1 /* 2 * Copyright (c) 1998, 2019, Oracle and/or its affiliates. All rights reserved. 3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 4 * 5 * This code is free software; you can redistribute it and/or modify it 6 * under the terms of the GNU General Public License version 2 only, as 7 * published by the Free Software Foundation. 8 * 9 * This code is distributed in the hope that it will be useful, but WITHOUT 10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 12 * version 2 for more details (a copy is included in the LICENSE file that 13 * accompanied this code). 14 * 15 * You should have received a copy of the GNU General Public License version 16 * 2 along with this work; if not, write to the Free Software Foundation, 17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 18 * 19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 20 * or visit www.oracle.com if you need additional information or have any 21 * questions. 22 * 23 */ 24 25 #include "precompiled.hpp" 26 #include "classfile/vmSymbols.hpp" 27 #include "jfr/jfrEvents.hpp" 28 #include "jfr/support/jfrThreadId.hpp" 29 #include "memory/allocation.inline.hpp" 30 #include "memory/resourceArea.hpp" 31 #include "oops/markWord.hpp" 32 #include "oops/oop.inline.hpp" 33 #include "runtime/atomic.hpp" 34 #include "runtime/handles.inline.hpp" 35 #include "runtime/interfaceSupport.inline.hpp" 36 #include "runtime/mutexLocker.hpp" 37 #include "runtime/objectMonitor.hpp" 38 #include "runtime/objectMonitor.inline.hpp" 39 #include "runtime/orderAccess.hpp" 40 #include "runtime/osThread.hpp" 41 #include "runtime/safepointMechanism.inline.hpp" 42 #include "runtime/sharedRuntime.hpp" 43 #include "runtime/stubRoutines.hpp" 44 #include "runtime/thread.inline.hpp" 45 #include "services/threadService.hpp" 46 #include "utilities/dtrace.hpp" 47 #include "utilities/macros.hpp" 48 #include "utilities/preserveException.hpp" 49 #if INCLUDE_JFR 50 #include "jfr/support/jfrFlush.hpp" 51 #endif 52 53 #ifdef DTRACE_ENABLED 54 55 // Only bother with this argument setup if dtrace is available 56 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly. 57 58 59 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \ 60 char* bytes = NULL; \ 61 int len = 0; \ 62 jlong jtid = SharedRuntime::get_java_tid(thread); \ 63 Symbol* klassname = ((oop)obj)->klass()->name(); \ 64 if (klassname != NULL) { \ 65 bytes = (char*)klassname->bytes(); \ 66 len = klassname->utf8_length(); \ 67 } 68 69 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \ 70 { \ 71 if (DTraceMonitorProbes) { \ 72 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \ 73 HOTSPOT_MONITOR_WAIT(jtid, \ 74 (monitor), bytes, len, (millis)); \ 75 } \ 76 } 77 78 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER 79 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED 80 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT 81 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY 82 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL 83 84 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \ 85 { \ 86 if (DTraceMonitorProbes) { \ 87 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \ 88 HOTSPOT_MONITOR_##probe(jtid, \ 89 (uintptr_t)(monitor), bytes, len); \ 90 } \ 91 } 92 93 #else // ndef DTRACE_ENABLED 94 95 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;} 96 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;} 97 98 #endif // ndef DTRACE_ENABLED 99 100 // Tunables ... 101 // The knob* variables are effectively final. Once set they should 102 // never be modified hence. Consider using __read_mostly with GCC. 103 104 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool - 105 106 static int Knob_Bonus = 100; // spin success bonus 107 static int Knob_BonusB = 100; // spin success bonus 108 static int Knob_Penalty = 200; // spin failure penalty 109 static int Knob_Poverty = 1000; 110 static int Knob_FixedSpin = 0; 111 static int Knob_PreSpin = 10; // 20-100 likely better 112 113 DEBUG_ONLY(static volatile bool InitDone = false;) 114 115 // ----------------------------------------------------------------------------- 116 // Theory of operations -- Monitors lists, thread residency, etc: 117 // 118 // * A thread acquires ownership of a monitor by successfully 119 // CAS()ing the _owner field from null to non-null. 120 // 121 // * Invariant: A thread appears on at most one monitor list -- 122 // cxq, EntryList or WaitSet -- at any one time. 123 // 124 // * Contending threads "push" themselves onto the cxq with CAS 125 // and then spin/park. 126 // 127 // * After a contending thread eventually acquires the lock it must 128 // dequeue itself from either the EntryList or the cxq. 129 // 130 // * The exiting thread identifies and unparks an "heir presumptive" 131 // tentative successor thread on the EntryList. Critically, the 132 // exiting thread doesn't unlink the successor thread from the EntryList. 133 // After having been unparked, the wakee will recontend for ownership of 134 // the monitor. The successor (wakee) will either acquire the lock or 135 // re-park itself. 136 // 137 // Succession is provided for by a policy of competitive handoff. 138 // The exiting thread does _not_ grant or pass ownership to the 139 // successor thread. (This is also referred to as "handoff" succession"). 140 // Instead the exiting thread releases ownership and possibly wakes 141 // a successor, so the successor can (re)compete for ownership of the lock. 142 // If the EntryList is empty but the cxq is populated the exiting 143 // thread will drain the cxq into the EntryList. It does so by 144 // by detaching the cxq (installing null with CAS) and folding 145 // the threads from the cxq into the EntryList. The EntryList is 146 // doubly linked, while the cxq is singly linked because of the 147 // CAS-based "push" used to enqueue recently arrived threads (RATs). 148 // 149 // * Concurrency invariants: 150 // 151 // -- only the monitor owner may access or mutate the EntryList. 152 // The mutex property of the monitor itself protects the EntryList 153 // from concurrent interference. 154 // -- Only the monitor owner may detach the cxq. 155 // 156 // * The monitor entry list operations avoid locks, but strictly speaking 157 // they're not lock-free. Enter is lock-free, exit is not. 158 // For a description of 'Methods and apparatus providing non-blocking access 159 // to a resource,' see U.S. Pat. No. 7844973. 160 // 161 // * The cxq can have multiple concurrent "pushers" but only one concurrent 162 // detaching thread. This mechanism is immune from the ABA corruption. 163 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious. 164 // 165 // * Taken together, the cxq and the EntryList constitute or form a 166 // single logical queue of threads stalled trying to acquire the lock. 167 // We use two distinct lists to improve the odds of a constant-time 168 // dequeue operation after acquisition (in the ::enter() epilogue) and 169 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm). 170 // A key desideratum is to minimize queue & monitor metadata manipulation 171 // that occurs while holding the monitor lock -- that is, we want to 172 // minimize monitor lock holds times. Note that even a small amount of 173 // fixed spinning will greatly reduce the # of enqueue-dequeue operations 174 // on EntryList|cxq. That is, spinning relieves contention on the "inner" 175 // locks and monitor metadata. 176 // 177 // Cxq points to the set of Recently Arrived Threads attempting entry. 178 // Because we push threads onto _cxq with CAS, the RATs must take the form of 179 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when 180 // the unlocking thread notices that EntryList is null but _cxq is != null. 181 // 182 // The EntryList is ordered by the prevailing queue discipline and 183 // can be organized in any convenient fashion, such as a doubly-linked list or 184 // a circular doubly-linked list. Critically, we want insert and delete operations 185 // to operate in constant-time. If we need a priority queue then something akin 186 // to Solaris' sleepq would work nicely. Viz., 187 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c. 188 // Queue discipline is enforced at ::exit() time, when the unlocking thread 189 // drains the cxq into the EntryList, and orders or reorders the threads on the 190 // EntryList accordingly. 191 // 192 // Barring "lock barging", this mechanism provides fair cyclic ordering, 193 // somewhat similar to an elevator-scan. 194 // 195 // * The monitor synchronization subsystem avoids the use of native 196 // synchronization primitives except for the narrow platform-specific 197 // park-unpark abstraction. See the comments in os_solaris.cpp regarding 198 // the semantics of park-unpark. Put another way, this monitor implementation 199 // depends only on atomic operations and park-unpark. The monitor subsystem 200 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the 201 // underlying OS manages the READY<->RUN transitions. 202 // 203 // * Waiting threads reside on the WaitSet list -- wait() puts 204 // the caller onto the WaitSet. 205 // 206 // * notify() or notifyAll() simply transfers threads from the WaitSet to 207 // either the EntryList or cxq. Subsequent exit() operations will 208 // unpark the notifyee. Unparking a notifee in notify() is inefficient - 209 // it's likely the notifyee would simply impale itself on the lock held 210 // by the notifier. 211 // 212 // * An interesting alternative is to encode cxq as (List,LockByte) where 213 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary 214 // variable, like _recursions, in the scheme. The threads or Events that form 215 // the list would have to be aligned in 256-byte addresses. A thread would 216 // try to acquire the lock or enqueue itself with CAS, but exiting threads 217 // could use a 1-0 protocol and simply STB to set the LockByte to 0. 218 // Note that is is *not* word-tearing, but it does presume that full-word 219 // CAS operations are coherent with intermix with STB operations. That's true 220 // on most common processors. 