1 /*
2 * Copyright (c) 1998, 2016, 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 "memory/resourceArea.hpp"
28 #include "oops/markOop.hpp"
29 #include "oops/oop.inline.hpp"
30 #include "runtime/atomic.hpp"
31 #include "runtime/handles.inline.hpp"
32 #include "runtime/interfaceSupport.hpp"
33 #include "runtime/mutexLocker.hpp"
34 #include "runtime/objectMonitor.hpp"
35 #include "runtime/objectMonitor.inline.hpp"
36 #include "runtime/orderAccess.inline.hpp"
37 #include "runtime/osThread.hpp"
38 #include "runtime/stubRoutines.hpp"
39 #include "runtime/thread.inline.hpp"
40 #include "services/threadService.hpp"
41 #include "trace/tracing.hpp"
42 #include "trace/traceMacros.hpp"
43 #include "utilities/dtrace.hpp"
44 #include "utilities/macros.hpp"
45 #include "utilities/preserveException.hpp"
46
47 #ifdef DTRACE_ENABLED
48
49 // Only bother with this argument setup if dtrace is available
50 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
51
52
53 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
54 char* bytes = NULL; \
55 int len = 0; \
56 jlong jtid = SharedRuntime::get_java_tid(thread); \
57 Symbol* klassname = ((oop)obj)->klass()->name(); \
58 if (klassname != NULL) { \
59 bytes = (char*)klassname->bytes(); \
60 len = klassname->utf8_length(); \
61 }
62
63 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
64 { \
65 if (DTraceMonitorProbes) { \
66 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
67 HOTSPOT_MONITOR_WAIT(jtid, \
68 (monitor), bytes, len, (millis)); \
69 } \
70 }
71
72 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
73 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
74 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
75 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
76 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
77
78 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
79 { \
80 if (DTraceMonitorProbes) { \
81 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
82 HOTSPOT_MONITOR_##probe(jtid, \
83 (uintptr_t)(monitor), bytes, len); \
84 } \
85 }
86
87 #else // ndef DTRACE_ENABLED
88
89 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
90 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
91
92 #endif // ndef DTRACE_ENABLED
93
94 // Tunables ...
95 // The knob* variables are effectively final. Once set they should
96 // never be modified hence. Consider using __read_mostly with GCC.
97
98 int ObjectMonitor::Knob_ExitRelease = 0;
99 int ObjectMonitor::Knob_Verbose = 0;
100 int ObjectMonitor::Knob_VerifyInUse = 0;
101 int ObjectMonitor::Knob_VerifyMatch = 0;
102 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool -
103 static int Knob_LogSpins = 0; // enable jvmstat tally for spins
104 static int Knob_HandOff = 0;
105 static int Knob_ReportSettings = 0;
106
107 static int Knob_SpinBase = 0; // Floor AKA SpinMin
108 static int Knob_SpinBackOff = 0; // spin-loop backoff
109 static int Knob_CASPenalty = -1; // Penalty for failed CAS
110 static int Knob_OXPenalty = -1; // Penalty for observed _owner change
111 static int Knob_SpinSetSucc = 1; // spinners set the _succ field
112 static int Knob_SpinEarly = 1;
113 static int Knob_SuccEnabled = 1; // futile wake throttling
114 static int Knob_SuccRestrict = 0; // Limit successors + spinners to at-most-one
115 static int Knob_MaxSpinners = -1; // Should be a function of # CPUs
116 static int Knob_Bonus = 100; // spin success bonus
117 static int Knob_BonusB = 100; // spin success bonus
118 static int Knob_Penalty = 200; // spin failure penalty
119 static int Knob_Poverty = 1000;
120 static int Knob_SpinAfterFutile = 1; // Spin after returning from park()
121 static int Knob_FixedSpin = 0;
122 static int Knob_OState = 3; // Spinner checks thread state of _owner
123 static int Knob_UsePause = 1;
124 static int Knob_ExitPolicy = 0;
125 static int Knob_PreSpin = 10; // 20-100 likely better
126 static int Knob_ResetEvent = 0;
127 static int BackOffMask = 0;
128
129 static int Knob_FastHSSEC = 0;
130 static int Knob_MoveNotifyee = 2; // notify() - disposition of notifyee
131 static int Knob_QMode = 0; // EntryList-cxq policy - queue discipline
132 static volatile int InitDone = 0;
133
134 // -----------------------------------------------------------------------------
135 // Theory of operations -- Monitors lists, thread residency, etc:
136 //
137 // * A thread acquires ownership of a monitor by successfully
138 // CAS()ing the _owner field from null to non-null.
139 //
140 // * Invariant: A thread appears on at most one monitor list --
141 // cxq, EntryList or WaitSet -- at any one time.
142 //
143 // * Contending threads "push" themselves onto the cxq with CAS
144 // and then spin/park.
145 //
146 // * After a contending thread eventually acquires the lock it must
147 // dequeue itself from either the EntryList or the cxq.
148 //
149 // * The exiting thread identifies and unparks an "heir presumptive"
150 // tentative successor thread on the EntryList. Critically, the
151 // exiting thread doesn't unlink the successor thread from the EntryList.
152 // After having been unparked, the wakee will recontend for ownership of
153 // the monitor. The successor (wakee) will either acquire the lock or
154 // re-park itself.
155 //
156 // Succession is provided for by a policy of competitive handoff.
157 // The exiting thread does _not_ grant or pass ownership to the
158 // successor thread. (This is also referred to as "handoff" succession").
159 // Instead the exiting thread releases ownership and possibly wakes
160 // a successor, so the successor can (re)compete for ownership of the lock.
161 // If the EntryList is empty but the cxq is populated the exiting
162 // thread will drain the cxq into the EntryList. It does so by
163 // by detaching the cxq (installing null with CAS) and folding
164 // the threads from the cxq into the EntryList. The EntryList is
165 // doubly linked, while the cxq is singly linked because of the
166 // CAS-based "push" used to enqueue recently arrived threads (RATs).
167 //
168 // * Concurrency invariants:
169 //
170 // -- only the monitor owner may access or mutate the EntryList.
171 // The mutex property of the monitor itself protects the EntryList
172 // from concurrent interference.
173 // -- Only the monitor owner may detach the cxq.
174 //
175 // * The monitor entry list operations avoid locks, but strictly speaking
176 // they're not lock-free. Enter is lock-free, exit is not.
177 // For a description of 'Methods and apparatus providing non-blocking access
178 // to a resource,' see U.S. Pat. No. 7844973.
179 //
180 // * The cxq can have multiple concurrent "pushers" but only one concurrent
181 // detaching thread. This mechanism is immune from the ABA corruption.
182 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
183 //
184 // * Taken together, the cxq and the EntryList constitute or form a
185 // single logical queue of threads stalled trying to acquire the lock.
186 // We use two distinct lists to improve the odds of a constant-time
187 // dequeue operation after acquisition (in the ::enter() epilogue) and
188 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
189 // A key desideratum is to minimize queue & monitor metadata manipulation
190 // that occurs while holding the monitor lock -- that is, we want to
191 // minimize monitor lock holds times. Note that even a small amount of
192 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
193 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
194 // locks and monitor metadata.
195 //
196 // Cxq points to the set of Recently Arrived Threads attempting entry.
197 // Because we push threads onto _cxq with CAS, the RATs must take the form of
198 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
199 // the unlocking thread notices that EntryList is null but _cxq is != null.
200 //
201 // The EntryList is ordered by the prevailing queue discipline and
202 // can be organized in any convenient fashion, such as a doubly-linked list or
203 // a circular doubly-linked list. Critically, we want insert and delete operations
204 // to operate in constant-time. If we need a priority queue then something akin
205 // to Solaris' sleepq would work nicely. Viz.,
206 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
207 // Queue discipline is enforced at ::exit() time, when the unlocking thread
208 // drains the cxq into the EntryList, and orders or reorders the threads on the
209 // EntryList accordingly.
210 //
211 // Barring "lock barging", this mechanism provides fair cyclic ordering,
212 // somewhat similar to an elevator-scan.
213 //
214 // * The monitor synchronization subsystem avoids the use of native
215 // synchronization primitives except for the narrow platform-specific
216 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
217 // the semantics of park-unpark. Put another way, this monitor implementation
218 // depends only on atomic operations and park-unpark. The monitor subsystem
219 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
220 // underlying OS manages the READY<->RUN transitions.
221 //
222 // * Waiting threads reside on the WaitSet list -- wait() puts
223 // the caller onto the WaitSet.
224 //
225 // * notify() or notifyAll() simply transfers threads from the WaitSet to
226 // either the EntryList or cxq. Subsequent exit() operations will
227 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
228 // it's likely the notifyee would simply impale itself on the lock held
229 // by the notifier.
230 //
231 // * An interesting alternative is to encode cxq as (List,LockByte) where
232 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
233 // variable, like _recursions, in the scheme. The threads or Events that form
234 // the list would have to be aligned in 256-byte addresses. A thread would
235 // try to acquire the lock or enqueue itself with CAS, but exiting threads
236 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
237 // Note that is is *not* word-tearing, but it does presume that full-word
238 // CAS operations are coherent with intermix with STB operations. That's true
239 // on most common processors.
240 //
241 // * See also http://blogs.sun.com/dave
242
243 #define DEFLATER_MARKER reinterpret_cast<void*>(-1)
244
245 // -----------------------------------------------------------------------------
246 // Enter support
247
248 bool ObjectMonitor::enter(TRAPS) {
249 // The following code is ordered to check the most common cases first
250 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
251 Thread * const Self = THREAD;
252
253 void * cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL);
254 if (cur == NULL) {
255 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
256 assert(_recursions == 0, "invariant");
257 assert(_owner == Self, "invariant");
258 return true;
259 }
260
261 if (cur == Self) {
262 // TODO-FIXME: check for integer overflow! BUGID 6557169.
263 _recursions++;
264 return true;
265 }
266
267 if (Self->is_lock_owned ((address)cur)) {
268 assert(_recursions == 0, "internal state error");
269 _recursions = 1;
270 // Commute owner from a thread-specific on-stack BasicLockObject address to
271 // a full-fledged "Thread *".
272 _owner = Self;
273 return true;
274 }
275
276 // We've encountered genuine contention.
277 assert(Self->_Stalled == 0, "invariant");
278 Self->_Stalled = intptr_t(this);
279
280 // Try one round of spinning *before* enqueueing Self
281 // and before going through the awkward and expensive state
282 // transitions. The following spin is strictly optional ...
283 // Note that if we acquire the monitor from an initial spin
284 // we forgo posting JVMTI events and firing DTRACE probes.
285 if (Knob_SpinEarly && TrySpin (Self) > 0) {
286 assert(_owner == Self, "invariant");
287 assert(_recursions == 0, "invariant");
288 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
289 Self->_Stalled = 0;
290 return true;
291 }
292
293 assert(_owner != Self, "invariant");
294 assert(_succ != Self, "invariant");
295 assert(Self->is_Java_thread(), "invariant");
296 JavaThread * jt = (JavaThread *) Self;
297 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
298 assert(jt->thread_state() != _thread_blocked, "invariant");
299 assert(this->object() != NULL, "invariant");
300
301 // Prevent deflation. See deflate_idle_monitors(), try_disable_monitor, and is_busy().
302 // Ensure the object-monitor relationship remains stable while there's contention.
303 const jint count = Atomic::add(1, &_count);
304 if (count <= 0 && _owner == DEFLATER_MARKER) {
305 // Deflation in progress.
306 // Help deflater thread install the mark word (in case deflater thread is slow).
307 install_displaced_markword_in_object();
308 Self->_Stalled = 0;
309 return false; // Caller should retry. Never mind about _count as this monitor has been deflated.
310 }
311 // The deflater thread will not deflate this monitor and the monitor is contended, continue.
312
313 EventJavaMonitorEnter event;
314
315 { // Change java thread status to indicate blocked on monitor enter.
316 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
317
318 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
319 if (JvmtiExport::should_post_monitor_contended_enter()) {
320 JvmtiExport::post_monitor_contended_enter(jt, this);
321
322 // The current thread does not yet own the monitor and does not
323 // yet appear on any queues that would get it made the successor.
324 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
325 // handler cannot accidentally consume an unpark() meant for the
326 // ParkEvent associated with this ObjectMonitor.
327 }
328
329 OSThreadContendState osts(Self->osthread());
330 ThreadBlockInVM tbivm(jt);
331
332 Self->set_current_pending_monitor(this);
333
334 // TODO-FIXME: change the following for(;;) loop to straight-line code.
335 for (;;) {
336 jt->set_suspend_equivalent();
337 // cleared by handle_special_suspend_equivalent_condition()
338 // or java_suspend_self()
339
340 EnterI(THREAD);
341
342 if (!ExitSuspendEquivalent(jt)) break;
343
344 // We have acquired the contended monitor, but while we were
345 // waiting another thread suspended us. We don't want to enter
346 // the monitor while suspended because that would surprise the
347 // thread that suspended us.
