1 /*
2 * Copyright (c) 1998, 2014, 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 "memory/resourceArea.hpp"
28 #include "oops/markOop.hpp"
29 #include "oops/oop.inline.hpp"
30 #include "runtime/biasedLocking.hpp"
31 #include "runtime/handles.inline.hpp"
32 #include "runtime/interfaceSupport.hpp"
33 #include "runtime/mutexLocker.hpp"
34 #include "runtime/objectMonitor.hpp"
35 #include "runtime/objectMonitor.inline.hpp"
36 #include "runtime/osThread.hpp"
37 #include "runtime/stubRoutines.hpp"
38 #include "runtime/synchronizer.hpp"
39 #include "runtime/thread.inline.hpp"
40 #include "utilities/dtrace.hpp"
41 #include "utilities/events.hpp"
42 #include "utilities/preserveException.hpp"
43 #ifdef TARGET_OS_FAMILY_linux
44 # include "os_linux.inline.hpp"
45 #endif
46 #ifdef TARGET_OS_FAMILY_solaris
47 # include "os_solaris.inline.hpp"
48 #endif
49 #ifdef TARGET_OS_FAMILY_windows
50 # include "os_windows.inline.hpp"
51 #endif
52 #ifdef TARGET_OS_FAMILY_bsd
53 # include "os_bsd.inline.hpp"
54 #endif
55
56 #if defined(__GNUC__)
57 // Need to inhibit inlining for older versions of GCC to avoid build-time failures
58 #define ATTR __attribute__((noinline))
59 #else
60 #define ATTR
61 #endif
62
63 // The "core" versions of monitor enter and exit reside in this file.
64 // The interpreter and compilers contain specialized transliterated
65 // variants of the enter-exit fast-path operations. See i486.ad fast_lock(),
66 // for instance. If you make changes here, make sure to modify the
67 // interpreter, and both C1 and C2 fast-path inline locking code emission.
68 //
69 //
70 // -----------------------------------------------------------------------------
71
72 #ifdef DTRACE_ENABLED
73
74 // Only bother with this argument setup if dtrace is available
75 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
76
77 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
78 char* bytes = NULL; \
79 int len = 0; \
80 jlong jtid = SharedRuntime::get_java_tid(thread); \
81 Symbol* klassname = ((oop)(obj))->klass()->name(); \
82 if (klassname != NULL) { \
83 bytes = (char*)klassname->bytes(); \
84 len = klassname->utf8_length(); \
85 }
86
87 #ifndef USDT2
88 HS_DTRACE_PROBE_DECL5(hotspot, monitor__wait,
89 jlong, uintptr_t, char*, int, long);
90 HS_DTRACE_PROBE_DECL4(hotspot, monitor__waited,
91 jlong, uintptr_t, char*, int);
92
93 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
94 { \
95 if (DTraceMonitorProbes) { \
96 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
97 HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid, \
98 (monitor), bytes, len, (millis)); \
99 } \
100 }
101
102 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
103 { \
104 if (DTraceMonitorProbes) { \
105 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
106 HS_DTRACE_PROBE4(hotspot, monitor__##probe, jtid, \
107 (uintptr_t)(monitor), bytes, len); \
108 } \
109 }
110
111 #else /* USDT2 */
112
113 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
114 { \
115 if (DTraceMonitorProbes) { \
116 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
117 HOTSPOT_MONITOR_WAIT(jtid, \
118 (uintptr_t)(monitor), bytes, len, (millis)); \
119 } \
120 }
121
122 #define HOTSPOT_MONITOR_PROBE_waited HOTSPOT_MONITOR_WAITED
123
124 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
125 { \
126 if (DTraceMonitorProbes) { \
127 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
128 HOTSPOT_MONITOR_PROBE_##probe(jtid, /* probe = waited */ \
129 (uintptr_t)(monitor), bytes, len); \
130 } \
131 }
132
133 #endif /* USDT2 */
134 #else // ndef DTRACE_ENABLED
135
136 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
137 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
138
139 #endif // ndef DTRACE_ENABLED
140
141 // This exists only as a workaround of dtrace bug 6254741
142 int dtrace_waited_probe(ObjectMonitor* monitor, Handle obj, Thread* thr) {
143 DTRACE_MONITOR_PROBE(waited, monitor, obj(), thr);
144 return 0;
145 }
146
147 #define NINFLATIONLOCKS 256
148 static volatile intptr_t InflationLocks [NINFLATIONLOCKS] ;
149
150 ObjectMonitor * ObjectSynchronizer::gBlockList = NULL ;
151 ObjectMonitor * volatile ObjectSynchronizer::gFreeList = NULL ;
152 ObjectMonitor * volatile ObjectSynchronizer::gOmInUseList = NULL ;
153 int ObjectSynchronizer::gOmInUseCount = 0;
154 static volatile intptr_t ListLock = 0 ; // protects global monitor free-list cache
155 static volatile int MonitorFreeCount = 0 ; // # on gFreeList
156 static volatile int MonitorPopulation = 0 ; // # Extant -- in circulation
157 #define CHAINMARKER (cast_to_oop<intptr_t>(-1))
158
159 // -----------------------------------------------------------------------------
160 // Fast Monitor Enter/Exit
161 // This the fast monitor enter. The interpreter and compiler use
162 // some assembly copies of this code. Make sure update those code
163 // if the following function is changed. The implementation is
164 // extremely sensitive to race condition. Be careful.
165
166 void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock, bool attempt_rebias, TRAPS) {
167 if (UseBiasedLocking) {
168 if (!SafepointSynchronize::is_at_safepoint()) {
169 BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD);
170 if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) {
171 return;
172 }
173 } else {
174 assert(!attempt_rebias, "can not rebias toward VM thread");
175 BiasedLocking::revoke_at_safepoint(obj);
176 }
177 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
178 }
179
180 slow_enter (obj, lock, THREAD) ;
181 }
182
183 void ObjectSynchronizer::fast_exit(oop object, BasicLock* lock, TRAPS) {
184 assert(!object->mark()->has_bias_pattern(), "should not see bias pattern here");
185 // if displaced header is null, the previous enter is recursive enter, no-op
186 markOop dhw = lock->displaced_header();
187 markOop mark ;
188 if (dhw == NULL) {
189 // Recursive stack-lock.
190 // Diagnostics -- Could be: stack-locked, inflating, inflated.
191 mark = object->mark() ;
192 assert (!mark->is_neutral(), "invariant") ;
193 if (mark->has_locker() && mark != markOopDesc::INFLATING()) {
194 assert(THREAD->is_lock_owned((address)mark->locker()), "invariant") ;
195 }
196 if (mark->has_monitor()) {
197 ObjectMonitor * m = mark->monitor() ;
198 assert(((oop)(m->object()))->mark() == mark, "invariant") ;
199 assert(m->is_entered(THREAD), "invariant") ;
200 }
201 return ;
202 }
203
204 mark = object->mark() ;
205
206 // If the object is stack-locked by the current thread, try to
207 // swing the displaced header from the box back to the mark.
208 if (mark == (markOop) lock) {
209 assert (dhw->is_neutral(), "invariant") ;
210 if ((markOop) Atomic::cmpxchg_ptr (dhw, object->mark_addr(), mark) == mark) {
211 TEVENT (fast_exit: release stacklock) ;
212 return;
213 }
214 }
215
216 ObjectSynchronizer::inflate(THREAD, object)->exit (true, THREAD) ;
217 }
218
219 // -----------------------------------------------------------------------------
220 // Interpreter/Compiler Slow Case
221 // This routine is used to handle interpreter/compiler slow case
222 // We don't need to use fast path here, because it must have been
223 // failed in the interpreter/compiler code.
224 void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) {
225 markOop mark = obj->mark();
226 assert(!mark->has_bias_pattern(), "should not see bias pattern here");
227
228 if (mark->is_neutral()) {
229 // Anticipate successful CAS -- the ST of the displaced mark must
230 // be visible <= the ST performed by the CAS.
231 lock->set_displaced_header(mark);
232 if (mark == (markOop) Atomic::cmpxchg_ptr(lock, obj()->mark_addr(), mark)) {
233 TEVENT (slow_enter: release stacklock) ;
234 return ;
235 }
236 // Fall through to inflate() ...
237 } else
238 if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
239 assert(lock != mark->locker(), "must not re-lock the same lock");
240 assert(lock != (BasicLock*)obj->mark(), "don't relock with same BasicLock");
241 lock->set_displaced_header(NULL);
242 return;
243 }
244
245 #if 0
246 // The following optimization isn't particularly useful.
247 if (mark->has_monitor() && mark->monitor()->is_entered(THREAD)) {
248 lock->set_displaced_header (NULL) ;
249 return ;
250 }
251 #endif
252
253 // The object header will never be displaced to this lock,
254 // so it does not matter what the value is, except that it
255 // must be non-zero to avoid looking like a re-entrant lock,
256 // and must not look locked either.