221 // 222 // * See also http://blogs.sun.com/dave 223 224 225 void* ObjectMonitor::operator new (size_t size) throw() { 226 return AllocateHeap(size, mtInternal); 227 } 228 void* ObjectMonitor::operator new[] (size_t size) throw() { 229 return operator new (size); 230 } 231 void ObjectMonitor::operator delete(void* p) { 232 FreeHeap(p); 233 } 234 void ObjectMonitor::operator delete[] (void *p) { 235 operator delete(p); 236 } 237 238 // ----------------------------------------------------------------------------- 239 // Enter support 240 241 void ObjectMonitor::enter(TRAPS) { 242 // The following code is ordered to check the most common cases first 243 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors. 244 Thread * const Self = THREAD; 245 246 void * cur = Atomic::cmpxchg(Self, &_owner, (void*)NULL); 247 if (cur == NULL) { 248 assert(_recursions == 0, "invariant"); 249 return; 250 } 251 252 if (cur == Self) { 253 // TODO-FIXME: check for integer overflow! BUGID 6557169. 254 _recursions++; 255 return; 256 } 257 258 if (Self->is_lock_owned ((address)cur)) { 259 assert(_recursions == 0, "internal state error"); 260 _recursions = 1; 261 // Commute owner from a thread-specific on-stack BasicLockObject address to 262 // a full-fledged "Thread *". 263 _owner = Self; 264 return; 265 } 266 267 // We've encountered genuine contention. 268 assert(Self->_Stalled == 0, "invariant"); 269 Self->_Stalled = intptr_t(this); 270 271 // Try one round of spinning *before* enqueueing Self 272 // and before going through the awkward and expensive state 273 // transitions. The following spin is strictly optional ... 274 // Note that if we acquire the monitor from an initial spin 275 // we forgo posting JVMTI events and firing DTRACE probes. 276 if (TrySpin(Self) > 0) { 277 assert(_owner == Self, "must be Self: owner=" INTPTR_FORMAT, p2i(_owner)); 278 assert(_recursions == 0, "must be 0: recursions=" INTPTR_FORMAT, 279 _recursions); 280 assert(((oop)object())->mark() == markWord::encode(this), 281 "object mark must match encoded this: mark=" INTPTR_FORMAT 282 ", encoded this=" INTPTR_FORMAT, ((oop)object())->mark().value(), 283 markWord::encode(this).value()); 284 Self->_Stalled = 0; 285 return; 286 } 287 288 assert(_owner != Self, "invariant"); 289 assert(_succ != Self, "invariant"); 290 assert(Self->is_Java_thread(), "invariant"); 291 JavaThread * jt = (JavaThread *) Self; 292 assert(!SafepointSynchronize::is_at_safepoint(), "invariant"); 293 assert(jt->thread_state() != _thread_blocked, "invariant"); 294 assert(this->object() != NULL, "invariant"); 295 assert(_contentions >= 0, "invariant"); 296 297 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy(). 298 // Ensure the object-monitor relationship remains stable while there's contention. 299 Atomic::inc(&_contentions); 300 301 JFR_ONLY(JfrConditionalFlushWithStacktrace<EventJavaMonitorEnter> flush(jt);) 302 EventJavaMonitorEnter event; 303 if (event.should_commit()) { 304 event.set_monitorClass(((oop)this->object())->klass()); 305 event.set_address((uintptr_t)(this->object_addr())); 306 } 307 308 { // Change java thread status to indicate blocked on monitor enter. 309 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this); 310 311 Self->set_current_pending_monitor(this); 312 313 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt); 314 if (JvmtiExport::should_post_monitor_contended_enter()) { 315 JvmtiExport::post_monitor_contended_enter(jt, this); 316 317 // The current thread does not yet own the monitor and does not 318 // yet appear on any queues that would get it made the successor. 319 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event 320 // handler cannot accidentally consume an unpark() meant for the 321 // ParkEvent associated with this ObjectMonitor. 322 } 323 324 OSThreadContendState osts(Self->osthread()); 325 ThreadBlockInVM tbivm(jt); 326 327 // TODO-FIXME: change the following for(;;) loop to straight-line code. 328 for (;;) { 329 jt->set_suspend_equivalent(); 330 // cleared by handle_special_suspend_equivalent_condition() 331 // or java_suspend_self() 332 333 EnterI(THREAD); 334 335 if (!ExitSuspendEquivalent(jt)) break; 336 337 // We have acquired the contended monitor, but while we were 338 // waiting another thread suspended us. We don't want to enter 339 // the monitor while suspended because that would surprise the 340 // thread that suspended us. 341 // 342 _recursions = 0; 343 _succ = NULL; 344 exit(false, Self); 345 346 jt->java_suspend_self(); 347 } 348 Self->set_current_pending_monitor(NULL); 349 350 // We cleared the pending monitor info since we've just gotten past 351 // the enter-check-for-suspend dance and we now own the monitor free 352 // and clear, i.e., it is no longer pending. The ThreadBlockInVM 353 // destructor can go to a safepoint at the end of this block. If we 354 // do a thread dump during that safepoint, then this thread will show 355 // as having "-locked" the monitor, but the OS and java.lang.Thread 356 // states will still report that the thread is blocked trying to 357 // acquire it. 358 } 359 360 Atomic::dec(&_contentions); 361 assert(_contentions >= 0, "invariant"); 362 Self->_Stalled = 0; 363 364 // Must either set _recursions = 0 or ASSERT _recursions == 0. 365 assert(_recursions == 0, "invariant"); 366 assert(_owner == Self, "invariant"); 367 assert(_succ != Self, "invariant"); 368 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant"); 369 370 // The thread -- now the owner -- is back in vm mode. 371 // Report the glorious news via TI,DTrace and jvmstat. 372 // The probe effect is non-trivial. All the reportage occurs 373 // while we hold the monitor, increasing the length of the critical 374 // section. Amdahl's parallel speedup law comes vividly into play. 375 // 376 // Another option might be to aggregate the events (thread local or 377 // per-monitor aggregation) and defer reporting until a more opportune 378 // time -- such as next time some thread encounters contention but has 379 // yet to acquire the lock. While spinning that thread could 380 // spinning we could increment JVMStat counters, etc. 381 382 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt); 383 if (JvmtiExport::should_post_monitor_contended_entered()) { 384 JvmtiExport::post_monitor_contended_entered(jt, this); 385 386 // The current thread already owns the monitor and is not going to 387 // call park() for the remainder of the monitor enter protocol. So 388 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED 389 // event handler consumed an unpark() issued by the thread that 390 // just exited the monitor. 391 } 392 if (event.should_commit()) { 393 event.set_previousOwner((uintptr_t)_previous_owner_tid); 394 event.commit(); 395 } 396 OM_PERFDATA_OP(ContendedLockAttempts, inc()); 397 } 398 399 // Caveat: TryLock() is not necessarily serializing if it returns failure. 400 // Callers must compensate as needed. 401 402 int ObjectMonitor::TryLock(Thread * Self) { 403 void * own = _owner; 404 if (own != NULL) return 0; 405 if (Atomic::replace_if_null(Self, &_owner)) { 406 assert(_recursions == 0, "invariant"); 407 return 1; 408 } 409 // The lock had been free momentarily, but we lost the race to the lock. 410 // Interference -- the CAS failed. 411 // We can either return -1 or retry. 412 // Retry doesn't make as much sense because the lock was just acquired. 413 return -1; 414 } 415 416 // Convert the fields used by is_busy() to a string that can be 417 // used for diagnostic output. 418 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) { 419 ss->print("is_busy: contentions=%d, waiters=%d, owner=" INTPTR_FORMAT 420 ", cxq=" INTPTR_FORMAT ", EntryList=" INTPTR_FORMAT, _contentions, 421 _waiters, p2i(_owner), p2i(_cxq), p2i(_EntryList)); 422 return ss->base(); 423 } 424 425 #define MAX_RECHECK_INTERVAL 1000 426 427 void ObjectMonitor::EnterI(TRAPS) { 428 Thread * const Self = THREAD; 429 assert(Self->is_Java_thread(), "invariant"); 430 assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant"); 431 432 // Try the lock - TATAS 433 if (TryLock (Self) > 0) { 434 assert(_succ != Self, "invariant"); 435 assert(_owner == Self, "invariant"); 436 assert(_Responsible != Self, "invariant"); 437 return; 438 } 439 440 assert(InitDone, "Unexpectedly not initialized"); 441 442 // We try one round of spinning *before* enqueueing Self. 443 // 444 // If the _owner is ready but OFFPROC we could use a YieldTo() 445 // operation to donate the remainder of this thread's quantum 446 // to the owner. This has subtle but beneficial affinity 447 // effects. 448 449 if (TrySpin(Self) > 0) { 450 assert(_owner == Self, "invariant"); 451 assert(_succ != Self, "invariant"); 452 assert(_Responsible != Self, "invariant"); 453 return; 454 } 455 456 // The Spin failed -- Enqueue and park the thread ... 457 assert(_succ != Self, "invariant"); 458 assert(_owner != Self, "invariant"); 459 assert(_Responsible != Self, "invariant"); 460 461 // Enqueue "Self" on ObjectMonitor's _cxq. 462 // 463 // Node acts as a proxy for Self. 464 // As an aside, if were to ever rewrite the synchronization code mostly 465 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class 466 // Java objects. This would avoid awkward lifecycle and liveness issues, 467 // as well as eliminate a subset of ABA issues. 468 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events. 469 470 ObjectWaiter node(Self); 471 Self->_ParkEvent->reset(); 472 node._prev = (ObjectWaiter *) 0xBAD; 473 node.TState = ObjectWaiter::TS_CXQ; 474 475 // Push "Self" onto the front of the _cxq. 476 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock. 477 // Note that spinning tends to reduce the rate at which threads 478 // enqueue and dequeue on EntryList|cxq. 479 ObjectWaiter * nxt; 480 for (;;) { 481 node._next = nxt = _cxq; 482 if (Atomic::cmpxchg(&node, &_cxq, nxt) == nxt) break; 483 484 // Interference - the CAS failed because _cxq changed. Just retry. 