348 //
349 _recursions = 0;
350 _succ = NULL;
351 exit(false, Self);
352
353 jt->java_suspend_self();
354 }
355 Self->set_current_pending_monitor(NULL);
356
357 // We cleared the pending monitor info since we've just gotten past
358 // the enter-check-for-suspend dance and we now own the monitor free
359 // and clear, i.e., it is no longer pending. The ThreadBlockInVM
360 // destructor can go to a safepoint at the end of this block. If we
361 // do a thread dump during that safepoint, then this thread will show
362 // as having "-locked" the monitor, but the OS and java.lang.Thread
363 // states will still report that the thread is blocked trying to
364 // acquire it.
365 }
366
367 Atomic::dec(&_count);
368 Self->_Stalled = 0;
369
370 // Must either set _recursions = 0 or ASSERT _recursions == 0.
371 assert(_recursions == 0, "invariant");
372 assert(_owner == Self, "invariant");
373 assert(_succ != Self, "invariant");
374 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
375
376 // The thread -- now the owner -- is back in vm mode.
377 // Report the glorious news via TI,DTrace and jvmstat.
378 // The probe effect is non-trivial. All the reportage occurs
379 // while we hold the monitor, increasing the length of the critical
380 // section. Amdahl's parallel speedup law comes vividly into play.
381 //
382 // Another option might be to aggregate the events (thread local or
383 // per-monitor aggregation) and defer reporting until a more opportune
384 // time -- such as next time some thread encounters contention but has
385 // yet to acquire the lock. While spinning that thread could
386 // spinning we could increment JVMStat counters, etc.
387
388 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
389 if (JvmtiExport::should_post_monitor_contended_entered()) {
390 JvmtiExport::post_monitor_contended_entered(jt, this);
391
392 // The current thread already owns the monitor and is not going to
393 // call park() for the remainder of the monitor enter protocol. So
394 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
395 // event handler consumed an unpark() issued by the thread that
396 // just exited the monitor.
397 }
398
399 if (event.should_commit()) {
400 event.set_monitorClass(((oop)this->object())->klass());
401 event.set_previousOwner((TYPE_THREAD)_previous_owner_tid);
402 event.set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
403 event.commit();
404 }
405
406 OM_PERFDATA_OP(ContendedLockAttempts, inc());
407 return true;
408 }
409
410
411 // Caveat: TryLock() is not necessarily serializing if it returns failure.
412 // Callers must compensate as needed.
413
414 int ObjectMonitor::TryLock(Thread * Self) {
415 void * own = _owner;
416 if (own != NULL) return 0;
417 if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
418 // Either guarantee _recursions == 0 or set _recursions = 0.
419 assert(_recursions == 0, "invariant");
420 assert(_owner == Self, "invariant");
421 return 1;
422 }
423 // The lock had been free momentarily, but we lost the race to the lock.
424 // Interference -- the CAS failed.
425 // We can either return -1 or retry.
426 // Retry doesn't make as much sense because the lock was just acquired.
427 return -1;
428 }
429
430 // Try disabling this monitor. Returns true iff successful.
431 // The method will install DEFLATER_MARKER (-1) as the owner of the monitor,
432 // check _waiters == 0, and make _count negative if it is currently 0. The
433 // monitor is successfully disabled if _count is negative and this monitor is
434 // still owned by DEFLATER_MARKER.
435 //
436 // All threads trying to acquire the monitor must before parking themselves
437 // increment _count and check that _owner != DEFLATER_MARKER. If _owner ==
438 // DEFLATER_MARKER and _count is positive, then a thread can still win the lock
439 // by atomically installing its thread pointer in _owner. If _count is
440 // negative and _owned == DEFLATER_MARKER, then the monitor has been
441 // successfully disabled and the acquiring threads should help install the
442 // displaced mark word back into the object and retry acquiring the lock.
443 //
444 // A thread wanting to wait on the monitor must increase _waiters while owning the monitor.
445 bool ObjectMonitor::try_disable_monitor() {
446 assert(Thread::current()->is_Java_thread(), "precondition");
447 // We don't want to disable newly allocated monitors as it could result in a endless inflate/deflate cycle.
448 assert(is_old(), "precondition");
449
450 // Set _owned to DEFLATER_MARKER if monitor is not owned by another thread.
451 // This forced contending thread through the slow path.
452 if (!is_busy() && Atomic::cmpxchg_ptr(DEFLATER_MARKER, &_owner, NULL) == NULL) {
453 // Another thread might still enter the monitor.
454 // Signal that other threads should retry if the owner is DEFLATER_MARKER by making _count negative.
455 if (_waiters == 0 && Atomic::cmpxchg(- max_jint, &_count, 0) == 0) {
456 // ABA problem with _count:
457 // Another thread might have acquired this monitor and finished using it.
458 // Check owner to see if that happened (no other thread installs DEFLATER_MARKER as owner).
459 if (_owner == DEFLATER_MARKER) {
460 // We successfully signalled to all threads entering that they should
461 // retry.
462 // Nobody acquired this monitor between installing DEFLATER_MARKER into
463 // _owner and now (such a thread would have changed _owner). If any
464 // thread is now waiting on the monitor, then _waiters must have been
465 // incremented as it was 0 before. _waiters is changed only when owning
466 // the monitor, but no other thread can have owned the monitor since we
467 // installed DEFLATER_MARKER, and thus _waiters must still be 0.
468 guarantee(_waiters == 0, "Not changed since the previous read");
469 guarantee(_cxq == NULL, "All contending threads should retry");
470 guarantee(_EntryList == NULL, "All contending threads should retry");
471 // Install the old mark word if nobody else has already done it.
472 install_displaced_markword_in_object();
473 set_allocation_state(Free);
474 // Leave this monitor locked to ensure acquiring threads take the slow-path and
475 // leave _count negative to make them retry.
476 return true; // Success, lock has been deflated.
477 }
478 // We do not own the monitor. Do not deflate.
479 Atomic::add(max_jint, &_count);
480 }
481 // Never mind. Another thread managed to acquire this monitor or there were
482 // threads waiting. The threads that saw (or will see) 0 <= _count and
483 // _owner == DEFLATER_MARKER will compete for ownership and one will
484 // eventually install itself as owner and subsequently run the exit protocol.
485 }
486 assert(0 <= _count, "Nobody else should be make _count negative");
487 return false;
488 }
489
490 // Install the displaced mark word of a disabled monitor into the object
491 // associated with the monitor.
492 // This method is idempotent and is expected to be executed by both mutators
493 // wanting to acquire a monitor for an object, mutators wanting to install a
494 // hashcode in an object, and the thread deflating monitors.
495 void ObjectMonitor::install_displaced_markword_in_object() {
496 markOop dmw = header();
497 assert(dmw->is_neutral() || dmw->hash() == 0 && dmw->is_marked(), "precondition");
498 if (dmw->hash() == 0 && !dmw->is_marked()) {
499 // Another thread might update the displaced mark word by computing a hash
500 // code for the oject or running this method.
501 // Signal to other threads that the object mark word should be installed
502 // and read from the object by marking the displaced mark word.
503 markOop marked_dmw = dmw->set_marked();
504 assert(marked_dmw->hash() == 0 && marked_dmw->is_marked(), "oops");
505 dmw = (markOop) Atomic::cmpxchg_ptr(marked_dmw, &_header, dmw);
506 // If the CAS failed because another thread installed a hash value, then dmw
507 // will contain the hash and be unmarked. If the CAS failed because another thread
508 // marked the displaced mark word, then dmw->hash() is zero and dmw is marked.
509 }
510 if (dmw->is_marked()) {
511 assert(dmw->hash() == 0, "invariant");
512 dmw = dmw->set_unmarked();
513 }
514 oop const obj = (oop) object();
515 // Install displaced mark word if object mark word still points to this monitor.
516 assert(dmw->is_neutral(), "Must not install non-neutral markword into object");
517 obj->cas_set_mark(dmw, markOopDesc::encode(this));
518 }
519
520 #define MAX_RECHECK_INTERVAL 1000
521
522 void ObjectMonitor::EnterI(TRAPS) {
523 Thread * const Self = THREAD;
524 assert(Self->is_Java_thread(), "invariant");
525 assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant");
526
527 // Try the lock - TATAS
528 if (TryLock (Self) > 0) {
529 assert(_succ != Self, "invariant");
530 assert(_owner == Self, "invariant");
531 assert(_Responsible != Self, "invariant");
532 return;
533 }
534
535 if (_owner == DEFLATER_MARKER) {
536 guarantee(0 < _count, "_owner == DEFLATER_MARKER && _count <= 0 should have been handled by the caller");
537 // Deflater thread tried to lock this monitor, but it failed to make _count negative and gave up.
538 // Try to acquire monitor.
539 if (Atomic::cmpxchg_ptr(Self, &_owner, DEFLATER_MARKER) == DEFLATER_MARKER) {
540 assert(_succ != Self, "invariant");
541 assert(_owner == Self, "invariant");
542 assert(_Responsible != Self, "invariant");
543 return;
544 }
545 }
546
547 DeferredInitialize();
548
549 // We try one round of spinning *before* enqueueing Self.
550 //
551 // If the _owner is ready but OFFPROC we could use a YieldTo()
552 // operation to donate the remainder of this thread's quantum
553 // to the owner. This has subtle but beneficial affinity
554 // effects.
555
556 if (TrySpin (Self) > 0) {
557 assert(_owner == Self, "invariant");
558 assert(_succ != Self, "invariant");
559 assert(_Responsible != Self, "invariant");
560 return;
561 }
562
563 // The Spin failed -- Enqueue and park the thread ...
564 assert(_succ != Self, "invariant");
565 assert(_owner != Self, "invariant");
566 assert(_Responsible != Self, "invariant");
567
568 // Enqueue "Self" on ObjectMonitor's _cxq.
569 //
570 // Node acts as a proxy for Self.
571 // As an aside, if were to ever rewrite the synchronization code mostly
572 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
573 // Java objects. This would avoid awkward lifecycle and liveness issues,
574 // as well as eliminate a subset of ABA issues.
575 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
576
577 ObjectWaiter node(Self);
578 Self->_ParkEvent->reset();
579 node._prev = (ObjectWaiter *) 0xBAD;
580 node.TState = ObjectWaiter::TS_CXQ;
581
582 // Push "Self" onto the front of the _cxq.
583 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
584 // Note that spinning tends to reduce the rate at which threads
585 // enqueue and dequeue on EntryList|cxq.
586 ObjectWaiter * nxt;
587 for (;;) {
588 node._next = nxt = _cxq;
589 if (Atomic::cmpxchg_ptr(&node, &_cxq, nxt) == nxt) break;
590
591 // Interference - the CAS failed because _cxq changed. Just retry.
592 // As an optional optimization we retry the lock.
593 if (TryLock (Self) > 0) {
594 assert(_succ != Self, "invariant");
595 assert(_owner == Self, "invariant");
596 assert(_Responsible != Self, "invariant");
597 return;
598 }
599 }
600
601 // Check for cxq|EntryList edge transition to non-null. This indicates
602 // the onset of contention. While contention persists exiting threads
603 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
604 // operations revert to the faster 1-0 mode. This enter operation may interleave
605 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
606 // arrange for one of the contending thread to use a timed park() operations
607 // to detect and recover from the race. (Stranding is form of progress failure
608 // where the monitor is unlocked but all the contending threads remain parked).
609 // That is, at least one of the contended threads will periodically poll _owner.
610 // One of the contending threads will become the designated "Responsible" thread.
611 // The Responsible thread uses a timed park instead of a normal indefinite park
612 // operation -- it periodically wakes and checks for and recovers from potential
613 // strandings admitted by 1-0 exit operations. We need at most one Responsible
614 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
615 // be responsible for a monitor.
616 //
617 // Currently, one of the contended threads takes on the added role of "Responsible".
618 // A viable alternative would be to use a dedicated "stranding checker" thread
619 // that periodically iterated over all the threads (or active monitors) and unparked
620 // successors where there was risk of stranding. This would help eliminate the
621 // timer scalability issues we see on some platforms as we'd only have one thread
622 // -- the checker -- parked on a timer.
623
624 if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
625 // Try to assume the role of responsible thread for the monitor.
626 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
627 Atomic::cmpxchg_ptr(Self, &_Responsible, NULL);
628 }
629
630 // The lock might have been released while this thread was occupied queueing
631 // itself onto _cxq. To close the race and avoid "stranding" and
632 // progress-liveness failure we must resample-retry _owner before parking.
633 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
634 // In this case the ST-MEMBAR is accomplished with CAS().
