1 /*
2 * Copyright (c) 1998, 2018, 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_ExitRelease = 0;
105 int ObjectMonitor::Knob_InlineNotify = 1;
106 int ObjectMonitor::Knob_Verbose = 0;
107 int ObjectMonitor::Knob_VerifyInUse = 0;
108 int ObjectMonitor::Knob_VerifyMatch = 0;
109 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool -
110
111 static int Knob_ReportSettings = 0;
112 static int Knob_SpinBase = 0; // Floor AKA SpinMin
113 static int Knob_SpinBackOff = 0; // spin-loop backoff
114 static int Knob_CASPenalty = -1; // Penalty for failed CAS
115 static int Knob_OXPenalty = -1; // Penalty for observed _owner change
116 static int Knob_SpinSetSucc = 1; // spinners set the _succ field
117 static int Knob_SpinEarly = 1;
118 static int Knob_SuccEnabled = 1; // futile wake throttling
119 static int Knob_SuccRestrict = 0; // Limit successors + spinners to at-most-one
120 static int Knob_MaxSpinners = -1; // Should be a function of # CPUs
121 static int Knob_Bonus = 100; // spin success bonus
122 static int Knob_BonusB = 100; // spin success bonus
123 static int Knob_Penalty = 200; // spin failure penalty
124 static int Knob_Poverty = 1000;
125 static int Knob_SpinAfterFutile = 1; // Spin after returning from park()
126 static int Knob_FixedSpin = 0;
127 static int Knob_OState = 3; // Spinner checks thread state of _owner
128 static int Knob_UsePause = 1;
129 static int Knob_ExitPolicy = 0;
130 static int Knob_PreSpin = 10; // 20-100 likely better
131 static int Knob_ResetEvent = 0;
132 static int BackOffMask = 0;
133
134 static int Knob_FastHSSEC = 0;
135 static int Knob_MoveNotifyee = 2; // notify() - disposition of notifyee
136 static int Knob_QMode = 0; // EntryList-cxq policy - queue discipline
137 static volatile int InitDone = 0;
138
139 // -----------------------------------------------------------------------------
140 // Theory of operations -- Monitors lists, thread residency, etc:
141 //
142 // * A thread acquires ownership of a monitor by successfully
143 // CAS()ing the _owner field from null to non-null.
144 //
145 // * Invariant: A thread appears on at most one monitor list --
146 // cxq, EntryList or WaitSet -- at any one time.
147 //
148 // * Contending threads "push" themselves onto the cxq with CAS
149 // and then spin/park.
150 //
151 // * After a contending thread eventually acquires the lock it must
152 // dequeue itself from either the EntryList or the cxq.
153 //
154 // * The exiting thread identifies and unparks an "heir presumptive"
155 // tentative successor thread on the EntryList. Critically, the
156 // exiting thread doesn't unlink the successor thread from the EntryList.
157 // After having been unparked, the wakee will recontend for ownership of
158 // the monitor. The successor (wakee) will either acquire the lock or
159 // re-park itself.
160 //
161 // Succession is provided for by a policy of competitive handoff.
162 // The exiting thread does _not_ grant or pass ownership to the
163 // successor thread. (This is also referred to as "handoff" succession").
164 // Instead the exiting thread releases ownership and possibly wakes
165 // a successor, so the successor can (re)compete for ownership of the lock.
166 // If the EntryList is empty but the cxq is populated the exiting
167 // thread will drain the cxq into the EntryList. It does so by
168 // by detaching the cxq (installing null with CAS) and folding
169 // the threads from the cxq into the EntryList. The EntryList is
170 // doubly linked, while the cxq is singly linked because of the
171 // CAS-based "push" used to enqueue recently arrived threads (RATs).
172 //
173 // * Concurrency invariants:
174 //
175 // -- only the monitor owner may access or mutate the EntryList.
176 // The mutex property of the monitor itself protects the EntryList
177 // from concurrent interference.
178 // -- Only the monitor owner may detach the cxq.
179 //
180 // * The monitor entry list operations avoid locks, but strictly speaking
181 // they're not lock-free. Enter is lock-free, exit is not.
182 // For a description of 'Methods and apparatus providing non-blocking access
183 // to a resource,' see U.S. Pat. No. 7844973.
184 //
185 // * The cxq can have multiple concurrent "pushers" but only one concurrent
186 // detaching thread. This mechanism is immune from the ABA corruption.
187 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
188 //
189 // * Taken together, the cxq and the EntryList constitute or form a
190 // single logical queue of threads stalled trying to acquire the lock.
191 // We use two distinct lists to improve the odds of a constant-time
192 // dequeue operation after acquisition (in the ::enter() epilogue) and
193 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
194 // A key desideratum is to minimize queue & monitor metadata manipulation
195 // that occurs while holding the monitor lock -- that is, we want to
196 // minimize monitor lock holds times. Note that even a small amount of
197 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
198 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
199 // locks and monitor metadata.
200 //
201 // Cxq points to the set of Recently Arrived Threads attempting entry.
202 // Because we push threads onto _cxq with CAS, the RATs must take the form of
203 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
204 // the unlocking thread notices that EntryList is null but _cxq is != null.
205 //
206 // The EntryList is ordered by the prevailing queue discipline and
207 // can be organized in any convenient fashion, such as a doubly-linked list or
208 // a circular doubly-linked list. Critically, we want insert and delete operations
209 // to operate in constant-time. If we need a priority queue then something akin
210 // to Solaris' sleepq would work nicely. Viz.,
211 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
212 // Queue discipline is enforced at ::exit() time, when the unlocking thread
213 // drains the cxq into the EntryList, and orders or reorders the threads on the
214 // EntryList accordingly.
215 //
216 // Barring "lock barging", this mechanism provides fair cyclic ordering,
217 // somewhat similar to an elevator-scan.
218 //
219 // * The monitor synchronization subsystem avoids the use of native
220 // synchronization primitives except for the narrow platform-specific
221 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
222 // the semantics of park-unpark. Put another way, this monitor implementation
223 // depends only on atomic operations and park-unpark. The monitor subsystem
224 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
225 // underlying OS manages the READY<->RUN transitions.
226 //
227 // * Waiting threads reside on the WaitSet list -- wait() puts
228 // the caller onto the WaitSet.
229 //
230 // * notify() or notifyAll() simply transfers threads from the WaitSet to
231 // either the EntryList or cxq. Subsequent exit() operations will
232 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
233 // it's likely the notifyee would simply impale itself on the lock held
234 // by the notifier.
235 //
236 // * An interesting alternative is to encode cxq as (List,LockByte) where
237 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
238 // variable, like _recursions, in the scheme. The threads or Events that form
239 // the list would have to be aligned in 256-byte addresses. A thread would
240 // try to acquire the lock or enqueue itself with CAS, but exiting threads
241 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
242 // Note that is is *not* word-tearing, but it does presume that full-word
243 // CAS operations are coherent with intermix with STB operations. That's true
244 // on most common processors.
245 //
246 // * See also http://blogs.sun.com/dave
247
248
249 void* ObjectMonitor::operator new (size_t size) throw() {
250 return AllocateHeap(size, mtInternal);
251 }
252 void* ObjectMonitor::operator new[] (size_t size) throw() {
253 return operator new (size);
254 }
255 void ObjectMonitor::operator delete(void* p) {
256 FreeHeap(p);
257 }
258 void ObjectMonitor::operator delete[] (void *p) {
259 operator delete(p);
260 }
261
262 // -----------------------------------------------------------------------------
263 // Enter support
264
265 void ObjectMonitor::enter(TRAPS) {
266 // The following code is ordered to check the most common cases first
267 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
268 Thread * const Self = THREAD;
269
270 void * cur = Atomic::cmpxchg(Self, &_owner, (void*)NULL);
271 if (cur == NULL) {
272 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
273 assert(_recursions == 0, "invariant");
274 assert(_owner == Self, "invariant");
275 return;
276 }
277
278 if (cur == Self) {
279 // TODO-FIXME: check for integer overflow! BUGID 6557169.
280 _recursions++;
281 return;
282 }
283
284 if (Self->is_lock_owned ((address)cur)) {
285 assert(_recursions == 0, "internal state error");
286 _recursions = 1;
287 // Commute owner from a thread-specific on-stack BasicLockObject address to
288 // a full-fledged "Thread *".
289 _owner = Self;
290 return;
291 }
292
293 // We've encountered genuine contention.
294 assert(Self->_Stalled == 0, "invariant");
295 Self->_Stalled = intptr_t(this);
296
297 // Try one round of spinning *before* enqueueing Self
298 // and before going through the awkward and expensive state
299 // transitions. The following spin is strictly optional ...
300 // Note that if we acquire the monitor from an initial spin
301 // we forgo posting JVMTI events and firing DTRACE probes.
302 if (Knob_SpinEarly && TrySpin (Self) > 0) {
303 assert(_owner == Self, "invariant");
304 assert(_recursions == 0, "invariant");
305 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
306 Self->_Stalled = 0;
307 return;
308 }
309
310 assert(_owner != Self, "invariant");
311 assert(_succ != Self, "invariant");
312 assert(Self->is_Java_thread(), "invariant");
313 JavaThread * jt = (JavaThread *) Self;
314 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
315 assert(jt->thread_state() != _thread_blocked, "invariant");
316 assert(this->object() != NULL, "invariant");
317 assert(_count >= 0, "invariant");
318
319 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
320 // Ensure the object-monitor relationship remains stable while there's contention.
321 Atomic::inc(&_count);
322
323 JFR_ONLY(JfrConditionalFlushWithStacktrace<EventJavaMonitorEnter> flush(jt);)
324 EventJavaMonitorEnter event;
325 if (event.should_commit()) {
326 event.set_monitorClass(((oop)this->object())->klass());
327 event.set_address((uintptr_t)(this->object_addr()));
328 }
329
330 { // Change java thread status to indicate blocked on monitor enter.
331 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
332
333 Self->set_current_pending_monitor(this);
334
335 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
336 if (JvmtiExport::should_post_monitor_contended_enter()) {
337 JvmtiExport::post_monitor_contended_enter(jt, this);
338
339 // The current thread does not yet own the monitor and does not
340 // yet appear on any queues that would get it made the successor.
341 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
342 // handler cannot accidentally consume an unpark() meant for the
343 // ParkEvent associated with this ObjectMonitor.
344 }
345
346 OSThreadContendState osts(Self->osthread());
347 ThreadBlockInVM tbivm(jt);
348
349 // TODO-FIXME: change the following for(;;) loop to straight-line code.
