735 // Relaxing assertion for bug 6320749.
736 assert(Universe::verify_in_progress() || DumpSharedSpaces ||
737 !SafepointSynchronize::is_at_safepoint(), "invariant");
738 assert(Universe::verify_in_progress() || DumpSharedSpaces ||
739 Self->is_Java_thread() , "invariant");
740 assert(Universe::verify_in_progress() || DumpSharedSpaces ||
741 ((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
742
743 ObjectMonitor* monitor = NULL;
744 markOop temp, test;
745 intptr_t hash;
746 markOop mark = ReadStableMark(obj);
747
748 // object should remain ineligible for biased locking
749 assert(!mark->has_bias_pattern(), "invariant");
750
751 if (mark->is_neutral()) {
752 hash = mark->hash(); // this is a normal header
753 if (hash) { // if it has hash, just return it
754 return hash;
755 }
756 hash = get_next_hash(Self, obj); // allocate a new hash code
757 temp = mark->copy_set_hash(hash); // merge the hash code into header
758 // use (machine word version) atomic operation to install the hash
759 test = (markOop) Atomic::cmpxchg_ptr(temp, obj->mark_addr(), mark);
760 if (test == mark) {
761 return hash;
762 }
763 // If atomic operation failed, we must inflate the header
764 // into heavy weight monitor. We could add more code here
765 // for fast path, but it does not worth the complexity.
766 } else if (mark->has_monitor()) {
767 monitor = mark->monitor();
768 temp = monitor->header();
769 assert(temp->is_neutral(), "invariant");
770 hash = temp->hash();
771 if (hash) {
772 return hash;
773 }
774 // Skip to the following code to reduce code size
775 } else if (Self->is_lock_owned((address)mark->locker())) {
776 temp = mark->displaced_mark_helper(); // this is a lightweight monitor owned
777 assert(temp->is_neutral(), "invariant");
778 hash = temp->hash(); // by current thread, check if the displaced
779 if (hash) { // header contains hash code
780 return hash;
781 }
782 // WARNING:
783 // The displaced header is strictly immutable.
784 // It can NOT be changed in ANY cases. So we have
785 // to inflate the header into heavyweight monitor
786 // even the current thread owns the lock. The reason
787 // is the BasicLock (stack slot) will be asynchronously
788 // read by other threads during the inflate() function.
789 // Any change to stack may not propagate to other threads
790 // correctly.
791 }
792
793 // Inflate the monitor to set hash code
794 monitor = ObjectSynchronizer::inflate(Self, obj, inflate_cause_hash_code);
795 // Load displaced header and check it has hash code
796 mark = monitor->header();
797 assert(mark->is_neutral(), "invariant");
798 hash = mark->hash();
799 if (hash == 0) {
800 hash = get_next_hash(Self, obj);
801 temp = mark->copy_set_hash(hash); // merge hash code into header
802 assert(temp->is_neutral(), "invariant");
803 test = (markOop) Atomic::cmpxchg_ptr(temp, monitor, mark);
804 if (test != mark) {
805 // The only update to the header in the monitor (outside GC)
806 // is install the hash code. If someone add new usage of
807 // displaced header, please update this code
808 hash = test->hash();
809 assert(test->is_neutral(), "invariant");
810 assert(hash != 0, "Trivial unexpected object/monitor header usage.");
811 }
812 }
813 // We finally get the hash
814 return hash;
815 }
816
817 // Deprecated -- use FastHashCode() instead.
818
819 intptr_t ObjectSynchronizer::identity_hash_value_for(Handle obj) {
820 return FastHashCode(Thread::current(), obj());
821 }
822
823
824 bool ObjectSynchronizer::current_thread_holds_lock(JavaThread* thread,
825 Handle h_obj) {
826 if (UseBiasedLocking) {
827 BiasedLocking::revoke_and_rebias(h_obj, false, thread);
828 assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
829 }
830
831 assert(thread == JavaThread::current(), "Can only be called on current thread");
832 oop obj = h_obj();
833
834 markOop mark = ReadStableMark(obj);
835
836 // Uncontended case, header points to stack
837 if (mark->has_locker()) {
838 return thread->is_lock_owned((address)mark->locker());
839 }
840 // Contended case, header points to ObjectMonitor (tagged pointer)
841 if (mark->has_monitor()) {
842 ObjectMonitor* monitor = mark->monitor();
843 return monitor->is_entered(thread) != 0;
844 }
845 // Unlocked case, header in place
846 assert(mark->is_neutral(), "sanity check");
847 return false;
848 }
849
850 // Be aware of this method could revoke bias of the lock object.
851 // This method queries the ownership of the lock handle specified by 'h_obj'.
852 // If the current thread owns the lock, it returns owner_self. If no
853 // thread owns the lock, it returns owner_none. Otherwise, it will return
854 // owner_other.
855 ObjectSynchronizer::LockOwnership ObjectSynchronizer::query_lock_ownership
856 (JavaThread *self, Handle h_obj) {
857 // The caller must beware this method can revoke bias, and
858 // revocation can result in a safepoint.
859 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
860 assert(self->thread_state() != _thread_blocked, "invariant");
861
862 // Possible mark states: neutral, biased, stack-locked, inflated
863
864 if (UseBiasedLocking && h_obj()->mark()->has_bias_pattern()) {
865 // CASE: biased
866 BiasedLocking::revoke_and_rebias(h_obj, false, self);
867 assert(!h_obj->mark()->has_bias_pattern(),
868 "biases should be revoked by now");
869 }
870
871 assert(self == JavaThread::current(), "Can only be called on current thread");
872 oop obj = h_obj();
873 markOop mark = ReadStableMark(obj);
874
875 // CASE: stack-locked. Mark points to a BasicLock on the owner's stack.
876 if (mark->has_locker()) {
877 return self->is_lock_owned((address)mark->locker()) ?
878 owner_self : owner_other;
879 }
880
881 // CASE: inflated. Mark (tagged pointer) points to an objectMonitor.
882 // The Object:ObjectMonitor relationship is stable as long as we're
883 // not at a safepoint.