257 lock->set_displaced_header(markOopDesc::unused_mark());
258 ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);
259 }
260
261 // This routine is used to handle interpreter/compiler slow case
262 // We don't need to use fast path here, because it must have
263 // failed in the interpreter/compiler code. Simply use the heavy
264 // weight monitor should be ok, unless someone find otherwise.
265 void ObjectSynchronizer::slow_exit(oop object, BasicLock* lock, TRAPS) {
266 fast_exit (object, lock, THREAD) ;
267 }
268
269 // -----------------------------------------------------------------------------
270 // Class Loader support to workaround deadlocks on the class loader lock objects
271 // Also used by GC
272 // complete_exit()/reenter() are used to wait on a nested lock
273 // i.e. to give up an outer lock completely and then re-enter
274 // Used when holding nested locks - lock acquisition order: lock1 then lock2
275 // 1) complete_exit lock1 - saving recursion count
276 // 2) wait on lock2
277 // 3) when notified on lock2, unlock lock2
278 // 4) reenter lock1 with original recursion count
279 // 5) lock lock2
280 // NOTE: must use heavy weight monitor to handle complete_exit/reenter()
281 intptr_t ObjectSynchronizer::complete_exit(Handle obj, TRAPS) {
282 TEVENT (complete_exit) ;
283 if (UseBiasedLocking) {
284 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
285 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
286 }
287
288 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
289
290 return monitor->complete_exit(THREAD);
291 }
292
293 // NOTE: must use heavy weight monitor to handle complete_exit/reenter()
294 void ObjectSynchronizer::reenter(Handle obj, intptr_t recursion, TRAPS) {
295 TEVENT (reenter) ;
296 if (UseBiasedLocking) {
297 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
298 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
299 }
300
301 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
302
303 monitor->reenter(recursion, THREAD);
304 }
305 // -----------------------------------------------------------------------------
306 // JNI locks on java objects
307 // NOTE: must use heavy weight monitor to handle jni monitor enter
308 void ObjectSynchronizer::jni_enter(Handle obj, TRAPS) { // possible entry from jni enter
309 // the current locking is from JNI instead of Java code
310 TEVENT (jni_enter) ;
311 if (UseBiasedLocking) {
312 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
313 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
314 }
315 THREAD->set_current_pending_monitor_is_from_java(false);
316 ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);
317 THREAD->set_current_pending_monitor_is_from_java(true);
318 }
319
320 // NOTE: must use heavy weight monitor to handle jni monitor enter
321 bool ObjectSynchronizer::jni_try_enter(Handle obj, Thread* THREAD) {
322 if (UseBiasedLocking) {
323 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
324 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
325 }
326
327 ObjectMonitor* monitor = ObjectSynchronizer::inflate_helper(obj());
328 return monitor->try_enter(THREAD);
329 }
330
331
332 // NOTE: must use heavy weight monitor to handle jni monitor exit
333 void ObjectSynchronizer::jni_exit(oop obj, Thread* THREAD) {
334 TEVENT (jni_exit) ;
335 if (UseBiasedLocking) {
336 Handle h_obj(THREAD, obj);
337 BiasedLocking::revoke_and_rebias(h_obj, false, THREAD);
338 obj = h_obj();
339 }
340 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
341
342 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj);
343 // If this thread has locked the object, exit the monitor. Note: can't use
344 // monitor->check(CHECK); must exit even if an exception is pending.
345 if (monitor->check(THREAD)) {
346 monitor->exit(true, THREAD);
347 }
348 }
349
350 // -----------------------------------------------------------------------------
351 // Internal VM locks on java objects
352 // standard constructor, allows locking failures
353 ObjectLocker::ObjectLocker(Handle obj, Thread* thread, bool doLock) {
354 _dolock = doLock;
355 _thread = thread;
356 debug_only(if (StrictSafepointChecks) _thread->check_for_valid_safepoint_state(false);)
357 _obj = obj;
358
359 if (_dolock) {
360 TEVENT (ObjectLocker) ;
361
362 ObjectSynchronizer::fast_enter(_obj, &_lock, false, _thread);
363 }
364 }
365
366 ObjectLocker::~ObjectLocker() {
367 if (_dolock) {
368 ObjectSynchronizer::fast_exit(_obj(), &_lock, _thread);
369 }
370 }
371
372
373 // -----------------------------------------------------------------------------
374 // Wait/Notify/NotifyAll
375 // NOTE: must use heavy weight monitor to handle wait()
376 int ObjectSynchronizer::wait(Handle obj, jlong millis, TRAPS) {
377 if (UseBiasedLocking) {
378 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
379 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
380 }
381 if (millis < 0) {
382 TEVENT (wait - throw IAX) ;
383 THROW_MSG_0(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
384 }
385 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
386 DTRACE_MONITOR_WAIT_PROBE(monitor, obj(), THREAD, millis);
387 monitor->wait(millis, true, THREAD);
388
389 // This dummy call is in place to get around dtrace bug 6254741. Once
390 // that's fixed we can uncomment the following line, remove the call
391 // and change this function back into a "void" func.
392 // DTRACE_MONITOR_PROBE(waited, monitor, obj(), THREAD);
393 return dtrace_waited_probe(monitor, obj, THREAD);
394 }
395
396 void ObjectSynchronizer::waitUninterruptibly (Handle obj, jlong millis, TRAPS) {
397 if (UseBiasedLocking) {
398 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
399 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
400 }
401 if (millis < 0) {
402 TEVENT (wait - throw IAX) ;
403 THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
404 }
405 ObjectSynchronizer::inflate(THREAD, obj()) -> wait(millis, false, THREAD) ;
406 }
407
408 void ObjectSynchronizer::notify(Handle obj, TRAPS) {
409 if (UseBiasedLocking) {
410 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
411 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
412 }
413
414 markOop mark = obj->mark();
415 if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
416 return;
417 }
418 ObjectSynchronizer::inflate(THREAD, obj())->notify(THREAD);
419 }
420
421 // NOTE: see comment of notify()
422 void ObjectSynchronizer::notifyall(Handle obj, TRAPS) {
423 if (UseBiasedLocking) {
424 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
425 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
426 }
427
428 markOop mark = obj->mark();
429 if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
430 return;
431 }
432 ObjectSynchronizer::inflate(THREAD, obj())->notifyAll(THREAD);
433 }
434
435 // -----------------------------------------------------------------------------
436 // Hash Code handling
437 //
438 // Performance concern:
439 // OrderAccess::storestore() calls release() which STs 0 into the global volatile
440 // OrderAccess::Dummy variable. This store is unnecessary for correctness.
441 // Many threads STing into a common location causes considerable cache migration
442 // or "sloshing" on large SMP system. As such, I avoid using OrderAccess::storestore()
443 // until it's repaired. In some cases OrderAccess::fence() -- which incurs local
444 // latency on the executing processor -- is a better choice as it scales on SMP
445 // systems. See http://blogs.sun.com/dave/entry/biased_locking_in_hotspot for a
446 // discussion of coherency costs. Note that all our current reference platforms
447 // provide strong ST-ST order, so the issue is moot on IA32, x64, and SPARC.
448 //
449 // As a general policy we use "volatile" to control compiler-based reordering
450 // and explicit fences (barriers) to control for architectural reordering performed
451 // by the CPU(s) or platform.
452
453 struct SharedGlobals {
454 // These are highly shared mostly-read variables.
455 // To avoid false-sharing they need to be the sole occupants of a $ line.
456 double padPrefix [8];
457 volatile int stwRandom ;
458 volatile int stwCycle ;
459
460 // Hot RW variables -- Sequester to avoid false-sharing
461 double padSuffix [16];
462 volatile int hcSequence ;
463 double padFinal [8] ;
464 } ;
465
466 static SharedGlobals GVars ;
467 static int MonitorScavengeThreshold = 1000000 ;
468 static volatile int ForceMonitorScavenge = 0 ; // Scavenge required and pending
469
470 static markOop ReadStableMark (oop obj) {
471 markOop mark = obj->mark() ;
472 if (!mark->is_being_inflated()) {
473 return mark ; // normal fast-path return
474 }
475
476 int its = 0 ;
477 for (;;) {
478 markOop mark = obj->mark() ;
479 if (!mark->is_being_inflated()) {
480 return mark ; // normal fast-path return
481 }
482
483 // The object is being inflated by some other thread.
484 // The caller of ReadStableMark() must wait for inflation to complete.
485 // Avoid live-lock
486 // TODO: consider calling SafepointSynchronize::do_call_back() while
487 // spinning to see if there's a safepoint pending. If so, immediately
488 // yielding or blocking would be appropriate. Avoid spinning while
489 // there is a safepoint pending.
490 // TODO: add inflation contention performance counters.
