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 ObjectMonitor * ObjectSynchronizer::gFreeListNextSafepoint = NULL;
119 ObjectMonitor * ObjectSynchronizer::gFreeListNextSafepointTail = NULL;
120 // global monitor in-use list, for moribund threads,
121 // monitors they inflated need to be scanned for deflation
122 ObjectMonitor * volatile ObjectSynchronizer::gOmInUseList = NULL;
123 // count of entries in gOmInUseList
124 int ObjectSynchronizer::gOmInUseCount = 0;
125
126 bool ObjectSynchronizer::_should_deflate_idle_monitors_conc = false;
127
128 static volatile intptr_t gListLock = 0; // protects global monitor lists
129 static volatile int gMonitorFreeCount = 0; // # on gFreeList
130 static int gMonitorFreeCountNextSafepoint = 0;
131 static volatile int gMonitorPopulation = 0; // # Extant -- in circulation
132
133 static void post_monitor_inflate_event(EventJavaMonitorInflate&,
134 const oop,
135 const ObjectSynchronizer::InflateCause);
136
137 #define CHAINMARKER (cast_to_oop<intptr_t>(-1))
138
139
140 // =====================> Quick functions
141
142 // The quick_* forms are special fast-path variants used to improve
143 // performance. In the simplest case, a "quick_*" implementation could
144 // simply return false, in which case the caller will perform the necessary
145 // state transitions and call the slow-path form.
146 // The fast-path is designed to handle frequently arising cases in an efficient
147 // manner and is just a degenerate "optimistic" variant of the slow-path.
148 // returns true -- to indicate the call was satisfied.
149 // returns false -- to indicate the call needs the services of the slow-path.
150 // A no-loitering ordinance is in effect for code in the quick_* family
151 // operators: safepoints or indefinite blocking (blocking that might span a
152 // safepoint) are forbidden. Generally the thread_state() is _in_Java upon
153 // entry.
154 //
155 // Consider: An interesting optimization is to have the JIT recognize the
156 // following common idiom:
157 // synchronized (someobj) { .... ; notify(); }
158 // That is, we find a notify() or notifyAll() call that immediately precedes
159 // the monitorexit operation. In that case the JIT could fuse the operations
160 // into a single notifyAndExit() runtime primitive.
161
162 bool ObjectSynchronizer::quick_notify(oopDesc * obj, Thread * self, bool all) {
163 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
164 assert(self->is_Java_thread(), "invariant");
165 assert(((JavaThread *) self)->thread_state() == _thread_in_Java, "invariant");
166 NoSafepointVerifier nsv;
167 if (obj == NULL) return false; // slow-path for invalid obj
168 const markOop mark = obj->mark();
169
170 if (mark->has_locker() && self->is_lock_owned((address)mark->locker())) {
171 // Degenerate notify
172 // stack-locked by caller so by definition the implied waitset is empty.
173 return true;
174 }
175
176 if (mark->has_monitor()) {
177 ObjectMonitor * const mon = mark->monitor();
178 assert(mon->object() == obj, "invariant");
179 if (mon->owner() != self) return false; // slow-path for IMS exception
180
181 if (mon->first_waiter() != NULL) {
182 // We have one or more waiters. Since this is an inflated monitor
183 // that we own, we can transfer one or more threads from the waitset
184 // to the entrylist here and now, avoiding the slow-path.
185 if (all) {
186 DTRACE_MONITOR_PROBE(notifyAll, mon, obj, self);
187 } else {
188 DTRACE_MONITOR_PROBE(notify, mon, obj, self);
189 }
190 int tally = 0;
191 do {
192 mon->INotify(self);
193 ++tally;
194 } while (mon->first_waiter() != NULL && all);
195 OM_PERFDATA_OP(Notifications, inc(tally));
196 }
197 return true;
198 }
199
200 // biased locking and any other IMS exception states take the slow-path
201 return false;
202 }
203
204
205 // The LockNode emitted directly at the synchronization site would have
206 // been too big if it were to have included support for the cases of inflated
207 // recursive enter and exit, so they go here instead.
208 // Note that we can't safely call AsyncPrintJavaStack() from within
209 // quick_enter() as our thread state remains _in_Java.
210
211 bool ObjectSynchronizer::quick_enter(oop obj, Thread * Self,
212 BasicLock * lock) {
213 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
214 assert(Self->is_Java_thread(), "invariant");
215 assert(((JavaThread *) Self)->thread_state() == _thread_in_Java, "invariant");
216 NoSafepointVerifier nsv;
217 if (obj == NULL) return false; // Need to throw NPE
218 const markOop mark = obj->mark();
219
220 if (mark->has_monitor()) {
221 ObjectMonitor * const m = mark->monitor();
222 assert(m->object() == obj, "invariant");
223 Thread * const owner = (Thread *) m->_owner;
224
225 // Lock contention and Transactional Lock Elision (TLE) diagnostics
226 // and observability
227 // Case: light contention possibly amenable to TLE
228 // Case: TLE inimical operations such as nested/recursive synchronization
229
230 if (owner == Self) {
231 m->_recursions++;
232 return true;
233 }
234
235 // This Java Monitor is inflated so obj's header will never be
236 // displaced to this thread's BasicLock. Make the displaced header
237 // non-NULL so this BasicLock is not seen as recursive nor as
238 // being locked. We do this unconditionally so that this thread's
239 // BasicLock cannot be mis-interpreted by any stack walkers. For
240 // performance reasons, stack walkers generally first check for
241 // Biased Locking in the object's header, the second check is for
242 // stack-locking in the object's header, the third check is for
243 // recursive stack-locking in the displaced header in the BasicLock,
244 // and last are the inflated Java Monitor (ObjectMonitor) checks.
245 lock->set_displaced_header(markOopDesc::unused_mark());
246
247 if (owner == NULL &&
248 Atomic::cmpxchg_ptr(Self, &(m->_owner), NULL) == NULL) {
249 assert(m->_recursions == 0, "invariant");
250 assert(m->_owner == Self, "invariant");
251 return true;
252 }
253 }
254
255 // Note that we could inflate in quick_enter.
256 // This is likely a useful optimization
257 // Critically, in quick_enter() we must not:
258 // -- perform bias revocation, or
259 // -- block indefinitely, or
260 // -- reach a safepoint
261
262 return false; // revert to slow-path
263 }
264
265 // -----------------------------------------------------------------------------
266 // Fast Monitor Enter/Exit
267 // This the fast monitor enter. The interpreter and compiler use
268 // some assembly copies of this code. Make sure update those code
269 // if the following function is changed. The implementation is
270 // extremely sensitive to race condition. Be careful.
271
272 void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock,
273 bool attempt_rebias, TRAPS) {
274 if (UseBiasedLocking) {
275 if (!SafepointSynchronize::is_at_safepoint()) {
276 BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD);
277 if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) {
278 return;
279 }
280 } else {
281 assert(!attempt_rebias, "can not rebias toward VM thread");
282 BiasedLocking::revoke_at_safepoint(obj);
283 }
284 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
285 }
286
287 slow_enter(obj, lock, THREAD);
288 }
289
290 void ObjectSynchronizer::fast_exit(oop object, BasicLock* lock, TRAPS) {
291 markOop mark = object->mark();
292 // We cannot check for Biased Locking if we are racing an inflation.
293 assert(mark == markOopDesc::INFLATING() ||
294 !mark->has_bias_pattern(), "should not see bias pattern here");
295
296 markOop dhw = lock->displaced_header();
297 if (dhw == NULL) {
298 // If the displaced header is NULL, then this exit matches up with
299 // a recursive enter. No real work to do here except for diagnostics.
300 #ifndef PRODUCT
301 if (mark != markOopDesc::INFLATING()) {
302 // Only do diagnostics if we are not racing an inflation. Simply
303 // exiting a recursive enter of a Java Monitor that is being
304 // inflated is safe; see the has_monitor() comment below.
305 assert(!mark->is_neutral(), "invariant");
306 assert(!mark->has_locker() ||
307 THREAD->is_lock_owned((address)mark->locker()), "invariant");
308 if (mark->has_monitor()) {
309 // The BasicLock's displaced_header is marked as a recursive
310 // enter and we have an inflated Java Monitor (ObjectMonitor).
311 // This is a special case where the Java Monitor was inflated
312 // after this thread entered the stack-lock recursively. When a
313 // Java Monitor is inflated, we cannot safely walk the Java
314 // Monitor owner's stack and update the BasicLocks because a
315 // Java Monitor can be asynchronously inflated by a thread that
316 // does not own the Java Monitor.
317 ObjectMonitor * m = mark->monitor();
318 assert(((oop)(m->object()))->mark() == mark, "invariant");
319 assert(m->is_entered(THREAD), "invariant");
320 }
321 }
322 #endif
323 return;
324 }
325
326 if (mark == (markOop) lock) {
327 // If the object is stack-locked by the current thread, try to
328 // swing the displaced header from the BasicLock back to the mark.
329 assert(dhw->is_neutral(), "invariant");
330 if ((markOop) Atomic::cmpxchg_ptr(dhw, object->mark_addr(), mark) == mark) {
331 TEVENT(fast_exit: release stack-lock);
332 return;
333 }
334 }
335
336 // We have to take the slow-path of possible inflation and then exit.
337 ObjectSynchronizer::inflate(THREAD,
338 object,
339 inflate_cause_vm_internal)->exit(true, THREAD);
340 }
341
342 // -----------------------------------------------------------------------------
343 // Interpreter/Compiler Slow Case
344 // This routine is used to handle interpreter/compiler slow case
345 // We don't need to use fast path here, because it must have been
346 // failed in the interpreter/compiler code.
347 void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) {
348 do {
349 markOop mark = obj->mark();
350 assert(!mark->has_bias_pattern(), "should not see bias pattern here");
351
352 if (mark->is_neutral()) {
353 // Anticipate successful CAS -- the ST of the displaced mark must
354 // be visible <= the ST performed by the CAS.
355 lock->set_displaced_header(mark);
356 if (mark == (markOop) Atomic::cmpxchg_ptr(lock, obj()->mark_addr(), mark)) {
357 TEVENT(slow_enter: release stacklock);
358 return;
359 }
360 // Fall through to inflate() ...
361 } else if (mark->has_locker() &&
362 THREAD->is_lock_owned((address)mark->locker())) {
363 assert(lock != mark->locker(), "must not re-lock the same lock");
364 assert(lock != (BasicLock*)obj->mark(), "don't relock with same BasicLock");
365 lock->set_displaced_header(NULL);
366 return;
367 }
368
369 // The object header will never be displaced to this lock,
370 // so it does not matter what the value is, except that it
371 // must be non-zero to avoid looking like a re-entrant lock,
372 // and must not look locked either.
