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
966 void ObjectSynchronizer::oops_do(OopClosure* f) {
967 if (MonitorInUseLists) {
968 // When using thread local monitor lists, we only scan the
969 // global used list here (for moribund threads), and
970 // the thread-local monitors in Thread::oops_do().
971 global_used_oops_do(f);
972 } else {
973 global_oops_do(f);
974 }
975 }
976
977 void ObjectSynchronizer::global_oops_do(OopClosure* f) {
978 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
979 PaddedEnd<ObjectMonitor> * block =
980 (PaddedEnd<ObjectMonitor> *)OrderAccess::load_ptr_acquire(&gBlockList);
981 for (; block != NULL; block = (PaddedEnd<ObjectMonitor> *)next(block)) {
982 assert(block->object() == CHAINMARKER, "must be a block header");
983 for (int i = 1; i < _BLOCKSIZE; i++) {
984 ObjectMonitor* mid = (ObjectMonitor *)&block[i];
985 if (mid->object() != NULL) {
986 f->do_oop((oop*)mid->object_addr());
987 }
988 }
989 }
990 }
991
992 void ObjectSynchronizer::global_used_oops_do(OopClosure* f) {
993 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
994 list_oops_do(gOmInUseList, f);
995 }
996
997 void ObjectSynchronizer::thread_local_used_oops_do(Thread* thread, OopClosure* f) {
998 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
999 list_oops_do(thread->omInUseList, f);
1000 }
1001
1002 void ObjectSynchronizer::list_oops_do(ObjectMonitor* list, OopClosure* f) {
1003 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1004 ObjectMonitor* mid;
1005 for (mid = list; mid != NULL; mid = mid->FreeNext) {
1006 if (mid->object() != NULL) {
1007 f->do_oop((oop*)mid->object_addr());
1008 }
1009 }
1010 }
1011
1012
1013 // -----------------------------------------------------------------------------
1014 // ObjectMonitor Lifecycle
1015 // -----------------------
1016 // Inflation unlinks monitors from the global gFreeList and
1017 // associates them with objects. Deflation -- which occurs at
1018 // STW-time -- disassociates idle monitors from objects. Such
1019 // scavenged monitors are returned to the gFreeList.
1020 //
1021 // The global list is protected by gListLock. All the critical sections
1022 // are short and operate in constant-time.
1023 //
1024 // ObjectMonitors reside in type-stable memory (TSM) and are immortal.
1025 //
1026 // Lifecycle:
1027 // -- unassigned and on the global free list
1028 // -- unassigned and on a thread's private omFreeList
1029 // -- assigned to an object. The object is inflated and the mark refers
1030 // to the objectmonitor.
1031
1032
1033 // Constraining monitor pool growth via MonitorBound ...
1034 //
1035 // The monitor pool is grow-only. We scavenge at STW safepoint-time, but the
1036 // the rate of scavenging is driven primarily by GC. As such, we can find
1037 // an inordinate number of monitors in circulation.
1038 // To avoid that scenario we can artificially induce a STW safepoint
1039 // if the pool appears to be growing past some reasonable bound.
1040 // Generally we favor time in space-time tradeoffs, but as there's no
1041 // natural back-pressure on the # of extant monitors we need to impose some
1042 // type of limit. Beware that if MonitorBound is set to too low a value
1043 // we could just loop. In addition, if MonitorBound is set to a low value
1044 // we'll incur more safepoints, which are harmful to performance.
1045 // See also: GuaranteedSafepointInterval
1046 //
1047 // The current implementation uses asynchronous VM operations.
1048
1049 static void InduceScavenge(Thread * Self, const char * Whence) {
1050 // Induce STW safepoint to trim monitors
1051 // Ultimately, this results in a call to deflate_idle_monitors() in the near future.
1052 // More precisely, trigger an asynchronous STW safepoint as the number
1053 // of active monitors passes the specified threshold.
1054 // TODO: assert thread state is reasonable
1055
1056 if (ForceMonitorScavenge == 0 && Atomic::xchg (1, &ForceMonitorScavenge) == 0) {
1057 if (ObjectMonitor::Knob_Verbose) {
1058 tty->print_cr("INFO: Monitor scavenge - Induced STW @%s (%d)",
1059 Whence, ForceMonitorScavenge) ;
1060 tty->flush();
1061 }
1062 // Induce a 'null' safepoint to scavenge monitors
1063 // Must VM_Operation instance be heap allocated as the op will be enqueue and posted
1064 // to the VMthread and have a lifespan longer than that of this activation record.
1065 // The VMThread will delete the op when completed.
1066 VMThread::execute(new VM_ScavengeMonitors());
1067
1068 if (ObjectMonitor::Knob_Verbose) {
1069 tty->print_cr("INFO: Monitor scavenge - STW posted @%s (%d)",
1070 Whence, ForceMonitorScavenge) ;
1071 tty->flush();
1072 }
1073 }
1074 }
1075
1076 void ObjectSynchronizer::verifyInUse(Thread *Self) {
1077 ObjectMonitor* mid;
1078 int in_use_tally = 0;
1079 for (mid = Self->omInUseList; mid != NULL; mid = mid->FreeNext) {
1080 in_use_tally++;
1081 }
1082 assert(in_use_tally == Self->omInUseCount, "in-use count off");
1083
1084 int free_tally = 0;
1085 for (mid = Self->omFreeList; mid != NULL; mid = mid->FreeNext) {
1086 free_tally++;
1087 }
1088 assert(free_tally == Self->omFreeCount, "free count off");
1089 }
1090
1091 ObjectMonitor* ObjectSynchronizer::omAlloc(Thread * Self) {
1092 // A large MAXPRIVATE value reduces both list lock contention
1093 // and list coherency traffic, but also tends to increase the
1094 // number of objectMonitors in circulation as well as the STW
1095 // scavenge costs. As usual, we lean toward time in space-time
1096 // tradeoffs.