350 for (;;) {
351 jt->set_suspend_equivalent();
352 // cleared by handle_special_suspend_equivalent_condition()
353 // or java_suspend_self()
354
355 EnterI(THREAD);
356
357 if (!ExitSuspendEquivalent(jt)) break;
358
359 // We have acquired the contended monitor, but while we were
360 // waiting another thread suspended us. We don't want to enter
361 // the monitor while suspended because that would surprise the
362 // thread that suspended us.
363 //
364 _recursions = 0;
365 _succ = NULL;
366 exit(false, Self);
367
368 jt->java_suspend_self();
369 }
370 Self->set_current_pending_monitor(NULL);
371
372 // We cleared the pending monitor info since we've just gotten past
373 // the enter-check-for-suspend dance and we now own the monitor free
374 // and clear, i.e., it is no longer pending. The ThreadBlockInVM
375 // destructor can go to a safepoint at the end of this block. If we
376 // do a thread dump during that safepoint, then this thread will show
377 // as having "-locked" the monitor, but the OS and java.lang.Thread
378 // states will still report that the thread is blocked trying to
379 // acquire it.
380 }
381
382 Atomic::dec(&_count);
383 assert(_count >= 0, "invariant");
384 Self->_Stalled = 0;
385
386 // Must either set _recursions = 0 or ASSERT _recursions == 0.
387 assert(_recursions == 0, "invariant");
388 assert(_owner == Self, "invariant");
389 assert(_succ != Self, "invariant");
390 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
391
392 // The thread -- now the owner -- is back in vm mode.
393 // Report the glorious news via TI,DTrace and jvmstat.
394 // The probe effect is non-trivial. All the reportage occurs
395 // while we hold the monitor, increasing the length of the critical
396 // section. Amdahl's parallel speedup law comes vividly into play.
397 //
398 // Another option might be to aggregate the events (thread local or
399 // per-monitor aggregation) and defer reporting until a more opportune
400 // time -- such as next time some thread encounters contention but has
401 // yet to acquire the lock. While spinning that thread could
402 // spinning we could increment JVMStat counters, etc.
403
404 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
405 if (JvmtiExport::should_post_monitor_contended_entered()) {
406 JvmtiExport::post_monitor_contended_entered(jt, this);
407
408 // The current thread already owns the monitor and is not going to
409 // call park() for the remainder of the monitor enter protocol. So
410 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
411 // event handler consumed an unpark() issued by the thread that
412 // just exited the monitor.
413 }
414 if (event.should_commit()) {
415 event.set_previousOwner((uintptr_t)_previous_owner_tid);
416 event.commit();
417 }
418 OM_PERFDATA_OP(ContendedLockAttempts, inc());
419 }
420
421 // Caveat: TryLock() is not necessarily serializing if it returns failure.
422 // Callers must compensate as needed.
423
424 int ObjectMonitor::TryLock(Thread * Self) {
425 void * own = _owner;
426 if (own != NULL) return 0;
427 if (Atomic::replace_if_null(Self, &_owner)) {
428 // Either guarantee _recursions == 0 or set _recursions = 0.
429 assert(_recursions == 0, "invariant");
430 assert(_owner == Self, "invariant");
431 return 1;
432 }
433 // The lock had been free momentarily, but we lost the race to the lock.
434 // Interference -- the CAS failed.
435 // We can either return -1 or retry.
436 // Retry doesn't make as much sense because the lock was just acquired.
437 return -1;
438 }
439
440 #define MAX_RECHECK_INTERVAL 1000
441
442 void ObjectMonitor::EnterI(TRAPS) {
443 Thread * const Self = THREAD;
444 assert(Self->is_Java_thread(), "invariant");
445 assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant");
446
447 // Try the lock - TATAS
448 if (TryLock (Self) > 0) {
449 assert(_succ != Self, "invariant");
450 assert(_owner == Self, "invariant");
451 assert(_Responsible != Self, "invariant");
452 return;
453 }
454
455 DeferredInitialize();
456
457 // We try one round of spinning *before* enqueueing Self.
458 //
459 // If the _owner is ready but OFFPROC we could use a YieldTo()
460 // operation to donate the remainder of this thread's quantum
461 // to the owner. This has subtle but beneficial affinity
462 // effects.
463
464 if (TrySpin (Self) > 0) {
465 assert(_owner == Self, "invariant");
466 assert(_succ != Self, "invariant");
467 assert(_Responsible != Self, "invariant");
468 return;
469 }
470
471 // The Spin failed -- Enqueue and park the thread ...
472 assert(_succ != Self, "invariant");
473 assert(_owner != Self, "invariant");
474 assert(_Responsible != Self, "invariant");
475
476 // Enqueue "Self" on ObjectMonitor's _cxq.
477 //
478 // Node acts as a proxy for Self.
479 // As an aside, if were to ever rewrite the synchronization code mostly
480 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
481 // Java objects. This would avoid awkward lifecycle and liveness issues,
482 // as well as eliminate a subset of ABA issues.
483 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
484
485 ObjectWaiter node(Self);
486 Self->_ParkEvent->reset();
487 node._prev = (ObjectWaiter *) 0xBAD;
488 node.TState = ObjectWaiter::TS_CXQ;
489
490 // Push "Self" onto the front of the _cxq.
491 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
492 // Note that spinning tends to reduce the rate at which threads
493 // enqueue and dequeue on EntryList|cxq.
494 ObjectWaiter * nxt;
495 for (;;) {
496 node._next = nxt = _cxq;
497 if (Atomic::cmpxchg(&node, &_cxq, nxt) == nxt) break;
498
499 // Interference - the CAS failed because _cxq changed. Just retry.
500 // As an optional optimization we retry the lock.
501 if (TryLock (Self) > 0) {
502 assert(_succ != Self, "invariant");
503 assert(_owner == Self, "invariant");
504 assert(_Responsible != Self, "invariant");
505 return;
506 }
507 }
508
509 // Check for cxq|EntryList edge transition to non-null. This indicates
510 // the onset of contention. While contention persists exiting threads
511 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
512 // operations revert to the faster 1-0 mode. This enter operation may interleave
513 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
514 // arrange for one of the contending thread to use a timed park() operations
515 // to detect and recover from the race. (Stranding is form of progress failure
516 // where the monitor is unlocked but all the contending threads remain parked).
517 // That is, at least one of the contended threads will periodically poll _owner.
518 // One of the contending threads will become the designated "Responsible" thread.
519 // The Responsible thread uses a timed park instead of a normal indefinite park
520 // operation -- it periodically wakes and checks for and recovers from potential
521 // strandings admitted by 1-0 exit operations. We need at most one Responsible
522 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
523 // be responsible for a monitor.
524 //
525 // Currently, one of the contended threads takes on the added role of "Responsible".
526 // A viable alternative would be to use a dedicated "stranding checker" thread
527 // that periodically iterated over all the threads (or active monitors) and unparked
528 // successors where there was risk of stranding. This would help eliminate the
529 // timer scalability issues we see on some platforms as we'd only have one thread
530 // -- the checker -- parked on a timer.
531
532 if (nxt == NULL && _EntryList == NULL) {
533 // Try to assume the role of responsible thread for the monitor.
534 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
535 Atomic::replace_if_null(Self, &_Responsible);
536 }
537
538 // The lock might have been released while this thread was occupied queueing
539 // itself onto _cxq. To close the race and avoid "stranding" and
540 // progress-liveness failure we must resample-retry _owner before parking.
541 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
542 // In this case the ST-MEMBAR is accomplished with CAS().
543 //
544 // TODO: Defer all thread state transitions until park-time.
545 // Since state transitions are heavy and inefficient we'd like
546 // to defer the state transitions until absolutely necessary,
547 // and in doing so avoid some transitions ...
548
549 int nWakeups = 0;
550 int recheckInterval = 1;
551
552 for (;;) {
553
554 if (TryLock(Self) > 0) break;
555 assert(_owner != Self, "invariant");
556
557 // park self
558 if (_Responsible == Self) {
559 Self->_ParkEvent->park((jlong) recheckInterval);
560 // Increase the recheckInterval, but clamp the value.
561 recheckInterval *= 8;
562 if (recheckInterval > MAX_RECHECK_INTERVAL) {
563 recheckInterval = MAX_RECHECK_INTERVAL;
564 }
565 } else {
566 Self->_ParkEvent->park();
567 }
568
569 if (TryLock(Self) > 0) break;
570
571 // The lock is still contested.
572 // Keep a tally of the # of futile wakeups.
573 // Note that the counter is not protected by a lock or updated by atomics.
574 // That is by design - we trade "lossy" counters which are exposed to
575 // races during updates for a lower probe effect.
576
577 // This PerfData object can be used in parallel with a safepoint.
578 // See the work around in PerfDataManager::destroy().
579 OM_PERFDATA_OP(FutileWakeups, inc());
580 ++nWakeups;
581
582 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
583 // We can defer clearing _succ until after the spin completes
584 // TrySpin() must tolerate being called with _succ == Self.
585 // Try yet another round of adaptive spinning.
586 if ((Knob_SpinAfterFutile & 1) && TrySpin(Self) > 0) break;
587
588 // We can find that we were unpark()ed and redesignated _succ while
589 // we were spinning. That's harmless. If we iterate and call park(),
590 // park() will consume the event and return immediately and we'll
591 // just spin again. This pattern can repeat, leaving _succ to simply
592 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks().
593 // Alternately, we can sample fired() here, and if set, forgo spinning
594 // in the next iteration.
595
596 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
597 Self->_ParkEvent->reset();
598 OrderAccess::fence();
599 }
600 if (_succ == Self) _succ = NULL;
601
602 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
603 OrderAccess::fence();
604 }
605
606 // Egress :
607 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
608 // Normally we'll find Self on the EntryList .
609 // From the perspective of the lock owner (this thread), the
610 // EntryList is stable and cxq is prepend-only.
611 // The head of cxq is volatile but the interior is stable.
612 // In addition, Self.TState is stable.
613
614 assert(_owner == Self, "invariant");
615 assert(object() != NULL, "invariant");
616 // I'd like to write:
617 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
618 // but as we're at a safepoint that's not safe.
619
620 UnlinkAfterAcquire(Self, &node);
621 if (_succ == Self) _succ = NULL;
622
623 assert(_succ != Self, "invariant");
624 if (_Responsible == Self) {
625 _Responsible = NULL;
626 OrderAccess::fence(); // Dekker pivot-point
627
628 // We may leave threads on cxq|EntryList without a designated
629 // "Responsible" thread. This is benign. When this thread subsequently
630 // exits the monitor it can "see" such preexisting "old" threads --
631 // threads that arrived on the cxq|EntryList before the fence, above --
632 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
633 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
634 // non-null and elect a new "Responsible" timer thread.
