1 /* 2 * Copyright (c) 1998, 2022, 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 "gc/shared/oopStorage.hpp" 28 #include "gc/shared/oopStorageSet.hpp" 29 #include "jfr/jfrEvents.hpp" 30 #include "jfr/support/jfrThreadId.hpp" 31 #include "logging/log.hpp" 32 #include "logging/logStream.hpp" 33 #include "memory/allocation.inline.hpp" 34 #include "memory/resourceArea.hpp" 35 #include "oops/markWord.hpp" 36 #include "oops/oop.inline.hpp" 37 #include "oops/oopHandle.inline.hpp" 38 #include "oops/weakHandle.inline.hpp" 39 #include "prims/jvmtiDeferredUpdates.hpp" 40 #include "prims/jvmtiExport.hpp" 41 #include "runtime/atomic.hpp" 42 #include "runtime/handles.inline.hpp" 43 #include "runtime/interfaceSupport.inline.hpp" 44 #include "runtime/mutexLocker.hpp" 45 #include "runtime/objectMonitor.hpp" 46 #include "runtime/objectMonitor.inline.hpp" 47 #include "runtime/orderAccess.hpp" 48 #include "runtime/osThread.hpp" 49 #include "runtime/perfData.hpp" 50 #include "runtime/safefetch.inline.hpp" 51 #include "runtime/safepointMechanism.inline.hpp" 52 #include "runtime/sharedRuntime.hpp" 53 #include "runtime/thread.inline.hpp" 54 #include "services/threadService.hpp" 55 #include "utilities/dtrace.hpp" 56 #include "utilities/macros.hpp" 57 #include "utilities/preserveException.hpp" 58 #if INCLUDE_JFR 59 #include "jfr/support/jfrFlush.hpp" 60 #endif 61 62 #ifdef DTRACE_ENABLED 63 64 // Only bother with this argument setup if dtrace is available 65 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly. 66 67 68 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \ 69 char* bytes = NULL; \ 70 int len = 0; \ 71 jlong jtid = SharedRuntime::get_java_tid(thread); \ 72 Symbol* klassname = obj->klass()->name(); \ 73 if (klassname != NULL) { \ 74 bytes = (char*)klassname->bytes(); \ 75 len = klassname->utf8_length(); \ 76 } 77 78 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \ 79 { \ 80 if (DTraceMonitorProbes) { \ 81 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \ 82 HOTSPOT_MONITOR_WAIT(jtid, \ 83 (monitor), bytes, len, (millis)); \ 84 } \ 85 } 86 87 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER 88 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED 89 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT 90 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY 91 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL 92 93 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \ 94 { \ 95 if (DTraceMonitorProbes) { \ 96 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \ 97 HOTSPOT_MONITOR_##probe(jtid, \ 98 (uintptr_t)(monitor), bytes, len); \ 99 } \ 100 } 101 102 #else // ndef DTRACE_ENABLED 103 104 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;} 105 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;} 106 107 #endif // ndef DTRACE_ENABLED 108 109 // Tunables ... 110 // The knob* variables are effectively final. Once set they should 111 // never be modified hence. Consider using __read_mostly with GCC. 112 113 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool - 114 115 static int Knob_Bonus = 100; // spin success bonus 116 static int Knob_BonusB = 100; // spin success bonus 117 static int Knob_Penalty = 200; // spin failure penalty 118 static int Knob_Poverty = 1000; 119 static int Knob_FixedSpin = 0; 120 static int Knob_PreSpin = 10; // 20-100 likely better 121 122 DEBUG_ONLY(static volatile bool InitDone = false;) 123 124 OopStorage* ObjectMonitor::_oop_storage = NULL; 125 126 // ----------------------------------------------------------------------------- 127 // Theory of operations -- Monitors lists, thread residency, etc: 128 // 129 // * A thread acquires ownership of a monitor by successfully 130 // CAS()ing the _owner field from null to non-null. 131 // 132 // * Invariant: A thread appears on at most one monitor list -- 133 // cxq, EntryList or WaitSet -- at any one time. 134 // 135 // * Contending threads "push" themselves onto the cxq with CAS 136 // and then spin/park. 137 // 138 // * After a contending thread eventually acquires the lock it must 139 // dequeue itself from either the EntryList or the cxq. 140 // 141 // * The exiting thread identifies and unparks an "heir presumptive" 142 // tentative successor thread on the EntryList. Critically, the 143 // exiting thread doesn't unlink the successor thread from the EntryList. 