373 lock->set_displaced_header(markOopDesc::unused_mark());
374 } while (!ObjectSynchronizer::inflate(THREAD,
375 obj(),
376 inflate_cause_monitor_enter)->enter(THREAD));
377 }
378
379 // This routine is used to handle interpreter/compiler slow case
380 // We don't need to use fast path here, because it must have
381 // failed in the interpreter/compiler code. Simply use the heavy
382 // weight monitor should be ok, unless someone find otherwise.
383 void ObjectSynchronizer::slow_exit(oop object, BasicLock* lock, TRAPS) {
384 fast_exit(object, lock, THREAD);
385 }
386
387 // -----------------------------------------------------------------------------
388 // Class Loader support to workaround deadlocks on the class loader lock objects
389 // Also used by GC
390 // complete_exit()/reenter() are used to wait on a nested lock
391 // i.e. to give up an outer lock completely and then re-enter
392 // Used when holding nested locks - lock acquisition order: lock1 then lock2
393 // 1) complete_exit lock1 - saving recursion count
394 // 2) wait on lock2
395 // 3) when notified on lock2, unlock lock2
396 // 4) reenter lock1 with original recursion count
397 // 5) lock lock2
398 // NOTE: must use heavy weight monitor to handle complete_exit/reenter()
399 intptr_t ObjectSynchronizer::complete_exit(Handle obj, TRAPS) {
400 TEVENT(complete_exit);
401 if (UseBiasedLocking) {
402 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
403 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
404 }
405
406 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD,
407 obj(),
408 inflate_cause_vm_internal);
409
410 return monitor->complete_exit(THREAD);
411 }
412
413 // NOTE: must use heavy weight monitor to handle complete_exit/reenter()
414 void ObjectSynchronizer::reenter(Handle obj, intptr_t recursion, TRAPS) {
415 TEVENT(reenter);
416 if (UseBiasedLocking) {
417 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
418 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
419 }
420
421 ObjectMonitor* monitor;
422 do {
423 monitor = ObjectSynchronizer::inflate(THREAD,
424 obj(),
425 inflate_cause_vm_internal);
426 } while(!monitor->reenter(recursion, THREAD));
427 }
428 // -----------------------------------------------------------------------------
429 // JNI locks on java objects
430 // NOTE: must use heavy weight monitor to handle jni monitor enter
431 void ObjectSynchronizer::jni_enter(Handle obj, TRAPS) {
432 // the current locking is from JNI instead of Java code
433 TEVENT(jni_enter);
434 if (UseBiasedLocking) {
435 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
436 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
437 }
438 THREAD->set_current_pending_monitor_is_from_java(false);
439 while (!ObjectSynchronizer::inflate(THREAD, obj(), inflate_cause_jni_enter)->enter(THREAD));
440 THREAD->set_current_pending_monitor_is_from_java(true);
441 }
442
443 // NOTE: must use heavy weight monitor to handle jni monitor exit
444 void ObjectSynchronizer::jni_exit(oop obj, Thread* THREAD) {
445 TEVENT(jni_exit);
446 if (UseBiasedLocking) {
447 Handle h_obj(THREAD, obj);
448 BiasedLocking::revoke_and_rebias(h_obj, false, THREAD);
449 obj = h_obj();
450 }
451 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
452
453 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD,
454 obj,
455 inflate_cause_jni_exit);
456 // If this thread has locked the object, exit the monitor. Note: can't use
457 // monitor->check(CHECK); must exit even if an exception is pending.
458 if (monitor->check(THREAD)) {
459 monitor->exit(true, THREAD);
460 }
461 }
462
463 // -----------------------------------------------------------------------------
464 // Internal VM locks on java objects
465 // standard constructor, allows locking failures
466 ObjectLocker::ObjectLocker(Handle obj, Thread* thread, bool doLock) {
467 _dolock = doLock;
468 _thread = thread;
469 debug_only(if (StrictSafepointChecks) _thread->check_for_valid_safepoint_state(false);)
470 _obj = obj;
471
472 if (_dolock) {
473 TEVENT(ObjectLocker);
474
475 ObjectSynchronizer::fast_enter(_obj, &_lock, false, _thread);
476 }
477 }
478
479 ObjectLocker::~ObjectLocker() {
480 if (_dolock) {
481 ObjectSynchronizer::fast_exit(_obj(), &_lock, _thread);
482 }
483 }
484
485
486 // -----------------------------------------------------------------------------
487 // Wait/Notify/NotifyAll
488 // NOTE: must use heavy weight monitor to handle wait()
489 int ObjectSynchronizer::wait(Handle obj, jlong millis, TRAPS) {
490 if (UseBiasedLocking) {
491 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
492 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
493 }
494 if (millis < 0) {
495 TEVENT(wait - throw IAX);
496 THROW_MSG_0(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
497 }
498 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD,
499 obj(),
500 inflate_cause_wait);
501
502 DTRACE_MONITOR_WAIT_PROBE(monitor, obj(), THREAD, millis);
503 monitor->wait(millis, true, THREAD);
504
505 // This dummy call is in place to get around dtrace bug 6254741. Once
506 // that's fixed we can uncomment the following line, remove the call
507 // and change this function back into a "void" func.
508 // DTRACE_MONITOR_PROBE(waited, monitor, obj(), THREAD);
509 return dtrace_waited_probe(monitor, obj, THREAD);
510 }
511
512 void ObjectSynchronizer::waitUninterruptibly(Handle obj, jlong millis, TRAPS) {
513 if (UseBiasedLocking) {
514 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
515 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
516 }
517 if (millis < 0) {
518 TEVENT(wait - throw IAX);
519 THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
520 }
521 ObjectSynchronizer::inflate(THREAD,
522 obj(),
523 inflate_cause_wait)->wait(millis, false, THREAD);
524 }
525
526 void ObjectSynchronizer::notify(Handle obj, TRAPS) {
527 if (UseBiasedLocking) {
528 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
529 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
530 }
531
532 markOop mark = obj->mark();
533 if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
534 return;
535 }
536 ObjectSynchronizer::inflate(THREAD,
537 obj(),
538 inflate_cause_notify)->notify(THREAD);
539 }
540
541 // NOTE: see comment of notify()
542 void ObjectSynchronizer::notifyall(Handle obj, TRAPS) {
543 if (UseBiasedLocking) {
544 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
545 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
546 }
547
548 markOop mark = obj->mark();
549 if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
550 return;
551 }
552 ObjectSynchronizer::inflate(THREAD,
553 obj(),
554 inflate_cause_notify)->notifyAll(THREAD);
555 }
556
557 // -----------------------------------------------------------------------------
558 // Hash Code handling
559 //
560 // Performance concern:
561 // OrderAccess::storestore() calls release() which at one time stored 0
562 // into the global volatile OrderAccess::dummy variable. This store was
563 // unnecessary for correctness. Many threads storing into a common location
564 // causes considerable cache migration or "sloshing" on large SMP systems.
565 // As such, I avoided using OrderAccess::storestore(). In some cases
566 // OrderAccess::fence() -- which incurs local latency on the executing
567 // processor -- is a better choice as it scales on SMP systems.
568 //
569 // See http://blogs.oracle.com/dave/entry/biased_locking_in_hotspot for
570 // a discussion of coherency costs. Note that all our current reference
571 // platforms provide strong ST-ST order, so the issue is moot on IA32,
572 // x64, and SPARC.
573 //
574 // As a general policy we use "volatile" to control compiler-based reordering
575 // and explicit fences (barriers) to control for architectural reordering
576 // performed by the CPU(s) or platform.
577
578 struct SharedGlobals {
579 char _pad_prefix[DEFAULT_CACHE_LINE_SIZE];
580 // These are highly shared mostly-read variables.
581 // To avoid false-sharing they need to be the sole occupants of a cache line.
582 volatile int stwRandom;
583 volatile int stwCycle;
584 DEFINE_PAD_MINUS_SIZE(1, DEFAULT_CACHE_LINE_SIZE, sizeof(volatile int) * 2);
585 // Hot RW variable -- Sequester to avoid false-sharing
586 volatile int hcSequence;
587 DEFINE_PAD_MINUS_SIZE(2, DEFAULT_CACHE_LINE_SIZE, sizeof(volatile int));
588 };
589
590 static SharedGlobals GVars;
591 static int MonitorScavengeThreshold = 1000000;
592 static volatile int ForceMonitorScavenge = 0; // Scavenge required and pending
593
594 static markOop ReadStableMark(oop obj) {
595 markOop mark = obj->mark();
596 if (!mark->is_being_inflated()) {
597 return mark; // normal fast-path return
598 }
599
600 int its = 0;
601 for (;;) {
602 markOop mark = obj->mark();
603 if (!mark->is_being_inflated()) {
604 return mark; // normal fast-path return
605 }
606
607 // The object is being inflated by some other thread.
608 // The caller of ReadStableMark() must wait for inflation to complete.
609 // Avoid live-lock
610 // TODO: consider calling SafepointSynchronize::do_call_back() while
611 // spinning to see if there's a safepoint pending. If so, immediately
612 // yielding or blocking would be appropriate. Avoid spinning while
613 // there is a safepoint pending.
614 // TODO: add inflation contention performance counters.
615 // TODO: restrict the aggregate number of spinners.
616
617 ++its;
618 if (its > 10000 || !os::is_MP()) {
619 if (its & 1) {
620 os::naked_yield();
621 TEVENT(Inflate: INFLATING - yield);
622 } else {
623 // Note that the following code attenuates the livelock problem but is not
624 // a complete remedy. A more complete solution would require that the inflating
625 // thread hold the associated inflation lock. The following code simply restricts
626 // the number of spinners to at most one. We'll have N-2 threads blocked
627 // on the inflationlock, 1 thread holding the inflation lock and using
628 // a yield/park strategy, and 1 thread in the midst of inflation.
629 // A more refined approach would be to change the encoding of INFLATING
630 // to allow encapsulation of a native thread pointer. Threads waiting for
631 // inflation to complete would use CAS to push themselves onto a singly linked
632 // list rooted at the markword. Once enqueued, they'd loop, checking a per-thread flag
633 // and calling park(). When inflation was complete the thread that accomplished inflation
634 // would detach the list and set the markword to inflated with a single CAS and
635 // then for each thread on the list, set the flag and unpark() the thread.
636 // This is conceptually similar to muxAcquire-muxRelease, except that muxRelease
637 // wakes at most one thread whereas we need to wake the entire list.
638 int ix = (cast_from_oop<intptr_t>(obj) >> 5) & (NINFLATIONLOCKS-1);
639 int YieldThenBlock = 0;
640 assert(ix >= 0 && ix < NINFLATIONLOCKS, "invariant");
641 assert((NINFLATIONLOCKS & (NINFLATIONLOCKS-1)) == 0, "invariant");
642 Thread::muxAcquire(gInflationLocks + ix, "gInflationLock");
643 while (obj->mark() == markOopDesc::INFLATING()) {
644 // Beware: NakedYield() is advisory and has almost no effect on some platforms
645 // so we periodically call Self->_ParkEvent->park(1).