635 //
636 // This thread executes:
637 // ST Responsible=null; MEMBAR (in enter epilogue - here)
638 // LD cxq|EntryList (in subsequent exit)
639 //
640 // Entering threads in the slow/contended path execute:
641 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
642 // The (ST cxq; MEMBAR) is accomplished with CAS().
643 //
644 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
645 // exit operation from floating above the ST Responsible=null.
646 }
647
648 // We've acquired ownership with CAS().
649 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
650 // But since the CAS() this thread may have also stored into _succ,
651 // EntryList, cxq or Responsible. These meta-data updates must be
652 // visible __before this thread subsequently drops the lock.
653 // Consider what could occur if we didn't enforce this constraint --
654 // STs to monitor meta-data and user-data could reorder with (become
655 // visible after) the ST in exit that drops ownership of the lock.
656 // Some other thread could then acquire the lock, but observe inconsistent
657 // or old monitor meta-data and heap data. That violates the JMM.
658 // To that end, the 1-0 exit() operation must have at least STST|LDST
659 // "release" barrier semantics. Specifically, there must be at least a
660 // STST|LDST barrier in exit() before the ST of null into _owner that drops
661 // the lock. The barrier ensures that changes to monitor meta-data and data
662 // protected by the lock will be visible before we release the lock, and
663 // therefore before some other thread (CPU) has a chance to acquire the lock.
664 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
665 //
666 // Critically, any prior STs to _succ or EntryList must be visible before
667 // the ST of null into _owner in the *subsequent* (following) corresponding
668 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
669 // execute a serializing instruction.
670
671 return;
672 }
673
674 // ReenterI() is a specialized inline form of the latter half of the
675 // contended slow-path from EnterI(). We use ReenterI() only for
676 // monitor reentry in wait().
677 //
678 // In the future we should reconcile EnterI() and ReenterI(), adding
679 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
680 // loop accordingly.
681
682 void ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) {
683 assert(Self != NULL, "invariant");
684 assert(SelfNode != NULL, "invariant");
685 assert(SelfNode->_thread == Self, "invariant");
686 assert(_waiters > 0, "invariant");
687 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
688 assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
689 JavaThread * jt = (JavaThread *) Self;
690
691 int nWakeups = 0;
692 for (;;) {
693 ObjectWaiter::TStates v = SelfNode->TState;
694 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
695 assert(_owner != Self, "invariant");
696
697 if (TryLock(Self) > 0) break;
698 if (TrySpin(Self) > 0) break;
699
700 // State transition wrappers around park() ...
701 // ReenterI() wisely defers state transitions until
702 // it's clear we must park the thread.
703 {
704 OSThreadContendState osts(Self->osthread());
705 ThreadBlockInVM tbivm(jt);
706
707 // cleared by handle_special_suspend_equivalent_condition()
708 // or java_suspend_self()
709 jt->set_suspend_equivalent();
710 Self->_ParkEvent->park();
711
712 // were we externally suspended while we were waiting?
713 for (;;) {
714 if (!ExitSuspendEquivalent(jt)) break;
715 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
716 jt->java_suspend_self();
717 jt->set_suspend_equivalent();
718 }
719 }
720
721 // Try again, but just so we distinguish between futile wakeups and
722 // successful wakeups. The following test isn't algorithmically
723 // necessary, but it helps us maintain sensible statistics.
724 if (TryLock(Self) > 0) break;
725
726 // The lock is still contested.
727 // Keep a tally of the # of futile wakeups.
728 // Note that the counter is not protected by a lock or updated by atomics.
729 // That is by design - we trade "lossy" counters which are exposed to
730 // races during updates for a lower probe effect.
731 ++nWakeups;
732
733 // Assuming this is not a spurious wakeup we'll normally
734 // find that _succ == Self.
735 if (_succ == Self) _succ = NULL;
736
737 // Invariant: after clearing _succ a contending thread
738 // *must* retry _owner before parking.
739 OrderAccess::fence();
740
741 // This PerfData object can be used in parallel with a safepoint.
742 // See the work around in PerfDataManager::destroy().
743 OM_PERFDATA_OP(FutileWakeups, inc());
744 }
745
746 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
747 // Normally we'll find Self on the EntryList.
748 // Unlinking from the EntryList is constant-time and atomic-free.
749 // From the perspective of the lock owner (this thread), the
750 // EntryList is stable and cxq is prepend-only.
751 // The head of cxq is volatile but the interior is stable.
752 // In addition, Self.TState is stable.
753
754 assert(_owner == Self, "invariant");
755 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
756 UnlinkAfterAcquire(Self, SelfNode);
757 if (_succ == Self) _succ = NULL;
758 assert(_succ != Self, "invariant");
759 SelfNode->TState = ObjectWaiter::TS_RUN;
760 OrderAccess::fence(); // see comments at the end of EnterI()
761 }
762
763 // By convention we unlink a contending thread from EntryList|cxq immediately
764 // after the thread acquires the lock in ::enter(). Equally, we could defer
765 // unlinking the thread until ::exit()-time.
766
767 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) {
768 assert(_owner == Self, "invariant");
769 assert(SelfNode->_thread == Self, "invariant");
770
771 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
772 // Normal case: remove Self from the DLL EntryList .
773 // This is a constant-time operation.
774 ObjectWaiter * nxt = SelfNode->_next;
775 ObjectWaiter * prv = SelfNode->_prev;
776 if (nxt != NULL) nxt->_prev = prv;
777 if (prv != NULL) prv->_next = nxt;
778 if (SelfNode == _EntryList) _EntryList = nxt;
779 assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
780 assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
781 } else {
782 assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant");
783 // Inopportune interleaving -- Self is still on the cxq.
784 // This usually means the enqueue of self raced an exiting thread.
785 // Normally we'll find Self near the front of the cxq, so
786 // dequeueing is typically fast. If needbe we can accelerate
787 // this with some MCS/CHL-like bidirectional list hints and advisory
788 // back-links so dequeueing from the interior will normally operate
789 // in constant-time.
790 // Dequeue Self from either the head (with CAS) or from the interior
791 // with a linear-time scan and normal non-atomic memory operations.
792 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
793 // and then unlink Self from EntryList. We have to drain eventually,
794 // so it might as well be now.
795
796 ObjectWaiter * v = _cxq;
797 assert(v != NULL, "invariant");
798 if (v != SelfNode || Atomic::cmpxchg(SelfNode->_next, &_cxq, v) != v) {
799 // The CAS above can fail from interference IFF a "RAT" arrived.
800 // In that case Self must be in the interior and can no longer be
801 // at the head of cxq.
802 if (v == SelfNode) {
803 assert(_cxq != v, "invariant");
804 v = _cxq; // CAS above failed - start scan at head of list
805 }
806 ObjectWaiter * p;
807 ObjectWaiter * q = NULL;
808 for (p = v; p != NULL && p != SelfNode; p = p->_next) {
809 q = p;
810 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
811 }
812 assert(v != SelfNode, "invariant");
813 assert(p == SelfNode, "Node not found on cxq");
814 assert(p != _cxq, "invariant");
815 assert(q != NULL, "invariant");
816 assert(q->_next == p, "invariant");
817 q->_next = p->_next;
818 }
819 }
820
821 #ifdef ASSERT
822 // Diagnostic hygiene ...
823 SelfNode->_prev = (ObjectWaiter *) 0xBAD;
824 SelfNode->_next = (ObjectWaiter *) 0xBAD;
825 SelfNode->TState = ObjectWaiter::TS_RUN;
826 #endif
827 }
828
829 // -----------------------------------------------------------------------------
830 // Exit support
831 //
832 // exit()
833 // ~~~~~~
834 // Note that the collector can't reclaim the objectMonitor or deflate
835 // the object out from underneath the thread calling ::exit() as the
836 // thread calling ::exit() never transitions to a stable state.
837 // This inhibits GC, which in turn inhibits asynchronous (and
838 // inopportune) reclamation of "this".
839 //
840 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
841 // There's one exception to the claim above, however. EnterI() can call
842 // exit() to drop a lock if the acquirer has been externally suspended.
843 // In that case exit() is called with _thread_state as _thread_blocked,
844 // but the monitor's _count field is > 0, which inhibits reclamation.
845 //
846 // 1-0 exit
847 // ~~~~~~~~
848 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
849 // the fast-path operators have been optimized so the common ::exit()
850 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
851 // The code emitted by fast_unlock() elides the usual MEMBAR. This
852 // greatly improves latency -- MEMBAR and CAS having considerable local
853 // latency on modern processors -- but at the cost of "stranding". Absent the
854 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
855 // ::enter() path, resulting in the entering thread being stranding
856 // and a progress-liveness failure. Stranding is extremely rare.
857 // We use timers (timed park operations) & periodic polling to detect
858 // and recover from stranding. Potentially stranded threads periodically
859 // wake up and poll the lock. See the usage of the _Responsible variable.
860 //
861 // The CAS() in enter provides for safety and exclusion, while the CAS or
862 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
863 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
864 // We detect and recover from stranding with timers.
865 //
866 // If a thread transiently strands it'll park until (a) another
867 // thread acquires the lock and then drops the lock, at which time the
868 // exiting thread will notice and unpark the stranded thread, or, (b)
869 // the timer expires. If the lock is high traffic then the stranding latency
870 // will be low due to (a). If the lock is low traffic then the odds of
871 // stranding are lower, although the worst-case stranding latency
872 // is longer. Critically, we don't want to put excessive load in the
873 // platform's timer subsystem. We want to minimize both the timer injection
874 // rate (timers created/sec) as well as the number of timers active at
875 // any one time. (more precisely, we want to minimize timer-seconds, which is
876 // the integral of the # of active timers at any instant over time).
877 // Both impinge on OS scalability. Given that, at most one thread parked on
878 // a monitor will use a timer.
879 //
880 // There is also the risk of a futile wake-up. If we drop the lock
881 // another thread can reacquire the lock immediately, and we can
882 // then wake a thread unnecessarily. This is benign, and we've
883 // structured the code so the windows are short and the frequency
884 // of such futile wakups is low.
885
886 void ObjectMonitor::exit(bool not_suspended, TRAPS) {
887 Thread * const Self = THREAD;
888 if (THREAD != _owner) {
889 if (THREAD->is_lock_owned((address) _owner)) {
890 // Transmute _owner from a BasicLock pointer to a Thread address.
891 // We don't need to hold _mutex for this transition.
892 // Non-null to Non-null is safe as long as all readers can
893 // tolerate either flavor.
894 assert(_recursions == 0, "invariant");
895 _owner = THREAD;
896 _recursions = 0;
897 } else {
898 // Apparent unbalanced locking ...