491 // TODO: restrict the aggregate number of spinners.
492
493 ++its ;
494 if (its > 10000 || !os::is_MP()) {
495 if (its & 1) {
496 os::NakedYield() ;
497 TEVENT (Inflate: INFLATING - yield) ;
498 } else {
499 // Note that the following code attenuates the livelock problem but is not
500 // a complete remedy. A more complete solution would require that the inflating
501 // thread hold the associated inflation lock. The following code simply restricts
502 // the number of spinners to at most one. We'll have N-2 threads blocked
503 // on the inflationlock, 1 thread holding the inflation lock and using
504 // a yield/park strategy, and 1 thread in the midst of inflation.
505 // A more refined approach would be to change the encoding of INFLATING
506 // to allow encapsulation of a native thread pointer. Threads waiting for
507 // inflation to complete would use CAS to push themselves onto a singly linked
508 // list rooted at the markword. Once enqueued, they'd loop, checking a per-thread flag
509 // and calling park(). When inflation was complete the thread that accomplished inflation
510 // would detach the list and set the markword to inflated with a single CAS and
511 // then for each thread on the list, set the flag and unpark() the thread.
512 // This is conceptually similar to muxAcquire-muxRelease, except that muxRelease
513 // wakes at most one thread whereas we need to wake the entire list.
514 int ix = (cast_from_oop<intptr_t>(obj) >> 5) & (NINFLATIONLOCKS-1) ;
515 int YieldThenBlock = 0 ;
516 assert (ix >= 0 && ix < NINFLATIONLOCKS, "invariant") ;
517 assert ((NINFLATIONLOCKS & (NINFLATIONLOCKS-1)) == 0, "invariant") ;
518 Thread::muxAcquire (InflationLocks + ix, "InflationLock") ;
519 while (obj->mark() == markOopDesc::INFLATING()) {
520 // Beware: NakedYield() is advisory and has almost no effect on some platforms
521 // so we periodically call Self->_ParkEvent->park(1).
522 // We use a mixed spin/yield/block mechanism.
523 if ((YieldThenBlock++) >= 16) {
524 Thread::current()->_ParkEvent->park(1) ;
525 } else {
526 os::NakedYield() ;
527 }
528 }
529 Thread::muxRelease (InflationLocks + ix ) ;
530 TEVENT (Inflate: INFLATING - yield/park) ;
531 }
532 } else {
533 SpinPause() ; // SMP-polite spinning
534 }
535 }
536 }
537
538 // hashCode() generation :
539 //
540 // Possibilities:
541 // * MD5Digest of {obj,stwRandom}
542 // * CRC32 of {obj,stwRandom} or any linear-feedback shift register function.
543 // * A DES- or AES-style SBox[] mechanism
544 // * One of the Phi-based schemes, such as:
545 // 2654435761 = 2^32 * Phi (golden ratio)
546 // HashCodeValue = ((uintptr_t(obj) >> 3) * 2654435761) ^ GVars.stwRandom ;
547 // * A variation of Marsaglia's shift-xor RNG scheme.
548 // * (obj ^ stwRandom) is appealing, but can result
549 // in undesirable regularity in the hashCode values of adjacent objects
550 // (objects allocated back-to-back, in particular). This could potentially
551 // result in hashtable collisions and reduced hashtable efficiency.
552 // There are simple ways to "diffuse" the middle address bits over the
553 // generated hashCode values:
554 //
555
556 static inline intptr_t get_next_hash(Thread * Self, oop obj) {
557 intptr_t value = 0 ;
558 if (hashCode == 0) {
559 // This form uses an unguarded global Park-Miller RNG,
560 // so it's possible for two threads to race and generate the same RNG.
561 // On MP system we'll have lots of RW access to a global, so the
562 // mechanism induces lots of coherency traffic.
563 value = os::random() ;
564 } else
565 if (hashCode == 1) {
566 // This variation has the property of being stable (idempotent)
567 // between STW operations. This can be useful in some of the 1-0
568 // synchronization schemes.
569 intptr_t addrBits = cast_from_oop<intptr_t>(obj) >> 3 ;
570 value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom ;
571 } else
572 if (hashCode == 2) {
573 value = 1 ; // for sensitivity testing
574 } else
575 if (hashCode == 3) {
576 value = ++GVars.hcSequence ;
577 } else
578 if (hashCode == 4) {
579 value = cast_from_oop<intptr_t>(obj) ;
580 } else {
581 // Marsaglia's xor-shift scheme with thread-specific state
582 // This is probably the best overall implementation -- we'll
583 // likely make this the default in future releases.
584 unsigned t = Self->_hashStateX ;
585 t ^= (t << 11) ;
586 Self->_hashStateX = Self->_hashStateY ;
587 Self->_hashStateY = Self->_hashStateZ ;
588 Self->_hashStateZ = Self->_hashStateW ;
589 unsigned v = Self->_hashStateW ;
590 v = (v ^ (v >> 19)) ^ (t ^ (t >> 8)) ;
591 Self->_hashStateW = v ;
592 value = v ;
593 }
594
595 value &= markOopDesc::hash_mask;
596 if (value == 0) value = 0xBAD ;
597 assert (value != markOopDesc::no_hash, "invariant") ;
598 TEVENT (hashCode: GENERATE) ;
599 return value;
600 }
601 //
602 intptr_t ObjectSynchronizer::FastHashCode (Thread * Self, oop obj) {
603 if (UseBiasedLocking) {
604 // NOTE: many places throughout the JVM do not expect a safepoint
605 // to be taken here, in particular most operations on perm gen
606 // objects. However, we only ever bias Java instances and all of
607 // the call sites of identity_hash that might revoke biases have
608 // been checked to make sure they can handle a safepoint. The
609 // added check of the bias pattern is to avoid useless calls to
610 // thread-local storage.
611 if (obj->mark()->has_bias_pattern()) {
612 // Box and unbox the raw reference just in case we cause a STW safepoint.
613 Handle hobj (Self, obj) ;
614 // Relaxing assertion for bug 6320749.
615 assert (Universe::verify_in_progress() ||
616 !SafepointSynchronize::is_at_safepoint(),
617 "biases should not be seen by VM thread here");
618 BiasedLocking::revoke_and_rebias(hobj, false, JavaThread::current());
619 obj = hobj() ;
620 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
621 }
622 }
623
624 // hashCode() is a heap mutator ...
625 // Relaxing assertion for bug 6320749.
626 assert (Universe::verify_in_progress() ||
627 !SafepointSynchronize::is_at_safepoint(), "invariant") ;
628 assert (Universe::verify_in_progress() ||
629 Self->is_Java_thread() , "invariant") ;
630 assert (Universe::verify_in_progress() ||
631 ((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
632
633 ObjectMonitor* monitor = NULL;
634 markOop temp, test;
635 intptr_t hash;
636 markOop mark = ReadStableMark (obj);
637
638 // object should remain ineligible for biased locking
639 assert (!mark->has_bias_pattern(), "invariant") ;
640
641 if (mark->is_neutral()) {
642 hash = mark->hash(); // this is a normal header
643 if (hash) { // if it has hash, just return it
644 return hash;
645 }
646 hash = get_next_hash(Self, obj); // allocate a new hash code
647 temp = mark->copy_set_hash(hash); // merge the hash code into header
648 // use (machine word version) atomic operation to install the hash
649 test = (markOop) Atomic::cmpxchg_ptr(temp, obj->mark_addr(), mark);
650 if (test == mark) {
651 return hash;
652 }
653 // If atomic operation failed, we must inflate the header
654 // into heavy weight monitor. We could add more code here
655 // for fast path, but it does not worth the complexity.
656 } else if (mark->has_monitor()) {
657 monitor = mark->monitor();
658 temp = monitor->header();
659 assert (temp->is_neutral(), "invariant") ;
660 hash = temp->hash();
661 if (hash) {
662 return hash;
663 }
664 // Skip to the following code to reduce code size
665 } else if (Self->is_lock_owned((address)mark->locker())) {
666 temp = mark->displaced_mark_helper(); // this is a lightweight monitor owned
667 assert (temp->is_neutral(), "invariant") ;
668 hash = temp->hash(); // by current thread, check if the displaced
669 if (hash) { // header contains hash code
670 return hash;
671 }
672 // WARNING:
673 // The displaced header is strictly immutable.
674 // It can NOT be changed in ANY cases. So we have
675 // to inflate the header into heavyweight monitor
676 // even the current thread owns the lock. The reason
677 // is the BasicLock (stack slot) will be asynchronously
678 // read by other threads during the inflate() function.
679 // Any change to stack may not propagate to other threads
680 // correctly.