884 if (mark->has_monitor()) {
885 void * owner = mark->monitor()->_owner;
886 if (owner == NULL) return owner_none;
887 return (owner == self ||
888 self->is_lock_owned((address)owner)) ? owner_self : owner_other;
889 }
890
891 // CASE: neutral
892 assert(mark->is_neutral(), "sanity check");
893 return owner_none; // it's unlocked
894 }
895
896 // FIXME: jvmti should call this
897 JavaThread* ObjectSynchronizer::get_lock_owner(Handle h_obj, bool doLock) {
898 if (UseBiasedLocking) {
899 if (SafepointSynchronize::is_at_safepoint()) {
900 BiasedLocking::revoke_at_safepoint(h_obj);
901 } else {
902 BiasedLocking::revoke_and_rebias(h_obj, false, JavaThread::current());
903 }
904 assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
905 }
906
907 oop obj = h_obj();
908 address owner = NULL;
909
910 markOop mark = ReadStableMark(obj);
911
912 // Uncontended case, header points to stack
913 if (mark->has_locker()) {
914 owner = (address) mark->locker();
915 }
916
917 // Contended case, header points to ObjectMonitor (tagged pointer)
918 if (mark->has_monitor()) {
919 ObjectMonitor* monitor = mark->monitor();
920 assert(monitor != NULL, "monitor should be non-null");
921 owner = (address) monitor->owner();
922 }
923
924 if (owner != NULL) {
925 // owning_thread_from_monitor_owner() may also return NULL here
926 return Threads::owning_thread_from_monitor_owner(owner, doLock);
927 }
928
929 // Unlocked case, header in place
930 // Cannot have assertion since this object may have been
931 // locked by another thread when reaching here.
932 // assert(mark->is_neutral(), "sanity check");
933
934 return NULL;
935 }
936
937 // Visitors ...
938
939 void ObjectSynchronizer::monitors_iterate(MonitorClosure* closure) {
940 PaddedEnd<ObjectMonitor> * block =
941 (PaddedEnd<ObjectMonitor> *)OrderAccess::load_ptr_acquire(&gBlockList);
942 while (block != NULL) {
943 assert(block->object() == CHAINMARKER, "must be a block header");
944 for (int i = _BLOCKSIZE - 1; i > 0; i--) {
945 ObjectMonitor* mid = (ObjectMonitor *)(block + i);
946 oop object = (oop)mid->object();
947 if (object != NULL) {
948 closure->do_monitor(mid);
949 }
950 }
951 block = (PaddedEnd<ObjectMonitor> *)block->FreeNext;
952 }
953 }
954
955 // Get the next block in the block list.
956 static inline ObjectMonitor* next(ObjectMonitor* block) {
957 assert(block->object() == CHAINMARKER, "must be a block header");
958 block = block->FreeNext;
959 assert(block == NULL || block->object() == CHAINMARKER, "must be a block header");
960 return block;
961 }
962
963 static bool monitors_used_above_threshold() {
964 if (gMonitorPopulation == 0) {
965 return false;
966 }
967 int monitors_used = gMonitorPopulation - gMonitorFreeCount;
968 int monitor_usage = (monitors_used * 100LL) / gMonitorPopulation;
969 return monitor_usage > MonitorUsedDeflationThreshold;
970 }
971
972 bool ObjectSynchronizer::is_cleanup_needed() {
973 if (MonitorUsedDeflationThreshold > 0) {
974 return monitors_used_above_threshold();
975 }
976 return false;
977 }
978
979 void ObjectSynchronizer::oops_do(OopClosure* f) {
980 if (MonitorInUseLists) {
981 // When using thread local monitor lists, we only scan the
982 // global used list here (for moribund threads), and
983 // the thread-local monitors in Thread::oops_do().
984 global_used_oops_do(f);
985 } else {
986 global_oops_do(f);
987 }
988 }
989
990 void ObjectSynchronizer::global_oops_do(OopClosure* f) {
991 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
992 PaddedEnd<ObjectMonitor> * block =
993 (PaddedEnd<ObjectMonitor> *)OrderAccess::load_ptr_acquire(&gBlockList);
994 for (; block != NULL; block = (PaddedEnd<ObjectMonitor> *)next(block)) {
995 assert(block->object() == CHAINMARKER, "must be a block header");
996 for (int i = 1; i < _BLOCKSIZE; i++) {
997 ObjectMonitor* mid = (ObjectMonitor *)&block[i];
998 if (mid->object() != NULL) {
999 f->do_oop((oop*)mid->object_addr());
1000 }
1001 }
1002 }
1003 }
1004
1005 void ObjectSynchronizer::global_used_oops_do(OopClosure* f) {
1006 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1007 list_oops_do(gOmInUseList, f);
1008 }
1009
1010 void ObjectSynchronizer::thread_local_used_oops_do(Thread* thread, OopClosure* f) {
1011 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1012 list_oops_do(thread->omInUseList, f);
1013 }
1014
1015 void ObjectSynchronizer::list_oops_do(ObjectMonitor* list, OopClosure* f) {
1016 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1017 ObjectMonitor* mid;
1018 for (mid = list; mid != NULL; mid = mid->FreeNext) {
1019 if (mid->object() != NULL) {
1020 f->do_oop((oop*)mid->object_addr());
1021 }
1022 }
1023 }
1024
1025
1026 // -----------------------------------------------------------------------------
1027 // ObjectMonitor Lifecycle
1028 // -----------------------
1029 // Inflation unlinks monitors from the global gFreeList and
1030 // associates them with objects. Deflation -- which occurs at
1031 // STW-time -- disassociates idle monitors from objects. Such
1032 // scavenged monitors are returned to the gFreeList.
1033 //
1034 // The global list is protected by gListLock. All the critical sections
1035 // are short and operate in constant-time.
1036 //
1037 // ObjectMonitors reside in type-stable memory (TSM) and are immortal.
1038 //
1039 // Lifecycle:
1040 // -- unassigned and on the global free list
1041 // -- unassigned and on a thread's private omFreeList
1042 // -- assigned to an object. The object is inflated and the mark refers
1043 // to the objectmonitor.
1044
1045
1046 // Constraining monitor pool growth via MonitorBound ...
1047 //
1048 // The monitor pool is grow-only. We scavenge at STW safepoint-time, but the
1049 // the rate of scavenging is driven primarily by GC. As such, we can find
1050 // an inordinate number of monitors in circulation.
1051 // To avoid that scenario we can artificially induce a STW safepoint
1052 // if the pool appears to be growing past some reasonable bound.
1053 // Generally we favor time in space-time tradeoffs, but as there's no
1054 // natural back-pressure on the # of extant monitors we need to impose some
1055 // type of limit. Beware that if MonitorBound is set to too low a value
1056 // we could just loop. In addition, if MonitorBound is set to a low value
1057 // we'll incur more safepoints, which are harmful to performance.
1058 // See also: GuaranteedSafepointInterval
1059 //
1060 // The current implementation uses asynchronous VM operations.
1061
1062 static void InduceScavenge(Thread * Self, const char * Whence) {
1063 // Induce STW safepoint to trim monitors
1064 // Ultimately, this results in a call to deflate_idle_monitors() in the near future.
1065 // More precisely, trigger an asynchronous STW safepoint as the number
1066 // of active monitors passes the specified threshold.
1067 // TODO: assert thread state is reasonable
1068
1069 if (ForceMonitorScavenge == 0 && Atomic::xchg (1, &ForceMonitorScavenge) == 0) {
1070 if (ObjectMonitor::Knob_Verbose) {
1071 tty->print_cr("INFO: Monitor scavenge - Induced STW @%s (%d)",
1072 Whence, ForceMonitorScavenge) ;
1073 tty->flush();
1074 }
1075 // Induce a 'null' safepoint to scavenge monitors
1076 // Must VM_Operation instance be heap allocated as the op will be enqueue and posted
1077 // to the VMthread and have a lifespan longer than that of this activation record.