646 // We use a mixed spin/yield/block mechanism.
647 if ((YieldThenBlock++) >= 16) {
648 Thread::current()->_ParkEvent->park(1);
649 } else {
650 os::naked_yield();
651 }
652 }
653 Thread::muxRelease(gInflationLocks + ix);
654 TEVENT(Inflate: INFLATING - yield/park);
655 }
656 } else {
657 SpinPause(); // SMP-polite spinning
658 }
659 }
660 }
661
662 // hashCode() generation :
663 //
664 // Possibilities:
665 // * MD5Digest of {obj,stwRandom}
666 // * CRC32 of {obj,stwRandom} or any linear-feedback shift register function.
667 // * A DES- or AES-style SBox[] mechanism
668 // * One of the Phi-based schemes, such as:
669 // 2654435761 = 2^32 * Phi (golden ratio)
670 // HashCodeValue = ((uintptr_t(obj) >> 3) * 2654435761) ^ GVars.stwRandom ;
671 // * A variation of Marsaglia's shift-xor RNG scheme.
672 // * (obj ^ stwRandom) is appealing, but can result
673 // in undesirable regularity in the hashCode values of adjacent objects
674 // (objects allocated back-to-back, in particular). This could potentially
675 // result in hashtable collisions and reduced hashtable efficiency.
676 // There are simple ways to "diffuse" the middle address bits over the
677 // generated hashCode values:
678
679 static inline intptr_t get_next_hash(Thread * Self, oop obj) {
680 intptr_t value = 0;
681 if (hashCode == 0) {
682 // This form uses an unguarded global Park-Miller RNG,
683 // so it's possible for two threads to race and generate the same RNG.
684 // On MP system we'll have lots of RW access to a global, so the
685 // mechanism induces lots of coherency traffic.
686 value = os::random();
687 } else if (hashCode == 1) {
688 // This variation has the property of being stable (idempotent)
689 // between STW operations. This can be useful in some of the 1-0
690 // synchronization schemes.
691 intptr_t addrBits = cast_from_oop<intptr_t>(obj) >> 3;
692 value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom;
693 } else if (hashCode == 2) {
694 value = 1; // for sensitivity testing
695 } else if (hashCode == 3) {
696 value = ++GVars.hcSequence;
697 } else if (hashCode == 4) {
698 value = cast_from_oop<intptr_t>(obj);
699 } else {
700 // Marsaglia's xor-shift scheme with thread-specific state
701 // This is probably the best overall implementation -- we'll
702 // likely make this the default in future releases.
703 unsigned t = Self->_hashStateX;
704 t ^= (t << 11);
705 Self->_hashStateX = Self->_hashStateY;
706 Self->_hashStateY = Self->_hashStateZ;
707 Self->_hashStateZ = Self->_hashStateW;
708 unsigned v = Self->_hashStateW;
709 v = (v ^ (v >> 19)) ^ (t ^ (t >> 8));
710 Self->_hashStateW = v;
711 value = v;
712 }
713
714 value &= markOopDesc::hash_mask;
715 if (value == 0) value = 0xBAD;
716 assert(value != markOopDesc::no_hash, "invariant");
717 TEVENT(hashCode: GENERATE);
718 return value;
719 }
720
721 intptr_t ObjectSynchronizer::FastHashCode(Thread * Self, oop obj) {
722 Retry:
723 if (UseBiasedLocking) {
724 // NOTE: many places throughout the JVM do not expect a safepoint
725 // to be taken here, in particular most operations on perm gen
726 // objects. However, we only ever bias Java instances and all of
727 // the call sites of identity_hash that might revoke biases have
728 // been checked to make sure they can handle a safepoint. The
729 // added check of the bias pattern is to avoid useless calls to
730 // thread-local storage.
731 if (obj->mark()->has_bias_pattern()) {
732 // Handle for oop obj in case of STW safepoint
733 Handle hobj(Self, obj);
734 // Relaxing assertion for bug 6320749.
735 assert(Universe::verify_in_progress() ||
736 !SafepointSynchronize::is_at_safepoint(),
737 "biases should not be seen by VM thread here");
738 BiasedLocking::revoke_and_rebias(hobj, false, JavaThread::current());
739 obj = hobj();
740 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
741 }
742 }
743
744 // hashCode() is a heap mutator ...
745 // Relaxing assertion for bug 6320749.
746 assert(Universe::verify_in_progress() || DumpSharedSpaces ||
747 !SafepointSynchronize::is_at_safepoint(), "invariant");
748 assert(Universe::verify_in_progress() || DumpSharedSpaces ||
749 Self->is_Java_thread() , "invariant");
750 assert(Universe::verify_in_progress() || DumpSharedSpaces ||
751 ((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
752
753 ObjectMonitor* monitor = NULL;
754 markOop temp, test;
755 intptr_t hash;
756 markOop mark = ReadStableMark(obj);
757
758 // object should remain ineligible for biased locking
759 assert(!mark->has_bias_pattern(), "invariant");
760
761 if (mark->is_neutral()) {
762 hash = mark->hash(); // this is a normal header
763 if (hash) { // if it has hash, just return it
764 return hash;
765 }
766 hash = get_next_hash(Self, obj); // allocate a new hash code
767 temp = mark->copy_set_hash(hash); // merge the hash code into header
768 // use (machine word version) atomic operation to install the hash
769 test = (markOop) Atomic::cmpxchg_ptr(temp, obj->mark_addr(), mark);
770 if (test == mark) {
771 return hash;
772 }
773 // If atomic operation failed, we must inflate the header
774 // into heavy weight monitor. We could add more code here
775 // for fast path, but it does not worth the complexity.
776 } else if (mark->has_monitor()) {
777 monitor = mark->monitor();
778 temp = monitor->header();
779 assert(temp->is_neutral() || temp->hash() == 0 && temp->is_marked(), "invariant");
780 hash = temp->hash();
781 if (hash) {
782 return hash;
783 }
784 // Skip to the following code to reduce code size
785 } else if (Self->is_lock_owned((address)mark->locker())) {
786 temp = mark->displaced_mark_helper(); // this is a lightweight monitor owned
787 assert(temp->is_neutral(), "invariant");
788 hash = temp->hash(); // by current thread, check if the displaced
789 if (hash) { // header contains hash code
790 return hash;
791 }
792 // WARNING:
793 // The displaced header is strictly immutable.
794 // It can NOT be changed in ANY cases. So we have
795 // to inflate the header into heavyweight monitor
796 // even the current thread owns the lock. The reason
797 // is the BasicLock (stack slot) will be asynchronously
798 // read by other threads during the inflate() function.
799 // Any change to stack may not propagate to other threads
800 // correctly.
801 }
802
803 // Inflate the monitor to set hash code
804 monitor = ObjectSynchronizer::inflate(Self, obj, inflate_cause_hash_code);
805 // Load displaced header and check it has hash code
806 mark = monitor->header();
807 assert(mark->is_neutral() || mark->hash() == 0 && mark->is_marked(), "invariant");
808 hash = mark->hash();
809 if (hash == 0) {
810 hash = get_next_hash(Self, obj);
811 temp = mark->set_unmarked()->copy_set_hash(hash); // merge hash code into header
812 assert(temp->is_neutral(), "invariant");
813 if (mark->is_marked()) {
814 // Monitor is being deflated. Try installing mark word with hash code into obj.
815 markOop monitor_mark = markOopDesc::encode(monitor);
816 if (obj->cas_set_mark(temp, monitor_mark) == monitor_mark) {
817 return hash;
818 } else {
819 // Somebody else installed a new mark word in obj. Start over. We are making progress,
820 // as the new mark word is not a pointer to monitor.
821 goto Retry;
822 }
823 }
824 test = (markOop) Atomic::cmpxchg_ptr(temp, monitor, mark);
825 if (test != mark) {
826 // The only update to the header in the monitor (outside GC) is install
827 // the hash code or mark the header to signal that the monitor is being
828 // deflated. If someone add new usage of displaced header, please update
829 // this code.
830 if (test->is_marked()) {
831 // Monitor is being deflated. Make progress by starting over.
832 assert(test->hash() == 0, "invariant");
833 goto Retry;
834 }
835 hash = test->hash();
836 assert(test->is_neutral(), "invariant");
837 assert(hash != 0, "Trivial unexpected object/monitor header usage.");
838 }
839 }
840 // We finally get the hash
841 return hash;
842 }
843
844 // Deprecated -- use FastHashCode() instead.
845
846 intptr_t ObjectSynchronizer::identity_hash_value_for(Handle obj) {
847 return FastHashCode(Thread::current(), obj());
848 }
849
850
851 bool ObjectSynchronizer::current_thread_holds_lock(JavaThread* thread,
852 Handle h_obj) {
853 if (UseBiasedLocking) {
854 BiasedLocking::revoke_and_rebias(h_obj, false, thread);
855 assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
856 }
857
858 assert(thread == JavaThread::current(), "Can only be called on current thread");
859 oop obj = h_obj();
860
861 markOop mark = ReadStableMark(obj);
862
863 // Uncontended case, header points to stack
864 if (mark->has_locker()) {
865 return thread->is_lock_owned((address)mark->locker());
866 }
867 // Contended case, header points to ObjectMonitor (tagged pointer)
868 if (mark->has_monitor()) {
869 ObjectMonitor* monitor = mark->monitor();
870 return monitor->is_entered(thread) != 0;
871 }
872 // Unlocked case, header in place
873 assert(mark->is_neutral(), "sanity check");
874 return false;
875 }
876
877 // Be aware of this method could revoke bias of the lock object.
878 // This method queries the ownership of the lock handle specified by 'h_obj'.
879 // If the current thread owns the lock, it returns owner_self. If no
880 // thread owns the lock, it returns owner_none. Otherwise, it will return
881 // owner_other.
882 ObjectSynchronizer::LockOwnership ObjectSynchronizer::query_lock_ownership
883 (JavaThread *self, Handle h_obj) {
884 // The caller must beware this method can revoke bias, and
885 // revocation can result in a safepoint.
886 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
887 assert(self->thread_state() != _thread_blocked, "invariant");
888
889 // Possible mark states: neutral, biased, stack-locked, inflated
890
891 if (UseBiasedLocking && h_obj()->mark()->has_bias_pattern()) {
892 // CASE: biased
893 BiasedLocking::revoke_and_rebias(h_obj, false, self);
894 assert(!h_obj->mark()->has_bias_pattern(),
895 "biases should be revoked by now");
896 }
897
898 assert(self == JavaThread::current(), "Can only be called on current thread");
899 oop obj = h_obj();
900 markOop mark = ReadStableMark(obj);
901
902 // CASE: stack-locked. Mark points to a BasicLock on the owner's stack.