485 // As an optional optimization we retry the lock. 486 if (TryLock (Self) > 0) { 487 assert(_succ != Self, "invariant"); 488 assert(_owner == Self, "invariant"); 489 assert(_Responsible != Self, "invariant"); 490 return; 491 } 492 } 493 494 // Check for cxq|EntryList edge transition to non-null. This indicates 495 // the onset of contention. While contention persists exiting threads 496 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit 497 // operations revert to the faster 1-0 mode. This enter operation may interleave 498 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we 499 // arrange for one of the contending thread to use a timed park() operations 500 // to detect and recover from the race. (Stranding is form of progress failure 501 // where the monitor is unlocked but all the contending threads remain parked). 502 // That is, at least one of the contended threads will periodically poll _owner. 503 // One of the contending threads will become the designated "Responsible" thread. 504 // The Responsible thread uses a timed park instead of a normal indefinite park 505 // operation -- it periodically wakes and checks for and recovers from potential 506 // strandings admitted by 1-0 exit operations. We need at most one Responsible 507 // thread per-monitor at any given moment. Only threads on cxq|EntryList may 508 // be responsible for a monitor. 509 // 510 // Currently, one of the contended threads takes on the added role of "Responsible". 511 // A viable alternative would be to use a dedicated "stranding checker" thread 512 // that periodically iterated over all the threads (or active monitors) and unparked 513 // successors where there was risk of stranding. This would help eliminate the 514 // timer scalability issues we see on some platforms as we'd only have one thread 515 // -- the checker -- parked on a timer. 516 517 if (nxt == NULL && _EntryList == NULL) { 518 // Try to assume the role of responsible thread for the monitor. 519 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self } 520 Atomic::replace_if_null(Self, &_Responsible); 521 } 522 523 // The lock might have been released while this thread was occupied queueing 524 // itself onto _cxq. To close the race and avoid "stranding" and 525 // progress-liveness failure we must resample-retry _owner before parking. 526 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner. 527 // In this case the ST-MEMBAR is accomplished with CAS(). 528 // 529 // TODO: Defer all thread state transitions until park-time. 530 // Since state transitions are heavy and inefficient we'd like 531 // to defer the state transitions until absolutely necessary, 532 // and in doing so avoid some transitions ... 533 534 int nWakeups = 0; 535 int recheckInterval = 1; 536 537 for (;;) { 538 539 if (TryLock(Self) > 0) break; 540 assert(_owner != Self, "invariant"); 541 542 // park self 543 if (_Responsible == Self) { 544 Self->_ParkEvent->park((jlong) recheckInterval); 545 // Increase the recheckInterval, but clamp the value. 546 recheckInterval *= 8; 547 if (recheckInterval > MAX_RECHECK_INTERVAL) { 548 recheckInterval = MAX_RECHECK_INTERVAL; 549 } 550 } else { 551 Self->_ParkEvent->park(); 552 } 553 554 if (TryLock(Self) > 0) break; 555 556 // The lock is still contested. 557 // Keep a tally of the # of futile wakeups. 558 // Note that the counter is not protected by a lock or updated by atomics. 559 // That is by design - we trade "lossy" counters which are exposed to 560 // races during updates for a lower probe effect. 561 562 // This PerfData object can be used in parallel with a safepoint. 563 // See the work around in PerfDataManager::destroy(). 564 OM_PERFDATA_OP(FutileWakeups, inc()); 565 ++nWakeups; 566 567 // Assuming this is not a spurious wakeup we'll normally find _succ == Self. 568 // We can defer clearing _succ until after the spin completes 569 // TrySpin() must tolerate being called with _succ == Self. 570 // Try yet another round of adaptive spinning. 571 if (TrySpin(Self) > 0) break; 572 573 // We can find that we were unpark()ed and redesignated _succ while 574 // we were spinning. That's harmless. If we iterate and call park(), 575 // park() will consume the event and return immediately and we'll 576 // just spin again. This pattern can repeat, leaving _succ to simply 577 // spin on a CPU. 578 579 if (_succ == Self) _succ = NULL; 580 581 // Invariant: after clearing _succ a thread *must* retry _owner before parking. 582 OrderAccess::fence(); 583 } 584 585 // Egress : 586 // Self has acquired the lock -- Unlink Self from the cxq or EntryList. 587 // Normally we'll find Self on the EntryList . 588 // From the perspective of the lock owner (this thread), the 589 // EntryList is stable and cxq is prepend-only. 590 // The head of cxq is volatile but the interior is stable. 591 // In addition, Self.TState is stable. 592 593 assert(_owner == Self, "invariant"); 594 assert(object() != NULL, "invariant"); 595 // I'd like to write: 596 // guarantee (((oop)(object()))->mark() == markWord::encode(this), "invariant") ; 597 // but as we're at a safepoint that's not safe. 598 599 UnlinkAfterAcquire(Self, &node); 600 if (_succ == Self) _succ = NULL; 601 602 assert(_succ != Self, "invariant"); 603 if (_Responsible == Self) { 604 _Responsible = NULL; 605 OrderAccess::fence(); // Dekker pivot-point 606 607 // We may leave threads on cxq|EntryList without a designated 608 // "Responsible" thread. This is benign. When this thread subsequently 609 // exits the monitor it can "see" such preexisting "old" threads -- 610 // threads that arrived on the cxq|EntryList before the fence, above -- 611 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads 612 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible 613 // non-null and elect a new "Responsible" timer thread. 614 // 615 // This thread executes: 616 // ST Responsible=null; MEMBAR (in enter epilogue - here) 617 // LD cxq|EntryList (in subsequent exit) 618 // 619 // Entering threads in the slow/contended path execute: 620 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog) 621 // The (ST cxq; MEMBAR) is accomplished with CAS(). 622 // 623 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent 624 // exit operation from floating above the ST Responsible=null. 625 } 626 627 // We've acquired ownership with CAS(). 628 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics. 629 // But since the CAS() this thread may have also stored into _succ, 630 // EntryList, cxq or Responsible. These meta-data updates must be 631 // visible __before this thread subsequently drops the lock. 632 // Consider what could occur if we didn't enforce this constraint -- 633 // STs to monitor meta-data and user-data could reorder with (become 634 // visible after) the ST in exit that drops ownership of the lock. 635 // Some other thread could then acquire the lock, but observe inconsistent 636 // or old monitor meta-data and heap data. That violates the JMM. 637 // To that end, the 1-0 exit() operation must have at least STST|LDST 638 // "release" barrier semantics. Specifically, there must be at least a 639 // STST|LDST barrier in exit() before the ST of null into _owner that drops 640 // the lock. The barrier ensures that changes to monitor meta-data and data 641 // protected by the lock will be visible before we release the lock, and 642 // therefore before some other thread (CPU) has a chance to acquire the lock. 643 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html. 644 // 645 // Critically, any prior STs to _succ or EntryList must be visible before 646 // the ST of null into _owner in the *subsequent* (following) corresponding 647 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily 648 // execute a serializing instruction. 649 650 return; 651 } 652 653 // ReenterI() is a specialized inline form of the latter half of the 654 // contended slow-path from EnterI(). We use ReenterI() only for 655 // monitor reentry in wait(). 656 // 657 // In the future we should reconcile EnterI() and ReenterI(). 658 659 void ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) { 660 assert(Self != NULL, "invariant"); 661 assert(SelfNode != NULL, "invariant"); 662 assert(SelfNode->_thread == Self, "invariant"); 663 assert(_waiters > 0, "invariant"); 664 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant"); 665 assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant"); 666 JavaThread * jt = (JavaThread *) Self; 667 668 int nWakeups = 0; 669 for (;;) { 670 ObjectWaiter::TStates v = SelfNode->TState; 671 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant"); 672 assert(_owner != Self, "invariant"); 673 674 if (TryLock(Self) > 0) break; 675 if (TrySpin(Self) > 0) break; 676 677 // State transition wrappers around park() ... 678 // ReenterI() wisely defers state transitions until 679 // it's clear we must park the thread. 680 { 681 OSThreadContendState osts(Self->osthread()); 682 ThreadBlockInVM tbivm(jt); 683 684 // cleared by handle_special_suspend_equivalent_condition() 685 // or java_suspend_self() 686 jt->set_suspend_equivalent(); 687 Self->_ParkEvent->park(); 688 689 // were we externally suspended while we were waiting? 690 for (;;) { 691 if (!ExitSuspendEquivalent(jt)) break; 692 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); } 693 jt->java_suspend_self(); 694 jt->set_suspend_equivalent(); 695 } 696 } 697 698 // Try again, but just so we distinguish between futile wakeups and 699 // successful wakeups. The following test isn't algorithmically 700 // necessary, but it helps us maintain sensible statistics. 701 if (TryLock(Self) > 0) break; 702 703 // The lock is still contested. 704 // Keep a tally of the # of futile wakeups. 705 // Note that the counter is not protected by a lock or updated by atomics. 706 // That is by design - we trade "lossy" counters which are exposed to 707 // races during updates for a lower probe effect. 708 ++nWakeups; 709 710 // Assuming this is not a spurious wakeup we'll normally 711 // find that _succ == Self. 712 if (_succ == Self) _succ = NULL; 713 714 // Invariant: after clearing _succ a contending thread 715 // *must* retry _owner before parking. 716 OrderAccess::fence(); 717 718 // This PerfData object can be used in parallel with a safepoint. 719 // See the work around in PerfDataManager::destroy(). 