899 // Naively we'd like to throw IllegalMonitorStateException.
900 // As a practical matter we can neither allocate nor throw an
901 // exception as ::exit() can be called from leaf routines.
902 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
903 // Upon deeper reflection, however, in a properly run JVM the only
904 // way we should encounter this situation is in the presence of
905 // unbalanced JNI locking. TODO: CheckJNICalls.
906 // See also: CR4414101
907 assert(false, "Non-balanced monitor enter/exit! Likely JNI locking");
908 return;
909 }
910 }
911
912 if (_recursions != 0) {
913 _recursions--; // this is simple recursive enter
914 return;
915 }
916
917 // Invariant: after setting Responsible=null an thread must execute
918 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
919 _Responsible = NULL;
920
921 #if INCLUDE_JFR
922 // get the owner's thread id for the MonitorEnter event
923 // if it is enabled and the thread isn't suspended
924 if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
925 _previous_owner_tid = JFR_THREAD_ID(Self);
926 }
927 #endif
928
929 for (;;) {
930 assert(THREAD == _owner, "invariant");
931
932 if (Knob_ExitPolicy == 0) {
933 // release semantics: prior loads and stores from within the critical section
934 // must not float (reorder) past the following store that drops the lock.
935 // On SPARC that requires MEMBAR #loadstore|#storestore.
936 // But of course in TSO #loadstore|#storestore is not required.
937 // I'd like to write one of the following:
938 // A. OrderAccess::release() ; _owner = NULL
939 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
940 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
941 // store into a _dummy variable. That store is not needed, but can result
942 // in massive wasteful coherency traffic on classic SMP systems.
943 // Instead, I use release_store(), which is implemented as just a simple
944 // ST on x64, x86 and SPARC.
945 OrderAccess::release_store(&_owner, (void*)NULL); // drop the lock
946 OrderAccess::storeload(); // See if we need to wake a successor
947 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
948 return;
949 }
950 // Other threads are blocked trying to acquire the lock.
951
952 // Normally the exiting thread is responsible for ensuring succession,
953 // but if other successors are ready or other entering threads are spinning
954 // then this thread can simply store NULL into _owner and exit without
955 // waking a successor. The existence of spinners or ready successors
956 // guarantees proper succession (liveness). Responsibility passes to the
957 // ready or running successors. The exiting thread delegates the duty.
958 // More precisely, if a successor already exists this thread is absolved
959 // of the responsibility of waking (unparking) one.
960 //
961 // The _succ variable is critical to reducing futile wakeup frequency.
962 // _succ identifies the "heir presumptive" thread that has been made
963 // ready (unparked) but that has not yet run. We need only one such
964 // successor thread to guarantee progress.
965 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
966 // section 3.3 "Futile Wakeup Throttling" for details.
967 //
968 // Note that spinners in Enter() also set _succ non-null.
969 // In the current implementation spinners opportunistically set
970 // _succ so that exiting threads might avoid waking a successor.
971 // Another less appealing alternative would be for the exiting thread
972 // to drop the lock and then spin briefly to see if a spinner managed
973 // to acquire the lock. If so, the exiting thread could exit
974 // immediately without waking a successor, otherwise the exiting
975 // thread would need to dequeue and wake a successor.
976 // (Note that we'd need to make the post-drop spin short, but no
977 // shorter than the worst-case round-trip cache-line migration time.
978 // The dropped lock needs to become visible to the spinner, and then
979 // the acquisition of the lock by the spinner must become visible to
980 // the exiting thread).
981
982 // It appears that an heir-presumptive (successor) must be made ready.
983 // Only the current lock owner can manipulate the EntryList or
984 // drain _cxq, so we need to reacquire the lock. If we fail
985 // to reacquire the lock the responsibility for ensuring succession
986 // falls to the new owner.
987 //
988 if (!Atomic::replace_if_null(THREAD, &_owner)) {
989 return;
990 }
991 } else {
992 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
993 OrderAccess::release_store(&_owner, (void*)NULL); // drop the lock
994 OrderAccess::storeload();
995 // Ratify the previously observed values.
996 if (_cxq == NULL || _succ != NULL) {
997 return;
998 }
999
1000 // inopportune interleaving -- the exiting thread (this thread)
1001 // in the fast-exit path raced an entering thread in the slow-enter
1002 // path.
1003 // We have two choices:
1004 // A. Try to reacquire the lock.
1005 // If the CAS() fails return immediately, otherwise
1006 // we either restart/rerun the exit operation, or simply
1007 // fall-through into the code below which wakes a successor.
1008 // B. If the elements forming the EntryList|cxq are TSM
1009 // we could simply unpark() the lead thread and return
1010 // without having set _succ.
1011 if (!Atomic::replace_if_null(THREAD, &_owner)) {
1012 return;
1013 }
1014 }
1015 }
1016
1017 guarantee(_owner == THREAD, "invariant");
1018
1019 ObjectWaiter * w = NULL;
1020 int QMode = Knob_QMode;
1021
1022 if (QMode == 2 && _cxq != NULL) {
1023 // QMode == 2 : cxq has precedence over EntryList.
1024 // Try to directly wake a successor from the cxq.
1025 // If successful, the successor will need to unlink itself from cxq.
1026 w = _cxq;
1027 assert(w != NULL, "invariant");
1028 assert(w->TState == ObjectWaiter::TS_CXQ, "Invariant");
1029 ExitEpilog(Self, w);
1030 return;
1031 }
1032
1033 if (QMode == 3 && _cxq != NULL) {
1034 // Aggressively drain cxq into EntryList at the first opportunity.
1035 // This policy ensure that recently-run threads live at the head of EntryList.
1036 // Drain _cxq into EntryList - bulk transfer.
1037 // First, detach _cxq.
1038 // The following loop is tantamount to: w = swap(&cxq, NULL)
1039 w = _cxq;
1040 for (;;) {
1041 assert(w != NULL, "Invariant");
1042 ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1043 if (u == w) break;
1044 w = u;
1045 }
1046 assert(w != NULL, "invariant");
1047
1048 ObjectWaiter * q = NULL;
1049 ObjectWaiter * p;
1050 for (p = w; p != NULL; p = p->_next) {
1051 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1052 p->TState = ObjectWaiter::TS_ENTER;
1053 p->_prev = q;
1054 q = p;
1055 }
1056
1057 // Append the RATs to the EntryList
1058 // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
1059 ObjectWaiter * Tail;
1060 for (Tail = _EntryList; Tail != NULL && Tail->_next != NULL;
1061 Tail = Tail->_next)
1062 /* empty */;
1063 if (Tail == NULL) {
1064 _EntryList = w;
1065 } else {
1066 Tail->_next = w;
1067 w->_prev = Tail;
1068 }
1069
1070 // Fall thru into code that tries to wake a successor from EntryList
1071 }
1072
1073 if (QMode == 4 && _cxq != NULL) {
1074 // Aggressively drain cxq into EntryList at the first opportunity.
1075 // This policy ensure that recently-run threads live at the head of EntryList.
1076
1077 // Drain _cxq into EntryList - bulk transfer.
1078 // First, detach _cxq.
1079 // The following loop is tantamount to: w = swap(&cxq, NULL)
1080 w = _cxq;
1081 for (;;) {
1082 assert(w != NULL, "Invariant");
1083 ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1084 if (u == w) break;
1085 w = u;
1086 }
1087 assert(w != NULL, "invariant");
1088
1089 ObjectWaiter * q = NULL;
1090 ObjectWaiter * p;
1091 for (p = w; p != NULL; p = p->_next) {
1092 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1093 p->TState = ObjectWaiter::TS_ENTER;
1094 p->_prev = q;
1095 q = p;
1096 }
1097
1098 // Prepend the RATs to the EntryList
1099 if (_EntryList != NULL) {
1100 q->_next = _EntryList;
1101 _EntryList->_prev = q;
1102 }
1103 _EntryList = w;
1104
1105 // Fall thru into code that tries to wake a successor from EntryList
1106 }
1107
1108 w = _EntryList;
1109 if (w != NULL) {
1110 // I'd like to write: guarantee (w->_thread != Self).
1111 // But in practice an exiting thread may find itself on the EntryList.
1112 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1113 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1114 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1115 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1116 // release the lock "O". T2 resumes immediately after the ST of null into
1117 // _owner, above. T2 notices that the EntryList is populated, so it
1118 // reacquires the lock and then finds itself on the EntryList.
1119 // Given all that, we have to tolerate the circumstance where "w" is
1120 // associated with Self.
1121 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1122 ExitEpilog(Self, w);
1123 return;
1124 }
1125
1126 // If we find that both _cxq and EntryList are null then just
1127 // re-run the exit protocol from the top.
1128 w = _cxq;
1129 if (w == NULL) continue;
1130
1131 // Drain _cxq into EntryList - bulk transfer.
1132 // First, detach _cxq.
1133 // The following loop is tantamount to: w = swap(&cxq, NULL)
1134 for (;;) {
1135 assert(w != NULL, "Invariant");
1136 ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1137 if (u == w) break;
1138 w = u;
1139 }
1140
1141 assert(w != NULL, "invariant");
1142 assert(_EntryList == NULL, "invariant");
1143
1144 // Convert the LIFO SLL anchored by _cxq into a DLL.
1145 // The list reorganization step operates in O(LENGTH(w)) time.
1146 // It's critical that this step operate quickly as
1147 // "Self" still holds the outer-lock, restricting parallelism
1148 // and effectively lengthening the critical section.
1149 // Invariant: s chases t chases u.
1150 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1151 // we have faster access to the tail.
1152
1153 if (QMode == 1) {
1154 // QMode == 1 : drain cxq to EntryList, reversing order
1155 // We also reverse the order of the list.
1156 ObjectWaiter * s = NULL;
1157 ObjectWaiter * t = w;
1158 ObjectWaiter * u = NULL;
1159 while (t != NULL) {
1160 guarantee(t->TState == ObjectWaiter::TS_CXQ, "invariant");
1161 t->TState = ObjectWaiter::TS_ENTER;
1162 u = t->_next;
1163 t->_prev = u;
1164 t->_next = s;
1165 s = t;
1166 t = u;
1167 }
1168 _EntryList = s;
1169 assert(s != NULL, "invariant");
1170 } else {
1171 // QMode == 0 or QMode == 2
1172 _EntryList = w;
1173 ObjectWaiter * q = NULL;
1174 ObjectWaiter * p;
1175 for (p = w; p != NULL; p = p->_next) {
1176 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1177 p->TState = ObjectWaiter::TS_ENTER;
1178 p->_prev = q;
1179 q = p;
1180 }
1181 }
1182
1183 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1184 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1185
1186 // See if we can abdicate to a spinner instead of waking a thread.