1078 // The VMThread will delete the op when completed.
1079 VMThread::execute(new VM_ScavengeMonitors());
1080
1081 if (ObjectMonitor::Knob_Verbose) {
1082 tty->print_cr("INFO: Monitor scavenge - STW posted @%s (%d)",
1083 Whence, ForceMonitorScavenge) ;
1084 tty->flush();
1085 }
1086 }
1087 }
1088
1089 void ObjectSynchronizer::verifyInUse(Thread *Self) {
1090 ObjectMonitor* mid;
1091 int in_use_tally = 0;
1092 for (mid = Self->omInUseList; mid != NULL; mid = mid->FreeNext) {
1093 in_use_tally++;
1094 }
1095 assert(in_use_tally == Self->omInUseCount, "in-use count off");
1096
1097 int free_tally = 0;
1098 for (mid = Self->omFreeList; mid != NULL; mid = mid->FreeNext) {
1099 free_tally++;
1100 }
1101 assert(free_tally == Self->omFreeCount, "free count off");
1102 }
1103
1104 ObjectMonitor* ObjectSynchronizer::omAlloc(Thread * Self) {
1105 // A large MAXPRIVATE value reduces both list lock contention
1106 // and list coherency traffic, but also tends to increase the
1107 // number of objectMonitors in circulation as well as the STW
1108 // scavenge costs. As usual, we lean toward time in space-time
1109 // tradeoffs.
1110 const int MAXPRIVATE = 1024;
1111 for (;;) {
1112 ObjectMonitor * m;
1113
1114 // 1: try to allocate from the thread's local omFreeList.
1115 // Threads will attempt to allocate first from their local list, then
1116 // from the global list, and only after those attempts fail will the thread
1117 // attempt to instantiate new monitors. Thread-local free lists take
1118 // heat off the gListLock and improve allocation latency, as well as reducing
1119 // coherency traffic on the shared global list.
1120 m = Self->omFreeList;
1121 if (m != NULL) {
1122 Self->omFreeList = m->FreeNext;
1123 Self->omFreeCount--;
1124 // CONSIDER: set m->FreeNext = BAD -- diagnostic hygiene
1125 guarantee(m->object() == NULL, "invariant");
1126 if (MonitorInUseLists) {
1127 m->FreeNext = Self->omInUseList;
1128 Self->omInUseList = m;
1129 Self->omInUseCount++;
1130 if (ObjectMonitor::Knob_VerifyInUse) {
1131 verifyInUse(Self);
1132 }
1133 } else {
1134 m->FreeNext = NULL;
1135 }
1136 return m;
1137 }
1138
1139 // 2: try to allocate from the global gFreeList
1140 // CONSIDER: use muxTry() instead of muxAcquire().
1141 // If the muxTry() fails then drop immediately into case 3.
1142 // If we're using thread-local free lists then try
1143 // to reprovision the caller's free list.
1144 if (gFreeList != NULL) {
1145 // Reprovision the thread's omFreeList.
1146 // Use bulk transfers to reduce the allocation rate and heat
1147 // on various locks.
1148 Thread::muxAcquire(&gListLock, "omAlloc");
1149 for (int i = Self->omFreeProvision; --i >= 0 && gFreeList != NULL;) {
1150 gMonitorFreeCount--;
1151 ObjectMonitor * take = gFreeList;
1152 gFreeList = take->FreeNext;
1153 guarantee(take->object() == NULL, "invariant");
1154 guarantee(!take->is_busy(), "invariant");
1155 take->Recycle();
1156 omRelease(Self, take, false);
1157 }
1158 Thread::muxRelease(&gListLock);
1159 Self->omFreeProvision += 1 + (Self->omFreeProvision/2);
1160 if (Self->omFreeProvision > MAXPRIVATE) Self->omFreeProvision = MAXPRIVATE;
1161 TEVENT(omFirst - reprovision);
1162
1163 const int mx = MonitorBound;
1164 if (mx > 0 && (gMonitorPopulation-gMonitorFreeCount) > mx) {
1165 // We can't safely induce a STW safepoint from omAlloc() as our thread
1166 // state may not be appropriate for such activities and callers may hold
1167 // naked oops, so instead we defer the action.
1168 InduceScavenge(Self, "omAlloc");
1169 }
1170 continue;
1171 }
1172
1173 // 3: allocate a block of new ObjectMonitors
1174 // Both the local and global free lists are empty -- resort to malloc().
1175 // In the current implementation objectMonitors are TSM - immortal.
1176 // Ideally, we'd write "new ObjectMonitor[_BLOCKSIZE], but we want
1177 // each ObjectMonitor to start at the beginning of a cache line,
1178 // so we use align_size_up().
1179 // A better solution would be to use C++ placement-new.
1180 // BEWARE: As it stands currently, we don't run the ctors!
1181 assert(_BLOCKSIZE > 1, "invariant");
1182 size_t neededsize = sizeof(PaddedEnd<ObjectMonitor>) * _BLOCKSIZE;
1183 PaddedEnd<ObjectMonitor> * temp;
1184 size_t aligned_size = neededsize + (DEFAULT_CACHE_LINE_SIZE - 1);
1185 void* real_malloc_addr = (void *)NEW_C_HEAP_ARRAY(char, aligned_size,
1186 mtInternal);
1187 temp = (PaddedEnd<ObjectMonitor> *)
1188 align_size_up((intptr_t)real_malloc_addr,
1189 DEFAULT_CACHE_LINE_SIZE);
1190
1191 // NOTE: (almost) no way to recover if allocation failed.
1192 // We might be able to induce a STW safepoint and scavenge enough
1193 // objectMonitors to permit progress.
1194 if (temp == NULL) {
1195 vm_exit_out_of_memory(neededsize, OOM_MALLOC_ERROR,
1196 "Allocate ObjectMonitors");
1197 }
1198 (void)memset((void *) temp, 0, neededsize);
1199
1200 // Format the block.
1201 // initialize the linked list, each monitor points to its next
1202 // forming the single linked free list, the very first monitor
1203 // will points to next block, which forms the block list.
1204 // The trick of using the 1st element in the block as gBlockList
1205 // linkage should be reconsidered. A better implementation would
1206 // look like: class Block { Block * next; int N; ObjectMonitor Body [N] ; }
1207
1208 for (int i = 1; i < _BLOCKSIZE; i++) {
1209 temp[i].FreeNext = (ObjectMonitor *)&temp[i+1];
1210 }
1211
1212 // terminate the last monitor as the end of list
1213 temp[_BLOCKSIZE - 1].FreeNext = NULL;
1214
1215 // Element [0] is reserved for global list linkage
1216 temp[0].set_object(CHAINMARKER);
1217
1218 // Consider carving out this thread's current request from the
1219 // block in hand. This avoids some lock traffic and redundant
1220 // list activity.