635 //
636 // TODO: Defer all thread state transitions until park-time.
637 // Since state transitions are heavy and inefficient we'd like
638 // to defer the state transitions until absolutely necessary,
639 // and in doing so avoid some transitions ...
640
641 TEVENT(Inflated enter - Contention);
642 int nWakeups = 0;
643 int recheckInterval = 1;
644
645 for (;;) {
646
647 if (TryLock(Self) > 0) break;
648 assert(_owner != Self, "invariant");
649
650 if ((SyncFlags & 2) && _Responsible == NULL) {
651 Atomic::cmpxchg_ptr(Self, &_Responsible, NULL);
652 }
653
654 // park self
655 if (_Responsible == Self || (SyncFlags & 1)) {
656 TEVENT(Inflated enter - park TIMED);
657 Self->_ParkEvent->park((jlong) recheckInterval);
658 // Increase the recheckInterval, but clamp the value.
659 recheckInterval *= 8;
660 if (recheckInterval > MAX_RECHECK_INTERVAL) {
661 recheckInterval = MAX_RECHECK_INTERVAL;
662 }
663 } else {
664 TEVENT(Inflated enter - park UNTIMED);
665 Self->_ParkEvent->park();
666 }
667
668 if (TryLock(Self) > 0) break;
669
670 if (_owner == DEFLATER_MARKER) {
671 guarantee(0 < _count, "_owner == DEFLATER_MARKER && _count <= 0 should have been handled by the caller");
672 // Deflater thread tried to lock this monitor, but it failed to make _count negative and gave up.
673 // Try to acquire monitor.
674 if (Atomic::cmpxchg_ptr(Self, &_owner, DEFLATER_MARKER) == DEFLATER_MARKER) {
675 break;
676 }
677 }
678
679 // The lock is still contested.
680 // Keep a tally of the # of futile wakeups.
681 // Note that the counter is not protected by a lock or updated by atomics.
682 // That is by design - we trade "lossy" counters which are exposed to
683 // races during updates for a lower probe effect.
684 TEVENT(Inflated enter - Futile wakeup);
685 // This PerfData object can be used in parallel with a safepoint.
686 // See the work around in PerfDataManager::destroy().
687 OM_PERFDATA_OP(FutileWakeups, inc());
688 ++nWakeups;
689
690 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
691 // We can defer clearing _succ until after the spin completes
692 // TrySpin() must tolerate being called with _succ == Self.
693 // Try yet another round of adaptive spinning.
694 if ((Knob_SpinAfterFutile & 1) && TrySpin(Self) > 0) break;
695
696 // We can find that we were unpark()ed and redesignated _succ while
697 // we were spinning. That's harmless. If we iterate and call park(),
698 // park() will consume the event and return immediately and we'll
699 // just spin again. This pattern can repeat, leaving _succ to simply
700 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks().
701 // Alternately, we can sample fired() here, and if set, forgo spinning
702 // in the next iteration.
703
704 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
705 Self->_ParkEvent->reset();
706 OrderAccess::fence();
707 }
708 if (_succ == Self) _succ = NULL;
709
710 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
711 OrderAccess::fence();
712 }
713
714 // Egress :
715 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
716 // Normally we'll find Self on the EntryList .
717 // From the perspective of the lock owner (this thread), the
718 // EntryList is stable and cxq is prepend-only.
719 // The head of cxq is volatile but the interior is stable.
720 // In addition, Self.TState is stable.
721
722 assert(_owner == Self, "invariant");
723 assert(object() != NULL, "invariant");
724 // I'd like to write:
725 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
726 // but as we're at a safepoint that's not safe.
727
728 UnlinkAfterAcquire(Self, &node);
729 if (_succ == Self) _succ = NULL;
730
731 assert(_succ != Self, "invariant");
732 if (_Responsible == Self) {
733 _Responsible = NULL;
734 OrderAccess::fence(); // Dekker pivot-point
735
736 // We may leave threads on cxq|EntryList without a designated
737 // "Responsible" thread. This is benign. When this thread subsequently
738 // exits the monitor it can "see" such preexisting "old" threads --
739 // threads that arrived on the cxq|EntryList before the fence, above --
740 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
741 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
742 // non-null and elect a new "Responsible" timer thread.
743 //
744 // This thread executes:
745 // ST Responsible=null; MEMBAR (in enter epilogue - here)
746 // LD cxq|EntryList (in subsequent exit)
747 //
748 // Entering threads in the slow/contended path execute:
749 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
750 // The (ST cxq; MEMBAR) is accomplished with CAS().
751 //
752 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
753 // exit operation from floating above the ST Responsible=null.
754 }
755
756 // We've acquired ownership with CAS().
757 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
758 // But since the CAS() this thread may have also stored into _succ,
759 // EntryList, cxq or Responsible. These meta-data updates must be
760 // visible __before this thread subsequently drops the lock.
761 // Consider what could occur if we didn't enforce this constraint --
762 // STs to monitor meta-data and user-data could reorder with (become
763 // visible after) the ST in exit that drops ownership of the lock.
764 // Some other thread could then acquire the lock, but observe inconsistent
765 // or old monitor meta-data and heap data. That violates the JMM.
766 // To that end, the 1-0 exit() operation must have at least STST|LDST
767 // "release" barrier semantics. Specifically, there must be at least a
768 // STST|LDST barrier in exit() before the ST of null into _owner that drops
769 // the lock. The barrier ensures that changes to monitor meta-data and data
770 // protected by the lock will be visible before we release the lock, and
771 // therefore before some other thread (CPU) has a chance to acquire the lock.
772 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
773 //
774 // Critically, any prior STs to _succ or EntryList must be visible before
775 // the ST of null into _owner in the *subsequent* (following) corresponding
776 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
777 // execute a serializing instruction.
778
779 if (SyncFlags & 8) {
780 OrderAccess::fence();
781 }
782 return;
783 }
784
785 // ReenterI() is a specialized inline form of the latter half of the
786 // contended slow-path from EnterI(). We use ReenterI() only for
787 // monitor reentry in wait().
788 //
789 // In the future we should reconcile EnterI() and ReenterI(), adding
790 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
791 // loop accordingly.
792
793 void ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) {
794 assert(Self != NULL, "invariant");
795 assert(SelfNode != NULL, "invariant");
796 assert(SelfNode->_thread == Self, "invariant");
797 assert(_waiters > 0, "invariant");
798 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
799 assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
800 JavaThread * jt = (JavaThread *) Self;
801
802 int nWakeups = 0;
803 for (;;) {
804 ObjectWaiter::TStates v = SelfNode->TState;
805 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
806 assert(_owner != Self, "invariant");
807
808 if (TryLock(Self) > 0) break;
809 if (TrySpin(Self) > 0) break;
810
811 if (_owner == DEFLATER_MARKER) {
812 guarantee(0 <= _count, "Impossible: _owner == DEFLATER_MARKER && _count < 0, monitor must not be owned by deflater thread here");
813 if (Atomic::cmpxchg_ptr(Self, &_owner, DEFLATER_MARKER) == DEFLATER_MARKER) {
814 break;
815 }
816 }
817
818 TEVENT(Wait Reentry - parking);
819
820 // State transition wrappers around park() ...
821 // ReenterI() wisely defers state transitions until
822 // it's clear we must park the thread.
823 {
824 OSThreadContendState osts(Self->osthread());
825 ThreadBlockInVM tbivm(jt);
826
827 // cleared by handle_special_suspend_equivalent_condition()
828 // or java_suspend_self()
829 jt->set_suspend_equivalent();
830 if (SyncFlags & 1) {
831 Self->_ParkEvent->park((jlong)MAX_RECHECK_INTERVAL);
832 } else {
833 Self->_ParkEvent->park();
834 }
835
836 // were we externally suspended while we were waiting?
837 for (;;) {
838 if (!ExitSuspendEquivalent(jt)) break;
839 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
840 jt->java_suspend_self();
841 jt->set_suspend_equivalent();
842 }
843 }
844
845 // Try again, but just so we distinguish between futile wakeups and
846 // successful wakeups. The following test isn't algorithmically
847 // necessary, but it helps us maintain sensible statistics.
848 if (TryLock(Self) > 0) break;
849
850 // The lock is still contested.
851 // Keep a tally of the # of futile wakeups.
852 // Note that the counter is not protected by a lock or updated by atomics.
853 // That is by design - we trade "lossy" counters which are exposed to
854 // races during updates for a lower probe effect.
855 TEVENT(Wait Reentry - futile wakeup);
856 ++nWakeups;
857
858 // Assuming this is not a spurious wakeup we'll normally
859 // find that _succ == Self.
860 if (_succ == Self) _succ = NULL;
861
862 // Invariant: after clearing _succ a contending thread
863 // *must* retry _owner before parking.
864 OrderAccess::fence();
865
866 // This PerfData object can be used in parallel with a safepoint.
867 // See the work around in PerfDataManager::destroy().
868 OM_PERFDATA_OP(FutileWakeups, inc());
869 }
870
871 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
872 // Normally we'll find Self on the EntryList.
873 // Unlinking from the EntryList is constant-time and atomic-free.
874 // From the perspective of the lock owner (this thread), the
875 // EntryList is stable and cxq is prepend-only.
876 // The head of cxq is volatile but the interior is stable.
877 // In addition, Self.TState is stable.
878
879 assert(_owner == Self, "invariant");
880 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
881 UnlinkAfterAcquire(Self, SelfNode);
882 if (_succ == Self) _succ = NULL;
883 assert(_succ != Self, "invariant");
884 SelfNode->TState = ObjectWaiter::TS_RUN;
885 OrderAccess::fence(); // see comments at the end of EnterI()
886 }
887
888 // By convention we unlink a contending thread from EntryList|cxq immediately
889 // after the thread acquires the lock in ::enter(). Equally, we could defer
890 // unlinking the thread until ::exit()-time.
891
892 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) {
893 assert(_owner == Self, "invariant");
894 assert(SelfNode->_thread == Self, "invariant");
895
896 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
897 // Normal case: remove Self from the DLL EntryList .
898 // This is a constant-time operation.
899 ObjectWaiter * nxt = SelfNode->_next;
900 ObjectWaiter * prv = SelfNode->_prev;
901 if (nxt != NULL) nxt->_prev = prv;
902 if (prv != NULL) prv->_next = nxt;
903 if (SelfNode == _EntryList) _EntryList = nxt;
904 assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
905 assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
906 TEVENT(Unlink from EntryList);
907 } else {
908 assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant");
909 // Inopportune interleaving -- Self is still on the cxq.
910 // This usually means the enqueue of self raced an exiting thread.
911 // Normally we'll find Self near the front of the cxq, so
912 // dequeueing is typically fast. If needbe we can accelerate
913 // this with some MCS/CHL-like bidirectional list hints and advisory
914 // back-links so dequeueing from the interior will normally operate
915 // in constant-time.
916 // Dequeue Self from either the head (with CAS) or from the interior
917 // with a linear-time scan and normal non-atomic memory operations.
918 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
919 // and then unlink Self from EntryList. We have to drain eventually,
920 // so it might as well be now.
921
922 ObjectWaiter * v = _cxq;
923 assert(v != NULL, "invariant");
924 if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) {
925 // The CAS above can fail from interference IFF a "RAT" arrived.
926 // In that case Self must be in the interior and can no longer be
927 // at the head of cxq.
928 if (v == SelfNode) {
929 assert(_cxq != v, "invariant");
930 v = _cxq; // CAS above failed - start scan at head of list
931 }
932 ObjectWaiter * p;
933 ObjectWaiter * q = NULL;
934 for (p = v; p != NULL && p != SelfNode; p = p->_next) {
935 q = p;
936 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
937 }
938 assert(v != SelfNode, "invariant");
939 assert(p == SelfNode, "Node not found on cxq");
940 assert(p != _cxq, "invariant");
941 assert(q != NULL, "invariant");
942 assert(q->_next == p, "invariant");
943 q->_next = p->_next;
944 }
945 TEVENT(Unlink from cxq);
946 }
947
948 #ifdef ASSERT
949 // Diagnostic hygiene ...
950 SelfNode->_prev = (ObjectWaiter *) 0xBAD;
951 SelfNode->_next = (ObjectWaiter *) 0xBAD;
952 SelfNode->TState = ObjectWaiter::TS_RUN;
953 #endif
954 }
955
956 // -----------------------------------------------------------------------------
957 // Exit support
958 //
959 // exit()
960 // ~~~~~~
961 // Note that the collector can't reclaim the objectMonitor or deflate
962 // the object out from underneath the thread calling ::exit() as the
963 // thread calling ::exit() never transitions to a stable state.
964 // This inhibits GC, which in turn inhibits asynchronous (and
965 // inopportune) reclamation of "this".
