1 /*
2 * Copyright (c) 1998, 2023, 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/oopStorage.hpp"
28 #include "gc/shared/oopStorageSet.hpp"
29 #include "jfr/jfrEvents.hpp"
30 #include "jfr/support/jfrThreadId.hpp"
31 #include "logging/log.hpp"
32 #include "logging/logStream.hpp"
33 #include "memory/allocation.inline.hpp"
34 #include "memory/resourceArea.hpp"
35 #include "oops/markWord.hpp"
36 #include "oops/oop.inline.hpp"
37 #include "oops/oopHandle.inline.hpp"
38 #include "oops/weakHandle.inline.hpp"
39 #include "prims/jvmtiDeferredUpdates.hpp"
40 #include "prims/jvmtiExport.hpp"
41 #include "runtime/atomic.hpp"
42 #include "runtime/handles.inline.hpp"
43 #include "runtime/interfaceSupport.inline.hpp"
44 #include "runtime/javaThread.inline.hpp"
45 #include "runtime/mutexLocker.hpp"
46 #include "runtime/objectMonitor.hpp"
47 #include "runtime/objectMonitor.inline.hpp"
48 #include "runtime/orderAccess.hpp"
49 #include "runtime/osThread.hpp"
50 #include "runtime/perfData.hpp"
51 #include "runtime/safefetch.hpp"
52 #include "runtime/safepointMechanism.inline.hpp"
53 #include "runtime/sharedRuntime.hpp"
54 #include "services/threadService.hpp"
55 #include "utilities/dtrace.hpp"
56 #include "utilities/macros.hpp"
57 #include "utilities/preserveException.hpp"
58 #if INCLUDE_JFR
59 #include "jfr/support/jfrFlush.hpp"
60 #endif
61
62 #ifdef DTRACE_ENABLED
63
64 // Only bother with this argument setup if dtrace is available
65 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
66
67
68 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
69 char* bytes = nullptr; \
70 int len = 0; \
71 jlong jtid = SharedRuntime::get_java_tid(thread); \
72 Symbol* klassname = obj->klass()->name(); \
73 if (klassname != nullptr) { \
74 bytes = (char*)klassname->bytes(); \
75 len = klassname->utf8_length(); \
76 }
77
78 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
79 { \
80 if (DTraceMonitorProbes) { \
81 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
82 HOTSPOT_MONITOR_WAIT(jtid, \
83 (monitor), bytes, len, (millis)); \
84 } \
85 }
86
87 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
88 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
89 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
90 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
91 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
92
93 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
94 { \
95 if (DTraceMonitorProbes) { \
96 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
97 HOTSPOT_MONITOR_##probe(jtid, \
98 (uintptr_t)(monitor), bytes, len); \
99 } \
100 }
101
102 #else // ndef DTRACE_ENABLED
103
104 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
105 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
106
107 #endif // ndef DTRACE_ENABLED
108
109 // Tunables ...
110 // The knob* variables are effectively final. Once set they should
111 // never be modified hence. Consider using __read_mostly with GCC.
112
113 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool -
114
115 static int Knob_Bonus = 100; // spin success bonus
116 static int Knob_BonusB = 100; // spin success bonus
117 static int Knob_Penalty = 200; // spin failure penalty
118 static int Knob_Poverty = 1000;
119 static int Knob_FixedSpin = 0;
120 static int Knob_PreSpin = 10; // 20-100 likely better
121
122 DEBUG_ONLY(static volatile bool InitDone = false;)
123
124 OopStorage* ObjectMonitor::_oop_storage = nullptr;
125
126 // -----------------------------------------------------------------------------
127 // Theory of operations -- Monitors lists, thread residency, etc:
128 //
129 // * A thread acquires ownership of a monitor by successfully
130 // CAS()ing the _owner field from null to non-null.
131 //
132 // * Invariant: A thread appears on at most one monitor list --
133 // cxq, EntryList or WaitSet -- at any one time.
134 //
135 // * Contending threads "push" themselves onto the cxq with CAS
136 // and then spin/park.
137 //
138 // * After a contending thread eventually acquires the lock it must
139 // dequeue itself from either the EntryList or the cxq.
140 //
141 // * The exiting thread identifies and unparks an "heir presumptive"
142 // tentative successor thread on the EntryList. Critically, the
143 // exiting thread doesn't unlink the successor thread from the EntryList.
144 // After having been unparked, the wakee will recontend for ownership of
145 // the monitor. The successor (wakee) will either acquire the lock or
146 // re-park itself.
147 //
148 // Succession is provided for by a policy of competitive handoff.
149 // The exiting thread does _not_ grant or pass ownership to the
150 // successor thread. (This is also referred to as "handoff" succession").
151 // Instead the exiting thread releases ownership and possibly wakes
152 // a successor, so the successor can (re)compete for ownership of the lock.
153 // If the EntryList is empty but the cxq is populated the exiting
154 // thread will drain the cxq into the EntryList. It does so by
155 // by detaching the cxq (installing null with CAS) and folding
156 // the threads from the cxq into the EntryList. The EntryList is
157 // doubly linked, while the cxq is singly linked because of the
158 // CAS-based "push" used to enqueue recently arrived threads (RATs).
159 //
160 // * Concurrency invariants:
161 //
162 // -- only the monitor owner may access or mutate the EntryList.
163 // The mutex property of the monitor itself protects the EntryList
164 // from concurrent interference.
165 // -- Only the monitor owner may detach the cxq.
166 //
167 // * The monitor entry list operations avoid locks, but strictly speaking
168 // they're not lock-free. Enter is lock-free, exit is not.
169 // For a description of 'Methods and apparatus providing non-blocking access
170 // to a resource,' see U.S. Pat. No. 7844973.
171 //
172 // * The cxq can have multiple concurrent "pushers" but only one concurrent
173 // detaching thread. This mechanism is immune from the ABA corruption.
174 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
175 //
176 // * Taken together, the cxq and the EntryList constitute or form a
177 // single logical queue of threads stalled trying to acquire the lock.
178 // We use two distinct lists to improve the odds of a constant-time
179 // dequeue operation after acquisition (in the ::enter() epilogue) and
180 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
181 // A key desideratum is to minimize queue & monitor metadata manipulation
182 // that occurs while holding the monitor lock -- that is, we want to
183 // minimize monitor lock holds times. Note that even a small amount of
184 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
185 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
186 // locks and monitor metadata.
187 //
188 // Cxq points to the set of Recently Arrived Threads attempting entry.
189 // Because we push threads onto _cxq with CAS, the RATs must take the form of
190 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
191 // the unlocking thread notices that EntryList is null but _cxq is != null.
192 //
193 // The EntryList is ordered by the prevailing queue discipline and
194 // can be organized in any convenient fashion, such as a doubly-linked list or
195 // a circular doubly-linked list. Critically, we want insert and delete operations
196 // to operate in constant-time. If we need a priority queue then something akin
197 // to Solaris' sleepq would work nicely. Viz.,
198 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
199 // Queue discipline is enforced at ::exit() time, when the unlocking thread
200 // drains the cxq into the EntryList, and orders or reorders the threads on the
201 // EntryList accordingly.
202 //
203 // Barring "lock barging", this mechanism provides fair cyclic ordering,
204 // somewhat similar to an elevator-scan.
205 //
206 // * The monitor synchronization subsystem avoids the use of native
207 // synchronization primitives except for the narrow platform-specific
208 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
209 // the semantics of park-unpark. Put another way, this monitor implementation
210 // depends only on atomic operations and park-unpark. The monitor subsystem
211 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
212 // underlying OS manages the READY<->RUN transitions.
213 //
214 // * Waiting threads reside on the WaitSet list -- wait() puts
215 // the caller onto the WaitSet.
216 //
217 // * notify() or notifyAll() simply transfers threads from the WaitSet to
218 // either the EntryList or cxq. Subsequent exit() operations will
219 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
220 // it's likely the notifyee would simply impale itself on the lock held
221 // by the notifier.
222 //
223 // * An interesting alternative is to encode cxq as (List,LockByte) where
224 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
225 // variable, like _recursions, in the scheme. The threads or Events that form
226 // the list would have to be aligned in 256-byte addresses. A thread would
227 // try to acquire the lock or enqueue itself with CAS, but exiting threads
228 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
229 // Note that is is *not* word-tearing, but it does presume that full-word
230 // CAS operations are coherent with intermix with STB operations. That's true
231 // on most common processors.
232 //
233 // * See also http://blogs.sun.com/dave
234
235
236 // Check that object() and set_object() are called from the right context:
237 static void check_object_context() {
238 #ifdef ASSERT
239 Thread* self = Thread::current();
240 if (self->is_Java_thread()) {
241 // Mostly called from JavaThreads so sanity check the thread state.
242 JavaThread* jt = JavaThread::cast(self);
243 switch (jt->thread_state()) {
244 case _thread_in_vm: // the usual case
245 case _thread_in_Java: // during deopt
246 break;
247 default:
248 fatal("called from an unsafe thread state");
249 }
250 assert(jt->is_active_Java_thread(), "must be active JavaThread");
251 } else {
252 // However, ThreadService::get_current_contended_monitor()
253 // can call here via the VMThread so sanity check it.
254 assert(self->is_VM_thread(), "must be");
255 }
256 #endif // ASSERT
257 }
258
259 ObjectMonitor::ObjectMonitor(oop object) :
260 _header(markWord::zero()),
261 _object(_oop_storage, object),
262 _owner(nullptr),
263 _previous_owner_tid(0),
264 _next_om(nullptr),
265 _recursions(0),
266 _EntryList(nullptr),
267 _cxq(nullptr),
268 _succ(nullptr),
269 _Responsible(nullptr),
270 _Spinner(0),
271 _SpinDuration(ObjectMonitor::Knob_SpinLimit),
272 _contentions(0),
273 _WaitSet(nullptr),
274 _waiters(0),
275 _WaitSetLock(0)
276 { }
277
278 ObjectMonitor::~ObjectMonitor() {
279 _object.release(_oop_storage);
280 }
281
282 oop ObjectMonitor::object() const {
283 check_object_context();
284 return _object.resolve();
285 }
286
287 oop ObjectMonitor::object_peek() const {
288 return _object.peek();
289 }
290
291 void ObjectMonitor::ExitOnSuspend::operator()(JavaThread* current) {
292 if (current->is_suspended()) {
293 _om->_recursions = 0;
294 _om->_succ = nullptr;
295 // Don't need a full fence after clearing successor here because of the call to exit().
296 _om->exit(current, false /* not_suspended */);
297 _om_exited = true;
298
299 current->set_current_pending_monitor(_om);
300 }
301 }
302
303 void ObjectMonitor::ClearSuccOnSuspend::operator()(JavaThread* current) {
304 if (current->is_suspended()) {
305 if (_om->_succ == current) {
306 _om->_succ = nullptr;
307 OrderAccess::fence(); // always do a full fence when successor is cleared
308 }
309 }
310 }
311
312 // -----------------------------------------------------------------------------
313 // Enter support
314
315 bool ObjectMonitor::enter(JavaThread* current) {
316 // The following code is ordered to check the most common cases first
317 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
318
319 void* cur = try_set_owner_from(nullptr, current);
320 if (cur == nullptr) {
321 assert(_recursions == 0, "invariant");
322 return true;
323 }
324
325 if (cur == current) {
326 // TODO-FIXME: check for integer overflow! BUGID 6557169.