1221
1222 // Acquire the gListLock to manipulate gBlockList and gFreeList.
1223 // An Oyama-Taura-Yonezawa scheme might be more efficient.
1224 Thread::muxAcquire(&gListLock, "omAlloc [2]");
1225 gMonitorPopulation += _BLOCKSIZE-1;
1226 gMonitorFreeCount += _BLOCKSIZE-1;
1227
1228 // Add the new block to the list of extant blocks (gBlockList).
1229 // The very first objectMonitor in a block is reserved and dedicated.
1230 // It serves as blocklist "next" linkage.
1231 temp[0].FreeNext = gBlockList;
1232 // There are lock-free uses of gBlockList so make sure that
1233 // the previous stores happen before we update gBlockList.
1234 OrderAccess::release_store_ptr(&gBlockList, temp);
1235
1236 // Add the new string of objectMonitors to the global free list
1237 temp[_BLOCKSIZE - 1].FreeNext = gFreeList;
1238 gFreeList = temp + 1;
1239 Thread::muxRelease(&gListLock);
1240 TEVENT(Allocate block of monitors);
1241 }
1242 }
1243
1244 // Place "m" on the caller's private per-thread omFreeList.
1245 // In practice there's no need to clamp or limit the number of
1246 // monitors on a thread's omFreeList as the only time we'll call
1247 // omRelease is to return a monitor to the free list after a CAS
1248 // attempt failed. This doesn't allow unbounded #s of monitors to
1249 // accumulate on a thread's free list.
1250 //
1251 // Key constraint: all ObjectMonitors on a thread's free list and the global
1252 // free list must have their object field set to null. This prevents the
1253 // scavenger -- deflate_idle_monitors -- from reclaiming them.
1254
1255 void ObjectSynchronizer::omRelease(Thread * Self, ObjectMonitor * m,
1256 bool fromPerThreadAlloc) {
1257 guarantee(m->object() == NULL, "invariant");
1258 guarantee(((m->is_busy()|m->_recursions) == 0), "freeing in-use monitor");
1259 // Remove from omInUseList
1260 if (MonitorInUseLists && fromPerThreadAlloc) {
1261 ObjectMonitor* cur_mid_in_use = NULL;
1262 bool extracted = false;
1263 for (ObjectMonitor* mid = Self->omInUseList; mid != NULL; cur_mid_in_use = mid, mid = mid->FreeNext) {
1264 if (m == mid) {
1265 // extract from per-thread in-use list
1266 if (mid == Self->omInUseList) {
1267 Self->omInUseList = mid->FreeNext;
1268 } else if (cur_mid_in_use != NULL) {
1269 cur_mid_in_use->FreeNext = mid->FreeNext; // maintain the current thread in-use list
1270 }
1271 extracted = true;
1272 Self->omInUseCount--;
1273 if (ObjectMonitor::Knob_VerifyInUse) {
1274 verifyInUse(Self);
1275 }
1276 break;
1277 }
1278 }
1279 assert(extracted, "Should have extracted from in-use list");
1280 }
1281
1282 // FreeNext is used for both omInUseList and omFreeList, so clear old before setting new
1283 m->FreeNext = Self->omFreeList;
1284 Self->omFreeList = m;
1285 Self->omFreeCount++;
1286 }
1287
1288 // Return the monitors of a moribund thread's local free list to
1289 // the global free list. Typically a thread calls omFlush() when
1290 // it's dying. We could also consider having the VM thread steal
1291 // monitors from threads that have not run java code over a few
1292 // consecutive STW safepoints. Relatedly, we might decay
1293 // omFreeProvision at STW safepoints.
1294 //
1295 // Also return the monitors of a moribund thread's omInUseList to
1296 // a global gOmInUseList under the global list lock so these
1297 // will continue to be scanned.
1298 //
1299 // We currently call omFlush() from Threads::remove() _before the thread
1300 // has been excised from the thread list and is no longer a mutator.
1301 // This means that omFlush() can not run concurrently with a safepoint and
1302 // interleave with the scavenge operator. In particular, this ensures that
1303 // the thread's monitors are scanned by a GC safepoint, either via
1304 // Thread::oops_do() (if safepoint happens before omFlush()) or via
1305 // ObjectSynchronizer::oops_do() (if it happens after omFlush() and the thread's
1306 // monitors have been transferred to the global in-use list).
1307
1308 void ObjectSynchronizer::omFlush(Thread * Self) {
1309 ObjectMonitor * list = Self->omFreeList; // Null-terminated SLL
1310 Self->omFreeList = NULL;
1311 ObjectMonitor * tail = NULL;
1312 int tally = 0;
1313 if (list != NULL) {
1314 ObjectMonitor * s;
1315 // The thread is going away, the per-thread free monitors
1316 // are freed via set_owner(NULL)
1317 // Link them to tail, which will be linked into the global free list
1318 // gFreeList below, under the gListLock
1319 for (s = list; s != NULL; s = s->FreeNext) {
1320 tally++;
1321 tail = s;
1322 guarantee(s->object() == NULL, "invariant");
1323 guarantee(!s->is_busy(), "invariant");
1324 s->set_owner(NULL); // redundant but good hygiene
1325 TEVENT(omFlush - Move one);
1326 }
1327 guarantee(tail != NULL && list != NULL, "invariant");
1328 }
1329
1330 ObjectMonitor * inUseList = Self->omInUseList;
1331 ObjectMonitor * inUseTail = NULL;
1332 int inUseTally = 0;
1333 if (inUseList != NULL) {
1334 Self->omInUseList = NULL;
1335 ObjectMonitor *cur_om;
1336 // The thread is going away, however the omInUseList inflated
1337 // monitors may still be in-use by other threads.