966 //
967 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
968 // There's one exception to the claim above, however. EnterI() can call
969 // exit() to drop a lock if the acquirer has been externally suspended.
970 // In that case exit() is called with _thread_state as _thread_blocked,
971 // but the monitor's _count field is > 0, which inhibits reclamation.
972 //
973 // 1-0 exit
974 // ~~~~~~~~
975 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
976 // the fast-path operators have been optimized so the common ::exit()
977 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
978 // The code emitted by fast_unlock() elides the usual MEMBAR. This
979 // greatly improves latency -- MEMBAR and CAS having considerable local
980 // latency on modern processors -- but at the cost of "stranding". Absent the
981 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
982 // ::enter() path, resulting in the entering thread being stranding
983 // and a progress-liveness failure. Stranding is extremely rare.
984 // We use timers (timed park operations) & periodic polling to detect
985 // and recover from stranding. Potentially stranded threads periodically
986 // wake up and poll the lock. See the usage of the _Responsible variable.
987 //
988 // The CAS() in enter provides for safety and exclusion, while the CAS or
989 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
990 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
991 // We detect and recover from stranding with timers.
992 //
993 // If a thread transiently strands it'll park until (a) another
994 // thread acquires the lock and then drops the lock, at which time the
995 // exiting thread will notice and unpark the stranded thread, or, (b)
996 // the timer expires. If the lock is high traffic then the stranding latency
997 // will be low due to (a). If the lock is low traffic then the odds of
998 // stranding are lower, although the worst-case stranding latency
999 // is longer. Critically, we don't want to put excessive load in the
1000 // platform's timer subsystem. We want to minimize both the timer injection
1001 // rate (timers created/sec) as well as the number of timers active at
1002 // any one time. (more precisely, we want to minimize timer-seconds, which is
1003 // the integral of the # of active timers at any instant over time).
1004 // Both impinge on OS scalability. Given that, at most one thread parked on
1005 // a monitor will use a timer.
1006 //
1007 // There is also the risk of a futile wake-up. If we drop the lock
1008 // another thread can reacquire the lock immediately, and we can
1009 // then wake a thread unnecessarily. This is benign, and we've
1010 // structured the code so the windows are short and the frequency
1011 // of such futile wakups is low.
1012
1013 void ObjectMonitor::exit(bool not_suspended, TRAPS) {
1014 Thread * const Self = THREAD;
1015 if (THREAD != _owner) {
1016 if (THREAD->is_lock_owned((address) _owner)) {
1017 // Transmute _owner from a BasicLock pointer to a Thread address.
1018 // We don't need to hold _mutex for this transition.
1019 // Non-null to Non-null is safe as long as all readers can
1020 // tolerate either flavor.
1021 assert(_recursions == 0, "invariant");
1022 _owner = THREAD;
1023 _recursions = 0;
1024 } else {
1025 // Apparent unbalanced locking ...
1026 // Naively we'd like to throw IllegalMonitorStateException.
1027 // As a practical matter we can neither allocate nor throw an
1028 // exception as ::exit() can be called from leaf routines.
1029 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
1030 // Upon deeper reflection, however, in a properly run JVM the only
1031 // way we should encounter this situation is in the presence of
1032 // unbalanced JNI locking. TODO: CheckJNICalls.
1033 // See also: CR4414101
1034 TEVENT(Exit - Throw IMSX);
1035 assert(false, "Non-balanced monitor enter/exit! Likely JNI locking");
1036 return;
1037 }
1038 }
1039
1040 if (_recursions != 0) {
1041 _recursions--; // this is simple recursive enter
1042 TEVENT(Inflated exit - recursive);
1043 return;
1044 }
1045
1046 // Invariant: after setting Responsible=null an thread must execute
1047 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
1048 if ((SyncFlags & 4) == 0) {
1049 _Responsible = NULL;
1050 }
1051
1052 #if INCLUDE_TRACE
1053 // get the owner's thread id for the MonitorEnter event
1054 // if it is enabled and the thread isn't suspended
1055 if (not_suspended && Tracing::is_event_enabled(TraceJavaMonitorEnterEvent)) {
1056 _previous_owner_tid = THREAD_TRACE_ID(Self);
1057 }
1058 #endif
1059
1060 for (;;) {
1061 assert(THREAD == _owner, "invariant");
1062
1063 if (Knob_ExitPolicy == 0) {
1064 // release semantics: prior loads and stores from within the critical section
1065 // must not float (reorder) past the following store that drops the lock.
1066 // On SPARC that requires MEMBAR #loadstore|#storestore.
1067 // But of course in TSO #loadstore|#storestore is not required.
1068 // I'd like to write one of the following:
1069 // A. OrderAccess::release() ; _owner = NULL
1070 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
1071 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
1072 // store into a _dummy variable. That store is not needed, but can result
1073 // in massive wasteful coherency traffic on classic SMP systems.
1074 // Instead, I use release_store(), which is implemented as just a simple
1075 // ST on x64, x86 and SPARC.
1076 OrderAccess::release_store_ptr(&_owner, NULL); // drop the lock
1077 OrderAccess::storeload(); // See if we need to wake a successor
1078 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1079 TEVENT(Inflated exit - simple egress);
1080 return;
1081 }
1082 TEVENT(Inflated exit - complex egress);
1083 // Other threads are blocked trying to acquire the lock.
1084
1085 // Normally the exiting thread is responsible for ensuring succession,
1086 // but if other successors are ready or other entering threads are spinning
1087 // then this thread can simply store NULL into _owner and exit without
1088 // waking a successor. The existence of spinners or ready successors
1089 // guarantees proper succession (liveness). Responsibility passes to the
1090 // ready or running successors. The exiting thread delegates the duty.
1091 // More precisely, if a successor already exists this thread is absolved
1092 // of the responsibility of waking (unparking) one.
1093 //
1094 // The _succ variable is critical to reducing futile wakeup frequency.
1095 // _succ identifies the "heir presumptive" thread that has been made
1096 // ready (unparked) but that has not yet run. We need only one such
1097 // successor thread to guarantee progress.
1098 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1099 // section 3.3 "Futile Wakeup Throttling" for details.
1100 //
1101 // Note that spinners in Enter() also set _succ non-null.
1102 // In the current implementation spinners opportunistically set
1103 // _succ so that exiting threads might avoid waking a successor.
1104 // Another less appealing alternative would be for the exiting thread
1105 // to drop the lock and then spin briefly to see if a spinner managed
1106 // to acquire the lock. If so, the exiting thread could exit
1107 // immediately without waking a successor, otherwise the exiting
1108 // thread would need to dequeue and wake a successor.
1109 // (Note that we'd need to make the post-drop spin short, but no
1110 // shorter than the worst-case round-trip cache-line migration time.
1111 // The dropped lock needs to become visible to the spinner, and then
1112 // the acquisition of the lock by the spinner must become visible to
1113 // the exiting thread).
1114
1115 // It appears that an heir-presumptive (successor) must be made ready.
1116 // Only the current lock owner can manipulate the EntryList or
1117 // drain _cxq, so we need to reacquire the lock. If we fail
1118 // to reacquire the lock the responsibility for ensuring succession
1119 // falls to the new owner.
1120 //
1121 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1122 return;
1123 }
1124 TEVENT(Exit - Reacquired);
1125 } else {
1126 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1127 OrderAccess::release_store_ptr(&_owner, NULL); // drop the lock
1128 OrderAccess::storeload();
1129 // Ratify the previously observed values.
1130 if (_cxq == NULL || _succ != NULL) {
1131 TEVENT(Inflated exit - simple egress);
1132 return;
1133 }
1134
1135 // inopportune interleaving -- the exiting thread (this thread)
1136 // in the fast-exit path raced an entering thread in the slow-enter
1137 // path.
1138 // We have two choices:
1139 // A. Try to reacquire the lock.
1140 // If the CAS() fails return immediately, otherwise
1141 // we either restart/rerun the exit operation, or simply
1142 // fall-through into the code below which wakes a successor.
1143 // B. If the elements forming the EntryList|cxq are TSM
1144 // we could simply unpark() the lead thread and return
1145 // without having set _succ.
1146 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
1147 TEVENT(Inflated exit - reacquired succeeded);
1148 return;
1149 }
1150 TEVENT(Inflated exit - reacquired failed);
1151 } else {
1152 TEVENT(Inflated exit - complex egress);
1153 }
1154 }
1155
1156 guarantee(_owner == THREAD, "invariant");
1157
1158 ObjectWaiter * w = NULL;
1159 int QMode = Knob_QMode;
1160
1161 if (QMode == 2 && _cxq != NULL) {
1162 // QMode == 2 : cxq has precedence over EntryList.
1163 // Try to directly wake a successor from the cxq.
1164 // If successful, the successor will need to unlink itself from cxq.
1165 w = _cxq;
1166 assert(w != NULL, "invariant");
1167 assert(w->TState == ObjectWaiter::TS_CXQ, "Invariant");
1168 ExitEpilog(Self, w);
1169 return;
1170 }
1171
1172 if (QMode == 3 && _cxq != NULL) {
1173 // Aggressively drain cxq into EntryList at the first opportunity.
1174 // This policy ensure that recently-run threads live at the head of EntryList.
1175 // Drain _cxq into EntryList - bulk transfer.
1176 // First, detach _cxq.
1177 // The following loop is tantamount to: w = swap(&cxq, NULL)
1178 w = _cxq;
1179 for (;;) {
1180 assert(w != NULL, "Invariant");
1181 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr(NULL, &_cxq, w);
1182 if (u == w) break;
1183 w = u;
1184 }
1185 assert(w != NULL, "invariant");
1186
1187 ObjectWaiter * q = NULL;
1188 ObjectWaiter * p;
1189 for (p = w; p != NULL; p = p->_next) {
1190 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1191 p->TState = ObjectWaiter::TS_ENTER;
1192 p->_prev = q;
1193 q = p;
1194 }
1195
1196 // Append the RATs to the EntryList
1197 // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
1198 ObjectWaiter * Tail;
1199 for (Tail = _EntryList; Tail != NULL && Tail->_next != NULL;
1200 Tail = Tail->_next)
1201 /* empty */;
1202 if (Tail == NULL) {
1203 _EntryList = w;
1204 } else {
1205 Tail->_next = w;
1206 w->_prev = Tail;
1207 }
1208
1209 // Fall thru into code that tries to wake a successor from EntryList
1210 }
1211
1212 if (QMode == 4 && _cxq != NULL) {
1213 // Aggressively drain cxq into EntryList at the first opportunity.
1214 // This policy ensure that recently-run threads live at the head of EntryList.
1215
1216 // Drain _cxq into EntryList - bulk transfer.
1217 // First, detach _cxq.
1218 // The following loop is tantamount to: w = swap(&cxq, NULL)
1219 w = _cxq;
1220 for (;;) {
1221 assert(w != NULL, "Invariant");
1222 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr(NULL, &_cxq, w);
1223 if (u == w) break;
1224 w = u;
1225 }
1226 assert(w != NULL, "invariant");
1227
1228 ObjectWaiter * q = NULL;
1229 ObjectWaiter * p;
1230 for (p = w; p != NULL; p = p->_next) {
1231 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1232 p->TState = ObjectWaiter::TS_ENTER;
1233 p->_prev = q;
1234 q = p;
1235 }
1236
1237 // Prepend the RATs to the EntryList
1238 if (_EntryList != NULL) {
1239 q->_next = _EntryList;
1240 _EntryList->_prev = q;
1241 }
1242 _EntryList = w;
1243
1244 // Fall thru into code that tries to wake a successor from EntryList
1245 }
1246
1247 w = _EntryList;
1248 if (w != NULL) {
1249 // I'd like to write: guarantee (w->_thread != Self).
1250 // But in practice an exiting thread may find itself on the EntryList.
1251 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1252 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1253 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1254 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1255 // release the lock "O". T2 resumes immediately after the ST of null into
1256 // _owner, above. T2 notices that the EntryList is populated, so it
1257 // reacquires the lock and then finds itself on the EntryList.
1258 // Given all that, we have to tolerate the circumstance where "w" is
1259 // associated with Self.
1260 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1261 ExitEpilog(Self, w);
1262 return;
1263 }
1264
1265 // If we find that both _cxq and EntryList are null then just
1266 // re-run the exit protocol from the top.
1267 w = _cxq;
1268 if (w == NULL) continue;
1269
1270 // Drain _cxq into EntryList - bulk transfer.
1271 // First, detach _cxq.
1272 // The following loop is tantamount to: w = swap(&cxq, NULL)
1273 for (;;) {
1274 assert(w != NULL, "Invariant");
1275 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr(NULL, &_cxq, w);
1276 if (u == w) break;
1277 w = u;
1278 }
1279 TEVENT(Inflated exit - drain cxq into EntryList);
1280
1281 assert(w != NULL, "invariant");
1282 assert(_EntryList == NULL, "invariant");
1283
1284 // Convert the LIFO SLL anchored by _cxq into a DLL.