681 }
682
683 // Inflate the monitor to set hash code
684 monitor = ObjectSynchronizer::inflate(Self, obj);
685 // Load displaced header and check it has hash code
686 mark = monitor->header();
687 assert (mark->is_neutral(), "invariant") ;
688 hash = mark->hash();
689 if (hash == 0) {
690 hash = get_next_hash(Self, obj);
691 temp = mark->copy_set_hash(hash); // merge hash code into header
692 assert (temp->is_neutral(), "invariant") ;
693 test = (markOop) Atomic::cmpxchg_ptr(temp, monitor, mark);
694 if (test != mark) {
695 // The only update to the header in the monitor (outside GC)
696 // is install the hash code. If someone add new usage of
697 // displaced header, please update this code
698 hash = test->hash();
699 assert (test->is_neutral(), "invariant") ;
700 assert (hash != 0, "Trivial unexpected object/monitor header usage.");
701 }
702 }
703 // We finally get the hash
704 return hash;
705 }
706
707 // Deprecated -- use FastHashCode() instead.
708
709 intptr_t ObjectSynchronizer::identity_hash_value_for(Handle obj) {
710 return FastHashCode (Thread::current(), obj()) ;
711 }
712
713
714 bool ObjectSynchronizer::current_thread_holds_lock(JavaThread* thread,
715 Handle h_obj) {
716 if (UseBiasedLocking) {
717 BiasedLocking::revoke_and_rebias(h_obj, false, thread);
718 assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
719 }
720
721 assert(thread == JavaThread::current(), "Can only be called on current thread");
722 oop obj = h_obj();
723
724 markOop mark = ReadStableMark (obj) ;
725
726 // Uncontended case, header points to stack
727 if (mark->has_locker()) {
728 return thread->is_lock_owned((address)mark->locker());
729 }
730 // Contended case, header points to ObjectMonitor (tagged pointer)
731 if (mark->has_monitor()) {
732 ObjectMonitor* monitor = mark->monitor();
733 return monitor->is_entered(thread) != 0 ;
734 }
735 // Unlocked case, header in place
736 assert(mark->is_neutral(), "sanity check");
737 return false;
738 }
739
740 // Be aware of this method could revoke bias of the lock object.
741 // This method queries the ownership of the lock handle specified by 'h_obj'.
742 // If the current thread owns the lock, it returns owner_self. If no
743 // thread owns the lock, it returns owner_none. Otherwise, it will return
744 // owner_other.
745 ObjectSynchronizer::LockOwnership ObjectSynchronizer::query_lock_ownership
746 (JavaThread *self, Handle h_obj) {
747 // The caller must beware this method can revoke bias, and
748 // revocation can result in a safepoint.
749 assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
750 assert (self->thread_state() != _thread_blocked , "invariant") ;
751
752 // Possible mark states: neutral, biased, stack-locked, inflated
753
754 if (UseBiasedLocking && h_obj()->mark()->has_bias_pattern()) {
755 // CASE: biased
756 BiasedLocking::revoke_and_rebias(h_obj, false, self);
757 assert(!h_obj->mark()->has_bias_pattern(),
758 "biases should be revoked by now");
759 }
760
761 assert(self == JavaThread::current(), "Can only be called on current thread");
762 oop obj = h_obj();
763 markOop mark = ReadStableMark (obj) ;
764
765 // CASE: stack-locked. Mark points to a BasicLock on the owner's stack.
766 if (mark->has_locker()) {
767 return self->is_lock_owned((address)mark->locker()) ?
768 owner_self : owner_other;
769 }
770
771 // CASE: inflated. Mark (tagged pointer) points to an objectMonitor.
772 // The Object:ObjectMonitor relationship is stable as long as we're
773 // not at a safepoint.
774 if (mark->has_monitor()) {
775 void * owner = mark->monitor()->_owner ;
776 if (owner == NULL) return owner_none ;
777 return (owner == self ||
778 self->is_lock_owned((address)owner)) ? owner_self : owner_other;
779 }
780
781 // CASE: neutral
782 assert(mark->is_neutral(), "sanity check");
783 return owner_none ; // it's unlocked
784 }
785
786 // FIXME: jvmti should call this
787 JavaThread* ObjectSynchronizer::get_lock_owner(Handle h_obj, bool doLock) {
788 if (UseBiasedLocking) {
789 if (SafepointSynchronize::is_at_safepoint()) {
790 BiasedLocking::revoke_at_safepoint(h_obj);
791 } else {
792 BiasedLocking::revoke_and_rebias(h_obj, false, JavaThread::current());
793 }
794 assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
795 }
796
797 oop obj = h_obj();
798 address owner = NULL;
799
800 markOop mark = ReadStableMark (obj) ;
801
802 // Uncontended case, header points to stack
803 if (mark->has_locker()) {
804 owner = (address) mark->locker();
805 }
806
807 // Contended case, header points to ObjectMonitor (tagged pointer)
808 if (mark->has_monitor()) {
809 ObjectMonitor* monitor = mark->monitor();
810 assert(monitor != NULL, "monitor should be non-null");
811 owner = (address) monitor->owner();
812 }
813
814 if (owner != NULL) {
815 // owning_thread_from_monitor_owner() may also return NULL here
816 return Threads::owning_thread_from_monitor_owner(owner, doLock);
817 }
818
819 // Unlocked case, header in place
820 // Cannot have assertion since this object may have been
821 // locked by another thread when reaching here.
822 // assert(mark->is_neutral(), "sanity check");
823
824 return NULL;
825 }
826 // Visitors ...
827
828 void ObjectSynchronizer::monitors_iterate(MonitorClosure* closure) {
829 ObjectMonitor* block = gBlockList;
830 ObjectMonitor* mid;
831 while (block) {
832 assert(block->object() == CHAINMARKER, "must be a block header");
833 for (int i = _BLOCKSIZE - 1; i > 0; i--) {
834 mid = block + i;
835 oop object = (oop) mid->object();
836 if (object != NULL) {
837 closure->do_monitor(mid);
838 }
839 }
840 block = (ObjectMonitor*) block->FreeNext;
841 }
842 }
843
844 // Get the next block in the block list.
845 static inline ObjectMonitor* next(ObjectMonitor* block) {
846 assert(block->object() == CHAINMARKER, "must be a block header");
847 block = block->FreeNext ;
848 assert(block == NULL || block->object() == CHAINMARKER, "must be a block header");
849 return block;
850 }
851
852
853 void ObjectSynchronizer::oops_do(OopClosure* f) {
854 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
855 for (ObjectMonitor* block = gBlockList; block != NULL; block = next(block)) {
856 assert(block->object() == CHAINMARKER, "must be a block header");
857 for (int i = 1; i < _BLOCKSIZE; i++) {
858 ObjectMonitor* mid = &block[i];
859 if (mid->object() != NULL) {
860 f->do_oop((oop*)mid->object_addr());
861 }
862 }
863 }
864 }
865
866
867 // -----------------------------------------------------------------------------
868 // ObjectMonitor Lifecycle
869 // -----------------------
870 // Inflation unlinks monitors from the global gFreeList and
871 // associates them with objects. Deflation -- which occurs at
872 // STW-time -- disassociates idle monitors from objects. Such
873 // scavenged monitors are returned to the gFreeList.
874 //
875 // The global list is protected by ListLock. All the critical sections
876 // are short and operate in constant-time.
877 //
878 // ObjectMonitors reside in type-stable memory (TSM) and are immortal.
879 //
880 // Lifecycle:
881 // -- unassigned and on the global free list
882 // -- unassigned and on a thread's private omFreeList
883 // -- assigned to an object. The object is inflated and the mark refers
884 // to the objectmonitor.
885 //
886
887
888 // Constraining monitor pool growth via MonitorBound ...
889 //
890 // The monitor pool is grow-only. We scavenge at STW safepoint-time, but the
891 // the rate of scavenging is driven primarily by GC. As such, we can find
892 // an inordinate number of monitors in circulation.
893 // To avoid that scenario we can artificially induce a STW safepoint
894 // if the pool appears to be growing past some reasonable bound.
895 // Generally we favor time in space-time tradeoffs, but as there's no
896 // natural back-pressure on the # of extant monitors we need to impose some
897 // type of limit. Beware that if MonitorBound is set to too low a value
898 // we could just loop. In addition, if MonitorBound is set to a low value
899 // we'll incur more safepoints, which are harmful to performance.
900 // See also: GuaranteedSafepointInterval
901 //
902 // The current implementation uses asynchronous VM operations.
903 //
904
905 static void InduceScavenge (Thread * Self, const char * Whence) {
906 // Induce STW safepoint to trim monitors
907 // Ultimately, this results in a call to deflate_idle_monitors() in the near future.
908 // More precisely, trigger an asynchronous STW safepoint as the number
909 // of active monitors passes the specified threshold.