1187 // A primary goal of the implementation is to reduce the
1188 // context-switch rate.
1189 if (_succ != NULL) continue;
1190
1191 w = _EntryList;
1192 if (w != NULL) {
1193 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1194 ExitEpilog(Self, w);
1195 return;
1196 }
1197 }
1198 }
1199
1200 // ExitSuspendEquivalent:
1201 // A faster alternate to handle_special_suspend_equivalent_condition()
1202 //
1203 // handle_special_suspend_equivalent_condition() unconditionally
1204 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1205 // operations have high latency. Note that in ::enter() we call HSSEC
1206 // while holding the monitor, so we effectively lengthen the critical sections.
1207 //
1208 // There are a number of possible solutions:
1209 //
1210 // A. To ameliorate the problem we might also defer state transitions
1211 // to as late as possible -- just prior to parking.
1212 // Given that, we'd call HSSEC after having returned from park(),
1213 // but before attempting to acquire the monitor. This is only a
1214 // partial solution. It avoids calling HSSEC while holding the
1215 // monitor (good), but it still increases successor reacquisition latency --
1216 // the interval between unparking a successor and the time the successor
1217 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1218 // If we use this technique we can also avoid EnterI()-exit() loop
1219 // in ::enter() where we iteratively drop the lock and then attempt
1220 // to reacquire it after suspending.
1221 //
1222 // B. In the future we might fold all the suspend bits into a
1223 // composite per-thread suspend flag and then update it with CAS().
1224 // Alternately, a Dekker-like mechanism with multiple variables
1225 // would suffice:
1226 // ST Self->_suspend_equivalent = false
1227 // MEMBAR
1228 // LD Self_>_suspend_flags
1229 //
1230 // UPDATE 2007-10-6: since I've replaced the native Mutex/Monitor subsystem
1231 // with a more efficient implementation, the need to use "FastHSSEC" has
1232 // decreased. - Dave
1233
1234
1235 bool ObjectMonitor::ExitSuspendEquivalent(JavaThread * jSelf) {
1236 const int Mode = Knob_FastHSSEC;
1237 if (Mode && !jSelf->is_external_suspend()) {
1238 assert(jSelf->is_suspend_equivalent(), "invariant");
1239 jSelf->clear_suspend_equivalent();
1240 if (2 == Mode) OrderAccess::storeload();
1241 if (!jSelf->is_external_suspend()) return false;
1242 // We raced a suspension -- fall thru into the slow path
1243 jSelf->set_suspend_equivalent();
1244 }
1245 return jSelf->handle_special_suspend_equivalent_condition();
1246 }
1247
1248
1249 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) {
1250 assert(_owner == Self, "invariant");
1251
1252 // Exit protocol:
1253 // 1. ST _succ = wakee
1254 // 2. membar #loadstore|#storestore;
1255 // 2. ST _owner = NULL
1256 // 3. unpark(wakee)
1257
1258 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL;
1259 ParkEvent * Trigger = Wakee->_event;
1260
1261 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1262 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1263 // out-of-scope (non-extant).
1264 Wakee = NULL;
1265
1266 // Drop the lock
1267 OrderAccess::release_store(&_owner, (void*)NULL);
1268 OrderAccess::fence(); // ST _owner vs LD in unpark()
1269
1270 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1271 Trigger->unpark();
1272
1273 // Maintain stats and report events to JVMTI
1274 OM_PERFDATA_OP(Parks, inc());
1275 }
1276
1277
1278 // -----------------------------------------------------------------------------
1279 // Class Loader deadlock handling.
1280 //
1281 // complete_exit exits a lock returning recursion count
1282 // complete_exit/reenter operate as a wait without waiting
1283 // complete_exit requires an inflated monitor
1284 // The _owner field is not always the Thread addr even with an
1285 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1286 // thread due to contention.
1287 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1288 Thread * const Self = THREAD;
1289 assert(Self->is_Java_thread(), "Must be Java thread!");
1290 JavaThread *jt = (JavaThread *)THREAD;
1291
1292 DeferredInitialize();
1293
1294 if (THREAD != _owner) {
1295 if (THREAD->is_lock_owned ((address)_owner)) {
1296 assert(_recursions == 0, "internal state error");
1297 _owner = THREAD; // Convert from basiclock addr to Thread addr
1298 _recursions = 0;
1299 }
1300 }
1301
1302 guarantee(Self == _owner, "complete_exit not owner");
1303 intptr_t save = _recursions; // record the old recursion count
1304 _recursions = 0; // set the recursion level to be 0
1305 exit(true, Self); // exit the monitor
1306 guarantee(_owner != Self, "invariant");
1307 return save;
1308 }
1309
1310 // reenter() enters a lock and sets recursion count
1311 // complete_exit/reenter operate as a wait without waiting
1312 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1313 Thread * const Self = THREAD;
1314 assert(Self->is_Java_thread(), "Must be Java thread!");
1315 JavaThread *jt = (JavaThread *)THREAD;
1316
1317 guarantee(_owner != Self, "reenter already owner");
1318 enter(THREAD); // enter the monitor
1319 guarantee(_recursions == 0, "reenter recursion");
1320 _recursions = recursions;
1321 return;
1322 }
1323
1324
1325 // -----------------------------------------------------------------------------
1326 // A macro is used below because there may already be a pending
1327 // exception which should not abort the execution of the routines
1328 // which use this (which is why we don't put this into check_slow and
1329 // call it with a CHECK argument).
1330
1331 #define CHECK_OWNER() \
1332 do { \
1333 if (THREAD != _owner) { \
1334 if (THREAD->is_lock_owned((address) _owner)) { \
1335 _owner = THREAD; /* Convert from basiclock addr to Thread addr */ \
1336 _recursions = 0; \
1337 } else { \
1338 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
1339 } \
1340 } \
1341 } while (false)
1342
1343 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
1344 // TODO-FIXME: remove check_slow() -- it's likely dead.
1345
1346 void ObjectMonitor::check_slow(TRAPS) {
1347 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1348 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1349 }
1350
1351 static int Adjust(volatile int * adr, int dx) {
1352 int v;
1353 for (v = *adr; Atomic::cmpxchg(v + dx, adr, v) != v; v = *adr) /* empty */;
1354 return v;
1355 }
1356
1357 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1358 ObjectMonitor* monitor,
1359 jlong notifier_tid,
1360 jlong timeout,
1361 bool timedout) {
1362 assert(event != NULL, "invariant");
1363 assert(monitor != NULL, "invariant");
1364 event->set_monitorClass(((oop)monitor->object())->klass());
1365 event->set_timeout(timeout);
1366 event->set_address((uintptr_t)monitor->object_addr());
1367 event->set_notifier(notifier_tid);
1368 event->set_timedOut(timedout);
1369 event->commit();
1370 }
1371
1372 // -----------------------------------------------------------------------------
1373 // Wait/Notify/NotifyAll
1374 //
1375 // Note: a subset of changes to ObjectMonitor::wait()
1376 // will need to be replicated in complete_exit
1377 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1378 Thread * const Self = THREAD;
1379 assert(Self->is_Java_thread(), "Must be Java thread!");
1380 JavaThread *jt = (JavaThread *)THREAD;
1381
1382 DeferredInitialize();
1383
1384 // Throw IMSX or IEX.
1385 CHECK_OWNER();
1386
1387 EventJavaMonitorWait event;
1388
1389 // check for a pending interrupt
1390 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1391 // post monitor waited event. Note that this is past-tense, we are done waiting.
1392 if (JvmtiExport::should_post_monitor_waited()) {
1393 // Note: 'false' parameter is passed here because the
1394 // wait was not timed out due to thread interrupt.
1395 JvmtiExport::post_monitor_waited(jt, this, false);
1396
1397 // In this short circuit of the monitor wait protocol, the
1398 // current thread never drops ownership of the monitor and
1399 // never gets added to the wait queue so the current thread
1400 // cannot be made the successor. This means that the
1401 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1402 // consume an unpark() meant for the ParkEvent associated with
1403 // this ObjectMonitor.
1404 }
1405 if (event.should_commit()) {
1406 post_monitor_wait_event(&event, this, 0, millis, false);
1407 }
1408 THROW(vmSymbols::java_lang_InterruptedException());
1409 return;
1410 }
1411
1412 assert(Self->_Stalled == 0, "invariant");
1413 Self->_Stalled = intptr_t(this);
1414 jt->set_current_waiting_monitor(this);
1415
1416 // create a node to be put into the queue
1417 // Critically, after we reset() the event but prior to park(), we must check
1418 // for a pending interrupt.
1419 ObjectWaiter node(Self);
1420 node.TState = ObjectWaiter::TS_WAIT;
1421 Self->_ParkEvent->reset();
1422 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1423
1424 // Enter the waiting queue, which is a circular doubly linked list in this case
1425 // but it could be a priority queue or any data structure.
1426 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1427 // by the the owner of the monitor *except* in the case where park()
1428 // returns because of a timeout of interrupt. Contention is exceptionally rare
1429 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1430
1431 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1432 AddWaiter(&node);
1433 Thread::SpinRelease(&_WaitSetLock);
1434
1435 _Responsible = NULL;
1436
1437 intptr_t save = _recursions; // record the old recursion count
1438 _waiters++; // increment the number of waiters
1439 _recursions = 0; // set the recursion level to be 1
1440 exit(true, Self); // exit the monitor
1441 guarantee(_owner != Self, "invariant");
1442
1443 // The thread is on the WaitSet list - now park() it.
1444 // On MP systems it's conceivable that a brief spin before we park
1445 // could be profitable.
1446 //
1447 // TODO-FIXME: change the following logic to a loop of the form
1448 // while (!timeout && !interrupted && _notified == 0) park()
1449
1450 int ret = OS_OK;
1451 int WasNotified = 0;
1452 { // State transition wrappers
1453 OSThread* osthread = Self->osthread();
1454 OSThreadWaitState osts(osthread, true);
1455 {
1456 ThreadBlockInVM tbivm(jt);
1457 // Thread is in thread_blocked state and oop access is unsafe.
1458 jt->set_suspend_equivalent();
1459
1460 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1461 // Intentionally empty
1462 } else if (node._notified == 0) {
1463 if (millis <= 0) {
1464 Self->_ParkEvent->park();
1465 } else {
1466 ret = Self->_ParkEvent->park(millis);
1467 }
1468 }
1469
1470 // were we externally suspended while we were waiting?
1471 if (ExitSuspendEquivalent (jt)) {
1472 // TODO-FIXME: add -- if succ == Self then succ = null.
1473 jt->java_suspend_self();
1474 }
1475
1476 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1477
1478 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1479 // from the WaitSet to the EntryList.