1097 const int MAXPRIVATE = 1024;
1098 for (;;) {
1099 ObjectMonitor * m;
1100
1101 // 1: try to allocate from the thread's local omFreeList.
1102 // Threads will attempt to allocate first from their local list, then
1103 // from the global list, and only after those attempts fail will the thread
1104 // attempt to instantiate new monitors. Thread-local free lists take
1105 // heat off the gListLock and improve allocation latency, as well as reducing
1106 // coherency traffic on the shared global list.
1107 m = Self->omFreeList;
1108 if (m != NULL) {
1109 Self->omFreeList = m->FreeNext;
1110 Self->omFreeCount--;
1111 // CONSIDER: set m->FreeNext = BAD -- diagnostic hygiene
1112 guarantee(m->object() == NULL, "invariant");
1113 if (MonitorInUseLists) {
1114 m->FreeNext = Self->omInUseList;
1115 Self->omInUseList = m;
1116 Self->omInUseCount++;
1117 if (ObjectMonitor::Knob_VerifyInUse) {
1118 verifyInUse(Self);
1119 }
1120 } else {
1121 m->FreeNext = NULL;
1122 }
1123 return m;
1124 }
1125
1126 // 2: try to allocate from the global gFreeList
1127 // CONSIDER: use muxTry() instead of muxAcquire().
1128 // If the muxTry() fails then drop immediately into case 3.
1129 // If we're using thread-local free lists then try
1130 // to reprovision the caller's free list.
1131 if (gFreeList != NULL) {
1132 // Reprovision the thread's omFreeList.
1133 // Use bulk transfers to reduce the allocation rate and heat
1134 // on various locks.
1135 Thread::muxAcquire(&gListLock, "omAlloc");
1136 for (int i = Self->omFreeProvision; --i >= 0 && gFreeList != NULL;) {
1137 gMonitorFreeCount--;
1138 ObjectMonitor * take = gFreeList;
1139 gFreeList = take->FreeNext;
1140 guarantee(take->object() == NULL, "invariant");
1141 guarantee(!take->is_busy(), "invariant");
1142 take->Recycle();
1143 omRelease(Self, take, false);
1144 }
1145 Thread::muxRelease(&gListLock);
1146 Self->omFreeProvision += 1 + (Self->omFreeProvision/2);
1147 if (Self->omFreeProvision > MAXPRIVATE) Self->omFreeProvision = MAXPRIVATE;
1148 TEVENT(omFirst - reprovision);
1149
1150 const int mx = MonitorBound;
1151 if (mx > 0 && (gMonitorPopulation-gMonitorFreeCount) > mx) {
1152 // We can't safely induce a STW safepoint from omAlloc() as our thread
1153 // state may not be appropriate for such activities and callers may hold
1154 // naked oops, so instead we defer the action.
1155 InduceScavenge(Self, "omAlloc");
1156 }
1157 continue;
1158 }
1159
1160 // 3: allocate a block of new ObjectMonitors
1161 // Both the local and global free lists are empty -- resort to malloc().
1162 // In the current implementation objectMonitors are TSM - immortal.
1163 // Ideally, we'd write "new ObjectMonitor[_BLOCKSIZE], but we want
1164 // each ObjectMonitor to start at the beginning of a cache line,
1165 // so we use align_size_up().
1166 // A better solution would be to use C++ placement-new.
1167 // BEWARE: As it stands currently, we don't run the ctors!
1168 assert(_BLOCKSIZE > 1, "invariant");
1169 size_t neededsize = sizeof(PaddedEnd<ObjectMonitor>) * _BLOCKSIZE;
1170 PaddedEnd<ObjectMonitor> * temp;
1171 size_t aligned_size = neededsize + (DEFAULT_CACHE_LINE_SIZE - 1);
1172 void* real_malloc_addr = (void *)NEW_C_HEAP_ARRAY(char, aligned_size,
1173 mtInternal);
1174 temp = (PaddedEnd<ObjectMonitor> *)
1175 align_size_up((intptr_t)real_malloc_addr,
1176 DEFAULT_CACHE_LINE_SIZE);
1177
1178 // NOTE: (almost) no way to recover if allocation failed.
1179 // We might be able to induce a STW safepoint and scavenge enough
1180 // objectMonitors to permit progress.
1181 if (temp == NULL) {
1182 vm_exit_out_of_memory(neededsize, OOM_MALLOC_ERROR,
1183 "Allocate ObjectMonitors");
1184 }
1185 (void)memset((void *) temp, 0, neededsize);
1186
1187 // Format the block.
1188 // initialize the linked list, each monitor points to its next
1189 // forming the single linked free list, the very first monitor
1190 // will points to next block, which forms the block list.
1191 // The trick of using the 1st element in the block as gBlockList
1192 // linkage should be reconsidered. A better implementation would
1193 // look like: class Block { Block * next; int N; ObjectMonitor Body [N] ; }
1194
1195 for (int i = 1; i < _BLOCKSIZE; i++) {
1196 temp[i].FreeNext = (ObjectMonitor *)&temp[i+1];
1197 }
1198
1199 // terminate the last monitor as the end of list
1200 temp[_BLOCKSIZE - 1].FreeNext = NULL;
1201
1202 // Element [0] is reserved for global list linkage
1203 temp[0].set_object(CHAINMARKER);
1204
1205 // Consider carving out this thread's current request from the
1206 // block in hand. This avoids some lock traffic and redundant
1207 // list activity.
