1 /*
2 * Copyright (c) 1998, 2024, 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 "gc/shared/collectedHeap.hpp"
28 #include "jfr/jfrEvents.hpp"
29 #include "logging/log.hpp"
30 #include "logging/logStream.hpp"
31 #include "memory/allocation.inline.hpp"
32 #include "memory/padded.hpp"
33 #include "memory/resourceArea.hpp"
34 #include "memory/universe.hpp"
35 #include "oops/markWord.hpp"
36 #include "oops/oop.inline.hpp"
37 #include "runtime/atomic.hpp"
38 #include "runtime/frame.inline.hpp"
39 #include "runtime/globals.hpp"
40 #include "runtime/handles.inline.hpp"
41 #include "runtime/handshake.hpp"
42 #include "runtime/interfaceSupport.inline.hpp"
43 #include "runtime/javaThread.hpp"
44 #include "runtime/lockStack.inline.hpp"
45 #include "runtime/mutexLocker.hpp"
46 #include "runtime/objectMonitor.hpp"
47 #include "runtime/objectMonitor.inline.hpp"
48 #include "runtime/os.inline.hpp"
49 #include "runtime/osThread.hpp"
50 #include "runtime/perfData.hpp"
51 #include "runtime/safepointMechanism.inline.hpp"
52 #include "runtime/safepointVerifiers.hpp"
53 #include "runtime/sharedRuntime.hpp"
54 #include "runtime/stubRoutines.hpp"
55 #include "runtime/synchronizer.hpp"
56 #include "runtime/threads.hpp"
57 #include "runtime/timer.hpp"
58 #include "runtime/trimNativeHeap.hpp"
59 #include "runtime/vframe.hpp"
60 #include "runtime/vmThread.hpp"
61 #include "utilities/align.hpp"
62 #include "utilities/dtrace.hpp"
63 #include "utilities/events.hpp"
64 #include "utilities/globalDefinitions.hpp"
65 #include "utilities/linkedlist.hpp"
66 #include "utilities/preserveException.hpp"
67
68 class ObjectMonitorsHashtable::PtrList :
69 public LinkedListImpl<ObjectMonitor*,
70 AnyObj::C_HEAP, mtThread,
71 AllocFailStrategy::RETURN_NULL> {};
72
73 class CleanupObjectMonitorsHashtable: StackObj {
74 public:
75 bool do_entry(void*& key, ObjectMonitorsHashtable::PtrList*& list) {
76 list->clear(); // clear the LinkListNodes
77 delete list; // then delete the LinkedList
78 return true;
79 }
80 };
81
82 ObjectMonitorsHashtable::~ObjectMonitorsHashtable() {
83 CleanupObjectMonitorsHashtable cleanup;
84 _ptrs->unlink(&cleanup); // cleanup the LinkedLists
85 delete _ptrs; // then delete the hash table
86 }
87
88 void ObjectMonitorsHashtable::add_entry(void* key, ObjectMonitor* om) {
89 ObjectMonitorsHashtable::PtrList* list = get_entry(key);
90 if (list == nullptr) {
91 // Create new list and add it to the hash table:
92 list = new (mtThread) ObjectMonitorsHashtable::PtrList;
93 add_entry(key, list);
94 }
95 list->add(om); // Add the ObjectMonitor to the list.
96 _om_count++;
97 }
98
99 bool ObjectMonitorsHashtable::has_entry(void* key, ObjectMonitor* om) {
100 ObjectMonitorsHashtable::PtrList* list = get_entry(key);
101 if (list == nullptr || list->find(om) == nullptr) {
102 return false;
103 }
104 return true;
105 }
106
107 void MonitorList::add(ObjectMonitor* m) {
108 ObjectMonitor* head;
109 do {
110 head = Atomic::load(&_head);
111 m->set_next_om(head);
112 } while (Atomic::cmpxchg(&_head, head, m) != head);
113
114 size_t count = Atomic::add(&_count, 1u);
115 if (count > max()) {
116 Atomic::inc(&_max);
117 }
118 }
119
120 size_t MonitorList::count() const {
121 return Atomic::load(&_count);
122 }
123
124 size_t MonitorList::max() const {
125 return Atomic::load(&_max);
126 }
127
128 // Walk the in-use list and unlink (at most MonitorDeflationMax) deflated
129 // ObjectMonitors. Returns the number of unlinked ObjectMonitors.
130 size_t MonitorList::unlink_deflated(Thread* current, LogStream* ls,
131 elapsedTimer* timer_p,
132 GrowableArray<ObjectMonitor*>* unlinked_list) {
133 size_t unlinked_count = 0;
134 ObjectMonitor* prev = nullptr;
135 ObjectMonitor* head = Atomic::load_acquire(&_head);
136 ObjectMonitor* m = head;
137 // The in-use list head can be null during the final audit.
138 while (m != nullptr) {
139 if (m->is_being_async_deflated()) {
140 // Find next live ObjectMonitor.
141 ObjectMonitor* next = m;
142 do {
143 ObjectMonitor* next_next = next->next_om();
144 unlinked_count++;
145 unlinked_list->append(next);
146 next = next_next;
147 if (unlinked_count >= (size_t)MonitorDeflationMax) {
148 // Reached the max so bail out on the gathering loop.
149 break;
150 }
151 } while (next != nullptr && next->is_being_async_deflated());
152 if (prev == nullptr) {
153 ObjectMonitor* prev_head = Atomic::cmpxchg(&_head, head, next);
154 if (prev_head != head) {
155 // Find new prev ObjectMonitor that just got inserted.
156 for (ObjectMonitor* n = prev_head; n != m; n = n->next_om()) {
157 prev = n;
158 }
159 prev->set_next_om(next);
160 }
161 } else {
162 prev->set_next_om(next);
163 }
164 if (unlinked_count >= (size_t)MonitorDeflationMax) {
165 // Reached the max so bail out on the searching loop.
166 break;
167 }
168 m = next;
169 } else {
170 prev = m;
171 m = m->next_om();
172 }
173
174 if (current->is_Java_thread()) {
175 // A JavaThread must check for a safepoint/handshake and honor it.
176 ObjectSynchronizer::chk_for_block_req(JavaThread::cast(current), "unlinking",
177 "unlinked_count", unlinked_count,
178 ls, timer_p);
179 }
180 }
181 Atomic::sub(&_count, unlinked_count);
182 return unlinked_count;
183 }
184
185 MonitorList::Iterator MonitorList::iterator() const {
186 return Iterator(Atomic::load_acquire(&_head));
187 }
188
189 ObjectMonitor* MonitorList::Iterator::next() {
190 ObjectMonitor* current = _current;
191 _current = current->next_om();
192 return current;
193 }
194
195 // The "core" versions of monitor enter and exit reside in this file.
196 // The interpreter and compilers contain specialized transliterated
197 // variants of the enter-exit fast-path operations. See c2_MacroAssembler_x86.cpp
198 // fast_lock(...) for instance. If you make changes here, make sure to modify the
199 // interpreter, and both C1 and C2 fast-path inline locking code emission.
200 //
201 // -----------------------------------------------------------------------------
202
203 #ifdef DTRACE_ENABLED
204
205 // Only bother with this argument setup if dtrace is available
206 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
207
208 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
209 char* bytes = nullptr; \
210 int len = 0; \
211 jlong jtid = SharedRuntime::get_java_tid(thread); \
212 Symbol* klassname = obj->klass()->name(); \
213 if (klassname != nullptr) { \
214 bytes = (char*)klassname->bytes(); \
215 len = klassname->utf8_length(); \
216 }
217
218 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
219 { \
220 if (DTraceMonitorProbes) { \
221 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
222 HOTSPOT_MONITOR_WAIT(jtid, \
223 (uintptr_t)(monitor), bytes, len, (millis)); \
224 } \
225 }
226
227 #define HOTSPOT_MONITOR_PROBE_notify HOTSPOT_MONITOR_NOTIFY
228 #define HOTSPOT_MONITOR_PROBE_notifyAll HOTSPOT_MONITOR_NOTIFYALL
229 #define HOTSPOT_MONITOR_PROBE_waited HOTSPOT_MONITOR_WAITED
230
231 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
232 { \
233 if (DTraceMonitorProbes) { \
234 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
235 HOTSPOT_MONITOR_PROBE_##probe(jtid, /* probe = waited */ \
236 (uintptr_t)(monitor), bytes, len); \
237 } \
238 }
239
240 #else // ndef DTRACE_ENABLED
241
242 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
243 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
244
245 #endif // ndef DTRACE_ENABLED
246
247 // This exists only as a workaround of dtrace bug 6254741
248 int dtrace_waited_probe(ObjectMonitor* monitor, Handle obj, JavaThread* thr) {
249 DTRACE_MONITOR_PROBE(waited, monitor, obj(), thr);
250 return 0;
251 }
252
253 static constexpr size_t inflation_lock_count() {
254 return 256;
255 }
256
257 // Static storage for an array of PlatformMutex.
258 alignas(PlatformMutex) static uint8_t _inflation_locks[inflation_lock_count()][sizeof(PlatformMutex)];
259
260 static inline PlatformMutex* inflation_lock(size_t index) {
261 return reinterpret_cast<PlatformMutex*>(_inflation_locks[index]);
262 }
263
264 void ObjectSynchronizer::initialize() {
265 for (size_t i = 0; i < inflation_lock_count(); i++) {
266 ::new(static_cast<void*>(inflation_lock(i))) PlatformMutex();
267 }
268 // Start the ceiling with the estimate for one thread.
269 set_in_use_list_ceiling(AvgMonitorsPerThreadEstimate);
270
271 // Start the timer for deflations, so it does not trigger immediately.
272 _last_async_deflation_time_ns = os::javaTimeNanos();
273 }
274
275 MonitorList ObjectSynchronizer::_in_use_list;
276 // monitors_used_above_threshold() policy is as follows:
277 //
278 // The ratio of the current _in_use_list count to the ceiling is used
279 // to determine if we are above MonitorUsedDeflationThreshold and need
280 // to do an async monitor deflation cycle. The ceiling is increased by
281 // AvgMonitorsPerThreadEstimate when a thread is added to the system
282 // and is decreased by AvgMonitorsPerThreadEstimate when a thread is
283 // removed from the system.
284 //
285 // Note: If the _in_use_list max exceeds the ceiling, then
286 // monitors_used_above_threshold() will use the in_use_list max instead
287 // of the thread count derived ceiling because we have used more
288 // ObjectMonitors than the estimated average.
289 //
290 // Note: If deflate_idle_monitors() has NoAsyncDeflationProgressMax
291 // no-progress async monitor deflation cycles in a row, then the ceiling
292 // is adjusted upwards by monitors_used_above_threshold().
293 //
294 // Start the ceiling with the estimate for one thread in initialize()
295 // which is called after cmd line options are processed.
296 static size_t _in_use_list_ceiling = 0;
297 bool volatile ObjectSynchronizer::_is_async_deflation_requested = false;
298 bool volatile ObjectSynchronizer::_is_final_audit = false;
299 jlong ObjectSynchronizer::_last_async_deflation_time_ns = 0;
300 static uintx _no_progress_cnt = 0;
301 static bool _no_progress_skip_increment = false;
302
303 // =====================> Quick functions
304
305 // The quick_* forms are special fast-path variants used to improve
306 // performance. In the simplest case, a "quick_*" implementation could
307 // simply return false, in which case the caller will perform the necessary
308 // state transitions and call the slow-path form.
309 // The fast-path is designed to handle frequently arising cases in an efficient
310 // manner and is just a degenerate "optimistic" variant of the slow-path.
311 // returns true -- to indicate the call was satisfied.
312 // returns false -- to indicate the call needs the services of the slow-path.
313 // A no-loitering ordinance is in effect for code in the quick_* family
314 // operators: safepoints or indefinite blocking (blocking that might span a
315 // safepoint) are forbidden. Generally the thread_state() is _in_Java upon
316 // entry.