1338 // Link them to inUseTail, which will be linked into the global in-use list
1339 // gOmInUseList below, under the gListLock
1340 for (cur_om = inUseList; cur_om != NULL; cur_om = cur_om->FreeNext) {
1341 inUseTail = cur_om;
1342 inUseTally++;
1343 }
1344 assert(Self->omInUseCount == inUseTally, "in-use count off");
1345 Self->omInUseCount = 0;
1346 guarantee(inUseTail != NULL && inUseList != NULL, "invariant");
1347 }
1348
1349 Thread::muxAcquire(&gListLock, "omFlush");
1350 if (tail != NULL) {
1351 tail->FreeNext = gFreeList;
1352 gFreeList = list;
1353 gMonitorFreeCount += tally;
1354 assert(Self->omFreeCount == tally, "free-count off");
1355 Self->omFreeCount = 0;
1356 }
1357
1358 if (inUseTail != NULL) {
1359 inUseTail->FreeNext = gOmInUseList;
1360 gOmInUseList = inUseList;
1361 gOmInUseCount += inUseTally;
1362 }
1363
1364 Thread::muxRelease(&gListLock);
1365 TEVENT(omFlush);
1366 }
1367
1368 // Fast path code shared by multiple functions
1369 ObjectMonitor* ObjectSynchronizer::inflate_helper(oop obj) {
1370 markOop mark = obj->mark();
1371 if (mark->has_monitor()) {
1372 assert(ObjectSynchronizer::verify_objmon_isinpool(mark->monitor()), "monitor is invalid");
1373 assert(mark->monitor()->header()->is_neutral(), "monitor must record a good object header");
1374 return mark->monitor();
1375 }
1376 return ObjectSynchronizer::inflate(Thread::current(),
1377 obj,
1378 inflate_cause_vm_internal);
1379 }
1380
1381 #if INCLUDE_TRACE
1382 static void post_monitor_inflate_event(EventJavaMonitorInflate* event,
1383 const oop obj,
1384 ObjectSynchronizer::InflateCause cause) {
1385 assert(event != NULL, "invariant");
1386 if (event->should_commit()) {
1387 event->set_monitorClass(obj->klass());
1388 event->set_address((TYPE_ADDRESS)(uintptr_t)(void*)obj);
1389 event->set_cause((u1)cause);
1390 event->commit();
1391 }
1392 }
1393 #endif
1394
1395 ObjectMonitor* ObjectSynchronizer::inflate(Thread * Self,
1396 oop object,
1397 const InflateCause cause) {
1398
1399 // Inflate mutates the heap ...
1400 // Relaxing assertion for bug 6320749.
1401 assert(Universe::verify_in_progress() ||
1402 !SafepointSynchronize::is_at_safepoint(), "invariant");
1403
1404 TRACE_ONLY(EventJavaMonitorInflate event;)
1405
1406 for (;;) {
1407 const markOop mark = object->mark();
1408 assert(!mark->has_bias_pattern(), "invariant");
1409
1410 // The mark can be in one of the following states:
1411 // * Inflated - just return
1412 // * Stack-locked - coerce it to inflated
1413 // * INFLATING - busy wait for conversion to complete
1414 // * Neutral - aggressively inflate the object.
1415 // * BIASED - Illegal. We should never see this
1416
1417 // CASE: inflated
1418 if (mark->has_monitor()) {
1419 ObjectMonitor * inf = mark->monitor();
1420 assert(inf->header()->is_neutral(), "invariant");
1421 assert(inf->object() == object, "invariant");
1422 assert(ObjectSynchronizer::verify_objmon_isinpool(inf), "monitor is invalid");
1423 return inf;
1424 }
1425
1426 // CASE: inflation in progress - inflating over a stack-lock.
1427 // Some other thread is converting from stack-locked to inflated.
1428 // Only that thread can complete inflation -- other threads must wait.
1429 // The INFLATING value is transient.
1430 // Currently, we spin/yield/park and poll the markword, waiting for inflation to finish.
1431 // We could always eliminate polling by parking the thread on some auxiliary list.
1432 if (mark == markOopDesc::INFLATING()) {
1433 TEVENT(Inflate: spin while INFLATING);
1434 ReadStableMark(object);
1435 continue;
1436 }
1437
1438 // CASE: stack-locked
1439 // Could be stack-locked either by this thread or by some other thread.
1440 //
1441 // Note that we allocate the objectmonitor speculatively, _before_ attempting
1442 // to install INFLATING into the mark word. We originally installed INFLATING,
1443 // allocated the objectmonitor, and then finally STed the address of the
1444 // objectmonitor into the mark. This was correct, but artificially lengthened
1445 // the interval in which INFLATED appeared in the mark, thus increasing
1446 // the odds of inflation contention.
1447 //
1448 // We now use per-thread private objectmonitor free lists.
1449 // These list are reprovisioned from the global free list outside the
1450 // critical INFLATING...ST interval. A thread can transfer
1451 // multiple objectmonitors en-mass from the global free list to its local free list.
1452 // This reduces coherency traffic and lock contention on the global free list.
1453 // Using such local free lists, it doesn't matter if the omAlloc() call appears
1454 // before or after the CAS(INFLATING) operation.
1455 // See the comments in omAlloc().
1456
1457 if (mark->has_locker()) {
1458 ObjectMonitor * m = omAlloc(Self);
1459 // Optimistically prepare the objectmonitor - anticipate successful CAS
1460 // We do this before the CAS in order to minimize the length of time
1461 // in which INFLATING appears in the mark.
1462 m->Recycle();
1463 m->_Responsible = NULL;
1464 m->_recursions = 0;
1465 m->_SpinDuration = ObjectMonitor::Knob_SpinLimit; // Consider: maintain by type/class
1466
1467 markOop cmp = (markOop) Atomic::cmpxchg_ptr(markOopDesc::INFLATING(), object->mark_addr(), mark);
1468 if (cmp != mark) {
1469 omRelease(Self, m, true);
1470 continue; // Interference -- just retry
1471 }
1472
1473 // We've successfully installed INFLATING (0) into the mark-word.
1474 // This is the only case where 0 will appear in a mark-word.
1475 // Only the singular thread that successfully swings the mark-word
1476 // to 0 can perform (or more precisely, complete) inflation.
1477 //
1478 // Why do we CAS a 0 into the mark-word instead of just CASing the
1479 // mark-word from the stack-locked value directly to the new inflated state?
1480 // Consider what happens when a thread unlocks a stack-locked object.
1481 // It attempts to use CAS to swing the displaced header value from the
1482 // on-stack basiclock back into the object header. Recall also that the
1483 // header value (hashcode, etc) can reside in (a) the object header, or
1484 // (b) a displaced header associated with the stack-lock, or (c) a displaced
1485 // header in an objectMonitor. The inflate() routine must copy the header
1486 // value from the basiclock on the owner's stack to the objectMonitor, all
1487 // the while preserving the hashCode stability invariants. If the owner
1488 // decides to release the lock while the value is 0, the unlock will fail
1489 // and control will eventually pass from slow_exit() to inflate. The owner
1490 // will then spin, waiting for the 0 value to disappear. Put another way,
1491 // the 0 causes the owner to stall if the owner happens to try to
1492 // drop the lock (restoring the header from the basiclock to the object)
1493 // while inflation is in-progress. This protocol avoids races that might
1494 // would otherwise permit hashCode values to change or "flicker" for an object.
1495 // Critically, while object->mark is 0 mark->displaced_mark_helper() is stable.
1496 // 0 serves as a "BUSY" inflate-in-progress indicator.
1497
1498
1499 // fetch the displaced mark from the owner's stack.
1500 // The owner can't die or unwind past the lock while our INFLATING
1501 // object is in the mark. Furthermore the owner can't complete
1502 // an unlock on the object, either.