144 // After having been unparked, the wakee will recontend for ownership of 145 // the monitor. The successor (wakee) will either acquire the lock or 146 // re-park itself. 147 // 148 // Succession is provided for by a policy of competitive handoff. 149 // The exiting thread does _not_ grant or pass ownership to the 150 // successor thread. (This is also referred to as "handoff" succession"). 151 // Instead the exiting thread releases ownership and possibly wakes 152 // a successor, so the successor can (re)compete for ownership of the lock. 153 // If the EntryList is empty but the cxq is populated the exiting 154 // thread will drain the cxq into the EntryList. It does so by 155 // by detaching the cxq (installing null with CAS) and folding 156 // the threads from the cxq into the EntryList. The EntryList is 157 // doubly linked, while the cxq is singly linked because of the 158 // CAS-based "push" used to enqueue recently arrived threads (RATs). 159 // 160 // * Concurrency invariants: 161 // 162 // -- only the monitor owner may access or mutate the EntryList. 163 // The mutex property of the monitor itself protects the EntryList 164 // from concurrent interference. 165 // -- Only the monitor owner may detach the cxq. 166 // 167 // * The monitor entry list operations avoid locks, but strictly speaking 168 // they're not lock-free. Enter is lock-free, exit is not. 169 // For a description of 'Methods and apparatus providing non-blocking access 170 // to a resource,' see U.S. Pat. No. 7844973. 171 // 172 // * The cxq can have multiple concurrent "pushers" but only one concurrent 173 // detaching thread. This mechanism is immune from the ABA corruption. 174 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious. 175 // 176 // * Taken together, the cxq and the EntryList constitute or form a 177 // single logical queue of threads stalled trying to acquire the lock. 178 // We use two distinct lists to improve the odds of a constant-time 179 // dequeue operation after acquisition (in the ::enter() epilogue) and 180 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm). 181 // A key desideratum is to minimize queue & monitor metadata manipulation 182 // that occurs while holding the monitor lock -- that is, we want to 183 // minimize monitor lock holds times. Note that even a small amount of 184 // fixed spinning will greatly reduce the # of enqueue-dequeue operations 185 // on EntryList|cxq. That is, spinning relieves contention on the "inner" 186 // locks and monitor metadata. 187 // 188 // Cxq points to the set of Recently Arrived Threads attempting entry. 189 // Because we push threads onto _cxq with CAS, the RATs must take the form of 190 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when 191 // the unlocking thread notices that EntryList is null but _cxq is != null. 192 // 193 // The EntryList is ordered by the prevailing queue discipline and 194 // can be organized in any convenient fashion, such as a doubly-linked list or 195 // a circular doubly-linked list. Critically, we want insert and delete operations 196 // to operate in constant-time. If we need a priority queue then something akin 197 // to Solaris' sleepq would work nicely. Viz., 198 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c. 199 // Queue discipline is enforced at ::exit() time, when the unlocking thread 200 // drains the cxq into the EntryList, and orders or reorders the threads on the 201 // EntryList accordingly. 