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/align.hpp"
48 #include "utilities/dtrace.hpp"
49 #include "utilities/events.hpp"
50 #include "utilities/preserveException.hpp"
51
52 // The "core" versions of monitor enter and exit reside in this file.
53 // The interpreter and compilers contain specialized transliterated
54 // variants of the enter-exit fast-path operations. See i486.ad fast_lock(),
55 // for instance. If you make changes here, make sure to modify the
56 // interpreter, and both C1 and C2 fast-path inline locking code emission.
57 //
58 // -----------------------------------------------------------------------------
59
60 #ifdef DTRACE_ENABLED
61
62 // Only bother with this argument setup if dtrace is available
63 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
64
65 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
66 char* bytes = NULL; \
67 int len = 0; \
68 jlong jtid = SharedRuntime::get_java_tid(thread); \
69 Symbol* klassname = ((oop)(obj))->klass()->name(); \
70 if (klassname != NULL) { \
71 bytes = (char*)klassname->bytes(); \
72 len = klassname->utf8_length(); \
73 }
74
75 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
76 { \
77 if (DTraceMonitorProbes) { \
78 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
79 HOTSPOT_MONITOR_WAIT(jtid, \
80 (uintptr_t)(monitor), bytes, len, (millis)); \
81 } \
82 }
83
84 #define HOTSPOT_MONITOR_PROBE_notify HOTSPOT_MONITOR_NOTIFY
85 #define HOTSPOT_MONITOR_PROBE_notifyAll HOTSPOT_MONITOR_NOTIFYALL
86 #define HOTSPOT_MONITOR_PROBE_waited HOTSPOT_MONITOR_WAITED
87
88 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
89 { \
90 if (DTraceMonitorProbes) { \
91 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
92 HOTSPOT_MONITOR_PROBE_##probe(jtid, /* probe = waited */ \
93 (uintptr_t)(monitor), bytes, len); \
94 } \
95 }
96
97 #else // ndef DTRACE_ENABLED
98
99 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
100 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
101
102 #endif // ndef DTRACE_ENABLED
103
104 // This exists only as a workaround of dtrace bug 6254741
105 int dtrace_waited_probe(ObjectMonitor* monitor, Handle obj, Thread* thr) {
106 DTRACE_MONITOR_PROBE(waited, monitor, obj(), thr);
107 return 0;
108 }
109
110 #define NINFLATIONLOCKS 256
111 static volatile intptr_t gInflationLocks[NINFLATIONLOCKS];
112
113 // global list of blocks of monitors
114 // gBlockList is really PaddedEnd<ObjectMonitor> *, but we don't
115 // want to expose the PaddedEnd template more than necessary.
116 ObjectMonitor * volatile ObjectSynchronizer::gBlockList = NULL;
117 // global monitor free list
118 ObjectMonitor * volatile ObjectSynchronizer::gFreeList = NULL;
119 // global monitor in-use list, for moribund threads,
120 // monitors they inflated need to be scanned for deflation
121 ObjectMonitor * volatile ObjectSynchronizer::gOmInUseList = NULL;
122 // count of entries in gOmInUseList
123 int ObjectSynchronizer::gOmInUseCount = 0;
124
125 static volatile intptr_t gListLock = 0; // protects global monitor lists
126 static volatile int gMonitorFreeCount = 0; // # on gFreeList
127 static volatile int gMonitorPopulation = 0; // # Extant -- in circulation
128
129 static void post_monitor_inflate_event(EventJavaMonitorInflate&,
130 const oop,
131 const ObjectSynchronizer::InflateCause);
132
133 #define CHAINMARKER (cast_to_oop<intptr_t>(-1))
134
135
136 // =====================> Quick functions
137
138 // The quick_* forms are special fast-path variants used to improve
139 // performance. In the simplest case, a "quick_*" implementation could
140 // simply return false, in which case the caller will perform the necessary
141 // state transitions and call the slow-path form.
142 // The fast-path is designed to handle frequently arising cases in an efficient
143 // manner and is just a degenerate "optimistic" variant of the slow-path.
144 // returns true -- to indicate the call was satisfied.
145 // returns false -- to indicate the call needs the services of the slow-path.
146 // A no-loitering ordinance is in effect for code in the quick_* family
147 // operators: safepoints or indefinite blocking (blocking that might span a
148 // safepoint) are forbidden. Generally the thread_state() is _in_Java upon
149 // entry.
150 //
151 // Consider: An interesting optimization is to have the JIT recognize the
152 // following common idiom:
153 // synchronized (someobj) { .... ; notify(); }
154 // That is, we find a notify() or notifyAll() call that immediately precedes
155 // the monitorexit operation. In that case the JIT could fuse the operations
156 // into a single notifyAndExit() runtime primitive.
157
158 bool ObjectSynchronizer::quick_notify(oopDesc * obj, Thread * self, bool all) {
159 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
160 assert(self->is_Java_thread(), "invariant");
161 assert(((JavaThread *) self)->thread_state() == _thread_in_Java, "invariant");
162 NoSafepointVerifier nsv;
163 if (obj == NULL) return false; // slow-path for invalid obj
164 const markOop mark = obj->mark();
165
166 if (mark->has_locker() && self->is_lock_owned((address)mark->locker())) {
167 // Degenerate notify
168 // stack-locked by caller so by definition the implied waitset is empty.
169 return true;
170 }
171
172 if (mark->has_monitor()) {
173 ObjectMonitor * const mon = mark->monitor();
174 assert(mon->object() == obj, "invariant");
175 if (mon->owner() != self) return false; // slow-path for IMS exception
176
177 if (mon->first_waiter() != NULL) {
178 // We have one or more waiters. Since this is an inflated monitor
179 // that we own, we can transfer one or more threads from the waitset
180 // to the entrylist here and now, avoiding the slow-path.
181 if (all) {
182 DTRACE_MONITOR_PROBE(notifyAll, mon, obj, self);
183 } else {
184 DTRACE_MONITOR_PROBE(notify, mon, obj, self);
185 }
186 int tally = 0;
187 do {
188 mon->INotify(self);
189 ++tally;
190 } while (mon->first_waiter() != NULL && all);
191 OM_PERFDATA_OP(Notifications, inc(tally));
192 }
193 return true;
194 }
195
196 // biased locking and any other IMS exception states take the slow-path
197 return false;
198 }
199
200
201 // The LockNode emitted directly at the synchronization site would have
202 // been too big if it were to have included support for the cases of inflated
203 // recursive enter and exit, so they go here instead.
204 // Note that we can't safely call AsyncPrintJavaStack() from within
205 // quick_enter() as our thread state remains _in_Java.
206
207 bool ObjectSynchronizer::quick_enter(oop obj, Thread * Self,
208 BasicLock * lock) {
209 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
210 assert(Self->is_Java_thread(), "invariant");
211 assert(((JavaThread *) Self)->thread_state() == _thread_in_Java, "invariant");
212 NoSafepointVerifier nsv;
213 if (obj == NULL) return false; // Need to throw NPE
214 const markOop mark = obj->mark();
215
216 if (mark->has_monitor()) {
217 ObjectMonitor * const m = mark->monitor();
218 assert(m->object() == obj, "invariant");
219 Thread * const owner = (Thread *) m->_owner;
220
221 // Lock contention and Transactional Lock Elision (TLE) diagnostics
222 // and observability
223 // Case: light contention possibly amenable to TLE
224 // Case: TLE inimical operations such as nested/recursive synchronization
225
226 if (owner == Self) {
227 m->_recursions++;
228 return true;
229 }
230
231 // This Java Monitor is inflated so obj's header will never be
232 // displaced to this thread's BasicLock. Make the displaced header
233 // non-NULL so this BasicLock is not seen as recursive nor as
234 // being locked. We do this unconditionally so that this thread's
235 // BasicLock cannot be mis-interpreted by any stack walkers. For
236 // performance reasons, stack walkers generally first check for
237 // Biased Locking in the object's header, the second check is for
238 // stack-locking in the object's header, the third check is for
239 // recursive stack-locking in the displaced header in the BasicLock,
240 // and last are the inflated Java Monitor (ObjectMonitor) checks.
241 lock->set_displaced_header(markOopDesc::unused_mark());
242
243 if (owner == NULL &&
244 Atomic::cmpxchg_ptr(Self, &(m->_owner), NULL) == NULL) {
245 assert(m->_recursions == 0, "invariant");
246 assert(m->_owner == Self, "invariant");
247 return true;
248 }
249 }
250
251 // Note that we could inflate in quick_enter.
252 // This is likely a useful optimization
253 // Critically, in quick_enter() we must not:
254 // -- perform bias revocation, or
255 // -- block indefinitely, or
256 // -- reach a safepoint
257
258 return false; // revert to slow-path
259 }
260
261 // -----------------------------------------------------------------------------
262 // Fast Monitor Enter/Exit
263 // This the fast monitor enter. The interpreter and compiler use
264 // some assembly copies of this code. Make sure update those code
265 // if the following function is changed. The implementation is
266 // extremely sensitive to race condition. Be careful.