327 _recursions++;
328 return true;
329 }
330
331 if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) {
332 assert(_recursions == 0, "internal state error");
333 _recursions = 1;
334 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*.
335 return true;
336 }
337
338 // We've encountered genuine contention.
339 assert(current->_Stalled == 0, "invariant");
340 current->_Stalled = intptr_t(this);
341
342 // Try one round of spinning *before* enqueueing current
343 // and before going through the awkward and expensive state
344 // transitions. The following spin is strictly optional ...
345 // Note that if we acquire the monitor from an initial spin
346 // we forgo posting JVMTI events and firing DTRACE probes.
347 if (TrySpin(current) > 0) {
348 assert(owner_raw() == current, "must be current: owner=" INTPTR_FORMAT, p2i(owner_raw()));
349 assert(_recursions == 0, "must be 0: recursions=" INTX_FORMAT, _recursions);
350 assert(object()->mark() == markWord::encode(this),
351 "object mark must match encoded this: mark=" INTPTR_FORMAT
352 ", encoded this=" INTPTR_FORMAT, object()->mark().value(),
353 markWord::encode(this).value());
354 current->_Stalled = 0;
355 return true;
356 }
357
358 assert(owner_raw() != current, "invariant");
359 assert(_succ != current, "invariant");
360 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
361 assert(current->thread_state() != _thread_blocked, "invariant");
362
363 // Keep track of contention for JVM/TI and M&M queries.
364 add_to_contentions(1);
365 if (is_being_async_deflated()) {
366 // Async deflation is in progress and our contentions increment
367 // above lost the race to async deflation. Undo the work and
368 // force the caller to retry.
369 const oop l_object = object();
370 if (l_object != nullptr) {
371 // Attempt to restore the header/dmw to the object's header so that
372 // we only retry once if the deflater thread happens to be slow.
373 install_displaced_markword_in_object(l_object);
374 }
375 current->_Stalled = 0;
376 add_to_contentions(-1);
377 return false;
378 }
379
380 JFR_ONLY(JfrConditionalFlush<EventJavaMonitorEnter> flush(current);)
381 EventJavaMonitorEnter event;
382 if (event.is_started()) {
383 event.set_monitorClass(object()->klass());
384 // Set an address that is 'unique enough', such that events close in
385 // time and with the same address are likely (but not guaranteed) to
386 // belong to the same object.
387 event.set_address((uintptr_t)this);
388 }
389
390 { // Change java thread status to indicate blocked on monitor enter.
391 JavaThreadBlockedOnMonitorEnterState jtbmes(current, this);
392
393 assert(current->current_pending_monitor() == nullptr, "invariant");
394 current->set_current_pending_monitor(this);
395
396 DTRACE_MONITOR_PROBE(contended__enter, this, object(), current);
397 if (JvmtiExport::should_post_monitor_contended_enter()) {
398 JvmtiExport::post_monitor_contended_enter(current, this);
399
400 // The current thread does not yet own the monitor and does not
401 // yet appear on any queues that would get it made the successor.
402 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
403 // handler cannot accidentally consume an unpark() meant for the
404 // ParkEvent associated with this ObjectMonitor.
405 }
406
407 OSThreadContendState osts(current->osthread());
408
409 assert(current->thread_state() == _thread_in_vm, "invariant");
410
411 for (;;) {
412 ExitOnSuspend eos(this);
413 {
414 ThreadBlockInVMPreprocess<ExitOnSuspend> tbivs(current, eos, true /* allow_suspend */);
415 EnterI(current);
416 current->set_current_pending_monitor(nullptr);
417 // We can go to a safepoint at the end of this block. If we
418 // do a thread dump during that safepoint, then this thread will show
419 // as having "-locked" the monitor, but the OS and java.lang.Thread
420 // states will still report that the thread is blocked trying to
421 // acquire it.
422 // If there is a suspend request, ExitOnSuspend will exit the OM
423 // and set the OM as pending.
424 }
425 if (!eos.exited()) {
426 // ExitOnSuspend did not exit the OM
427 assert(owner_raw() == current, "invariant");
428 break;
429 }
430 }
431
432 // We've just gotten past the enter-check-for-suspend dance and we now own
433 // the monitor free and clear.
434 }
435
436 add_to_contentions(-1);
437 assert(contentions() >= 0, "must not be negative: contentions=%d", contentions());
438 current->_Stalled = 0;
439
440 // Must either set _recursions = 0 or ASSERT _recursions == 0.
441 assert(_recursions == 0, "invariant");
442 assert(owner_raw() == current, "invariant");
443 assert(_succ != current, "invariant");
444 assert(object()->mark() == markWord::encode(this), "invariant");
445
446 // The thread -- now the owner -- is back in vm mode.
447 // Report the glorious news via TI,DTrace and jvmstat.
448 // The probe effect is non-trivial. All the reportage occurs
449 // while we hold the monitor, increasing the length of the critical
450 // section. Amdahl's parallel speedup law comes vividly into play.
451 //
452 // Another option might be to aggregate the events (thread local or
453 // per-monitor aggregation) and defer reporting until a more opportune
454 // time -- such as next time some thread encounters contention but has
455 // yet to acquire the lock. While spinning that thread could
456 // spinning we could increment JVMStat counters, etc.
457
458 DTRACE_MONITOR_PROBE(contended__entered, this, object(), current);
459 if (JvmtiExport::should_post_monitor_contended_entered()) {
460 JvmtiExport::post_monitor_contended_entered(current, this);
461
462 // The current thread already owns the monitor and is not going to
463 // call park() for the remainder of the monitor enter protocol. So
464 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
465 // event handler consumed an unpark() issued by the thread that
466 // just exited the monitor.
467 }
468 if (event.should_commit()) {
469 event.set_previousOwner(_previous_owner_tid);
470 event.commit();
471 }
472 OM_PERFDATA_OP(ContendedLockAttempts, inc());
473 return true;
474 }
475
476 // Caveat: TryLock() is not necessarily serializing if it returns failure.
477 // Callers must compensate as needed.
478
479 int ObjectMonitor::TryLock(JavaThread* current) {
480 void* own = owner_raw();
481 if (own != nullptr) return 0;
482 if (try_set_owner_from(nullptr, current) == nullptr) {
483 assert(_recursions == 0, "invariant");
484 return 1;
485 }
486 // The lock had been free momentarily, but we lost the race to the lock.
487 // Interference -- the CAS failed.
488 // We can either return -1 or retry.
489 // Retry doesn't make as much sense because the lock was just acquired.
490 return -1;
491 }
492
493 // Deflate the specified ObjectMonitor if not in-use. Returns true if it
494 // was deflated and false otherwise.
495 //
496 // The async deflation protocol sets owner to DEFLATER_MARKER and
497 // makes contentions negative as signals to contending threads that
498 // an async deflation is in progress. There are a number of checks
499 // as part of the protocol to make sure that the calling thread has
500 // not lost the race to a contending thread.
501 //
502 // The ObjectMonitor has been successfully async deflated when:
503 // (contentions < 0)
504 // Contending threads that see that condition know to retry their operation.
505 //
506 bool ObjectMonitor::deflate_monitor() {
507 if (is_busy()) {
508 // Easy checks are first - the ObjectMonitor is busy so no deflation.
509 return false;
510 }
511
512 const oop obj = object_peek();
513
514 if (obj == nullptr) {
515 // If the object died, we can recycle the monitor without racing with
516 // Java threads. The GC already broke the association with the object.
517 set_owner_from(nullptr, DEFLATER_MARKER);
518 assert(contentions() >= 0, "must be non-negative: contentions=%d", contentions());
519 _contentions = INT_MIN; // minimum negative int
520 } else {
521 // Attempt async deflation protocol.
522
523 // Set a null owner to DEFLATER_MARKER to force any contending thread
524 // through the slow path. This is just the first part of the async
525 // deflation dance.
526 if (try_set_owner_from(nullptr, DEFLATER_MARKER) != nullptr) {
527 // The owner field is no longer null so we lost the race since the
528 // ObjectMonitor is now busy.
529 return false;
530 }
531
532 if (contentions() > 0 || _waiters != 0) {
533 // Another thread has raced to enter the ObjectMonitor after
534 // is_busy() above or has already entered and waited on
535 // it which makes it busy so no deflation. Restore owner to
536 // null if it is still DEFLATER_MARKER.
537 if (try_set_owner_from(DEFLATER_MARKER, nullptr) != DEFLATER_MARKER) {
538 // Deferred decrement for the JT EnterI() that cancelled the async deflation.
539 add_to_contentions(-1);
540 }
541 return false;
542 }
543
544 // Make a zero contentions field negative to force any contending threads
545 // to retry. This is the second part of the async deflation dance.
546 if (Atomic::cmpxchg(&_contentions, 0, INT_MIN) != 0) {
547 // Contentions was no longer 0 so we lost the race since the
548 // ObjectMonitor is now busy. Restore owner to null if it is
549 // still DEFLATER_MARKER:
550 if (try_set_owner_from(DEFLATER_MARKER, nullptr) != DEFLATER_MARKER) {
551 // Deferred decrement for the JT EnterI() that cancelled the async deflation.
552 add_to_contentions(-1);
553 }
554 return false;
555 }
556 }
557
558 // Sanity checks for the races:
559 guarantee(owner_is_DEFLATER_MARKER(), "must be deflater marker");
560 guarantee(contentions() < 0, "must be negative: contentions=%d",
561 contentions());
562 guarantee(_waiters == 0, "must be 0: waiters=%d", _waiters);
563 guarantee(_cxq == nullptr, "must be no contending threads: cxq="
564 INTPTR_FORMAT, p2i(_cxq));
565 guarantee(_EntryList == nullptr,
566 "must be no entering threads: EntryList=" INTPTR_FORMAT,
567 p2i(_EntryList));
568
569 if (obj != nullptr) {
570 if (log_is_enabled(Trace, monitorinflation)) {
571 ResourceMark rm;
572 log_trace(monitorinflation)("deflate_monitor: object=" INTPTR_FORMAT
573 ", mark=" INTPTR_FORMAT ", type='%s'",
574 p2i(obj), obj->mark().value(),
575 obj->klass()->external_name());
576 }
577
578 // Install the old mark word if nobody else has already done it.
579 install_displaced_markword_in_object(obj);
580 }
581
582 // We leave owner == DEFLATER_MARKER and contentions < 0
583 // to force any racing threads to retry.
584 return true; // Success, ObjectMonitor has been deflated.
585 }
586
587 // Install the displaced mark word (dmw) of a deflating ObjectMonitor
588 // into the header of the object associated with the monitor. This
589 // idempotent method is called by a thread that is deflating a
590 // monitor and by other threads that have detected a race with the
591 // deflation process.