910 // TODO: assert thread state is reasonable
911
912 if (ForceMonitorScavenge == 0 && Atomic::xchg (1, &ForceMonitorScavenge) == 0) {
913 if (ObjectMonitor::Knob_Verbose) {
914 ::printf ("Monitor scavenge - Induced STW @%s (%d)\n", Whence, ForceMonitorScavenge) ;
915 ::fflush(stdout) ;
916 }
917 // Induce a 'null' safepoint to scavenge monitors
918 // Must VM_Operation instance be heap allocated as the op will be enqueue and posted
919 // to the VMthread and have a lifespan longer than that of this activation record.
920 // The VMThread will delete the op when completed.
921 VMThread::execute (new VM_ForceAsyncSafepoint()) ;
922
923 if (ObjectMonitor::Knob_Verbose) {
924 ::printf ("Monitor scavenge - STW posted @%s (%d)\n", Whence, ForceMonitorScavenge) ;
925 ::fflush(stdout) ;
926 }
927 }
928 }
929 /* Too slow for general assert or debug
930 void ObjectSynchronizer::verifyInUse (Thread *Self) {
931 ObjectMonitor* mid;
932 int inusetally = 0;
933 for (mid = Self->omInUseList; mid != NULL; mid = mid->FreeNext) {
934 inusetally ++;
935 }
936 assert(inusetally == Self->omInUseCount, "inuse count off");
937
938 int freetally = 0;
939 for (mid = Self->omFreeList; mid != NULL; mid = mid->FreeNext) {
940 freetally ++;
941 }
942 assert(freetally == Self->omFreeCount, "free count off");
943 }
944 */
945 ObjectMonitor * ATTR ObjectSynchronizer::omAlloc (Thread * Self) {
946 // A large MAXPRIVATE value reduces both list lock contention
947 // and list coherency traffic, but also tends to increase the
948 // number of objectMonitors in circulation as well as the STW
949 // scavenge costs. As usual, we lean toward time in space-time
950 // tradeoffs.
951 const int MAXPRIVATE = 1024 ;
952 for (;;) {
953 ObjectMonitor * m ;
954
955 // 1: try to allocate from the thread's local omFreeList.
956 // Threads will attempt to allocate first from their local list, then
957 // from the global list, and only after those attempts fail will the thread
958 // attempt to instantiate new monitors. Thread-local free lists take
959 // heat off the ListLock and improve allocation latency, as well as reducing
960 // coherency traffic on the shared global list.
961 m = Self->omFreeList ;
962 if (m != NULL) {
963 Self->omFreeList = m->FreeNext ;
964 Self->omFreeCount -- ;
965 // CONSIDER: set m->FreeNext = BAD -- diagnostic hygiene
966 guarantee (m->object() == NULL, "invariant") ;
967 if (MonitorInUseLists) {
968 m->FreeNext = Self->omInUseList;
969 Self->omInUseList = m;
970 Self->omInUseCount ++;
971 // verifyInUse(Self);
972 } else {
973 m->FreeNext = NULL;
974 }
975 return m ;
976 }
977
978 // 2: try to allocate from the global gFreeList
979 // CONSIDER: use muxTry() instead of muxAcquire().
980 // If the muxTry() fails then drop immediately into case 3.
981 // If we're using thread-local free lists then try
982 // to reprovision the caller's free list.
983 if (gFreeList != NULL) {
984 // Reprovision the thread's omFreeList.
985 // Use bulk transfers to reduce the allocation rate and heat
986 // on various locks.
987 Thread::muxAcquire (&ListLock, "omAlloc") ;
988 for (int i = Self->omFreeProvision; --i >= 0 && gFreeList != NULL; ) {
989 MonitorFreeCount --;
990 ObjectMonitor * take = gFreeList ;
991 gFreeList = take->FreeNext ;
992 guarantee (take->object() == NULL, "invariant") ;
993 guarantee (!take->is_busy(), "invariant") ;
994 take->Recycle() ;
995 omRelease (Self, take, false) ;
996 }
997 Thread::muxRelease (&ListLock) ;
998 Self->omFreeProvision += 1 + (Self->omFreeProvision/2) ;
999 if (Self->omFreeProvision > MAXPRIVATE ) Self->omFreeProvision = MAXPRIVATE ;
1000 TEVENT (omFirst - reprovision) ;
1001
1002 const int mx = MonitorBound ;
1003 if (mx > 0 && (MonitorPopulation-MonitorFreeCount) > mx) {
1004 // We can't safely induce a STW safepoint from omAlloc() as our thread
1005 // state may not be appropriate for such activities and callers may hold
1006 // naked oops, so instead we defer the action.
1007 InduceScavenge (Self, "omAlloc") ;
1008 }
1009 continue;
1010 }
1011
1012 // 3: allocate a block of new ObjectMonitors
1013 // Both the local and global free lists are empty -- resort to malloc().
1014 // In the current implementation objectMonitors are TSM - immortal.
1015 assert (_BLOCKSIZE > 1, "invariant") ;
1016 ObjectMonitor * temp = new ObjectMonitor[_BLOCKSIZE];
1017
1018 // NOTE: (almost) no way to recover if allocation failed.
1019 // We might be able to induce a STW safepoint and scavenge enough
1020 // objectMonitors to permit progress.
1021 if (temp == NULL) {
1022 vm_exit_out_of_memory (sizeof (ObjectMonitor[_BLOCKSIZE]), OOM_MALLOC_ERROR,
1023 "Allocate ObjectMonitors");
1024 }
1025
1026 // Format the block.
1027 // initialize the linked list, each monitor points to its next
1028 // forming the single linked free list, the very first monitor
1029 // will points to next block, which forms the block list.
1030 // The trick of using the 1st element in the block as gBlockList
1031 // linkage should be reconsidered. A better implementation would
1032 // look like: class Block { Block * next; int N; ObjectMonitor Body [N] ; }
1033
1034 for (int i = 1; i < _BLOCKSIZE ; i++) {
1035 temp[i].FreeNext = &temp[i+1];
1036 }
1037
1038 // terminate the last monitor as the end of list
1039 temp[_BLOCKSIZE - 1].FreeNext = NULL ;
1040
1041 // Element [0] is reserved for global list linkage
1042 temp[0].set_object(CHAINMARKER);
1043
1044 // Consider carving out this thread's current request from the
1045 // block in hand. This avoids some lock traffic and redundant
1046 // list activity.
1047
1048 // Acquire the ListLock to manipulate BlockList and FreeList.
1049 // An Oyama-Taura-Yonezawa scheme might be more efficient.
1050 Thread::muxAcquire (&ListLock, "omAlloc [2]") ;
1051 MonitorPopulation += _BLOCKSIZE-1;
1052 MonitorFreeCount += _BLOCKSIZE-1;
1053
1054 // Add the new block to the list of extant blocks (gBlockList).
1055 // The very first objectMonitor in a block is reserved and dedicated.
1056 // It serves as blocklist "next" linkage.
1057 temp[0].FreeNext = gBlockList;
1058 gBlockList = temp;
1059
1060 // Add the new string of objectMonitors to the global free list
1061 temp[_BLOCKSIZE - 1].FreeNext = gFreeList ;
1062 gFreeList = temp + 1;
1063 Thread::muxRelease (&ListLock) ;
1064 TEVENT (Allocate block of monitors) ;
1065 }
1066 }
1067
1068 // Place "m" on the caller's private per-thread omFreeList.
1069 // In practice there's no need to clamp or limit the number of
1070 // monitors on a thread's omFreeList as the only time we'll call
1071 // omRelease is to return a monitor to the free list after a CAS
1072 // attempt failed. This doesn't allow unbounded #s of monitors to
1073 // accumulate on a thread's free list.
1074 //
1075
1076 void ObjectSynchronizer::omRelease (Thread * Self, ObjectMonitor * m, bool fromPerThreadAlloc) {
1077 guarantee (m->object() == NULL, "invariant") ;
1078
1079 // Remove from omInUseList
1080 if (MonitorInUseLists && fromPerThreadAlloc) {
1081 ObjectMonitor* curmidinuse = NULL;
1082 for (ObjectMonitor* mid = Self->omInUseList; mid != NULL; ) {
1083 if (m == mid) {
1084 // extract from per-thread in-use-list
1085 if (mid == Self->omInUseList) {
1086 Self->omInUseList = mid->FreeNext;
1087 } else if (curmidinuse != NULL) {
1088 curmidinuse->FreeNext = mid->FreeNext; // maintain the current thread inuselist
1089 }
1090 Self->omInUseCount --;
1091 // verifyInUse(Self);
1092 break;
1093 } else {
1094 curmidinuse = mid;
1095 mid = mid->FreeNext;
1096 }
1097 }
1098 }
1099
1100 // FreeNext is used for both onInUseList and omFreeList, so clear old before setting new
1101 m->FreeNext = Self->omFreeList ;
1102 Self->omFreeList = m ;
1103 Self->omFreeCount ++ ;
1104 }
1105
1106 // Return the monitors of a moribund thread's local free list to
1107 // the global free list. Typically a thread calls omFlush() when
1108 // it's dying. We could also consider having the VM thread steal
1109 // monitors from threads that have not run java code over a few
1110 // consecutive STW safepoints. Relatedly, we might decay
1111 // omFreeProvision at STW safepoints.