903 if (mark->has_locker()) {
904 return self->is_lock_owned((address)mark->locker()) ?
905 owner_self : owner_other;
906 }
907
908 // CASE: inflated. Mark (tagged pointer) points to an objectMonitor.
909 // The Object:ObjectMonitor relationship is stable as long as we're
910 // not at a safepoint.
911 if (mark->has_monitor()) {
912 void * owner = mark->monitor()->_owner;
913 if (owner == NULL) return owner_none;
914 return (owner == self ||
915 self->is_lock_owned((address)owner)) ? owner_self : owner_other;
916 }
917
918 // CASE: neutral
919 assert(mark->is_neutral(), "sanity check");
920 return owner_none; // it's unlocked
921 }
922
923 // FIXME: jvmti should call this
924 JavaThread* ObjectSynchronizer::get_lock_owner(Handle h_obj, bool doLock) {
925 if (UseBiasedLocking) {
926 if (SafepointSynchronize::is_at_safepoint()) {
927 BiasedLocking::revoke_at_safepoint(h_obj);
928 } else {
929 BiasedLocking::revoke_and_rebias(h_obj, false, JavaThread::current());
930 }
931 assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
932 }
933
934 oop obj = h_obj();
935 address owner = NULL;
936
937 markOop mark = ReadStableMark(obj);
938
939 // Uncontended case, header points to stack
940 if (mark->has_locker()) {
941 owner = (address) mark->locker();
942 }
943
944 // Contended case, header points to ObjectMonitor (tagged pointer)
945 if (mark->has_monitor()) {
946 ObjectMonitor* monitor = mark->monitor();
947 assert(monitor != NULL, "monitor should be non-null");
948 owner = (address) monitor->owner();
949 }
950
951 if (owner != NULL) {
952 // owning_thread_from_monitor_owner() may also return NULL here
953 return Threads::owning_thread_from_monitor_owner(owner, doLock);
954 }
955
956 // Unlocked case, header in place
957 // Cannot have assertion since this object may have been
958 // locked by another thread when reaching here.
959 // assert(mark->is_neutral(), "sanity check");
960
961 return NULL;
962 }
963
964 // Visitors ...
965
966 void ObjectSynchronizer::monitors_iterate(MonitorClosure* closure) {
967 PaddedEnd<ObjectMonitor> * block =
968 (PaddedEnd<ObjectMonitor> *)OrderAccess::load_ptr_acquire(&gBlockList);
969 while (block != NULL) {
970 assert(block->object() == CHAINMARKER, "must be a block header");
971 for (int i = _BLOCKSIZE - 1; i > 0; i--) {
972 ObjectMonitor* mid = (ObjectMonitor *)(block + i);
973 oop object = (oop)mid->object();
974 if (object != NULL) {
975 closure->do_monitor(mid);
976 }
977 }
978 block = (PaddedEnd<ObjectMonitor> *)block->FreeNext;
979 }
980 }
981
982 // Get the next block in the block list.
983 static inline ObjectMonitor* next(ObjectMonitor* block) {
984 assert(block->object() == CHAINMARKER, "must be a block header");
985 block = block->FreeNext;
986 assert(block == NULL || block->object() == CHAINMARKER, "must be a block header");
987 return block;
988 }
989
990
991 void ObjectSynchronizer::oops_do(OopClosure* f) {
992 if (MonitorInUseLists) {
993 // When using thread local monitor lists, we only scan the
994 // global used list here (for moribund threads), and
995 // the thread-local monitors in Thread::oops_do().
996 global_used_oops_do(f);
997 } else {
998 global_oops_do(f);
999 }
1000 }
1001
1002 void ObjectSynchronizer::global_oops_do(OopClosure* f) {
1003 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1004 PaddedEnd<ObjectMonitor> * block =
1005 (PaddedEnd<ObjectMonitor> *)OrderAccess::load_ptr_acquire(&gBlockList);
1006 for (; block != NULL; block = (PaddedEnd<ObjectMonitor> *)next(block)) {
1007 assert(block->object() == CHAINMARKER, "must be a block header");
1008 for (int i = 1; i < _BLOCKSIZE; i++) {
1009 ObjectMonitor* mid = (ObjectMonitor *)&block[i];
1010 if (mid->is_active()) {
1011 assert(mid->object() != NULL, "invariant");
1012 f->do_oop((oop*)mid->object_addr());
1013 }
1014 }
1015 }
1016 }
1017
1018 void ObjectSynchronizer::global_used_oops_do(OopClosure* f) {
1019 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1020 list_oops_do(gOmInUseList, f);
1021 }
1022
1023 void ObjectSynchronizer::thread_local_used_oops_do(Thread* thread, OopClosure* f) {
1024 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1025 list_oops_do(thread->omInUseList, f);
1026 }
1027
1028 void ObjectSynchronizer::list_oops_do(ObjectMonitor* list, OopClosure* f) {
1029 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1030 ObjectMonitor* mid;
1031 for (mid = list; mid != NULL; mid = mid->FreeNext) {
1032 if (mid->object() != NULL) {
1033 f->do_oop((oop*)mid->object_addr());
1034 }
1035 }
1036 }
1037
1038
1039 // -----------------------------------------------------------------------------
1040 // ObjectMonitor Lifecycle
1041 // -----------------------
1042 // Inflation unlinks monitors from the global gFreeList and
1043 // associates them with objects. Deflation -- which occurs at
1044 // STW-time -- disassociates idle monitors from objects. Such
1045 // scavenged monitors are returned to the gFreeList.
1046 //
1047 // The global list is protected by gListLock. All the critical sections
1048 // are short and operate in constant-time.
1049 //
1050 // ObjectMonitors reside in type-stable memory (TSM) and are immortal.
1051 //
1052 // Lifecycle:
1053 // -- unassigned and on the global free list
1054 // -- unassigned and on a thread's private omFreeList
1055 // -- assigned to an object. The object is inflated and the mark refers
1056 // to the objectmonitor.
1057
1058
1059 // Constraining monitor pool growth via MonitorBound ...
1060 //
1061 // The monitor pool is grow-only. We scavenge at STW safepoint-time, but the
1062 // the rate of scavenging is driven primarily by GC. As such, we can find
1063 // an inordinate number of monitors in circulation.
1064 // To avoid that scenario we can artificially induce a STW safepoint
1065 // if the pool appears to be growing past some reasonable bound.
1066 // Generally we favor time in space-time tradeoffs, but as there's no
1067 // natural back-pressure on the # of extant monitors we need to impose some
1068 // type of limit. Beware that if MonitorBound is set to too low a value
1069 // we could just loop. In addition, if MonitorBound is set to a low value
1070 // we'll incur more safepoints, which are harmful to performance.
1071 // See also: GuaranteedSafepointInterval
1072 //
1073 // The current implementation uses asynchronous VM operations.
1074
1075 static void InduceScavenge(Thread * Self, const char * Whence) {
1076 // Induce STW safepoint to trim monitors
1077 // Ultimately, this results in a call to deflate_idle_monitors() in the near future.
1078 // More precisely, trigger an asynchronous STW safepoint as the number
1079 // of active monitors passes the specified threshold.
1080 // TODO: assert thread state is reasonable
1081
1082 if (ForceMonitorScavenge == 0 && Atomic::xchg (1, &ForceMonitorScavenge) == 0) {
1083 if (ObjectMonitor::Knob_Verbose) {
1084 tty->print_cr("INFO: Monitor scavenge - Induced STW @%s (%d)",
1085 Whence, ForceMonitorScavenge) ;
1086 tty->flush();
1087 }
1088 // Induce a 'null' safepoint to scavenge monitors
1089 // Must VM_Operation instance be heap allocated as the op will be enqueue and posted
1090 // to the VMthread and have a lifespan longer than that of this activation record.
1091 // The VMThread will delete the op when completed.
1092 VMThread::execute(new VM_ScavengeMonitors());
1093
1094 if (ObjectMonitor::Knob_Verbose) {
1095 tty->print_cr("INFO: Monitor scavenge - STW posted @%s (%d)",
1096 Whence, ForceMonitorScavenge) ;
1097 tty->flush();
1098 }
1099 }
1100 }
1101
1102 void ObjectSynchronizer::verifyInUse(Thread *Self) {
1103 ObjectMonitor* mid;
1104 int in_use_tally = 0;
1105 for (mid = Self->omInUseList; mid != NULL; mid = mid->FreeNext) {
1106 in_use_tally++;
1107 guarantee(mid->is_active(), "invariant");
1108 }
1109 guarantee(in_use_tally == Self->omInUseCount, "in-use count off");
1110
1111 int free_tally = 0;
1112 for (mid = Self->omFreeList; mid != NULL; mid = mid->FreeNext) {
1113 free_tally++;
1114 guarantee(mid->is_free(), "invariant");
1115 }
1116 guarantee(free_tally == Self->omFreeCount, "free count off");
1117 }
1118
1119 ObjectMonitor* ObjectSynchronizer::omAlloc(Thread * Self) {
1120 // A large MAXPRIVATE value reduces both list lock contention
1121 // and list coherency traffic, but also tends to increase the
1122 // number of objectMonitors in circulation as well as the STW
1123 // scavenge costs. As usual, we lean toward time in space-time
1124 // tradeoffs.
1125 const int MAXPRIVATE = 1024;
1126 for (;;) {
1127 ObjectMonitor * m;
1128
1129 // 1: try to allocate from the thread's local omFreeList.
1130 // Threads will attempt to allocate first from their local list, then
1131 // from the global list, and only after those attempts fail will the thread
1132 // attempt to instantiate new monitors. Thread-local free lists take
1133 // heat off the gListLock and improve allocation latency, as well as reducing
1134 // coherency traffic on the shared global list.
1135 m = Self->omFreeList;
1136 if (m != NULL) {
1137 Self->omFreeList = m->FreeNext;
1138 Self->omFreeCount--;
1139 // CONSIDER: set m->FreeNext = BAD -- diagnostic hygiene
1140 guarantee(m->object() == NULL, "invariant");
1141 m->set_allocation_state(ObjectMonitor::New);
1142 if (MonitorInUseLists) {
1143 m->FreeNext = Self->omInUseList;
1144 Self->omInUseList = m;
1145 Self->omInUseCount++;
1146 if (ObjectMonitor::Knob_VerifyInUse) {
1147 verifyInUse(Self);
1148 }
1149 } else {
1150 m->FreeNext = NULL;
1151 }
1152 assert(!m->is_free(), "post-condition");
1153 return m;
1154 }
1155
1156 // 2: try to allocate from the global gFreeList
1157 // CONSIDER: use muxTry() instead of muxAcquire().
1158 // If the muxTry() fails then drop immediately into case 3.
1159 // If we're using thread-local free lists then try
1160 // to reprovision the caller's free list.
1161 if (gFreeList != NULL) {
1162 // Reprovision the thread's omFreeList.