592 void ObjectMonitor::install_displaced_markword_in_object(const oop obj) {
593 // This function must only be called when (owner == DEFLATER_MARKER
594 // && contentions <= 0), but we can't guarantee that here because
595 // those values could change when the ObjectMonitor gets moved from
596 // the global free list to a per-thread free list.
597
598 guarantee(obj != nullptr, "must be non-null");
599
600 // Separate loads in is_being_async_deflated(), which is almost always
601 // called before this function, from the load of dmw/header below.
602
603 // _contentions and dmw/header may get written by different threads.
604 // Make sure to observe them in the same order when having several observers.
605 OrderAccess::loadload_for_IRIW();
606
607 const oop l_object = object_peek();
608 if (l_object == nullptr) {
609 // ObjectMonitor's object ref has already been cleared by async
610 // deflation or GC so we're done here.
611 return;
612 }
613 assert(l_object == obj, "object=" INTPTR_FORMAT " must equal obj="
614 INTPTR_FORMAT, p2i(l_object), p2i(obj));
615
616 markWord dmw = header();
617 // The dmw has to be neutral (not null, not locked and not marked).
618 assert(dmw.is_neutral(), "must be neutral: dmw=" INTPTR_FORMAT, dmw.value());
619
620 // Install displaced mark word if the object's header still points
621 // to this ObjectMonitor. More than one racing caller to this function
622 // can rarely reach this point, but only one can win.
623 markWord res = obj->cas_set_mark(dmw, markWord::encode(this));
624 if (res != markWord::encode(this)) {
625 // This should be rare so log at the Info level when it happens.
626 log_info(monitorinflation)("install_displaced_markword_in_object: "
627 "failed cas_set_mark: new_mark=" INTPTR_FORMAT
628 ", old_mark=" INTPTR_FORMAT ", res=" INTPTR_FORMAT,
629 dmw.value(), markWord::encode(this).value(),
630 res.value());
631 }
632
633 // Note: It does not matter which thread restored the header/dmw
634 // into the object's header. The thread deflating the monitor just
635 // wanted the object's header restored and it is. The threads that
636 // detected a race with the deflation process also wanted the
637 // object's header restored before they retry their operation and
638 // because it is restored they will only retry once.
639 }
640
641 // Convert the fields used by is_busy() to a string that can be
642 // used for diagnostic output.
643 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) {
644 ss->print("is_busy: waiters=%d, ", _waiters);
645 if (contentions() > 0) {
646 ss->print("contentions=%d, ", contentions());
647 } else {
648 ss->print("contentions=0");
649 }
650 if (!owner_is_DEFLATER_MARKER()) {
651 ss->print("owner=" INTPTR_FORMAT, p2i(owner_raw()));
652 } else {
653 // We report null instead of DEFLATER_MARKER here because is_busy()
654 // ignores DEFLATER_MARKER values.
655 ss->print("owner=" INTPTR_FORMAT, NULL_WORD);
656 }
657 ss->print(", cxq=" INTPTR_FORMAT ", EntryList=" INTPTR_FORMAT, p2i(_cxq),
658 p2i(_EntryList));
659 return ss->base();
660 }
661
662 #define MAX_RECHECK_INTERVAL 1000
663
664 void ObjectMonitor::EnterI(JavaThread* current) {
665 assert(current->thread_state() == _thread_blocked, "invariant");
666
667 // Try the lock - TATAS
668 if (TryLock (current) > 0) {
669 assert(_succ != current, "invariant");
670 assert(owner_raw() == current, "invariant");
671 assert(_Responsible != current, "invariant");
672 return;
673 }
674
675 if (try_set_owner_from(DEFLATER_MARKER, current) == DEFLATER_MARKER) {
676 // Cancelled the in-progress async deflation by changing owner from
677 // DEFLATER_MARKER to current. As part of the contended enter protocol,
678 // contentions was incremented to a positive value before EnterI()
679 // was called and that prevents the deflater thread from winning the
680 // last part of the 2-part async deflation protocol. After EnterI()
681 // returns to enter(), contentions is decremented because the caller
682 // now owns the monitor. We bump contentions an extra time here to
683 // prevent the deflater thread from winning the last part of the
684 // 2-part async deflation protocol after the regular decrement
685 // occurs in enter(). The deflater thread will decrement contentions
686 // after it recognizes that the async deflation was cancelled.
687 add_to_contentions(1);
688 assert(_succ != current, "invariant");
689 assert(_Responsible != current, "invariant");
690 return;
691 }
692
693 assert(InitDone, "Unexpectedly not initialized");
694
695 // We try one round of spinning *before* enqueueing current.
696 //
697 // If the _owner is ready but OFFPROC we could use a YieldTo()
698 // operation to donate the remainder of this thread's quantum
699 // to the owner. This has subtle but beneficial affinity
700 // effects.
701
702 if (TrySpin(current) > 0) {
703 assert(owner_raw() == current, "invariant");
704 assert(_succ != current, "invariant");
705 assert(_Responsible != current, "invariant");
706 return;
707 }
708
709 // The Spin failed -- Enqueue and park the thread ...
710 assert(_succ != current, "invariant");
711 assert(owner_raw() != current, "invariant");
712 assert(_Responsible != current, "invariant");
713
714 // Enqueue "current" on ObjectMonitor's _cxq.
715 //
716 // Node acts as a proxy for current.
717 // As an aside, if were to ever rewrite the synchronization code mostly
718 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
719 // Java objects. This would avoid awkward lifecycle and liveness issues,
720 // as well as eliminate a subset of ABA issues.
721 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
722
723 ObjectWaiter node(current);
724 current->_ParkEvent->reset();
725 node._prev = (ObjectWaiter*) 0xBAD;
726 node.TState = ObjectWaiter::TS_CXQ;
727
728 // Push "current" onto the front of the _cxq.
729 // Once on cxq/EntryList, current stays on-queue until it acquires the lock.
730 // Note that spinning tends to reduce the rate at which threads
731 // enqueue and dequeue on EntryList|cxq.
732 ObjectWaiter* nxt;
733 for (;;) {
734 node._next = nxt = _cxq;
735 if (Atomic::cmpxchg(&_cxq, nxt, &node) == nxt) break;
736
737 // Interference - the CAS failed because _cxq changed. Just retry.
738 // As an optional optimization we retry the lock.
739 if (TryLock (current) > 0) {
740 assert(_succ != current, "invariant");
741 assert(owner_raw() == current, "invariant");
742 assert(_Responsible != current, "invariant");
743 return;
744 }
745 }
746
747 // Check for cxq|EntryList edge transition to non-null. This indicates
748 // the onset of contention. While contention persists exiting threads
749 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
750 // operations revert to the faster 1-0 mode. This enter operation may interleave
751 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
752 // arrange for one of the contending thread to use a timed park() operations
753 // to detect and recover from the race. (Stranding is form of progress failure
754 // where the monitor is unlocked but all the contending threads remain parked).
755 // That is, at least one of the contended threads will periodically poll _owner.
756 // One of the contending threads will become the designated "Responsible" thread.
757 // The Responsible thread uses a timed park instead of a normal indefinite park
758 // operation -- it periodically wakes and checks for and recovers from potential
759 // strandings admitted by 1-0 exit operations. We need at most one Responsible
760 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
761 // be responsible for a monitor.
762 //
763 // Currently, one of the contended threads takes on the added role of "Responsible".
764 // A viable alternative would be to use a dedicated "stranding checker" thread
765 // that periodically iterated over all the threads (or active monitors) and unparked
766 // successors where there was risk of stranding. This would help eliminate the
767 // timer scalability issues we see on some platforms as we'd only have one thread
768 // -- the checker -- parked on a timer.
769
770 if (nxt == nullptr && _EntryList == nullptr) {
771 // Try to assume the role of responsible thread for the monitor.
772 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=current }
773 Atomic::replace_if_null(&_Responsible, current);
774 }
775
776 // The lock might have been released while this thread was occupied queueing
777 // itself onto _cxq. To close the race and avoid "stranding" and
778 // progress-liveness failure we must resample-retry _owner before parking.
779 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
780 // In this case the ST-MEMBAR is accomplished with CAS().
781 //
782 // TODO: Defer all thread state transitions until park-time.
783 // Since state transitions are heavy and inefficient we'd like
784 // to defer the state transitions until absolutely necessary,
785 // and in doing so avoid some transitions ...
786
787 int nWakeups = 0;
788 int recheckInterval = 1;
789
790 for (;;) {
791
792 if (TryLock(current) > 0) break;
793 assert(owner_raw() != current, "invariant");
794
795 // park self
796 if (_Responsible == current) {
797 current->_ParkEvent->park((jlong) recheckInterval);
798 // Increase the recheckInterval, but clamp the value.
799 recheckInterval *= 8;
800 if (recheckInterval > MAX_RECHECK_INTERVAL) {
801 recheckInterval = MAX_RECHECK_INTERVAL;
802 }
803 } else {
804 current->_ParkEvent->park();
805 }
806
807 if (TryLock(current) > 0) break;
808
809 if (try_set_owner_from(DEFLATER_MARKER, current) == DEFLATER_MARKER) {
810 // Cancelled the in-progress async deflation by changing owner from
811 // DEFLATER_MARKER to current. As part of the contended enter protocol,
812 // contentions was incremented to a positive value before EnterI()
813 // was called and that prevents the deflater thread from winning the
814 // last part of the 2-part async deflation protocol. After EnterI()
815 // returns to enter(), contentions is decremented because the caller
816 // now owns the monitor. We bump contentions an extra time here to
817 // prevent the deflater thread from winning the last part of the
818 // 2-part async deflation protocol after the regular decrement
819 // occurs in enter(). The deflater thread will decrement contentions
820 // after it recognizes that the async deflation was cancelled.
821 add_to_contentions(1);
822 break;
823 }
824
825 // The lock is still contested.
826 // Keep a tally of the # of futile wakeups.
827 // Note that the counter is not protected by a lock or updated by atomics.
828 // That is by design - we trade "lossy" counters which are exposed to
829 // races during updates for a lower probe effect.
830
831 // This PerfData object can be used in parallel with a safepoint.
832 // See the work around in PerfDataManager::destroy().
833 OM_PERFDATA_OP(FutileWakeups, inc());
834 ++nWakeups;
835
836 // Assuming this is not a spurious wakeup we'll normally find _succ == current.
837 // We can defer clearing _succ until after the spin completes
838 // TrySpin() must tolerate being called with _succ == current.
839 // Try yet another round of adaptive spinning.
840 if (TrySpin(current) > 0) break;
841
842 // We can find that we were unpark()ed and redesignated _succ while
843 // we were spinning. That's harmless. If we iterate and call park(),
844 // park() will consume the event and return immediately and we'll
845 // just spin again. This pattern can repeat, leaving _succ to simply
846 // spin on a CPU.
847
848 if (_succ == current) _succ = nullptr;
849
850 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
851 OrderAccess::fence();
852 }
853
854 // Egress :
855 // current has acquired the lock -- Unlink current from the cxq or EntryList.
856 // Normally we'll find current on the EntryList .
