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