1163 // Use bulk transfers to reduce the allocation rate and heat
1164 // on various locks.
1165 Thread::muxAcquire(&gListLock, "omAlloc");
1166 for (int i = Self->omFreeProvision; --i >= 0 && gFreeList != NULL;) {
1167 gMonitorFreeCount--;
1168 ObjectMonitor * take = gFreeList;
1169 gFreeList = take->FreeNext;
1170 take->set_object(NULL);
1171 take->set_owner(NULL);
1172 take->_count = 0;
1173 guarantee(!take->is_busy(), "invariant");
1174 take->Recycle();
1175 assert(take->is_free(), "invariant");
1176 omRelease(Self, take, false);
1177 }
1178 Thread::muxRelease(&gListLock);
1179 Self->omFreeProvision += 1 + (Self->omFreeProvision/2);
1180 if (Self->omFreeProvision > MAXPRIVATE) Self->omFreeProvision = MAXPRIVATE;
1181 TEVENT(omFirst - reprovision);
1182
1183 const int mx = MonitorBound;
1184 if (mx > 0 && (gMonitorPopulation-gMonitorFreeCount) > mx) {
1185 // We can't safely induce a STW safepoint from omAlloc() as our thread
1186 // state may not be appropriate for such activities and callers may hold
1187 // naked oops, so instead we defer the action.
1188 InduceScavenge(Self, "omAlloc");
1189 }
1190 continue;
1191 }
1192
1193 // 3: allocate a block of new ObjectMonitors
1194 // Both the local and global free lists are empty -- resort to malloc().
1195 // In the current implementation objectMonitors are TSM - immortal.
1196 // Ideally, we'd write "new ObjectMonitor[_BLOCKSIZE], but we want
1197 // each ObjectMonitor to start at the beginning of a cache line,
1198 // so we use align_size_up().
1199 // A better solution would be to use C++ placement-new.
1200 // BEWARE: As it stands currently, we don't run the ctors!
1201 assert(_BLOCKSIZE > 1, "invariant");
1202 size_t neededsize = sizeof(PaddedEnd<ObjectMonitor>) * _BLOCKSIZE;
1203 PaddedEnd<ObjectMonitor> * temp;
1204 size_t aligned_size = neededsize + (DEFAULT_CACHE_LINE_SIZE - 1);
1205 void* real_malloc_addr = (void *)NEW_C_HEAP_ARRAY(char, aligned_size,
1206 mtInternal);
1207 temp = (PaddedEnd<ObjectMonitor> *)
1208 align_size_up((intptr_t)real_malloc_addr,
1209 DEFAULT_CACHE_LINE_SIZE);
1210
1211 // NOTE: (almost) no way to recover if allocation failed.
1212 // We might be able to induce a STW safepoint and scavenge enough
1213 // objectMonitors to permit progress.
1214 if (temp == NULL) {
1215 vm_exit_out_of_memory(neededsize, OOM_MALLOC_ERROR,
1216 "Allocate ObjectMonitors");
1217 }
1218 (void)memset((void *) temp, 0, neededsize);
1219
1220 // Format the block.
1221 // initialize the linked list, each monitor points to its next
1222 // forming the single linked free list, the very first monitor
1223 // will points to next block, which forms the block list.
1224 // The trick of using the 1st element in the block as gBlockList
1225 // linkage should be reconsidered. A better implementation would
1226 // look like: class Block { Block * next; int N; ObjectMonitor Body [N] ; }
1227
1228 for (int i = 1; i < _BLOCKSIZE; i++) {
1229 temp[i].FreeNext = (ObjectMonitor *)&temp[i+1];
1230 assert(temp[i].is_free(), "invariant");
1231 }
1232
1233 // terminate the last monitor as the end of list
1234 temp[_BLOCKSIZE - 1].FreeNext = NULL;
1235
1236 // Element [0] is reserved for global list linkage
1237 temp[0].set_object(CHAINMARKER);
1238
1239 // Consider carving out this thread's current request from the
1240 // block in hand. This avoids some lock traffic and redundant
1241 // list activity.
1242
1243 // Acquire the gListLock to manipulate gBlockList and gFreeList.
1244 // An Oyama-Taura-Yonezawa scheme might be more efficient.
1245 Thread::muxAcquire(&gListLock, "omAlloc [2]");
1246 gMonitorPopulation += _BLOCKSIZE-1;
1247 gMonitorFreeCount += _BLOCKSIZE-1;
1248
1249 // Add the new block to the list of extant blocks (gBlockList).
1250 // The very first objectMonitor in a block is reserved and dedicated.
1251 // It serves as blocklist "next" linkage.
1252 temp[0].FreeNext = gBlockList;
1253 // There are lock-free uses of gBlockList so make sure that
1254 // the previous stores happen before we update gBlockList.
1255 OrderAccess::release_store_ptr(&gBlockList, temp);
1256
1257 // Add the new string of objectMonitors to the global free list
1258 temp[_BLOCKSIZE - 1].FreeNext = gFreeList;
1259 gFreeList = temp + 1;
1260 Thread::muxRelease(&gListLock);
1261 TEVENT(Allocate block of monitors);
1262 }
1263 }
1264
1265 // Place "m" on the caller's private per-thread omFreeList.
1266 // In practice there's no need to clamp or limit the number of
1267 // monitors on a thread's omFreeList as the only time we'll call
1268 // omRelease is to return a monitor to the free list after a CAS
1269 // attempt failed. This doesn't allow unbounded #s of monitors to
1270 // accumulate on a thread's free list.
1271 //
1272 // Key constraint: all ObjectMonitors on a thread's free list and the global
1273 // free list must have their object field set to null. This prevents the
1274 // scavenger -- deflate_idle_monitors -- from reclaiming them.
1275
1276 void ObjectSynchronizer::omRelease(Thread * Self, ObjectMonitor * m,
1277 bool fromPerThreadAlloc) {
1278 guarantee(m->object() == NULL, "invariant");
1279 guarantee(((m->is_busy()|m->_recursions) == 0), "freeing in-use monitor");
1280 m->set_allocation_state(ObjectMonitor::Free);
1281 // Remove from omInUseList
1282 if (MonitorInUseLists && fromPerThreadAlloc) {
1283 ObjectMonitor* cur_mid_in_use = NULL;
1284 bool extracted = false;
1285 for (ObjectMonitor* mid = Self->omInUseList; mid != NULL; cur_mid_in_use = mid, mid = mid->FreeNext) {
1286 if (m == mid) {
1287 // extract from per-thread in-use list
1288 if (mid == Self->omInUseList) {
1289 Self->omInUseList = mid->FreeNext;
1290 } else if (cur_mid_in_use != NULL) {
1291 cur_mid_in_use->FreeNext = mid->FreeNext; // maintain the current thread in-use list
1292 }
1293 extracted = true;
1294 Self->omInUseCount--;
1295 if (ObjectMonitor::Knob_VerifyInUse) {
1296 verifyInUse(Self);
1297 }
1298 break;
1299 }
1300 }
1301 assert(extracted, "Should have extracted from in-use list");
1302 }
1303
1304 // FreeNext is used for both omInUseList and omFreeList, so clear old before setting new
1305 m->FreeNext = Self->omFreeList;
1306 Self->omFreeList = m;
1307 Self->omFreeCount++;
1308 }
1309
1310 // Return the monitors of a moribund thread's local free list to
1311 // the global free list. Typically a thread calls omFlush() when
1312 // it's dying. We could also consider having the VM thread steal
1313 // monitors from threads that have not run java code over a few
1314 // consecutive STW safepoints. Relatedly, we might decay
1315 // omFreeProvision at STW safepoints.
1316 //
1317 // Also return the monitors of a moribund thread's omInUseList to
1318 // a global gOmInUseList under the global list lock so these
1319 // will continue to be scanned.
1320 //
1321 // We currently call omFlush() from Threads::remove() _before the thread
1322 // has been excised from the thread list and is no longer a mutator.
1323 // This means that omFlush() can not run concurrently with a safepoint and
1324 // interleave with the scavenge operator. In particular, this ensures that
1325 // the thread's monitors are scanned by a GC safepoint, either via
1326 // Thread::oops_do() (if safepoint happens before omFlush()) or via
1327 // ObjectSynchronizer::oops_do() (if it happens after omFlush() and the thread's
1328 // monitors have been transferred to the global in-use list).
1329
1330 void ObjectSynchronizer::omFlush(Thread * Self) {
1331 ObjectMonitor * list = Self->omFreeList; // Null-terminated SLL
1332 Self->omFreeList = NULL;
1333 ObjectMonitor * tail = NULL;
1334 int tally = 0;
1335 if (list != NULL) {
1336 ObjectMonitor * s;
1337 // The thread is going away, the per-thread free monitors
1338 // are freed via set_owner(NULL)
1339 // Link them to tail, which will be linked into the global free list
1340 // gFreeList below, under the gListLock
1341 for (s = list; s != NULL; s = s->FreeNext) {
1342 tally++;
1343 tail = s;
1344 guarantee(s->object() == NULL, "invariant");
1345 guarantee(!s->is_busy(), "invariant");
1346 s->set_owner(NULL); // redundant but good hygiene
1347 TEVENT(omFlush - Move one);
1348 }
1349 guarantee(tail != NULL && list != NULL, "invariant");
1350 }
1351
1352 ObjectMonitor * inUseList = Self->omInUseList;
1353 ObjectMonitor * inUseTail = NULL;
1354 int inUseTally = 0;
1355 if (inUseList != NULL) {
1356 Self->omInUseList = NULL;
1357 ObjectMonitor *cur_om;
1358 // The thread is going away, however the omInUseList inflated
1359 // monitors may still be in-use by other threads.
1360 // Link them to inUseTail, which will be linked into the global in-use list
1361 // gOmInUseList below, under the gListLock
1362 for (cur_om = inUseList; cur_om != NULL; cur_om = cur_om->FreeNext) {
1363 inUseTail = cur_om;
1364 inUseTally++;
1365 }
1366 assert(Self->omInUseCount == inUseTally, "in-use count off");
1367 Self->omInUseCount = 0;
1368 guarantee(inUseTail != NULL && inUseList != NULL, "invariant");
1369 }
1370
1371 Thread::muxAcquire(&gListLock, "omFlush");
1372 if (tail != NULL) {
1373 tail->FreeNext = gFreeList;
1374 gFreeList = list;
1375 gMonitorFreeCount += tally;
1376 assert(Self->omFreeCount == tally, "free-count off");
1377 Self->omFreeCount = 0;
1378 }
1379
1380 if (inUseTail != NULL) {
1381 inUseTail->FreeNext = gOmInUseList;
1382 gOmInUseList = inUseList;
1383 gOmInUseCount += inUseTally;
1384 }
1385
1386 Thread::muxRelease(&gListLock);
1387 TEVENT(omFlush);
1388 }
1389
1390 // Fast path code shared by multiple functions
1391 ObjectMonitor* ObjectSynchronizer::inflate_helper(oop obj) {
1392 markOop mark = obj->mark();
1393 if (mark->has_monitor()) {
1394 assert(ObjectSynchronizer::verify_objmon_isinpool(mark->monitor()), "monitor is invalid");
1395 markOop dmw = mark->monitor()->header();
1396 assert(dmw->is_neutral() || dmw->hash() == 0 && dmw->is_marked(), "monitor must record a good object header");
1397 if (dmw->is_neutral()) {
1398 return mark->monitor();
1399 }
1400 }
1401 return ObjectSynchronizer::inflate(Thread::current(),
1402 obj,
1403 inflate_cause_vm_internal);
1404 }
1405
1406 ObjectMonitor* ObjectSynchronizer::inflate(Thread * Self,
1407 oop object,
1408 const InflateCause cause) {
1409 Retry:
1410 // Inflate mutates the heap ...