857 // From the perspective of the lock owner (this thread), the
858 // EntryList is stable and cxq is prepend-only.
859 // The head of cxq is volatile but the interior is stable.
860 // In addition, current.TState is stable.
861
862 assert(owner_raw() == current, "invariant");
863
864 UnlinkAfterAcquire(current, &node);
865 if (_succ == current) _succ = nullptr;
866
867 assert(_succ != current, "invariant");
868 if (_Responsible == current) {
869 _Responsible = nullptr;
870 OrderAccess::fence(); // Dekker pivot-point
871
872 // We may leave threads on cxq|EntryList without a designated
873 // "Responsible" thread. This is benign. When this thread subsequently
874 // exits the monitor it can "see" such preexisting "old" threads --
875 // threads that arrived on the cxq|EntryList before the fence, above --
876 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
877 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
878 // non-null and elect a new "Responsible" timer thread.
879 //
880 // This thread executes:
881 // ST Responsible=null; MEMBAR (in enter epilogue - here)
882 // LD cxq|EntryList (in subsequent exit)
883 //
884 // Entering threads in the slow/contended path execute:
885 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
886 // The (ST cxq; MEMBAR) is accomplished with CAS().
887 //
888 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
889 // exit operation from floating above the ST Responsible=null.
890 }
891
892 // We've acquired ownership with CAS().
893 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
894 // But since the CAS() this thread may have also stored into _succ,
895 // EntryList, cxq or Responsible. These meta-data updates must be
896 // visible __before this thread subsequently drops the lock.
897 // Consider what could occur if we didn't enforce this constraint --
898 // STs to monitor meta-data and user-data could reorder with (become
899 // visible after) the ST in exit that drops ownership of the lock.
900 // Some other thread could then acquire the lock, but observe inconsistent
901 // or old monitor meta-data and heap data. That violates the JMM.
902 // To that end, the 1-0 exit() operation must have at least STST|LDST
903 // "release" barrier semantics. Specifically, there must be at least a
904 // STST|LDST barrier in exit() before the ST of null into _owner that drops
905 // the lock. The barrier ensures that changes to monitor meta-data and data
906 // protected by the lock will be visible before we release the lock, and
907 // therefore before some other thread (CPU) has a chance to acquire the lock.
908 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
909 //
910 // Critically, any prior STs to _succ or EntryList must be visible before
911 // the ST of null into _owner in the *subsequent* (following) corresponding
912 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
913 // execute a serializing instruction.
914
915 return;
916 }
917
918 // ReenterI() is a specialized inline form of the latter half of the
919 // contended slow-path from EnterI(). We use ReenterI() only for
920 // monitor reentry in wait().
921 //
922 // In the future we should reconcile EnterI() and ReenterI().
923
924 void ObjectMonitor::ReenterI(JavaThread* current, ObjectWaiter* currentNode) {
925 assert(current != nullptr, "invariant");
926 assert(currentNode != nullptr, "invariant");
927 assert(currentNode->_thread == current, "invariant");
928 assert(_waiters > 0, "invariant");
929 assert(object()->mark() == markWord::encode(this), "invariant");
930
931 assert(current->thread_state() != _thread_blocked, "invariant");
932
933 int nWakeups = 0;
934 for (;;) {
935 ObjectWaiter::TStates v = currentNode->TState;
936 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
937 assert(owner_raw() != current, "invariant");
938
939 if (TryLock(current) > 0) break;
940 if (TrySpin(current) > 0) break;
941
942 {
943 OSThreadContendState osts(current->osthread());
944
945 assert(current->thread_state() == _thread_in_vm, "invariant");
946
947 {
948 ClearSuccOnSuspend csos(this);
949 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
950 current->_ParkEvent->park();
951 }
952 }
953
954 // Try again, but just so we distinguish between futile wakeups and
955 // successful wakeups. The following test isn't algorithmically
956 // necessary, but it helps us maintain sensible statistics.
957 if (TryLock(current) > 0) break;
958
959 // The lock is still contested.
960 // Keep a tally of the # of futile wakeups.
961 // Note that the counter is not protected by a lock or updated by atomics.
962 // That is by design - we trade "lossy" counters which are exposed to
963 // races during updates for a lower probe effect.
964 ++nWakeups;
965
966 // Assuming this is not a spurious wakeup we'll normally
967 // find that _succ == current.
968 if (_succ == current) _succ = nullptr;
969
970 // Invariant: after clearing _succ a contending thread
971 // *must* retry _owner before parking.
972 OrderAccess::fence();
973
974 // This PerfData object can be used in parallel with a safepoint.
975 // See the work around in PerfDataManager::destroy().
976 OM_PERFDATA_OP(FutileWakeups, inc());
977 }
978
979 // current has acquired the lock -- Unlink current from the cxq or EntryList .
980 // Normally we'll find current on the EntryList.
981 // Unlinking from the EntryList is constant-time and atomic-free.
982 // From the perspective of the lock owner (this thread), the
983 // EntryList is stable and cxq is prepend-only.
984 // The head of cxq is volatile but the interior is stable.
985 // In addition, current.TState is stable.
986
987 assert(owner_raw() == current, "invariant");
988 assert(object()->mark() == markWord::encode(this), "invariant");
989 UnlinkAfterAcquire(current, currentNode);
990 if (_succ == current) _succ = nullptr;
991 assert(_succ != current, "invariant");
992 currentNode->TState = ObjectWaiter::TS_RUN;
993 OrderAccess::fence(); // see comments at the end of EnterI()
994 }
995
996 // By convention we unlink a contending thread from EntryList|cxq immediately
997 // after the thread acquires the lock in ::enter(). Equally, we could defer
998 // unlinking the thread until ::exit()-time.
999
1000 void ObjectMonitor::UnlinkAfterAcquire(JavaThread* current, ObjectWaiter* currentNode) {
1001 assert(owner_raw() == current, "invariant");
1002 assert(currentNode->_thread == current, "invariant");
1003
1004 if (currentNode->TState == ObjectWaiter::TS_ENTER) {
1005 // Normal case: remove current from the DLL EntryList .
1006 // This is a constant-time operation.
1007 ObjectWaiter* nxt = currentNode->_next;
1008 ObjectWaiter* prv = currentNode->_prev;
1009 if (nxt != nullptr) nxt->_prev = prv;
1010 if (prv != nullptr) prv->_next = nxt;
1011 if (currentNode == _EntryList) _EntryList = nxt;
1012 assert(nxt == nullptr || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
1013 assert(prv == nullptr || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
1014 } else {
1015 assert(currentNode->TState == ObjectWaiter::TS_CXQ, "invariant");
1016 // Inopportune interleaving -- current is still on the cxq.
1017 // This usually means the enqueue of self raced an exiting thread.
1018 // Normally we'll find current near the front of the cxq, so
1019 // dequeueing is typically fast. If needbe we can accelerate
1020 // this with some MCS/CHL-like bidirectional list hints and advisory
1021 // back-links so dequeueing from the interior will normally operate
1022 // in constant-time.
1023 // Dequeue current from either the head (with CAS) or from the interior
1024 // with a linear-time scan and normal non-atomic memory operations.
1025 // CONSIDER: if current is on the cxq then simply drain cxq into EntryList
1026 // and then unlink current from EntryList. We have to drain eventually,
1027 // so it might as well be now.
1028
1029 ObjectWaiter* v = _cxq;
1030 assert(v != nullptr, "invariant");
1031 if (v != currentNode || Atomic::cmpxchg(&_cxq, v, currentNode->_next) != v) {
1032 // The CAS above can fail from interference IFF a "RAT" arrived.
1033 // In that case current must be in the interior and can no longer be
1034 // at the head of cxq.
1035 if (v == currentNode) {
1036 assert(_cxq != v, "invariant");
1037 v = _cxq; // CAS above failed - start scan at head of list
1038 }
1039 ObjectWaiter* p;
1040 ObjectWaiter* q = nullptr;
1041 for (p = v; p != nullptr && p != currentNode; p = p->_next) {
1042 q = p;
1043 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
1044 }
1045 assert(v != currentNode, "invariant");
1046 assert(p == currentNode, "Node not found on cxq");
1047 assert(p != _cxq, "invariant");
1048 assert(q != nullptr, "invariant");
1049 assert(q->_next == p, "invariant");
1050 q->_next = p->_next;
1051 }
1052 }
1053
1054 #ifdef ASSERT
1055 // Diagnostic hygiene ...
1056 currentNode->_prev = (ObjectWaiter*) 0xBAD;
1057 currentNode->_next = (ObjectWaiter*) 0xBAD;
1058 currentNode->TState = ObjectWaiter::TS_RUN;
1059 #endif
1060 }
1061
1062 // -----------------------------------------------------------------------------
1063 // Exit support
1064 //
1065 // exit()
1066 // ~~~~~~
1067 // Note that the collector can't reclaim the objectMonitor or deflate
1068 // the object out from underneath the thread calling ::exit() as the
1069 // thread calling ::exit() never transitions to a stable state.
1070 // This inhibits GC, which in turn inhibits asynchronous (and
1071 // inopportune) reclamation of "this".
1072 //
1073 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
1074 // There's one exception to the claim above, however. EnterI() can call
1075 // exit() to drop a lock if the acquirer has been externally suspended.
1076 // In that case exit() is called with _thread_state == _thread_blocked,
1077 // but the monitor's _contentions field is > 0, which inhibits reclamation.
1078 //
1079 // 1-0 exit
1080 // ~~~~~~~~
1081 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
1082 // the fast-path operators have been optimized so the common ::exit()
1083 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
1084 // The code emitted by fast_unlock() elides the usual MEMBAR. This
1085 // greatly improves latency -- MEMBAR and CAS having considerable local
1086 // latency on modern processors -- but at the cost of "stranding". Absent the
1087 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
1088 // ::enter() path, resulting in the entering thread being stranding
1089 // and a progress-liveness failure. Stranding is extremely rare.
1090 // We use timers (timed park operations) & periodic polling to detect
1091 // and recover from stranding. Potentially stranded threads periodically
1092 // wake up and poll the lock. See the usage of the _Responsible variable.
1093 //
1094 // The CAS() in enter provides for safety and exclusion, while the CAS or
1095 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
1096 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
1097 // We detect and recover from stranding with timers.
1098 //
1099 // If a thread transiently strands it'll park until (a) another
1100 // thread acquires the lock and then drops the lock, at which time the
1101 // exiting thread will notice and unpark the stranded thread, or, (b)
1102 // the timer expires. If the lock is high traffic then the stranding latency
1103 // will be low due to (a). If the lock is low traffic then the odds of
1104 // stranding are lower, although the worst-case stranding latency
1105 // is longer. Critically, we don't want to put excessive load in the
1106 // platform's timer subsystem. We want to minimize both the timer injection
1107 // rate (timers created/sec) as well as the number of timers active at
1108 // any one time. (more precisely, we want to minimize timer-seconds, which is
1109 // the integral of the # of active timers at any instant over time).
1110 // Both impinge on OS scalability. Given that, at most one thread parked on
1111 // a monitor will use a timer.
