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