1411 // Relaxing assertion for bug 6320749.
1412 assert(Universe::verify_in_progress() ||
1413 !SafepointSynchronize::is_at_safepoint(), "invariant");
1414
1415 EventJavaMonitorInflate event;
1416
1417 for (;;) {
1418 const markOop mark = object->mark();
1419 assert(!mark->has_bias_pattern(), "invariant");
1420
1421 // The mark can be in one of the following states:
1422 // * Inflated - just return
1423 // * Stack-locked - coerce it to inflated
1424 // * INFLATING - busy wait for conversion to complete
1425 // * Neutral - aggressively inflate the object.
1426 // * BIASED - Illegal. We should never see this
1427
1428 // CASE: inflated
1429 if (mark->has_monitor()) {
1430 ObjectMonitor * inf = mark->monitor();
1431 markOop dmw = inf->header();
1432 assert(dmw->is_neutral() || dmw->hash() == 0 && dmw->is_marked(), "invariant");
1433 if (dmw->is_marked()) {
1434 goto Retry;
1435 }
1436 assert(inf->object() == object, "invariant");
1437 assert(ObjectSynchronizer::verify_objmon_isinpool(inf), "monitor is invalid");
1438 event.cancel(); // let's not post an inflation event, unless we did the deed ourselves
1439 return inf;
1440 }
1441
1442 // CASE: inflation in progress - inflating over a stack-lock.
1443 // Some other thread is converting from stack-locked to inflated.
1444 // Only that thread can complete inflation -- other threads must wait.
1445 // The INFLATING value is transient.
1446 // Currently, we spin/yield/park and poll the markword, waiting for inflation to finish.
1447 // We could always eliminate polling by parking the thread on some auxiliary list.
1448 if (mark == markOopDesc::INFLATING()) {
1449 TEVENT(Inflate: spin while INFLATING);
1450 ReadStableMark(object);
1451 continue;
1452 }
1453
1454 // CASE: stack-locked
1455 // Could be stack-locked either by this thread or by some other thread.
1456 //
1457 // Note that we allocate the objectmonitor speculatively, _before_ attempting
1458 // to install INFLATING into the mark word. We originally installed INFLATING,
1459 // allocated the objectmonitor, and then finally STed the address of the
1460 // objectmonitor into the mark. This was correct, but artificially lengthened
1461 // the interval in which INFLATED appeared in the mark, thus increasing
1462 // the odds of inflation contention.
1463 //
1464 // We now use per-thread private objectmonitor free lists.
1465 // These list are reprovisioned from the global free list outside the
1466 // critical INFLATING...ST interval. A thread can transfer
1467 // multiple objectmonitors en-mass from the global free list to its local free list.
1468 // This reduces coherency traffic and lock contention on the global free list.
1469 // Using such local free lists, it doesn't matter if the omAlloc() call appears
1470 // before or after the CAS(INFLATING) operation.
1471 // See the comments in omAlloc().
1472
1473 if (mark->has_locker()) {
1474 ObjectMonitor * m = omAlloc(Self);
1475 // Optimistically prepare the objectmonitor - anticipate successful CAS
1476 // We do this before the CAS in order to minimize the length of time
1477 // in which INFLATING appears in the mark.
1478 m->Recycle();
1479 m->_Responsible = NULL;
1480 m->_recursions = 0;
1481 m->_SpinDuration = ObjectMonitor::Knob_SpinLimit; // Consider: maintain by type/class
1482
1483 markOop cmp = (markOop) Atomic::cmpxchg_ptr(markOopDesc::INFLATING(), object->mark_addr(), mark);
1484 if (cmp != mark) {
1485 omRelease(Self, m, true);
1486 continue; // Interference -- just retry
1487 }
1488
1489 // We've successfully installed INFLATING (0) into the mark-word.
1490 // This is the only case where 0 will appear in a mark-word.
1491 // Only the singular thread that successfully swings the mark-word
1492 // to 0 can perform (or more precisely, complete) inflation.
1493 //
1494 // Why do we CAS a 0 into the mark-word instead of just CASing the
1495 // mark-word from the stack-locked value directly to the new inflated state?
1496 // Consider what happens when a thread unlocks a stack-locked object.
1497 // It attempts to use CAS to swing the displaced header value from the
1498 // on-stack basiclock back into the object header. Recall also that the
1499 // header value (hashcode, etc) can reside in (a) the object header, or
1500 // (b) a displaced header associated with the stack-lock, or (c) a displaced
1501 // header in an objectMonitor. The inflate() routine must copy the header
1502 // value from the basiclock on the owner's stack to the objectMonitor, all
1503 // the while preserving the hashCode stability invariants. If the owner
1504 // decides to release the lock while the value is 0, the unlock will fail
1505 // and control will eventually pass from slow_exit() to inflate. The owner
1506 // will then spin, waiting for the 0 value to disappear. Put another way,
1507 // the 0 causes the owner to stall if the owner happens to try to
1508 // drop the lock (restoring the header from the basiclock to the object)
1509 // while inflation is in-progress. This protocol avoids races that might
1510 // would otherwise permit hashCode values to change or "flicker" for an object.
1511 // Critically, while object->mark is 0 mark->displaced_mark_helper() is stable.
1512 // 0 serves as a "BUSY" inflate-in-progress indicator.
1513
1514
1515 // fetch the displaced mark from the owner's stack.
1516 // The owner can't die or unwind past the lock while our INFLATING
1517 // object is in the mark. Furthermore the owner can't complete
1518 // an unlock on the object, either.
1519 markOop dmw = mark->displaced_mark_helper();
1520 assert(dmw->is_neutral(), "invariant");
1521
1522 // Setup monitor fields to proper values -- prepare the monitor
1523 m->set_header(dmw);
1524
1525 // Optimization: if the mark->locker stack address is associated
1526 // with this thread we could simply set m->_owner = Self.
1527 // Note that a thread can inflate an object
1528 // that it has stack-locked -- as might happen in wait() -- directly
1529 // with CAS. That is, we can avoid the xchg-NULL .... ST idiom.
1530 m->set_owner(mark->locker());
1531 m->set_object(object);
1532 // TODO-FIXME: assert BasicLock->dhw != 0.
1533
1534 // Must preserve store ordering. The monitor state must
1535 // be stable at the time of publishing the monitor address.
1536 guarantee(object->mark() == markOopDesc::INFLATING(), "invariant");
1537 object->release_set_mark(markOopDesc::encode(m));
1538
1539 // Hopefully the performance counters are allocated on distinct cache lines
1540 // to avoid false sharing on MP systems ...
1541 OM_PERFDATA_OP(Inflations, inc());
1542 TEVENT(Inflate: overwrite stacklock);
1543 if (log_is_enabled(Debug, monitorinflation)) {
1544 if (object->is_instance()) {
1545 ResourceMark rm;
1546 log_debug(monitorinflation)("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1547 p2i(object), p2i(object->mark()),
1548 object->klass()->external_name());
1549 }
1550 }
1551 if (event.should_commit()) {
1552 post_monitor_inflate_event(event, object, cause);
1553 }
1554 return m;
1555 }
1556
1557 // CASE: neutral
1558 // TODO-FIXME: for entry we currently inflate and then try to CAS _owner.
1559 // If we know we're inflating for entry it's better to inflate by swinging a
1560 // pre-locked objectMonitor pointer into the object header. A successful
1561 // CAS inflates the object *and* confers ownership to the inflating thread.
1562 // In the current implementation we use a 2-step mechanism where we CAS()
1563 // to inflate and then CAS() again to try to swing _owner from NULL to Self.
1564 // An inflateTry() method that we could call from fast_enter() and slow_enter()
1565 // would be useful.
1566
1567 assert(mark->is_neutral(), "invariant");
1568 ObjectMonitor * m = omAlloc(Self);
1569 // prepare m for installation - set monitor to initial state
1570 m->Recycle();
1571 m->set_header(mark);
1572 m->set_owner(NULL);
1573 m->set_object(object);
1574 m->_recursions = 0;
1575 m->_Responsible = NULL;
1576 m->_SpinDuration = ObjectMonitor::Knob_SpinLimit; // consider: keep metastats by type/class
1577
1578 if (Atomic::cmpxchg_ptr (markOopDesc::encode(m), object->mark_addr(), mark) != mark) {
1579 m->set_object(NULL);
1580 m->set_owner(NULL);
1581 m->Recycle();
1582 omRelease(Self, m, true);
1583 m = NULL;
1584 continue;
1585 // interference - the markword changed - just retry.
1586 // The state-transitions are one-way, so there's no chance of
1587 // live-lock -- "Inflated" is an absorbing state.
1588 }
1589
1590 // Hopefully the performance counters are allocated on distinct
1591 // cache lines to avoid false sharing on MP systems ...
1592 OM_PERFDATA_OP(Inflations, inc());
1593 TEVENT(Inflate: overwrite neutral);
1594 if (log_is_enabled(Debug, monitorinflation)) {
1595 if (object->is_instance()) {
1596 ResourceMark rm;
1597 log_debug(monitorinflation)("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1598 p2i(object), p2i(object->mark()),
1599 object->klass()->external_name());
1600 }
1601 }
1602 if (event.should_commit()) {
1603 post_monitor_inflate_event(event, object, cause);
1604 }
1605 return m;
1606 }
1607 }
1608
1609
1610 // Deflate_idle_monitors() is called at all safepoints, immediately
1611 // after all mutators are stopped, but before any objects have moved.
1612 // It traverses the list of known monitors, deflating where possible.
1613 // The scavenged monitor are returned to the monitor free list.
1614 //
1615 // Beware that we scavenge at *every* stop-the-world point.
1616 // Having a large number of monitors in-circulation negatively
1617 // impacts the performance of some applications (e.g., PointBase).
1618 // Broadly, we want to minimize the # of monitors in circulation.