1112 //
1113 // There is also the risk of a futile wake-up. If we drop the lock
1114 // another thread can reacquire the lock immediately, and we can
1115 // then wake a thread unnecessarily. This is benign, and we've
1116 // structured the code so the windows are short and the frequency
1117 // of such futile wakups is low.
1118
1119 void ObjectMonitor::exit(JavaThread* current, bool not_suspended) {
1120 void* cur = owner_raw();
1121 if (current != cur) {
1122 if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) {
1123 assert(_recursions == 0, "invariant");
1124 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*.
1125 _recursions = 0;
1126 } else {
1127 // Apparent unbalanced locking ...
1128 // Naively we'd like to throw IllegalMonitorStateException.
1129 // As a practical matter we can neither allocate nor throw an
1130 // exception as ::exit() can be called from leaf routines.
1131 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
1132 // Upon deeper reflection, however, in a properly run JVM the only
1133 // way we should encounter this situation is in the presence of
1134 // unbalanced JNI locking. TODO: CheckJNICalls.
1135 // See also: CR4414101
1136 #ifdef ASSERT
1137 LogStreamHandle(Error, monitorinflation) lsh;
1138 lsh.print_cr("ERROR: ObjectMonitor::exit(): thread=" INTPTR_FORMAT
1139 " is exiting an ObjectMonitor it does not own.", p2i(current));
1140 lsh.print_cr("The imbalance is possibly caused by JNI locking.");
1141 print_debug_style_on(&lsh);
1142 assert(false, "Non-balanced monitor enter/exit!");
1143 #endif
1144 return;
1145 }
1146 }
1147
1148 if (_recursions != 0) {
1149 _recursions--; // this is simple recursive enter
1150 return;
1151 }
1152
1153 // Invariant: after setting Responsible=null an thread must execute
1154 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
1155 _Responsible = nullptr;
1156
1157 #if INCLUDE_JFR
1158 // get the owner's thread id for the MonitorEnter event
1159 // if it is enabled and the thread isn't suspended
1160 if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
1161 _previous_owner_tid = JFR_THREAD_ID(current);
1162 }
1163 #endif
1164
1165 for (;;) {
1166 assert(current == owner_raw(), "invariant");
1167
1168 // Drop the lock.
1169 // release semantics: prior loads and stores from within the critical section
1170 // must not float (reorder) past the following store that drops the lock.
1171 // Uses a storeload to separate release_store(owner) from the
1172 // successor check. The try_set_owner() below uses cmpxchg() so
1173 // we get the fence down there.
1174 release_clear_owner(current);
1175 OrderAccess::storeload();
1176
1177 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != nullptr) {
1178 return;
1179 }
1180 // Other threads are blocked trying to acquire the lock.
1181
1182 // Normally the exiting thread is responsible for ensuring succession,
1183 // but if other successors are ready or other entering threads are spinning
1184 // then this thread can simply store null into _owner and exit without
1185 // waking a successor. The existence of spinners or ready successors
1186 // guarantees proper succession (liveness). Responsibility passes to the
1187 // ready or running successors. The exiting thread delegates the duty.
1188 // More precisely, if a successor already exists this thread is absolved
1189 // of the responsibility of waking (unparking) one.
1190 //
1191 // The _succ variable is critical to reducing futile wakeup frequency.
1192 // _succ identifies the "heir presumptive" thread that has been made
1193 // ready (unparked) but that has not yet run. We need only one such
1194 // successor thread to guarantee progress.
1195 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1196 // section 3.3 "Futile Wakeup Throttling" for details.
1197 //
1198 // Note that spinners in Enter() also set _succ non-null.
1199 // In the current implementation spinners opportunistically set
1200 // _succ so that exiting threads might avoid waking a successor.
1201 // Another less appealing alternative would be for the exiting thread
1202 // to drop the lock and then spin briefly to see if a spinner managed
1203 // to acquire the lock. If so, the exiting thread could exit
1204 // immediately without waking a successor, otherwise the exiting
1205 // thread would need to dequeue and wake a successor.
1206 // (Note that we'd need to make the post-drop spin short, but no
1207 // shorter than the worst-case round-trip cache-line migration time.
1208 // The dropped lock needs to become visible to the spinner, and then
1209 // the acquisition of the lock by the spinner must become visible to
1210 // the exiting thread).
1211
1212 // It appears that an heir-presumptive (successor) must be made ready.
1213 // Only the current lock owner can manipulate the EntryList or
1214 // drain _cxq, so we need to reacquire the lock. If we fail
1215 // to reacquire the lock the responsibility for ensuring succession
1216 // falls to the new owner.
1217 //
1218 if (try_set_owner_from(nullptr, current) != nullptr) {
1219 return;
1220 }
1221
1222 guarantee(owner_raw() == current, "invariant");
1223
1224 ObjectWaiter* w = nullptr;
1225
1226 w = _EntryList;
1227 if (w != nullptr) {
1228 // I'd like to write: guarantee (w->_thread != current).
1229 // But in practice an exiting thread may find itself on the EntryList.
1230 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1231 // then calls exit(). Exit release the lock by setting O._owner to null.
1232 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1233 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1234 // release the lock "O". T2 resumes immediately after the ST of null into
1235 // _owner, above. T2 notices that the EntryList is populated, so it
1236 // reacquires the lock and then finds itself on the EntryList.
1237 // Given all that, we have to tolerate the circumstance where "w" is
1238 // associated with current.
1239 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1240 ExitEpilog(current, w);
1241 return;
1242 }
1243
1244 // If we find that both _cxq and EntryList are null then just
1245 // re-run the exit protocol from the top.
1246 w = _cxq;
1247 if (w == nullptr) continue;
1248
1249 // Drain _cxq into EntryList - bulk transfer.
1250 // First, detach _cxq.
1251 // The following loop is tantamount to: w = swap(&cxq, nullptr)
1252 for (;;) {
1253 assert(w != nullptr, "Invariant");
1254 ObjectWaiter* u = Atomic::cmpxchg(&_cxq, w, (ObjectWaiter*)nullptr);
1255 if (u == w) break;
1256 w = u;
1257 }
1258
1259 assert(w != nullptr, "invariant");
1260 assert(_EntryList == nullptr, "invariant");
1261
1262 // Convert the LIFO SLL anchored by _cxq into a DLL.
1263 // The list reorganization step operates in O(LENGTH(w)) time.
1264 // It's critical that this step operate quickly as
1265 // "current" still holds the outer-lock, restricting parallelism
1266 // and effectively lengthening the critical section.
1267 // Invariant: s chases t chases u.
1268 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1269 // we have faster access to the tail.
1270
1271 _EntryList = w;
1272 ObjectWaiter* q = nullptr;
1273 ObjectWaiter* p;
1274 for (p = w; p != nullptr; p = p->_next) {
1275 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1276 p->TState = ObjectWaiter::TS_ENTER;
1277 p->_prev = q;
1278 q = p;
1279 }
1280
1281 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = nullptr
1282 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1283
1284 // See if we can abdicate to a spinner instead of waking a thread.
1285 // A primary goal of the implementation is to reduce the
1286 // context-switch rate.
1287 if (_succ != nullptr) continue;
1288
1289 w = _EntryList;
1290 if (w != nullptr) {
1291 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1292 ExitEpilog(current, w);
1293 return;
1294 }
1295 }
1296 }
1297
1298 void ObjectMonitor::ExitEpilog(JavaThread* current, ObjectWaiter* Wakee) {
1299 assert(owner_raw() == current, "invariant");
1300
1301 // Exit protocol:
1302 // 1. ST _succ = wakee
1303 // 2. membar #loadstore|#storestore;
1304 // 2. ST _owner = nullptr
1305 // 3. unpark(wakee)
1306
1307 _succ = Wakee->_thread;
1308 ParkEvent * Trigger = Wakee->_event;
1309
1310 // Hygiene -- once we've set _owner = nullptr we can't safely dereference Wakee again.
1311 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1312 // out-of-scope (non-extant).
1313 Wakee = nullptr;
1314
1315 // Drop the lock.
1316 // Uses a fence to separate release_store(owner) from the LD in unpark().
1317 release_clear_owner(current);
1318 OrderAccess::fence();
1319
1320 DTRACE_MONITOR_PROBE(contended__exit, this, object(), current);
1321 Trigger->unpark();
1322
1323 // Maintain stats and report events to JVMTI
1324 OM_PERFDATA_OP(Parks, inc());
1325 }
1326
1327 // complete_exit exits a lock returning recursion count
1328 // complete_exit requires an inflated monitor
1329 // The _owner field is not always the Thread addr even with an
1330 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1331 // thread due to contention.
1332 intx ObjectMonitor::complete_exit(JavaThread* current) {
1333 assert(InitDone, "Unexpectedly not initialized");
1334
1335 void* cur = owner_raw();
1336 if (current != cur) {
1337 if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) {
1338 assert(_recursions == 0, "internal state error");
1339 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*.
1340 _recursions = 0;
1341 }
1342 }
1343
1344 guarantee(current == owner_raw(), "complete_exit not owner");
1345 intx save = _recursions; // record the old recursion count
1346 _recursions = 0; // set the recursion level to be 0
1347 exit(current); // exit the monitor
1348 guarantee(owner_raw() != current, "invariant");
1349 return save;
1350 }
1351
1352 // Checks that the current THREAD owns this monitor and causes an
1353 // immediate return if it doesn't. We don't use the CHECK macro
1354 // because we want the IMSE to be the only exception that is thrown
1355 // from the call site when false is returned. Any other pending
1356 // exception is ignored.
1357 #define CHECK_OWNER() \
1358 do { \
1359 if (!check_owner(THREAD)) { \
1360 assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \
1361 return; \
1362 } \
1363 } while (false)
1364
1365 // Returns true if the specified thread owns the ObjectMonitor.
1366 // Otherwise returns false and throws IllegalMonitorStateException
1367 // (IMSE). If there is a pending exception and the specified thread
1368 // is not the owner, that exception will be replaced by the IMSE.
1369 bool ObjectMonitor::check_owner(TRAPS) {
1370 JavaThread* current = THREAD;
1371 void* cur = owner_raw();
1372 assert(cur != anon_owner_ptr(), "no anon owner here");
1373 if (cur == current) {
1374 return true;
1375 }
1376 if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) {
1377 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*.
1378 _recursions = 0;
1379 return true;
1380 }
1381 THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(),
1382 "current thread is not owner", false);
1383 }
1384
1385 static inline bool is_excluded(const Klass* monitor_klass) {
1386 assert(monitor_klass != nullptr, "invariant");
1387 NOT_JFR_RETURN_(false);
1388 JFR_ONLY(return vmSymbols::jfr_chunk_rotation_monitor() == monitor_klass->name());
1389 }
1390
1391 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1392 ObjectMonitor* monitor,
1393 uint64_t notifier_tid,
1394 jlong timeout,
1395 bool timedout) {
1396 assert(event != nullptr, "invariant");
1397 assert(monitor != nullptr, "invariant");
1398 const Klass* monitor_klass = monitor->object()->klass();
1399 if (is_excluded(monitor_klass)) {
1400 return;
1401 }
1402 event->set_monitorClass(monitor_klass);
1403 event->set_timeout(timeout);
1404 // Set an address that is 'unique enough', such that events close in
1405 // time and with the same address are likely (but not guaranteed) to
1406 // belong to the same object.