202 // 203 // Barring "lock barging", this mechanism provides fair cyclic ordering, 204 // somewhat similar to an elevator-scan. 205 // 206 // * The monitor synchronization subsystem avoids the use of native 207 // synchronization primitives except for the narrow platform-specific 208 // park-unpark abstraction. See the comments in os_solaris.cpp regarding 209 // the semantics of park-unpark. Put another way, this monitor implementation 210 // depends only on atomic operations and park-unpark. The monitor subsystem 211 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the 212 // underlying OS manages the READY<->RUN transitions. 213 // 214 // * Waiting threads reside on the WaitSet list -- wait() puts 215 // the caller onto the WaitSet. 216 // 217 // * notify() or notifyAll() simply transfers threads from the WaitSet to 218 // either the EntryList or cxq. Subsequent exit() operations will 219 // unpark the notifyee. Unparking a notifee in notify() is inefficient - 220 // it's likely the notifyee would simply impale itself on the lock held 221 // by the notifier. 222 // 223 // * An interesting alternative is to encode cxq as (List,LockByte) where 224 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary 225 // variable, like _recursions, in the scheme. The threads or Events that form 226 // the list would have to be aligned in 256-byte addresses. A thread would 227 // try to acquire the lock or enqueue itself with CAS, but exiting threads 228 // could use a 1-0 protocol and simply STB to set the LockByte to 0. 229 // Note that is is *not* word-tearing, but it does presume that full-word 230 // CAS operations are coherent with intermix with STB operations. That's true 231 // on most common processors. 232 // 233 // * See also http://blogs.sun.com/dave 234 235 236 // Check that object() and set_object() are called from the right context: 237 static void check_object_context() { 238 #ifdef ASSERT 239 Thread* self = Thread::current(); 240 if (self->is_Java_thread()) { 241 // Mostly called from JavaThreads so sanity check the thread state. 242 JavaThread* jt = self->as_Java_thread(); 243 switch (jt->thread_state()) { 244 case _thread_in_vm: // the usual case 245 case _thread_in_Java: // during deopt 246 break; 247 default: 248 fatal("called from an unsafe thread state"); 249 } 250 assert(jt->is_active_Java_thread(), "must be active JavaThread"); 251 } else { 252 // However, ThreadService::get_current_contended_monitor() 253 // can call here via the VMThread so sanity check it. 254 assert(self->is_VM_thread(), "must be"); 255 } 256 #endif // ASSERT 257 } 258 259 ObjectMonitor::ObjectMonitor(oop object) : 260 _header(markWord::zero()), 261 _object(_oop_storage, object), 262 _owner(NULL), 263 _previous_owner_tid(0), 264 _next_om(NULL), 265 _recursions(0), 266 _EntryList(NULL), 267 _cxq(NULL), 268 _succ(NULL), 269 _Responsible(NULL), 270 _Spinner(0), 271 _SpinDuration(ObjectMonitor::Knob_SpinLimit), 272 _contentions(0), 273 _WaitSet(NULL), 274 _waiters(0), 275 _WaitSetLock(0) 276 { } 277 278 ObjectMonitor::~ObjectMonitor() { 279 _object.release(_oop_storage); 280 } 281 282 oop ObjectMonitor::object() const { 283 check_object_context(); 284 if (_object.is_null()) { 285 return NULL; 286 } 287 return _object.resolve(); 288 } 289 290 oop ObjectMonitor::object_peek() const { 291 if (_object.is_null()) { 292 return NULL; 293 } 294 return _object.peek(); 295 } 296 297 void ObjectMonitor::ExitOnSuspend::operator()(JavaThread* current) { 298 if (current->is_suspended()) { 299 _om->_recursions = 0; 300 _om->_succ = NULL; 301 // Don't need a full fence after clearing successor here because of the call to exit(). 