317 //
318 // Consider: An interesting optimization is to have the JIT recognize the
319 // following common idiom:
320 // synchronized (someobj) { .... ; notify(); }
321 // That is, we find a notify() or notifyAll() call that immediately precedes
322 // the monitorexit operation. In that case the JIT could fuse the operations
323 // into a single notifyAndExit() runtime primitive.
324
325 bool ObjectSynchronizer::quick_notify(oopDesc* obj, JavaThread* current, bool all) {
326 assert(current->thread_state() == _thread_in_Java, "invariant");
327 NoSafepointVerifier nsv;
328 if (obj == nullptr) return false; // slow-path for invalid obj
329 const markWord mark = obj->mark();
330
331 if (LockingMode == LM_LIGHTWEIGHT) {
332 if (mark.is_fast_locked() && current->lock_stack().contains(cast_to_oop(obj))) {
333 // Degenerate notify
334 // fast-locked by caller so by definition the implied waitset is empty.
335 return true;
336 }
337 } else if (LockingMode == LM_LEGACY) {
338 if (mark.has_locker() && current->is_lock_owned((address)mark.locker())) {
339 // Degenerate notify
340 // stack-locked by caller so by definition the implied waitset is empty.
341 return true;
342 }
343 }
344
345 if (mark.has_monitor()) {
346 ObjectMonitor* const mon = mark.monitor();
347 assert(mon->object() == oop(obj), "invariant");
348 if (mon->owner() != current) return false; // slow-path for IMS exception
349
350 if (mon->first_waiter() != nullptr) {
351 // We have one or more waiters. Since this is an inflated monitor
352 // that we own, we can transfer one or more threads from the waitset
353 // to the entrylist here and now, avoiding the slow-path.
354 if (all) {
355 DTRACE_MONITOR_PROBE(notifyAll, mon, obj, current);
356 } else {
357 DTRACE_MONITOR_PROBE(notify, mon, obj, current);
358 }
359 int free_count = 0;
360 do {
361 mon->INotify(current);
362 ++free_count;
363 } while (mon->first_waiter() != nullptr && all);
364 OM_PERFDATA_OP(Notifications, inc(free_count));
365 }
366 return true;
367 }
368
369 // other IMS exception states take the slow-path
370 return false;
371 }
372
373
374 // The LockNode emitted directly at the synchronization site would have
375 // been too big if it were to have included support for the cases of inflated
376 // recursive enter and exit, so they go here instead.
377 // Note that we can't safely call AsyncPrintJavaStack() from within
378 // quick_enter() as our thread state remains _in_Java.
379
380 bool ObjectSynchronizer::quick_enter(oop obj, JavaThread* current,
381 BasicLock * lock) {
382 assert(current->thread_state() == _thread_in_Java, "invariant");
383 NoSafepointVerifier nsv;
384 if (obj == nullptr) return false; // Need to throw NPE
385
386 if (obj->klass()->is_value_based()) {
387 return false;
388 }
389
390 if (LockingMode == LM_LIGHTWEIGHT) {
391 LockStack& lock_stack = current->lock_stack();
392 if (lock_stack.is_full()) {
393 // Always go into runtime if the lock stack is full.
394 return false;
395 }
396 if (lock_stack.try_recursive_enter(obj)) {
397 // Recursive lock successful.
398 current->inc_held_monitor_count();
399 return true;
400 }
401 }
402
403 const markWord mark = obj->mark();
404
405 if (mark.has_monitor()) {
406 ObjectMonitor* const m = mark.monitor();
407 // An async deflation or GC can race us before we manage to make
408 // the ObjectMonitor busy by setting the owner below. If we detect
409 // that race we just bail out to the slow-path here.
410 if (m->object_peek() == nullptr) {
411 return false;
412 }
413 JavaThread* const owner = static_cast<JavaThread*>(m->owner_raw());
414
415 // Lock contention and Transactional Lock Elision (TLE) diagnostics
416 // and observability
417 // Case: light contention possibly amenable to TLE
418 // Case: TLE inimical operations such as nested/recursive synchronization
419
420 if (owner == current) {
421 m->_recursions++;
422 current->inc_held_monitor_count();
423 return true;
424 }
425
426 if (LockingMode != LM_LIGHTWEIGHT) {
427 // This Java Monitor is inflated so obj's header will never be
428 // displaced to this thread's BasicLock. Make the displaced header
429 // non-null so this BasicLock is not seen as recursive nor as
430 // being locked. We do this unconditionally so that this thread's
431 // BasicLock cannot be mis-interpreted by any stack walkers. For
432 // performance reasons, stack walkers generally first check for
433 // stack-locking in the object's header, the second check is for
434 // recursive stack-locking in the displaced header in the BasicLock,
435 // and last are the inflated Java Monitor (ObjectMonitor) checks.
436 lock->set_displaced_header(markWord::unused_mark());
437 }
438
439 if (owner == nullptr && m->try_set_owner_from(nullptr, current) == nullptr) {
440 assert(m->_recursions == 0, "invariant");
441 current->inc_held_monitor_count();
442 return true;
443 }
444 }
445
446 // Note that we could inflate in quick_enter.
447 // This is likely a useful optimization
448 // Critically, in quick_enter() we must not:
449 // -- block indefinitely, or
450 // -- reach a safepoint
451
452 return false; // revert to slow-path
453 }
454
455 // Handle notifications when synchronizing on value based classes
456 void ObjectSynchronizer::handle_sync_on_value_based_class(Handle obj, JavaThread* locking_thread) {
457 assert(locking_thread == Thread::current() || locking_thread->is_obj_deopt_suspend(), "must be");
458 frame last_frame = locking_thread->last_frame();
459 bool bcp_was_adjusted = false;
460 // Don't decrement bcp if it points to the frame's first instruction. This happens when
461 // handle_sync_on_value_based_class() is called because of a synchronized method. There
462 // is no actual monitorenter instruction in the byte code in this case.
463 if (last_frame.is_interpreted_frame() &&
464 (last_frame.interpreter_frame_method()->code_base() < last_frame.interpreter_frame_bcp())) {
465 // adjust bcp to point back to monitorenter so that we print the correct line numbers
466 last_frame.interpreter_frame_set_bcp(last_frame.interpreter_frame_bcp() - 1);
467 bcp_was_adjusted = true;
468 }
469
470 if (DiagnoseSyncOnValueBasedClasses == FATAL_EXIT) {
471 ResourceMark rm;
472 stringStream ss;
473 locking_thread->print_active_stack_on(&ss);
474 char* base = (char*)strstr(ss.base(), "at");
475 char* newline = (char*)strchr(ss.base(), '\n');
476 if (newline != nullptr) {
477 *newline = '\0';
478 }
479 fatal("Synchronizing on object " INTPTR_FORMAT " of klass %s %s", p2i(obj()), obj->klass()->external_name(), base);
480 } else {
481 assert(DiagnoseSyncOnValueBasedClasses == LOG_WARNING, "invalid value for DiagnoseSyncOnValueBasedClasses");
482 ResourceMark rm;
483 Log(valuebasedclasses) vblog;
484
485 vblog.info("Synchronizing on object " INTPTR_FORMAT " of klass %s", p2i(obj()), obj->klass()->external_name());
486 if (locking_thread->has_last_Java_frame()) {
487 LogStream info_stream(vblog.info());
488 locking_thread->print_active_stack_on(&info_stream);
489 } else {
490 vblog.info("Cannot find the last Java frame");
491 }
492
493 EventSyncOnValueBasedClass event;
494 if (event.should_commit()) {
495 event.set_valueBasedClass(obj->klass());
496 event.commit();
497 }
498 }
499
500 if (bcp_was_adjusted) {
501 last_frame.interpreter_frame_set_bcp(last_frame.interpreter_frame_bcp() + 1);
502 }
503 }
504
505 static bool useHeavyMonitors() {
506 #if defined(X86) || defined(AARCH64) || defined(PPC64) || defined(RISCV64) || defined(S390)
507 return LockingMode == LM_MONITOR;
508 #else
509 return false;
510 #endif
511 }
512
513 // -----------------------------------------------------------------------------
514 // Monitor Enter/Exit
515
516 void ObjectSynchronizer::enter_for(Handle obj, BasicLock* lock, JavaThread* locking_thread) {
517 // When called with locking_thread != Thread::current() some mechanism must synchronize
518 // the locking_thread with respect to the current thread. Currently only used when
519 // deoptimizing and re-locking locks. See Deoptimization::relock_objects
520 assert(locking_thread == Thread::current() || locking_thread->is_obj_deopt_suspend(), "must be");
521 if (!enter_fast_impl(obj, lock, locking_thread)) {
522 // Inflated ObjectMonitor::enter_for is required
523
524 // An async deflation can race after the inflate_for() call and before
525 // enter_for() can make the ObjectMonitor busy. enter_for() returns false
526 // if we have lost the race to async deflation and we simply try again.
527 while (true) {
528 ObjectMonitor* monitor = inflate_for(locking_thread, obj(), inflate_cause_monitor_enter);
529 if (monitor->enter_for(locking_thread)) {
530 return;
531 }
532 assert(monitor->is_being_async_deflated(), "must be");
533 }
534 }
535 }
536
537 void ObjectSynchronizer::enter(Handle obj, BasicLock* lock, JavaThread* current) {
538 assert(current == Thread::current(), "must be");
539 if (!enter_fast_impl(obj, lock, current)) {
540 // Inflated ObjectMonitor::enter is required
541
542 // An async deflation can race after the inflate() call and before
543 // enter() can make the ObjectMonitor busy. enter() returns false if
544 // we have lost the race to async deflation and we simply try again.
545 while (true) {
546 ObjectMonitor* monitor = inflate(current, obj(), inflate_cause_monitor_enter);
547 if (monitor->enter(current)) {
548 return;
549 }
550 }
551 }
552 }
553
554 // The interpreter and compiler assembly code tries to lock using the fast path
555 // of this algorithm. Make sure to update that code if the following function is
556 // changed. The implementation is extremely sensitive to race condition. Be careful.
557 bool ObjectSynchronizer::enter_fast_impl(Handle obj, BasicLock* lock, JavaThread* locking_thread) {
558
559 if (obj->klass()->is_value_based()) {
560 handle_sync_on_value_based_class(obj, locking_thread);
561 }
562
563 locking_thread->inc_held_monitor_count();
564
565 if (!useHeavyMonitors()) {
566 if (LockingMode == LM_LIGHTWEIGHT) {
567 // Fast-locking does not use the 'lock' argument.
568 LockStack& lock_stack = locking_thread->lock_stack();
569 if (lock_stack.is_full()) {
570 // We unconditionally make room on the lock stack by inflating
571 // the least recently locked object on the lock stack.
572
573 // About the choice to inflate least recently locked object.
574 // First we must chose to inflate a lock, either some lock on
575 // the lock-stack or the lock that is currently being entered
576 // (which may or may not be on the lock-stack).
577 // Second the best lock to inflate is a lock which is entered
578 // in a control flow where there are only a very few locks being
579 // used, as the costly part of inflated locking is inflation,
580 // not locking. But this property is entirely program dependent.
581 // Third inflating the lock currently being entered on when it
582 // is not present on the lock-stack will result in a still full
583 // lock-stack. This creates a scenario where every deeper nested
584 // monitorenter must call into the runtime.
585 // The rational here is as follows:
586 // Because we cannot (currently) figure out the second, and want
587 // to avoid the third, we inflate a lock on the lock-stack.
588 // The least recently locked lock is chosen as it is the lock
589 // with the longest critical section.