1407 event->set_address((uintptr_t)monitor);
1408 event->set_notifier(notifier_tid);
1409 event->set_timedOut(timedout);
1410 event->commit();
1411 }
1412
1413 // -----------------------------------------------------------------------------
1414 // Wait/Notify/NotifyAll
1415 //
1416 // Note: a subset of changes to ObjectMonitor::wait()
1417 // will need to be replicated in complete_exit
1418 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1419 JavaThread* current = THREAD;
1420
1421 assert(InitDone, "Unexpectedly not initialized");
1422
1423 CHECK_OWNER(); // Throws IMSE if not owner.
1424
1425 EventJavaMonitorWait event;
1426
1427 // check for a pending interrupt
1428 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1429 // post monitor waited event. Note that this is past-tense, we are done waiting.
1430 if (JvmtiExport::should_post_monitor_waited()) {
1431 // Note: 'false' parameter is passed here because the
1432 // wait was not timed out due to thread interrupt.
1433 JvmtiExport::post_monitor_waited(current, this, false);
1434
1435 // In this short circuit of the monitor wait protocol, the
1436 // current thread never drops ownership of the monitor and
1437 // never gets added to the wait queue so the current thread
1438 // cannot be made the successor. This means that the
1439 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1440 // consume an unpark() meant for the ParkEvent associated with
1441 // this ObjectMonitor.
1442 }
1443 if (event.should_commit()) {
1444 post_monitor_wait_event(&event, this, 0, millis, false);
1445 }
1446 THROW(vmSymbols::java_lang_InterruptedException());
1447 return;
1448 }
1449
1450 assert(current->_Stalled == 0, "invariant");
1451 current->_Stalled = intptr_t(this);
1452 current->set_current_waiting_monitor(this);
1453
1454 // create a node to be put into the queue
1455 // Critically, after we reset() the event but prior to park(), we must check
1456 // for a pending interrupt.
1457 ObjectWaiter node(current);
1458 node.TState = ObjectWaiter::TS_WAIT;
1459 current->_ParkEvent->reset();
1460 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1461
1462 // Enter the waiting queue, which is a circular doubly linked list in this case
1463 // but it could be a priority queue or any data structure.
1464 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1465 // by the owner of the monitor *except* in the case where park()
1466 // returns because of a timeout of interrupt. Contention is exceptionally rare
1467 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1468
1469 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1470 AddWaiter(&node);
1471 Thread::SpinRelease(&_WaitSetLock);
1472
1473 _Responsible = nullptr;
1474
1475 intx save = _recursions; // record the old recursion count
1476 _waiters++; // increment the number of waiters
1477 _recursions = 0; // set the recursion level to be 1
1478 exit(current); // exit the monitor
1479 guarantee(owner_raw() != current, "invariant");
1480
1481 // The thread is on the WaitSet list - now park() it.
1482 // On MP systems it's conceivable that a brief spin before we park
1483 // could be profitable.
1484 //
1485 // TODO-FIXME: change the following logic to a loop of the form
1486 // while (!timeout && !interrupted && _notified == 0) park()
1487
1488 int ret = OS_OK;
1489 int WasNotified = 0;
1490
1491 // Need to check interrupt state whilst still _thread_in_vm
1492 bool interrupted = interruptible && current->is_interrupted(false);
1493
1494 { // State transition wrappers
1495 OSThread* osthread = current->osthread();
1496 OSThreadWaitState osts(osthread, true);
1497
1498 assert(current->thread_state() == _thread_in_vm, "invariant");
1499
1500 {
1501 ClearSuccOnSuspend csos(this);
1502 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
1503 if (interrupted || HAS_PENDING_EXCEPTION) {
1504 // Intentionally empty
1505 } else if (node._notified == 0) {
1506 if (millis <= 0) {
1507 current->_ParkEvent->park();
1508 } else {
1509 ret = current->_ParkEvent->park(millis);
1510 }
1511 }
1512 }
1513
1514 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1515 // from the WaitSet to the EntryList.
1516 // See if we need to remove Node from the WaitSet.
1517 // We use double-checked locking to avoid grabbing _WaitSetLock
1518 // if the thread is not on the wait queue.
1519 //
1520 // Note that we don't need a fence before the fetch of TState.
1521 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1522 // written by the is thread. (perhaps the fetch might even be satisfied
1523 // by a look-aside into the processor's own store buffer, although given
1524 // the length of the code path between the prior ST and this load that's
1525 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1526 // then we'll acquire the lock and then re-fetch a fresh TState value.
1527 // That is, we fail toward safety.
1528
1529 if (node.TState == ObjectWaiter::TS_WAIT) {
1530 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1531 if (node.TState == ObjectWaiter::TS_WAIT) {
1532 DequeueSpecificWaiter(&node); // unlink from WaitSet
1533 assert(node._notified == 0, "invariant");
1534 node.TState = ObjectWaiter::TS_RUN;
1535 }
1536 Thread::SpinRelease(&_WaitSetLock);
1537 }
1538
1539 // The thread is now either on off-list (TS_RUN),
1540 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1541 // The Node's TState variable is stable from the perspective of this thread.
1542 // No other threads will asynchronously modify TState.
1543 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1544 OrderAccess::loadload();
1545 if (_succ == current) _succ = nullptr;
1546 WasNotified = node._notified;
1547
1548 // Reentry phase -- reacquire the monitor.
1549 // re-enter contended monitor after object.wait().
1550 // retain OBJECT_WAIT state until re-enter successfully completes
1551 // Thread state is thread_in_vm and oop access is again safe,
1552 // although the raw address of the object may have changed.
1553 // (Don't cache naked oops over safepoints, of course).
1554
1555 // post monitor waited event. Note that this is past-tense, we are done waiting.
1556 if (JvmtiExport::should_post_monitor_waited()) {
1557 JvmtiExport::post_monitor_waited(current, this, ret == OS_TIMEOUT);
1558
1559 if (node._notified != 0 && _succ == current) {
1560 // In this part of the monitor wait-notify-reenter protocol it
1561 // is possible (and normal) for another thread to do a fastpath
1562 // monitor enter-exit while this thread is still trying to get
1563 // to the reenter portion of the protocol.
1564 //
1565 // The ObjectMonitor was notified and the current thread is
1566 // the successor which also means that an unpark() has already
1567 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1568 // consume the unpark() that was done when the successor was
1569 // set because the same ParkEvent is shared between Java
1570 // monitors and JVM/TI RawMonitors (for now).
1571 //
1572 // We redo the unpark() to ensure forward progress, i.e., we
1573 // don't want all pending threads hanging (parked) with none
1574 // entering the unlocked monitor.
1575 node._event->unpark();
1576 }
1577 }
1578
1579 if (event.should_commit()) {
1580 post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1581 }
1582
1583 OrderAccess::fence();
1584
1585 assert(current->_Stalled != 0, "invariant");
1586 current->_Stalled = 0;
1587
1588 assert(owner_raw() != current, "invariant");
1589 ObjectWaiter::TStates v = node.TState;
1590 if (v == ObjectWaiter::TS_RUN) {
1591 enter(current);
1592 } else {
1593 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1594 ReenterI(current, &node);
1595 node.wait_reenter_end(this);
1596 }
1597
1598 // current has reacquired the lock.
1599 // Lifecycle - the node representing current must not appear on any queues.
1600 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1601 // want residual elements associated with this thread left on any lists.
1602 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1603 assert(owner_raw() == current, "invariant");
1604 assert(_succ != current, "invariant");
1605 } // OSThreadWaitState()
1606
1607 current->set_current_waiting_monitor(nullptr);
1608
1609 guarantee(_recursions == 0, "invariant");
1610 int relock_count = JvmtiDeferredUpdates::get_and_reset_relock_count_after_wait(current);
1611 _recursions = save // restore the old recursion count
1612 + relock_count; // increased by the deferred relock count
1613 current->inc_held_monitor_count(relock_count); // Deopt never entered these counts.
1614 _waiters--; // decrement the number of waiters
1615
1616 // Verify a few postconditions
1617 assert(owner_raw() == current, "invariant");
1618 assert(_succ != current, "invariant");
1619 assert(object()->mark() == markWord::encode(this), "invariant");
1620
1621 // check if the notification happened
1622 if (!WasNotified) {
1623 // no, it could be timeout or Thread.interrupt() or both
1624 // check for interrupt event, otherwise it is timeout
1625 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1626 THROW(vmSymbols::java_lang_InterruptedException());
1627 }
1628 }
1629
1630 // NOTE: Spurious wake up will be consider as timeout.
1631 // Monitor notify has precedence over thread interrupt.
1632 }
1633
1634
1635 // Consider:
1636 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1637 // then instead of transferring a thread from the WaitSet to the EntryList
1638 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1639
1640 void ObjectMonitor::INotify(JavaThread* current) {
1641 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1642 ObjectWaiter* iterator = DequeueWaiter();
1643 if (iterator != nullptr) {
1644 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1645 guarantee(iterator->_notified == 0, "invariant");
1646 // Disposition - what might we do with iterator ?
1647 // a. add it directly to the EntryList - either tail (policy == 1)
1648 // or head (policy == 0).
1649 // b. push it onto the front of the _cxq (policy == 2).
1650 // For now we use (b).
1651
1652 iterator->TState = ObjectWaiter::TS_ENTER;
1653
1654 iterator->_notified = 1;
1655 iterator->_notifier_tid = JFR_THREAD_ID(current);
1656
1657 ObjectWaiter* list = _EntryList;
1658 if (list != nullptr) {
1659 assert(list->_prev == nullptr, "invariant");
1660 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1661 assert(list != iterator, "invariant");
1662 }
1663
1664 // prepend to cxq
1665 if (list == nullptr) {
1666 iterator->_next = iterator->_prev = nullptr;
1667 _EntryList = iterator;
1668 } else {
1669 iterator->TState = ObjectWaiter::TS_CXQ;
1670 for (;;) {
1671 ObjectWaiter* front = _cxq;
1672 iterator->_next = front;
1673 if (Atomic::cmpxchg(&_cxq, front, iterator) == front) {
1674 break;
1675 }
1676 }
1677 }
1678
1679 // _WaitSetLock protects the wait queue, not the EntryList. We could
1680 // move the add-to-EntryList operation, above, outside the critical section
1681 // protected by _WaitSetLock. In practice that's not useful. With the
1682 // exception of wait() timeouts and interrupts the monitor owner
1683 // is the only thread that grabs _WaitSetLock. There's almost no contention
1684 // on _WaitSetLock so it's not profitable to reduce the length of the
1685 // critical section.
1686
1687 iterator->wait_reenter_begin(this);
1688 }
1689 Thread::SpinRelease(&_WaitSetLock);
1690 }
1691
1692 // Consider: a not-uncommon synchronization bug is to use notify() when
1693 // notifyAll() is more appropriate, potentially resulting in stranded
1694 // threads; this is one example of a lost wakeup. A useful diagnostic
1695 // option is to force all notify() operations to behave as notifyAll().