720 OM_PERFDATA_OP(FutileWakeups, inc()); 721 } 722 723 // Self has acquired the lock -- Unlink Self from the cxq or EntryList . 724 // Normally we'll find Self on the EntryList. 725 // Unlinking from the EntryList is constant-time and atomic-free. 726 // From the perspective of the lock owner (this thread), the 727 // EntryList is stable and cxq is prepend-only. 728 // The head of cxq is volatile but the interior is stable. 729 // In addition, Self.TState is stable. 730 731 assert(_owner == Self, "invariant"); 732 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant"); 733 UnlinkAfterAcquire(Self, SelfNode); 734 if (_succ == Self) _succ = NULL; 735 assert(_succ != Self, "invariant"); 736 SelfNode->TState = ObjectWaiter::TS_RUN; 737 OrderAccess::fence(); // see comments at the end of EnterI() 738 } 739 740 // By convention we unlink a contending thread from EntryList|cxq immediately 741 // after the thread acquires the lock in ::enter(). Equally, we could defer 742 // unlinking the thread until ::exit()-time. 743 744 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) { 745 assert(_owner == Self, "invariant"); 746 assert(SelfNode->_thread == Self, "invariant"); 747 748 if (SelfNode->TState == ObjectWaiter::TS_ENTER) { 749 // Normal case: remove Self from the DLL EntryList . 750 // This is a constant-time operation. 751 ObjectWaiter * nxt = SelfNode->_next; 752 ObjectWaiter * prv = SelfNode->_prev; 753 if (nxt != NULL) nxt->_prev = prv; 754 if (prv != NULL) prv->_next = nxt; 755 if (SelfNode == _EntryList) _EntryList = nxt; 756 assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant"); 757 assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant"); 758 } else { 759 assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant"); 760 // Inopportune interleaving -- Self is still on the cxq. 761 // This usually means the enqueue of self raced an exiting thread. 762 // Normally we'll find Self near the front of the cxq, so 763 // dequeueing is typically fast. If needbe we can accelerate 764 // this with some MCS/CHL-like bidirectional list hints and advisory 765 // back-links so dequeueing from the interior will normally operate 766 // in constant-time. 767 // Dequeue Self from either the head (with CAS) or from the interior 768 // with a linear-time scan and normal non-atomic memory operations. 769 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList 770 // and then unlink Self from EntryList. We have to drain eventually, 771 // so it might as well be now. 772 773 ObjectWaiter * v = _cxq; 774 assert(v != NULL, "invariant"); 775 if (v != SelfNode || Atomic::cmpxchg(SelfNode->_next, &_cxq, v) != v) { 776 // The CAS above can fail from interference IFF a "RAT" arrived. 777 // In that case Self must be in the interior and can no longer be 778 // at the head of cxq. 779 if (v == SelfNode) { 780 assert(_cxq != v, "invariant"); 781 v = _cxq; // CAS above failed - start scan at head of list 782 } 783 ObjectWaiter * p; 784 ObjectWaiter * q = NULL; 785 for (p = v; p != NULL && p != SelfNode; p = p->_next) { 786 q = p; 787 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant"); 788 } 789 assert(v != SelfNode, "invariant"); 790 assert(p == SelfNode, "Node not found on cxq"); 791 assert(p != _cxq, "invariant"); 792 assert(q != NULL, "invariant"); 793 assert(q->_next == p, "invariant"); 794 q->_next = p->_next; 795 } 796 } 797 798 #ifdef ASSERT 799 // Diagnostic hygiene ... 800 SelfNode->_prev = (ObjectWaiter *) 0xBAD; 801 SelfNode->_next = (ObjectWaiter *) 0xBAD; 802 SelfNode->TState = ObjectWaiter::TS_RUN; 803 #endif 804 } 805 806 // ----------------------------------------------------------------------------- 807 // Exit support 808 // 809 // exit() 810 // ~~~~~~ 811 // Note that the collector can't reclaim the objectMonitor or deflate 812 // the object out from underneath the thread calling ::exit() as the 813 // thread calling ::exit() never transitions to a stable state. 814 // This inhibits GC, which in turn inhibits asynchronous (and 815 // inopportune) reclamation of "this". 816 // 817 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ; 818 // There's one exception to the claim above, however. EnterI() can call 819 // exit() to drop a lock if the acquirer has been externally suspended. 820 // In that case exit() is called with _thread_state as _thread_blocked, 821 // but the monitor's _contentions field is > 0, which inhibits reclamation. 822 // 823 // 1-0 exit 824 // ~~~~~~~~ 825 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of 826 // the fast-path operators have been optimized so the common ::exit() 827 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock(). 828 // The code emitted by fast_unlock() elides the usual MEMBAR. This 829 // greatly improves latency -- MEMBAR and CAS having considerable local 830 // latency on modern processors -- but at the cost of "stranding". Absent the 831 // MEMBAR, a thread in fast_unlock() can race a thread in the slow 832 // ::enter() path, resulting in the entering thread being stranding 833 // and a progress-liveness failure. Stranding is extremely rare. 834 // We use timers (timed park operations) & periodic polling to detect 835 // and recover from stranding. Potentially stranded threads periodically 836 // wake up and poll the lock. See the usage of the _Responsible variable. 837 // 838 // The CAS() in enter provides for safety and exclusion, while the CAS or 839 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking 840 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding. 841 // We detect and recover from stranding with timers. 842 // 843 // If a thread transiently strands it'll park until (a) another 844 // thread acquires the lock and then drops the lock, at which time the 845 // exiting thread will notice and unpark the stranded thread, or, (b) 846 // the timer expires. If the lock is high traffic then the stranding latency 847 // will be low due to (a). If the lock is low traffic then the odds of 848 // stranding are lower, although the worst-case stranding latency 849 // is longer. Critically, we don't want to put excessive load in the 850 // platform's timer subsystem. We want to minimize both the timer injection 851 // rate (timers created/sec) as well as the number of timers active at 852 // any one time. (more precisely, we want to minimize timer-seconds, which is 853 // the integral of the # of active timers at any instant over time). 854 // Both impinge on OS scalability. Given that, at most one thread parked on 855 // a monitor will use a timer. 856 // 857 // There is also the risk of a futile wake-up. If we drop the lock 858 // another thread can reacquire the lock immediately, and we can 859 // then wake a thread unnecessarily. This is benign, and we've 860 // structured the code so the windows are short and the frequency 861 // of such futile wakups is low. 862 863 void ObjectMonitor::exit(bool not_suspended, TRAPS) { 864 Thread * const Self = THREAD; 865 if (THREAD != _owner) { 866 if (THREAD->is_lock_owned((address) _owner)) { 867 // Transmute _owner from a BasicLock pointer to a Thread address. 868 // We don't need to hold _mutex for this transition. 869 // Non-null to Non-null is safe as long as all readers can 870 // tolerate either flavor. 871 assert(_recursions == 0, "invariant"); 872 _owner = THREAD; 873 _recursions = 0; 874 } else { 875 // Apparent unbalanced locking ... 876 // Naively we'd like to throw IllegalMonitorStateException. 877 // As a practical matter we can neither allocate nor throw an 878 // exception as ::exit() can be called from leaf routines. 879 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties. 880 // Upon deeper reflection, however, in a properly run JVM the only 881 // way we should encounter this situation is in the presence of 882 // unbalanced JNI locking. TODO: CheckJNICalls. 883 // See also: CR4414101 884 assert(false, "Non-balanced monitor enter/exit! Likely JNI locking"); 885 return; 886 } 887 } 888 889 if (_recursions != 0) { 890 _recursions--; // this is simple recursive enter 891 return; 892 } 893 894 // Invariant: after setting Responsible=null an thread must execute 895 // a MEMBAR or other serializing instruction before fetching EntryList|cxq. 896 _Responsible = NULL; 897 898 #if INCLUDE_JFR 899 // get the owner's thread id for the MonitorEnter event 900 // if it is enabled and the thread isn't suspended 901 if (not_suspended && EventJavaMonitorEnter::is_enabled()) { 902 _previous_owner_tid = JFR_THREAD_ID(Self); 903 } 904 #endif 905 906 for (;;) { 907 assert(THREAD == _owner, "invariant"); 908 909 // release semantics: prior loads and stores from within the critical section 910 // must not float (reorder) past the following store that drops the lock. 911 // On SPARC that requires MEMBAR #loadstore|#storestore. 912 // But of course in TSO #loadstore|#storestore is not required. 913 OrderAccess::release_store(&_owner, (void*)NULL); // drop the lock 914 OrderAccess::storeload(); // See if we need to wake a successor 915 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) { 916 return; 917 } 918 // Other threads are blocked trying to acquire the lock. 919 920 // Normally the exiting thread is responsible for ensuring succession, 921 // but if other successors are ready or other entering threads are spinning 922 // then this thread can simply store NULL into _owner and exit without 923 // waking a successor. The existence of spinners or ready successors 924 // guarantees proper succession (liveness). Responsibility passes to the 925 // ready or running successors. The exiting thread delegates the duty. 926 // More precisely, if a successor already exists this thread is absolved 927 // of the responsibility of waking (unparking) one. 928 // 929 // The _succ variable is critical to reducing futile wakeup frequency. 930 // _succ identifies the "heir presumptive" thread that has been made 931 // ready (unparked) but that has not yet run. We need only one such 932 // successor thread to guarantee progress. 933 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf 934 // section 3.3 "Futile Wakeup Throttling" for details. 