1619 //
1620 // We have added a flag, MonitorInUseLists, which creates a list
1621 // of active monitors for each thread. deflate_idle_monitors()
1622 // only scans the per-thread in-use lists. omAlloc() puts all
1623 // assigned monitors on the per-thread list. deflate_idle_monitors()
1624 // returns the non-busy monitors to the global free list.
1625 // When a thread dies, omFlush() adds the list of active monitors for
1626 // that thread to a global gOmInUseList acquiring the
1627 // global list lock. deflate_idle_monitors() acquires the global
1628 // list lock to scan for non-busy monitors to the global free list.
1629 // An alternative could have used a single global in-use list. The
1630 // downside would have been the additional cost of acquiring the global list lock
1631 // for every omAlloc().
1632 //
1633 // Perversely, the heap size -- and thus the STW safepoint rate --
1634 // typically drives the scavenge rate. Large heaps can mean infrequent GC,
1635 // which in turn can mean large(r) numbers of objectmonitors in circulation.
1636 // This is an unfortunate aspect of this design.
1637
1638 enum ManifestConstants {
1639 ClearResponsibleAtSTW = 0
1640 };
1641
1642 void ObjectSynchronizer::do_safepoint_work() {
1643 if (MonitorInUseLists || !AsyncDeflateIdleMonitors) {
1644 ObjectSynchronizer::deflate_idle_monitors();
1645 return;
1646 }
1647 assert(AsyncDeflateIdleMonitors, "oops");
1648 if (gFreeListNextSafepoint != NULL) {
1649 #ifdef ASSERT
1650 for (ObjectMonitor* monitor = gFreeListNextSafepoint; monitor != NULL; monitor = monitor->FreeNext) {
1651 guarantee(monitor->owner() == NULL, "invariant");
1652 guarantee(monitor->waiters() == 0, "invariant");
1653 guarantee(monitor->recursions() == 0, "invariant");
1654 guarantee(monitor->object() != NULL, "invariant");
1655 guarantee(monitor->header() != 0, "invariant");
1656 guarantee(monitor->is_free(), "invariant");
1657 }
1658 guarantee(gFreeListNextSafepointTail != NULL, "invariant");
1659 #endif // def ASSERT
1660
1661 Thread::muxAcquire(&gListLock, "do_safepoint_work");
1662 gFreeListNextSafepointTail->FreeNext = gFreeList;
1663 gFreeList = gFreeListNextSafepoint;
1664 gMonitorFreeCount += gMonitorFreeCountNextSafepoint;
1665 Thread::muxRelease(&gListLock);
1666
1667 gFreeListNextSafepoint = NULL;
1668 gFreeListNextSafepointTail = NULL;
1669 gMonitorFreeCountNextSafepoint = 0;
1670 }
1671 set_should_deflate_idle_monitors_conc();
1672 MutexLockerEx ml(Service_lock, Mutex::_no_safepoint_check_flag);
1673 Service_lock->notify_all();
1674 }
1675
1676 void ObjectSynchronizer::append_to_freelist_for_after_safepoint(int nScavenged, ObjectMonitor* const head, ObjectMonitor* const tail) {
1677 #ifdef ASSERT
1678 int count = 0;
1679 for(ObjectMonitor* m = head; m != NULL; m = m->FreeNext) { count++; }
1680 guarantee(count == nScavenged, "invariant");
1681 #endif // def ASSERT
1682 if (head != NULL) {
1683 assert(tail->FreeNext == NULL, "invariant");
1684 tail->FreeNext = gFreeListNextSafepoint;
1685 gFreeListNextSafepoint = head;
1686 }
1687 if (gFreeListNextSafepointTail == NULL) {
1688 gFreeListNextSafepointTail = tail;
1689 }
1690 gMonitorFreeCountNextSafepoint += nScavenged;
1691 OM_PERFDATA_OP(Deflations, inc(nScavenged));
1692 }
1693
1694 void ObjectSynchronizer::deflate_idle_monitors_conc() {
1695 assert(Thread::current()->is_Java_thread(), "precondition");
1696 _should_deflate_idle_monitors_conc = false;
1697 if (MonitorInUseLists) {
1698 return; // Don't want to run over the thread list for now.
1699 }
1700
1701 ObjectMonitor* freeHeadp = NULL;
1702 ObjectMonitor* freeTailp = NULL;
1703 int nScavenged = 0;
1704 int nInuse = 0;
1705 int nInCirculation = 0;
1706
1707 PaddedEnd<ObjectMonitor> * block =
1708 (PaddedEnd<ObjectMonitor> *)OrderAccess::load_ptr_acquire(&gBlockList);
1709 for (; block != NULL; block = (PaddedEnd<ObjectMonitor> *)next(block)) {
1710 // Iterate over all extant monitors - Scavenge all idle monitors.
1711 assert(block->object() == CHAINMARKER, "must be a block header");
1712 if (SafepointSynchronize::is_synchronizing()) {
1713 append_to_freelist_for_after_safepoint(nScavenged, freeHeadp, freeTailp);
1714 nScavenged = 0;
1715 freeHeadp = NULL;
1716 freeTailp = NULL;
1717 JavaThread* const jt = (JavaThread*) Thread::current();
1718 ThreadBlockInVM blocker(jt);
1719 }
1720 nInCirculation += _BLOCKSIZE;
1721 for (int i = 1; i < _BLOCKSIZE; i++) {
1722 ObjectMonitor* mid = (ObjectMonitor*)&block[i];
1723 if (!mid->is_old()) {
1724 // Skip deflating newly allocated or free monitors.
1725 if (mid->is_new()) {
1726 // Mark mid as "old".
1727 mid->set_allocation_state(ObjectMonitor::Old);
1728 }
1729 continue;
1730 }
1731
1732 oop obj = (oop)mid->object();
1733 assert(obj != NULL, "invariant");
1734
1735 if (mid->try_disable_monitor()) {
1736 mid->FreeNext = NULL;
1737 if (freeHeadp == NULL) { freeHeadp = mid; }
1738 if (freeTailp != NULL) { freeTailp->FreeNext = mid; }
1739 freeTailp = mid;
1740 nScavenged++;
1741 } else {
1742 nInuse++;
1743 }
1744 }
1745 }
1746 append_to_freelist_for_after_safepoint(nScavenged, freeHeadp, freeTailp);
1747 OM_PERFDATA_OP(MonExtant, set_value(nInCirculation));
1748 }
1749
1750 // Deflate a single monitor if not in-use
1751 // Return true if deflated, false if in-use
1752 bool ObjectSynchronizer::deflate_monitor(ObjectMonitor* mid, oop obj,
1753 ObjectMonitor** freeHeadp,
1754 ObjectMonitor** freeTailp) {
1755 bool deflated;
1756 // Normal case ... The monitor is associated with obj.
1757 guarantee(obj->mark() == markOopDesc::encode(mid), "invariant");
1758 guarantee(mid == obj->mark()->monitor(), "invariant");
1759 guarantee(mid->header()->is_neutral(), "invariant");
1760
1761 if (mid->is_busy()) {
1762 if (ClearResponsibleAtSTW) mid->_Responsible = NULL;
1763 deflated = false;
1764 } else {
1765 // Deflate the monitor if it is no longer being used
1766 // It's idle - scavenge and return to the global free list
1767 // plain old deflation ...
1768 TEVENT(deflate_idle_monitors - scavenge1);
1769 if (log_is_enabled(Debug, monitorinflation)) {
1770 if (obj->is_instance()) {
1771 ResourceMark rm;
1772 log_debug(monitorinflation)("Deflating object " INTPTR_FORMAT " , "
1773 "mark " INTPTR_FORMAT " , type %s",
1774 p2i(obj), p2i(obj->mark()),
1775 obj->klass()->external_name());
1776 }
1777 }
1778
1779 // Restore the header back to obj
1780 obj->release_set_mark(mid->header());
1781 mid->clear();
1782
1783 assert(mid->object() == NULL, "invariant");
1784 assert(mid->is_free(), "invariant");
1785
1786 // Move the object to the working free list defined by freeHeadp, freeTailp
1787 if (*freeHeadp == NULL) *freeHeadp = mid;
1788 if (*freeTailp != NULL) {
1789 ObjectMonitor * prevtail = *freeTailp;
1790 assert(prevtail->FreeNext == NULL, "cleaned up deflated?");
1791 prevtail->FreeNext = mid;
1792 }
1793 *freeTailp = mid;
1794 deflated = true;
1795 }
1796 return deflated;
1797 }
1798
1799 // Walk a given monitor list, and deflate idle monitors
1800 // The given list could be a per-thread list or a global list
1801 // Caller acquires gListLock
1802 int ObjectSynchronizer::deflate_monitor_list(ObjectMonitor** listHeadp,
1803 ObjectMonitor** freeHeadp,
1804 ObjectMonitor** freeTailp) {
1805 ObjectMonitor* mid;
1806 ObjectMonitor* next;
1807 ObjectMonitor* cur_mid_in_use = NULL;
1808 int deflated_count = 0;
1809
1810 for (mid = *listHeadp; mid != NULL;) {
1811 oop obj = (oop) mid->object();
1812 if (obj != NULL && deflate_monitor(mid, obj, freeHeadp, freeTailp)) {
1813 // if deflate_monitor succeeded,
1814 // extract from per-thread in-use list
1815 if (mid == *listHeadp) {
1816 *listHeadp = mid->FreeNext;
1817 } else if (cur_mid_in_use != NULL) {
1818 cur_mid_in_use->FreeNext = mid->FreeNext; // maintain the current thread in-use list
1819 }
1820 next = mid->FreeNext;
1821 mid->FreeNext = NULL; // This mid is current tail in the freeHeadp list
1822 mid = next;
1823 deflated_count++;
1824 } else {
1825 cur_mid_in_use = mid;
1826 mid = mid->FreeNext;
1827 }
1828 }
1829 return deflated_count;
1830 }
1831
1832 void ObjectSynchronizer::deflate_idle_monitors() {
1833 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1834 int nInuse = 0; // currently associated with objects
1835 int nInCirculation = 0; // extant
1836 int nScavenged = 0; // reclaimed
1837 bool deflated = false;
1838
1839 ObjectMonitor * freeHeadp = NULL; // Local SLL of scavenged monitors
1840 ObjectMonitor * freeTailp = NULL;
1841
1842 TEVENT(deflate_idle_monitors);
1843 // Prevent omFlush from changing mids in Thread dtor's during deflation
1844 // And in case the vm thread is acquiring a lock during a safepoint
1845 // See e.g. 6320749
1846 Thread::muxAcquire(&gListLock, "scavenge - return");
1847
1848 if (MonitorInUseLists) {
1849 int inUse = 0;
1850 for (JavaThread* cur = Threads::first(); cur != NULL; cur = cur->next()) {
1851 nInCirculation+= cur->omInUseCount;
1852 int deflated_count = deflate_monitor_list(cur->omInUseList_addr(), &freeHeadp, &freeTailp);
1853 cur->omInUseCount-= deflated_count;
1854 if (ObjectMonitor::Knob_VerifyInUse) {
1855 verifyInUse(cur);
1856 }
1857 nScavenged += deflated_count;
1858 nInuse += cur->omInUseCount;
1859 }
1860
1861 // For moribund threads, scan gOmInUseList
1862 if (gOmInUseList) {
1863 nInCirculation += gOmInUseCount;
1864 int deflated_count = deflate_monitor_list((ObjectMonitor **)&gOmInUseList, &freeHeadp, &freeTailp);
1865 gOmInUseCount-= deflated_count;
1866 nScavenged += deflated_count;
1867 nInuse += gOmInUseCount;
1868 }
1869
1870 } else {
1871 PaddedEnd<ObjectMonitor> * block =
1872 (PaddedEnd<ObjectMonitor> *)OrderAccess::load_ptr_acquire(&gBlockList);
1873 for (; block != NULL; block = (PaddedEnd<ObjectMonitor> *)next(block)) {
1874 // Iterate over all extant monitors - Scavenge all idle monitors.