1503 markOop dmw = mark->displaced_mark_helper();
1504 assert(dmw->is_neutral(), "invariant");
1505
1506 // Setup monitor fields to proper values -- prepare the monitor
1507 m->set_header(dmw);
1508
1509 // Optimization: if the mark->locker stack address is associated
1510 // with this thread we could simply set m->_owner = Self.
1511 // Note that a thread can inflate an object
1512 // that it has stack-locked -- as might happen in wait() -- directly
1513 // with CAS. That is, we can avoid the xchg-NULL .... ST idiom.
1514 m->set_owner(mark->locker());
1515 m->set_object(object);
1516 // TODO-FIXME: assert BasicLock->dhw != 0.
1517
1518 // Must preserve store ordering. The monitor state must
1519 // be stable at the time of publishing the monitor address.
1520 guarantee(object->mark() == markOopDesc::INFLATING(), "invariant");
1521 object->release_set_mark(markOopDesc::encode(m));
1522
1523 // Hopefully the performance counters are allocated on distinct cache lines
1524 // to avoid false sharing on MP systems ...
1525 OM_PERFDATA_OP(Inflations, inc());
1526 TEVENT(Inflate: overwrite stacklock);
1527 if (log_is_enabled(Debug, monitorinflation)) {
1528 if (object->is_instance()) {
1529 ResourceMark rm;
1530 log_debug(monitorinflation)("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1531 p2i(object), p2i(object->mark()),
1532 object->klass()->external_name());
1533 }
1534 }
1535 TRACE_ONLY(post_monitor_inflate_event(&event, object, cause);)
1536 return m;
1537 }
1538
1539 // CASE: neutral
1540 // TODO-FIXME: for entry we currently inflate and then try to CAS _owner.
1541 // If we know we're inflating for entry it's better to inflate by swinging a
1542 // pre-locked objectMonitor pointer into the object header. A successful
1543 // CAS inflates the object *and* confers ownership to the inflating thread.
1544 // In the current implementation we use a 2-step mechanism where we CAS()
1545 // to inflate and then CAS() again to try to swing _owner from NULL to Self.
1546 // An inflateTry() method that we could call from fast_enter() and slow_enter()
1547 // would be useful.
1548
1549 assert(mark->is_neutral(), "invariant");
1550 ObjectMonitor * m = omAlloc(Self);
1551 // prepare m for installation - set monitor to initial state
1552 m->Recycle();
1553 m->set_header(mark);
1554 m->set_owner(NULL);
1555 m->set_object(object);
1556 m->_recursions = 0;
1557 m->_Responsible = NULL;
1558 m->_SpinDuration = ObjectMonitor::Knob_SpinLimit; // consider: keep metastats by type/class
1559
1560 if (Atomic::cmpxchg_ptr (markOopDesc::encode(m), object->mark_addr(), mark) != mark) {
1561 m->set_object(NULL);
1562 m->set_owner(NULL);
1563 m->Recycle();
1564 omRelease(Self, m, true);
1565 m = NULL;
1566 continue;
1567 // interference - the markword changed - just retry.
1568 // The state-transitions are one-way, so there's no chance of
1569 // live-lock -- "Inflated" is an absorbing state.
1570 }
1571
1572 // Hopefully the performance counters are allocated on distinct
1573 // cache lines to avoid false sharing on MP systems ...
1574 OM_PERFDATA_OP(Inflations, inc());
1575 TEVENT(Inflate: overwrite neutral);
1576 if (log_is_enabled(Debug, monitorinflation)) {
1577 if (object->is_instance()) {
1578 ResourceMark rm;
1579 log_debug(monitorinflation)("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1580 p2i(object), p2i(object->mark()),
1581 object->klass()->external_name());
1582 }
1583 }
1584 TRACE_ONLY(post_monitor_inflate_event(&event, object, cause);)
1585 return m;
1586 }
1587 }
1588
1589
1590 // Deflate_idle_monitors() is called at all safepoints, immediately
1591 // after all mutators are stopped, but before any objects have moved.
1592 // It traverses the list of known monitors, deflating where possible.
1593 // The scavenged monitor are returned to the monitor free list.
1594 //
1595 // Beware that we scavenge at *every* stop-the-world point.
1596 // Having a large number of monitors in-circulation negatively
1597 // impacts the performance of some applications (e.g., PointBase).
1598 // Broadly, we want to minimize the # of monitors in circulation.
1599 //
1600 // We have added a flag, MonitorInUseLists, which creates a list
1601 // of active monitors for each thread. deflate_idle_monitors()
1602 // only scans the per-thread in-use lists. omAlloc() puts all
1603 // assigned monitors on the per-thread list. deflate_idle_monitors()
1604 // returns the non-busy monitors to the global free list.
1605 // When a thread dies, omFlush() adds the list of active monitors for
1606 // that thread to a global gOmInUseList acquiring the
1607 // global list lock. deflate_idle_monitors() acquires the global
1608 // list lock to scan for non-busy monitors to the global free list.
1609 // An alternative could have used a single global in-use list. The
1610 // downside would have been the additional cost of acquiring the global list lock
1611 // for every omAlloc().
1612 //
1613 // Perversely, the heap size -- and thus the STW safepoint rate --
1614 // typically drives the scavenge rate. Large heaps can mean infrequent GC,
1615 // which in turn can mean large(r) numbers of objectmonitors in circulation.
1616 // This is an unfortunate aspect of this design.
1617
1618 enum ManifestConstants {
1619 ClearResponsibleAtSTW = 0
1620 };
1621
1622 // Deflate a single monitor if not in-use
1623 // Return true if deflated, false if in-use
1624 bool ObjectSynchronizer::deflate_monitor(ObjectMonitor* mid, oop obj,
1625 ObjectMonitor** freeHeadp,
1626 ObjectMonitor** freeTailp) {
1627 bool deflated;
1628 // Normal case ... The monitor is associated with obj.
1629 guarantee(obj->mark() == markOopDesc::encode(mid), "invariant");
1630 guarantee(mid == obj->mark()->monitor(), "invariant");
1631 guarantee(mid->header()->is_neutral(), "invariant");
1632
1633 if (mid->is_busy()) {
1634 if (ClearResponsibleAtSTW) mid->_Responsible = NULL;
1635 deflated = false;
1636 } else {
1637 // Deflate the monitor if it is no longer being used
1638 // It's idle - scavenge and return to the global free list
1639 // plain old deflation ...