935 // 936 // Note that spinners in Enter() also set _succ non-null. 937 // In the current implementation spinners opportunistically set 938 // _succ so that exiting threads might avoid waking a successor. 939 // Another less appealing alternative would be for the exiting thread 940 // to drop the lock and then spin briefly to see if a spinner managed 941 // to acquire the lock. If so, the exiting thread could exit 942 // immediately without waking a successor, otherwise the exiting 943 // thread would need to dequeue and wake a successor. 944 // (Note that we'd need to make the post-drop spin short, but no 945 // shorter than the worst-case round-trip cache-line migration time. 946 // The dropped lock needs to become visible to the spinner, and then 947 // the acquisition of the lock by the spinner must become visible to 948 // the exiting thread). 949 950 // It appears that an heir-presumptive (successor) must be made ready. 951 // Only the current lock owner can manipulate the EntryList or 952 // drain _cxq, so we need to reacquire the lock. If we fail 953 // to reacquire the lock the responsibility for ensuring succession 954 // falls to the new owner. 955 // 956 if (!Atomic::replace_if_null(THREAD, &_owner)) { 957 return; 958 } 959 960 guarantee(_owner == THREAD, "invariant"); 961 962 ObjectWaiter * w = NULL; 963 964 w = _EntryList; 965 if (w != NULL) { 966 // I'd like to write: guarantee (w->_thread != Self). 967 // But in practice an exiting thread may find itself on the EntryList. 968 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and 969 // then calls exit(). Exit release the lock by setting O._owner to NULL. 970 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The 971 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then 972 // release the lock "O". T2 resumes immediately after the ST of null into 973 // _owner, above. T2 notices that the EntryList is populated, so it 974 // reacquires the lock and then finds itself on the EntryList. 975 // Given all that, we have to tolerate the circumstance where "w" is 976 // associated with Self. 977 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant"); 978 ExitEpilog(Self, w); 979 return; 980 } 981 982 // If we find that both _cxq and EntryList are null then just 983 // re-run the exit protocol from the top. 984 w = _cxq; 985 if (w == NULL) continue; 986 987 // Drain _cxq into EntryList - bulk transfer. 988 // First, detach _cxq. 989 // The following loop is tantamount to: w = swap(&cxq, NULL) 990 for (;;) { 991 assert(w != NULL, "Invariant"); 992 ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w); 993 if (u == w) break; 994 w = u; 995 } 996 997 assert(w != NULL, "invariant"); 998 assert(_EntryList == NULL, "invariant"); 999 1000 // Convert the LIFO SLL anchored by _cxq into a DLL. 1001 // The list reorganization step operates in O(LENGTH(w)) time. 1002 // It's critical that this step operate quickly as 1003 // "Self" still holds the outer-lock, restricting parallelism 1004 // and effectively lengthening the critical section. 1005 // Invariant: s chases t chases u. 1006 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so 1007 // we have faster access to the tail. 1008 1009 _EntryList = w; 1010 ObjectWaiter * q = NULL; 1011 ObjectWaiter * p; 1012 for (p = w; p != NULL; p = p->_next) { 1013 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant"); 1014 p->TState = ObjectWaiter::TS_ENTER; 1015 p->_prev = q; 1016 q = p; 1017 } 1018 1019 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL 1020 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog(). 1021 1022 // See if we can abdicate to a spinner instead of waking a thread. 1023 // A primary goal of the implementation is to reduce the 1024 // context-switch rate. 1025 if (_succ != NULL) continue; 1026 1027 w = _EntryList; 1028 if (w != NULL) { 1029 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant"); 1030 ExitEpilog(Self, w); 1031 return; 1032 } 1033 } 1034 } 1035 1036 // ExitSuspendEquivalent: 1037 // A faster alternate to handle_special_suspend_equivalent_condition() 1038 // 1039 // handle_special_suspend_equivalent_condition() unconditionally 1040 // acquires the SR_lock. On some platforms uncontended MutexLocker() 1041 // operations have high latency. Note that in ::enter() we call HSSEC 1042 // while holding the monitor, so we effectively lengthen the critical sections. 1043 // 1044 // There are a number of possible solutions: 1045 // 1046 // A. To ameliorate the problem we might also defer state transitions 1047 // to as late as possible -- just prior to parking. 1048 // Given that, we'd call HSSEC after having returned from park(), 1049 // but before attempting to acquire the monitor. This is only a 1050 // partial solution. It avoids calling HSSEC while holding the 1051 // monitor (good), but it still increases successor reacquisition latency -- 1052 // the interval between unparking a successor and the time the successor 1053 // resumes and retries the lock. See ReenterI(), which defers state transitions. 1054 // If we use this technique we can also avoid EnterI()-exit() loop 1055 // in ::enter() where we iteratively drop the lock and then attempt 1056 // to reacquire it after suspending. 1057 // 1058 // B. In the future we might fold all the suspend bits into a 1059 // composite per-thread suspend flag and then update it with CAS(). 1060 // Alternately, a Dekker-like mechanism with multiple variables 1061 // would suffice: 1062 // ST Self->_suspend_equivalent = false 1063 // MEMBAR 1064 // LD Self_>_suspend_flags 1065 1066 bool ObjectMonitor::ExitSuspendEquivalent(JavaThread * jSelf) { 1067 return jSelf->handle_special_suspend_equivalent_condition(); 1068 } 1069 1070 1071 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) { 1072 assert(_owner == Self, "invariant"); 1073 1074 // Exit protocol: 1075 // 1. ST _succ = wakee 1076 // 2. membar #loadstore|#storestore; 1077 // 2. ST _owner = NULL 1078 // 3. unpark(wakee) 1079 1080 _succ = Wakee->_thread; 1081 ParkEvent * Trigger = Wakee->_event; 1082 1083 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again. 1084 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be 1085 // out-of-scope (non-extant). 1086 Wakee = NULL; 1087 1088 // Drop the lock 1089 OrderAccess::release_store(&_owner, (void*)NULL); 1090 OrderAccess::fence(); // ST _owner vs LD in unpark() 1091 1092 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self); 1093 Trigger->unpark(); 1094 1095 // Maintain stats and report events to JVMTI 1096 OM_PERFDATA_OP(Parks, inc()); 1097 } 1098 1099 1100 // ----------------------------------------------------------------------------- 1101 // Class Loader deadlock handling. 1102 // 1103 // complete_exit exits a lock returning recursion count 1104 // complete_exit/reenter operate as a wait without waiting 1105 // complete_exit requires an inflated monitor 1106 // The _owner field is not always the Thread addr even with an 1107 // inflated monitor, e.g. the monitor can be inflated by a non-owning 1108 // thread due to contention. 1109 intptr_t ObjectMonitor::complete_exit(TRAPS) { 1110 Thread * const Self = THREAD; 1111 assert(Self->is_Java_thread(), "Must be Java thread!"); 1112 JavaThread *jt = (JavaThread *)THREAD; 1113 1114 assert(InitDone, "Unexpectedly not initialized"); 1115 1116 if (THREAD != _owner) { 1117 if (THREAD->is_lock_owned ((address)_owner)) { 1118 assert(_recursions == 0, "internal state error"); 1119 _owner = THREAD; // Convert from basiclock addr to Thread addr 1120 _recursions = 0; 1121 } 1122 } 1123 1124 guarantee(Self == _owner, "complete_exit not owner"); 1125 intptr_t save = _recursions; // record the old recursion count 1126 _recursions = 0; // set the recursion level to be 0 1127 exit(true, Self); // exit the monitor 1128 guarantee(_owner != Self, "invariant"); 1129 return save; 1130 } 1131 1132 // reenter() enters a lock and sets recursion count 1133 // complete_exit/reenter operate as a wait without waiting 1134 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) { 1135 Thread * const Self = THREAD; 1136 assert(Self->is_Java_thread(), "Must be Java thread!"); 1137 JavaThread *jt = (JavaThread *)THREAD; 1138 1139 guarantee(_owner != Self, "reenter already owner"); 1140 enter(THREAD); // enter the monitor 1141 guarantee(_recursions == 0, "reenter recursion"); 1142 _recursions = recursions; 1143 return; 1144 } 1145 1146 // Checks that the current THREAD owns this monitor and causes an 1147 // immediate return if it doesn't. We don't use the CHECK macro 1148 // because we want the IMSE to be the only exception that is thrown 1149 // from the call site when false is returned. Any other pending 1150 // exception is ignored. 1151 #define CHECK_OWNER() \ 1152 do { \ 1153 if (!check_owner(THREAD)) { \ 1154 assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \ 1155 return; \ 1156 } \ 1157 } while (false) 1158 1159 // Returns true if the specified thread owns the ObjectMonitor. 1160 // Otherwise returns false and throws IllegalMonitorStateException 1161 // (IMSE). If there is a pending exception and the specified thread 1162 // is not the owner, that exception will be replaced by the IMSE. 1163 bool ObjectMonitor::check_owner(Thread* THREAD) { 1164 if (_owner == THREAD) { 1165 return true; 1166 } 1167 if (THREAD->is_lock_owned((address)_owner)) { 1168 _owner = THREAD; // convert from BasicLock addr to Thread addr 1169 _recursions = 0; 1170 return true; 1171 } 1172 THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(), 1173 "current thread is not owner", false); 1174 } 1175 1176 static void post_monitor_wait_event(EventJavaMonitorWait* event, 1177 ObjectMonitor* monitor, 1178 jlong notifier_tid, 1179 jlong timeout, 1180 bool timedout) { 1181 assert(event != NULL, "invariant"); 1182 assert(monitor != NULL, "invariant"); 1183 event->set_monitorClass(((oop)monitor->object())->klass()); 1184 event->set_timeout(timeout); 1185 event->set_address((uintptr_t)monitor->object_addr()); 1186 event->set_notifier(notifier_tid); 1187 event->set_timedOut(timedout); 1188 event->commit(); 1189 } 1190 1191 // ----------------------------------------------------------------------------- 1192 // Wait/Notify/NotifyAll 1193 // 1194 // Note: a subset of changes to ObjectMonitor::wait() 1195 // will need to be replicated in complete_exit 1196 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) { 1197 Thread * const Self = THREAD; 1198 assert(Self->is_Java_thread(), "Must be Java thread!"); 1199 JavaThread *jt = (JavaThread *)THREAD; 1200 1201 assert(InitDone, "Unexpectedly not initialized"); 1202 1203 CHECK_OWNER(); // Throws IMSE if not owner. 