267
268 void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock,
269 bool attempt_rebias, TRAPS) {
270 if (UseBiasedLocking) {
271 if (!SafepointSynchronize::is_at_safepoint()) {
272 BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD);
273 if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) {
274 return;
275 }
276 } else {
277 assert(!attempt_rebias, "can not rebias toward VM thread");
278 BiasedLocking::revoke_at_safepoint(obj);
279 }
280 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
281 }
282
283 slow_enter(obj, lock, THREAD);
284 }
285
286 void ObjectSynchronizer::fast_exit(oop object, BasicLock* lock, TRAPS) {
287 markOop mark = object->mark();
288 // We cannot check for Biased Locking if we are racing an inflation.
289 assert(mark == markOopDesc::INFLATING() ||
290 !mark->has_bias_pattern(), "should not see bias pattern here");
291
292 markOop dhw = lock->displaced_header();
293 if (dhw == NULL) {
294 // If the displaced header is NULL, then this exit matches up with
295 // a recursive enter. No real work to do here except for diagnostics.
296 #ifndef PRODUCT
297 if (mark != markOopDesc::INFLATING()) {
298 // Only do diagnostics if we are not racing an inflation. Simply
299 // exiting a recursive enter of a Java Monitor that is being
300 // inflated is safe; see the has_monitor() comment below.
301 assert(!mark->is_neutral(), "invariant");
302 assert(!mark->has_locker() ||
303 THREAD->is_lock_owned((address)mark->locker()), "invariant");
304 if (mark->has_monitor()) {
305 // The BasicLock's displaced_header is marked as a recursive
306 // enter and we have an inflated Java Monitor (ObjectMonitor).
307 // This is a special case where the Java Monitor was inflated
308 // after this thread entered the stack-lock recursively. When a
309 // Java Monitor is inflated, we cannot safely walk the Java
310 // Monitor owner's stack and update the BasicLocks because a
311 // Java Monitor can be asynchronously inflated by a thread that
312 // does not own the Java Monitor.
313 ObjectMonitor * m = mark->monitor();
314 assert(((oop)(m->object()))->mark() == mark, "invariant");
315 assert(m->is_entered(THREAD), "invariant");
316 }
317 }
318 #endif
319 return;
320 }
321
322 if (mark == (markOop) lock) {
323 // If the object is stack-locked by the current thread, try to
324 // swing the displaced header from the BasicLock back to the mark.
325 assert(dhw->is_neutral(), "invariant");
326 if ((markOop) Atomic::cmpxchg_ptr(dhw, object->mark_addr(), mark) == mark) {
327 TEVENT(fast_exit: release stack-lock);
328 return;
329 }
330 }
331
332 // We have to take the slow-path of possible inflation and then exit.
333 ObjectSynchronizer::inflate(THREAD,
334 object,
335 inflate_cause_vm_internal)->exit(true, THREAD);
336 }
337
338 // -----------------------------------------------------------------------------
339 // Interpreter/Compiler Slow Case
340 // This routine is used to handle interpreter/compiler slow case
341 // We don't need to use fast path here, because it must have been
342 // failed in the interpreter/compiler code.
343 void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) {
344 markOop mark = obj->mark();
345 assert(!mark->has_bias_pattern(), "should not see bias pattern here");
346
347 if (mark->is_neutral()) {
348 // Anticipate successful CAS -- the ST of the displaced mark must
349 // be visible <= the ST performed by the CAS.
350 lock->set_displaced_header(mark);
351 if (mark == (markOop) Atomic::cmpxchg_ptr(lock, obj()->mark_addr(), mark)) {
352 TEVENT(slow_enter: release stacklock);
353 return;
354 }
355 // Fall through to inflate() ...
356 } else if (mark->has_locker() &&
357 THREAD->is_lock_owned((address)mark->locker())) {
358 assert(lock != mark->locker(), "must not re-lock the same lock");
359 assert(lock != (BasicLock*)obj->mark(), "don't relock with same BasicLock");
360 lock->set_displaced_header(NULL);
361 return;
362 }
363
364 // The object header will never be displaced to this lock,
365 // so it does not matter what the value is, except that it
366 // must be non-zero to avoid looking like a re-entrant lock,
367 // and must not look locked either.
368 lock->set_displaced_header(markOopDesc::unused_mark());
369 ObjectSynchronizer::inflate(THREAD,
370 obj(),
371 inflate_cause_monitor_enter)->enter(THREAD);
372 }
373
374 // This routine is used to handle interpreter/compiler slow case
375 // We don't need to use fast path here, because it must have
376 // failed in the interpreter/compiler code. Simply use the heavy
377 // weight monitor should be ok, unless someone find otherwise.
378 void ObjectSynchronizer::slow_exit(oop object, BasicLock* lock, TRAPS) {
379 fast_exit(object, lock, THREAD);
380 }
381
382 // -----------------------------------------------------------------------------
383 // Class Loader support to workaround deadlocks on the class loader lock objects
384 // Also used by GC
385 // complete_exit()/reenter() are used to wait on a nested lock
386 // i.e. to give up an outer lock completely and then re-enter
387 // Used when holding nested locks - lock acquisition order: lock1 then lock2
388 // 1) complete_exit lock1 - saving recursion count
389 // 2) wait on lock2
390 // 3) when notified on lock2, unlock lock2
391 // 4) reenter lock1 with original recursion count
392 // 5) lock lock2
393 // NOTE: must use heavy weight monitor to handle complete_exit/reenter()
394 intptr_t ObjectSynchronizer::complete_exit(Handle obj, TRAPS) {
395 TEVENT(complete_exit);
396 if (UseBiasedLocking) {
397 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
398 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
399 }
400
401 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD,
402 obj(),
403 inflate_cause_vm_internal);
404
405 return monitor->complete_exit(THREAD);
406 }
407
408 // NOTE: must use heavy weight monitor to handle complete_exit/reenter()
409 void ObjectSynchronizer::reenter(Handle obj, intptr_t recursion, TRAPS) {
410 TEVENT(reenter);
411 if (UseBiasedLocking) {
412 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
413 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
414 }
415
416 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD,
417 obj(),
418 inflate_cause_vm_internal);
419
420 monitor->reenter(recursion, THREAD);
421 }
422 // -----------------------------------------------------------------------------
423 // JNI locks on java objects
424 // NOTE: must use heavy weight monitor to handle jni monitor enter
425 void ObjectSynchronizer::jni_enter(Handle obj, TRAPS) {
426 // the current locking is from JNI instead of Java code
427 TEVENT(jni_enter);
428 if (UseBiasedLocking) {
429 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
430 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
431 }
432 THREAD->set_current_pending_monitor_is_from_java(false);
433 ObjectSynchronizer::inflate(THREAD, obj(), inflate_cause_jni_enter)->enter(THREAD);
434 THREAD->set_current_pending_monitor_is_from_java(true);
435 }
436
437 // NOTE: must use heavy weight monitor to handle jni monitor exit
438 void ObjectSynchronizer::jni_exit(oop obj, Thread* THREAD) {
439 TEVENT(jni_exit);
440 if (UseBiasedLocking) {
441 Handle h_obj(THREAD, obj);
442 BiasedLocking::revoke_and_rebias(h_obj, false, THREAD);
443 obj = h_obj();
444 }
445 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
446
447 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD,
448 obj,
449 inflate_cause_jni_exit);
450 // If this thread has locked the object, exit the monitor. Note: can't use
451 // monitor->check(CHECK); must exit even if an exception is pending.
452 if (monitor->check(THREAD)) {
453 monitor->exit(true, THREAD);
454 }
455 }
456
457 // -----------------------------------------------------------------------------
458 // Internal VM locks on java objects
459 // standard constructor, allows locking failures
460 ObjectLocker::ObjectLocker(Handle obj, Thread* thread, bool doLock) {
461 _dolock = doLock;
462 _thread = thread;
463 debug_only(if (StrictSafepointChecks) _thread->check_for_valid_safepoint_state(false);)
464 _obj = obj;
465
466 if (_dolock) {
467 TEVENT(ObjectLocker);
468
469 ObjectSynchronizer::fast_enter(_obj, &_lock, false, _thread);
470 }
471 }
472
473 ObjectLocker::~ObjectLocker() {
474 if (_dolock) {
475 ObjectSynchronizer::fast_exit(_obj(), &_lock, _thread);
476 }
477 }
478
479
480 // -----------------------------------------------------------------------------
481 // Wait/Notify/NotifyAll
482 // NOTE: must use heavy weight monitor to handle wait()
483 int ObjectSynchronizer::wait(Handle obj, jlong millis, TRAPS) {
484 if (UseBiasedLocking) {
485 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
486 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
487 }
488 if (millis < 0) {
489 TEVENT(wait - throw IAX);
490 THROW_MSG_0(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
491 }
492 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD,
493 obj(),
494 inflate_cause_wait);
495
496 DTRACE_MONITOR_WAIT_PROBE(monitor, obj(), THREAD, millis);
497 monitor->wait(millis, true, THREAD);
498
499 // This dummy call is in place to get around dtrace bug 6254741. Once
500 // that's fixed we can uncomment the following line, remove the call
501 // and change this function back into a "void" func.