1208
1209 // Acquire the gListLock to manipulate gBlockList and gFreeList.
1210 // An Oyama-Taura-Yonezawa scheme might be more efficient.
1211 Thread::muxAcquire(&gListLock, "omAlloc [2]");
1212 gMonitorPopulation += _BLOCKSIZE-1;
1213 gMonitorFreeCount += _BLOCKSIZE-1;
1214
1215 // Add the new block to the list of extant blocks (gBlockList).
1216 // The very first objectMonitor in a block is reserved and dedicated.
1217 // It serves as blocklist "next" linkage.
1218 temp[0].FreeNext = gBlockList;
1219 // There are lock-free uses of gBlockList so make sure that
1220 // the previous stores happen before we update gBlockList.
1221 OrderAccess::release_store_ptr(&gBlockList, temp);
1222
1223 // Add the new string of objectMonitors to the global free list
1224 temp[_BLOCKSIZE - 1].FreeNext = gFreeList;
1225 gFreeList = temp + 1;
1226 Thread::muxRelease(&gListLock);
1227 TEVENT(Allocate block of monitors);
1228 }
1229 }
1230
1231 // Place "m" on the caller's private per-thread omFreeList.
1232 // In practice there's no need to clamp or limit the number of
1233 // monitors on a thread's omFreeList as the only time we'll call
1234 // omRelease is to return a monitor to the free list after a CAS
1235 // attempt failed. This doesn't allow unbounded #s of monitors to
1236 // accumulate on a thread's free list.
1237 //
1238 // Key constraint: all ObjectMonitors on a thread's free list and the global
1239 // free list must have their object field set to null. This prevents the
1240 // scavenger -- deflate_idle_monitors -- from reclaiming them.
1241
1242 void ObjectSynchronizer::omRelease(Thread * Self, ObjectMonitor * m,
1243 bool fromPerThreadAlloc) {
1244 guarantee(m->object() == NULL, "invariant");
1245 guarantee(((m->is_busy()|m->_recursions) == 0), "freeing in-use monitor");
1246 // Remove from omInUseList
1247 if (MonitorInUseLists && fromPerThreadAlloc) {
1248 ObjectMonitor* cur_mid_in_use = NULL;
1249 bool extracted = false;
1250 for (ObjectMonitor* mid = Self->omInUseList; mid != NULL; cur_mid_in_use = mid, mid = mid->FreeNext) {
1251 if (m == mid) {
1252 // extract from per-thread in-use list
1253 if (mid == Self->omInUseList) {
1254 Self->omInUseList = mid->FreeNext;
1255 } else if (cur_mid_in_use != NULL) {
1256 cur_mid_in_use->FreeNext = mid->FreeNext; // maintain the current thread in-use list
1257 }
1258 extracted = true;
1259 Self->omInUseCount--;
1260 if (ObjectMonitor::Knob_VerifyInUse) {
1261 verifyInUse(Self);
1262 }
1263 break;
1264 }
1265 }
1266 assert(extracted, "Should have extracted from in-use list");
1267 }
1268
1269 // FreeNext is used for both omInUseList and omFreeList, so clear old before setting new
1270 m->FreeNext = Self->omFreeList;
1271 Self->omFreeList = m;
1272 Self->omFreeCount++;
1273 }
1274
1275 // Return the monitors of a moribund thread's local free list to
1276 // the global free list. Typically a thread calls omFlush() when
1277 // it's dying. We could also consider having the VM thread steal
1278 // monitors from threads that have not run java code over a few
1279 // consecutive STW safepoints. Relatedly, we might decay
1280 // omFreeProvision at STW safepoints.
1281 //
1282 // Also return the monitors of a moribund thread's omInUseList to
1283 // a global gOmInUseList under the global list lock so these
1284 // will continue to be scanned.
1285 //
1286 // We currently call omFlush() from Threads::remove() _before the thread
1287 // has been excised from the thread list and is no longer a mutator.
1288 // This means that omFlush() can not run concurrently with a safepoint and
1289 // interleave with the scavenge operator. In particular, this ensures that
1290 // the thread's monitors are scanned by a GC safepoint, either via
1291 // Thread::oops_do() (if safepoint happens before omFlush()) or via
1292 // ObjectSynchronizer::oops_do() (if it happens after omFlush() and the thread's
1293 // monitors have been transferred to the global in-use list).
1294
1295 void ObjectSynchronizer::omFlush(Thread * Self) {
1296 ObjectMonitor * list = Self->omFreeList; // Null-terminated SLL
1297 Self->omFreeList = NULL;
1298 ObjectMonitor * tail = NULL;
1299 int tally = 0;
1300 if (list != NULL) {
1301 ObjectMonitor * s;
1302 // The thread is going away, the per-thread free monitors
1303 // are freed via set_owner(NULL)
1304 // Link them to tail, which will be linked into the global free list
1305 // gFreeList below, under the gListLock
1306 for (s = list; s != NULL; s = s->FreeNext) {
1307 tally++;
1308 tail = s;
1309 guarantee(s->object() == NULL, "invariant");
1310 guarantee(!s->is_busy(), "invariant");
1311 s->set_owner(NULL); // redundant but good hygiene
1312 TEVENT(omFlush - Move one);
1313 }
1314 guarantee(tail != NULL && list != NULL, "invariant");
1315 }
1316
1317 ObjectMonitor * inUseList = Self->omInUseList;
1318 ObjectMonitor * inUseTail = NULL;
1319 int inUseTally = 0;
1320 if (inUseList != NULL) {
1321 Self->omInUseList = NULL;
1322 ObjectMonitor *cur_om;
1323 // The thread is going away, however the omInUseList inflated
1324 // monitors may still be in-use by other threads.