590
591 log_info(monitorinflation)("LockStack capacity exceeded, inflating.");
592 ObjectMonitor* monitor = inflate_for(locking_thread, lock_stack.bottom(), inflate_cause_vm_internal);
593 assert(monitor->owner() == Thread::current(), "must be owner=" PTR_FORMAT " current=" PTR_FORMAT " mark=" PTR_FORMAT,
594 p2i(monitor->owner()), p2i(Thread::current()), monitor->object()->mark_acquire().value());
595 assert(!lock_stack.is_full(), "must have made room here");
596 }
597
598 markWord mark = obj()->mark_acquire();
599 while (mark.is_neutral()) {
600 // Retry until a lock state change has been observed. cas_set_mark() may collide with non lock bits modifications.
601 // Try to swing into 'fast-locked' state.
602 assert(!lock_stack.contains(obj()), "thread must not already hold the lock");
603 const markWord locked_mark = mark.set_fast_locked();
604 const markWord old_mark = obj()->cas_set_mark(locked_mark, mark);
605 if (old_mark == mark) {
606 // Successfully fast-locked, push object to lock-stack and return.
607 lock_stack.push(obj());
608 return true;
609 }
610 mark = old_mark;
611 }
612
613 if (mark.is_fast_locked() && lock_stack.try_recursive_enter(obj())) {
614 // Recursive lock successful.
615 return true;
616 }
617
618 // Failed to fast lock.
619 return false;
620 } else if (LockingMode == LM_LEGACY) {
621 markWord mark = obj->mark();
622 if (mark.is_neutral()) {
623 // Anticipate successful CAS -- the ST of the displaced mark must
624 // be visible <= the ST performed by the CAS.
625 lock->set_displaced_header(mark);
626 if (mark == obj()->cas_set_mark(markWord::from_pointer(lock), mark)) {
627 return true;
628 }
629 } else if (mark.has_locker() &&
630 locking_thread->is_lock_owned((address) mark.locker())) {
631 assert(lock != mark.locker(), "must not re-lock the same lock");
632 assert(lock != (BasicLock*) obj->mark().value(), "don't relock with same BasicLock");
633 lock->set_displaced_header(markWord::from_pointer(nullptr));
634 return true;
635 }
636
637 // The object header will never be displaced to this lock,
638 // so it does not matter what the value is, except that it
639 // must be non-zero to avoid looking like a re-entrant lock,
640 // and must not look locked either.
641 lock->set_displaced_header(markWord::unused_mark());
642
643 // Failed to fast lock.
644 return false;
645 }
646 } else if (VerifyHeavyMonitors) {
647 guarantee((obj->mark().value() & markWord::lock_mask_in_place) != markWord::locked_value, "must not be lightweight/stack-locked");
648 }
649
650 return false;
651 }
652
653 void ObjectSynchronizer::exit(oop object, BasicLock* lock, JavaThread* current) {
654 current->dec_held_monitor_count();
655
656 if (!useHeavyMonitors()) {
657 markWord mark = object->mark();
658 if (LockingMode == LM_LIGHTWEIGHT) {
659 // Fast-locking does not use the 'lock' argument.
660 LockStack& lock_stack = current->lock_stack();
661 if (mark.is_fast_locked() && lock_stack.try_recursive_exit(object)) {
662 // Recursively unlocked.
663 return;
664 }
665
666 if (mark.is_fast_locked() && lock_stack.is_recursive(object)) {
667 // This lock is recursive but is not at the top of the lock stack so we're
668 // doing an unbalanced exit. We have to fall thru to inflation below and
669 // let ObjectMonitor::exit() do the unlock.
670 } else {
671 while (mark.is_fast_locked()) {
672 // Retry until a lock state change has been observed. cas_set_mark() may collide with non lock bits modifications.
673 const markWord unlocked_mark = mark.set_unlocked();
674 const markWord old_mark = object->cas_set_mark(unlocked_mark, mark);
675 if (old_mark == mark) {
676 size_t recursions = lock_stack.remove(object) - 1;
677 assert(recursions == 0, "must not be recursive here");
678 return;
679 }
680 mark = old_mark;
681 }
682 }
683 } else if (LockingMode == LM_LEGACY) {
684 markWord dhw = lock->displaced_header();
685 if (dhw.value() == 0) {
686 // If the displaced header is null, then this exit matches up with
687 // a recursive enter. No real work to do here except for diagnostics.
688 #ifndef PRODUCT
689 if (mark != markWord::INFLATING()) {
690 // Only do diagnostics if we are not racing an inflation. Simply
691 // exiting a recursive enter of a Java Monitor that is being
692 // inflated is safe; see the has_monitor() comment below.
693 assert(!mark.is_neutral(), "invariant");
694 assert(!mark.has_locker() ||
695 current->is_lock_owned((address)mark.locker()), "invariant");
696 if (mark.has_monitor()) {
697 // The BasicLock's displaced_header is marked as a recursive
698 // enter and we have an inflated Java Monitor (ObjectMonitor).
699 // This is a special case where the Java Monitor was inflated
700 // after this thread entered the stack-lock recursively. When a
701 // Java Monitor is inflated, we cannot safely walk the Java
702 // Monitor owner's stack and update the BasicLocks because a
703 // Java Monitor can be asynchronously inflated by a thread that
704 // does not own the Java Monitor.
705 ObjectMonitor* m = mark.monitor();
706 assert(m->object()->mark() == mark, "invariant");
707 assert(m->is_entered(current), "invariant");
708 }
709 }
710 #endif
711 return;
712 }
713
714 if (mark == markWord::from_pointer(lock)) {
715 // If the object is stack-locked by the current thread, try to
716 // swing the displaced header from the BasicLock back to the mark.
717 assert(dhw.is_neutral(), "invariant");
718 if (object->cas_set_mark(dhw, mark) == mark) {
719 return;
720 }
721 }
722 }
723 } else if (VerifyHeavyMonitors) {
724 guarantee((object->mark().value() & markWord::lock_mask_in_place) != markWord::locked_value, "must not be lightweight/stack-locked");
725 }
726
727 // We have to take the slow-path of possible inflation and then exit.
728 // The ObjectMonitor* can't be async deflated until ownership is
729 // dropped inside exit() and the ObjectMonitor* must be !is_busy().
730 ObjectMonitor* monitor = inflate(current, object, inflate_cause_vm_internal);
731 assert(!monitor->is_owner_anonymous(), "must not be");
732 monitor->exit(current);
733 }
734
735 // -----------------------------------------------------------------------------
736 // JNI locks on java objects
737 // NOTE: must use heavy weight monitor to handle jni monitor enter
738 void ObjectSynchronizer::jni_enter(Handle obj, JavaThread* current) {
739 if (obj->klass()->is_value_based()) {
740 handle_sync_on_value_based_class(obj, current);
741 }
742
743 // the current locking is from JNI instead of Java code
744 current->set_current_pending_monitor_is_from_java(false);
745 // An async deflation can race after the inflate() call and before
746 // enter() can make the ObjectMonitor busy. enter() returns false if
747 // we have lost the race to async deflation and we simply try again.
748 while (true) {
749 ObjectMonitor* monitor = inflate(current, obj(), inflate_cause_jni_enter);
750 if (monitor->enter(current)) {
751 current->inc_held_monitor_count(1, true);
752 break;
753 }
754 }
755 current->set_current_pending_monitor_is_from_java(true);
756 }
757
758 // NOTE: must use heavy weight monitor to handle jni monitor exit
759 void ObjectSynchronizer::jni_exit(oop obj, TRAPS) {
760 JavaThread* current = THREAD;
761
762 // The ObjectMonitor* can't be async deflated until ownership is
763 // dropped inside exit() and the ObjectMonitor* must be !is_busy().
764 ObjectMonitor* monitor = inflate(current, obj, inflate_cause_jni_exit);
765 // If this thread has locked the object, exit the monitor. We
766 // intentionally do not use CHECK on check_owner because we must exit the
767 // monitor even if an exception was already pending.
768 if (monitor->check_owner(THREAD)) {
769 monitor->exit(current);
770 current->dec_held_monitor_count(1, true);
771 }
772 }
773
774 // -----------------------------------------------------------------------------
775 // Internal VM locks on java objects
776 // standard constructor, allows locking failures
777 ObjectLocker::ObjectLocker(Handle obj, JavaThread* thread) {
778 _thread = thread;
779 _thread->check_for_valid_safepoint_state();
780 _obj = obj;
781
782 if (_obj() != nullptr) {
783 ObjectSynchronizer::enter(_obj, &_lock, _thread);
784 }
785 }
786
787 ObjectLocker::~ObjectLocker() {
788 if (_obj() != nullptr) {
789 ObjectSynchronizer::exit(_obj(), &_lock, _thread);
790 }
791 }
792
793
794 // -----------------------------------------------------------------------------
795 // Wait/Notify/NotifyAll
796 // NOTE: must use heavy weight monitor to handle wait()
797 int ObjectSynchronizer::wait(Handle obj, jlong millis, TRAPS) {
798 JavaThread* current = THREAD;
799 if (millis < 0) {
800 THROW_MSG_0(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
801 }
802 // The ObjectMonitor* can't be async deflated because the _waiters
803 // field is incremented before ownership is dropped and decremented
804 // after ownership is regained.
805 ObjectMonitor* monitor = inflate(current, obj(), inflate_cause_wait);
806
807 DTRACE_MONITOR_WAIT_PROBE(monitor, obj(), current, millis);
808 monitor->wait(millis, true, THREAD); // Not CHECK as we need following code
809
810 // This dummy call is in place to get around dtrace bug 6254741. Once
811 // that's fixed we can uncomment the following line, remove the call
812 // and change this function back into a "void" func.
813 // DTRACE_MONITOR_PROBE(waited, monitor, obj(), THREAD);
814 int ret_code = dtrace_waited_probe(monitor, obj, THREAD);
815 return ret_code;
816 }
817
818 void ObjectSynchronizer::notify(Handle obj, TRAPS) {
819 JavaThread* current = THREAD;
820
821 markWord mark = obj->mark();
822 if (LockingMode == LM_LIGHTWEIGHT) {
823 if ((mark.is_fast_locked() && current->lock_stack().contains(obj()))) {
824 // Not inflated so there can't be any waiters to notify.
825 return;
826 }
827 } else if (LockingMode == LM_LEGACY) {
828 if (mark.has_locker() && current->is_lock_owned((address)mark.locker())) {
829 // Not inflated so there can't be any waiters to notify.
830 return;
831 }
832 }
833 // The ObjectMonitor* can't be async deflated until ownership is
834 // dropped by the calling thread.
835 ObjectMonitor* monitor = inflate(current, obj(), inflate_cause_notify);
836 monitor->notify(CHECK);
837 }
838
839 // NOTE: see comment of notify()
840 void ObjectSynchronizer::notifyall(Handle obj, TRAPS) {
841 JavaThread* current = THREAD;
842
843 markWord mark = obj->mark();
844 if (LockingMode == LM_LIGHTWEIGHT) {
845 if ((mark.is_fast_locked() && current->lock_stack().contains(obj()))) {
846 // Not inflated so there can't be any waiters to notify.
847 return;
848 }
849 } else if (LockingMode == LM_LEGACY) {
850 if (mark.has_locker() && current->is_lock_owned((address)mark.locker())) {
851 // Not inflated so there can't be any waiters to notify.
852 return;
853 }
854 }
855 // The ObjectMonitor* can't be async deflated until ownership is
856 // dropped by the calling thread.
857 ObjectMonitor* monitor = inflate(current, obj(), inflate_cause_notify);
858 monitor->notifyAll(CHECK);
859 }
860
861 // -----------------------------------------------------------------------------
862 // Hash Code handling
863
864 struct SharedGlobals {
865 char _pad_prefix[OM_CACHE_LINE_SIZE];
866 // This is a highly shared mostly-read variable.
867 // To avoid false-sharing it needs to be the sole occupant of a cache line.
