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