1112 //
1113 // Also return the monitors of a moribund thread"s omInUseList to
1114 // a global gOmInUseList under the global list lock so these
1115 // will continue to be scanned.
1116 //
1117 // We currently call omFlush() from the Thread:: dtor _after the thread
1118 // has been excised from the thread list and is no longer a mutator.
1119 // That means that omFlush() can run concurrently with a safepoint and
1120 // the scavenge operator. Calling omFlush() from JavaThread::exit() might
1121 // be a better choice as we could safely reason that that the JVM is
1122 // not at a safepoint at the time of the call, and thus there could
1123 // be not inopportune interleavings between omFlush() and the scavenge
1124 // operator.
1125
1126 void ObjectSynchronizer::omFlush (Thread * Self) {
1127 ObjectMonitor * List = Self->omFreeList ; // Null-terminated SLL
1128 Self->omFreeList = NULL ;
1129 ObjectMonitor * Tail = NULL ;
1130 int Tally = 0;
1131 if (List != NULL) {
1132 ObjectMonitor * s ;
1133 for (s = List ; s != NULL ; s = s->FreeNext) {
1134 Tally ++ ;
1135 Tail = s ;
1136 guarantee (s->object() == NULL, "invariant") ;
1137 guarantee (!s->is_busy(), "invariant") ;
1138 s->set_owner (NULL) ; // redundant but good hygiene
1139 TEVENT (omFlush - Move one) ;
1140 }
1141 guarantee (Tail != NULL && List != NULL, "invariant") ;
1142 }
1143
1144 ObjectMonitor * InUseList = Self->omInUseList;
1145 ObjectMonitor * InUseTail = NULL ;
1146 int InUseTally = 0;
1147 if (InUseList != NULL) {
1148 Self->omInUseList = NULL;
1149 ObjectMonitor *curom;
1150 for (curom = InUseList; curom != NULL; curom = curom->FreeNext) {
1151 InUseTail = curom;
1152 InUseTally++;
1153 }
1154 // TODO debug
1155 assert(Self->omInUseCount == InUseTally, "inuse count off");
1156 Self->omInUseCount = 0;
1157 guarantee (InUseTail != NULL && InUseList != NULL, "invariant");
1158 }
1159
1160 Thread::muxAcquire (&ListLock, "omFlush") ;
1161 if (Tail != NULL) {
1162 Tail->FreeNext = gFreeList ;
1163 gFreeList = List ;
1164 MonitorFreeCount += Tally;
1165 }
1166
1167 if (InUseTail != NULL) {
1168 InUseTail->FreeNext = gOmInUseList;
1169 gOmInUseList = InUseList;
1170 gOmInUseCount += InUseTally;
1171 }
1172
1173 Thread::muxRelease (&ListLock) ;
1174 TEVENT (omFlush) ;
1175 }
1176
1177 // Fast path code shared by multiple functions
1178 ObjectMonitor* ObjectSynchronizer::inflate_helper(oop obj) {
1179 markOop mark = obj->mark();
1180 if (mark->has_monitor()) {
1181 assert(ObjectSynchronizer::verify_objmon_isinpool(mark->monitor()), "monitor is invalid");
1182 assert(mark->monitor()->header()->is_neutral(), "monitor must record a good object header");
1183 return mark->monitor();
1184 }
1185 return ObjectSynchronizer::inflate(Thread::current(), obj);
1186 }
1187
1188
1189 // Note that we could encounter some performance loss through false-sharing as
1190 // multiple locks occupy the same $ line. Padding might be appropriate.
1191
1192
1193 ObjectMonitor * ATTR ObjectSynchronizer::inflate (Thread * Self, oop object) {
1194 // Inflate mutates the heap ...
1195 // Relaxing assertion for bug 6320749.
1196 assert (Universe::verify_in_progress() ||
1197 !SafepointSynchronize::is_at_safepoint(), "invariant") ;
1198
1199 for (;;) {
1200 const markOop mark = object->mark() ;
1201 assert (!mark->has_bias_pattern(), "invariant") ;
1202
1203 // The mark can be in one of the following states:
1204 // * Inflated - just return
1205 // * Stack-locked - coerce it to inflated
1206 // * INFLATING - busy wait for conversion to complete
1207 // * Neutral - aggressively inflate the object.
1208 // * BIASED - Illegal. We should never see this
1209
1210 // CASE: inflated
1211 if (mark->has_monitor()) {
1212 ObjectMonitor * inf = mark->monitor() ;
1213 assert (inf->header()->is_neutral(), "invariant");
1214 assert (inf->object() == object, "invariant") ;
1215 assert (ObjectSynchronizer::verify_objmon_isinpool(inf), "monitor is invalid");
1216 return inf ;
1217 }
1218
1219 // CASE: inflation in progress - inflating over a stack-lock.
1220 // Some other thread is converting from stack-locked to inflated.
1221 // Only that thread can complete inflation -- other threads must wait.
1222 // The INFLATING value is transient.
1223 // Currently, we spin/yield/park and poll the markword, waiting for inflation to finish.
1224 // We could always eliminate polling by parking the thread on some auxiliary list.
1225 if (mark == markOopDesc::INFLATING()) {
1226 TEVENT (Inflate: spin while INFLATING) ;
1227 ReadStableMark(object) ;
1228 continue ;
1229 }
1230
1231 // CASE: stack-locked
1232 // Could be stack-locked either by this thread or by some other thread.
1233 //
1234 // Note that we allocate the objectmonitor speculatively, _before_ attempting
1235 // to install INFLATING into the mark word. We originally installed INFLATING,
1236 // allocated the objectmonitor, and then finally STed the address of the
1237 // objectmonitor into the mark. This was correct, but artificially lengthened
1238 // the interval in which INFLATED appeared in the mark, thus increasing
1239 // the odds of inflation contention.
1240 //
1241 // We now use per-thread private objectmonitor free lists.
1242 // These list are reprovisioned from the global free list outside the
1243 // critical INFLATING...ST interval. A thread can transfer
1244 // multiple objectmonitors en-mass from the global free list to its local free list.
1245 // This reduces coherency traffic and lock contention on the global free list.
1246 // Using such local free lists, it doesn't matter if the omAlloc() call appears
1247 // before or after the CAS(INFLATING) operation.
1248 // See the comments in omAlloc().
1249
1250 if (mark->has_locker()) {
1251 ObjectMonitor * m = omAlloc (Self) ;
1252 // Optimistically prepare the objectmonitor - anticipate successful CAS
1253 // We do this before the CAS in order to minimize the length of time
1254 // in which INFLATING appears in the mark.
1255 m->Recycle();
1256 m->_Responsible = NULL ;
1257 m->OwnerIsThread = 0 ;
1258 m->_recursions = 0 ;
1259 m->_SpinDuration = ObjectMonitor::Knob_SpinLimit ; // Consider: maintain by type/class
1260
1261 markOop cmp = (markOop) Atomic::cmpxchg_ptr (markOopDesc::INFLATING(), object->mark_addr(), mark) ;
1262 if (cmp != mark) {
1263 omRelease (Self, m, true) ;
1264 continue ; // Interference -- just retry
1265 }
1266
1267 // We've successfully installed INFLATING (0) into the mark-word.
1268 // This is the only case where 0 will appear in a mark-work.
1269 // Only the singular thread that successfully swings the mark-word
1270 // to 0 can perform (or more precisely, complete) inflation.
1271 //
1272 // Why do we CAS a 0 into the mark-word instead of just CASing the
1273 // mark-word from the stack-locked value directly to the new inflated state?
1274 // Consider what happens when a thread unlocks a stack-locked object.
1275 // It attempts to use CAS to swing the displaced header value from the
1276 // on-stack basiclock back into the object header. Recall also that the
1277 // header value (hashcode, etc) can reside in (a) the object header, or
1278 // (b) a displaced header associated with the stack-lock, or (c) a displaced
1279 // header in an objectMonitor. The inflate() routine must copy the header
1280 // value from the basiclock on the owner's stack to the objectMonitor, all
1281 // the while preserving the hashCode stability invariants. If the owner
1282 // decides to release the lock while the value is 0, the unlock will fail
1283 // and control will eventually pass from slow_exit() to inflate. The owner
1284 // will then spin, waiting for the 0 value to disappear. Put another way,
1285 // the 0 causes the owner to stall if the owner happens to try to
1286 // drop the lock (restoring the header from the basiclock to the object)
1287 // while inflation is in-progress. This protocol avoids races that might
1288 // would otherwise permit hashCode values to change or "flicker" for an object.
