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