1325 // Link them to inUseTail, which will be linked into the global in-use list
1326 // gOmInUseList below, under the gListLock
1327 for (cur_om = inUseList; cur_om != NULL; cur_om = cur_om->FreeNext) {
1328 inUseTail = cur_om;
1329 inUseTally++;
1330 }
1331 assert(Self->omInUseCount == inUseTally, "in-use count off");
1332 Self->omInUseCount = 0;
1333 guarantee(inUseTail != NULL && inUseList != NULL, "invariant");
1334 }
1335
1336 Thread::muxAcquire(&gListLock, "omFlush");
1337 if (tail != NULL) {
1338 tail->FreeNext = gFreeList;
1339 gFreeList = list;
1340 gMonitorFreeCount += tally;
1341 assert(Self->omFreeCount == tally, "free-count off");
1342 Self->omFreeCount = 0;
1343 }
1344
1345 if (inUseTail != NULL) {
1346 inUseTail->FreeNext = gOmInUseList;
1347 gOmInUseList = inUseList;
1348 gOmInUseCount += inUseTally;
1349 }
1350
1351 Thread::muxRelease(&gListLock);
1352 TEVENT(omFlush);
1353 }
1354
1355 // Fast path code shared by multiple functions
1356 ObjectMonitor* ObjectSynchronizer::inflate_helper(oop obj) {
1357 markOop mark = obj->mark();
1358 if (mark->has_monitor()) {
1359 assert(ObjectSynchronizer::verify_objmon_isinpool(mark->monitor()), "monitor is invalid");
1360 assert(mark->monitor()->header()->is_neutral(), "monitor must record a good object header");
1361 return mark->monitor();
1362 }
1363 return ObjectSynchronizer::inflate(Thread::current(),
1364 obj,
1365 inflate_cause_vm_internal);
1366 }
1367
1368 ObjectMonitor* ObjectSynchronizer::inflate(Thread * Self,
1369 oop object,
1370 const InflateCause cause) {
1371
1372 // Inflate mutates the heap ...
1373 // Relaxing assertion for bug 6320749.
1374 assert(Universe::verify_in_progress() ||
1375 !SafepointSynchronize::is_at_safepoint(), "invariant");
1376
1377 EventJavaMonitorInflate event;
1378
1379 for (;;) {
1380 const markOop mark = object->mark();
1381 assert(!mark->has_bias_pattern(), "invariant");
1382
1383 // The mark can be in one of the following states:
1384 // * Inflated - just return
1385 // * Stack-locked - coerce it to inflated
1386 // * INFLATING - busy wait for conversion to complete
1387 // * Neutral - aggressively inflate the object.
1388 // * BIASED - Illegal. We should never see this
1389
1390 // CASE: inflated
1391 if (mark->has_monitor()) {
1392 ObjectMonitor * inf = mark->monitor();
1393 assert(inf->header()->is_neutral(), "invariant");
1394 assert(inf->object() == object, "invariant");
1395 assert(ObjectSynchronizer::verify_objmon_isinpool(inf), "monitor is invalid");
1396 event.cancel(); // let's not post an inflation event, unless we did the deed ourselves
1397 return inf;
1398 }
1399
1400 // CASE: inflation in progress - inflating over a stack-lock.
1401 // Some other thread is converting from stack-locked to inflated.
1402 // Only that thread can complete inflation -- other threads must wait.
1403 // The INFLATING value is transient.
1404 // Currently, we spin/yield/park and poll the markword, waiting for inflation to finish.
1405 // We could always eliminate polling by parking the thread on some auxiliary list.
1406 if (mark == markOopDesc::INFLATING()) {
1407 TEVENT(Inflate: spin while INFLATING);
1408 ReadStableMark(object);
1409 continue;
1410 }
1411
1412 // CASE: stack-locked
1413 // Could be stack-locked either by this thread or by some other thread.
1414 //
1415 // Note that we allocate the objectmonitor speculatively, _before_ attempting
1416 // to install INFLATING into the mark word. We originally installed INFLATING,
1417 // allocated the objectmonitor, and then finally STed the address of the
1418 // objectmonitor into the mark. This was correct, but artificially lengthened
1419 // the interval in which INFLATED appeared in the mark, thus increasing
1420 // the odds of inflation contention.
1421 //
1422 // We now use per-thread private objectmonitor free lists.
1423 // These list are reprovisioned from the global free list outside the
1424 // critical INFLATING...ST interval. A thread can transfer
1425 // multiple objectmonitors en-mass from the global free list to its local free list.
1426 // This reduces coherency traffic and lock contention on the global free list.
1427 // Using such local free lists, it doesn't matter if the omAlloc() call appears
1428 // before or after the CAS(INFLATING) operation.
1429 // See the comments in omAlloc().
1430
1431 if (mark->has_locker()) {
1432 ObjectMonitor * m = omAlloc(Self);
1433 // Optimistically prepare the objectmonitor - anticipate successful CAS
1434 // We do this before the CAS in order to minimize the length of time
1435 // in which INFLATING appears in the mark.
1436 m->Recycle();
1437 m->_Responsible = NULL;
1438 m->_recursions = 0;
1439 m->_SpinDuration = ObjectMonitor::Knob_SpinLimit; // Consider: maintain by type/class
1440
1441 markOop cmp = (markOop) Atomic::cmpxchg_ptr(markOopDesc::INFLATING(), object->mark_addr(), mark);
1442 if (cmp != mark) {
1443 omRelease(Self, m, true);
1444 continue; // Interference -- just retry
1445 }
1446
1447 // We've successfully installed INFLATING (0) into the mark-word.
1448 // This is the only case where 0 will appear in a mark-word.
