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