868 volatile int stw_random;
869 DEFINE_PAD_MINUS_SIZE(1, OM_CACHE_LINE_SIZE, sizeof(volatile int));
870 // Hot RW variable -- Sequester to avoid false-sharing
871 volatile int hc_sequence;
872 DEFINE_PAD_MINUS_SIZE(2, OM_CACHE_LINE_SIZE, sizeof(volatile int));
873 };
874
875 static SharedGlobals GVars;
876
877 static markWord read_stable_mark(oop obj) {
878 markWord mark = obj->mark_acquire();
879 if (!mark.is_being_inflated() || LockingMode == LM_LIGHTWEIGHT) {
880 // New lightweight locking does not use the markWord::INFLATING() protocol.
881 return mark; // normal fast-path return
882 }
883
884 int its = 0;
885 for (;;) {
886 markWord mark = obj->mark_acquire();
887 if (!mark.is_being_inflated()) {
888 return mark; // normal fast-path return
889 }
890
891 // The object is being inflated by some other thread.
892 // The caller of read_stable_mark() must wait for inflation to complete.
893 // Avoid live-lock.
894
895 ++its;
896 if (its > 10000 || !os::is_MP()) {
897 if (its & 1) {
898 os::naked_yield();
899 } else {
900 // Note that the following code attenuates the livelock problem but is not
901 // a complete remedy. A more complete solution would require that the inflating
902 // thread hold the associated inflation lock. The following code simply restricts
903 // the number of spinners to at most one. We'll have N-2 threads blocked
904 // on the inflationlock, 1 thread holding the inflation lock and using
905 // a yield/park strategy, and 1 thread in the midst of inflation.
906 // A more refined approach would be to change the encoding of INFLATING
907 // to allow encapsulation of a native thread pointer. Threads waiting for
908 // inflation to complete would use CAS to push themselves onto a singly linked
909 // list rooted at the markword. Once enqueued, they'd loop, checking a per-thread flag
910 // and calling park(). When inflation was complete the thread that accomplished inflation
911 // would detach the list and set the markword to inflated with a single CAS and
912 // then for each thread on the list, set the flag and unpark() the thread.
913
914 // Index into the lock array based on the current object address.
915 static_assert(is_power_of_2(inflation_lock_count()), "must be");
916 size_t ix = (cast_from_oop<intptr_t>(obj) >> 5) & (inflation_lock_count() - 1);
917 int YieldThenBlock = 0;
918 assert(ix < inflation_lock_count(), "invariant");
919 inflation_lock(ix)->lock();
920 while (obj->mark_acquire() == markWord::INFLATING()) {
921 // Beware: naked_yield() is advisory and has almost no effect on some platforms
922 // so we periodically call current->_ParkEvent->park(1).
923 // We use a mixed spin/yield/block mechanism.
924 if ((YieldThenBlock++) >= 16) {
925 Thread::current()->_ParkEvent->park(1);
926 } else {
927 os::naked_yield();
928 }
929 }
930 inflation_lock(ix)->unlock();
931 }
932 } else {
933 SpinPause(); // SMP-polite spinning
934 }
935 }
936 }
937
938 // hashCode() generation :
939 //
940 // Possibilities:
941 // * MD5Digest of {obj,stw_random}
942 // * CRC32 of {obj,stw_random} or any linear-feedback shift register function.
943 // * A DES- or AES-style SBox[] mechanism
944 // * One of the Phi-based schemes, such as:
945 // 2654435761 = 2^32 * Phi (golden ratio)
946 // HashCodeValue = ((uintptr_t(obj) >> 3) * 2654435761) ^ GVars.stw_random ;
947 // * A variation of Marsaglia's shift-xor RNG scheme.
948 // * (obj ^ stw_random) is appealing, but can result
949 // in undesirable regularity in the hashCode values of adjacent objects
950 // (objects allocated back-to-back, in particular). This could potentially
951 // result in hashtable collisions and reduced hashtable efficiency.
952 // There are simple ways to "diffuse" the middle address bits over the
953 // generated hashCode values:
954
955 static inline intptr_t get_next_hash(Thread* current, oop obj) {
956 intptr_t value = 0;
957 if (hashCode == 0) {
958 // This form uses global Park-Miller RNG.
959 // On MP system we'll have lots of RW access to a global, so the
960 // mechanism induces lots of coherency traffic.
961 value = os::random();
962 } else if (hashCode == 1) {
963 // This variation has the property of being stable (idempotent)
964 // between STW operations. This can be useful in some of the 1-0
965 // synchronization schemes.
966 intptr_t addr_bits = cast_from_oop<intptr_t>(obj) >> 3;
967 value = addr_bits ^ (addr_bits >> 5) ^ GVars.stw_random;
968 } else if (hashCode == 2) {
969 value = 1; // for sensitivity testing
970 } else if (hashCode == 3) {
971 value = ++GVars.hc_sequence;
972 } else if (hashCode == 4) {
973 value = cast_from_oop<intptr_t>(obj);
974 } else {
975 // Marsaglia's xor-shift scheme with thread-specific state
976 // This is probably the best overall implementation -- we'll
977 // likely make this the default in future releases.
978 unsigned t = current->_hashStateX;
979 t ^= (t << 11);
980 current->_hashStateX = current->_hashStateY;
981 current->_hashStateY = current->_hashStateZ;
982 current->_hashStateZ = current->_hashStateW;
983 unsigned v = current->_hashStateW;
984 v = (v ^ (v >> 19)) ^ (t ^ (t >> 8));
985 current->_hashStateW = v;
986 value = v;
987 }
988
989 value &= UseCompactObjectHeaders ? markWord::hash_mask_compact : markWord::hash_mask;
990 if (value == 0) value = 0xBAD;
991 assert(value != markWord::no_hash, "invariant");
992 return value;
993 }
994
995 intptr_t ObjectSynchronizer::FastHashCode(Thread* current, oop obj) {
996
997 while (true) {
998 ObjectMonitor* monitor = nullptr;
999 markWord temp, test;
1000 intptr_t hash;
1001 markWord mark = read_stable_mark(obj);
1002 if (VerifyHeavyMonitors) {
1003 assert(LockingMode == LM_MONITOR, "+VerifyHeavyMonitors requires LockingMode == 0 (LM_MONITOR)");
1004 guarantee((obj->mark().value() & markWord::lock_mask_in_place) != markWord::locked_value, "must not be lightweight/stack-locked");
1005 }
1006 if (mark.is_neutral() || (LockingMode == LM_LIGHTWEIGHT && mark.is_fast_locked())) {
1007 hash = mark.hash();
1008 if (hash != 0) { // if it has a hash, just return it
1009 return hash;
1010 }
1011 hash = get_next_hash(current, obj); // get a new hash
1012 temp = mark.copy_set_hash(hash); // merge the hash into header
1013 // try to install the hash
1014 test = obj->cas_set_mark(temp, mark);
1015 if (test == mark) { // if the hash was installed, return it
1016 return hash;
1017 }
1018 if (LockingMode == LM_LIGHTWEIGHT) {
1019 // CAS failed, retry
1020 continue;
1021 }
1022 // Failed to install the hash. It could be that another thread
1023 // installed the hash just before our attempt or inflation has
1024 // occurred or... so we fall thru to inflate the monitor for
1025 // stability and then install the hash.
1026 } else if (mark.has_monitor()) {
1027 monitor = mark.monitor();
1028 temp = monitor->header();
1029 assert(temp.is_neutral(), "invariant: header=" INTPTR_FORMAT, temp.value());
1030 hash = temp.hash();
1031 if (hash != 0) {
1032 // It has a hash.
1033
1034 // Separate load of dmw/header above from the loads in
1035 // is_being_async_deflated().
1036
1037 // dmw/header and _contentions may get written by different threads.
1038 // Make sure to observe them in the same order when having several observers.
1039 OrderAccess::loadload_for_IRIW();
1040
1041 if (monitor->is_being_async_deflated()) {
1042 // But we can't safely use the hash if we detect that async
1043 // deflation has occurred. So we attempt to restore the
1044 // header/dmw to the object's header so that we only retry
1045 // once if the deflater thread happens to be slow.
1046 monitor->install_displaced_markword_in_object(obj);
1047 continue;
1048 }
1049 return hash;
1050 }
1051 // Fall thru so we only have one place that installs the hash in
1052 // the ObjectMonitor.
1053 } else if (LockingMode == LM_LEGACY && mark.has_locker() && current->is_lock_owned((address)mark.locker())) {
1054 // This is a stack-lock owned by the calling thread so fetch the
1055 // displaced markWord from the BasicLock on the stack.
1056 temp = mark.displaced_mark_helper();
1057 assert(temp.is_neutral(), "invariant: header=" INTPTR_FORMAT, temp.value());
1058 hash = temp.hash();
1059 if (hash != 0) { // if it has a hash, just return it
1060 return hash;
1061 }
1062 // WARNING:
1063 // The displaced header in the BasicLock on a thread's stack
1064 // is strictly immutable. It CANNOT be changed in ANY cases.
1065 // So we have to inflate the stack-lock into an ObjectMonitor
1066 // even if the current thread owns the lock. The BasicLock on
1067 // a thread's stack can be asynchronously read by other threads
1068 // during an inflate() call so any change to that stack memory
1069 // may not propagate to other threads correctly.
1070 }
1071
1072 // Inflate the monitor to set the hash.
1073
1074 // An async deflation can race after the inflate() call and before we
1075 // can update the ObjectMonitor's header with the hash value below.
1076 monitor = inflate(current, obj, inflate_cause_hash_code);
1077 // Load ObjectMonitor's header/dmw field and see if it has a hash.
1078 mark = monitor->header();
1079 assert(mark.is_neutral(), "invariant: header=" INTPTR_FORMAT, mark.value());
1080 hash = mark.hash();
1081 if (hash == 0) { // if it does not have a hash
1082 hash = get_next_hash(current, obj); // get a new hash
1083 temp = mark.copy_set_hash(hash) ; // merge the hash into header
1084 assert(temp.is_neutral(), "invariant: header=" INTPTR_FORMAT, temp.value());
1085 uintptr_t v = Atomic::cmpxchg((volatile uintptr_t*)monitor->header_addr(), mark.value(), temp.value());
1086 test = markWord(v);
1087 if (test != mark) {
1088 // The attempt to update the ObjectMonitor's header/dmw field
1089 // did not work. This can happen if another thread managed to
1090 // merge in the hash just before our cmpxchg().
1091 // If we add any new usages of the header/dmw field, this code
1092 // will need to be updated.
1093 hash = test.hash();
1094 assert(test.is_neutral(), "invariant: header=" INTPTR_FORMAT, test.value());
1095 assert(hash != 0, "should only have lost the race to a thread that set a non-zero hash");
1096 }
1097 if (monitor->is_being_async_deflated()) {
1098 // If we detect that async deflation has occurred, then we
1099 // attempt to restore the header/dmw to the object's header
1100 // so that we only retry once if the deflater thread happens
1101 // to be slow.
1102 monitor->install_displaced_markword_in_object(obj);
1103 continue;
1104 }
1105 }
1106 // We finally get the hash.
1107 return hash;
1108 }
1109 }
1110
1111 bool ObjectSynchronizer::current_thread_holds_lock(JavaThread* current,
1112 Handle h_obj) {
1113 assert(current == JavaThread::current(), "Can only be called on current thread");
1114 oop obj = h_obj();
1115
1116 markWord mark = read_stable_mark(obj);
1117
1118 if (LockingMode == LM_LEGACY && mark.has_locker()) {
1119 // stack-locked case, header points into owner's stack
1120 return current->is_lock_owned((address)mark.locker());
1121 }
1122
1123 if (LockingMode == LM_LIGHTWEIGHT && mark.is_fast_locked()) {
1124 // fast-locking case, see if lock is in current's lock stack
1125 return current->lock_stack().contains(h_obj());
1126 }
1127
1128 if (mark.has_monitor()) {
1129 // Inflated monitor so header points to ObjectMonitor (tagged pointer).