1289 // Critically, while object->mark is 0 mark->displaced_mark_helper() is stable.
1290 // 0 serves as a "BUSY" inflate-in-progress indicator.
1291
1292
1293 // fetch the displaced mark from the owner's stack.
1294 // The owner can't die or unwind past the lock while our INFLATING
1295 // object is in the mark. Furthermore the owner can't complete
1296 // an unlock on the object, either.
1297 markOop dmw = mark->displaced_mark_helper() ;
1298 assert (dmw->is_neutral(), "invariant") ;
1299
1300 // Setup monitor fields to proper values -- prepare the monitor
1301 m->set_header(dmw) ;
1302
1303 // Optimization: if the mark->locker stack address is associated
1304 // with this thread we could simply set m->_owner = Self and
1305 // m->OwnerIsThread = 1. Note that a thread can inflate an object
1306 // that it has stack-locked -- as might happen in wait() -- directly
1307 // with CAS. That is, we can avoid the xchg-NULL .... ST idiom.
1308 m->set_owner(mark->locker());
1309 m->set_object(object);
1310 // TODO-FIXME: assert BasicLock->dhw != 0.
1311
1312 // Must preserve store ordering. The monitor state must
1313 // be stable at the time of publishing the monitor address.
1314 guarantee (object->mark() == markOopDesc::INFLATING(), "invariant") ;
1315 object->release_set_mark(markOopDesc::encode(m));
1316
1317 // Hopefully the performance counters are allocated on distinct cache lines
1318 // to avoid false sharing on MP systems ...
1319 if (ObjectMonitor::_sync_Inflations != NULL) ObjectMonitor::_sync_Inflations->inc() ;
1320 TEVENT(Inflate: overwrite stacklock) ;
1321 if (TraceMonitorInflation) {
1322 if (object->is_instance()) {
1323 ResourceMark rm;
1324 tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1325 (void *) object, (intptr_t) object->mark(),
1326 object->klass()->external_name());
1327 }
1328 }
1329 return m ;
1330 }
1331
1332 // CASE: neutral
1333 // TODO-FIXME: for entry we currently inflate and then try to CAS _owner.
1334 // If we know we're inflating for entry it's better to inflate by swinging a
1335 // pre-locked objectMonitor pointer into the object header. A successful
1336 // CAS inflates the object *and* confers ownership to the inflating thread.
1337 // In the current implementation we use a 2-step mechanism where we CAS()
1338 // to inflate and then CAS() again to try to swing _owner from NULL to Self.
1339 // An inflateTry() method that we could call from fast_enter() and slow_enter()
1340 // would be useful.
1341
1342 assert (mark->is_neutral(), "invariant");
1343 ObjectMonitor * m = omAlloc (Self) ;
1344 // prepare m for installation - set monitor to initial state
1345 m->Recycle();
1346 m->set_header(mark);
1347 m->set_owner(NULL);
1348 m->set_object(object);
1349 m->OwnerIsThread = 1 ;
1350 m->_recursions = 0 ;
1351 m->_Responsible = NULL ;
1352 m->_SpinDuration = ObjectMonitor::Knob_SpinLimit ; // consider: keep metastats by type/class
1353
1354 if (Atomic::cmpxchg_ptr (markOopDesc::encode(m), object->mark_addr(), mark) != mark) {
1355 m->set_object (NULL) ;
1356 m->set_owner (NULL) ;
1357 m->OwnerIsThread = 0 ;
1358 m->Recycle() ;
1359 omRelease (Self, m, true) ;
1360 m = NULL ;
1361 continue ;
1362 // interference - the markword changed - just retry.
1363 // The state-transitions are one-way, so there's no chance of
1364 // live-lock -- "Inflated" is an absorbing state.
1365 }
1366
1367 // Hopefully the performance counters are allocated on distinct
1368 // cache lines to avoid false sharing on MP systems ...
1369 if (ObjectMonitor::_sync_Inflations != NULL) ObjectMonitor::_sync_Inflations->inc() ;
1370 TEVENT(Inflate: overwrite neutral) ;
1371 if (TraceMonitorInflation) {
1372 if (object->is_instance()) {
1373 ResourceMark rm;
1374 tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1375 (void *) object, (intptr_t) object->mark(),
1376 object->klass()->external_name());
1377 }
1378 }
1379 return m ;
1380 }
1381 }
1382
1383 // Note that we could encounter some performance loss through false-sharing as
1384 // multiple locks occupy the same $ line. Padding might be appropriate.
1385
1386
1387 // Deflate_idle_monitors() is called at all safepoints, immediately
1388 // after all mutators are stopped, but before any objects have moved.
1389 // It traverses the list of known monitors, deflating where possible.
1390 // The scavenged monitor are returned to the monitor free list.
1391 //
1392 // Beware that we scavenge at *every* stop-the-world point.
1393 // Having a large number of monitors in-circulation negatively
1394 // impacts the performance of some applications (e.g., PointBase).
1395 // Broadly, we want to minimize the # of monitors in circulation.
1396 //
1397 // We have added a flag, MonitorInUseLists, which creates a list
1398 // of active monitors for each thread. deflate_idle_monitors()
1399 // only scans the per-thread inuse lists. omAlloc() puts all
1400 // assigned monitors on the per-thread list. deflate_idle_monitors()
1401 // returns the non-busy monitors to the global free list.
1402 // When a thread dies, omFlush() adds the list of active monitors for
1403 // that thread to a global gOmInUseList acquiring the
1404 // global list lock. deflate_idle_monitors() acquires the global
1405 // list lock to scan for non-busy monitors to the global free list.
1406 // An alternative could have used a single global inuse list. The
1407 // downside would have been the additional cost of acquiring the global list lock
1408 // for every omAlloc().
1409 //
1410 // Perversely, the heap size -- and thus the STW safepoint rate --
1411 // typically drives the scavenge rate. Large heaps can mean infrequent GC,
1412 // which in turn can mean large(r) numbers of objectmonitors in circulation.
1413 // This is an unfortunate aspect of this design.
1414 //
1415
1416 enum ManifestConstants {
1417 ClearResponsibleAtSTW = 0,
1418 MaximumRecheckInterval = 1000
1419 } ;
1420
1421 // Deflate a single monitor if not in use
1422 // Return true if deflated, false if in use
1423 bool ObjectSynchronizer::deflate_monitor(ObjectMonitor* mid, oop obj,
1424 ObjectMonitor** FreeHeadp, ObjectMonitor** FreeTailp) {
1425 bool deflated;
1426 // Normal case ... The monitor is associated with obj.
1427 guarantee (obj->mark() == markOopDesc::encode(mid), "invariant") ;
1428 guarantee (mid == obj->mark()->monitor(), "invariant");
1429 guarantee (mid->header()->is_neutral(), "invariant");
1430
1431 if (mid->is_busy()) {
1432 if (ClearResponsibleAtSTW) mid->_Responsible = NULL ;
1433 deflated = false;
1434 } else {
1435 // Deflate the monitor if it is no longer being used
1436 // It's idle - scavenge and return to the global free list
1437 // plain old deflation ...
1438 TEVENT (deflate_idle_monitors - scavenge1) ;
1439 if (TraceMonitorInflation) {
1440 if (obj->is_instance()) {
1441 ResourceMark rm;
1442 tty->print_cr("Deflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1443 (void *) obj, (intptr_t) obj->mark(), obj->klass()->external_name());
1444 }
1445 }
1446
1447 // Restore the header back to obj
1448 obj->release_set_mark(mid->header());
1449 mid->clear();
1450
1451 assert (mid->object() == NULL, "invariant") ;
1452
1453 // Move the object to the working free list defined by FreeHead,FreeTail.