1480 // See if we need to remove Node from the WaitSet.
1481 // We use double-checked locking to avoid grabbing _WaitSetLock
1482 // if the thread is not on the wait queue.
1483 //
1484 // Note that we don't need a fence before the fetch of TState.
1485 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1486 // written by the is thread. (perhaps the fetch might even be satisfied
1487 // by a look-aside into the processor's own store buffer, although given
1488 // the length of the code path between the prior ST and this load that's
1489 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1490 // then we'll acquire the lock and then re-fetch a fresh TState value.
1491 // That is, we fail toward safety.
1492
1493 if (node.TState == ObjectWaiter::TS_WAIT) {
1494 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1495 if (node.TState == ObjectWaiter::TS_WAIT) {
1496 DequeueSpecificWaiter(&node); // unlink from WaitSet
1497 assert(node._notified == 0, "invariant");
1498 node.TState = ObjectWaiter::TS_RUN;
1499 }
1500 Thread::SpinRelease(&_WaitSetLock);
1501 }
1502
1503 // The thread is now either on off-list (TS_RUN),
1504 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1505 // The Node's TState variable is stable from the perspective of this thread.
1506 // No other threads will asynchronously modify TState.
1507 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1508 OrderAccess::loadload();
1509 if (_succ == Self) _succ = NULL;
1510 WasNotified = node._notified;
1511
1512 // Reentry phase -- reacquire the monitor.
1513 // re-enter contended monitor after object.wait().
1514 // retain OBJECT_WAIT state until re-enter successfully completes
1515 // Thread state is thread_in_vm and oop access is again safe,
1516 // although the raw address of the object may have changed.
1517 // (Don't cache naked oops over safepoints, of course).
1518
1519 // post monitor waited event. Note that this is past-tense, we are done waiting.
1520 if (JvmtiExport::should_post_monitor_waited()) {
1521 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1522
1523 if (node._notified != 0 && _succ == Self) {
1524 // In this part of the monitor wait-notify-reenter protocol it
1525 // is possible (and normal) for another thread to do a fastpath
1526 // monitor enter-exit while this thread is still trying to get
1527 // to the reenter portion of the protocol.
1528 //
1529 // The ObjectMonitor was notified and the current thread is
1530 // the successor which also means that an unpark() has already
1531 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1532 // consume the unpark() that was done when the successor was
1533 // set because the same ParkEvent is shared between Java
1534 // monitors and JVM/TI RawMonitors (for now).
1535 //
1536 // We redo the unpark() to ensure forward progress, i.e., we
1537 // don't want all pending threads hanging (parked) with none
1538 // entering the unlocked monitor.
1539 node._event->unpark();
1540 }
1541 }
1542
1543 if (event.should_commit()) {
1544 post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1545 }
1546
1547 OrderAccess::fence();
1548
1549 assert(Self->_Stalled != 0, "invariant");
1550 Self->_Stalled = 0;
1551
1552 assert(_owner != Self, "invariant");
1553 ObjectWaiter::TStates v = node.TState;
1554 if (v == ObjectWaiter::TS_RUN) {
1555 enter(Self);
1556 } else {
1557 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1558 ReenterI(Self, &node);
1559 node.wait_reenter_end(this);
1560 }
1561
1562 // Self has reacquired the lock.
1563 // Lifecycle - the node representing Self must not appear on any queues.
1564 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1565 // want residual elements associated with this thread left on any lists.
1566 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1567 assert(_owner == Self, "invariant");
1568 assert(_succ != Self, "invariant");
1569 } // OSThreadWaitState()
1570
1571 jt->set_current_waiting_monitor(NULL);
1572
1573 guarantee(_recursions == 0, "invariant");
1574 _recursions = save; // restore the old recursion count
1575 _waiters--; // decrement the number of waiters
1576
1577 // Verify a few postconditions
1578 assert(_owner == Self, "invariant");
1579 assert(_succ != Self, "invariant");
1580 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
1581
1582 // check if the notification happened
1583 if (!WasNotified) {
1584 // no, it could be timeout or Thread.interrupt() or both
1585 // check for interrupt event, otherwise it is timeout
1586 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1587 THROW(vmSymbols::java_lang_InterruptedException());
1588 }
1589 }
1590
1591 // NOTE: Spurious wake up will be consider as timeout.
1592 // Monitor notify has precedence over thread interrupt.
1593 }
1594
1595
1596 // Consider:
1597 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1598 // then instead of transferring a thread from the WaitSet to the EntryList
1599 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1600
1601 void ObjectMonitor::INotify(Thread * Self) {
1602 const int policy = Knob_MoveNotifyee;
1603
1604 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1605 ObjectWaiter * iterator = DequeueWaiter();
1606 if (iterator != NULL) {
1607 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1608 guarantee(iterator->_notified == 0, "invariant");
1609 // Disposition - what might we do with iterator ?
1610 // a. add it directly to the EntryList - either tail (policy == 1)
1611 // or head (policy == 0).
1612 // b. push it onto the front of the _cxq (policy == 2).
1613 // For now we use (b).
1614 if (policy != 4) {
1615 iterator->TState = ObjectWaiter::TS_ENTER;
1616 }
1617 iterator->_notified = 1;
1618 iterator->_notifier_tid = JFR_THREAD_ID(Self);
1619
1620 ObjectWaiter * list = _EntryList;
1621 if (list != NULL) {
1622 assert(list->_prev == NULL, "invariant");
1623 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1624 assert(list != iterator, "invariant");
1625 }
1626
1627 if (policy == 0) { // prepend to EntryList
1628 if (list == NULL) {
1629 iterator->_next = iterator->_prev = NULL;
1630 _EntryList = iterator;
1631 } else {
1632 list->_prev = iterator;
1633 iterator->_next = list;
1634 iterator->_prev = NULL;
1635 _EntryList = iterator;
1636 }
1637 } else if (policy == 1) { // append to EntryList
1638 if (list == NULL) {
1639 iterator->_next = iterator->_prev = NULL;
1640 _EntryList = iterator;
1641 } else {
1642 // CONSIDER: finding the tail currently requires a linear-time walk of
1643 // the EntryList. We can make tail access constant-time by converting to
1644 // a CDLL instead of using our current DLL.
1645 ObjectWaiter * tail;
1646 for (tail = list; tail->_next != NULL; tail = tail->_next) {}
1647 assert(tail != NULL && tail->_next == NULL, "invariant");
1648 tail->_next = iterator;
1649 iterator->_prev = tail;
1650 iterator->_next = NULL;
1651 }
1652 } else if (policy == 2) { // prepend to cxq
1653 if (list == NULL) {
1654 iterator->_next = iterator->_prev = NULL;
1655 _EntryList = iterator;
1656 } else {
1657 iterator->TState = ObjectWaiter::TS_CXQ;
1658 for (;;) {
1659 ObjectWaiter * front = _cxq;
1660 iterator->_next = front;
1661 if (Atomic::cmpxchg(iterator, &_cxq, front) == front) {
1662 break;
1663 }
1664 }
1665 }
1666 } else if (policy == 3) { // append to cxq
1667 iterator->TState = ObjectWaiter::TS_CXQ;
1668 for (;;) {
1669 ObjectWaiter * tail = _cxq;
1670 if (tail == NULL) {
1671 iterator->_next = NULL;
1672 if (Atomic::replace_if_null(iterator, &_cxq)) {
1673 break;
1674 }
1675 } else {
1676 while (tail->_next != NULL) tail = tail->_next;
1677 tail->_next = iterator;
1678 iterator->_prev = tail;
1679 iterator->_next = NULL;
1680 break;
1681 }
1682 }
1683 } else {
1684 ParkEvent * ev = iterator->_event;
1685 iterator->TState = ObjectWaiter::TS_RUN;
1686 OrderAccess::fence();
1687 ev->unpark();
1688 }
1689
1690 // _WaitSetLock protects the wait queue, not the EntryList. We could
1691 // move the add-to-EntryList operation, above, outside the critical section
1692 // protected by _WaitSetLock. In practice that's not useful. With the
1693 // exception of wait() timeouts and interrupts the monitor owner
1694 // is the only thread that grabs _WaitSetLock. There's almost no contention
1695 // on _WaitSetLock so it's not profitable to reduce the length of the
1696 // critical section.
1697
1698 if (policy < 4) {
1699 iterator->wait_reenter_begin(this);
1700 }
1701 }
1702 Thread::SpinRelease(&_WaitSetLock);
1703 }
1704
1705 // Consider: a not-uncommon synchronization bug is to use notify() when
1706 // notifyAll() is more appropriate, potentially resulting in stranded
1707 // threads; this is one example of a lost wakeup. A useful diagnostic
1708 // option is to force all notify() operations to behave as notifyAll().
1709 //
1710 // Note: We can also detect many such problems with a "minimum wait".
1711 // When the "minimum wait" is set to a small non-zero timeout value
1712 // and the program does not hang whereas it did absent "minimum wait",
1713 // that suggests a lost wakeup bug.
1714
1715 void ObjectMonitor::notify(TRAPS) {
1716 CHECK_OWNER();
1717 if (_WaitSet == NULL) {
1718 return;
1719 }
1720 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1721 INotify(THREAD);
1722 OM_PERFDATA_OP(Notifications, inc(1));
1723 }
1724
1725
1726 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1727 // from the waitset to the EntryList. This could be done more efficiently with a
1728 // single bulk transfer but in practice it's not time-critical. Beware too,
1729 // that in prepend-mode we invert the order of the waiters. Let's say that the
1730 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1731 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1732
1733 void ObjectMonitor::notifyAll(TRAPS) {
1734 CHECK_OWNER();
1735 if (_WaitSet == NULL) {
1736 return;
1737 }
1738
1739 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1740 int tally = 0;
1741 while (_WaitSet != NULL) {
1742 tally++;
1743 INotify(THREAD);
1744 }
1745
1746 OM_PERFDATA_OP(Notifications, inc(tally));
1747 }
1748
1749 // -----------------------------------------------------------------------------
1750 // Adaptive Spinning Support
1751 //
1752 // Adaptive spin-then-block - rational spinning
1753 //
1754 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1755 // algorithm. On high order SMP systems it would be better to start with
1756 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1757 // a contending thread could enqueue itself on the cxq and then spin locally
1758 // on a thread-specific variable such as its ParkEvent._Event flag.
1759 // That's left as an exercise for the reader. Note that global spinning is
1760 // not problematic on Niagara, as the L2 cache serves the interconnect and
1761 // has both low latency and massive bandwidth.