1130 // The first stage of async deflation does not affect any field
1131 // used by this comparison so the ObjectMonitor* is usable here.
1132 ObjectMonitor* monitor = mark.monitor();
1133 return monitor->is_entered(current) != 0;
1134 }
1135 // Unlocked case, header in place
1136 assert(mark.is_neutral(), "sanity check");
1137 return false;
1138 }
1139
1140 JavaThread* ObjectSynchronizer::get_lock_owner(ThreadsList * t_list, Handle h_obj) {
1141 oop obj = h_obj();
1142 markWord mark = read_stable_mark(obj);
1143
1144 if (LockingMode == LM_LEGACY && mark.has_locker()) {
1145 // stack-locked so header points into owner's stack.
1146 // owning_thread_from_monitor_owner() may also return null here:
1147 return Threads::owning_thread_from_monitor_owner(t_list, (address) mark.locker());
1148 }
1149
1150 if (LockingMode == LM_LIGHTWEIGHT && mark.is_fast_locked()) {
1151 // fast-locked so get owner from the object.
1152 // owning_thread_from_object() may also return null here:
1153 return Threads::owning_thread_from_object(t_list, h_obj());
1154 }
1155
1156 if (mark.has_monitor()) {
1157 // Inflated monitor so header points to ObjectMonitor (tagged pointer).
1158 // The first stage of async deflation does not affect any field
1159 // used by this comparison so the ObjectMonitor* is usable here.
1160 ObjectMonitor* monitor = mark.monitor();
1161 assert(monitor != nullptr, "monitor should be non-null");
1162 // owning_thread_from_monitor() may also return null here:
1163 return Threads::owning_thread_from_monitor(t_list, monitor);
1164 }
1165
1166 // Unlocked case, header in place
1167 // Cannot have assertion since this object may have been
1168 // locked by another thread when reaching here.
1169 // assert(mark.is_neutral(), "sanity check");
1170
1171 return nullptr;
1172 }
1173
1174 // Visitors ...
1175
1176 // Iterate ObjectMonitors where the owner == thread; this does NOT include
1177 // ObjectMonitors where owner is set to a stack-lock address in thread.
1178 //
1179 // This version of monitors_iterate() works with the in-use monitor list.
1180 //
1181 void ObjectSynchronizer::monitors_iterate(MonitorClosure* closure, JavaThread* thread) {
1182 MonitorList::Iterator iter = _in_use_list.iterator();
1183 while (iter.has_next()) {
1184 ObjectMonitor* mid = iter.next();
1185 if (mid->owner() != thread) {
1186 // Not owned by the target thread and intentionally skips when owner
1187 // is set to a stack-lock address in the target thread.
1188 continue;
1189 }
1190 if (!mid->is_being_async_deflated() && mid->object_peek() != nullptr) {
1191 // Only process with closure if the object is set.
1192
1193 // monitors_iterate() is only called at a safepoint or when the
1194 // target thread is suspended or when the target thread is
1195 // operating on itself. The current closures in use today are
1196 // only interested in an owned ObjectMonitor and ownership
1197 // cannot be dropped under the calling contexts so the
1198 // ObjectMonitor cannot be async deflated.
1199 closure->do_monitor(mid);
1200 }
1201 }
1202 }
1203
1204 // This version of monitors_iterate() works with the specified linked list.
1205 //
1206 void ObjectSynchronizer::monitors_iterate(MonitorClosure* closure,
1207 ObjectMonitorsHashtable::PtrList* list,
1208 JavaThread* thread) {
1209 typedef LinkedListIterator<ObjectMonitor*> ObjectMonitorIterator;
1210 ObjectMonitorIterator iter(list->head());
1211 while (!iter.is_empty()) {
1212 ObjectMonitor* mid = *iter.next();
1213 // Owner set to a stack-lock address in thread should never be seen here:
1214 assert(mid->owner() == thread, "must be");
1215 if (!mid->is_being_async_deflated() && mid->object_peek() != nullptr) {
1216 // Only process with closure if the object is set.
1217
1218 // monitors_iterate() is only called at a safepoint or when the
1219 // target thread is suspended or when the target thread is
1220 // operating on itself. The current closures in use today are
1221 // only interested in an owned ObjectMonitor and ownership
1222 // cannot be dropped under the calling contexts so the
1223 // ObjectMonitor cannot be async deflated.
1224 closure->do_monitor(mid);
1225 }
1226 }
1227 }
1228
1229 static bool monitors_used_above_threshold(MonitorList* list) {
1230 if (MonitorUsedDeflationThreshold == 0) { // disabled case is easy
1231 return false;
1232 }
1233 // Start with ceiling based on a per-thread estimate:
1234 size_t ceiling = ObjectSynchronizer::in_use_list_ceiling();
1235 size_t old_ceiling = ceiling;
1236 if (ceiling < list->max()) {
1237 // The max used by the system has exceeded the ceiling so use that:
1238 ceiling = list->max();
1239 }
1240 size_t monitors_used = list->count();
1241 if (monitors_used == 0) { // empty list is easy
1242 return false;
1243 }
1244 if (NoAsyncDeflationProgressMax != 0 &&
1245 _no_progress_cnt >= NoAsyncDeflationProgressMax) {
1246 float remainder = (100.0 - MonitorUsedDeflationThreshold) / 100.0;
1247 size_t new_ceiling = ceiling + (ceiling * remainder) + 1;
1248 ObjectSynchronizer::set_in_use_list_ceiling(new_ceiling);
1249 log_info(monitorinflation)("Too many deflations without progress; "
1250 "bumping in_use_list_ceiling from " SIZE_FORMAT
1251 " to " SIZE_FORMAT, old_ceiling, new_ceiling);
1252 _no_progress_cnt = 0;
1253 ceiling = new_ceiling;
1254 }
1255
1256 // Check if our monitor usage is above the threshold:
1257 size_t monitor_usage = (monitors_used * 100LL) / ceiling;
1258 if (int(monitor_usage) > MonitorUsedDeflationThreshold) {
1259 log_info(monitorinflation)("monitors_used=" SIZE_FORMAT ", ceiling=" SIZE_FORMAT
1260 ", monitor_usage=" SIZE_FORMAT ", threshold=" INTX_FORMAT,
1261 monitors_used, ceiling, monitor_usage, MonitorUsedDeflationThreshold);
1262 return true;
1263 }
1264
1265 return false;
1266 }
1267
1268 size_t ObjectSynchronizer::in_use_list_ceiling() {
1269 return _in_use_list_ceiling;
1270 }
1271
1272 void ObjectSynchronizer::dec_in_use_list_ceiling() {
1273 Atomic::sub(&_in_use_list_ceiling, AvgMonitorsPerThreadEstimate);
1274 }
1275
1276 void ObjectSynchronizer::inc_in_use_list_ceiling() {
1277 Atomic::add(&_in_use_list_ceiling, AvgMonitorsPerThreadEstimate);
1278 }
1279
1280 void ObjectSynchronizer::set_in_use_list_ceiling(size_t new_value) {
1281 _in_use_list_ceiling = new_value;
1282 }
1283
1284 bool ObjectSynchronizer::is_async_deflation_needed() {
1285 if (is_async_deflation_requested()) {
1286 // Async deflation request.
1287 log_info(monitorinflation)("Async deflation needed: explicit request");
1288 return true;
1289 }
1290
1291 jlong time_since_last = time_since_last_async_deflation_ms();
1292
1293 if (AsyncDeflationInterval > 0 &&
1294 time_since_last > AsyncDeflationInterval &&
1295 monitors_used_above_threshold(&_in_use_list)) {
1296 // It's been longer than our specified deflate interval and there
1297 // are too many monitors in use. We don't deflate more frequently
1298 // than AsyncDeflationInterval (unless is_async_deflation_requested)
1299 // in order to not swamp the MonitorDeflationThread.
1300 log_info(monitorinflation)("Async deflation needed: monitors used are above the threshold");
1301 return true;
1302 }
1303
1304 if (GuaranteedAsyncDeflationInterval > 0 &&
1305 time_since_last > GuaranteedAsyncDeflationInterval) {
1306 // It's been longer than our specified guaranteed deflate interval.
1307 // We need to clean up the used monitors even if the threshold is
1308 // not reached, to keep the memory utilization at bay when many threads
1309 // touched many monitors.
1310 log_info(monitorinflation)("Async deflation needed: guaranteed interval (" INTX_FORMAT " ms) "
1311 "is greater than time since last deflation (" JLONG_FORMAT " ms)",
1312 GuaranteedAsyncDeflationInterval, time_since_last);
1313
1314 // If this deflation has no progress, then it should not affect the no-progress
1315 // tracking, otherwise threshold heuristics would think it was triggered, experienced
1316 // no progress, and needs to backoff more aggressively. In this "no progress" case,
1317 // the generic code would bump the no-progress counter, and we compensate for that
1318 // by telling it to skip the update.
1319 //
1320 // If this deflation has progress, then it should let non-progress tracking
1321 // know about this, otherwise the threshold heuristics would kick in, potentially
1322 // experience no-progress due to aggressive cleanup by this deflation, and think
1323 // it is still in no-progress stride. In this "progress" case, the generic code would
1324 // zero the counter, and we allow it to happen.
1325 _no_progress_skip_increment = true;
1326
1327 return true;
1328 }
1329
1330 return false;
1331 }
1332
1333 bool ObjectSynchronizer::request_deflate_idle_monitors() {
1334 JavaThread* current = JavaThread::current();
1335 bool ret_code = false;
1336
1337 jlong last_time = last_async_deflation_time_ns();
1338 set_is_async_deflation_requested(true);
1339 {
1340 MonitorLocker ml(MonitorDeflation_lock, Mutex::_no_safepoint_check_flag);
1341 ml.notify_all();
1342 }
1343 const int N_CHECKS = 5;
1344 for (int i = 0; i < N_CHECKS; i++) { // sleep for at most 5 seconds
1345 if (last_async_deflation_time_ns() > last_time) {
1346 log_info(monitorinflation)("Async Deflation happened after %d check(s).", i);
1347 ret_code = true;
1348 break;
1349 }
1350 {
1351 // JavaThread has to honor the blocking protocol.
1352 ThreadBlockInVM tbivm(current);
1353 os::naked_short_sleep(999); // sleep for almost 1 second
1354 }
1355 }
1356 if (!ret_code) {
1357 log_info(monitorinflation)("Async Deflation DID NOT happen after %d checks.", N_CHECKS);
1358 }
1359
1360 return ret_code;
1361 }
1362
1363 jlong ObjectSynchronizer::time_since_last_async_deflation_ms() {
1364 return (os::javaTimeNanos() - last_async_deflation_time_ns()) / (NANOUNITS / MILLIUNITS);
1365 }
1366
1367 static void post_monitor_inflate_event(EventJavaMonitorInflate* event,
1368 const oop obj,
1369 ObjectSynchronizer::InflateCause cause) {
1370 assert(event != nullptr, "invariant");
1371 event->set_monitorClass(obj->klass());
1372 event->set_address((uintptr_t)(void*)obj);
1373 event->set_cause((u1)cause);
1374 event->commit();
1375 }
1376
1377 // Fast path code shared by multiple functions
1378 void ObjectSynchronizer::inflate_helper(oop obj) {
1379 markWord mark = obj->mark_acquire();
1380 if (mark.has_monitor()) {
1381 ObjectMonitor* monitor = mark.monitor();
1382 markWord dmw = monitor->header();
1383 assert(dmw.is_neutral(), "sanity check: header=" INTPTR_FORMAT, dmw.value());
1384 return;
1385 }
1386 (void)inflate(Thread::current(), obj, inflate_cause_vm_internal);
1387 }
1388
1389 ObjectMonitor* ObjectSynchronizer::inflate(Thread* current, oop obj, const InflateCause cause) {
1390 assert(current == Thread::current(), "must be");
1391 if (LockingMode == LM_LIGHTWEIGHT && current->is_Java_thread()) {
1392 return inflate_impl(JavaThread::cast(current), obj, cause);
1393 }
1394 return inflate_impl(nullptr, obj, cause);
1395 }
1396
1397 ObjectMonitor* ObjectSynchronizer::inflate_for(JavaThread* thread, oop obj, const InflateCause cause) {
1398 assert(thread == Thread::current() || thread->is_obj_deopt_suspend(), "must be");
1399 return inflate_impl(thread, obj, cause);
1400 }
1401
1402 ObjectMonitor* ObjectSynchronizer::inflate_impl(JavaThread* inflating_thread, oop object, const InflateCause cause) {
1403 // The JavaThread* inflating_thread parameter is only used by LM_LIGHTWEIGHT and requires
1404 // that the inflating_thread == Thread::current() or is suspended throughout the call by
1405 // some other mechanism.