502 // DTRACE_MONITOR_PROBE(waited, monitor, obj(), THREAD);
503 return dtrace_waited_probe(monitor, obj, THREAD);
504 }
505
506 void ObjectSynchronizer::waitUninterruptibly(Handle obj, jlong millis, TRAPS) {
507 if (UseBiasedLocking) {
508 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
509 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
510 }
511 if (millis < 0) {
512 TEVENT(wait - throw IAX);
513 THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
514 }
515 ObjectSynchronizer::inflate(THREAD,
516 obj(),
517 inflate_cause_wait)->wait(millis, false, THREAD);
518 }
519
520 void ObjectSynchronizer::notify(Handle obj, TRAPS) {
521 if (UseBiasedLocking) {
522 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
523 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
524 }
525
526 markOop mark = obj->mark();
527 if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
528 return;
529 }
530 ObjectSynchronizer::inflate(THREAD,
531 obj(),
532 inflate_cause_notify)->notify(THREAD);
533 }
534
535 // NOTE: see comment of notify()
536 void ObjectSynchronizer::notifyall(Handle obj, TRAPS) {
537 if (UseBiasedLocking) {
538 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
539 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
540 }
541
542 markOop mark = obj->mark();
543 if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
544 return;
545 }
546 ObjectSynchronizer::inflate(THREAD,
547 obj(),
548 inflate_cause_notify)->notifyAll(THREAD);
549 }
550
551 // -----------------------------------------------------------------------------
552 // Hash Code handling
553 //
554 // Performance concern:
555 // OrderAccess::storestore() calls release() which at one time stored 0
556 // into the global volatile OrderAccess::dummy variable. This store was
557 // unnecessary for correctness. Many threads storing into a common location
558 // causes considerable cache migration or "sloshing" on large SMP systems.
559 // As such, I avoided using OrderAccess::storestore(). In some cases
560 // OrderAccess::fence() -- which incurs local latency on the executing
561 // processor -- is a better choice as it scales on SMP systems.
562 //
563 // See http://blogs.oracle.com/dave/entry/biased_locking_in_hotspot for
564 // a discussion of coherency costs. Note that all our current reference
565 // platforms provide strong ST-ST order, so the issue is moot on IA32,
566 // x64, and SPARC.
567 //
568 // As a general policy we use "volatile" to control compiler-based reordering
569 // and explicit fences (barriers) to control for architectural reordering
570 // performed by the CPU(s) or platform.
571
572 struct SharedGlobals {
573 char _pad_prefix[DEFAULT_CACHE_LINE_SIZE];
574 // These are highly shared mostly-read variables.
575 // To avoid false-sharing they need to be the sole occupants of a cache line.
576 volatile int stwRandom;
577 volatile int stwCycle;
578 DEFINE_PAD_MINUS_SIZE(1, DEFAULT_CACHE_LINE_SIZE, sizeof(volatile int) * 2);
579 // Hot RW variable -- Sequester to avoid false-sharing
580 volatile int hcSequence;
581 DEFINE_PAD_MINUS_SIZE(2, DEFAULT_CACHE_LINE_SIZE, sizeof(volatile int));
582 };
583
584 static SharedGlobals GVars;
585 static int MonitorScavengeThreshold = 1000000;
586 static volatile int ForceMonitorScavenge = 0; // Scavenge required and pending
587
588 static markOop ReadStableMark(oop obj) {
589 markOop mark = obj->mark();
590 if (!mark->is_being_inflated()) {
591 return mark; // normal fast-path return
592 }
593
594 int its = 0;
595 for (;;) {
596 markOop mark = obj->mark();
597 if (!mark->is_being_inflated()) {
598 return mark; // normal fast-path return
599 }
600
601 // The object is being inflated by some other thread.
602 // The caller of ReadStableMark() must wait for inflation to complete.
603 // Avoid live-lock
604 // TODO: consider calling SafepointSynchronize::do_call_back() while
605 // spinning to see if there's a safepoint pending. If so, immediately
606 // yielding or blocking would be appropriate. Avoid spinning while
607 // there is a safepoint pending.
608 // TODO: add inflation contention performance counters.
609 // TODO: restrict the aggregate number of spinners.
610
611 ++its;
612 if (its > 10000 || !os::is_MP()) {
613 if (its & 1) {
614 os::naked_yield();
615 TEVENT(Inflate: INFLATING - yield);
616 } else {
617 // Note that the following code attenuates the livelock problem but is not
618 // a complete remedy. A more complete solution would require that the inflating
619 // thread hold the associated inflation lock. The following code simply restricts
620 // the number of spinners to at most one. We'll have N-2 threads blocked
621 // on the inflationlock, 1 thread holding the inflation lock and using
622 // a yield/park strategy, and 1 thread in the midst of inflation.
623 // A more refined approach would be to change the encoding of INFLATING
624 // to allow encapsulation of a native thread pointer. Threads waiting for
625 // inflation to complete would use CAS to push themselves onto a singly linked
626 // list rooted at the markword. Once enqueued, they'd loop, checking a per-thread flag
627 // and calling park(). When inflation was complete the thread that accomplished inflation
628 // would detach the list and set the markword to inflated with a single CAS and
629 // then for each thread on the list, set the flag and unpark() the thread.
630 // This is conceptually similar to muxAcquire-muxRelease, except that muxRelease
631 // wakes at most one thread whereas we need to wake the entire list.
632 int ix = (cast_from_oop<intptr_t>(obj) >> 5) & (NINFLATIONLOCKS-1);
633 int YieldThenBlock = 0;
634 assert(ix >= 0 && ix < NINFLATIONLOCKS, "invariant");
635 assert((NINFLATIONLOCKS & (NINFLATIONLOCKS-1)) == 0, "invariant");
636 Thread::muxAcquire(gInflationLocks + ix, "gInflationLock");
637 while (obj->mark() == markOopDesc::INFLATING()) {
638 // Beware: NakedYield() is advisory and has almost no effect on some platforms
639 // so we periodically call Self->_ParkEvent->park(1).
640 // We use a mixed spin/yield/block mechanism.
641 if ((YieldThenBlock++) >= 16) {
642 Thread::current()->_ParkEvent->park(1);
643 } else {
644 os::naked_yield();
645 }
646 }
647 Thread::muxRelease(gInflationLocks + ix);
648 TEVENT(Inflate: INFLATING - yield/park);
649 }
650 } else {
651 SpinPause(); // SMP-polite spinning
652 }
653 }
654 }
655
656 // hashCode() generation :
657 //
658 // Possibilities:
659 // * MD5Digest of {obj,stwRandom}
660 // * CRC32 of {obj,stwRandom} or any linear-feedback shift register function.
661 // * A DES- or AES-style SBox[] mechanism
662 // * One of the Phi-based schemes, such as:
663 // 2654435761 = 2^32 * Phi (golden ratio)
664 // HashCodeValue = ((uintptr_t(obj) >> 3) * 2654435761) ^ GVars.stwRandom ;
665 // * A variation of Marsaglia's shift-xor RNG scheme.
666 // * (obj ^ stwRandom) is appealing, but can result
667 // in undesirable regularity in the hashCode values of adjacent objects
668 // (objects allocated back-to-back, in particular). This could potentially
669 // result in hashtable collisions and reduced hashtable efficiency.
670 // There are simple ways to "diffuse" the middle address bits over the
671 // generated hashCode values:
672
673 static inline intptr_t get_next_hash(Thread * Self, oop obj) {
674 intptr_t value = 0;
675 if (hashCode == 0) {
676 // This form uses global Park-Miller 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 static bool monitors_used_above_threshold() {
966 if (gMonitorPopulation == 0) {
967 return false;
968 }
969 int monitors_used = gMonitorPopulation - gMonitorFreeCount;
970 int monitor_usage = (monitors_used * 100LL) / gMonitorPopulation;
971 return monitor_usage > MonitorUsedDeflationThreshold;
972 }
973
974 bool ObjectSynchronizer::is_cleanup_needed() {
975 if (MonitorUsedDeflationThreshold > 0) {
976 return monitors_used_above_threshold();
977 }
978 return false;
979 }
980
981 void ObjectSynchronizer::oops_do(OopClosure* f) {
982 if (MonitorInUseLists) {
983 // When using thread local monitor lists, we only scan the
984 // global used list here (for moribund threads), and
985 // the thread-local monitors in Thread::oops_do().