1762 //
1763 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1764 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1765 // (duration) or we can fix the count at approximately the duration of
1766 // a context switch and vary the frequency. Of course we could also
1767 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1768 // For a description of 'Adaptive spin-then-block mutual exclusion in
1769 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1770 //
1771 // This implementation varies the duration "D", where D varies with
1772 // the success rate of recent spin attempts. (D is capped at approximately
1773 // length of a round-trip context switch). The success rate for recent
1774 // spin attempts is a good predictor of the success rate of future spin
1775 // attempts. The mechanism adapts automatically to varying critical
1776 // section length (lock modality), system load and degree of parallelism.
1777 // D is maintained per-monitor in _SpinDuration and is initialized
1778 // optimistically. Spin frequency is fixed at 100%.
1779 //
1780 // Note that _SpinDuration is volatile, but we update it without locks
1781 // or atomics. The code is designed so that _SpinDuration stays within
1782 // a reasonable range even in the presence of races. The arithmetic
1783 // operations on _SpinDuration are closed over the domain of legal values,
1784 // so at worst a race will install and older but still legal value.
1785 // At the very worst this introduces some apparent non-determinism.
1786 // We might spin when we shouldn't or vice-versa, but since the spin
1787 // count are relatively short, even in the worst case, the effect is harmless.
1788 //
1789 // Care must be taken that a low "D" value does not become an
1790 // an absorbing state. Transient spinning failures -- when spinning
1791 // is overall profitable -- should not cause the system to converge
1792 // on low "D" values. We want spinning to be stable and predictable
1793 // and fairly responsive to change and at the same time we don't want
1794 // it to oscillate, become metastable, be "too" non-deterministic,
1795 // or converge on or enter undesirable stable absorbing states.
1796 //
1797 // We implement a feedback-based control system -- using past behavior
1798 // to predict future behavior. We face two issues: (a) if the
1799 // input signal is random then the spin predictor won't provide optimal
1800 // results, and (b) if the signal frequency is too high then the control
1801 // system, which has some natural response lag, will "chase" the signal.
1802 // (b) can arise from multimodal lock hold times. Transient preemption
1803 // can also result in apparent bimodal lock hold times.
1804 // Although sub-optimal, neither condition is particularly harmful, as
1805 // in the worst-case we'll spin when we shouldn't or vice-versa.
1806 // The maximum spin duration is rather short so the failure modes aren't bad.
1807 // To be conservative, I've tuned the gain in system to bias toward
1808 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1809 // "rings" or oscillates between spinning and not spinning. This happens
1810 // when spinning is just on the cusp of profitability, however, so the
1811 // situation is not dire. The state is benign -- there's no need to add
1812 // hysteresis control to damp the transition rate between spinning and
1813 // not spinning.
1814
1815 // Spinning: Fixed frequency (100%), vary duration
1816 int ObjectMonitor::TrySpin(Thread * Self) {
1817 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1818 int ctr = Knob_FixedSpin;
1819 if (ctr != 0) {
1820 while (--ctr >= 0) {
1821 if (TryLock(Self) > 0) return 1;
1822 SpinPause();
1823 }
1824 return 0;
1825 }
1826
1827 for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1828 if (TryLock(Self) > 0) {
1829 // Increase _SpinDuration ...
1830 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1831 // Raising _SpurDuration to the poverty line is key.
1832 int x = _SpinDuration;
1833 if (x < Knob_SpinLimit) {
1834 if (x < Knob_Poverty) x = Knob_Poverty;
1835 _SpinDuration = x + Knob_BonusB;
1836 }
1837 return 1;
1838 }
1839 SpinPause();
1840 }
1841
1842 // Admission control - verify preconditions for spinning
1843 //
1844 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1845 // becoming an absorbing state. Put another way, we spin briefly to
1846 // sample, just in case the system load, parallelism, contention, or lock
1847 // modality changed.
1848 //
1849 // Consider the following alternative:
1850 // Periodically set _SpinDuration = _SpinLimit and try a long/full
1851 // spin attempt. "Periodically" might mean after a tally of
1852 // the # of failed spin attempts (or iterations) reaches some threshold.
1853 // This takes us into the realm of 1-out-of-N spinning, where we
1854 // hold the duration constant but vary the frequency.
1855
1856 ctr = _SpinDuration;
1857 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase;
1858 if (ctr <= 0) return 0;
1859
1860 if (Knob_SuccRestrict && _succ != NULL) return 0;
1861 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
1862 return 0;
1863 }
1864
1865 int MaxSpin = Knob_MaxSpinners;
1866 if (MaxSpin >= 0) {
1867 if (_Spinner > MaxSpin) {
1868 return 0;
1869 }
1870 // Slightly racy, but benign ...
1871 Adjust(&_Spinner, 1);
1872 }
1873
1874 // We're good to spin ... spin ingress.
1875 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1876 // when preparing to LD...CAS _owner, etc and the CAS is likely
1877 // to succeed.
1878 int hits = 0;
1879 int msk = 0;
1880 int caspty = Knob_CASPenalty;
1881 int oxpty = Knob_OXPenalty;
1882 int sss = Knob_SpinSetSucc;
1883 if (sss && _succ == NULL) _succ = Self;
1884 Thread * prv = NULL;
1885
1886 // There are three ways to exit the following loop:
1887 // 1. A successful spin where this thread has acquired the lock.
1888 // 2. Spin failure with prejudice
1889 // 3. Spin failure without prejudice
1890
1891 while (--ctr >= 0) {
1892
1893 // Periodic polling -- Check for pending GC
1894 // Threads may spin while they're unsafe.
1895 // We don't want spinning threads to delay the JVM from reaching
1896 // a stop-the-world safepoint or to steal cycles from GC.
1897 // If we detect a pending safepoint we abort in order that
1898 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1899 // this thread, if safe, doesn't steal cycles from GC.
1900 // This is in keeping with the "no loitering in runtime" rule.
1901 // We periodically check to see if there's a safepoint pending.
1902 if ((ctr & 0xFF) == 0) {
1903 if (SafepointMechanism::poll(Self)) {
1904 goto Abort; // abrupt spin egress
1905 }
1906 if (Knob_UsePause & 1) SpinPause();
1907 }
1908
1909 if (Knob_UsePause & 2) SpinPause();
1910
1911 // Exponential back-off ... Stay off the bus to reduce coherency traffic.
1912 // This is useful on classic SMP systems, but is of less utility on
1913 // N1-style CMT platforms.
1914 //
1915 // Trade-off: lock acquisition latency vs coherency bandwidth.
1916 // Lock hold times are typically short. A histogram
1917 // of successful spin attempts shows that we usually acquire
1918 // the lock early in the spin. That suggests we want to
1919 // sample _owner frequently in the early phase of the spin,
1920 // but then back-off and sample less frequently as the spin
1921 // progresses. The back-off makes a good citizen on SMP big
1922 // SMP systems. Oversampling _owner can consume excessive
1923 // coherency bandwidth. Relatedly, if we _oversample _owner we
1924 // can inadvertently interfere with the the ST m->owner=null.
1925 // executed by the lock owner.
1926 if (ctr & msk) continue;
1927 ++hits;
1928 if ((hits & 0xF) == 0) {
1929 // The 0xF, above, corresponds to the exponent.
1930 // Consider: (msk+1)|msk
1931 msk = ((msk << 2)|3) & BackOffMask;
1932 }
1933
1934 // Probe _owner with TATAS
1935 // If this thread observes the monitor transition or flicker
1936 // from locked to unlocked to locked, then the odds that this
1937 // thread will acquire the lock in this spin attempt go down
1938 // considerably. The same argument applies if the CAS fails
1939 // or if we observe _owner change from one non-null value to
1940 // another non-null value. In such cases we might abort
1941 // the spin without prejudice or apply a "penalty" to the
1942 // spin count-down variable "ctr", reducing it by 100, say.
1943
1944 Thread * ox = (Thread *) _owner;
1945 if (ox == NULL) {
1946 ox = (Thread*)Atomic::cmpxchg(Self, &_owner, (void*)NULL);
1947 if (ox == NULL) {
1948 // The CAS succeeded -- this thread acquired ownership
1949 // Take care of some bookkeeping to exit spin state.
1950 if (sss && _succ == Self) {
1951 _succ = NULL;
1952 }
1953 if (MaxSpin > 0) Adjust(&_Spinner, -1);
1954
1955 // Increase _SpinDuration :
1956 // The spin was successful (profitable) so we tend toward
1957 // longer spin attempts in the future.
1958 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
1959 // If we acquired the lock early in the spin cycle it
1960 // makes sense to increase _SpinDuration proportionally.
1961 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1962 int x = _SpinDuration;
1963 if (x < Knob_SpinLimit) {
1964 if (x < Knob_Poverty) x = Knob_Poverty;
1965 _SpinDuration = x + Knob_Bonus;
1966 }
1967 return 1;
1968 }
1969
1970 // The CAS failed ... we can take any of the following actions:
1971 // * penalize: ctr -= Knob_CASPenalty
1972 // * exit spin with prejudice -- goto Abort;
1973 // * exit spin without prejudice.
1974 // * Since CAS is high-latency, retry again immediately.
1975 prv = ox;
1976 if (caspty == -2) break;
1977 if (caspty == -1) goto Abort;
1978 ctr -= caspty;
1979 continue;
1980 }
1981
1982 // Did lock ownership change hands ?
1983 if (ox != prv && prv != NULL) {
1984 if (oxpty == -2) break;
1985 if (oxpty == -1) goto Abort;
1986 ctr -= oxpty;
1987 }
1988 prv = ox;
1989
1990 // Abort the spin if the owner is not executing.
1991 // The owner must be executing in order to drop the lock.
1992 // Spinning while the owner is OFFPROC is idiocy.
1993 // Consider: ctr -= RunnablePenalty ;
1994 if (Knob_OState && NotRunnable (Self, ox)) {
1995 goto Abort;
1996 }
1997 if (sss && _succ == NULL) _succ = Self;
1998 }
1999
2000 // Spin failed with prejudice -- reduce _SpinDuration.
2001 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2002 // AIMD is globally stable.
2003 {
2004 int x = _SpinDuration;
2005 if (x > 0) {
2006 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2007 // This is globally sample and tends to damp the response.
2008 x -= Knob_Penalty;
2009 if (x < 0) x = 0;
2010 _SpinDuration = x;
2011 }
2012 }
2013
2014 Abort:
2015 if (MaxSpin >= 0) Adjust(&_Spinner, -1);
2016 if (sss && _succ == Self) {
2017 _succ = NULL;
2018 // Invariant: after setting succ=null a contending thread
2019 // must recheck-retry _owner before parking. This usually happens
2020 // in the normal usage of TrySpin(), but it's safest
2021 // to make TrySpin() as foolproof as possible.