1406 // Even with LM_LIGHTWEIGHT the thread might be nullptr when called from a non
1407 // JavaThread. (As may still be the case from FastHashCode). However it is only
1408 // important for the correctness of the LM_LIGHTWEIGHT algorithm that the thread
1409 // is set when called from ObjectSynchronizer::enter from the owning thread,
1410 // ObjectSynchronizer::enter_for from any thread, or ObjectSynchronizer::exit.
1411 EventJavaMonitorInflate event;
1412
1413 for (;;) {
1414 const markWord mark = object->mark_acquire();
1415
1416 // The mark can be in one of the following states:
1417 // * inflated - Just return if using stack-locking.
1418 // If using fast-locking and the ObjectMonitor owner
1419 // is anonymous and the inflating_thread owns the
1420 // object lock, then we make the inflating_thread
1421 // the ObjectMonitor owner and remove the lock from
1422 // the inflating_thread's lock stack.
1423 // * fast-locked - Coerce it to inflated from fast-locked.
1424 // * stack-locked - Coerce it to inflated from stack-locked.
1425 // * INFLATING - Busy wait for conversion from stack-locked to
1426 // inflated.
1427 // * neutral - Aggressively inflate the object.
1428
1429 // CASE: inflated
1430 if (mark.has_monitor()) {
1431 ObjectMonitor* inf = mark.monitor();
1432 markWord dmw = inf->header();
1433 assert(dmw.is_neutral(), "invariant: header=" INTPTR_FORMAT, dmw.value());
1434 if (LockingMode == LM_LIGHTWEIGHT && inf->is_owner_anonymous() &&
1435 inflating_thread != nullptr && inflating_thread->lock_stack().contains(object)) {
1436 inf->set_owner_from_anonymous(inflating_thread);
1437 size_t removed = inflating_thread->lock_stack().remove(object);
1438 inf->set_recursions(removed - 1);
1439 }
1440 return inf;
1441 }
1442
1443 if (LockingMode != LM_LIGHTWEIGHT) {
1444 // New lightweight locking does not use INFLATING.
1445 // CASE: inflation in progress - inflating over a stack-lock.
1446 // Some other thread is converting from stack-locked to inflated.
1447 // Only that thread can complete inflation -- other threads must wait.
1448 // The INFLATING value is transient.
1449 // Currently, we spin/yield/park and poll the markword, waiting for inflation to finish.
1450 // We could always eliminate polling by parking the thread on some auxiliary list.
1451 if (mark == markWord::INFLATING()) {
1452 read_stable_mark(object);
1453 continue;
1454 }
1455 }
1456
1457 // CASE: fast-locked
1458 // Could be fast-locked either by the inflating_thread or by some other thread.
1459 //
1460 // Note that we allocate the ObjectMonitor speculatively, _before_
1461 // attempting to set the object's mark to the new ObjectMonitor. If
1462 // the inflating_thread owns the monitor, then we set the ObjectMonitor's
1463 // owner to the inflating_thread. Otherwise, we set the ObjectMonitor's owner
1464 // to anonymous. If we lose the race to set the object's mark to the
1465 // new ObjectMonitor, then we just delete it and loop around again.
1466 //
1467 LogStreamHandle(Trace, monitorinflation) lsh;
1468 if (LockingMode == LM_LIGHTWEIGHT && mark.is_fast_locked()) {
1469 ObjectMonitor* monitor = new ObjectMonitor(object);
1470 monitor->set_header(mark.set_unlocked());
1471 bool own = inflating_thread != nullptr && inflating_thread->lock_stack().contains(object);
1472 if (own) {
1473 // Owned by inflating_thread.
1474 monitor->set_owner_from(nullptr, inflating_thread);
1475 } else {
1476 // Owned by somebody else.
1477 monitor->set_owner_anonymous();
1478 }
1479 markWord monitor_mark = markWord::encode(monitor);
1480 markWord old_mark = object->cas_set_mark(monitor_mark, mark);
1481 if (old_mark == mark) {
1482 // Success! Return inflated monitor.
1483 if (own) {
1484 size_t removed = inflating_thread->lock_stack().remove(object);
1485 monitor->set_recursions(removed - 1);
1486 }
1487 // Once the ObjectMonitor is configured and object is associated
1488 // with the ObjectMonitor, it is safe to allow async deflation:
1489 _in_use_list.add(monitor);
1490
1491 // Hopefully the performance counters are allocated on distinct
1492 // cache lines to avoid false sharing on MP systems ...
1493 OM_PERFDATA_OP(Inflations, inc());
1494 if (log_is_enabled(Trace, monitorinflation)) {
1495 ResourceMark rm;
1496 lsh.print_cr("inflate(has_locker): object=" INTPTR_FORMAT ", mark="
1497 INTPTR_FORMAT ", type='%s'", p2i(object),
1498 object->mark().value(), object->klass()->external_name());
1499 }
1500 if (event.should_commit()) {
1501 post_monitor_inflate_event(&event, object, cause);
1502 }
1503 return monitor;
1504 } else {
1505 delete monitor;
1506 continue; // Interference -- just retry
1507 }
1508 }
1509
1510 // CASE: stack-locked
1511 // Could be stack-locked either by current or by some other thread.
1512 //
1513 // Note that we allocate the ObjectMonitor speculatively, _before_ attempting
1514 // to install INFLATING into the mark word. We originally installed INFLATING,
1515 // allocated the ObjectMonitor, and then finally STed the address of the
1516 // ObjectMonitor into the mark. This was correct, but artificially lengthened
1517 // the interval in which INFLATING appeared in the mark, thus increasing
1518 // the odds of inflation contention. If we lose the race to set INFLATING,
1519 // then we just delete the ObjectMonitor and loop around again.
1520 //
1521 if (LockingMode == LM_LEGACY && mark.has_locker()) {
1522 assert(LockingMode != LM_LIGHTWEIGHT, "cannot happen with new lightweight locking");
1523 ObjectMonitor* m = new ObjectMonitor(object);
1524 // Optimistically prepare the ObjectMonitor - anticipate successful CAS
1525 // We do this before the CAS in order to minimize the length of time
1526 // in which INFLATING appears in the mark.
1527
1528 markWord cmp = object->cas_set_mark(markWord::INFLATING(), mark);
1529 if (cmp != mark) {
1530 delete m;
1531 continue; // Interference -- just retry
1532 }
1533
1534 // We've successfully installed INFLATING (0) into the mark-word.
1535 // This is the only case where 0 will appear in a mark-word.
1536 // Only the singular thread that successfully swings the mark-word
1537 // to 0 can perform (or more precisely, complete) inflation.
1538 //
1539 // Why do we CAS a 0 into the mark-word instead of just CASing the
1540 // mark-word from the stack-locked value directly to the new inflated state?
1541 // Consider what happens when a thread unlocks a stack-locked object.
1542 // It attempts to use CAS to swing the displaced header value from the
1543 // on-stack BasicLock back into the object header. Recall also that the
1544 // header value (hash code, etc) can reside in (a) the object header, or
1545 // (b) a displaced header associated with the stack-lock, or (c) a displaced
1546 // header in an ObjectMonitor. The inflate() routine must copy the header
1547 // value from the BasicLock on the owner's stack to the ObjectMonitor, all
1548 // the while preserving the hashCode stability invariants. If the owner
1549 // decides to release the lock while the value is 0, the unlock will fail
1550 // and control will eventually pass from slow_exit() to inflate. The owner
1551 // will then spin, waiting for the 0 value to disappear. Put another way,
1552 // the 0 causes the owner to stall if the owner happens to try to
1553 // drop the lock (restoring the header from the BasicLock to the object)
1554 // while inflation is in-progress. This protocol avoids races that might
1555 // would otherwise permit hashCode values to change or "flicker" for an object.
1556 // Critically, while object->mark is 0 mark.displaced_mark_helper() is stable.
1557 // 0 serves as a "BUSY" inflate-in-progress indicator.
1558
1559
1560 // fetch the displaced mark from the owner's stack.
1561 // The owner can't die or unwind past the lock while our INFLATING
1562 // object is in the mark. Furthermore the owner can't complete
1563 // an unlock on the object, either.
1564 markWord dmw = mark.displaced_mark_helper();
1565 // Catch if the object's header is not neutral (not locked and
1566 // not marked is what we care about here).
1567 assert(dmw.is_neutral(), "invariant: header=" INTPTR_FORMAT, dmw.value());
1568
1569 // Setup monitor fields to proper values -- prepare the monitor
1570 m->set_header(dmw);
1571
1572 // Optimization: if the mark.locker stack address is associated
1573 // with this thread we could simply set m->_owner = current.
1574 // Note that a thread can inflate an object
1575 // that it has stack-locked -- as might happen in wait() -- directly
1576 // with CAS. That is, we can avoid the xchg-nullptr .... ST idiom.
1577 m->set_owner_from(nullptr, mark.locker());
1578 // TODO-FIXME: assert BasicLock->dhw != 0.
1579
1580 // Must preserve store ordering. The monitor state must
1581 // be stable at the time of publishing the monitor address.
1582 guarantee(object->mark() == markWord::INFLATING(), "invariant");
1583 // Release semantics so that above set_object() is seen first.
1584 object->release_set_mark(markWord::encode(m));
1585
1586 // Once ObjectMonitor is configured and the object is associated
1587 // with the ObjectMonitor, it is safe to allow async deflation:
1588 _in_use_list.add(m);
1589
1590 // Hopefully the performance counters are allocated on distinct cache lines
1591 // to avoid false sharing on MP systems ...
1592 OM_PERFDATA_OP(Inflations, inc());
1593 if (log_is_enabled(Trace, monitorinflation)) {
1594 ResourceMark rm;
1595 lsh.print_cr("inflate(has_locker): object=" INTPTR_FORMAT ", mark="
1596 INTPTR_FORMAT ", type='%s'", p2i(object),
1597 object->mark().value(), object->klass()->external_name());
1598 }
1599 if (event.should_commit()) {
1600 post_monitor_inflate_event(&event, object, cause);
1601 }
1602 return m;
1603 }
1604
1605 // CASE: neutral
1606 // TODO-FIXME: for entry we currently inflate and then try to CAS _owner.
1607 // If we know we're inflating for entry it's better to inflate by swinging a
1608 // pre-locked ObjectMonitor pointer into the object header. A successful
1609 // CAS inflates the object *and* confers ownership to the inflating thread.
1610 // In the current implementation we use a 2-step mechanism where we CAS()
1611 // to inflate and then CAS() again to try to swing _owner from null to current.
1612 // An inflateTry() method that we could call from enter() would be useful.
1613
1614 // Catch if the object's header is not neutral (not locked and
1615 // not marked is what we care about here).