986 global_used_oops_do(f);
987 } else {
988 global_oops_do(f);
989 }
990 }
991
992 void ObjectSynchronizer::global_oops_do(OopClosure* f) {
993 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
994 PaddedEnd<ObjectMonitor> * block =
995 (PaddedEnd<ObjectMonitor> *)OrderAccess::load_ptr_acquire(&gBlockList);
996 for (; block != NULL; block = (PaddedEnd<ObjectMonitor> *)next(block)) {
997 assert(block->object() == CHAINMARKER, "must be a block header");
998 for (int i = 1; i < _BLOCKSIZE; i++) {
999 ObjectMonitor* mid = (ObjectMonitor *)&block[i];
1000 if (mid->object() != NULL) {
1001 f->do_oop((oop*)mid->object_addr());
1002 }
1003 }
1004 }
1005 }
1006
1007 void ObjectSynchronizer::global_used_oops_do(OopClosure* f) {
1008 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1009 list_oops_do(gOmInUseList, f);
1010 }
1011
1012 void ObjectSynchronizer::thread_local_used_oops_do(Thread* thread, OopClosure* f) {
1013 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1014 list_oops_do(thread->omInUseList, f);
1015 }
1016
1017 void ObjectSynchronizer::list_oops_do(ObjectMonitor* list, OopClosure* f) {
1018 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1019 ObjectMonitor* mid;
1020 for (mid = list; mid != NULL; mid = mid->FreeNext) {
1021 if (mid->object() != NULL) {
1022 f->do_oop((oop*)mid->object_addr());
1023 }
1024 }
1025 }
1026
1027
1028 // -----------------------------------------------------------------------------
1029 // ObjectMonitor Lifecycle
1030 // -----------------------
1031 // Inflation unlinks monitors from the global gFreeList and
1032 // associates them with objects. Deflation -- which occurs at
1033 // STW-time -- disassociates idle monitors from objects. Such
1034 // scavenged monitors are returned to the gFreeList.
1035 //
1036 // The global list is protected by gListLock. All the critical sections
1037 // are short and operate in constant-time.
1038 //
1039 // ObjectMonitors reside in type-stable memory (TSM) and are immortal.
1040 //
1041 // Lifecycle:
1042 // -- unassigned and on the global free list
1043 // -- unassigned and on a thread's private omFreeList
1044 // -- assigned to an object. The object is inflated and the mark refers
1045 // to the objectmonitor.
1046
1047
1048 // Constraining monitor pool growth via MonitorBound ...
1049 //
1050 // The monitor pool is grow-only. We scavenge at STW safepoint-time, but the
1051 // the rate of scavenging is driven primarily by GC. As such, we can find
1052 // an inordinate number of monitors in circulation.
1053 // To avoid that scenario we can artificially induce a STW safepoint
1054 // if the pool appears to be growing past some reasonable bound.
1055 // Generally we favor time in space-time tradeoffs, but as there's no
1056 // natural back-pressure on the # of extant monitors we need to impose some
1057 // type of limit. Beware that if MonitorBound is set to too low a value
1058 // we could just loop. In addition, if MonitorBound is set to a low value
1059 // we'll incur more safepoints, which are harmful to performance.
1060 // See also: GuaranteedSafepointInterval
1061 //
1062 // The current implementation uses asynchronous VM operations.
1063
1064 static void InduceScavenge(Thread * Self, const char * Whence) {
1065 // Induce STW safepoint to trim monitors
1066 // Ultimately, this results in a call to deflate_idle_monitors() in the near future.
1067 // More precisely, trigger an asynchronous STW safepoint as the number
1068 // of active monitors passes the specified threshold.
1069 // TODO: assert thread state is reasonable
1070
1071 if (ForceMonitorScavenge == 0 && Atomic::xchg (1, &ForceMonitorScavenge) == 0) {
1072 if (ObjectMonitor::Knob_Verbose) {
1073 tty->print_cr("INFO: Monitor scavenge - Induced STW @%s (%d)",
1074 Whence, ForceMonitorScavenge) ;
1075 tty->flush();
1076 }
1077 // Induce a 'null' safepoint to scavenge monitors
1078 // Must VM_Operation instance be heap allocated as the op will be enqueue and posted
1079 // to the VMthread and have a lifespan longer than that of this activation record.
1080 // The VMThread will delete the op when completed.
1081 VMThread::execute(new VM_ScavengeMonitors());
1082
1083 if (ObjectMonitor::Knob_Verbose) {
1084 tty->print_cr("INFO: Monitor scavenge - STW posted @%s (%d)",
1085 Whence, ForceMonitorScavenge) ;
1086 tty->flush();
1087 }
1088 }
1089 }
1090
1091 void ObjectSynchronizer::verifyInUse(Thread *Self) {
1092 ObjectMonitor* mid;
1093 int in_use_tally = 0;
1094 for (mid = Self->omInUseList; mid != NULL; mid = mid->FreeNext) {
1095 in_use_tally++;
1096 }
1097 assert(in_use_tally == Self->omInUseCount, "in-use count off");
1098
1099 int free_tally = 0;
1100 for (mid = Self->omFreeList; mid != NULL; mid = mid->FreeNext) {
1101 free_tally++;
1102 }
1103 assert(free_tally == Self->omFreeCount, "free count off");
1104 }
1105
1106 ObjectMonitor* ObjectSynchronizer::omAlloc(Thread * Self) {
1107 // A large MAXPRIVATE value reduces both list lock contention
1108 // and list coherency traffic, but also tends to increase the
1109 // number of objectMonitors in circulation as well as the STW
1110 // scavenge costs. As usual, we lean toward time in space-time
1111 // tradeoffs.
1112 const int MAXPRIVATE = 1024;
1113 for (;;) {
1114 ObjectMonitor * m;
1115
1116 // 1: try to allocate from the thread's local omFreeList.
1117 // Threads will attempt to allocate first from their local list, then
1118 // from the global list, and only after those attempts fail will the thread
1119 // attempt to instantiate new monitors. Thread-local free lists take
1120 // heat off the gListLock and improve allocation latency, as well as reducing
1121 // coherency traffic on the shared global list.
1122 m = Self->omFreeList;
1123 if (m != NULL) {
1124 Self->omFreeList = m->FreeNext;
1125 Self->omFreeCount--;
1126 // CONSIDER: set m->FreeNext = BAD -- diagnostic hygiene
1127 guarantee(m->object() == NULL, "invariant");
1128 if (MonitorInUseLists) {
1129 m->FreeNext = Self->omInUseList;
1130 Self->omInUseList = m;
1131 Self->omInUseCount++;
1132 if (ObjectMonitor::Knob_VerifyInUse) {
1133 verifyInUse(Self);
1134 }
1135 } else {
1136 m->FreeNext = NULL;
1137 }
1138 return m;
1139 }
1140
1141 // 2: try to allocate from the global gFreeList
1142 // CONSIDER: use muxTry() instead of muxAcquire().
1143 // If the muxTry() fails then drop immediately into case 3.
1144 // If we're using thread-local free lists then try
1145 // to reprovision the caller's free list.
1146 if (gFreeList != NULL) {
1147 // Reprovision the thread's omFreeList.
1148 // Use bulk transfers to reduce the allocation rate and heat
1149 // on various locks.
1150 Thread::muxAcquire(&gListLock, "omAlloc");
1151 for (int i = Self->omFreeProvision; --i >= 0 && gFreeList != NULL;) {
1152 gMonitorFreeCount--;
1153 ObjectMonitor * take = gFreeList;
1154 gFreeList = take->FreeNext;
1155 guarantee(take->object() == NULL, "invariant");
1156 guarantee(!take->is_busy(), "invariant");
1157 take->Recycle();
1158 omRelease(Self, take, false);
1159 }
1160 Thread::muxRelease(&gListLock);
1161 Self->omFreeProvision += 1 + (Self->omFreeProvision/2);
1162 if (Self->omFreeProvision > MAXPRIVATE) Self->omFreeProvision = MAXPRIVATE;
1163 TEVENT(omFirst - reprovision);
1164
1165 const int mx = MonitorBound;
1166 if (mx > 0 && (gMonitorPopulation-gMonitorFreeCount) > mx) {
1167 // We can't safely induce a STW safepoint from omAlloc() as our thread
1168 // state may not be appropriate for such activities and callers may hold
1169 // naked oops, so instead we defer the action.
1170 InduceScavenge(Self, "omAlloc");
1171 }
1172 continue;
1173 }
1174
1175 // 3: allocate a block of new ObjectMonitors
1176 // Both the local and global free lists are empty -- resort to malloc().
1177 // In the current implementation objectMonitors are TSM - immortal.
1178 // Ideally, we'd write "new ObjectMonitor[_BLOCKSIZE], but we want
1179 // each ObjectMonitor to start at the beginning of a cache line,
1180 // so we use align_up().
1181 // A better solution would be to use C++ placement-new.
1182 // BEWARE: As it stands currently, we don't run the ctors!