1285 // The list reorganization step operates in O(LENGTH(w)) time.
1286 // It's critical that this step operate quickly as
1287 // "Self" still holds the outer-lock, restricting parallelism
1288 // and effectively lengthening the critical section.
1289 // Invariant: s chases t chases u.
1290 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1291 // we have faster access to the tail.
1292
1293 if (QMode == 1) {
1294 // QMode == 1 : drain cxq to EntryList, reversing order
1295 // We also reverse the order of the list.
1296 ObjectWaiter * s = NULL;
1297 ObjectWaiter * t = w;
1298 ObjectWaiter * u = NULL;
1299 while (t != NULL) {
1300 guarantee(t->TState == ObjectWaiter::TS_CXQ, "invariant");
1301 t->TState = ObjectWaiter::TS_ENTER;
1302 u = t->_next;
1303 t->_prev = u;
1304 t->_next = s;
1305 s = t;
1306 t = u;
1307 }
1308 _EntryList = s;
1309 assert(s != NULL, "invariant");
1310 } else {
1311 // QMode == 0 or QMode == 2
1312 _EntryList = w;
1313 ObjectWaiter * q = NULL;
1314 ObjectWaiter * p;
1315 for (p = w; p != NULL; p = p->_next) {
1316 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1317 p->TState = ObjectWaiter::TS_ENTER;
1318 p->_prev = q;
1319 q = p;
1320 }
1321 }
1322
1323 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1324 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1325
1326 // See if we can abdicate to a spinner instead of waking a thread.
1327 // A primary goal of the implementation is to reduce the
1328 // context-switch rate.
1329 if (_succ != NULL) continue;
1330
1331 w = _EntryList;
1332 if (w != NULL) {
1333 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1334 ExitEpilog(Self, w);
1335 return;
1336 }
1337 }
1338 }
1339
1340 // ExitSuspendEquivalent:
1341 // A faster alternate to handle_special_suspend_equivalent_condition()
1342 //
1343 // handle_special_suspend_equivalent_condition() unconditionally
1344 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1345 // operations have high latency. Note that in ::enter() we call HSSEC
1346 // while holding the monitor, so we effectively lengthen the critical sections.
1347 //
1348 // There are a number of possible solutions:
1349 //
1350 // A. To ameliorate the problem we might also defer state transitions
1351 // to as late as possible -- just prior to parking.
1352 // Given that, we'd call HSSEC after having returned from park(),
1353 // but before attempting to acquire the monitor. This is only a
1354 // partial solution. It avoids calling HSSEC while holding the
1355 // monitor (good), but it still increases successor reacquisition latency --
1356 // the interval between unparking a successor and the time the successor
1357 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1358 // If we use this technique we can also avoid EnterI()-exit() loop
1359 // in ::enter() where we iteratively drop the lock and then attempt
1360 // to reacquire it after suspending.
1361 //
1362 // B. In the future we might fold all the suspend bits into a
1363 // composite per-thread suspend flag and then update it with CAS().
1364 // Alternately, a Dekker-like mechanism with multiple variables
1365 // would suffice:
1366 // ST Self->_suspend_equivalent = false
1367 // MEMBAR
1368 // LD Self_>_suspend_flags
1369 //
1370 // UPDATE 2007-10-6: since I've replaced the native Mutex/Monitor subsystem
1371 // with a more efficient implementation, the need to use "FastHSSEC" has
1372 // decreased. - Dave
1373
1374
1375 bool ObjectMonitor::ExitSuspendEquivalent(JavaThread * jSelf) {
1376 const int Mode = Knob_FastHSSEC;
1377 if (Mode && !jSelf->is_external_suspend()) {
1378 assert(jSelf->is_suspend_equivalent(), "invariant");
1379 jSelf->clear_suspend_equivalent();
1380 if (2 == Mode) OrderAccess::storeload();
1381 if (!jSelf->is_external_suspend()) return false;
1382 // We raced a suspension -- fall thru into the slow path
1383 TEVENT(ExitSuspendEquivalent - raced);
1384 jSelf->set_suspend_equivalent();
1385 }
1386 return jSelf->handle_special_suspend_equivalent_condition();
1387 }
1388
1389
1390 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) {
1391 assert(_owner == Self, "invariant");
1392
1393 // Exit protocol:
1394 // 1. ST _succ = wakee
1395 // 2. membar #loadstore|#storestore;
1396 // 2. ST _owner = NULL
1397 // 3. unpark(wakee)
1398
1399 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL;
1400 ParkEvent * Trigger = Wakee->_event;
1401
1402 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1403 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1404 // out-of-scope (non-extant).
1405 Wakee = NULL;
1406
1407 // Drop the lock
1408 OrderAccess::release_store_ptr(&_owner, NULL);
1409 OrderAccess::fence(); // ST _owner vs LD in unpark()
1410
1411 if (SafepointSynchronize::do_call_back()) {
1412 TEVENT(unpark before SAFEPOINT);
1413 }
1414
1415 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1416 Trigger->unpark();
1417
1418 // Maintain stats and report events to JVMTI
1419 OM_PERFDATA_OP(Parks, inc());
1420 }
1421
1422
1423 // -----------------------------------------------------------------------------
1424 // Class Loader deadlock handling.
1425 //
1426 // complete_exit exits a lock returning recursion count
1427 // complete_exit/reenter operate as a wait without waiting
1428 // complete_exit requires an inflated monitor
1429 // The _owner field is not always the Thread addr even with an
1430 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1431 // thread due to contention.
1432 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1433 Thread * const Self = THREAD;
1434 assert(Self->is_Java_thread(), "Must be Java thread!");
1435 JavaThread *jt = (JavaThread *)THREAD;
1436
1437 DeferredInitialize();
1438
1439 if (THREAD != _owner) {
1440 if (THREAD->is_lock_owned ((address)_owner)) {
1441 assert(_recursions == 0, "internal state error");
1442 _owner = THREAD; // Convert from basiclock addr to Thread addr
1443 _recursions = 0;
1444 }
1445 }
1446
1447 guarantee(Self == _owner, "complete_exit not owner");
1448 intptr_t save = _recursions; // record the old recursion count
1449 _recursions = 0; // set the recursion level to be 0
1450 exit(true, Self); // exit the monitor
1451 guarantee(_owner != Self, "invariant");
1452 return save;
1453 }
1454
1455 // reenter() enters a lock and sets recursion count
1456 // complete_exit/reenter operate as a wait without waiting
1457 bool ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1458 Thread * const Self = THREAD;
1459 assert(Self->is_Java_thread(), "Must be Java thread!");
1460 JavaThread *jt = (JavaThread *)THREAD;
1461
1462 guarantee(_owner != Self, "reenter already owner");
1463 if (!enter(THREAD)) { return false; } // enter the monitor
1464 guarantee(_recursions == 0, "reenter recursion");
1465 _recursions = recursions;
1466 return true;
1467 }
1468
1469
1470 // -----------------------------------------------------------------------------
1471 // A macro is used below because there may already be a pending
1472 // exception which should not abort the execution of the routines
1473 // which use this (which is why we don't put this into check_slow and
1474 // call it with a CHECK argument).
1475
1476 #define CHECK_OWNER() \
1477 do { \
1478 if (THREAD != _owner) { \
1479 if (THREAD->is_lock_owned((address) _owner)) { \
1480 _owner = THREAD; /* Convert from basiclock addr to Thread addr */ \
1481 _recursions = 0; \
1482 } else { \
1483 TEVENT(Throw IMSX); \
1484 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
1485 } \
1486 } \
1487 } while (false)
1488
1489 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
1490 // TODO-FIXME: remove check_slow() -- it's likely dead.
1491
1492 void ObjectMonitor::check_slow(TRAPS) {
1493 TEVENT(check_slow - throw IMSX);
1494 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1495 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1496 }
1497
1498 static int Adjust(volatile int * adr, int dx) {
1499 int v;
1500 for (v = *adr; Atomic::cmpxchg(v + dx, adr, v) != v; v = *adr) /* empty */;
1501 return v;
1502 }
1503
1504 // helper method for posting a monitor wait event
1505 void ObjectMonitor::post_monitor_wait_event(EventJavaMonitorWait* event,
1506 jlong notifier_tid,
1507 jlong timeout,
1508 bool timedout) {
1509 assert(event != NULL, "invariant");
1510 event->set_monitorClass(((oop)this->object())->klass());
1511 event->set_timeout(timeout);
1512 event->set_address((TYPE_ADDRESS)this->object_addr());
1513 event->set_notifier(notifier_tid);
1514 event->set_timedOut(timedout);
1515 event->commit();
1516 }
1517
1518 // -----------------------------------------------------------------------------
1519 // Wait/Notify/NotifyAll
1520 //
1521 // Note: a subset of changes to ObjectMonitor::wait()
1522 // will need to be replicated in complete_exit
1523 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1524 Thread * const Self = THREAD;
1525 assert(Self->is_Java_thread(), "Must be Java thread!");
1526 JavaThread *jt = (JavaThread *)THREAD;
1527
1528 DeferredInitialize();
1529
1530 // Throw IMSX or IEX.
1531 CHECK_OWNER();
1532
1533 EventJavaMonitorWait event;
1534
1535 // check for a pending interrupt
1536 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1537 // post monitor waited event. Note that this is past-tense, we are done waiting.
1538 if (JvmtiExport::should_post_monitor_waited()) {
1539 // Note: 'false' parameter is passed here because the
1540 // wait was not timed out due to thread interrupt.
1541 JvmtiExport::post_monitor_waited(jt, this, false);
1542
1543 // In this short circuit of the monitor wait protocol, the
1544 // current thread never drops ownership of the monitor and
1545 // never gets added to the wait queue so the current thread
1546 // cannot be made the successor. This means that the
1547 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1548 // consume an unpark() meant for the ParkEvent associated with
1549 // this ObjectMonitor.
1550 }
1551 if (event.should_commit()) {
1552 post_monitor_wait_event(&event, 0, millis, false);
1553 }
1554 TEVENT(Wait - Throw IEX);
1555 THROW(vmSymbols::java_lang_InterruptedException());
1556 return;
1557 }
1558
1559 TEVENT(Wait);
1560
1561 assert(Self->_Stalled == 0, "invariant");
1562 Self->_Stalled = intptr_t(this);
1563 jt->set_current_waiting_monitor(this);
1564
1565 // create a node to be put into the queue
1566 // Critically, after we reset() the event but prior to park(), we must check
1567 // for a pending interrupt.
1568 ObjectWaiter node(Self);
1569 node.TState = ObjectWaiter::TS_WAIT;
1570 Self->_ParkEvent->reset();
1571 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1572
1573 // Enter the waiting queue, which is a circular doubly linked list in this case
1574 // but it could be a priority queue or any data structure.
1575 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1576 // by the the owner of the monitor *except* in the case where park()
1577 // returns because of a timeout of interrupt. Contention is exceptionally rare
1578 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1579
1580 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1581 AddWaiter(&node);
1582 Thread::SpinRelease(&_WaitSetLock);
1583
1584 if ((SyncFlags & 4) == 0) {
1585 _Responsible = NULL;
1586 }
1587 intptr_t save = _recursions; // record the old recursion count
1588 _waiters++; // increment the number of waiters
1589 _recursions = 0; // set the recursion level to be 1
1590 exit(true, Self); // exit the monitor
1591 guarantee(_owner != Self, "invariant");
1592
1593 // The thread is on the WaitSet list - now park() it.
1594 // On MP systems it's conceivable that a brief spin before we park
1595 // could be profitable.
1596 //
1597 // TODO-FIXME: change the following logic to a loop of the form
1598 // while (!timeout && !interrupted && _notified == 0) park()
1599
1600 int ret = OS_OK;
1601 int WasNotified = 0;
1602 { // State transition wrappers
1603 OSThread* osthread = Self->osthread();
1604 OSThreadWaitState osts(osthread, true);
1605 {
1606 ThreadBlockInVM tbivm(jt);
1607 // Thread is in thread_blocked state and oop access is unsafe.
1608 jt->set_suspend_equivalent();
1609
1610 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1611 // Intentionally empty
1612 } else if (node._notified == 0) {
1613 if (millis <= 0) {
1614 Self->_ParkEvent->park();
1615 } else {
1616 ret = Self->_ParkEvent->park(millis);
1617 }
1618 }
1619
1620 // were we externally suspended while we were waiting?
1621 if (ExitSuspendEquivalent (jt)) {
1622 // TODO-FIXME: add -- if succ == Self then succ = null.
1623 jt->java_suspend_self();
1624 }
1625
1626 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1627
1628 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1629 // from the WaitSet to the EntryList.