1616 assert(mark.is_neutral(), "invariant: header=" INTPTR_FORMAT, mark.value());
1617 ObjectMonitor* m = new ObjectMonitor(object);
1618 // prepare m for installation - set monitor to initial state
1619 m->set_header(mark);
1620
1621 if (object->cas_set_mark(markWord::encode(m), mark) != mark) {
1622 delete m;
1623 m = nullptr;
1624 continue;
1625 // interference - the markword changed - just retry.
1626 // The state-transitions are one-way, so there's no chance of
1627 // live-lock -- "Inflated" is an absorbing state.
1628 }
1629
1630 // Once the ObjectMonitor is configured and object is associated
1631 // with the ObjectMonitor, it is safe to allow async deflation:
1632 _in_use_list.add(m);
1633
1634 // Hopefully the performance counters are allocated on distinct
1635 // cache lines to avoid false sharing on MP systems ...
1636 OM_PERFDATA_OP(Inflations, inc());
1637 if (log_is_enabled(Trace, monitorinflation)) {
1638 ResourceMark rm;
1639 lsh.print_cr("inflate(neutral): object=" INTPTR_FORMAT ", mark="
1640 INTPTR_FORMAT ", type='%s'", p2i(object),
1641 object->mark().value(), object->klass()->external_name());
1642 }
1643 if (event.should_commit()) {
1644 post_monitor_inflate_event(&event, object, cause);
1645 }
1646 return m;
1647 }
1648 }
1649
1650 void ObjectSynchronizer::chk_for_block_req(JavaThread* current, const char* op_name,
1651 const char* cnt_name, size_t cnt,
1652 LogStream* ls, elapsedTimer* timer_p) {
1653 if (!SafepointMechanism::should_process(current)) {
1654 return;
1655 }
1656
1657 // A safepoint/handshake has started.
1658 if (ls != nullptr) {
1659 timer_p->stop();
1660 ls->print_cr("pausing %s: %s=" SIZE_FORMAT ", in_use_list stats: ceiling="
1661 SIZE_FORMAT ", count=" SIZE_FORMAT ", max=" SIZE_FORMAT,
1662 op_name, cnt_name, cnt, in_use_list_ceiling(),
1663 _in_use_list.count(), _in_use_list.max());
1664 }
1665
1666 {
1667 // Honor block request.
1668 ThreadBlockInVM tbivm(current);
1669 }
1670
1671 if (ls != nullptr) {
1672 ls->print_cr("resuming %s: in_use_list stats: ceiling=" SIZE_FORMAT
1673 ", count=" SIZE_FORMAT ", max=" SIZE_FORMAT, op_name,
1674 in_use_list_ceiling(), _in_use_list.count(), _in_use_list.max());
1675 timer_p->start();
1676 }
1677 }
1678
1679 // Walk the in-use list and deflate (at most MonitorDeflationMax) idle
1680 // ObjectMonitors. Returns the number of deflated ObjectMonitors.
1681 //
1682 // If table != nullptr, we gather owned ObjectMonitors indexed by the
1683 // owner in the table. Please note that ObjectMonitors where the owner
1684 // is set to a stack-lock address are NOT associated with the JavaThread
1685 // that holds that stack-lock. All of the current consumers of
1686 // ObjectMonitorsHashtable info only care about JNI locked monitors and
1687 // those do not have the owner set to a stack-lock address.
1688 //
1689 size_t ObjectSynchronizer::deflate_monitor_list(Thread* current, LogStream* ls,
1690 elapsedTimer* timer_p,
1691 ObjectMonitorsHashtable* table) {
1692 MonitorList::Iterator iter = _in_use_list.iterator();
1693 size_t deflated_count = 0;
1694
1695 while (iter.has_next()) {
1696 if (deflated_count >= (size_t)MonitorDeflationMax) {
1697 break;
1698 }
1699 ObjectMonitor* mid = iter.next();
1700 if (mid->deflate_monitor()) {
1701 deflated_count++;
1702 } else if (table != nullptr) {
1703 // The caller is interested in the owned ObjectMonitors. This does
1704 // not include when owner is set to a stack-lock address in thread.
1705 // This also does not capture unowned ObjectMonitors that cannot be
1706 // deflated because of a waiter.
1707 void* key = mid->owner();
1708 // Since deflate_idle_monitors() and deflate_monitor_list() can be
1709 // called more than once, we have to make sure the entry has not
1710 // already been added.
1711 if (key != nullptr && !table->has_entry(key, mid)) {
1712 table->add_entry(key, mid);
1713 }
1714 }
1715
1716 if (current->is_Java_thread()) {
1717 // A JavaThread must check for a safepoint/handshake and honor it.
1718 chk_for_block_req(JavaThread::cast(current), "deflation", "deflated_count",
1719 deflated_count, ls, timer_p);
1720 }
1721 }
1722
1723 return deflated_count;
1724 }
1725
1726 class HandshakeForDeflation : public HandshakeClosure {
1727 public:
1728 HandshakeForDeflation() : HandshakeClosure("HandshakeForDeflation") {}
1729
1730 void do_thread(Thread* thread) {
1731 log_trace(monitorinflation)("HandshakeForDeflation::do_thread: thread="
1732 INTPTR_FORMAT, p2i(thread));
1733 }
1734 };
1735
1736 class VM_RendezvousGCThreads : public VM_Operation {
1737 public:
1738 bool evaluate_at_safepoint() const override { return false; }
1739 VMOp_Type type() const override { return VMOp_RendezvousGCThreads; }
1740 void doit() override {
1741 Universe::heap()->safepoint_synchronize_begin();
1742 Universe::heap()->safepoint_synchronize_end();
1743 };
1744 };
1745
1746 static size_t delete_monitors(Thread* current, GrowableArray<ObjectMonitor*>* delete_list,
1747 LogStream* ls, elapsedTimer* timer_p) {
1748 NativeHeapTrimmer::SuspendMark sm("monitor deletion");
1749 size_t deleted_count = 0;
1750 for (ObjectMonitor* monitor: *delete_list) {
1751 delete monitor;
1752 deleted_count++;
1753 if (current->is_Java_thread()) {
1754 // A JavaThread must check for a safepoint/handshake and honor it.
1755 ObjectSynchronizer::chk_for_block_req(JavaThread::cast(current), "deletion", "deleted_count",
1756 deleted_count, ls, timer_p);
1757 }
1758 }
1759 return deleted_count;
1760 }
1761
1762 // This function is called by the MonitorDeflationThread to deflate
1763 // ObjectMonitors. It is also called via do_final_audit_and_print_stats()
1764 // and VM_ThreadDump::doit() by the VMThread.
1765 size_t ObjectSynchronizer::deflate_idle_monitors(ObjectMonitorsHashtable* table) {
1766 Thread* current = Thread::current();
1767 if (current->is_Java_thread()) {
1768 // The async deflation request has been processed.
1769 _last_async_deflation_time_ns = os::javaTimeNanos();
1770 set_is_async_deflation_requested(false);
1771 }
1772
1773 LogStreamHandle(Debug, monitorinflation) lsh_debug;
1774 LogStreamHandle(Info, monitorinflation) lsh_info;
1775 LogStream* ls = nullptr;
1776 if (log_is_enabled(Debug, monitorinflation)) {
1777 ls = &lsh_debug;
1778 } else if (log_is_enabled(Info, monitorinflation)) {
1779 ls = &lsh_info;
1780 }
1781
1782 elapsedTimer timer;
1783 if (ls != nullptr) {
1784 ls->print_cr("begin deflating: in_use_list stats: ceiling=" SIZE_FORMAT ", count=" SIZE_FORMAT ", max=" SIZE_FORMAT,
1785 in_use_list_ceiling(), _in_use_list.count(), _in_use_list.max());
1786 timer.start();
1787 }
1788
1789 // Deflate some idle ObjectMonitors.
1790 size_t deflated_count = deflate_monitor_list(current, ls, &timer, table);
1791 size_t unlinked_count = 0;
1792 size_t deleted_count = 0;
1793 if (deflated_count > 0 || is_final_audit()) {
1794 // There are ObjectMonitors that have been deflated or this is the
1795 // final audit and all the remaining ObjectMonitors have been
1796 // deflated, BUT the MonitorDeflationThread blocked for the final
1797 // safepoint during unlinking.
1798
1799 // Unlink deflated ObjectMonitors from the in-use list.
1800 ResourceMark rm;
1801 GrowableArray<ObjectMonitor*> delete_list((int)deflated_count);
1802 unlinked_count = _in_use_list.unlink_deflated(current, ls, &timer, &delete_list);
1803 if (current->is_monitor_deflation_thread()) {
1804 if (ls != nullptr) {
1805 timer.stop();
1806 ls->print_cr("before handshaking: unlinked_count=" SIZE_FORMAT
1807 ", in_use_list stats: ceiling=" SIZE_FORMAT ", count="
1808 SIZE_FORMAT ", max=" SIZE_FORMAT,
1809 unlinked_count, in_use_list_ceiling(),
1810 _in_use_list.count(), _in_use_list.max());
1811 }
1812
1813 // A JavaThread needs to handshake in order to safely free the
1814 // ObjectMonitors that were deflated in this cycle.
1815 HandshakeForDeflation hfd_hc;
1816 Handshake::execute(&hfd_hc);
1817 // Also, we sync and desync GC threads around the handshake, so that they can
1818 // safely read the mark-word and look-through to the object-monitor, without
1819 // being afraid that the object-monitor is going away.
1820 VM_RendezvousGCThreads sync_gc;
1821 VMThread::execute(&sync_gc);
1822
1823 if (ls != nullptr) {
1824 ls->print_cr("after handshaking: in_use_list stats: ceiling="
1825 SIZE_FORMAT ", count=" SIZE_FORMAT ", max=" SIZE_FORMAT,
1826 in_use_list_ceiling(), _in_use_list.count(), _in_use_list.max());
1827 timer.start();
1828 }
1829 } else {
1830 // This is not a monitor deflation thread.
1831 // No handshake or rendezvous is needed when we are already at safepoint.
1832 assert_at_safepoint();
1833 }
1834
1835 // After the handshake, safely free the ObjectMonitors that were
1836 // deflated and unlinked in this cycle.
1837 deleted_count = delete_monitors(current, &delete_list, ls, &timer);
1838 assert(unlinked_count == deleted_count, "must be");
1839 }
1840
1841 if (ls != nullptr) {
1842 timer.stop();
1843 if (deflated_count != 0 || unlinked_count != 0 || log_is_enabled(Debug, monitorinflation)) {
1844 ls->print_cr("deflated_count=" SIZE_FORMAT ", {unlinked,deleted}_count=" SIZE_FORMAT " monitors in %3.7f secs",
1845 deflated_count, unlinked_count, timer.seconds());
1846 }
1847 ls->print_cr("end deflating: in_use_list stats: ceiling=" SIZE_FORMAT ", count=" SIZE_FORMAT ", max=" SIZE_FORMAT,
1848 in_use_list_ceiling(), _in_use_list.count(), _in_use_list.max());
1849 if (table != nullptr) {
1850 ls->print_cr("ObjectMonitorsHashtable: key_count=" SIZE_FORMAT ", om_count=" SIZE_FORMAT,
1851 table->key_count(), table->om_count());
1852 }
1853 }
1854
1855 OM_PERFDATA_OP(MonExtant, set_value(_in_use_list.count()));
1856 OM_PERFDATA_OP(Deflations, inc(deflated_count));
1857
1858 GVars.stw_random = os::random();
1859
1860 if (deflated_count != 0) {
1861 _no_progress_cnt = 0;
1862 } else if (_no_progress_skip_increment) {
1863 _no_progress_skip_increment = false;
1864 } else {
1865 _no_progress_cnt++;
1866 }
1867
1868 return deflated_count;
1869 }
1870
1871 // Monitor cleanup on JavaThread::exit
1872
1873 // Iterate through monitor cache and attempt to release thread's monitors
1874 class ReleaseJavaMonitorsClosure: public MonitorClosure {
1875 private:
1876 JavaThread* _thread;
1877
1878 public:
1879 ReleaseJavaMonitorsClosure(JavaThread* thread) : _thread(thread) {}
1880 void do_monitor(ObjectMonitor* mid) {
1881 intx rec = mid->complete_exit(_thread);
1882 _thread->dec_held_monitor_count(rec + 1);
1883 }
1884 };
1885
1886 // Release all inflated monitors owned by current thread. Lightweight monitors are
1887 // ignored. This is meant to be called during JNI thread detach which assumes
1888 // all remaining monitors are heavyweight. All exceptions are swallowed.