1183 assert(_BLOCKSIZE > 1, "invariant");
1184 size_t neededsize = sizeof(PaddedEnd<ObjectMonitor>) * _BLOCKSIZE;
1185 PaddedEnd<ObjectMonitor> * temp;
1186 size_t aligned_size = neededsize + (DEFAULT_CACHE_LINE_SIZE - 1);
1187 void* real_malloc_addr = (void *)NEW_C_HEAP_ARRAY(char, aligned_size,
1188 mtInternal);
1189 temp = (PaddedEnd<ObjectMonitor> *)
1190 align_up(real_malloc_addr, DEFAULT_CACHE_LINE_SIZE);
1191
1192 // NOTE: (almost) no way to recover if allocation failed.
1193 // We might be able to induce a STW safepoint and scavenge enough
1194 // objectMonitors to permit progress.
1195 if (temp == NULL) {
1196 vm_exit_out_of_memory(neededsize, OOM_MALLOC_ERROR,
1197 "Allocate ObjectMonitors");
1198 }
1199 (void)memset((void *) temp, 0, neededsize);
1200
1201 // Format the block.
1202 // initialize the linked list, each monitor points to its next
1203 // forming the single linked free list, the very first monitor
1204 // will points to next block, which forms the block list.
1205 // The trick of using the 1st element in the block as gBlockList
1206 // linkage should be reconsidered. A better implementation would
1207 // look like: class Block { Block * next; int N; ObjectMonitor Body [N] ; }
1208
1209 for (int i = 1; i < _BLOCKSIZE; i++) {
1210 temp[i].FreeNext = (ObjectMonitor *)&temp[i+1];
1211 }
1212
1213 // terminate the last monitor as the end of list
1214 temp[_BLOCKSIZE - 1].FreeNext = NULL;
1215
1216 // Element [0] is reserved for global list linkage
1217 temp[0].set_object(CHAINMARKER);
1218
1219 // Consider carving out this thread's current request from the
1220 // block in hand. This avoids some lock traffic and redundant
1221 // list activity.
1222
1223 // Acquire the gListLock to manipulate gBlockList and gFreeList.
1224 // An Oyama-Taura-Yonezawa scheme might be more efficient.
1225 Thread::muxAcquire(&gListLock, "omAlloc [2]");
1226 gMonitorPopulation += _BLOCKSIZE-1;
1227 gMonitorFreeCount += _BLOCKSIZE-1;
1228
1229 // Add the new block to the list of extant blocks (gBlockList).
1230 // The very first objectMonitor in a block is reserved and dedicated.
1231 // It serves as blocklist "next" linkage.
1232 temp[0].FreeNext = gBlockList;
1233 // There are lock-free uses of gBlockList so make sure that
1234 // the previous stores happen before we update gBlockList.
1235 OrderAccess::release_store_ptr(&gBlockList, temp);
1236
1237 // Add the new string of objectMonitors to the global free list
1238 temp[_BLOCKSIZE - 1].FreeNext = gFreeList;
1239 gFreeList = temp + 1;
1240 Thread::muxRelease(&gListLock);
1241 TEVENT(Allocate block of monitors);
1242 }
1243 }
1244
1245 // Place "m" on the caller's private per-thread omFreeList.
1246 // In practice there's no need to clamp or limit the number of
1247 // monitors on a thread's omFreeList as the only time we'll call
1248 // omRelease is to return a monitor to the free list after a CAS
1249 // attempt failed. This doesn't allow unbounded #s of monitors to
1250 // accumulate on a thread's free list.
1251 //
1252 // Key constraint: all ObjectMonitors on a thread's free list and the global
1253 // free list must have their object field set to null. This prevents the
1254 // scavenger -- deflate_idle_monitors -- from reclaiming them.
1255
1256 void ObjectSynchronizer::omRelease(Thread * Self, ObjectMonitor * m,
1257 bool fromPerThreadAlloc) {
1258 guarantee(m->object() == NULL, "invariant");
1259 guarantee(((m->is_busy()|m->_recursions) == 0), "freeing in-use monitor");
1260 // Remove from omInUseList
1261 if (MonitorInUseLists && fromPerThreadAlloc) {
1262 ObjectMonitor* cur_mid_in_use = NULL;
1263 bool extracted = false;
1264 for (ObjectMonitor* mid = Self->omInUseList; mid != NULL; cur_mid_in_use = mid, mid = mid->FreeNext) {
1265 if (m == mid) {
1266 // extract from per-thread in-use list
1267 if (mid == Self->omInUseList) {
1268 Self->omInUseList = mid->FreeNext;
1269 } else if (cur_mid_in_use != NULL) {
1270 cur_mid_in_use->FreeNext = mid->FreeNext; // maintain the current thread in-use list
1271 }
1272 extracted = true;
1273 Self->omInUseCount--;
1274 if (ObjectMonitor::Knob_VerifyInUse) {
1275 verifyInUse(Self);
1276 }
1277 break;
1278 }
1279 }
1280 assert(extracted, "Should have extracted from in-use list");
1281 }
1282
1283 // FreeNext is used for both omInUseList and omFreeList, so clear old before setting new
1284 m->FreeNext = Self->omFreeList;
1285 Self->omFreeList = m;
1286 Self->omFreeCount++;
1287 }
1288
1289 // Return the monitors of a moribund thread's local free list to
1290 // the global free list. Typically a thread calls omFlush() when
1291 // it's dying. We could also consider having the VM thread steal
1292 // monitors from threads that have not run java code over a few
1293 // consecutive STW safepoints. Relatedly, we might decay
1294 // omFreeProvision at STW safepoints.
1295 //
1296 // Also return the monitors of a moribund thread's omInUseList to
1297 // a global gOmInUseList under the global list lock so these
1298 // will continue to be scanned.
1299 //
1300 // We currently call omFlush() from Threads::remove() _before the thread
1301 // has been excised from the thread list and is no longer a mutator.
1302 // This means that omFlush() can not run concurrently with a safepoint and
1303 // interleave with the scavenge operator. In particular, this ensures that
1304 // the thread's monitors are scanned by a GC safepoint, either via
1305 // Thread::oops_do() (if safepoint happens before omFlush()) or via
1306 // ObjectSynchronizer::oops_do() (if it happens after omFlush() and the thread's
1307 // monitors have been transferred to the global in-use list).
1308
1309 void ObjectSynchronizer::omFlush(Thread * Self) {
1310 ObjectMonitor * list = Self->omFreeList; // Null-terminated SLL
1311 Self->omFreeList = NULL;
1312 ObjectMonitor * tail = NULL;
1313 int tally = 0;
1314 if (list != NULL) {
1315 ObjectMonitor * s;
1316 // The thread is going away, the per-thread free monitors
1317 // are freed via set_owner(NULL)
1318 // Link them to tail, which will be linked into the global free list
1319 // gFreeList below, under the gListLock
1320 for (s = list; s != NULL; s = s->FreeNext) {
1321 tally++;
1322 tail = s;
1323 guarantee(s->object() == NULL, "invariant");
1324 guarantee(!s->is_busy(), "invariant");
1325 s->set_owner(NULL); // redundant but good hygiene
1326 TEVENT(omFlush - Move one);
1327 }
1328 guarantee(tail != NULL && list != NULL, "invariant");
1329 }
1330
1331 ObjectMonitor * inUseList = Self->omInUseList;
1332 ObjectMonitor * inUseTail = NULL;
1333 int inUseTally = 0;
1334 if (inUseList != NULL) {
1335 Self->omInUseList = NULL;
1336 ObjectMonitor *cur_om;
1337 // The thread is going away, however the omInUseList inflated
1338 // monitors may still be in-use by other threads.
1339 // Link them to inUseTail, which will be linked into the global in-use list
1340 // gOmInUseList below, under the gListLock
1341 for (cur_om = inUseList; cur_om != NULL; cur_om = cur_om->FreeNext) {
1342 inUseTail = cur_om;
1343 inUseTally++;
1344 }
1345 assert(Self->omInUseCount == inUseTally, "in-use count off");
1346 Self->omInUseCount = 0;
1347 guarantee(inUseTail != NULL && inUseList != NULL, "invariant");
1348 }
1349
1350 Thread::muxAcquire(&gListLock, "omFlush");
1351 if (tail != NULL) {
1352 tail->FreeNext = gFreeList;
1353 gFreeList = list;
1354 gMonitorFreeCount += tally;
1355 assert(Self->omFreeCount == tally, "free-count off");
1356 Self->omFreeCount = 0;
1357 }
1358
1359 if (inUseTail != NULL) {
1360 inUseTail->FreeNext = gOmInUseList;
1361 gOmInUseList = inUseList;
1362 gOmInUseCount += inUseTally;
1363 }
1364
1365 Thread::muxRelease(&gListLock);
1366 TEVENT(omFlush);
1367 }
1368
1369 // Fast path code shared by multiple functions
1370 ObjectMonitor* ObjectSynchronizer::inflate_helper(oop obj) {
1371 markOop mark = obj->mark();
1372 if (mark->has_monitor()) {
1373 assert(ObjectSynchronizer::verify_objmon_isinpool(mark->monitor()), "monitor is invalid");
1374 assert(mark->monitor()->header()->is_neutral(), "monitor must record a good object header");
1375 return mark->monitor();
1376 }
1377 return ObjectSynchronizer::inflate(Thread::current(),
1378 obj,
1379 inflate_cause_vm_internal);
1380 }
1381
1382 ObjectMonitor* ObjectSynchronizer::inflate(Thread * Self,
1383 oop object,
1384 const InflateCause cause) {
1385
1386 // Inflate mutates the heap ...