1204 1205 EventJavaMonitorWait event; 1206 1207 // check for a pending interrupt 1208 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) { 1209 // post monitor waited event. Note that this is past-tense, we are done waiting. 1210 if (JvmtiExport::should_post_monitor_waited()) { 1211 // Note: 'false' parameter is passed here because the 1212 // wait was not timed out due to thread interrupt. 1213 JvmtiExport::post_monitor_waited(jt, this, false); 1214 1215 // In this short circuit of the monitor wait protocol, the 1216 // current thread never drops ownership of the monitor and 1217 // never gets added to the wait queue so the current thread 1218 // cannot be made the successor. This means that the 1219 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally 1220 // consume an unpark() meant for the ParkEvent associated with 1221 // this ObjectMonitor. 1222 } 1223 if (event.should_commit()) { 1224 post_monitor_wait_event(&event, this, 0, millis, false); 1225 } 1226 THROW(vmSymbols::java_lang_InterruptedException()); 1227 return; 1228 } 1229 1230 assert(Self->_Stalled == 0, "invariant"); 1231 Self->_Stalled = intptr_t(this); 1232 jt->set_current_waiting_monitor(this); 1233 1234 // create a node to be put into the queue 1235 // Critically, after we reset() the event but prior to park(), we must check 1236 // for a pending interrupt. 1237 ObjectWaiter node(Self); 1238 node.TState = ObjectWaiter::TS_WAIT; 1239 Self->_ParkEvent->reset(); 1240 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag 1241 1242 // Enter the waiting queue, which is a circular doubly linked list in this case 1243 // but it could be a priority queue or any data structure. 1244 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only 1245 // by the the owner of the monitor *except* in the case where park() 1246 // returns because of a timeout of interrupt. Contention is exceptionally rare 1247 // so we use a simple spin-lock instead of a heavier-weight blocking lock. 1248 1249 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add"); 1250 AddWaiter(&node); 1251 Thread::SpinRelease(&_WaitSetLock); 1252 1253 _Responsible = NULL; 1254 1255 intptr_t save = _recursions; // record the old recursion count 1256 _waiters++; // increment the number of waiters 1257 _recursions = 0; // set the recursion level to be 1 1258 exit(true, Self); // exit the monitor 1259 guarantee(_owner != Self, "invariant"); 1260 1261 // The thread is on the WaitSet list - now park() it. 1262 // On MP systems it's conceivable that a brief spin before we park 1263 // could be profitable. 1264 // 1265 // TODO-FIXME: change the following logic to a loop of the form 1266 // while (!timeout && !interrupted && _notified == 0) park() 1267 1268 int ret = OS_OK; 1269 int WasNotified = 0; 1270 { // State transition wrappers 1271 OSThread* osthread = Self->osthread(); 1272 OSThreadWaitState osts(osthread, true); 1273 { 1274 ThreadBlockInVM tbivm(jt); 1275 // Thread is in thread_blocked state and oop access is unsafe. 1276 jt->set_suspend_equivalent(); 1277 1278 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) { 1279 // Intentionally empty 1280 } else if (node._notified == 0) { 1281 if (millis <= 0) { 1282 Self->_ParkEvent->park(); 1283 } else { 1284 ret = Self->_ParkEvent->park(millis); 1285 } 1286 } 1287 1288 // were we externally suspended while we were waiting? 1289 if (ExitSuspendEquivalent (jt)) { 1290 // TODO-FIXME: add -- if succ == Self then succ = null. 1291 jt->java_suspend_self(); 1292 } 1293 1294 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm 1295 1296 // Node may be on the WaitSet, the EntryList (or cxq), or in transition 1297 // from the WaitSet to the EntryList. 1298 // See if we need to remove Node from the WaitSet. 1299 // We use double-checked locking to avoid grabbing _WaitSetLock 1300 // if the thread is not on the wait queue. 1301 // 1302 // Note that we don't need a fence before the fetch of TState. 1303 // In the worst case we'll fetch a old-stale value of TS_WAIT previously 1304 // written by the is thread. (perhaps the fetch might even be satisfied 1305 // by a look-aside into the processor's own store buffer, although given 1306 // the length of the code path between the prior ST and this load that's 1307 // highly unlikely). If the following LD fetches a stale TS_WAIT value 1308 // then we'll acquire the lock and then re-fetch a fresh TState value. 1309 // That is, we fail toward safety. 1310 1311 if (node.TState == ObjectWaiter::TS_WAIT) { 1312 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink"); 1313 if (node.TState == ObjectWaiter::TS_WAIT) { 1314 DequeueSpecificWaiter(&node); // unlink from WaitSet 1315 assert(node._notified == 0, "invariant"); 1316 node.TState = ObjectWaiter::TS_RUN; 1317 } 1318 Thread::SpinRelease(&_WaitSetLock); 1319 } 1320 1321 // The thread is now either on off-list (TS_RUN), 1322 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ). 1323 // The Node's TState variable is stable from the perspective of this thread. 1324 // No other threads will asynchronously modify TState. 1325 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant"); 1326 OrderAccess::loadload(); 1327 if (_succ == Self) _succ = NULL; 1328 WasNotified = node._notified; 1329 1330 // Reentry phase -- reacquire the monitor. 1331 // re-enter contended monitor after object.wait(). 1332 // retain OBJECT_WAIT state until re-enter successfully completes 1333 // Thread state is thread_in_vm and oop access is again safe, 1334 // although the raw address of the object may have changed. 1335 // (Don't cache naked oops over safepoints, of course). 1336 1337 // post monitor waited event. Note that this is past-tense, we are done waiting. 1338 if (JvmtiExport::should_post_monitor_waited()) { 1339 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT); 1340 1341 if (node._notified != 0 && _succ == Self) { 1342 // In this part of the monitor wait-notify-reenter protocol it 1343 // is possible (and normal) for another thread to do a fastpath 1344 // monitor enter-exit while this thread is still trying to get 1345 // to the reenter portion of the protocol. 1346 // 1347 // The ObjectMonitor was notified and the current thread is 1348 // the successor which also means that an unpark() has already 1349 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can 1350 // consume the unpark() that was done when the successor was 1351 // set because the same ParkEvent is shared between Java 1352 // monitors and JVM/TI RawMonitors (for now). 1353 // 1354 // We redo the unpark() to ensure forward progress, i.e., we 1355 // don't want all pending threads hanging (parked) with none 1356 // entering the unlocked monitor. 1357 node._event->unpark(); 1358 } 1359 } 1360 1361 if (event.should_commit()) { 1362 post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT); 1363 } 1364 1365 OrderAccess::fence(); 1366 1367 assert(Self->_Stalled != 0, "invariant"); 1368 Self->_Stalled = 0; 1369 1370 assert(_owner != Self, "invariant"); 1371 ObjectWaiter::TStates v = node.TState; 1372 if (v == ObjectWaiter::TS_RUN) { 1373 enter(Self); 1374 } else { 1375 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant"); 1376 ReenterI(Self, &node); 1377 node.wait_reenter_end(this); 1378 } 1379 1380 // Self has reacquired the lock. 1381 // Lifecycle - the node representing Self must not appear on any queues. 1382 // Node is about to go out-of-scope, but even if it were immortal we wouldn't 1383 // want residual elements associated with this thread left on any lists. 1384 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant"); 1385 assert(_owner == Self, "invariant"); 1386 assert(_succ != Self, "invariant"); 1387 } // OSThreadWaitState() 1388 1389 jt->set_current_waiting_monitor(NULL); 1390 1391 guarantee(_recursions == 0, "invariant"); 1392 _recursions = save // restore the old recursion count 1393 + jt->get_and_reset_relock_count_after_wait(); // increased by the deferred relock count 1394 _waiters--; // decrement the number of waiters 1395 1396 // Verify a few postconditions 1397 assert(_owner == Self, "invariant"); 1398 assert(_succ != Self, "invariant"); 1399 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant"); 1400 1401 // check if the notification happened 1402 if (!WasNotified) { 1403 // no, it could be timeout or Thread.interrupt() or both 1404 // check for interrupt event, otherwise it is timeout 1405 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) { 1406 THROW(vmSymbols::java_lang_InterruptedException()); 1407 } 1408 } 1409 1410 // NOTE: Spurious wake up will be consider as timeout. 1411 // Monitor notify has precedence over thread interrupt. 1412 } 1413 1414 1415 // Consider: 1416 // If the lock is cool (cxq == null && succ == null) and we're on an MP system 1417 // then instead of transferring a thread from the WaitSet to the EntryList 1418 // we might just dequeue a thread from the WaitSet and directly unpark() it. 1419 1420 void ObjectMonitor::INotify(Thread * Self) { 1421 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify"); 1422 ObjectWaiter * iterator = DequeueWaiter(); 1423 if (iterator != NULL) { 1424 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant"); 1425 guarantee(iterator->_notified == 0, "invariant"); 1426 // Disposition - what might we do with iterator ? 1427 // a. add it directly to the EntryList - either tail (policy == 1) 1428 // or head (policy == 0). 1429 // b. push it onto the front of the _cxq (policy == 2). 1430 // For now we use (b). 