1630 // See if we need to remove Node from the WaitSet.
1631 // We use double-checked locking to avoid grabbing _WaitSetLock
1632 // if the thread is not on the wait queue.
1633 //
1634 // Note that we don't need a fence before the fetch of TState.
1635 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1636 // written by the is thread. (perhaps the fetch might even be satisfied
1637 // by a look-aside into the processor's own store buffer, although given
1638 // the length of the code path between the prior ST and this load that's
1639 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1640 // then we'll acquire the lock and then re-fetch a fresh TState value.
1641 // That is, we fail toward safety.
1642
1643 if (node.TState == ObjectWaiter::TS_WAIT) {
1644 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1645 if (node.TState == ObjectWaiter::TS_WAIT) {
1646 DequeueSpecificWaiter(&node); // unlink from WaitSet
1647 assert(node._notified == 0, "invariant");
1648 node.TState = ObjectWaiter::TS_RUN;
1649 }
1650 Thread::SpinRelease(&_WaitSetLock);
1651 }
1652
1653 // The thread is now either on off-list (TS_RUN),
1654 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1655 // The Node's TState variable is stable from the perspective of this thread.
1656 // No other threads will asynchronously modify TState.
1657 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1658 OrderAccess::loadload();
1659 if (_succ == Self) _succ = NULL;
1660 WasNotified = node._notified;
1661
1662 // Reentry phase -- reacquire the monitor.
1663 // re-enter contended monitor after object.wait().
1664 // retain OBJECT_WAIT state until re-enter successfully completes
1665 // Thread state is thread_in_vm and oop access is again safe,
1666 // although the raw address of the object may have changed.
1667 // (Don't cache naked oops over safepoints, of course).
1668
1669 // post monitor waited event. Note that this is past-tense, we are done waiting.
1670 if (JvmtiExport::should_post_monitor_waited()) {
1671 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1672
1673 if (node._notified != 0 && _succ == Self) {
1674 // In this part of the monitor wait-notify-reenter protocol it
1675 // is possible (and normal) for another thread to do a fastpath
1676 // monitor enter-exit while this thread is still trying to get
1677 // to the reenter portion of the protocol.
1678 //
1679 // The ObjectMonitor was notified and the current thread is
1680 // the successor which also means that an unpark() has already
1681 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1682 // consume the unpark() that was done when the successor was
1683 // set because the same ParkEvent is shared between Java
1684 // monitors and JVM/TI RawMonitors (for now).
1685 //
1686 // We redo the unpark() to ensure forward progress, i.e., we
1687 // don't want all pending threads hanging (parked) with none
1688 // entering the unlocked monitor.
1689 node._event->unpark();
1690 }
1691 }
1692
1693 if (event.should_commit()) {
1694 post_monitor_wait_event(&event, node._notifier_tid, millis, ret == OS_TIMEOUT);
1695 }
1696
1697 OrderAccess::fence();
1698
1699 assert(Self->_Stalled != 0, "invariant");
1700 Self->_Stalled = 0;
1701
1702 assert(_owner != Self, "invariant");
1703 ObjectWaiter::TStates v = node.TState;
1704 if (v == ObjectWaiter::TS_RUN) {
1705 DEBUG_ONLY(const bool success = ) enter(Self);
1706 assert(success, "enter signalled that we should retry, but monitor should not be deflated as waiters > 0");
1707 } else {
1708 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1709 ReenterI(Self, &node);
1710 node.wait_reenter_end(this);
1711 }
1712
1713 // Self has reacquired the lock.
1714 // Lifecycle - the node representing Self must not appear on any queues.
1715 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1716 // want residual elements associated with this thread left on any lists.
1717 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1718 assert(_owner == Self, "invariant");
1719 assert(_succ != Self, "invariant");
1720 } // OSThreadWaitState()
1721
1722 jt->set_current_waiting_monitor(NULL);
1723
1724 guarantee(_recursions == 0, "invariant");
1725 _recursions = save; // restore the old recursion count
1726 _waiters--; // decrement the number of waiters
1727
1728 // Verify a few postconditions
1729 assert(_owner == Self, "invariant");
1730 assert(_succ != Self, "invariant");
1731 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
1732
1733 if (SyncFlags & 32) {
1734 OrderAccess::fence();
1735 }
1736
1737 // check if the notification happened
1738 if (!WasNotified) {
1739 // no, it could be timeout or Thread.interrupt() or both
1740 // check for interrupt event, otherwise it is timeout
1741 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1742 TEVENT(Wait - throw IEX from epilog);
1743 THROW(vmSymbols::java_lang_InterruptedException());
1744 }
1745 }
1746
1747 // NOTE: Spurious wake up will be consider as timeout.
1748 // Monitor notify has precedence over thread interrupt.
1749 }
1750
1751
1752 // Consider:
1753 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1754 // then instead of transferring a thread from the WaitSet to the EntryList
1755 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1756
1757 void ObjectMonitor::INotify(Thread * Self) {
1758 const int policy = Knob_MoveNotifyee;
1759
1760 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1761 ObjectWaiter * iterator = DequeueWaiter();
1762 if (iterator != NULL) {
1763 TEVENT(Notify1 - Transfer);
1764 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1765 guarantee(iterator->_notified == 0, "invariant");
1766 // Disposition - what might we do with iterator ?
1767 // a. add it directly to the EntryList - either tail (policy == 1)
1768 // or head (policy == 0).
1769 // b. push it onto the front of the _cxq (policy == 2).
1770 // For now we use (b).
1771 if (policy != 4) {
1772 iterator->TState = ObjectWaiter::TS_ENTER;
1773 }
1774 iterator->_notified = 1;
1775 iterator->_notifier_tid = THREAD_TRACE_ID(Self);
1776
1777 ObjectWaiter * list = _EntryList;
1778 if (list != NULL) {
1779 assert(list->_prev == NULL, "invariant");
1780 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1781 assert(list != iterator, "invariant");
1782 }
1783
1784 if (policy == 0) { // prepend to EntryList
1785 if (list == NULL) {
1786 iterator->_next = iterator->_prev = NULL;
1787 _EntryList = iterator;
1788 } else {
1789 list->_prev = iterator;
1790 iterator->_next = list;
1791 iterator->_prev = NULL;
1792 _EntryList = iterator;
1793 }
1794 } else if (policy == 1) { // append to EntryList
1795 if (list == NULL) {
1796 iterator->_next = iterator->_prev = NULL;
1797 _EntryList = iterator;
1798 } else {
1799 // CONSIDER: finding the tail currently requires a linear-time walk of
1800 // the EntryList. We can make tail access constant-time by converting to
1801 // a CDLL instead of using our current DLL.
1802 ObjectWaiter * tail;
1803 for (tail = list; tail->_next != NULL; tail = tail->_next) /* empty */;
1804 assert(tail != NULL && tail->_next == NULL, "invariant");
1805 tail->_next = iterator;
1806 iterator->_prev = tail;
1807 iterator->_next = NULL;
1808 }
1809 } else if (policy == 2) { // prepend to cxq
1810 if (list == NULL) {
1811 iterator->_next = iterator->_prev = NULL;
1812 _EntryList = iterator;
1813 } else {
1814 iterator->TState = ObjectWaiter::TS_CXQ;
1815 for (;;) {
1816 ObjectWaiter * front = _cxq;
1817 iterator->_next = front;
1818 if (Atomic::cmpxchg_ptr(iterator, &_cxq, front) == front) {
1819 break;
1820 }
1821 }
1822 }
1823 } else if (policy == 3) { // append to cxq
1824 iterator->TState = ObjectWaiter::TS_CXQ;
1825 for (;;) {
1826 ObjectWaiter * tail = _cxq;
1827 if (tail == NULL) {
1828 iterator->_next = NULL;
1829 if (Atomic::cmpxchg_ptr(iterator, &_cxq, NULL) == NULL) {
1830 break;
1831 }
1832 } else {
1833 while (tail->_next != NULL) tail = tail->_next;
1834 tail->_next = iterator;
1835 iterator->_prev = tail;
1836 iterator->_next = NULL;
1837 break;
1838 }
1839 }
1840 } else {
1841 ParkEvent * ev = iterator->_event;
1842 iterator->TState = ObjectWaiter::TS_RUN;
1843 OrderAccess::fence();
1844 ev->unpark();
1845 }
1846
1847 // _WaitSetLock protects the wait queue, not the EntryList. We could
1848 // move the add-to-EntryList operation, above, outside the critical section
1849 // protected by _WaitSetLock. In practice that's not useful. With the
1850 // exception of wait() timeouts and interrupts the monitor owner
1851 // is the only thread that grabs _WaitSetLock. There's almost no contention
1852 // on _WaitSetLock so it's not profitable to reduce the length of the
1853 // critical section.
1854
1855 if (policy < 4) {
1856 iterator->wait_reenter_begin(this);
1857 }
1858 }
1859 Thread::SpinRelease(&_WaitSetLock);
1860 }
1861
1862 // Consider: a not-uncommon synchronization bug is to use notify() when
1863 // notifyAll() is more appropriate, potentially resulting in stranded
1864 // threads; this is one example of a lost wakeup. A useful diagnostic
1865 // option is to force all notify() operations to behave as notifyAll().
1866 //
1867 // Note: We can also detect many such problems with a "minimum wait".
1868 // When the "minimum wait" is set to a small non-zero timeout value
1869 // and the program does not hang whereas it did absent "minimum wait",
1870 // that suggests a lost wakeup bug. The '-XX:SyncFlags=1' option uses
1871 // a "minimum wait" for all park() operations; see the recheckInterval
1872 // variable and MAX_RECHECK_INTERVAL.
1873
1874 void ObjectMonitor::notify(TRAPS) {
1875 CHECK_OWNER();
1876 if (_WaitSet == NULL) {
1877 TEVENT(Empty-Notify);
1878 return;
1879 }
1880 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1881 INotify(THREAD);
1882 OM_PERFDATA_OP(Notifications, inc(1));
1883 }
1884
1885
1886 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1887 // from the waitset to the EntryList. This could be done more efficiently with a
1888 // single bulk transfer but in practice it's not time-critical. Beware too,
1889 // that in prepend-mode we invert the order of the waiters. Let's say that the
1890 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1891 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1892
1893 void ObjectMonitor::notifyAll(TRAPS) {
1894 CHECK_OWNER();
1895 if (_WaitSet == NULL) {
1896 TEVENT(Empty-NotifyAll);
1897 return;
1898 }
1899
1900 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1901 int tally = 0;
1902 while (_WaitSet != NULL) {
1903 tally++;
1904 INotify(THREAD);
1905 }
1906
1907 OM_PERFDATA_OP(Notifications, inc(tally));
1908 }
1909
1910 // -----------------------------------------------------------------------------
1911 // Adaptive Spinning Support
1912 //
1913 // Adaptive spin-then-block - rational spinning
1914 //
1915 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1916 // algorithm. On high order SMP systems it would be better to start with
1917 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1918 // a contending thread could enqueue itself on the cxq and then spin locally
1919 // on a thread-specific variable such as its ParkEvent._Event flag.
1920 // That's left as an exercise for the reader. Note that global spinning is
1921 // not problematic on Niagara, as the L2 cache serves the interconnect and
1922 // has both low latency and massive bandwidth.
1923 //
1924 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1925 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1926 // (duration) or we can fix the count at approximately the duration of
1927 // a context switch and vary the frequency. Of course we could also
1928 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1929 // For a description of 'Adaptive spin-then-block mutual exclusion in
1930 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1931 //
1932 // This implementation varies the duration "D", where D varies with
1933 // the success rate of recent spin attempts. (D is capped at approximately
1934 // length of a round-trip context switch). The success rate for recent
1935 // spin attempts is a good predictor of the success rate of future spin
1936 // attempts. The mechanism adapts automatically to varying critical
1937 // section length (lock modality), system load and degree of parallelism.
1938 // D is maintained per-monitor in _SpinDuration and is initialized
1939 // optimistically. Spin frequency is fixed at 100%.
1940 //
1941 // Note that _SpinDuration is volatile, but we update it without locks
1942 // or atomics. The code is designed so that _SpinDuration stays within
1943 // a reasonable range even in the presence of races. The arithmetic
1944 // operations on _SpinDuration are closed over the domain of legal values,
1945 // so at worst a race will install and older but still legal value.
1946 // At the very worst this introduces some apparent non-determinism.
1947 // We might spin when we shouldn't or vice-versa, but since the spin
1948 // count are relatively short, even in the worst case, the effect is harmless.
1949 //
1950 // Care must be taken that a low "D" value does not become an
1951 // an absorbing state. Transient spinning failures -- when spinning
1952 // is overall profitable -- should not cause the system to converge
1953 // on low "D" values. We want spinning to be stable and predictable
1954 // and fairly responsive to change and at the same time we don't want
1955 // it to oscillate, become metastable, be "too" non-deterministic,
1956 // or converge on or enter undesirable stable absorbing states.