1387 // Relaxing assertion for bug 6320749.
1388 assert(Universe::verify_in_progress() ||
1389 !SafepointSynchronize::is_at_safepoint(), "invariant");
1390
1391 EventJavaMonitorInflate event;
1392
1393 for (;;) {
1394 const markOop mark = object->mark();
1395 assert(!mark->has_bias_pattern(), "invariant");
1396
1397 // The mark can be in one of the following states:
1398 // * Inflated - just return
1399 // * Stack-locked - coerce it to inflated
1400 // * INFLATING - busy wait for conversion to complete
1401 // * Neutral - aggressively inflate the object.
1402 // * BIASED - Illegal. We should never see this
1403
1404 // CASE: inflated
1405 if (mark->has_monitor()) {
1406 ObjectMonitor * inf = mark->monitor();
1407 assert(inf->header()->is_neutral(), "invariant");
1408 assert(inf->object() == object, "invariant");
1409 assert(ObjectSynchronizer::verify_objmon_isinpool(inf), "monitor is invalid");
1410 event.cancel(); // let's not post an inflation event, unless we did the deed ourselves
1411 return inf;
1412 }
1413
1414 // CASE: inflation in progress - inflating over a stack-lock.
1415 // Some other thread is converting from stack-locked to inflated.
1416 // Only that thread can complete inflation -- other threads must wait.
1417 // The INFLATING value is transient.
1418 // Currently, we spin/yield/park and poll the markword, waiting for inflation to finish.
1419 // We could always eliminate polling by parking the thread on some auxiliary list.
1420 if (mark == markOopDesc::INFLATING()) {
1421 TEVENT(Inflate: spin while INFLATING);
1422 ReadStableMark(object);
1423 continue;
1424 }
1425
1426 // CASE: stack-locked
1427 // Could be stack-locked either by this thread or by some other thread.
1428 //
1429 // Note that we allocate the objectmonitor speculatively, _before_ attempting
1430 // to install INFLATING into the mark word. We originally installed INFLATING,
1431 // allocated the objectmonitor, and then finally STed the address of the
1432 // objectmonitor into the mark. This was correct, but artificially lengthened
1433 // the interval in which INFLATED appeared in the mark, thus increasing
1434 // the odds of inflation contention.
1435 //
1436 // We now use per-thread private objectmonitor free lists.
1437 // These list are reprovisioned from the global free list outside the
1438 // critical INFLATING...ST interval. A thread can transfer
1439 // multiple objectmonitors en-mass from the global free list to its local free list.
1440 // This reduces coherency traffic and lock contention on the global free list.
1441 // Using such local free lists, it doesn't matter if the omAlloc() call appears
1442 // before or after the CAS(INFLATING) operation.
1443 // See the comments in omAlloc().
1444
1445 if (mark->has_locker()) {
1446 ObjectMonitor * m = omAlloc(Self);
1447 // Optimistically prepare the objectmonitor - anticipate successful CAS
1448 // We do this before the CAS in order to minimize the length of time
1449 // in which INFLATING appears in the mark.
1450 m->Recycle();
1451 m->_Responsible = NULL;
1452 m->_recursions = 0;
1453 m->_SpinDuration = ObjectMonitor::Knob_SpinLimit; // Consider: maintain by type/class
1454
1455 markOop cmp = (markOop) Atomic::cmpxchg_ptr(markOopDesc::INFLATING(), object->mark_addr(), mark);
1456 if (cmp != mark) {
1457 omRelease(Self, m, true);
1458 continue; // Interference -- just retry
1459 }
1460
1461 // We've successfully installed INFLATING (0) into the mark-word.
1462 // This is the only case where 0 will appear in a mark-word.
1463 // Only the singular thread that successfully swings the mark-word
1464 // to 0 can perform (or more precisely, complete) inflation.
1465 //
1466 // Why do we CAS a 0 into the mark-word instead of just CASing the
1467 // mark-word from the stack-locked value directly to the new inflated state?
1468 // Consider what happens when a thread unlocks a stack-locked object.
1469 // It attempts to use CAS to swing the displaced header value from the
1470 // on-stack basiclock back into the object header. Recall also that the
1471 // header value (hashcode, etc) can reside in (a) the object header, or
1472 // (b) a displaced header associated with the stack-lock, or (c) a displaced
1473 // header in an objectMonitor. The inflate() routine must copy the header
1474 // value from the basiclock on the owner's stack to the objectMonitor, all
1475 // the while preserving the hashCode stability invariants. If the owner
1476 // decides to release the lock while the value is 0, the unlock will fail
1477 // and control will eventually pass from slow_exit() to inflate. The owner
1478 // will then spin, waiting for the 0 value to disappear. Put another way,
1479 // the 0 causes the owner to stall if the owner happens to try to
1480 // drop the lock (restoring the header from the basiclock to the object)
1481 // while inflation is in-progress. This protocol avoids races that might
1482 // would otherwise permit hashCode values to change or "flicker" for an object.
1483 // Critically, while object->mark is 0 mark->displaced_mark_helper() is stable.
1484 // 0 serves as a "BUSY" inflate-in-progress indicator.
1485
1486
1487 // fetch the displaced mark from the owner's stack.
1488 // The owner can't die or unwind past the lock while our INFLATING
1489 // object is in the mark. Furthermore the owner can't complete
1490 // an unlock on the object, either.
1491 markOop dmw = mark->displaced_mark_helper();
1492 assert(dmw->is_neutral(), "invariant");
1493
1494 // Setup monitor fields to proper values -- prepare the monitor
1495 m->set_header(dmw);
1496
1497 // Optimization: if the mark->locker stack address is associated
1498 // with this thread we could simply set m->_owner = Self.
1499 // Note that a thread can inflate an object
1500 // that it has stack-locked -- as might happen in wait() -- directly
1501 // with CAS. That is, we can avoid the xchg-NULL .... ST idiom.
1502 m->set_owner(mark->locker());
1503 m->set_object(object);
1504 // TODO-FIXME: assert BasicLock->dhw != 0.
1505
1506 // Must preserve store ordering. The monitor state must
1507 // be stable at the time of publishing the monitor address.
1508 guarantee(object->mark() == markOopDesc::INFLATING(), "invariant");
1509 object->release_set_mark(markOopDesc::encode(m));
1510
1511 // Hopefully the performance counters are allocated on distinct cache lines
1512 // to avoid false sharing on MP systems ...
1513 OM_PERFDATA_OP(Inflations, inc());
1514 TEVENT(Inflate: overwrite stacklock);
1515 if (log_is_enabled(Debug, monitorinflation)) {
1516 if (object->is_instance()) {
1517 ResourceMark rm;
1518 log_debug(monitorinflation)("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1519 p2i(object), p2i(object->mark()),
1520 object->klass()->external_name());
1521 }
1522 }
1523 if (event.should_commit()) {
1524 post_monitor_inflate_event(event, object, cause);
1525 }
1526 return m;
1527 }
1528
1529 // CASE: neutral
1530 // TODO-FIXME: for entry we currently inflate and then try to CAS _owner.
1531 // If we know we're inflating for entry it's better to inflate by swinging a
1532 // pre-locked objectMonitor pointer into the object header. A successful
1533 // CAS inflates the object *and* confers ownership to the inflating thread.
1534 // In the current implementation we use a 2-step mechanism where we CAS()
1535 // to inflate and then CAS() again to try to swing _owner from NULL to Self.
1536 // An inflateTry() method that we could call from fast_enter() and slow_enter()
1537 // would be useful.
1538
1539 assert(mark->is_neutral(), "invariant");
1540 ObjectMonitor * m = omAlloc(Self);
1541 // prepare m for installation - set monitor to initial state
1542 m->Recycle();
1543 m->set_header(mark);
1544 m->set_owner(NULL);
1545 m->set_object(object);
1546 m->_recursions = 0;
1547 m->_Responsible = NULL;
1548 m->_SpinDuration = ObjectMonitor::Knob_SpinLimit; // consider: keep metastats by type/class
1549
1550 if (Atomic::cmpxchg_ptr (markOopDesc::encode(m), object->mark_addr(), mark) != mark) {
1551 m->set_object(NULL);
1552 m->set_owner(NULL);
1553 m->Recycle();
1554 omRelease(Self, m, true);
1555 m = NULL;
1556 continue;
1557 // interference - the markword changed - just retry.