1696 //
1697 // Note: We can also detect many such problems with a "minimum wait".
1698 // When the "minimum wait" is set to a small non-zero timeout value
1699 // and the program does not hang whereas it did absent "minimum wait",
1700 // that suggests a lost wakeup bug.
1701
1702 void ObjectMonitor::notify(TRAPS) {
1703 JavaThread* current = THREAD;
1704 CHECK_OWNER(); // Throws IMSE if not owner.
1705 if (_WaitSet == nullptr) {
1706 return;
1707 }
1708 DTRACE_MONITOR_PROBE(notify, this, object(), current);
1709 INotify(current);
1710 OM_PERFDATA_OP(Notifications, inc(1));
1711 }
1712
1713
1714 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1715 // from the waitset to the EntryList. This could be done more efficiently with a
1716 // single bulk transfer but in practice it's not time-critical. Beware too,
1717 // that in prepend-mode we invert the order of the waiters. Let's say that the
1718 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1719 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1720
1721 void ObjectMonitor::notifyAll(TRAPS) {
1722 JavaThread* current = THREAD;
1723 CHECK_OWNER(); // Throws IMSE if not owner.
1724 if (_WaitSet == nullptr) {
1725 return;
1726 }
1727
1728 DTRACE_MONITOR_PROBE(notifyAll, this, object(), current);
1729 int tally = 0;
1730 while (_WaitSet != nullptr) {
1731 tally++;
1732 INotify(current);
1733 }
1734
1735 OM_PERFDATA_OP(Notifications, inc(tally));
1736 }
1737
1738 // -----------------------------------------------------------------------------
1739 // Adaptive Spinning Support
1740 //
1741 // Adaptive spin-then-block - rational spinning
1742 //
1743 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1744 // algorithm. On high order SMP systems it would be better to start with
1745 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1746 // a contending thread could enqueue itself on the cxq and then spin locally
1747 // on a thread-specific variable such as its ParkEvent._Event flag.
1748 // That's left as an exercise for the reader. Note that global spinning is
1749 // not problematic on Niagara, as the L2 cache serves the interconnect and
1750 // has both low latency and massive bandwidth.
1751 //
1752 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1753 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1754 // (duration) or we can fix the count at approximately the duration of
1755 // a context switch and vary the frequency. Of course we could also
1756 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1757 // For a description of 'Adaptive spin-then-block mutual exclusion in
1758 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1759 //
1760 // This implementation varies the duration "D", where D varies with
1761 // the success rate of recent spin attempts. (D is capped at approximately
1762 // length of a round-trip context switch). The success rate for recent
1763 // spin attempts is a good predictor of the success rate of future spin
1764 // attempts. The mechanism adapts automatically to varying critical
1765 // section length (lock modality), system load and degree of parallelism.
1766 // D is maintained per-monitor in _SpinDuration and is initialized
1767 // optimistically. Spin frequency is fixed at 100%.
1768 //
1769 // Note that _SpinDuration is volatile, but we update it without locks
1770 // or atomics. The code is designed so that _SpinDuration stays within
1771 // a reasonable range even in the presence of races. The arithmetic
1772 // operations on _SpinDuration are closed over the domain of legal values,
1773 // so at worst a race will install and older but still legal value.
1774 // At the very worst this introduces some apparent non-determinism.
1775 // We might spin when we shouldn't or vice-versa, but since the spin
1776 // count are relatively short, even in the worst case, the effect is harmless.
1777 //
1778 // Care must be taken that a low "D" value does not become an
1779 // an absorbing state. Transient spinning failures -- when spinning
1780 // is overall profitable -- should not cause the system to converge
1781 // on low "D" values. We want spinning to be stable and predictable
1782 // and fairly responsive to change and at the same time we don't want
1783 // it to oscillate, become metastable, be "too" non-deterministic,
1784 // or converge on or enter undesirable stable absorbing states.
1785 //
1786 // We implement a feedback-based control system -- using past behavior
1787 // to predict future behavior. We face two issues: (a) if the
1788 // input signal is random then the spin predictor won't provide optimal
1789 // results, and (b) if the signal frequency is too high then the control
1790 // system, which has some natural response lag, will "chase" the signal.
1791 // (b) can arise from multimodal lock hold times. Transient preemption
1792 // can also result in apparent bimodal lock hold times.
1793 // Although sub-optimal, neither condition is particularly harmful, as
1794 // in the worst-case we'll spin when we shouldn't or vice-versa.
1795 // The maximum spin duration is rather short so the failure modes aren't bad.
1796 // To be conservative, I've tuned the gain in system to bias toward
1797 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1798 // "rings" or oscillates between spinning and not spinning. This happens
1799 // when spinning is just on the cusp of profitability, however, so the
1800 // situation is not dire. The state is benign -- there's no need to add
1801 // hysteresis control to damp the transition rate between spinning and
1802 // not spinning.
1803
1804 // Spinning: Fixed frequency (100%), vary duration
1805 int ObjectMonitor::TrySpin(JavaThread* current) {
1806 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1807 int ctr = Knob_FixedSpin;
1808 if (ctr != 0) {
1809 while (--ctr >= 0) {
1810 if (TryLock(current) > 0) return 1;
1811 SpinPause();
1812 }
1813 return 0;
1814 }
1815
1816 for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1817 if (TryLock(current) > 0) {
1818 // Increase _SpinDuration ...
1819 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1820 // Raising _SpurDuration to the poverty line is key.
1821 int x = _SpinDuration;
1822 if (x < Knob_SpinLimit) {
1823 if (x < Knob_Poverty) x = Knob_Poverty;
1824 _SpinDuration = x + Knob_BonusB;
1825 }
1826 return 1;
1827 }
1828 SpinPause();
1829 }
1830
1831 // Admission control - verify preconditions for spinning
1832 //
1833 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1834 // becoming an absorbing state. Put another way, we spin briefly to
1835 // sample, just in case the system load, parallelism, contention, or lock
1836 // modality changed.
1837 //
1838 // Consider the following alternative:
1839 // Periodically set _SpinDuration = _SpinLimit and try a long/full
1840 // spin attempt. "Periodically" might mean after a tally of
1841 // the # of failed spin attempts (or iterations) reaches some threshold.
1842 // This takes us into the realm of 1-out-of-N spinning, where we
1843 // hold the duration constant but vary the frequency.
1844
1845 ctr = _SpinDuration;
1846 if (ctr <= 0) return 0;
1847
1848 if (NotRunnable(current, static_cast<JavaThread*>(owner_raw()))) {
1849 return 0;
1850 }
1851
1852 // We're good to spin ... spin ingress.
1853 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1854 // when preparing to LD...CAS _owner, etc and the CAS is likely
1855 // to succeed.
1856 if (_succ == nullptr) {
1857 _succ = current;
1858 }
1859 Thread* prv = nullptr;
1860
1861 // There are three ways to exit the following loop:
1862 // 1. A successful spin where this thread has acquired the lock.
1863 // 2. Spin failure with prejudice
1864 // 3. Spin failure without prejudice
1865
1866 while (--ctr >= 0) {
1867
1868 // Periodic polling -- Check for pending GC
1869 // Threads may spin while they're unsafe.
1870 // We don't want spinning threads to delay the JVM from reaching
1871 // a stop-the-world safepoint or to steal cycles from GC.
1872 // If we detect a pending safepoint we abort in order that
1873 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1874 // this thread, if safe, doesn't steal cycles from GC.
1875 // This is in keeping with the "no loitering in runtime" rule.
1876 // We periodically check to see if there's a safepoint pending.
1877 if ((ctr & 0xFF) == 0) {
1878 // Can't call SafepointMechanism::should_process() since that
1879 // might update the poll values and we could be in a thread_blocked
1880 // state here which is not allowed so just check the poll.
1881 if (SafepointMechanism::local_poll_armed(current)) {
1882 goto Abort; // abrupt spin egress
1883 }
1884 SpinPause();
1885 }
1886
1887 // Probe _owner with TATAS
1888 // If this thread observes the monitor transition or flicker
1889 // from locked to unlocked to locked, then the odds that this
1890 // thread will acquire the lock in this spin attempt go down
1891 // considerably. The same argument applies if the CAS fails
1892 // or if we observe _owner change from one non-null value to
1893 // another non-null value. In such cases we might abort
1894 // the spin without prejudice or apply a "penalty" to the
1895 // spin count-down variable "ctr", reducing it by 100, say.
1896
1897 JavaThread* ox = static_cast<JavaThread*>(owner_raw());
1898 if (ox == nullptr) {
1899 ox = static_cast<JavaThread*>(try_set_owner_from(nullptr, current));
1900 if (ox == nullptr) {
1901 // The CAS succeeded -- this thread acquired ownership
1902 // Take care of some bookkeeping to exit spin state.
1903 if (_succ == current) {
1904 _succ = nullptr;
1905 }
1906
1907 // Increase _SpinDuration :
1908 // The spin was successful (profitable) so we tend toward
1909 // longer spin attempts in the future.
1910 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
1911 // If we acquired the lock early in the spin cycle it
1912 // makes sense to increase _SpinDuration proportionally.
1913 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1914 int x = _SpinDuration;
1915 if (x < Knob_SpinLimit) {
1916 if (x < Knob_Poverty) x = Knob_Poverty;
1917 _SpinDuration = x + Knob_Bonus;
1918 }
1919 return 1;
1920 }
1921
1922 // The CAS failed ... we can take any of the following actions:
1923 // * penalize: ctr -= CASPenalty
1924 // * exit spin with prejudice -- goto Abort;
1925 // * exit spin without prejudice.
1926 // * Since CAS is high-latency, retry again immediately.
1927 prv = ox;
1928 goto Abort;
1929 }
1930
1931 // Did lock ownership change hands ?
1932 if (ox != prv && prv != nullptr) {
1933 goto Abort;
1934 }
1935 prv = ox;
1936
1937 // Abort the spin if the owner is not executing.
1938 // The owner must be executing in order to drop the lock.
1939 // Spinning while the owner is OFFPROC is idiocy.
1940 // Consider: ctr -= RunnablePenalty ;
1941 if (NotRunnable(current, ox)) {
1942 goto Abort;
1943 }
1944 if (_succ == nullptr) {
1945 _succ = current;
1946 }
1947 }
1948
1949 // Spin failed with prejudice -- reduce _SpinDuration.
1950 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
1951 // AIMD is globally stable.
1952 {
1953 int x = _SpinDuration;
1954 if (x > 0) {
1955 // Consider an AIMD scheme like: x -= (x >> 3) + 100
1956 // This is globally sample and tends to damp the response.
1957 x -= Knob_Penalty;
1958 if (x < 0) x = 0;
1959 _SpinDuration = x;
1960 }
1961 }
1962
1963 Abort:
1964 if (_succ == current) {
1965 _succ = nullptr;
1966 // Invariant: after setting succ=null a contending thread
1967 // must recheck-retry _owner before parking. This usually happens
1968 // in the normal usage of TrySpin(), but it's safest
1969 // to make TrySpin() as foolproof as possible.