1957 //
1958 // We implement a feedback-based control system -- using past behavior
1959 // to predict future behavior. We face two issues: (a) if the
1960 // input signal is random then the spin predictor won't provide optimal
1961 // results, and (b) if the signal frequency is too high then the control
1962 // system, which has some natural response lag, will "chase" the signal.
1963 // (b) can arise from multimodal lock hold times. Transient preemption
1964 // can also result in apparent bimodal lock hold times.
1965 // Although sub-optimal, neither condition is particularly harmful, as
1966 // in the worst-case we'll spin when we shouldn't or vice-versa.
1967 // The maximum spin duration is rather short so the failure modes aren't bad.
1968 // To be conservative, I've tuned the gain in system to bias toward
1969 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1970 // "rings" or oscillates between spinning and not spinning. This happens
1971 // when spinning is just on the cusp of profitability, however, so the
1972 // situation is not dire. The state is benign -- there's no need to add
1973 // hysteresis control to damp the transition rate between spinning and
1974 // not spinning.
1975
1976 // Spinning: Fixed frequency (100%), vary duration
1977 int ObjectMonitor::TrySpin(Thread * Self) {
1978 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1979 int ctr = Knob_FixedSpin;
1980 if (ctr != 0) {
1981 while (--ctr >= 0) {
1982 if (TryLock(Self) > 0) return 1;
1983 SpinPause();
1984 }
1985 return 0;
1986 }
1987
1988 for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1989 if (TryLock(Self) > 0) {
1990 // Increase _SpinDuration ...
1991 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1992 // Raising _SpurDuration to the poverty line is key.
1993 int x = _SpinDuration;
1994 if (x < Knob_SpinLimit) {
1995 if (x < Knob_Poverty) x = Knob_Poverty;
1996 _SpinDuration = x + Knob_BonusB;
1997 }
1998 return 1;
1999 }
2000 SpinPause();
2001 }
2002
2003 // Admission control - verify preconditions for spinning
2004 //
2005 // We always spin a little bit, just to prevent _SpinDuration == 0 from
2006 // becoming an absorbing state. Put another way, we spin briefly to
2007 // sample, just in case the system load, parallelism, contention, or lock
2008 // modality changed.
2009 //
2010 // Consider the following alternative:
2011 // Periodically set _SpinDuration = _SpinLimit and try a long/full
2012 // spin attempt. "Periodically" might mean after a tally of
2013 // the # of failed spin attempts (or iterations) reaches some threshold.
2014 // This takes us into the realm of 1-out-of-N spinning, where we
2015 // hold the duration constant but vary the frequency.
2016
2017 ctr = _SpinDuration;
2018 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase;
2019 if (ctr <= 0) return 0;
2020
2021 if (Knob_SuccRestrict && _succ != NULL) return 0;
2022 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
2023 TEVENT(Spin abort - notrunnable [TOP]);
2024 return 0;
2025 }
2026
2027 int MaxSpin = Knob_MaxSpinners;
2028 if (MaxSpin >= 0) {
2029 if (_Spinner > MaxSpin) {
2030 TEVENT(Spin abort -- too many spinners);
2031 return 0;
2032 }
2033 // Slightly racy, but benign ...
2034 Adjust(&_Spinner, 1);
2035 }
2036
2037 // We're good to spin ... spin ingress.
2038 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
2039 // when preparing to LD...CAS _owner, etc and the CAS is likely
2040 // to succeed.
2041 int hits = 0;
2042 int msk = 0;
2043 int caspty = Knob_CASPenalty;
2044 int oxpty = Knob_OXPenalty;
2045 int sss = Knob_SpinSetSucc;
2046 if (sss && _succ == NULL) _succ = Self;
2047 Thread * prv = NULL;
2048
2049 // There are three ways to exit the following loop:
2050 // 1. A successful spin where this thread has acquired the lock.
2051 // 2. Spin failure with prejudice
2052 // 3. Spin failure without prejudice
2053
2054 while (--ctr >= 0) {
2055
2056 // Periodic polling -- Check for pending GC
2057 // Threads may spin while they're unsafe.
2058 // We don't want spinning threads to delay the JVM from reaching
2059 // a stop-the-world safepoint or to steal cycles from GC.
2060 // If we detect a pending safepoint we abort in order that
2061 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
2062 // this thread, if safe, doesn't steal cycles from GC.
2063 // This is in keeping with the "no loitering in runtime" rule.
2064 // We periodically check to see if there's a safepoint pending.
2065 if ((ctr & 0xFF) == 0) {
2066 if (SafepointSynchronize::do_call_back()) {
2067 TEVENT(Spin: safepoint);
2068 goto Abort; // abrupt spin egress
2069 }
2070 if (Knob_UsePause & 1) SpinPause();
2071 }
2072
2073 if (Knob_UsePause & 2) SpinPause();
2074
2075 // Exponential back-off ... Stay off the bus to reduce coherency traffic.
2076 // This is useful on classic SMP systems, but is of less utility on
2077 // N1-style CMT platforms.
2078 //
2079 // Trade-off: lock acquisition latency vs coherency bandwidth.
2080 // Lock hold times are typically short. A histogram
2081 // of successful spin attempts shows that we usually acquire
2082 // the lock early in the spin. That suggests we want to
2083 // sample _owner frequently in the early phase of the spin,
2084 // but then back-off and sample less frequently as the spin
2085 // progresses. The back-off makes a good citizen on SMP big
2086 // SMP systems. Oversampling _owner can consume excessive
2087 // coherency bandwidth. Relatedly, if we _oversample _owner we
2088 // can inadvertently interfere with the the ST m->owner=null.
2089 // executed by the lock owner.
2090 if (ctr & msk) continue;
2091 ++hits;
2092 if ((hits & 0xF) == 0) {
2093 // The 0xF, above, corresponds to the exponent.
2094 // Consider: (msk+1)|msk
2095 msk = ((msk << 2)|3) & BackOffMask;
2096 }
2097
2098 // Probe _owner with TATAS
2099 // If this thread observes the monitor transition or flicker
2100 // from locked to unlocked to locked, then the odds that this
2101 // thread will acquire the lock in this spin attempt go down
2102 // considerably. The same argument applies if the CAS fails
2103 // or if we observe _owner change from one non-null value to
2104 // another non-null value. In such cases we might abort
2105 // the spin without prejudice or apply a "penalty" to the
2106 // spin count-down variable "ctr", reducing it by 100, say.
2107
2108 Thread * ox = (Thread *) _owner;
2109 if (ox == NULL) {
2110 ox = (Thread *) Atomic::cmpxchg_ptr(Self, &_owner, NULL);
2111 if (ox == NULL) {
2112 // The CAS succeeded -- this thread acquired ownership
2113 // Take care of some bookkeeping to exit spin state.
2114 if (sss && _succ == Self) {
2115 _succ = NULL;
2116 }
2117 if (MaxSpin > 0) Adjust(&_Spinner, -1);
2118
2119 // Increase _SpinDuration :
2120 // The spin was successful (profitable) so we tend toward
2121 // longer spin attempts in the future.
2122 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2123 // If we acquired the lock early in the spin cycle it
2124 // makes sense to increase _SpinDuration proportionally.
2125 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2126 int x = _SpinDuration;
2127 if (x < Knob_SpinLimit) {
2128 if (x < Knob_Poverty) x = Knob_Poverty;
2129 _SpinDuration = x + Knob_Bonus;
2130 }
2131 return 1;
2132 }
2133
2134 // The CAS failed ... we can take any of the following actions:
2135 // * penalize: ctr -= Knob_CASPenalty
2136 // * exit spin with prejudice -- goto Abort;
2137 // * exit spin without prejudice.
2138 // * Since CAS is high-latency, retry again immediately.
2139 prv = ox;
2140 TEVENT(Spin: cas failed);
2141 if (caspty == -2) break;
2142 if (caspty == -1) goto Abort;
2143 ctr -= caspty;
2144 continue;
2145 }
2146
2147 // Did lock ownership change hands ?
2148 if (ox != prv && prv != NULL) {
2149 TEVENT(spin: Owner changed)
2150 if (oxpty == -2) break;
2151 if (oxpty == -1) goto Abort;
2152 ctr -= oxpty;
2153 }
2154 prv = ox;
2155
2156 // Abort the spin if the owner is not executing.
2157 // The owner must be executing in order to drop the lock.
2158 // Spinning while the owner is OFFPROC is idiocy.
2159 // Consider: ctr -= RunnablePenalty ;
2160 if (Knob_OState && NotRunnable (Self, ox)) {
2161 TEVENT(Spin abort - notrunnable);
2162 goto Abort;
2163 }
2164 if (sss && _succ == NULL) _succ = Self;
2165 }
2166
2167 // Spin failed with prejudice -- reduce _SpinDuration.
2168 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2169 // AIMD is globally stable.
2170 TEVENT(Spin failure);
2171 {
2172 int x = _SpinDuration;
2173 if (x > 0) {
2174 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2175 // This is globally sample and tends to damp the response.
2176 x -= Knob_Penalty;
2177 if (x < 0) x = 0;
2178 _SpinDuration = x;
2179 }
2180 }
2181
2182 Abort:
2183 if (MaxSpin >= 0) Adjust(&_Spinner, -1);
2184 if (sss && _succ == Self) {
2185 _succ = NULL;
2186 // Invariant: after setting succ=null a contending thread
2187 // must recheck-retry _owner before parking. This usually happens
2188 // in the normal usage of TrySpin(), but it's safest
2189 // to make TrySpin() as foolproof as possible.
2190 OrderAccess::fence();
2191 if (TryLock(Self) > 0) return 1;
2192 }
2193 return 0;
2194 }
2195
2196 // NotRunnable() -- informed spinning
2197 //
2198 // Don't bother spinning if the owner is not eligible to drop the lock.
2199 // Peek at the owner's schedctl.sc_state and Thread._thread_values and
2200 // spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2201 // The thread must be runnable in order to drop the lock in timely fashion.
2202 // If the _owner is not runnable then spinning will not likely be
2203 // successful (profitable).
2204 //
2205 // Beware -- the thread referenced by _owner could have died
2206 // so a simply fetch from _owner->_thread_state might trap.
2207 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2208 // Because of the lifecycle issues the schedctl and _thread_state values
2209 // observed by NotRunnable() might be garbage. NotRunnable must
2210 // tolerate this and consider the observed _thread_state value
2211 // as advisory.
2212 //
2213 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2214 // a thread pointer.
2215 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2216 // with the LSB of _owner. Another option would be to probablistically probe
2217 // the putative _owner->TypeTag value.
2218 //
2219 // Checking _thread_state isn't perfect. Even if the thread is
2220 // in_java it might be blocked on a page-fault or have been preempted
2221 // and sitting on a ready/dispatch queue. _thread state in conjunction
2222 // with schedctl.sc_state gives us a good picture of what the
2223 // thread is doing, however.
2224 //
2225 // TODO: check schedctl.sc_state.
2226 // We'll need to use SafeFetch32() to read from the schedctl block.
2227 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/
2228 //
2229 // The return value from NotRunnable() is *advisory* -- the
2230 // result is based on sampling and is not necessarily coherent.
2231 // The caller must tolerate false-negative and false-positive errors.
2232 // Spinning, in general, is probabilistic anyway.
2233
2234
2235 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) {
2236 // Check ox->TypeTag == 2BAD.
2237 if (ox == NULL) return 0;
2238
2239 // Avoid transitive spinning ...
2240 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2241 // Immediately after T1 acquires L it's possible that T2, also
2242 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2243 // This occurs transiently after T1 acquired L but before
2244 // T1 managed to clear T1.Stalled. T2 does not need to abort
2245 // its spin in this circumstance.
2246 intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
2247
2248 if (BlockedOn == 1) return 1;
2249 if (BlockedOn != 0) {
2250 return BlockedOn != intptr_t(this) && _owner == ox;
2251 }
2252
2253 assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant");
2254 int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);;
2255 // consider also: jst != _thread_in_Java -- but that's overspecific.
2256 return jst == _thread_blocked || jst == _thread_in_native;
2257 }
2258
2259
2260 // -----------------------------------------------------------------------------
2261 // WaitSet management ...