1558 // The state-transitions are one-way, so there's no chance of
1559 // live-lock -- "Inflated" is an absorbing state.
1560 }
1561
1562 // Hopefully the performance counters are allocated on distinct
1563 // cache lines to avoid false sharing on MP systems ...
1564 OM_PERFDATA_OP(Inflations, inc());
1565 TEVENT(Inflate: overwrite neutral);
1566 if (log_is_enabled(Debug, monitorinflation)) {
1567 if (object->is_instance()) {
1568 ResourceMark rm;
1569 log_debug(monitorinflation)("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1570 p2i(object), p2i(object->mark()),
1571 object->klass()->external_name());
1572 }
1573 }
1574 if (event.should_commit()) {
1575 post_monitor_inflate_event(event, object, cause);
1576 }
1577 return m;
1578 }
1579 }
1580
1581
1582 // Deflate_idle_monitors() is called at all safepoints, immediately
1583 // after all mutators are stopped, but before any objects have moved.
1584 // It traverses the list of known monitors, deflating where possible.
1585 // The scavenged monitor are returned to the monitor free list.
1586 //
1587 // Beware that we scavenge at *every* stop-the-world point.
1588 // Having a large number of monitors in-circulation negatively
1589 // impacts the performance of some applications (e.g., PointBase).
1590 // Broadly, we want to minimize the # of monitors in circulation.
1591 //
1592 // We have added a flag, MonitorInUseLists, which creates a list
1593 // of active monitors for each thread. deflate_idle_monitors()
1594 // only scans the per-thread in-use lists. omAlloc() puts all
1595 // assigned monitors on the per-thread list. deflate_idle_monitors()
1596 // returns the non-busy monitors to the global free list.
1597 // When a thread dies, omFlush() adds the list of active monitors for
1598 // that thread to a global gOmInUseList acquiring the
1599 // global list lock. deflate_idle_monitors() acquires the global
1600 // list lock to scan for non-busy monitors to the global free list.
1601 // An alternative could have used a single global in-use list. The
1602 // downside would have been the additional cost of acquiring the global list lock
1603 // for every omAlloc().
1604 //
1605 // Perversely, the heap size -- and thus the STW safepoint rate --
1606 // typically drives the scavenge rate. Large heaps can mean infrequent GC,
1607 // which in turn can mean large(r) numbers of objectmonitors in circulation.
1608 // This is an unfortunate aspect of this design.
1609
1610 enum ManifestConstants {
1611 ClearResponsibleAtSTW = 0
1612 };
1613
1614 // Deflate a single monitor if not in-use
1615 // Return true if deflated, false if in-use
1616 bool ObjectSynchronizer::deflate_monitor(ObjectMonitor* mid, oop obj,
1617 ObjectMonitor** freeHeadp,
1618 ObjectMonitor** freeTailp) {
1619 bool deflated;
1620 // Normal case ... The monitor is associated with obj.
1621 guarantee(obj->mark() == markOopDesc::encode(mid), "invariant");
1622 guarantee(mid == obj->mark()->monitor(), "invariant");
1623 guarantee(mid->header()->is_neutral(), "invariant");
1624
1625 if (mid->is_busy()) {
1626 if (ClearResponsibleAtSTW) mid->_Responsible = NULL;
1627 deflated = false;
1628 } else {
1629 // Deflate the monitor if it is no longer being used
1630 // It's idle - scavenge and return to the global free list
1631 // plain old deflation ...
1632 TEVENT(deflate_idle_monitors - scavenge1);
1633 if (log_is_enabled(Debug, monitorinflation)) {
1634 if (obj->is_instance()) {
1635 ResourceMark rm;
1636 log_debug(monitorinflation)("Deflating object " INTPTR_FORMAT " , "
1637 "mark " INTPTR_FORMAT " , type %s",
1638 p2i(obj), p2i(obj->mark()),
1639 obj->klass()->external_name());
1640 }
1641 }
1642
1643 // Restore the header back to obj
1644 obj->release_set_mark(mid->header());
1645 mid->clear();
1646
1647 assert(mid->object() == NULL, "invariant");
1648
1649 // Move the object to the working free list defined by freeHeadp, freeTailp
1650 if (*freeHeadp == NULL) *freeHeadp = mid;
1651 if (*freeTailp != NULL) {
1652 ObjectMonitor * prevtail = *freeTailp;
1653 assert(prevtail->FreeNext == NULL, "cleaned up deflated?");
1654 prevtail->FreeNext = mid;
1655 }
1656 *freeTailp = mid;
1657 deflated = true;
1658 }
1659 return deflated;
1660 }
1661
1662 // Walk a given monitor list, and deflate idle monitors
1663 // The given list could be a per-thread list or a global list
1664 // Caller acquires gListLock
1665 int ObjectSynchronizer::deflate_monitor_list(ObjectMonitor** listHeadp,
1666 ObjectMonitor** freeHeadp,
1667 ObjectMonitor** freeTailp) {
1668 ObjectMonitor* mid;
1669 ObjectMonitor* next;
1670 ObjectMonitor* cur_mid_in_use = NULL;
1671 int deflated_count = 0;
1672
1673 for (mid = *listHeadp; mid != NULL;) {
1674 oop obj = (oop) mid->object();
1675 if (obj != NULL && deflate_monitor(mid, obj, freeHeadp, freeTailp)) {
1676 // if deflate_monitor succeeded,
1677 // extract from per-thread in-use list
1678 if (mid == *listHeadp) {
1679 *listHeadp = mid->FreeNext;
1680 } else if (cur_mid_in_use != NULL) {
1681 cur_mid_in_use->FreeNext = mid->FreeNext; // maintain the current thread in-use list
1682 }
1683 next = mid->FreeNext;
1684 mid->FreeNext = NULL; // This mid is current tail in the freeHeadp list
1685 mid = next;
1686 deflated_count++;
1687 } else {
1688 cur_mid_in_use = mid;
1689 mid = mid->FreeNext;
1690 }
1691 }
1692 return deflated_count;
1693 }
1694
1695 void ObjectSynchronizer::deflate_idle_monitors() {
1696 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1697 int nInuse = 0; // currently associated with objects
1698 int nInCirculation = 0; // extant
1699 int nScavenged = 0; // reclaimed
1700 bool deflated = false;
1701
1702 ObjectMonitor * freeHeadp = NULL; // Local SLL of scavenged monitors
1703 ObjectMonitor * freeTailp = NULL;
1704
1705 TEVENT(deflate_idle_monitors);
1706 // Prevent omFlush from changing mids in Thread dtor's during deflation
1707 // And in case the vm thread is acquiring a lock during a safepoint
1708 // See e.g. 6320749
1709 Thread::muxAcquire(&gListLock, "scavenge - return");
1710
1711 if (MonitorInUseLists) {
1712 int inUse = 0;
1713 for (JavaThread* cur = Threads::first(); cur != NULL; cur = cur->next()) {
1714 nInCirculation+= cur->omInUseCount;
1715 int deflated_count = deflate_monitor_list(cur->omInUseList_addr(), &freeHeadp, &freeTailp);
1716 cur->omInUseCount-= deflated_count;
1717 if (ObjectMonitor::Knob_VerifyInUse) {
1718 verifyInUse(cur);
1719 }
1720 nScavenged += deflated_count;
1721 nInuse += cur->omInUseCount;
1722 }
1723
1724 // For moribund threads, scan gOmInUseList
1725 if (gOmInUseList) {
1726 nInCirculation += gOmInUseCount;
1727 int deflated_count = deflate_monitor_list((ObjectMonitor **)&gOmInUseList, &freeHeadp, &freeTailp);
1728 gOmInUseCount-= deflated_count;
1729 nScavenged += deflated_count;
1730 nInuse += gOmInUseCount;
1731 }
1732
1733 } else {
1734 PaddedEnd<ObjectMonitor> * block =
1735 (PaddedEnd<ObjectMonitor> *)OrderAccess::load_ptr_acquire(&gBlockList);
1736 for (; block != NULL; block = (PaddedEnd<ObjectMonitor> *)next(block)) {
1737 // Iterate over all extant monitors - Scavenge all idle monitors.
1738 assert(block->object() == CHAINMARKER, "must be a block header");
1739 nInCirculation += _BLOCKSIZE;
1740 for (int i = 1; i < _BLOCKSIZE; i++) {
1741 ObjectMonitor* mid = (ObjectMonitor*)&block[i];
1742 oop obj = (oop)mid->object();
1743
1744 if (obj == NULL) {
1745 // The monitor is not associated with an object.
1746 // The monitor should either be a thread-specific private
1747 // free list or the global free list.
1748 // obj == NULL IMPLIES mid->is_busy() == 0
1749 guarantee(!mid->is_busy(), "invariant");
1750 continue;
1751 }
1752 deflated = deflate_monitor(mid, obj, &freeHeadp, &freeTailp);
1753
1754 if (deflated) {
1755 mid->FreeNext = NULL;
1756 nScavenged++;
1757 } else {
1758 nInuse++;
1759 }
1760 }
1761 }
1762 }
1763
1764 gMonitorFreeCount += nScavenged;
1765
1766 // Consider: audit gFreeList to ensure that gMonitorFreeCount and list agree.