1640 TEVENT(deflate_idle_monitors - scavenge1);
1641 if (log_is_enabled(Debug, monitorinflation)) {
1642 if (obj->is_instance()) {
1643 ResourceMark rm;
1644 log_debug(monitorinflation)("Deflating object " INTPTR_FORMAT " , "
1645 "mark " INTPTR_FORMAT " , type %s",
1646 p2i(obj), p2i(obj->mark()),
1647 obj->klass()->external_name());
1648 }
1649 }
1650
1651 // Restore the header back to obj
1652 obj->release_set_mark(mid->header());
1653 mid->clear();
1654
1655 assert(mid->object() == NULL, "invariant");
1656
1657 // Move the object to the working free list defined by freeHeadp, freeTailp
1658 if (*freeHeadp == NULL) *freeHeadp = mid;
1659 if (*freeTailp != NULL) {
1660 ObjectMonitor * prevtail = *freeTailp;
1661 assert(prevtail->FreeNext == NULL, "cleaned up deflated?");
1662 prevtail->FreeNext = mid;
1663 }
1664 *freeTailp = mid;
1665 deflated = true;
1666 }
1667 return deflated;
1668 }
1669
1670 // Walk a given monitor list, and deflate idle monitors
1671 // The given list could be a per-thread list or a global list
1672 // Caller acquires gListLock
1673 int ObjectSynchronizer::deflate_monitor_list(ObjectMonitor** listHeadp,
1674 ObjectMonitor** freeHeadp,
1675 ObjectMonitor** freeTailp) {
1676 ObjectMonitor* mid;
1677 ObjectMonitor* next;
1678 ObjectMonitor* cur_mid_in_use = NULL;
1679 int deflated_count = 0;
1680
1681 for (mid = *listHeadp; mid != NULL;) {
1682 oop obj = (oop) mid->object();
1683 if (obj != NULL && deflate_monitor(mid, obj, freeHeadp, freeTailp)) {
1684 // if deflate_monitor succeeded,
1685 // extract from per-thread in-use list
1686 if (mid == *listHeadp) {
1687 *listHeadp = mid->FreeNext;
1688 } else if (cur_mid_in_use != NULL) {
1689 cur_mid_in_use->FreeNext = mid->FreeNext; // maintain the current thread in-use list
1690 }
1691 next = mid->FreeNext;
1692 mid->FreeNext = NULL; // This mid is current tail in the freeHeadp list
1693 mid = next;
1694 deflated_count++;
1695 } else {
1696 cur_mid_in_use = mid;
1697 mid = mid->FreeNext;
1698 }
1699 }
1700 return deflated_count;
1701 }
1702
1703 void ObjectSynchronizer::deflate_idle_monitors() {
1704 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1705 int nInuse = 0; // currently associated with objects
1706 int nInCirculation = 0; // extant
1707 int nScavenged = 0; // reclaimed
1708 bool deflated = false;
1709
1710 ObjectMonitor * freeHeadp = NULL; // Local SLL of scavenged monitors
1711 ObjectMonitor * freeTailp = NULL;
1712
1713 TEVENT(deflate_idle_monitors);
1714 // Prevent omFlush from changing mids in Thread dtor's during deflation
1715 // And in case the vm thread is acquiring a lock during a safepoint
1716 // See e.g. 6320749
1717 Thread::muxAcquire(&gListLock, "scavenge - return");
1718
1719 if (MonitorInUseLists) {
1720 int inUse = 0;
1721 for (JavaThread* cur = Threads::first(); cur != NULL; cur = cur->next()) {
1722 nInCirculation+= cur->omInUseCount;
1723 int deflated_count = deflate_monitor_list(cur->omInUseList_addr(), &freeHeadp, &freeTailp);
1724 cur->omInUseCount-= deflated_count;
1725 if (ObjectMonitor::Knob_VerifyInUse) {
1726 verifyInUse(cur);
1727 }
1728 nScavenged += deflated_count;
1729 nInuse += cur->omInUseCount;
1730 }
1731
1732 // For moribund threads, scan gOmInUseList
1733 if (gOmInUseList) {
1734 nInCirculation += gOmInUseCount;
1735 int deflated_count = deflate_monitor_list((ObjectMonitor **)&gOmInUseList, &freeHeadp, &freeTailp);
1736 gOmInUseCount-= deflated_count;
1737 nScavenged += deflated_count;
1738 nInuse += gOmInUseCount;
1739 }
1740
1741 } else {
1742 PaddedEnd<ObjectMonitor> * block =
1743 (PaddedEnd<ObjectMonitor> *)OrderAccess::load_ptr_acquire(&gBlockList);
1744 for (; block != NULL; block = (PaddedEnd<ObjectMonitor> *)next(block)) {
1745 // Iterate over all extant monitors - Scavenge all idle monitors.
1746 assert(block->object() == CHAINMARKER, "must be a block header");
1747 nInCirculation += _BLOCKSIZE;
1748 for (int i = 1; i < _BLOCKSIZE; i++) {
1749 ObjectMonitor* mid = (ObjectMonitor*)&block[i];
1750 oop obj = (oop)mid->object();
1751
1752 if (obj == NULL) {
1753 // The monitor is not associated with an object.
1754 // The monitor should either be a thread-specific private
1755 // free list or the global free list.
1756 // obj == NULL IMPLIES mid->is_busy() == 0
1757 guarantee(!mid->is_busy(), "invariant");
1758 continue;
1759 }
1760 deflated = deflate_monitor(mid, obj, &freeHeadp, &freeTailp);
1761
1762 if (deflated) {
1763 mid->FreeNext = NULL;
1764 nScavenged++;
1765 } else {
1766 nInuse++;
1767 }
1768 }
1769 }
1770 }
1771
1772 gMonitorFreeCount += nScavenged;
1773
1774 // Consider: audit gFreeList to ensure that gMonitorFreeCount and list agree.
1775
1776 if (ObjectMonitor::Knob_Verbose) {
1777 tty->print_cr("INFO: Deflate: InCirc=%d InUse=%d Scavenged=%d "
1778 "ForceMonitorScavenge=%d : pop=%d free=%d",
1779 nInCirculation, nInuse, nScavenged, ForceMonitorScavenge,
1780 gMonitorPopulation, gMonitorFreeCount);
1781 tty->flush();
1782 }
1783
1784 ForceMonitorScavenge = 0; // Reset
1785
1786 // Move the scavenged monitors back to the global free list.
1787 if (freeHeadp != NULL) {
1788 guarantee(freeTailp != NULL && nScavenged > 0, "invariant");
1789 assert(freeTailp->FreeNext == NULL, "invariant");
1790 // constant-time list splice - prepend scavenged segment to gFreeList
1791 freeTailp->FreeNext = gFreeList;
1792 gFreeList = freeHeadp;
1793 }
1794 Thread::muxRelease(&gListLock);
1795
1796 OM_PERFDATA_OP(Deflations, inc(nScavenged));
1797 OM_PERFDATA_OP(MonExtant, set_value(nInCirculation));
1798
1799 // TODO: Add objectMonitor leak detection.
1800 // Audit/inventory the objectMonitors -- make sure they're all accounted for.