1970 OrderAccess::fence();
1971 if (TryLock(current) > 0) return 1;
1972 }
1973 return 0;
1974 }
1975
1976 // NotRunnable() -- informed spinning
1977 //
1978 // Don't bother spinning if the owner is not eligible to drop the lock.
1979 // Spin only if the owner thread is _thread_in_Java or _thread_in_vm.
1980 // The thread must be runnable in order to drop the lock in timely fashion.
1981 // If the _owner is not runnable then spinning will not likely be
1982 // successful (profitable).
1983 //
1984 // Beware -- the thread referenced by _owner could have died
1985 // so a simply fetch from _owner->_thread_state might trap.
1986 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
1987 // Because of the lifecycle issues, the _thread_state values
1988 // observed by NotRunnable() might be garbage. NotRunnable must
1989 // tolerate this and consider the observed _thread_state value
1990 // as advisory.
1991 //
1992 // Beware too, that _owner is sometimes a BasicLock address and sometimes
1993 // a thread pointer.
1994 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
1995 // with the LSB of _owner. Another option would be to probabilistically probe
1996 // the putative _owner->TypeTag value.
1997 //
1998 // Checking _thread_state isn't perfect. Even if the thread is
1999 // in_java it might be blocked on a page-fault or have been preempted
2000 // and sitting on a ready/dispatch queue.
2001 //
2002 // The return value from NotRunnable() is *advisory* -- the
2003 // result is based on sampling and is not necessarily coherent.
2004 // The caller must tolerate false-negative and false-positive errors.
2005 // Spinning, in general, is probabilistic anyway.
2006
2007
2008 int ObjectMonitor::NotRunnable(JavaThread* current, JavaThread* ox) {
2009 // Check ox->TypeTag == 2BAD.
2010 if (ox == nullptr) return 0;
2011
2012 // Avoid transitive spinning ...
2013 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2014 // Immediately after T1 acquires L it's possible that T2, also
2015 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2016 // This occurs transiently after T1 acquired L but before
2017 // T1 managed to clear T1.Stalled. T2 does not need to abort
2018 // its spin in this circumstance.
2019 intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
2020
2021 if (BlockedOn == 1) return 1;
2022 if (BlockedOn != 0) {
2023 return BlockedOn != intptr_t(this) && owner_raw() == ox;
2024 }
2025
2026 assert(sizeof(ox->_thread_state == sizeof(int)), "invariant");
2027 int jst = SafeFetch32((int *) &ox->_thread_state, -1);;
2028 // consider also: jst != _thread_in_Java -- but that's overspecific.
2029 return jst == _thread_blocked || jst == _thread_in_native;
2030 }
2031
2032
2033 // -----------------------------------------------------------------------------
2034 // WaitSet management ...
2035
2036 ObjectWaiter::ObjectWaiter(JavaThread* current) {
2037 _next = nullptr;
2038 _prev = nullptr;
2039 _notified = 0;
2040 _notifier_tid = 0;
2041 TState = TS_RUN;
2042 _thread = current;
2043 _event = _thread->_ParkEvent;
2044 _active = false;
2045 assert(_event != nullptr, "invariant");
2046 }
2047
2048 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2049 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(_thread, mon);
2050 }
2051
2052 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2053 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(_thread, _active);
2054 }
2055
2056 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2057 assert(node != nullptr, "should not add null node");
2058 assert(node->_prev == nullptr, "node already in list");
2059 assert(node->_next == nullptr, "node already in list");
2060 // put node at end of queue (circular doubly linked list)
2061 if (_WaitSet == nullptr) {
2062 _WaitSet = node;
2063 node->_prev = node;
2064 node->_next = node;
2065 } else {
2066 ObjectWaiter* head = _WaitSet;
2067 ObjectWaiter* tail = head->_prev;
2068 assert(tail->_next == head, "invariant check");
2069 tail->_next = node;
2070 head->_prev = node;
2071 node->_next = head;
2072 node->_prev = tail;
2073 }
2074 }
2075
2076 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2077 // dequeue the very first waiter
2078 ObjectWaiter* waiter = _WaitSet;
2079 if (waiter) {
2080 DequeueSpecificWaiter(waiter);
2081 }
2082 return waiter;
2083 }
2084
2085 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2086 assert(node != nullptr, "should not dequeue nullptr node");
2087 assert(node->_prev != nullptr, "node already removed from list");
2088 assert(node->_next != nullptr, "node already removed from list");
2089 // when the waiter has woken up because of interrupt,
2090 // timeout or other spurious wake-up, dequeue the
2091 // waiter from waiting list
2092 ObjectWaiter* next = node->_next;
2093 if (next == node) {
2094 assert(node->_prev == node, "invariant check");
2095 _WaitSet = nullptr;
2096 } else {
2097 ObjectWaiter* prev = node->_prev;
2098 assert(prev->_next == node, "invariant check");
2099 assert(next->_prev == node, "invariant check");
2100 next->_prev = prev;
2101 prev->_next = next;
2102 if (_WaitSet == node) {
2103 _WaitSet = next;
2104 }
2105 }
2106 node->_next = nullptr;
2107 node->_prev = nullptr;
2108 }
2109
2110 // -----------------------------------------------------------------------------
2111 // PerfData support
2112 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = nullptr;
2113 PerfCounter * ObjectMonitor::_sync_FutileWakeups = nullptr;
2114 PerfCounter * ObjectMonitor::_sync_Parks = nullptr;
2115 PerfCounter * ObjectMonitor::_sync_Notifications = nullptr;
2116 PerfCounter * ObjectMonitor::_sync_Inflations = nullptr;
2117 PerfCounter * ObjectMonitor::_sync_Deflations = nullptr;
2118 PerfLongVariable * ObjectMonitor::_sync_MonExtant = nullptr;
2119
2120 // One-shot global initialization for the sync subsystem.
2121 // We could also defer initialization and initialize on-demand
2122 // the first time we call ObjectSynchronizer::inflate().
2123 // Initialization would be protected - like so many things - by
2124 // the MonitorCache_lock.
2125
2126 void ObjectMonitor::Initialize() {
2127 assert(!InitDone, "invariant");
2128
2129 if (!os::is_MP()) {
2130 Knob_SpinLimit = 0;
2131 Knob_PreSpin = 0;
2132 Knob_FixedSpin = -1;
2133 }
2134
2135 if (UsePerfData) {
2136 EXCEPTION_MARK;
2137 #define NEWPERFCOUNTER(n) \
2138 { \
2139 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \
2140 CHECK); \
2141 }
2142 #define NEWPERFVARIABLE(n) \
2143 { \
2144 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \
2145 CHECK); \
2146 }
2147 NEWPERFCOUNTER(_sync_Inflations);
2148 NEWPERFCOUNTER(_sync_Deflations);
2149 NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2150 NEWPERFCOUNTER(_sync_FutileWakeups);
2151 NEWPERFCOUNTER(_sync_Parks);
2152 NEWPERFCOUNTER(_sync_Notifications);
2153 NEWPERFVARIABLE(_sync_MonExtant);
2154 #undef NEWPERFCOUNTER
2155 #undef NEWPERFVARIABLE
2156 }
2157
2158 _oop_storage = OopStorageSet::create_weak("ObjectSynchronizer Weak", mtSynchronizer);
2159
2160 DEBUG_ONLY(InitDone = true;)
2161 }
2162
2163 void ObjectMonitor::print_on(outputStream* st) const {
2164 // The minimal things to print for markWord printing, more can be added for debugging and logging.
2165 st->print("{contentions=0x%08x,waiters=0x%08x"
2166 ",recursions=" INTX_FORMAT ",owner=" INTPTR_FORMAT "}",
2167 contentions(), waiters(), recursions(),
2168 p2i(owner()));
2169 }
2170 void ObjectMonitor::print() const { print_on(tty); }
2171
2172 #ifdef ASSERT
2173 // Print the ObjectMonitor like a debugger would:
2174 //
2175 // (ObjectMonitor) 0x00007fdfb6012e40 = {
2176 // _header = 0x0000000000000001
2177 // _object = 0x000000070ff45fd0
2178 // _pad_buf0 = {
2179 // [0] = '\0'
2180 // ...
2181 // [43] = '\0'
2182 // }
2183 // _owner = 0x0000000000000000
2184 // _previous_owner_tid = 0
2185 // _pad_buf1 = {
2186 // [0] = '\0'
2187 // ...
2188 // [47] = '\0'
2189 // }
2190 // _next_om = 0x0000000000000000
2191 // _recursions = 0
2192 // _EntryList = 0x0000000000000000
2193 // _cxq = 0x0000000000000000
2194 // _succ = 0x0000000000000000
2195 // _Responsible = 0x0000000000000000
2196 // _Spinner = 0
2197 // _SpinDuration = 5000
2198 // _contentions = 0
2199 // _WaitSet = 0x0000700009756248
2200 // _waiters = 1
2201 // _WaitSetLock = 0
2202 // }
2203 //
2204 void ObjectMonitor::print_debug_style_on(outputStream* st) const {
2205 st->print_cr("(ObjectMonitor*) " INTPTR_FORMAT " = {", p2i(this));
2206 st->print_cr(" _header = " INTPTR_FORMAT, header().value());
2207 st->print_cr(" _object = " INTPTR_FORMAT, p2i(object_peek()));
2208 st->print_cr(" _pad_buf0 = {");
2209 st->print_cr(" [0] = '\\0'");
2210 st->print_cr(" ...");
2211 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf0) - 1);
2212 st->print_cr(" }");
2213 st->print_cr(" _owner = " INTPTR_FORMAT, p2i(owner_raw()));
2214 st->print_cr(" _previous_owner_tid = " UINT64_FORMAT, _previous_owner_tid);
2215 st->print_cr(" _pad_buf1 = {");
2216 st->print_cr(" [0] = '\\0'");
2217 st->print_cr(" ...");
2218 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf1) - 1);
2219 st->print_cr(" }");
2220 st->print_cr(" _next_om = " INTPTR_FORMAT, p2i(next_om()));
2221 st->print_cr(" _recursions = " INTX_FORMAT, _recursions);
2222 st->print_cr(" _EntryList = " INTPTR_FORMAT, p2i(_EntryList));
2223 st->print_cr(" _cxq = " INTPTR_FORMAT, p2i(_cxq));
2224 st->print_cr(" _succ = " INTPTR_FORMAT, p2i(_succ));
2225 st->print_cr(" _Responsible = " INTPTR_FORMAT, p2i(_Responsible));
2226 st->print_cr(" _Spinner = %d", _Spinner);
2227 st->print_cr(" _SpinDuration = %d", _SpinDuration);
2228 st->print_cr(" _contentions = %d", contentions());
2229 st->print_cr(" _WaitSet = " INTPTR_FORMAT, p2i(_WaitSet));
2230 st->print_cr(" _waiters = %d", _waiters);
2231 st->print_cr(" _WaitSetLock = %d", _WaitSetLock);
2232 st->print_cr("}");
2233 }
2234 #endif