1767
1768 if (ObjectMonitor::Knob_Verbose) {
1769 tty->print_cr("INFO: Deflate: InCirc=%d InUse=%d Scavenged=%d "
1770 "ForceMonitorScavenge=%d : pop=%d free=%d",
1771 nInCirculation, nInuse, nScavenged, ForceMonitorScavenge,
1772 gMonitorPopulation, gMonitorFreeCount);
1773 tty->flush();
1774 }
1775
1776 ForceMonitorScavenge = 0; // Reset
1777
1778 // Move the scavenged monitors back to the global free list.
1779 if (freeHeadp != NULL) {
1780 guarantee(freeTailp != NULL && nScavenged > 0, "invariant");
1781 assert(freeTailp->FreeNext == NULL, "invariant");
1782 // constant-time list splice - prepend scavenged segment to gFreeList
1783 freeTailp->FreeNext = gFreeList;
1784 gFreeList = freeHeadp;
1785 }
1786 Thread::muxRelease(&gListLock);
1787
1788 OM_PERFDATA_OP(Deflations, inc(nScavenged));
1789 OM_PERFDATA_OP(MonExtant, set_value(nInCirculation));
1790
1791 // TODO: Add objectMonitor leak detection.
1792 // Audit/inventory the objectMonitors -- make sure they're all accounted for.
1793 GVars.stwRandom = os::random();
1794 GVars.stwCycle++;
1795 }
1796
1797 // Monitor cleanup on JavaThread::exit
1798
1799 // Iterate through monitor cache and attempt to release thread's monitors
1800 // Gives up on a particular monitor if an exception occurs, but continues
1801 // the overall iteration, swallowing the exception.
1802 class ReleaseJavaMonitorsClosure: public MonitorClosure {
1803 private:
1804 TRAPS;
1805
1806 public:
1807 ReleaseJavaMonitorsClosure(Thread* thread) : THREAD(thread) {}
1808 void do_monitor(ObjectMonitor* mid) {
1809 if (mid->owner() == THREAD) {
1810 if (ObjectMonitor::Knob_VerifyMatch != 0) {
1811 ResourceMark rm;
1812 Handle obj(THREAD, (oop) mid->object());
1813 tty->print("INFO: unexpected locked object:");
1814 javaVFrame::print_locked_object_class_name(tty, obj, "locked");
1815 fatal("exiting JavaThread=" INTPTR_FORMAT
1816 " unexpectedly owns ObjectMonitor=" INTPTR_FORMAT,
1817 p2i(THREAD), p2i(mid));
1818 }
1819 (void)mid->complete_exit(CHECK);
1820 }
1821 }
1822 };
1823
1824 // Release all inflated monitors owned by THREAD. Lightweight monitors are
1825 // ignored. This is meant to be called during JNI thread detach which assumes
1826 // all remaining monitors are heavyweight. All exceptions are swallowed.
1827 // Scanning the extant monitor list can be time consuming.
1828 // A simple optimization is to add a per-thread flag that indicates a thread
1829 // called jni_monitorenter() during its lifetime.
1830 //
1831 // Instead of No_Savepoint_Verifier it might be cheaper to
1832 // use an idiom of the form:
1833 // auto int tmp = SafepointSynchronize::_safepoint_counter ;
1834 // <code that must not run at safepoint>
1835 // guarantee (((tmp ^ _safepoint_counter) | (tmp & 1)) == 0) ;
1836 // Since the tests are extremely cheap we could leave them enabled
1837 // for normal product builds.
1838
1839 void ObjectSynchronizer::release_monitors_owned_by_thread(TRAPS) {
1840 assert(THREAD == JavaThread::current(), "must be current Java thread");
1841 NoSafepointVerifier nsv;
1842 ReleaseJavaMonitorsClosure rjmc(THREAD);
1843 Thread::muxAcquire(&gListLock, "release_monitors_owned_by_thread");
1844 ObjectSynchronizer::monitors_iterate(&rjmc);
1845 Thread::muxRelease(&gListLock);
1846 THREAD->clear_pending_exception();
1847 }
1848
1849 const char* ObjectSynchronizer::inflate_cause_name(const InflateCause cause) {
1850 switch (cause) {
1851 case inflate_cause_vm_internal: return "VM Internal";
1852 case inflate_cause_monitor_enter: return "Monitor Enter";
1853 case inflate_cause_wait: return "Monitor Wait";
1854 case inflate_cause_notify: return "Monitor Notify";
1855 case inflate_cause_hash_code: return "Monitor Hash Code";
1856 case inflate_cause_jni_enter: return "JNI Monitor Enter";
1857 case inflate_cause_jni_exit: return "JNI Monitor Exit";
1858 default:
1859 ShouldNotReachHere();
1860 }
1861 return "Unknown";
1862 }
1863
1864 static void post_monitor_inflate_event(EventJavaMonitorInflate& event,
1865 const oop obj,
1866 const ObjectSynchronizer::InflateCause cause) {
1867 #if INCLUDE_TRACE
1868 assert(event.should_commit(), "check outside");
1869 event.set_monitorClass(obj->klass());
1870 event.set_address((TYPE_ADDRESS)(uintptr_t)(void*)obj);
1871 event.set_cause((u1)cause);
1872 event.commit();
1873 #endif
1874 }
1875
1876 //------------------------------------------------------------------------------
1877 // Debugging code
1878
1879 void ObjectSynchronizer::sanity_checks(const bool verbose,
1880 const uint cache_line_size,
1881 int *error_cnt_ptr,
1882 int *warning_cnt_ptr) {
1883 u_char *addr_begin = (u_char*)&GVars;
1884 u_char *addr_stwRandom = (u_char*)&GVars.stwRandom;
1885 u_char *addr_hcSequence = (u_char*)&GVars.hcSequence;
1886
1887 if (verbose) {
1888 tty->print_cr("INFO: sizeof(SharedGlobals)=" SIZE_FORMAT,
1889 sizeof(SharedGlobals));
1890 }
1891
1892 uint offset_stwRandom = (uint)(addr_stwRandom - addr_begin);
1893 if (verbose) tty->print_cr("INFO: offset(stwRandom)=%u", offset_stwRandom);
1894
1895 uint offset_hcSequence = (uint)(addr_hcSequence - addr_begin);
1896 if (verbose) {
1897 tty->print_cr("INFO: offset(_hcSequence)=%u", offset_hcSequence);
1898 }
1899
1900 if (cache_line_size != 0) {
1901 // We were able to determine the L1 data cache line size so
1902 // do some cache line specific sanity checks
1903
1904 if (offset_stwRandom < cache_line_size) {
1905 tty->print_cr("WARNING: the SharedGlobals.stwRandom field is closer "
1906 "to the struct beginning than a cache line which permits "
1907 "false sharing.");
1908 (*warning_cnt_ptr)++;
1909 }
1910
1911 if ((offset_hcSequence - offset_stwRandom) < cache_line_size) {
1912 tty->print_cr("WARNING: the SharedGlobals.stwRandom and "
1913 "SharedGlobals.hcSequence fields are closer than a cache "
1914 "line which permits false sharing.");
1915 (*warning_cnt_ptr)++;
1916 }
1917
1918 if ((sizeof(SharedGlobals) - offset_hcSequence) < cache_line_size) {
1919 tty->print_cr("WARNING: the SharedGlobals.hcSequence field is closer "
1920 "to the struct end than a cache line which permits false "
1921 "sharing.");
1922 (*warning_cnt_ptr)++;
1923 }
1924 }
1925 }
1926
1927 #ifndef PRODUCT
1928
1929 // Check if monitor belongs to the monitor cache
1930 // The list is grow-only so it's *relatively* safe to traverse
1931 // the list of extant blocks without taking a lock.
1932
1933 int ObjectSynchronizer::verify_objmon_isinpool(ObjectMonitor *monitor) {
1934 PaddedEnd<ObjectMonitor> * block =
1935 (PaddedEnd<ObjectMonitor> *)OrderAccess::load_ptr_acquire(&gBlockList);
1936 while (block != NULL) {
1937 assert(block->object() == CHAINMARKER, "must be a block header");
1938 if (monitor > (ObjectMonitor *)&block[0] &&
1939 monitor < (ObjectMonitor *)&block[_BLOCKSIZE]) {
1940 address mon = (address)monitor;
1941 address blk = (address)block;
1942 size_t diff = mon - blk;
1943 assert((diff % sizeof(PaddedEnd<ObjectMonitor>)) == 0, "must be aligned");
1944 return 1;
1945 }
1946 block = (PaddedEnd<ObjectMonitor> *)block->FreeNext;
1947 }
1948 return 0;
1949 }
1950
1951 #endif