1449 // Only the singular thread that successfully swings the mark-word
1450 // to 0 can perform (or more precisely, complete) inflation.
1451 //
1452 // Why do we CAS a 0 into the mark-word instead of just CASing the
1453 // mark-word from the stack-locked value directly to the new inflated state?
1454 // Consider what happens when a thread unlocks a stack-locked object.
1455 // It attempts to use CAS to swing the displaced header value from the
1456 // on-stack basiclock back into the object header. Recall also that the
1457 // header value (hashcode, etc) can reside in (a) the object header, or
1458 // (b) a displaced header associated with the stack-lock, or (c) a displaced
1459 // header in an objectMonitor. The inflate() routine must copy the header
1460 // value from the basiclock on the owner's stack to the objectMonitor, all
1461 // the while preserving the hashCode stability invariants. If the owner
1462 // decides to release the lock while the value is 0, the unlock will fail
1463 // and control will eventually pass from slow_exit() to inflate. The owner
1464 // will then spin, waiting for the 0 value to disappear. Put another way,
1465 // the 0 causes the owner to stall if the owner happens to try to
1466 // drop the lock (restoring the header from the basiclock to the object)
1467 // while inflation is in-progress. This protocol avoids races that might
1468 // would otherwise permit hashCode values to change or "flicker" for an object.
1469 // Critically, while object->mark is 0 mark->displaced_mark_helper() is stable.
1470 // 0 serves as a "BUSY" inflate-in-progress indicator.
1471
1472
1473 // fetch the displaced mark from the owner's stack.
1474 // The owner can't die or unwind past the lock while our INFLATING
1475 // object is in the mark. Furthermore the owner can't complete
1476 // an unlock on the object, either.
1477 markOop dmw = mark->displaced_mark_helper();
1478 assert(dmw->is_neutral(), "invariant");
1479
1480 // Setup monitor fields to proper values -- prepare the monitor
1481 m->set_header(dmw);
1482
1483 // Optimization: if the mark->locker stack address is associated
1484 // with this thread we could simply set m->_owner = Self.
1485 // Note that a thread can inflate an object
1486 // that it has stack-locked -- as might happen in wait() -- directly
1487 // with CAS. That is, we can avoid the xchg-NULL .... ST idiom.
1488 m->set_owner(mark->locker());
1489 m->set_object(object);
1490 // TODO-FIXME: assert BasicLock->dhw != 0.
1491
1492 // Must preserve store ordering. The monitor state must
1493 // be stable at the time of publishing the monitor address.
1494 guarantee(object->mark() == markOopDesc::INFLATING(), "invariant");
1495 object->release_set_mark(markOopDesc::encode(m));
1496
1497 // Hopefully the performance counters are allocated on distinct cache lines
1498 // to avoid false sharing on MP systems ...
1499 OM_PERFDATA_OP(Inflations, inc());
1500 TEVENT(Inflate: overwrite stacklock);
1501 if (log_is_enabled(Debug, monitorinflation)) {
1502 if (object->is_instance()) {
1503 ResourceMark rm;
1504 log_debug(monitorinflation)("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1505 p2i(object), p2i(object->mark()),
1506 object->klass()->external_name());
1507 }
1508 }
1509 if (event.should_commit()) {
1510 post_monitor_inflate_event(event, object, cause);
1511 }
1512 return m;
1513 }
1514
1515 // CASE: neutral
1516 // TODO-FIXME: for entry we currently inflate and then try to CAS _owner.
1517 // If we know we're inflating for entry it's better to inflate by swinging a
1518 // pre-locked objectMonitor pointer into the object header. A successful
1519 // CAS inflates the object *and* confers ownership to the inflating thread.
1520 // In the current implementation we use a 2-step mechanism where we CAS()
1521 // to inflate and then CAS() again to try to swing _owner from NULL to Self.
1522 // An inflateTry() method that we could call from fast_enter() and slow_enter()
1523 // would be useful.
1524
1525 assert(mark->is_neutral(), "invariant");
1526 ObjectMonitor * m = omAlloc(Self);
1527 // prepare m for installation - set monitor to initial state
1528 m->Recycle();
1529 m->set_header(mark);
1530 m->set_owner(NULL);
1531 m->set_object(object);
1532 m->_recursions = 0;
1533 m->_Responsible = NULL;
1534 m->_SpinDuration = ObjectMonitor::Knob_SpinLimit; // consider: keep metastats by type/class
1535
1536 if (Atomic::cmpxchg_ptr (markOopDesc::encode(m), object->mark_addr(), mark) != mark) {
1537 m->set_object(NULL);
1538 m->set_owner(NULL);
1539 m->Recycle();
1540 omRelease(Self, m, true);
1541 m = NULL;
1542 continue;
1543 // interference - the markword changed - just retry.
1544 // The state-transitions are one-way, so there's no chance of
1545 // live-lock -- "Inflated" is an absorbing state.
1546 }
1547
1548 // Hopefully the performance counters are allocated on distinct
1549 // cache lines to avoid false sharing on MP systems ...
1550 OM_PERFDATA_OP(Inflations, inc());
1551 TEVENT(Inflate: overwrite neutral);
1552 if (log_is_enabled(Debug, monitorinflation)) {
1553 if (object->is_instance()) {
1554 ResourceMark rm;
1555 log_debug(monitorinflation)("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1556 p2i(object), p2i(object->mark()),
1557 object->klass()->external_name());
1558 }
1559 }
1560 if (event.should_commit()) {
1561 post_monitor_inflate_event(event, object, cause);
1562 }
1563 return m;
1564 }
1565 }
1566
1567
1568 // Deflate_idle_monitors() is called at all safepoints, immediately
1569 // after all mutators are stopped, but before any objects have moved.