2022 OrderAccess::fence();
2023 if (TryLock(Self) > 0) return 1;
2024 }
2025 return 0;
2026 }
2027
2028 // NotRunnable() -- informed spinning
2029 //
2030 // Don't bother spinning if the owner is not eligible to drop the lock.
2031 // Spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2032 // The thread must be runnable in order to drop the lock in timely fashion.
2033 // If the _owner is not runnable then spinning will not likely be
2034 // successful (profitable).
2035 //
2036 // Beware -- the thread referenced by _owner could have died
2037 // so a simply fetch from _owner->_thread_state might trap.
2038 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2039 // Because of the lifecycle issues, the _thread_state values
2040 // observed by NotRunnable() might be garbage. NotRunnable must
2041 // tolerate this and consider the observed _thread_state value
2042 // as advisory.
2043 //
2044 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2045 // a thread pointer.
2046 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2047 // with the LSB of _owner. Another option would be to probabilistically probe
2048 // the putative _owner->TypeTag value.
2049 //
2050 // Checking _thread_state isn't perfect. Even if the thread is
2051 // in_java it might be blocked on a page-fault or have been preempted
2052 // and sitting on a ready/dispatch queue.
2053 //
2054 // The return value from NotRunnable() is *advisory* -- the
2055 // result is based on sampling and is not necessarily coherent.
2056 // The caller must tolerate false-negative and false-positive errors.
2057 // Spinning, in general, is probabilistic anyway.
2058
2059
2060 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) {
2061 // Check ox->TypeTag == 2BAD.
2062 if (ox == NULL) return 0;
2063
2064 // Avoid transitive spinning ...
2065 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2066 // Immediately after T1 acquires L it's possible that T2, also
2067 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2068 // This occurs transiently after T1 acquired L but before
2069 // T1 managed to clear T1.Stalled. T2 does not need to abort
2070 // its spin in this circumstance.
2071 intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
2072
2073 if (BlockedOn == 1) return 1;
2074 if (BlockedOn != 0) {
2075 return BlockedOn != intptr_t(this) && _owner == ox;
2076 }
2077
2078 assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant");
2079 int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);;
2080 // consider also: jst != _thread_in_Java -- but that's overspecific.
2081 return jst == _thread_blocked || jst == _thread_in_native;
2082 }
2083
2084
2085 // -----------------------------------------------------------------------------
2086 // WaitSet management ...
2087
2088 ObjectWaiter::ObjectWaiter(Thread* thread) {
2089 _next = NULL;
2090 _prev = NULL;
2091 _notified = 0;
2092 _notifier_tid = 0;
2093 TState = TS_RUN;
2094 _thread = thread;
2095 _event = thread->_ParkEvent;
2096 _active = false;
2097 assert(_event != NULL, "invariant");
2098 }
2099
2100 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2101 JavaThread *jt = (JavaThread *)this->_thread;
2102 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2103 }
2104
2105 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2106 JavaThread *jt = (JavaThread *)this->_thread;
2107 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2108 }
2109
2110 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2111 assert(node != NULL, "should not add NULL node");
2112 assert(node->_prev == NULL, "node already in list");
2113 assert(node->_next == NULL, "node already in list");
2114 // put node at end of queue (circular doubly linked list)
2115 if (_WaitSet == NULL) {
2116 _WaitSet = node;
2117 node->_prev = node;
2118 node->_next = node;
2119 } else {
2120 ObjectWaiter* head = _WaitSet;
2121 ObjectWaiter* tail = head->_prev;
2122 assert(tail->_next == head, "invariant check");
2123 tail->_next = node;
2124 head->_prev = node;
2125 node->_next = head;
2126 node->_prev = tail;
2127 }
2128 }
2129
2130 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2131 // dequeue the very first waiter
2132 ObjectWaiter* waiter = _WaitSet;
2133 if (waiter) {
2134 DequeueSpecificWaiter(waiter);
2135 }
2136 return waiter;
2137 }
2138
2139 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2140 assert(node != NULL, "should not dequeue NULL node");
2141 assert(node->_prev != NULL, "node already removed from list");
2142 assert(node->_next != NULL, "node already removed from list");
2143 // when the waiter has woken up because of interrupt,
2144 // timeout or other spurious wake-up, dequeue the
2145 // waiter from waiting list
2146 ObjectWaiter* next = node->_next;
2147 if (next == node) {
2148 assert(node->_prev == node, "invariant check");
2149 _WaitSet = NULL;
2150 } else {
2151 ObjectWaiter* prev = node->_prev;
2152 assert(prev->_next == node, "invariant check");
2153 assert(next->_prev == node, "invariant check");
2154 next->_prev = prev;
2155 prev->_next = next;
2156 if (_WaitSet == node) {
2157 _WaitSet = next;
2158 }
2159 }
2160 node->_next = NULL;
2161 node->_prev = NULL;
2162 }
2163
2164 // -----------------------------------------------------------------------------
2165 // PerfData support
2166 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL;
2167 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL;
2168 PerfCounter * ObjectMonitor::_sync_Parks = NULL;
2169 PerfCounter * ObjectMonitor::_sync_Notifications = NULL;
2170 PerfCounter * ObjectMonitor::_sync_Inflations = NULL;
2171 PerfCounter * ObjectMonitor::_sync_Deflations = NULL;
2172 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL;
2173
2174 // One-shot global initialization for the sync subsystem.
2175 // We could also defer initialization and initialize on-demand
2176 // the first time we call inflate(). Initialization would
2177 // be protected - like so many things - by the MonitorCache_lock.
2178
2179 void ObjectMonitor::Initialize() {
2180 static int InitializationCompleted = 0;
2181 assert(InitializationCompleted == 0, "invariant");
2182 InitializationCompleted = 1;
2183 if (UsePerfData) {
2184 EXCEPTION_MARK;
2185 #define NEWPERFCOUNTER(n) \
2186 { \
2187 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \
2188 CHECK); \
2189 }
2190 #define NEWPERFVARIABLE(n) \
2191 { \
2192 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \
2193 CHECK); \
2194 }
2195 NEWPERFCOUNTER(_sync_Inflations);
2196 NEWPERFCOUNTER(_sync_Deflations);
2197 NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2198 NEWPERFCOUNTER(_sync_FutileWakeups);
2199 NEWPERFCOUNTER(_sync_Parks);
2200 NEWPERFCOUNTER(_sync_Notifications);
2201 NEWPERFVARIABLE(_sync_MonExtant);
2202 #undef NEWPERFCOUNTER
2203 #undef NEWPERFVARIABLE
2204 }
2205 }
2206
2207 static char * kvGet(char * kvList, const char * Key) {
2208 if (kvList == NULL) return NULL;
2209 size_t n = strlen(Key);
2210 char * Search;
2211 for (Search = kvList; *Search; Search += strlen(Search) + 1) {
2212 if (strncmp (Search, Key, n) == 0) {
2213 if (Search[n] == '=') return Search + n + 1;
2214 if (Search[n] == 0) return(char *) "1";
2215 }
2216 }
2217 return NULL;
2218 }
2219
2220 static int kvGetInt(char * kvList, const char * Key, int Default) {
2221 char * v = kvGet(kvList, Key);
2222 int rslt = v ? ::strtol(v, NULL, 0) : Default;
2223 if (Knob_ReportSettings && v != NULL) {
2224 tty->print_cr("INFO: SyncKnob: %s %d(%d)", Key, rslt, Default) ;
2225 tty->flush();
2226 }
2227 return rslt;
2228 }
2229
2230 void ObjectMonitor::DeferredInitialize() {
2231 if (InitDone > 0) return;
2232 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2233 while (InitDone != 1) /* empty */;
2234 return;
2235 }
2236
2237 // One-shot global initialization ...
2238 // The initialization is idempotent, so we don't need locks.
2239 // In the future consider doing this via os::init_2().
2240 // SyncKnobs consist of <Key>=<Value> pairs in the style
2241 // of environment variables. Start by converting ':' to NUL.
2242
2243 if (SyncKnobs == NULL) SyncKnobs = "";
2244
2245 size_t sz = strlen(SyncKnobs);
2246 char * knobs = (char *) os::malloc(sz + 2, mtInternal);
2247 if (knobs == NULL) {
2248 vm_exit_out_of_memory(sz + 2, OOM_MALLOC_ERROR, "Parse SyncKnobs");
2249 guarantee(0, "invariant");
2250 }
2251 strcpy(knobs, SyncKnobs);
2252 knobs[sz+1] = 0;
2253 for (char * p = knobs; *p; p++) {
2254 if (*p == ':') *p = 0;
2255 }
2256
2257 #define SETKNOB(x) { Knob_##x = kvGetInt(knobs, #x, Knob_##x); }
2258 SETKNOB(ReportSettings);
2259 SETKNOB(ExitRelease);
2260 SETKNOB(InlineNotify);
2261 SETKNOB(Verbose);
2262 SETKNOB(VerifyInUse);
2263 SETKNOB(VerifyMatch);
2264 SETKNOB(FixedSpin);
2265 SETKNOB(SpinLimit);
2266 SETKNOB(SpinBase);
2267 SETKNOB(SpinBackOff);
2268 SETKNOB(CASPenalty);
2269 SETKNOB(OXPenalty);
2270 SETKNOB(SpinSetSucc);
2271 SETKNOB(SuccEnabled);
2272 SETKNOB(SuccRestrict);
2273 SETKNOB(Penalty);
2274 SETKNOB(Bonus);
2275 SETKNOB(BonusB);
2276 SETKNOB(Poverty);
2277 SETKNOB(SpinAfterFutile);
2278 SETKNOB(UsePause);
2279 SETKNOB(SpinEarly);
2280 SETKNOB(OState);
2281 SETKNOB(MaxSpinners);
2282 SETKNOB(PreSpin);
2283 SETKNOB(ExitPolicy);
2284 SETKNOB(QMode);
2285 SETKNOB(ResetEvent);
2286 SETKNOB(MoveNotifyee);
2287 SETKNOB(FastHSSEC);
2288 #undef SETKNOB
2289
2290 if (os::is_MP()) {
2291 BackOffMask = (1 << Knob_SpinBackOff) - 1;
2292 if (Knob_ReportSettings) {
2293 tty->print_cr("INFO: BackOffMask=0x%X", BackOffMask);
2294 }
2295 // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
2296 } else {
2297 Knob_SpinLimit = 0;
2298 Knob_SpinBase = 0;
2299 Knob_PreSpin = 0;
2300 Knob_FixedSpin = -1;
2301 }
2302
2303 os::free(knobs);
2304 OrderAccess::fence();
2305 InitDone = 1;
2306 }
2307