2262
2263 ObjectWaiter::ObjectWaiter(Thread* thread) {
2264 _next = NULL;
2265 _prev = NULL;
2266 _notified = 0;
2267 TState = TS_RUN;
2268 _thread = thread;
2269 _event = thread->_ParkEvent;
2270 _active = false;
2271 assert(_event != NULL, "invariant");
2272 }
2273
2274 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2275 JavaThread *jt = (JavaThread *)this->_thread;
2276 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2277 }
2278
2279 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2280 JavaThread *jt = (JavaThread *)this->_thread;
2281 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2282 }
2283
2284 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2285 assert(node != NULL, "should not add NULL node");
2286 assert(node->_prev == NULL, "node already in list");
2287 assert(node->_next == NULL, "node already in list");
2288 // put node at end of queue (circular doubly linked list)
2289 if (_WaitSet == NULL) {
2290 _WaitSet = node;
2291 node->_prev = node;
2292 node->_next = node;
2293 } else {
2294 ObjectWaiter* head = _WaitSet;
2295 ObjectWaiter* tail = head->_prev;
2296 assert(tail->_next == head, "invariant check");
2297 tail->_next = node;
2298 head->_prev = node;
2299 node->_next = head;
2300 node->_prev = tail;
2301 }
2302 }
2303
2304 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2305 // dequeue the very first waiter
2306 ObjectWaiter* waiter = _WaitSet;
2307 if (waiter) {
2308 DequeueSpecificWaiter(waiter);
2309 }
2310 return waiter;
2311 }
2312
2313 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2314 assert(node != NULL, "should not dequeue NULL node");
2315 assert(node->_prev != NULL, "node already removed from list");
2316 assert(node->_next != NULL, "node already removed from list");
2317 // when the waiter has woken up because of interrupt,
2318 // timeout or other spurious wake-up, dequeue the
2319 // waiter from waiting list
2320 ObjectWaiter* next = node->_next;
2321 if (next == node) {
2322 assert(node->_prev == node, "invariant check");
2323 _WaitSet = NULL;
2324 } else {
2325 ObjectWaiter* prev = node->_prev;
2326 assert(prev->_next == node, "invariant check");
2327 assert(next->_prev == node, "invariant check");
2328 next->_prev = prev;
2329 prev->_next = next;
2330 if (_WaitSet == node) {
2331 _WaitSet = next;
2332 }
2333 }
2334 node->_next = NULL;
2335 node->_prev = NULL;
2336 }
2337
2338 // -----------------------------------------------------------------------------
2339 // PerfData support
2340 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL;
2341 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL;
2342 PerfCounter * ObjectMonitor::_sync_Parks = NULL;
2343 PerfCounter * ObjectMonitor::_sync_EmptyNotifications = NULL;
2344 PerfCounter * ObjectMonitor::_sync_Notifications = NULL;
2345 PerfCounter * ObjectMonitor::_sync_PrivateA = NULL;
2346 PerfCounter * ObjectMonitor::_sync_PrivateB = NULL;
2347 PerfCounter * ObjectMonitor::_sync_SlowExit = NULL;
2348 PerfCounter * ObjectMonitor::_sync_SlowEnter = NULL;
2349 PerfCounter * ObjectMonitor::_sync_SlowNotify = NULL;
2350 PerfCounter * ObjectMonitor::_sync_SlowNotifyAll = NULL;
2351 PerfCounter * ObjectMonitor::_sync_FailedSpins = NULL;
2352 PerfCounter * ObjectMonitor::_sync_SuccessfulSpins = NULL;
2353 PerfCounter * ObjectMonitor::_sync_MonInCirculation = NULL;
2354 PerfCounter * ObjectMonitor::_sync_MonScavenged = NULL;
2355 PerfCounter * ObjectMonitor::_sync_Inflations = NULL;
2356 PerfCounter * ObjectMonitor::_sync_Deflations = NULL;
2357 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL;
2358
2359 // One-shot global initialization for the sync subsystem.
2360 // We could also defer initialization and initialize on-demand
2361 // the first time we call inflate(). Initialization would
2362 // be protected - like so many things - by the MonitorCache_lock.
2363
2364 void ObjectMonitor::Initialize() {
2365 static int InitializationCompleted = 0;
2366 assert(InitializationCompleted == 0, "invariant");
2367 InitializationCompleted = 1;
2368 if (UsePerfData) {
2369 EXCEPTION_MARK;
2370 #define NEWPERFCOUNTER(n) \
2371 { \
2372 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \
2373 CHECK); \
2374 }
2375 #define NEWPERFVARIABLE(n) \
2376 { \
2377 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \
2378 CHECK); \
2379 }
2380 NEWPERFCOUNTER(_sync_Inflations);
2381 NEWPERFCOUNTER(_sync_Deflations);
2382 NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2383 NEWPERFCOUNTER(_sync_FutileWakeups);
2384 NEWPERFCOUNTER(_sync_Parks);
2385 NEWPERFCOUNTER(_sync_EmptyNotifications);
2386 NEWPERFCOUNTER(_sync_Notifications);
2387 NEWPERFCOUNTER(_sync_SlowEnter);
2388 NEWPERFCOUNTER(_sync_SlowExit);
2389 NEWPERFCOUNTER(_sync_SlowNotify);
2390 NEWPERFCOUNTER(_sync_SlowNotifyAll);
2391 NEWPERFCOUNTER(_sync_FailedSpins);
2392 NEWPERFCOUNTER(_sync_SuccessfulSpins);
2393 NEWPERFCOUNTER(_sync_PrivateA);
2394 NEWPERFCOUNTER(_sync_PrivateB);
2395 NEWPERFCOUNTER(_sync_MonInCirculation);
2396 NEWPERFCOUNTER(_sync_MonScavenged);
2397 NEWPERFVARIABLE(_sync_MonExtant);
2398 #undef NEWPERFCOUNTER
2399 #undef NEWPERFVARIABLE
2400 }
2401 }
2402
2403 static char * kvGet(char * kvList, const char * Key) {
2404 if (kvList == NULL) return NULL;
2405 size_t n = strlen(Key);
2406 char * Search;
2407 for (Search = kvList; *Search; Search += strlen(Search) + 1) {
2408 if (strncmp (Search, Key, n) == 0) {
2409 if (Search[n] == '=') return Search + n + 1;
2410 if (Search[n] == 0) return(char *) "1";
2411 }
2412 }
2413 return NULL;
2414 }
2415
2416 static int kvGetInt(char * kvList, const char * Key, int Default) {
2417 char * v = kvGet(kvList, Key);
2418 int rslt = v ? ::strtol(v, NULL, 0) : Default;
2419 if (Knob_ReportSettings && v != NULL) {
2420 tty->print_cr("INFO: SyncKnob: %s %d(%d)", Key, rslt, Default) ;
2421 tty->flush();
2422 }
2423 return rslt;
2424 }
2425
2426 void ObjectMonitor::DeferredInitialize() {
2427 if (InitDone > 0) return;
2428 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2429 while (InitDone != 1) /* empty */;
2430 return;
2431 }
2432
2433 // One-shot global initialization ...
2434 // The initialization is idempotent, so we don't need locks.
2435 // In the future consider doing this via os::init_2().
2436 // SyncKnobs consist of <Key>=<Value> pairs in the style
2437 // of environment variables. Start by converting ':' to NUL.
2438
2439 if (SyncKnobs == NULL) SyncKnobs = "";
2440
2441 size_t sz = strlen(SyncKnobs);
2442 char * knobs = (char *) malloc(sz + 2);
2443 if (knobs == NULL) {
2444 vm_exit_out_of_memory(sz + 2, OOM_MALLOC_ERROR, "Parse SyncKnobs");
2445 guarantee(0, "invariant");
2446 }
2447 strcpy(knobs, SyncKnobs);
2448 knobs[sz+1] = 0;
2449 for (char * p = knobs; *p; p++) {
2450 if (*p == ':') *p = 0;
2451 }
2452
2453 #define SETKNOB(x) { Knob_##x = kvGetInt(knobs, #x, Knob_##x); }
2454 SETKNOB(ReportSettings);
2455 SETKNOB(ExitRelease);
2456 SETKNOB(Verbose);
2457 SETKNOB(VerifyInUse);
2458 SETKNOB(VerifyMatch);
2459 SETKNOB(FixedSpin);
2460 SETKNOB(SpinLimit);
2461 SETKNOB(SpinBase);
2462 SETKNOB(SpinBackOff);
2463 SETKNOB(CASPenalty);
2464 SETKNOB(OXPenalty);
2465 SETKNOB(LogSpins);
2466 SETKNOB(SpinSetSucc);
2467 SETKNOB(SuccEnabled);
2468 SETKNOB(SuccRestrict);
2469 SETKNOB(Penalty);
2470 SETKNOB(Bonus);
2471 SETKNOB(BonusB);
2472 SETKNOB(Poverty);
2473 SETKNOB(SpinAfterFutile);
2474 SETKNOB(UsePause);
2475 SETKNOB(SpinEarly);
2476 SETKNOB(OState);
2477 SETKNOB(MaxSpinners);
2478 SETKNOB(PreSpin);
2479 SETKNOB(ExitPolicy);
2480 SETKNOB(QMode);
2481 SETKNOB(ResetEvent);
2482 SETKNOB(MoveNotifyee);
2483 SETKNOB(FastHSSEC);
2484 #undef SETKNOB
2485
2486 if (Knob_Verbose) {
2487 sanity_checks();
2488 }
2489
2490 if (os::is_MP()) {
2491 BackOffMask = (1 << Knob_SpinBackOff) - 1;
2492 if (Knob_ReportSettings) {
2493 tty->print_cr("INFO: BackOffMask=0x%X", BackOffMask);
2494 }
2495 // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
2496 } else {
2497 Knob_SpinLimit = 0;
2498 Knob_SpinBase = 0;
2499 Knob_PreSpin = 0;
2500 Knob_FixedSpin = -1;
2501 }
2502
2503 if (Knob_LogSpins == 0) {
2504 ObjectMonitor::_sync_FailedSpins = NULL;
2505 }
2506
2507 free(knobs);
2508 OrderAccess::fence();
2509 InitDone = 1;
2510 }
2511
2512 void ObjectMonitor::sanity_checks() {
2513 int error_cnt = 0;
2514 int warning_cnt = 0;
2515 bool verbose = Knob_Verbose != 0 NOT_PRODUCT(|| VerboseInternalVMTests);
2516
2517 if (verbose) {
2518 tty->print_cr("INFO: sizeof(ObjectMonitor)=" SIZE_FORMAT,
2519 sizeof(ObjectMonitor));
2520 tty->print_cr("INFO: sizeof(PaddedEnd<ObjectMonitor>)=" SIZE_FORMAT,
2521 sizeof(PaddedEnd<ObjectMonitor>));
2522 }
2523
2524 uint cache_line_size = VM_Version::L1_data_cache_line_size();
2525 if (verbose) {
2526 tty->print_cr("INFO: L1_data_cache_line_size=%u", cache_line_size);
2527 }
2528
2529 ObjectMonitor dummy;
2530 u_char *addr_begin = (u_char*)&dummy;
2531 u_char *addr_header = (u_char*)&dummy._header;
2532 u_char *addr_owner = (u_char*)&dummy._owner;
2533
2534 uint offset_header = (uint)(addr_header - addr_begin);
2535 if (verbose) tty->print_cr("INFO: offset(_header)=%u", offset_header);
2536
2537 uint offset_owner = (uint)(addr_owner - addr_begin);
2538 if (verbose) tty->print_cr("INFO: offset(_owner)=%u", offset_owner);
2539
2540 if ((uint)(addr_header - addr_begin) != 0) {
2541 tty->print_cr("ERROR: offset(_header) must be zero (0).");
2542 error_cnt++;
2543 }
2544
2545 if (cache_line_size != 0) {
2546 // We were able to determine the L1 data cache line size so
2547 // do some cache line specific sanity checks
2548
2549 if ((offset_owner - offset_header) < cache_line_size) {
2550 tty->print_cr("WARNING: the _header and _owner fields are closer "
2551 "than a cache line which permits false sharing.");
2552 warning_cnt++;
2553 }
2554
2555 if ((sizeof(PaddedEnd<ObjectMonitor>) % cache_line_size) != 0) {
2556 tty->print_cr("WARNING: PaddedEnd<ObjectMonitor> size is not a "
2557 "multiple of a cache line which permits false sharing.");
2558 warning_cnt++;
2559 }
2560 }
2561
2562 ObjectSynchronizer::sanity_checks(verbose, cache_line_size, &error_cnt,
2563 &warning_cnt);
2564
2565 if (verbose || error_cnt != 0 || warning_cnt != 0) {
2566 tty->print_cr("INFO: error_cnt=%d", error_cnt);
2567 tty->print_cr("INFO: warning_cnt=%d", warning_cnt);
2568 }
2569
2570 guarantee(error_cnt == 0,
2571 "Fatal error(s) found in ObjectMonitor::sanity_checks()");
2572 }
2573
2574 #ifndef PRODUCT
2575 void ObjectMonitor_test() {
2576 ObjectMonitor::sanity_checks();
2577 }
2578 #endif