1570 // It traverses the list of known monitors, deflating where possible.
1571 // The scavenged monitor are returned to the monitor free list.
1572 //
1573 // Beware that we scavenge at *every* stop-the-world point.
1574 // Having a large number of monitors in-circulation negatively
1575 // impacts the performance of some applications (e.g., PointBase).
1576 // Broadly, we want to minimize the # of monitors in circulation.
1577 //
1578 // We have added a flag, MonitorInUseLists, which creates a list
1579 // of active monitors for each thread. deflate_idle_monitors()
1580 // only scans the per-thread in-use lists. omAlloc() puts all
1581 // assigned monitors on the per-thread list. deflate_idle_monitors()
1582 // returns the non-busy monitors to the global free list.
1583 // When a thread dies, omFlush() adds the list of active monitors for
1584 // that thread to a global gOmInUseList acquiring the
1585 // global list lock. deflate_idle_monitors() acquires the global
1586 // list lock to scan for non-busy monitors to the global free list.
1587 // An alternative could have used a single global in-use list. The
1588 // downside would have been the additional cost of acquiring the global list lock
1589 // for every omAlloc().
1590 //
1591 // Perversely, the heap size -- and thus the STW safepoint rate --
1592 // typically drives the scavenge rate. Large heaps can mean infrequent GC,
1593 // which in turn can mean large(r) numbers of objectmonitors in circulation.
1594 // This is an unfortunate aspect of this design.
1595
1596 enum ManifestConstants {
1597 ClearResponsibleAtSTW = 0
1598 };
1599
1600 // Deflate a single monitor if not in-use
1601 // Return true if deflated, false if in-use
1602 bool ObjectSynchronizer::deflate_monitor(ObjectMonitor* mid, oop obj,
1603 ObjectMonitor** freeHeadp,
1604 ObjectMonitor** freeTailp) {
1605 bool deflated;
1606 // Normal case ... The monitor is associated with obj.
1607 guarantee(obj->mark() == markOopDesc::encode(mid), "invariant");
1608 guarantee(mid == obj->mark()->monitor(), "invariant");
1609 guarantee(mid->header()->is_neutral(), "invariant");
1610
1611 if (mid->is_busy()) {
1612 if (ClearResponsibleAtSTW) mid->_Responsible = NULL;
1613 deflated = false;
1614 } else {
1615 // Deflate the monitor if it is no longer being used
1616 // It's idle - scavenge and return to the global free list
1617 // plain old deflation ...
1618 TEVENT(deflate_idle_monitors - scavenge1);
1619 if (log_is_enabled(Debug, monitorinflation)) {
1620 if (obj->is_instance()) {
1621 ResourceMark rm;
1622 log_debug(monitorinflation)("Deflating object " INTPTR_FORMAT " , "
1623 "mark " INTPTR_FORMAT " , type %s",
1624 p2i(obj), p2i(obj->mark()),
1625 obj->klass()->external_name());
1626 }
1627 }
1628
1629 // Restore the header back to obj
1630 obj->release_set_mark(mid->header());
1631 mid->clear();
1632
1633 assert(mid->object() == NULL, "invariant");
1634
1635 // Move the object to the working free list defined by freeHeadp, freeTailp
1636 if (*freeHeadp == NULL) *freeHeadp = mid;
1637 if (*freeTailp != NULL) {
1638 ObjectMonitor * prevtail = *freeTailp;
1639 assert(prevtail->FreeNext == NULL, "cleaned up deflated?");
1640 prevtail->FreeNext = mid;
1641 }
1642 *freeTailp = mid;
1643 deflated = true;
1644 }
1645 return deflated;
1646 }
1647
1648 // Walk a given monitor list, and deflate idle monitors
1649 // The given list could be a per-thread list or a global list
1650 // Caller acquires gListLock
1651 int ObjectSynchronizer::deflate_monitor_list(ObjectMonitor** listHeadp,
1652 ObjectMonitor** freeHeadp,
1653 ObjectMonitor** freeTailp) {
1654 ObjectMonitor* mid;
1655 ObjectMonitor* next;
1656 ObjectMonitor* cur_mid_in_use = NULL;
1657 int deflated_count = 0;
1658
1659 for (mid = *listHeadp; mid != NULL;) {
1660 oop obj = (oop) mid->object();
1661 if (obj != NULL && deflate_monitor(mid, obj, freeHeadp, freeTailp)) {
1662 // if deflate_monitor succeeded,
1663 // extract from per-thread in-use list
1664 if (mid == *listHeadp) {
1665 *listHeadp = mid->FreeNext;
1666 } else if (cur_mid_in_use != NULL) {
1667 cur_mid_in_use->FreeNext = mid->FreeNext; // maintain the current thread in-use list
1668 }
1669 next = mid->FreeNext;
1670 mid->FreeNext = NULL; // This mid is current tail in the freeHeadp list
1671 mid = next;
1672 deflated_count++;
1673 } else {
1674 cur_mid_in_use = mid;
1675 mid = mid->FreeNext;
1676 }
1677 }
1678 return deflated_count;
1679 }
1680
1681 void ObjectSynchronizer::deflate_idle_monitors() {
1682 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1683 int nInuse = 0; // currently associated with objects
1684 int nInCirculation = 0; // extant
1685 int nScavenged = 0; // reclaimed
1686 bool deflated = false;
1687
1688 ObjectMonitor * freeHeadp = NULL; // Local SLL of scavenged monitors
1689 ObjectMonitor * freeTailp = NULL;
1690
1691 TEVENT(deflate_idle_monitors);
1692 // Prevent omFlush from changing mids in Thread dtor's during deflation
1693 // And in case the vm thread is acquiring a lock during a safepoint
1694 // See e.g. 6320749
1695 Thread::muxAcquire(&gListLock, "scavenge - return");
1696
1697 if (MonitorInUseLists) {
1698 int inUse = 0;
1699 for (JavaThread* cur = Threads::first(); cur != NULL; cur = cur->next()) {
1700 nInCirculation+= cur->omInUseCount;
1701 int deflated_count = deflate_monitor_list(cur->omInUseList_addr(), &freeHeadp, &freeTailp);
1702 cur->omInUseCount-= deflated_count;
1703 if (ObjectMonitor::Knob_VerifyInUse) {
1704 verifyInUse(cur);
1705 }
1706 nScavenged += deflated_count;
1707 nInuse += cur->omInUseCount;
1708 }
1709
1710 // For moribund threads, scan gOmInUseList
1711 if (gOmInUseList) {
1712 nInCirculation += gOmInUseCount;
1713 int deflated_count = deflate_monitor_list((ObjectMonitor **)&gOmInUseList, &freeHeadp, &freeTailp);
1714 gOmInUseCount-= deflated_count;
1715 nScavenged += deflated_count;
1716 nInuse += gOmInUseCount;
1717 }
1718
1719 } else {
1720 PaddedEnd<ObjectMonitor> * block =
1721 (PaddedEnd<ObjectMonitor> *)OrderAccess::load_ptr_acquire(&gBlockList);
1722 for (; block != NULL; block = (PaddedEnd<ObjectMonitor> *)next(block)) {
1723 // Iterate over all extant monitors - Scavenge all idle monitors.