1454 if (*FreeHeadp == NULL) *FreeHeadp = mid;
1455 if (*FreeTailp != NULL) {
1456 ObjectMonitor * prevtail = *FreeTailp;
1457 assert(prevtail->FreeNext == NULL, "cleaned up deflated?"); // TODO KK
1458 prevtail->FreeNext = mid;
1459 }
1460 *FreeTailp = mid;
1461 deflated = true;
1462 }
1463 return deflated;
1464 }
1465
1466 // Caller acquires ListLock
1467 int ObjectSynchronizer::walk_monitor_list(ObjectMonitor** listheadp,
1468 ObjectMonitor** FreeHeadp, ObjectMonitor** FreeTailp) {
1469 ObjectMonitor* mid;
1470 ObjectMonitor* next;
1471 ObjectMonitor* curmidinuse = NULL;
1472 int deflatedcount = 0;
1473
1474 for (mid = *listheadp; mid != NULL; ) {
1475 oop obj = (oop) mid->object();
1476 bool deflated = false;
1477 if (obj != NULL) {
1478 deflated = deflate_monitor(mid, obj, FreeHeadp, FreeTailp);
1479 }
1480 if (deflated) {
1481 // extract from per-thread in-use-list
1482 if (mid == *listheadp) {
1483 *listheadp = mid->FreeNext;
1484 } else if (curmidinuse != NULL) {
1485 curmidinuse->FreeNext = mid->FreeNext; // maintain the current thread inuselist
1486 }
1487 next = mid->FreeNext;
1488 mid->FreeNext = NULL; // This mid is current tail in the FreeHead list
1489 mid = next;
1490 deflatedcount++;
1491 } else {
1492 curmidinuse = mid;
1493 mid = mid->FreeNext;
1494 }
1495 }
1496 return deflatedcount;
1497 }
1498
1499 void ObjectSynchronizer::deflate_idle_monitors() {
1500 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1501 int nInuse = 0 ; // currently associated with objects
1502 int nInCirculation = 0 ; // extant
1503 int nScavenged = 0 ; // reclaimed
1504 bool deflated = false;
1505
1506 ObjectMonitor * FreeHead = NULL ; // Local SLL of scavenged monitors
1507 ObjectMonitor * FreeTail = NULL ;
1508
1509 TEVENT (deflate_idle_monitors) ;
1510 // Prevent omFlush from changing mids in Thread dtor's during deflation
1511 // And in case the vm thread is acquiring a lock during a safepoint
1512 // See e.g. 6320749
1513 Thread::muxAcquire (&ListLock, "scavenge - return") ;
1514
1515 if (MonitorInUseLists) {
1516 int inUse = 0;
1517 for (JavaThread* cur = Threads::first(); cur != NULL; cur = cur->next()) {
1518 nInCirculation+= cur->omInUseCount;
1519 int deflatedcount = walk_monitor_list(cur->omInUseList_addr(), &FreeHead, &FreeTail);
1520 cur->omInUseCount-= deflatedcount;
1521 // verifyInUse(cur);
1522 nScavenged += deflatedcount;
1523 nInuse += cur->omInUseCount;
1524 }
1525
1526 // For moribund threads, scan gOmInUseList
1527 if (gOmInUseList) {
1528 nInCirculation += gOmInUseCount;
1529 int deflatedcount = walk_monitor_list((ObjectMonitor **)&gOmInUseList, &FreeHead, &FreeTail);
1530 gOmInUseCount-= deflatedcount;
1531 nScavenged += deflatedcount;
1532 nInuse += gOmInUseCount;
1533 }
1534
1535 } else for (ObjectMonitor* block = gBlockList; block != NULL; block = next(block)) {
1536 // Iterate over all extant monitors - Scavenge all idle monitors.
1537 assert(block->object() == CHAINMARKER, "must be a block header");
1538 nInCirculation += _BLOCKSIZE ;
1539 for (int i = 1 ; i < _BLOCKSIZE; i++) {
1540 ObjectMonitor* mid = &block[i];
1541 oop obj = (oop) mid->object();
1542
1543 if (obj == NULL) {
1544 // The monitor is not associated with an object.
1545 // The monitor should either be a thread-specific private
1546 // free list or the global free list.
1547 // obj == NULL IMPLIES mid->is_busy() == 0
1548 guarantee (!mid->is_busy(), "invariant") ;
1549 continue ;
1550 }
1551 deflated = deflate_monitor(mid, obj, &FreeHead, &FreeTail);
1552
1553 if (deflated) {
1554 mid->FreeNext = NULL ;
1555 nScavenged ++ ;
1556 } else {
1557 nInuse ++;
1558 }
1559 }
1560 }
1561
1562 MonitorFreeCount += nScavenged;
1563
1564 // Consider: audit gFreeList to ensure that MonitorFreeCount and list agree.
1565
1566 if (ObjectMonitor::Knob_Verbose) {
1567 ::printf ("Deflate: InCirc=%d InUse=%d Scavenged=%d ForceMonitorScavenge=%d : pop=%d free=%d\n",
1568 nInCirculation, nInuse, nScavenged, ForceMonitorScavenge,
1569 MonitorPopulation, MonitorFreeCount) ;
1570 ::fflush(stdout) ;
1571 }
1572
1573 ForceMonitorScavenge = 0; // Reset
1574
1575 // Move the scavenged monitors back to the global free list.
1576 if (FreeHead != NULL) {
1577 guarantee (FreeTail != NULL && nScavenged > 0, "invariant") ;
1578 assert (FreeTail->FreeNext == NULL, "invariant") ;
1579 // constant-time list splice - prepend scavenged segment to gFreeList
1580 FreeTail->FreeNext = gFreeList ;
1581 gFreeList = FreeHead ;
1582 }
1583 Thread::muxRelease (&ListLock) ;
1584
1585 if (ObjectMonitor::_sync_Deflations != NULL) ObjectMonitor::_sync_Deflations->inc(nScavenged) ;
1586 if (ObjectMonitor::_sync_MonExtant != NULL) ObjectMonitor::_sync_MonExtant ->set_value(nInCirculation);
1587
1588 // TODO: Add objectMonitor leak detection.
1589 // Audit/inventory the objectMonitors -- make sure they're all accounted for.
1590 GVars.stwRandom = os::random() ;
1591 GVars.stwCycle ++ ;
1592 }
1593
1594 // Monitor cleanup on JavaThread::exit
1595
1596 // Iterate through monitor cache and attempt to release thread's monitors
1597 // Gives up on a particular monitor if an exception occurs, but continues
1598 // the overall iteration, swallowing the exception.
1599 class ReleaseJavaMonitorsClosure: public MonitorClosure {
1600 private:
1601 TRAPS;
1602
1603 public:
1604 ReleaseJavaMonitorsClosure(Thread* thread) : THREAD(thread) {}
1605 void do_monitor(ObjectMonitor* mid) {
1606 if (mid->owner() == THREAD) {
1607 (void)mid->complete_exit(CHECK);
1608 }
1609 }
1610 };
1611
1612 // Release all inflated monitors owned by THREAD. Lightweight monitors are
1613 // ignored. This is meant to be called during JNI thread detach which assumes
1614 // all remaining monitors are heavyweight. All exceptions are swallowed.
1615 // Scanning the extant monitor list can be time consuming.
1616 // A simple optimization is to add a per-thread flag that indicates a thread
1617 // called jni_monitorenter() during its lifetime.
1618 //
1619 // Instead of No_Savepoint_Verifier it might be cheaper to
1620 // use an idiom of the form:
1621 // auto int tmp = SafepointSynchronize::_safepoint_counter ;
1622 // <code that must not run at safepoint>
1623 // guarantee (((tmp ^ _safepoint_counter) | (tmp & 1)) == 0) ;
1624 // Since the tests are extremely cheap we could leave them enabled
1625 // for normal product builds.
1626
1627 void ObjectSynchronizer::release_monitors_owned_by_thread(TRAPS) {
1628 assert(THREAD == JavaThread::current(), "must be current Java thread");
1629 No_Safepoint_Verifier nsv ;
1630 ReleaseJavaMonitorsClosure rjmc(THREAD);
1631 Thread::muxAcquire(&ListLock, "release_monitors_owned_by_thread");
1632 ObjectSynchronizer::monitors_iterate(&rjmc);
1633 Thread::muxRelease(&ListLock);
1634 THREAD->clear_pending_exception();
1635 }
1636
1637 //------------------------------------------------------------------------------
1638 // Non-product code
1639
1640 #ifndef PRODUCT
1641
1642 // Verify all monitors in the monitor cache, the verification is weak.
1643 void ObjectSynchronizer::verify() {
1644 ObjectMonitor* block = gBlockList;
1645 ObjectMonitor* mid;
1646 while (block) {
1647 assert(block->object() == CHAINMARKER, "must be a block header");
1648 for (int i = 1; i < _BLOCKSIZE; i++) {
1649 mid = block + i;
1650 oop object = (oop) mid->object();
1651 if (object != NULL) {
1652 mid->verify();
1653 }
1654 }
1655 block = (ObjectMonitor*) block->FreeNext;
1656 }
1657 }
1658
1659 // Check if monitor belongs to the monitor cache
1660 // The list is grow-only so it's *relatively* safe to traverse
1661 // the list of extant blocks without taking a lock.
1662
1663 int ObjectSynchronizer::verify_objmon_isinpool(ObjectMonitor *monitor) {
1664 ObjectMonitor* block = gBlockList;
1665
1666 while (block) {
1667 assert(block->object() == CHAINMARKER, "must be a block header");
1668 if (monitor > &block[0] && monitor < &block[_BLOCKSIZE]) {
1669 address mon = (address) monitor;
1670 address blk = (address) block;
1671 size_t diff = mon - blk;
1672 assert((diff % sizeof(ObjectMonitor)) == 0, "check");
1673 return 1;
1674 }
1675 block = (ObjectMonitor*) block->FreeNext;
1676 }
1677 return 0;
1678 }
1679
1680 #endif