1431 1432 iterator->TState = ObjectWaiter::TS_ENTER; 1433 1434 iterator->_notified = 1; 1435 iterator->_notifier_tid = JFR_THREAD_ID(Self); 1436 1437 ObjectWaiter * list = _EntryList; 1438 if (list != NULL) { 1439 assert(list->_prev == NULL, "invariant"); 1440 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant"); 1441 assert(list != iterator, "invariant"); 1442 } 1443 1444 // prepend to cxq 1445 if (list == NULL) { 1446 iterator->_next = iterator->_prev = NULL; 1447 _EntryList = iterator; 1448 } else { 1449 iterator->TState = ObjectWaiter::TS_CXQ; 1450 for (;;) { 1451 ObjectWaiter * front = _cxq; 1452 iterator->_next = front; 1453 if (Atomic::cmpxchg(iterator, &_cxq, front) == front) { 1454 break; 1455 } 1456 } 1457 } 1458 1459 // _WaitSetLock protects the wait queue, not the EntryList. We could 1460 // move the add-to-EntryList operation, above, outside the critical section 1461 // protected by _WaitSetLock. In practice that's not useful. With the 1462 // exception of wait() timeouts and interrupts the monitor owner 1463 // is the only thread that grabs _WaitSetLock. There's almost no contention 1464 // on _WaitSetLock so it's not profitable to reduce the length of the 1465 // critical section. 1466 1467 iterator->wait_reenter_begin(this); 1468 } 1469 Thread::SpinRelease(&_WaitSetLock); 1470 } 1471 1472 // Consider: a not-uncommon synchronization bug is to use notify() when 1473 // notifyAll() is more appropriate, potentially resulting in stranded 1474 // threads; this is one example of a lost wakeup. A useful diagnostic 1475 // option is to force all notify() operations to behave as notifyAll(). 1476 // 1477 // Note: We can also detect many such problems with a "minimum wait". 1478 // When the "minimum wait" is set to a small non-zero timeout value 1479 // and the program does not hang whereas it did absent "minimum wait", 1480 // that suggests a lost wakeup bug. 1481 1482 void ObjectMonitor::notify(TRAPS) { 1483 CHECK_OWNER(); // Throws IMSE if not owner. 1484 if (_WaitSet == NULL) { 1485 return; 1486 } 1487 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD); 1488 INotify(THREAD); 1489 OM_PERFDATA_OP(Notifications, inc(1)); 1490 } 1491 1492 1493 // The current implementation of notifyAll() transfers the waiters one-at-a-time 1494 // from the waitset to the EntryList. This could be done more efficiently with a 1495 // single bulk transfer but in practice it's not time-critical. Beware too, 1496 // that in prepend-mode we invert the order of the waiters. Let's say that the 1497 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend 1498 // mode the waitset will be empty and the EntryList will be "DCBAXYZ". 1499 1500 void ObjectMonitor::notifyAll(TRAPS) { 1501 CHECK_OWNER(); // Throws IMSE if not owner. 1502 if (_WaitSet == NULL) { 1503 return; 1504 } 1505 1506 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD); 1507 int tally = 0; 1508 while (_WaitSet != NULL) { 1509 tally++; 1510 INotify(THREAD); 1511 } 1512 1513 OM_PERFDATA_OP(Notifications, inc(tally)); 1514 } 1515 1516 // ----------------------------------------------------------------------------- 1517 // Adaptive Spinning Support 1518 // 1519 // Adaptive spin-then-block - rational spinning 1520 // 1521 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS 1522 // algorithm. On high order SMP systems it would be better to start with 1523 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH, 1524 // a contending thread could enqueue itself on the cxq and then spin locally 1525 // on a thread-specific variable such as its ParkEvent._Event flag. 1526 // That's left as an exercise for the reader. Note that global spinning is 1527 // not problematic on Niagara, as the L2 cache serves the interconnect and 1528 // has both low latency and massive bandwidth. 1529 // 1530 // Broadly, we can fix the spin frequency -- that is, the % of contended lock 1531 // acquisition attempts where we opt to spin -- at 100% and vary the spin count 1532 // (duration) or we can fix the count at approximately the duration of 1533 // a context switch and vary the frequency. Of course we could also 1534 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor. 1535 // For a description of 'Adaptive spin-then-block mutual exclusion in 1536 // multi-threaded processing,' see U.S. Pat. No. 8046758. 1537 // 1538 // This implementation varies the duration "D", where D varies with 1539 // the success rate of recent spin attempts. (D is capped at approximately 1540 // length of a round-trip context switch). The success rate for recent 1541 // spin attempts is a good predictor of the success rate of future spin 1542 // attempts. The mechanism adapts automatically to varying critical 1543 // section length (lock modality), system load and degree of parallelism. 1544 // D is maintained per-monitor in _SpinDuration and is initialized 1545 // optimistically. Spin frequency is fixed at 100%. 1546 // 1547 // Note that _SpinDuration is volatile, but we update it without locks 1548 // or atomics. The code is designed so that _SpinDuration stays within 1549 // a reasonable range even in the presence of races. The arithmetic 1550 // operations on _SpinDuration are closed over the domain of legal values, 1551 // so at worst a race will install and older but still legal value. 1552 // At the very worst this introduces some apparent non-determinism. 1553 // We might spin when we shouldn't or vice-versa, but since the spin 1554 // count are relatively short, even in the worst case, the effect is harmless. 1555 // 1556 // Care must be taken that a low "D" value does not become an 1557 // an absorbing state. Transient spinning failures -- when spinning 1558 // is overall profitable -- should not cause the system to converge 1559 // on low "D" values. We want spinning to be stable and predictable 1560 // and fairly responsive to change and at the same time we don't want 1561 // it to oscillate, become metastable, be "too" non-deterministic, 1562 // or converge on or enter undesirable stable absorbing states. 1563 // 1564 // We implement a feedback-based control system -- using past behavior 1565 // to predict future behavior. We face two issues: (a) if the 1566 // input signal is random then the spin predictor won't provide optimal 1567 // results, and (b) if the signal frequency is too high then the control 1568 // system, which has some natural response lag, will "chase" the signal. 1569 // (b) can arise from multimodal lock hold times. Transient preemption 1570 // can also result in apparent bimodal lock hold times. 1571 // Although sub-optimal, neither condition is particularly harmful, as 1572 // in the worst-case we'll spin when we shouldn't or vice-versa. 1573 // The maximum spin duration is rather short so the failure modes aren't bad. 1574 // To be conservative, I've tuned the gain in system to bias toward 1575 // _not spinning. Relatedly, the system can sometimes enter a mode where it 1576 // "rings" or oscillates between spinning and not spinning. This happens 1577 // when spinning is just on the cusp of profitability, however, so the 1578 // situation is not dire. The state is benign -- there's no need to add 1579 // hysteresis control to damp the transition rate between spinning and 1580 // not spinning. 1581 1582 // Spinning: Fixed frequency (100%), vary duration 1583 int ObjectMonitor::TrySpin(Thread * Self) { 1584 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning. 1585 int ctr = Knob_FixedSpin; 1586 if (ctr != 0) { 1587 while (--ctr >= 0) { 1588 if (TryLock(Self) > 0) return 1; 1589 SpinPause(); 1590 } 1591 return 0; 1592 } 1593 1594 for (ctr = Knob_PreSpin + 1; --ctr >= 0;) { 1595 if (TryLock(Self) > 0) { 1596 // Increase _SpinDuration ... 1597 // Note that we don't clamp SpinDuration precisely at SpinLimit. 1598 // Raising _SpurDuration to the poverty line is key. 1599 int x = _SpinDuration; 1600 if (x < Knob_SpinLimit) { 1601 if (x < Knob_Poverty) x = Knob_Poverty; 1602 _SpinDuration = x + Knob_BonusB; 1603 } 1604 return 1; 1605 } 1606 SpinPause(); 1607 } 1608 1609 // Admission control - verify preconditions for spinning 1610 // 1611 // We always spin a little bit, just to prevent _SpinDuration == 0 from 1612 // becoming an absorbing state. Put another way, we spin briefly to 1613 // sample, just in case the system load, parallelism, contention, or lock 1614 // modality changed. 1615 // 1616 // Consider the following alternative: 1617 // Periodically set _SpinDuration = _SpinLimit and try a long/full 1618 // spin attempt. "Periodically" might mean after a tally of 1619 // the # of failed spin attempts (or iterations) reaches some threshold. 1620 // This takes us into the realm of 1-out-of-N spinning, where we 1621 // hold the duration constant but vary the frequency. 1622 1623 ctr = _SpinDuration; 1624 if (ctr <= 0) return 0; 1625 1626 if (NotRunnable(Self, (Thread *) _owner)) { 1627 return 0; 1628 } 1629 1630 // We're good to spin ... spin ingress. 1631 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades 1632 // when preparing to LD...CAS _owner, etc and the CAS is likely 1633 // to succeed. 1634 if (_succ == NULL) { 1635 _succ = Self; 1636 } 1637 Thread * prv = NULL; 1638 1639 // There are three ways to exit the following loop: 1640 // 1. A successful spin where this thread has acquired the lock. 1641 // 2. Spin failure with prejudice 1642 // 3. Spin failure without prejudice 1643 1644 while (--ctr >= 0) { 1645 1646 // Periodic polling -- Check for pending GC 1647 // Threads may spin while they're unsafe. 1648 // We don't want spinning threads to delay the JVM from reaching 1649 // a stop-the-world safepoint or to steal cycles from GC. 1650 // If we detect a pending safepoint we abort in order that 1651 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b) 1652 // this thread, if safe, doesn't steal cycles from GC. 1653 // This is in keeping with the "no loitering in runtime" rule. 1654 // We periodically check to see if there's a safepoint pending. 