1724 assert(block->object() == CHAINMARKER, "must be a block header");
1725 nInCirculation += _BLOCKSIZE;
1726 for (int i = 1; i < _BLOCKSIZE; i++) {
1727 ObjectMonitor* mid = (ObjectMonitor*)&block[i];
1728 oop obj = (oop)mid->object();
1729
1730 if (obj == NULL) {
1731 // The monitor is not associated with an object.
1732 // The monitor should either be a thread-specific private
1733 // free list or the global free list.
1734 // obj == NULL IMPLIES mid->is_busy() == 0
1735 guarantee(!mid->is_busy(), "invariant");
1736 continue;
1737 }
1738 deflated = deflate_monitor(mid, obj, &freeHeadp, &freeTailp);
1739
1740 if (deflated) {
1741 mid->FreeNext = NULL;
1742 nScavenged++;
1743 } else {
1744 nInuse++;
1745 }
1746 }
1747 }
1748 }
1749
1750 gMonitorFreeCount += nScavenged;
1751
1752 // Consider: audit gFreeList to ensure that gMonitorFreeCount and list agree.
1753
1754 if (ObjectMonitor::Knob_Verbose) {
1755 tty->print_cr("INFO: Deflate: InCirc=%d InUse=%d Scavenged=%d "
1756 "ForceMonitorScavenge=%d : pop=%d free=%d",
1757 nInCirculation, nInuse, nScavenged, ForceMonitorScavenge,
1758 gMonitorPopulation, gMonitorFreeCount);
1759 tty->flush();
1760 }
1761
1762 ForceMonitorScavenge = 0; // Reset
1763
1764 // Move the scavenged monitors back to the global free list.
1765 if (freeHeadp != NULL) {
1766 guarantee(freeTailp != NULL && nScavenged > 0, "invariant");
1767 assert(freeTailp->FreeNext == NULL, "invariant");
1768 // constant-time list splice - prepend scavenged segment to gFreeList
1769 freeTailp->FreeNext = gFreeList;
1770 gFreeList = freeHeadp;
1771 }
1772 Thread::muxRelease(&gListLock);
1773
1774 OM_PERFDATA_OP(Deflations, inc(nScavenged));
1775 OM_PERFDATA_OP(MonExtant, set_value(nInCirculation));
1776
1777 // TODO: Add objectMonitor leak detection.
1778 // Audit/inventory the objectMonitors -- make sure they're all accounted for.
1779 GVars.stwRandom = os::random();
1780 GVars.stwCycle++;
1781 }
1782
1783 // Monitor cleanup on JavaThread::exit
1784
1785 // Iterate through monitor cache and attempt to release thread's monitors
1786 // Gives up on a particular monitor if an exception occurs, but continues
1787 // the overall iteration, swallowing the exception.
1788 class ReleaseJavaMonitorsClosure: public MonitorClosure {
1789 private:
1790 TRAPS;
1791
1792 public:
1793 ReleaseJavaMonitorsClosure(Thread* thread) : THREAD(thread) {}
1794 void do_monitor(ObjectMonitor* mid) {
1795 if (mid->owner() == THREAD) {
1796 if (ObjectMonitor::Knob_VerifyMatch != 0) {
1797 ResourceMark rm;
1798 Handle obj(THREAD, (oop) mid->object());
1799 tty->print("INFO: unexpected locked object:");
1800 javaVFrame::print_locked_object_class_name(tty, obj, "locked");
1801 fatal("exiting JavaThread=" INTPTR_FORMAT
1802 " unexpectedly owns ObjectMonitor=" INTPTR_FORMAT,
1803 p2i(THREAD), p2i(mid));
1804 }
1805 (void)mid->complete_exit(CHECK);
1806 }
1807 }
1808 };
1809
1810 // Release all inflated monitors owned by THREAD. Lightweight monitors are
1811 // ignored. This is meant to be called during JNI thread detach which assumes
1812 // all remaining monitors are heavyweight. All exceptions are swallowed.
1813 // Scanning the extant monitor list can be time consuming.
1814 // A simple optimization is to add a per-thread flag that indicates a thread
1815 // called jni_monitorenter() during its lifetime.