1801 GVars.stwRandom = os::random();
1802 GVars.stwCycle++;
1803 }
1804
1805 // Monitor cleanup on JavaThread::exit
1806
1807 // Iterate through monitor cache and attempt to release thread's monitors
1808 // Gives up on a particular monitor if an exception occurs, but continues
1809 // the overall iteration, swallowing the exception.
1810 class ReleaseJavaMonitorsClosure: public MonitorClosure {
1811 private:
1812 TRAPS;
1813
1814 public:
1815 ReleaseJavaMonitorsClosure(Thread* thread) : THREAD(thread) {}
1816 void do_monitor(ObjectMonitor* mid) {
1817 if (mid->owner() == THREAD) {
1818 if (ObjectMonitor::Knob_VerifyMatch != 0) {
1819 ResourceMark rm;
1820 Handle obj(THREAD, (oop) mid->object());
1821 tty->print("INFO: unexpected locked object:");
1822 javaVFrame::print_locked_object_class_name(tty, obj, "locked");
1823 fatal("exiting JavaThread=" INTPTR_FORMAT
1824 " unexpectedly owns ObjectMonitor=" INTPTR_FORMAT,
1825 p2i(THREAD), p2i(mid));
1826 }
1827 (void)mid->complete_exit(CHECK);
1828 }
1829 }
1830 };
1831
1832 // Release all inflated monitors owned by THREAD. Lightweight monitors are
1833 // ignored. This is meant to be called during JNI thread detach which assumes
1834 // all remaining monitors are heavyweight. All exceptions are swallowed.
1835 // Scanning the extant monitor list can be time consuming.
1836 // A simple optimization is to add a per-thread flag that indicates a thread
1837 // called jni_monitorenter() during its lifetime.
1838 //
1839 // Instead of No_Savepoint_Verifier it might be cheaper to
1840 // use an idiom of the form:
1841 // auto int tmp = SafepointSynchronize::_safepoint_counter ;
1842 // <code that must not run at safepoint>
1843 // guarantee (((tmp ^ _safepoint_counter) | (tmp & 1)) == 0) ;
1844 // Since the tests are extremely cheap we could leave them enabled
1845 // for normal product builds.
1846
1847 void ObjectSynchronizer::release_monitors_owned_by_thread(TRAPS) {
1848 assert(THREAD == JavaThread::current(), "must be current Java thread");
1849 NoSafepointVerifier nsv;
1850 ReleaseJavaMonitorsClosure rjmc(THREAD);
1851 Thread::muxAcquire(&gListLock, "release_monitors_owned_by_thread");
1852 ObjectSynchronizer::monitors_iterate(&rjmc);
1853 Thread::muxRelease(&gListLock);
1854 THREAD->clear_pending_exception();
1855 }
1856
1857 const char* ObjectSynchronizer::inflate_cause_name(const InflateCause cause) {
1858 switch (cause) {
1859 case inflate_cause_vm_internal: return "VM Internal";
1860 case inflate_cause_monitor_enter: return "Monitor Enter";
1861 case inflate_cause_wait: return "Monitor Wait";
1862 case inflate_cause_notify: return "Monitor Notify";
1863 case inflate_cause_hash_code: return "Monitor Hash Code";
1864 case inflate_cause_jni_enter: return "JNI Monitor Enter";
1865 case inflate_cause_jni_exit: return "JNI Monitor Exit";
1866 default:
1867 ShouldNotReachHere();
1868 }
1869 return "Unknown";
1870 }
1871
1872 //------------------------------------------------------------------------------
1873 // Debugging code
1874
1875 void ObjectSynchronizer::sanity_checks(const bool verbose,
1876 const uint cache_line_size,
1877 int *error_cnt_ptr,
1878 int *warning_cnt_ptr) {
1879 u_char *addr_begin = (u_char*)&GVars;
1880 u_char *addr_stwRandom = (u_char*)&GVars.stwRandom;
1881 u_char *addr_hcSequence = (u_char*)&GVars.hcSequence;
1882
1883 if (verbose) {
1884 tty->print_cr("INFO: sizeof(SharedGlobals)=" SIZE_FORMAT,
1885 sizeof(SharedGlobals));
1886 }
1887
1888 uint offset_stwRandom = (uint)(addr_stwRandom - addr_begin);
1889 if (verbose) tty->print_cr("INFO: offset(stwRandom)=%u", offset_stwRandom);
1890
1891 uint offset_hcSequence = (uint)(addr_hcSequence - addr_begin);
1892 if (verbose) {
1893 tty->print_cr("INFO: offset(_hcSequence)=%u", offset_hcSequence);
1894 }
1895
1896 if (cache_line_size != 0) {
1897 // We were able to determine the L1 data cache line size so
1898 // do some cache line specific sanity checks
1899
1900 if (offset_stwRandom < cache_line_size) {
1901 tty->print_cr("WARNING: the SharedGlobals.stwRandom field is closer "
1902 "to the struct beginning than a cache line which permits "
1903 "false sharing.");
1904 (*warning_cnt_ptr)++;
1905 }
1906
1907 if ((offset_hcSequence - offset_stwRandom) < cache_line_size) {
1908 tty->print_cr("WARNING: the SharedGlobals.stwRandom and "
1909 "SharedGlobals.hcSequence fields are closer than a cache "
1910 "line which permits false sharing.");
1911 (*warning_cnt_ptr)++;
1912 }
1913
1914 if ((sizeof(SharedGlobals) - offset_hcSequence) < cache_line_size) {
1915 tty->print_cr("WARNING: the SharedGlobals.hcSequence field is closer "
1916 "to the struct end than a cache line which permits false "
1917 "sharing.");
1918 (*warning_cnt_ptr)++;
1919 }
1920 }
1921 }
1922
1923 #ifndef PRODUCT
1924
1925 // Check if monitor belongs to the monitor cache
1926 // The list is grow-only so it's *relatively* safe to traverse
1927 // the list of extant blocks without taking a lock.
1928
1929 int ObjectSynchronizer::verify_objmon_isinpool(ObjectMonitor *monitor) {
1930 PaddedEnd<ObjectMonitor> * block =
1931 (PaddedEnd<ObjectMonitor> *)OrderAccess::load_ptr_acquire(&gBlockList);
1932 while (block != NULL) {
1933 assert(block->object() == CHAINMARKER, "must be a block header");
1934 if (monitor > (ObjectMonitor *)&block[0] &&
1935 monitor < (ObjectMonitor *)&block[_BLOCKSIZE]) {
1936 address mon = (address)monitor;
1937 address blk = (address)block;
1938 size_t diff = mon - blk;
1939 assert((diff % sizeof(PaddedEnd<ObjectMonitor>)) == 0, "must be aligned");
1940 return 1;
1941 }
1942 block = (PaddedEnd<ObjectMonitor> *)block->FreeNext;
1943 }
1944 return 0;
1945 }
1946
1947 #endif