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