1875 assert(block->object() == CHAINMARKER, "must be a block header");
1876 nInCirculation += _BLOCKSIZE;
1877 for (int i = 1; i < _BLOCKSIZE; i++) {
1878 ObjectMonitor* mid = (ObjectMonitor*)&block[i];
1879 oop obj = (oop)mid->object();
1880
1881 if (obj == NULL) {
1882 // The monitor is not associated with an object.
1883 // The monitor should either be a thread-specific private
1884 // free list or the global free list.
1885 // obj == NULL IMPLIES mid->is_busy() == 0
1886 guarantee(!mid->is_busy(), "invariant");
1887 continue;
1888 }
1889 deflated = deflate_monitor(mid, obj, &freeHeadp, &freeTailp);
1890
1891 if (deflated) {
1892 mid->FreeNext = NULL;
1893 nScavenged++;
1894 } else {
1895 nInuse++;
1896 }
1897 }
1898 }
1899 }
1900
1901 gMonitorFreeCount += nScavenged;
1902
1903 // Consider: audit gFreeList to ensure that gMonitorFreeCount and list agree.
1904
1905 if (ObjectMonitor::Knob_Verbose) {
1906 tty->print_cr("INFO: Deflate: InCirc=%d InUse=%d Scavenged=%d "
1907 "ForceMonitorScavenge=%d : pop=%d free=%d",
1908 nInCirculation, nInuse, nScavenged, ForceMonitorScavenge,
1909 gMonitorPopulation, gMonitorFreeCount);
1910 tty->flush();
1911 }
1912
1913 ForceMonitorScavenge = 0; // Reset
1914
1915 // Move the scavenged monitors back to the global free list.
1916 if (freeHeadp != NULL) {
1917 guarantee(freeTailp != NULL && nScavenged > 0, "invariant");
1918 assert(freeTailp->FreeNext == NULL, "invariant");
1919 // constant-time list splice - prepend scavenged segment to gFreeList
1920 freeTailp->FreeNext = gFreeList;
1921 gFreeList = freeHeadp;
1922 }
1923 Thread::muxRelease(&gListLock);
1924
1925 OM_PERFDATA_OP(Deflations, inc(nScavenged));
1926 OM_PERFDATA_OP(MonExtant, set_value(nInCirculation));
1927
1928 // TODO: Add objectMonitor leak detection.
1929 // Audit/inventory the objectMonitors -- make sure they're all accounted for.
1930 GVars.stwRandom = os::random();
1931 GVars.stwCycle++;
1932 }
1933
1934 // Monitor cleanup on JavaThread::exit
1935
1936 // Iterate through monitor cache and attempt to release thread's monitors
1937 // Gives up on a particular monitor if an exception occurs, but continues
1938 // the overall iteration, swallowing the exception.
1939 class ReleaseJavaMonitorsClosure: public MonitorClosure {
1940 private:
1941 TRAPS;
1942
1943 public:
1944 ReleaseJavaMonitorsClosure(Thread* thread) : THREAD(thread) {}
1945 void do_monitor(ObjectMonitor* mid) {
1946 if (mid->owner() == THREAD) {
1947 if (ObjectMonitor::Knob_VerifyMatch != 0) {
1948 ResourceMark rm;
1949 Handle obj(THREAD, (oop) mid->object());
1950 tty->print("INFO: unexpected locked object:");
1951 javaVFrame::print_locked_object_class_name(tty, obj, "locked");
1952 fatal("exiting JavaThread=" INTPTR_FORMAT
1953 " unexpectedly owns ObjectMonitor=" INTPTR_FORMAT,
1954 p2i(THREAD), p2i(mid));
1955 }
1956 (void)mid->complete_exit(CHECK);
1957 }
1958 }
1959 };
1960
1961 // Release all inflated monitors owned by THREAD. Lightweight monitors are
1962 // ignored. This is meant to be called during JNI thread detach which assumes
1963 // all remaining monitors are heavyweight. All exceptions are swallowed.
1964 // Scanning the extant monitor list can be time consuming.
1965 // A simple optimization is to add a per-thread flag that indicates a thread
1966 // called jni_monitorenter() during its lifetime.
1967 //
1968 // Instead of No_Savepoint_Verifier it might be cheaper to
1969 // use an idiom of the form:
1970 // auto int tmp = SafepointSynchronize::_safepoint_counter ;
1971 // <code that must not run at safepoint>
1972 // guarantee (((tmp ^ _safepoint_counter) | (tmp & 1)) == 0) ;
1973 // Since the tests are extremely cheap we could leave them enabled
1974 // for normal product builds.
1975
1976 void ObjectSynchronizer::release_monitors_owned_by_thread(TRAPS) {
1977 assert(THREAD == JavaThread::current(), "must be current Java thread");
1978 NoSafepointVerifier nsv;
1979 ReleaseJavaMonitorsClosure rjmc(THREAD);
1980 Thread::muxAcquire(&gListLock, "release_monitors_owned_by_thread");
1981 ObjectSynchronizer::monitors_iterate(&rjmc);
1982 Thread::muxRelease(&gListLock);
1983 THREAD->clear_pending_exception();
1984 }
1985
1986 const char* ObjectSynchronizer::inflate_cause_name(const InflateCause cause) {
1987 switch (cause) {
1988 case inflate_cause_vm_internal: return "VM Internal";
1989 case inflate_cause_monitor_enter: return "Monitor Enter";
1990 case inflate_cause_wait: return "Monitor Wait";
1991 case inflate_cause_notify: return "Monitor Notify";
1992 case inflate_cause_hash_code: return "Monitor Hash Code";
1993 case inflate_cause_jni_enter: return "JNI Monitor Enter";
1994 case inflate_cause_jni_exit: return "JNI Monitor Exit";
1995 default:
1996 ShouldNotReachHere();
1997 }
1998 return "Unknown";
1999 }
2000
2001 static void post_monitor_inflate_event(EventJavaMonitorInflate& event,
2002 const oop obj,
2003 const ObjectSynchronizer::InflateCause cause) {
2004 #if INCLUDE_TRACE
2005 assert(event.should_commit(), "check outside");
2006 event.set_monitorClass(obj->klass());
2007 event.set_address((TYPE_ADDRESS)(uintptr_t)(void*)obj);
2008 event.set_cause((u1)cause);
2009 event.commit();
2010 #endif
2011 }
2012
2013 //------------------------------------------------------------------------------
2014 // Debugging code
2015
2016 void ObjectSynchronizer::sanity_checks(const bool verbose,
2017 const uint cache_line_size,
2018 int *error_cnt_ptr,
2019 int *warning_cnt_ptr) {
2020 u_char *addr_begin = (u_char*)&GVars;
2021 u_char *addr_stwRandom = (u_char*)&GVars.stwRandom;
2022 u_char *addr_hcSequence = (u_char*)&GVars.hcSequence;
2023
2024 if (verbose) {
2025 tty->print_cr("INFO: sizeof(SharedGlobals)=" SIZE_FORMAT,
2026 sizeof(SharedGlobals));
2027 }
2028
2029 uint offset_stwRandom = (uint)(addr_stwRandom - addr_begin);
2030 if (verbose) tty->print_cr("INFO: offset(stwRandom)=%u", offset_stwRandom);
2031
2032 uint offset_hcSequence = (uint)(addr_hcSequence - addr_begin);
2033 if (verbose) {
2034 tty->print_cr("INFO: offset(_hcSequence)=%u", offset_hcSequence);
2035 }
2036
2037 if (cache_line_size != 0) {
2038 // We were able to determine the L1 data cache line size so
2039 // do some cache line specific sanity checks
2040
2041 if (offset_stwRandom < cache_line_size) {
2042 tty->print_cr("WARNING: the SharedGlobals.stwRandom field is closer "
2043 "to the struct beginning than a cache line which permits "
2044 "false sharing.");
2045 (*warning_cnt_ptr)++;
2046 }
2047
2048 if ((offset_hcSequence - offset_stwRandom) < cache_line_size) {
2049 tty->print_cr("WARNING: the SharedGlobals.stwRandom and "
2050 "SharedGlobals.hcSequence fields are closer than a cache "
2051 "line which permits false sharing.");
2052 (*warning_cnt_ptr)++;
2053 }
2054
2055 if ((sizeof(SharedGlobals) - offset_hcSequence) < cache_line_size) {
2056 tty->print_cr("WARNING: the SharedGlobals.hcSequence field is closer "
2057 "to the struct end than a cache line which permits false "
2058 "sharing.");
2059 (*warning_cnt_ptr)++;
2060 }
2061 }
2062 }
2063
2064 #ifndef PRODUCT
2065
2066 // Check if monitor belongs to the monitor cache
2067 // The list is grow-only so it's *relatively* safe to traverse
2068 // the list of extant blocks without taking a lock.
2069
2070 int ObjectSynchronizer::verify_objmon_isinpool(ObjectMonitor *monitor) {
2071 PaddedEnd<ObjectMonitor> * block =
2072 (PaddedEnd<ObjectMonitor> *)OrderAccess::load_ptr_acquire(&gBlockList);
2073 while (block != NULL) {
2074 assert(block->object() == CHAINMARKER, "must be a block header");
2075 if (monitor > (ObjectMonitor *)&block[0] &&
2076 monitor < (ObjectMonitor *)&block[_BLOCKSIZE]) {
2077 address mon = (address)monitor;
2078 address blk = (address)block;
2079 size_t diff = mon - blk;
2080 assert((diff % sizeof(PaddedEnd<ObjectMonitor>)) == 0, "must be aligned");
2081 return 1;
2082 }
2083 block = (PaddedEnd<ObjectMonitor> *)block->FreeNext;
2084 }
2085 return 0;
2086 }
2087
2088 #endif