1816 //
1817 // Instead of No_Savepoint_Verifier it might be cheaper to
1818 // use an idiom of the form:
1819 // auto int tmp = SafepointSynchronize::_safepoint_counter ;
1820 // <code that must not run at safepoint>
1821 // guarantee (((tmp ^ _safepoint_counter) | (tmp & 1)) == 0) ;
1822 // Since the tests are extremely cheap we could leave them enabled
1823 // for normal product builds.
1824
1825 void ObjectSynchronizer::release_monitors_owned_by_thread(TRAPS) {
1826 assert(THREAD == JavaThread::current(), "must be current Java thread");
1827 NoSafepointVerifier nsv;
1828 ReleaseJavaMonitorsClosure rjmc(THREAD);
1829 Thread::muxAcquire(&gListLock, "release_monitors_owned_by_thread");
1830 ObjectSynchronizer::monitors_iterate(&rjmc);
1831 Thread::muxRelease(&gListLock);
1832 THREAD->clear_pending_exception();
1833 }
1834
1835 const char* ObjectSynchronizer::inflate_cause_name(const InflateCause cause) {
1836 switch (cause) {
1837 case inflate_cause_vm_internal: return "VM Internal";
1838 case inflate_cause_monitor_enter: return "Monitor Enter";
1839 case inflate_cause_wait: return "Monitor Wait";
1840 case inflate_cause_notify: return "Monitor Notify";
1841 case inflate_cause_hash_code: return "Monitor Hash Code";
1842 case inflate_cause_jni_enter: return "JNI Monitor Enter";
1843 case inflate_cause_jni_exit: return "JNI Monitor Exit";
1844 default:
1845 ShouldNotReachHere();
1846 }
1847 return "Unknown";
1848 }
1849
1850 static void post_monitor_inflate_event(EventJavaMonitorInflate& event,
1851 const oop obj,
1852 const ObjectSynchronizer::InflateCause cause) {
1853 #if INCLUDE_TRACE
1854 assert(event.should_commit(), "check outside");
1855 event.set_monitorClass(obj->klass());
1856 event.set_address((TYPE_ADDRESS)(uintptr_t)(void*)obj);
1857 event.set_cause((u1)cause);
1858 event.commit();
1859 #endif
1860 }
1861
1862 //------------------------------------------------------------------------------
1863 // Debugging code
1864
1865 void ObjectSynchronizer::sanity_checks(const bool verbose,
1866 const uint cache_line_size,
1867 int *error_cnt_ptr,
1868 int *warning_cnt_ptr) {
1869 u_char *addr_begin = (u_char*)&GVars;
1870 u_char *addr_stwRandom = (u_char*)&GVars.stwRandom;
1871 u_char *addr_hcSequence = (u_char*)&GVars.hcSequence;
1872
1873 if (verbose) {
1874 tty->print_cr("INFO: sizeof(SharedGlobals)=" SIZE_FORMAT,
1875 sizeof(SharedGlobals));
1876 }
1877
1878 uint offset_stwRandom = (uint)(addr_stwRandom - addr_begin);
1879 if (verbose) tty->print_cr("INFO: offset(stwRandom)=%u", offset_stwRandom);
1880
1881 uint offset_hcSequence = (uint)(addr_hcSequence - addr_begin);
1882 if (verbose) {
1883 tty->print_cr("INFO: offset(_hcSequence)=%u", offset_hcSequence);
1884 }
1885
1886 if (cache_line_size != 0) {
1887 // We were able to determine the L1 data cache line size so
1888 // do some cache line specific sanity checks
1889
1890 if (offset_stwRandom < cache_line_size) {
1891 tty->print_cr("WARNING: the SharedGlobals.stwRandom field is closer "
1892 "to the struct beginning than a cache line which permits "
1893 "false sharing.");
1894 (*warning_cnt_ptr)++;
1895 }
1896
1897 if ((offset_hcSequence - offset_stwRandom) < cache_line_size) {
1898 tty->print_cr("WARNING: the SharedGlobals.stwRandom and "
1899 "SharedGlobals.hcSequence fields are closer than a cache "
1900 "line which permits false sharing.");
1901 (*warning_cnt_ptr)++;
1902 }
1903
1904 if ((sizeof(SharedGlobals) - offset_hcSequence) < cache_line_size) {
1905 tty->print_cr("WARNING: the SharedGlobals.hcSequence field is closer "
1906 "to the struct end than a cache line which permits false "
1907 "sharing.");
1908 (*warning_cnt_ptr)++;
1909 }
1910 }
1911 }
1912
1913 #ifndef PRODUCT
1914
1915 // Check if monitor belongs to the monitor cache
1916 // The list is grow-only so it's *relatively* safe to traverse
1917 // the list of extant blocks without taking a lock.
1918
1919 int ObjectSynchronizer::verify_objmon_isinpool(ObjectMonitor *monitor) {
1920 PaddedEnd<ObjectMonitor> * block =
1921 (PaddedEnd<ObjectMonitor> *)OrderAccess::load_ptr_acquire(&gBlockList);
1922 while (block != NULL) {
1923 assert(block->object() == CHAINMARKER, "must be a block header");
1924 if (monitor > (ObjectMonitor *)&block[0] &&
1925 monitor < (ObjectMonitor *)&block[_BLOCKSIZE]) {
1926 address mon = (address)monitor;
1927 address blk = (address)block;
1928 size_t diff = mon - blk;
1929 assert((diff % sizeof(PaddedEnd<ObjectMonitor>)) == 0, "must be aligned");
1930 return 1;
1931 }
1932 block = (PaddedEnd<ObjectMonitor> *)block->FreeNext;
1933 }
1934 return 0;
1935 }
1936
1937 #endif