1655 if ((ctr & 0xFF) == 0) { 1656 if (SafepointMechanism::should_block(Self)) { 1657 goto Abort; // abrupt spin egress 1658 } 1659 SpinPause(); 1660 } 1661 1662 // Probe _owner with TATAS 1663 // If this thread observes the monitor transition or flicker 1664 // from locked to unlocked to locked, then the odds that this 1665 // thread will acquire the lock in this spin attempt go down 1666 // considerably. The same argument applies if the CAS fails 1667 // or if we observe _owner change from one non-null value to 1668 // another non-null value. In such cases we might abort 1669 // the spin without prejudice or apply a "penalty" to the 1670 // spin count-down variable "ctr", reducing it by 100, say. 1671 1672 Thread * ox = (Thread *) _owner; 1673 if (ox == NULL) { 1674 ox = (Thread*)Atomic::cmpxchg(Self, &_owner, (void*)NULL); 1675 if (ox == NULL) { 1676 // The CAS succeeded -- this thread acquired ownership 1677 // Take care of some bookkeeping to exit spin state. 1678 if (_succ == Self) { 1679 _succ = NULL; 1680 } 1681 1682 // Increase _SpinDuration : 1683 // The spin was successful (profitable) so we tend toward 1684 // longer spin attempts in the future. 1685 // CONSIDER: factor "ctr" into the _SpinDuration adjustment. 1686 // If we acquired the lock early in the spin cycle it 1687 // makes sense to increase _SpinDuration proportionally. 1688 // Note that we don't clamp SpinDuration precisely at SpinLimit. 1689 int x = _SpinDuration; 1690 if (x < Knob_SpinLimit) { 1691 if (x < Knob_Poverty) x = Knob_Poverty; 1692 _SpinDuration = x + Knob_Bonus; 1693 } 1694 return 1; 1695 } 1696 1697 // The CAS failed ... we can take any of the following actions: 1698 // * penalize: ctr -= CASPenalty 1699 // * exit spin with prejudice -- goto Abort; 1700 // * exit spin without prejudice. 1701 // * Since CAS is high-latency, retry again immediately. 1702 prv = ox; 1703 goto Abort; 1704 } 1705 1706 // Did lock ownership change hands ? 1707 if (ox != prv && prv != NULL) { 1708 goto Abort; 1709 } 1710 prv = ox; 1711 1712 // Abort the spin if the owner is not executing. 1713 // The owner must be executing in order to drop the lock. 1714 // Spinning while the owner is OFFPROC is idiocy. 1715 // Consider: ctr -= RunnablePenalty ; 1716 if (NotRunnable(Self, ox)) { 1717 goto Abort; 1718 } 1719 if (_succ == NULL) { 1720 _succ = Self; 1721 } 1722 } 1723 1724 // Spin failed with prejudice -- reduce _SpinDuration. 1725 // TODO: Use an AIMD-like policy to adjust _SpinDuration. 1726 // AIMD is globally stable. 1727 { 1728 int x = _SpinDuration; 1729 if (x > 0) { 1730 // Consider an AIMD scheme like: x -= (x >> 3) + 100 1731 // This is globally sample and tends to damp the response. 1732 x -= Knob_Penalty; 1733 if (x < 0) x = 0; 1734 _SpinDuration = x; 1735 } 1736 } 1737 1738 Abort: 1739 if (_succ == Self) { 1740 _succ = NULL; 1741 // Invariant: after setting succ=null a contending thread 1742 // must recheck-retry _owner before parking. This usually happens 1743 // in the normal usage of TrySpin(), but it's safest 1744 // to make TrySpin() as foolproof as possible. 1745 OrderAccess::fence(); 1746 if (TryLock(Self) > 0) return 1; 1747 } 1748 return 0; 1749 } 1750 1751 // NotRunnable() -- informed spinning 1752 // 1753 // Don't bother spinning if the owner is not eligible to drop the lock. 1754 // Spin only if the owner thread is _thread_in_Java or _thread_in_vm. 1755 // The thread must be runnable in order to drop the lock in timely fashion. 1756 // If the _owner is not runnable then spinning will not likely be 1757 // successful (profitable). 1758 // 1759 // Beware -- the thread referenced by _owner could have died 1760 // so a simply fetch from _owner->_thread_state might trap. 1761 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state. 1762 // Because of the lifecycle issues, the _thread_state values 1763 // observed by NotRunnable() might be garbage. NotRunnable must 1764 // tolerate this and consider the observed _thread_state value 1765 // as advisory. 1766 // 1767 // Beware too, that _owner is sometimes a BasicLock address and sometimes 1768 // a thread pointer. 1769 // Alternately, we might tag the type (thread pointer vs basiclock pointer) 1770 // with the LSB of _owner. Another option would be to probabilistically probe 1771 // the putative _owner->TypeTag value. 1772 // 1773 // Checking _thread_state isn't perfect. Even if the thread is 1774 // in_java it might be blocked on a page-fault or have been preempted 1775 // and sitting on a ready/dispatch queue. 1776 // 1777 // The return value from NotRunnable() is *advisory* -- the 1778 // result is based on sampling and is not necessarily coherent. 1779 // The caller must tolerate false-negative and false-positive errors. 1780 // Spinning, in general, is probabilistic anyway. 1781 1782 1783 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) { 1784 // Check ox->TypeTag == 2BAD. 1785 if (ox == NULL) return 0; 1786 1787 // Avoid transitive spinning ... 1788 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L. 1789 // Immediately after T1 acquires L it's possible that T2, also 1790 // spinning on L, will see L.Owner=T1 and T1._Stalled=L. 1791 // This occurs transiently after T1 acquired L but before 1792 // T1 managed to clear T1.Stalled. T2 does not need to abort 1793 // its spin in this circumstance. 1794 intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1)); 1795 1796 if (BlockedOn == 1) return 1; 1797 if (BlockedOn != 0) { 1798 return BlockedOn != intptr_t(this) && _owner == ox; 1799 } 1800 1801 assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant"); 1802 int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);; 1803 // consider also: jst != _thread_in_Java -- but that's overspecific. 1804 return jst == _thread_blocked || jst == _thread_in_native; 1805 } 1806 1807 1808 // ----------------------------------------------------------------------------- 1809 // WaitSet management ... 1810 1811 ObjectWaiter::ObjectWaiter(Thread* thread) { 1812 _next = NULL; 1813 _prev = NULL; 1814 _notified = 0; 1815 _notifier_tid = 0; 1816 TState = TS_RUN; 1817 _thread = thread; 1818 _event = thread->_ParkEvent; 1819 _active = false; 1820 assert(_event != NULL, "invariant"); 1821 } 1822 1823 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) { 1824 JavaThread *jt = (JavaThread *)this->_thread; 1825 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon); 1826 } 1827 1828 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) { 1829 JavaThread *jt = (JavaThread *)this->_thread; 1830 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active); 1831 } 1832 1833 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) { 1834 assert(node != NULL, "should not add NULL node"); 1835 assert(node->_prev == NULL, "node already in list"); 1836 assert(node->_next == NULL, "node already in list"); 1837 // put node at end of queue (circular doubly linked list) 1838 if (_WaitSet == NULL) { 1839 _WaitSet = node; 1840 node->_prev = node; 1841 node->_next = node; 1842 } else { 1843 ObjectWaiter* head = _WaitSet; 1844 ObjectWaiter* tail = head->_prev; 1845 assert(tail->_next == head, "invariant check"); 1846 tail->_next = node; 1847 head->_prev = node; 1848 node->_next = head; 1849 node->_prev = tail; 1850 } 1851 } 1852 1853 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() { 1854 // dequeue the very first waiter 1855 ObjectWaiter* waiter = _WaitSet; 1856 if (waiter) { 1857 DequeueSpecificWaiter(waiter); 1858 } 1859 return waiter; 1860 } 1861 1862 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) { 1863 assert(node != NULL, "should not dequeue NULL node"); 1864 assert(node->_prev != NULL, "node already removed from list"); 1865 assert(node->_next != NULL, "node already removed from list"); 1866 // when the waiter has woken up because of interrupt, 1867 // timeout or other spurious wake-up, dequeue the 1868 // waiter from waiting list 1869 ObjectWaiter* next = node->_next; 1870 if (next == node) { 1871 assert(node->_prev == node, "invariant check"); 1872 _WaitSet = NULL; 1873 } else { 1874 ObjectWaiter* prev = node->_prev; 1875 assert(prev->_next == node, "invariant check"); 1876 assert(next->_prev == node, "invariant check"); 1877 next->_prev = prev; 1878 prev->_next = next; 1879 if (_WaitSet == node) { 1880 _WaitSet = next; 1881 } 1882 } 1883 node->_next = NULL; 1884 node->_prev = NULL; 1885 } 1886 1887 // ----------------------------------------------------------------------------- 1888 // PerfData support 1889 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL; 1890 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL; 1891 PerfCounter * ObjectMonitor::_sync_Parks = NULL; 1892 PerfCounter * ObjectMonitor::_sync_Notifications = NULL; 1893 PerfCounter * ObjectMonitor::_sync_Inflations = NULL; 1894 PerfCounter * ObjectMonitor::_sync_Deflations = NULL; 1895 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL; 1896 1897 // One-shot global initialization for the sync subsystem. 1898 // We could also defer initialization and initialize on-demand 1899 // the first time we call ObjectSynchronizer::inflate(). 1900 // Initialization would be protected - like so many things - by 1901 // the MonitorCache_lock. 1902 1903 void ObjectMonitor::Initialize() { 1904 assert(!InitDone, "invariant"); 1905 1906 if (!os::is_MP()) { 1907 Knob_SpinLimit = 0; 1908 Knob_PreSpin = 0; 1909 Knob_FixedSpin = -1; 1910 } 1911 1912 if (UsePerfData) { 1913 EXCEPTION_MARK; 1914 #define NEWPERFCOUNTER(n) \ 1915 { \ 1916 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \ 1917 CHECK); \ 1918 } 1919 #define NEWPERFVARIABLE(n) \ 1920 { \ 1921 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \ 1922 CHECK); \ 1923 } 1924 NEWPERFCOUNTER(_sync_Inflations); 1925 NEWPERFCOUNTER(_sync_Deflations); 1926 NEWPERFCOUNTER(_sync_ContendedLockAttempts); 1927 NEWPERFCOUNTER(_sync_FutileWakeups); 1928 NEWPERFCOUNTER(_sync_Parks); 1929 NEWPERFCOUNTER(_sync_Notifications); 1930 NEWPERFVARIABLE(_sync_MonExtant); 1931 #undef NEWPERFCOUNTER 1932 #undef NEWPERFVARIABLE 1933 } 1934 1935 DEBUG_ONLY(InitDone = true;) 1936 } 1937 1938 void ObjectMonitor::print_on(outputStream* st) const { 1939 // The minimal things to print for markWord printing, more can be added for debugging and logging. 1940 st->print("{contentions=0x%08x,waiters=0x%08x" 1941 ",recursions=" INTPTR_FORMAT ",owner=" INTPTR_FORMAT "}", 1942 contentions(), waiters(), recursions(), 1943 p2i(owner())); 1944 } 1945 void ObjectMonitor::print() const { print_on(tty); }