1889 // Scanning the extant monitor list can be time consuming.
1890 // A simple optimization is to add a per-thread flag that indicates a thread
1891 // called jni_monitorenter() during its lifetime.
1892 //
1893 // Instead of NoSafepointVerifier it might be cheaper to
1894 // use an idiom of the form:
1895 // auto int tmp = SafepointSynchronize::_safepoint_counter ;
1896 // <code that must not run at safepoint>
1897 // guarantee (((tmp ^ _safepoint_counter) | (tmp & 1)) == 0) ;
1898 // Since the tests are extremely cheap we could leave them enabled
1899 // for normal product builds.
1900
1901 void ObjectSynchronizer::release_monitors_owned_by_thread(JavaThread* current) {
1902 assert(current == JavaThread::current(), "must be current Java thread");
1903 NoSafepointVerifier nsv;
1904 ReleaseJavaMonitorsClosure rjmc(current);
1905 ObjectSynchronizer::monitors_iterate(&rjmc, current);
1906 assert(!current->has_pending_exception(), "Should not be possible");
1907 current->clear_pending_exception();
1908 assert(current->held_monitor_count() == 0, "Should not be possible");
1909 // All monitors (including entered via JNI) have been unlocked above, so we need to clear jni count.
1910 current->clear_jni_monitor_count();
1911 }
1912
1913 const char* ObjectSynchronizer::inflate_cause_name(const InflateCause cause) {
1914 switch (cause) {
1915 case inflate_cause_vm_internal: return "VM Internal";
1916 case inflate_cause_monitor_enter: return "Monitor Enter";
1917 case inflate_cause_wait: return "Monitor Wait";
1918 case inflate_cause_notify: return "Monitor Notify";
1919 case inflate_cause_hash_code: return "Monitor Hash Code";
1920 case inflate_cause_jni_enter: return "JNI Monitor Enter";
1921 case inflate_cause_jni_exit: return "JNI Monitor Exit";
1922 default:
1923 ShouldNotReachHere();
1924 }
1925 return "Unknown";
1926 }
1927
1928 //------------------------------------------------------------------------------
1929 // Debugging code
1930
1931 u_char* ObjectSynchronizer::get_gvars_addr() {
1932 return (u_char*)&GVars;
1933 }
1934
1935 u_char* ObjectSynchronizer::get_gvars_hc_sequence_addr() {
1936 return (u_char*)&GVars.hc_sequence;
1937 }
1938
1939 size_t ObjectSynchronizer::get_gvars_size() {
1940 return sizeof(SharedGlobals);
1941 }
1942
1943 u_char* ObjectSynchronizer::get_gvars_stw_random_addr() {
1944 return (u_char*)&GVars.stw_random;
1945 }
1946
1947 // Do the final audit and print of ObjectMonitor stats; must be done
1948 // by the VMThread at VM exit time.
1949 void ObjectSynchronizer::do_final_audit_and_print_stats() {
1950 assert(Thread::current()->is_VM_thread(), "sanity check");
1951
1952 if (is_final_audit()) { // Only do the audit once.
1953 return;
1954 }
1955 set_is_final_audit();
1956 log_info(monitorinflation)("Starting the final audit.");
1957
1958 if (log_is_enabled(Info, monitorinflation)) {
1959 // Do deflations in order to reduce the in-use monitor population
1960 // that is reported by ObjectSynchronizer::log_in_use_monitor_details()
1961 // which is called by ObjectSynchronizer::audit_and_print_stats().
1962 while (deflate_idle_monitors(/* ObjectMonitorsHashtable is not needed here */ nullptr) > 0) {
1963 ; // empty
1964 }
1965 // The other audit_and_print_stats() call is done at the Debug
1966 // level at a safepoint in SafepointSynchronize::do_cleanup_tasks.
1967 audit_and_print_stats(true /* on_exit */);
1968 }
1969 }
1970
1971 // This function can be called at a safepoint or it can be called when
1972 // we are trying to exit the VM. When we are trying to exit the VM, the
1973 // list walker functions can run in parallel with the other list
1974 // operations so spin-locking is used for safety.
1975 //
1976 // Calls to this function can be added in various places as a debugging
1977 // aid; pass 'true' for the 'on_exit' parameter to have in-use monitor
1978 // details logged at the Info level and 'false' for the 'on_exit'
1979 // parameter to have in-use monitor details logged at the Trace level.
1980 //
1981 void ObjectSynchronizer::audit_and_print_stats(bool on_exit) {
1982 assert(on_exit || SafepointSynchronize::is_at_safepoint(), "invariant");
1983
1984 LogStreamHandle(Debug, monitorinflation) lsh_debug;
1985 LogStreamHandle(Info, monitorinflation) lsh_info;
1986 LogStreamHandle(Trace, monitorinflation) lsh_trace;
1987 LogStream* ls = nullptr;
1988 if (log_is_enabled(Trace, monitorinflation)) {
1989 ls = &lsh_trace;
1990 } else if (log_is_enabled(Debug, monitorinflation)) {
1991 ls = &lsh_debug;
1992 } else if (log_is_enabled(Info, monitorinflation)) {
1993 ls = &lsh_info;
1994 }
1995 assert(ls != nullptr, "sanity check");
1996
1997 int error_cnt = 0;
1998
1999 ls->print_cr("Checking in_use_list:");
2000 chk_in_use_list(ls, &error_cnt);
2001
2002 if (error_cnt == 0) {
2003 ls->print_cr("No errors found in in_use_list checks.");
2004 } else {
2005 log_error(monitorinflation)("found in_use_list errors: error_cnt=%d", error_cnt);
2006 }
2007
2008 if ((on_exit && log_is_enabled(Info, monitorinflation)) ||
2009 (!on_exit && log_is_enabled(Trace, monitorinflation))) {
2010 // When exiting this log output is at the Info level. When called
2011 // at a safepoint, this log output is at the Trace level since
2012 // there can be a lot of it.
2013 log_in_use_monitor_details(ls);
2014 }
2015
2016 ls->flush();
2017
2018 guarantee(error_cnt == 0, "ERROR: found monitor list errors: error_cnt=%d", error_cnt);
2019 }
2020
2021 // Check the in_use_list; log the results of the checks.
2022 void ObjectSynchronizer::chk_in_use_list(outputStream* out, int *error_cnt_p) {
2023 size_t l_in_use_count = _in_use_list.count();
2024 size_t l_in_use_max = _in_use_list.max();
2025 out->print_cr("count=" SIZE_FORMAT ", max=" SIZE_FORMAT, l_in_use_count,
2026 l_in_use_max);
2027
2028 size_t ck_in_use_count = 0;
2029 MonitorList::Iterator iter = _in_use_list.iterator();
2030 while (iter.has_next()) {
2031 ObjectMonitor* mid = iter.next();
2032 chk_in_use_entry(mid, out, error_cnt_p);
2033 ck_in_use_count++;
2034 }
2035
2036 if (l_in_use_count == ck_in_use_count) {
2037 out->print_cr("in_use_count=" SIZE_FORMAT " equals ck_in_use_count="
2038 SIZE_FORMAT, l_in_use_count, ck_in_use_count);
2039 } else {
2040 out->print_cr("WARNING: in_use_count=" SIZE_FORMAT " is not equal to "
2041 "ck_in_use_count=" SIZE_FORMAT, l_in_use_count,
2042 ck_in_use_count);
2043 }
2044
2045 size_t ck_in_use_max = _in_use_list.max();
2046 if (l_in_use_max == ck_in_use_max) {
2047 out->print_cr("in_use_max=" SIZE_FORMAT " equals ck_in_use_max="
2048 SIZE_FORMAT, l_in_use_max, ck_in_use_max);
2049 } else {
2050 out->print_cr("WARNING: in_use_max=" SIZE_FORMAT " is not equal to "
2051 "ck_in_use_max=" SIZE_FORMAT, l_in_use_max, ck_in_use_max);
2052 }
2053 }
2054
2055 // Check an in-use monitor entry; log any errors.
2056 void ObjectSynchronizer::chk_in_use_entry(ObjectMonitor* n, outputStream* out,
2057 int* error_cnt_p) {
2058 if (n->owner_is_DEFLATER_MARKER()) {
2059 // This should not happen, but if it does, it is not fatal.
2060 out->print_cr("WARNING: monitor=" INTPTR_FORMAT ": in-use monitor is "
2061 "deflated.", p2i(n));
2062 return;
2063 }
2064 if (n->header().value() == 0) {
2065 out->print_cr("ERROR: monitor=" INTPTR_FORMAT ": in-use monitor must "
2066 "have non-null _header field.", p2i(n));
2067 *error_cnt_p = *error_cnt_p + 1;
2068 }
2069 const oop obj = n->object_peek();
2070 if (obj != nullptr) {
2071 const markWord mark = obj->mark();
2072 if (!mark.has_monitor()) {
2073 out->print_cr("ERROR: monitor=" INTPTR_FORMAT ": in-use monitor's "
2074 "object does not think it has a monitor: obj="
2075 INTPTR_FORMAT ", mark=" INTPTR_FORMAT, p2i(n),
2076 p2i(obj), mark.value());
2077 *error_cnt_p = *error_cnt_p + 1;
2078 }
2079 ObjectMonitor* const obj_mon = mark.monitor();
2080 if (n != obj_mon) {
2081 out->print_cr("ERROR: monitor=" INTPTR_FORMAT ": in-use monitor's "
2082 "object does not refer to the same monitor: obj="
2083 INTPTR_FORMAT ", mark=" INTPTR_FORMAT ", obj_mon="
2084 INTPTR_FORMAT, p2i(n), p2i(obj), mark.value(), p2i(obj_mon));
2085 *error_cnt_p = *error_cnt_p + 1;
2086 }
2087 }
2088 }
2089
2090 // Log details about ObjectMonitors on the in_use_list. The 'BHL'
2091 // flags indicate why the entry is in-use, 'object' and 'object type'
2092 // indicate the associated object and its type.
2093 void ObjectSynchronizer::log_in_use_monitor_details(outputStream* out) {
2094 stringStream ss;
2095 if (_in_use_list.count() > 0) {
2096 out->print_cr("In-use monitor info:");
2097 out->print_cr("(B -> is_busy, H -> has hash code, L -> lock status)");
2098 out->print_cr("%18s %s %18s %18s",
2099 "monitor", "BHL", "object", "object type");
2100 out->print_cr("================== === ================== ==================");
2101 MonitorList::Iterator iter = _in_use_list.iterator();
2102 while (iter.has_next()) {
2103 ObjectMonitor* mid = iter.next();
2104 const oop obj = mid->object_peek();
2105 const markWord mark = mid->header();
2106 ResourceMark rm;
2107 out->print(INTPTR_FORMAT " %d%d%d " INTPTR_FORMAT " %s", p2i(mid),
2108 mid->is_busy(), mark.hash() != 0, mid->owner() != nullptr,
2109 p2i(obj), obj == nullptr ? "" : obj->klass()->external_name());
2110 if (mid->is_busy()) {
2111 out->print(" (%s)", mid->is_busy_to_string(&ss));
2112 ss.reset();
2113 }
2114 out->cr();
2115 }
2116 }
2117
2118 out->flush();
2119 }