302 _om->exit(current, false /* not_suspended */); 303 _om_exited = true; 304 305 current->set_current_pending_monitor(_om); 306 } 307 } 308 309 void ObjectMonitor::ClearSuccOnSuspend::operator()(JavaThread* current) { 310 if (current->is_suspended()) { 311 if (_om->_succ == current) { 312 _om->_succ = NULL; 313 OrderAccess::fence(); // always do a full fence when successor is cleared 314 } 315 } 316 } 317 318 // ----------------------------------------------------------------------------- 319 // Enter support 320 321 bool ObjectMonitor::enter(JavaThread* current) { 322 // The following code is ordered to check the most common cases first 323 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors. 324 325 void* cur = try_set_owner_from(NULL, current); 326 if (cur == NULL) { 327 assert(_recursions == 0, "invariant"); 328 return true; 329 } 330 331 if (cur == current) { 332 // TODO-FIXME: check for integer overflow! BUGID 6557169. 333 _recursions++; 334 return true; 335 } 336 337 if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) { 338 assert(_recursions == 0, "internal state error"); 339 _recursions = 1; 340 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*. 341 return true; 342 } 343 344 // We've encountered genuine contention. 345 assert(current->_Stalled == 0, "invariant"); 346 current->_Stalled = intptr_t(this); 347 348 // Try one round of spinning *before* enqueueing current 349 // and before going through the awkward and expensive state 350 // transitions. The following spin is strictly optional ... 351 // Note that if we acquire the monitor from an initial spin 352 // we forgo posting JVMTI events and firing DTRACE probes. 353 if (TrySpin(current) > 0) { 354 assert(owner_raw() == current, "must be current: owner=" INTPTR_FORMAT, p2i(owner_raw())); 355 assert(_recursions == 0, "must be 0: recursions=" INTX_FORMAT, _recursions); 356 assert(object()->mark() == markWord::encode(this), 357 "object mark must match encoded this: mark=" INTPTR_FORMAT 358 ", encoded this=" INTPTR_FORMAT, object()->mark().value(), 359 markWord::encode(this).value()); 360 current->_Stalled = 0; 361 return true; 362 } 363 364 assert(owner_raw() != current, "invariant"); 365 assert(_succ != current, "invariant"); 366 assert(!SafepointSynchronize::is_at_safepoint(), "invariant"); 367 assert(current->thread_state() != _thread_blocked, "invariant"); 368 369 // Keep track of contention for JVM/TI and M&M queries. 370 add_to_contentions(1); 371 if (is_being_async_deflated()) { 372 // Async deflation is in progress and our contentions increment 373 // above lost the race to async deflation. Undo the work and 374 // force the caller to retry. 375 const oop l_object = object(); 376 if (l_object != NULL) { 377 // Attempt to restore the header/dmw to the object's header so that 378 // we only retry once if the deflater thread happens to be slow. 379 install_displaced_markword_in_object(l_object); 380 } 381 current->_Stalled = 0; 382 add_to_contentions(-1); 383 return false; 384 } 385 386 JFR_ONLY(JfrConditionalFlushWithStacktrace<EventJavaMonitorEnter> flush(current);) 387 EventJavaMonitorEnter event; 388 if (event.is_started()) { 389 event.set_monitorClass(object()->klass()); 390 // Set an address that is 'unique enough', such that events close in 391 // time and with the same address are likely (but not guaranteed) to 392 // belong to the same object. 393 event.set_address((uintptr_t)this); 394 } 395 396 { // Change java thread status to indicate blocked on monitor enter. 397 JavaThreadBlockedOnMonitorEnterState jtbmes(current, this); 398 399 assert(current->current_pending_monitor() == NULL, "invariant"); 400 current->set_current_pending_monitor(this); 401 402 DTRACE_MONITOR_PROBE(contended__enter, this, object(), current); 403 if (JvmtiExport::should_post_monitor_contended_enter()) { 404 JvmtiExport::post_monitor_contended_enter(current, this); 405 406 // The current thread does not yet own the monitor and does not 407 // yet appear on any queues that would get it made the successor. 408 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event 409 // handler cannot accidentally consume an unpark() meant for the 410 // ParkEvent associated with this ObjectMonitor. 411 } 412 413 OSThreadContendState osts(current->osthread()); 414 415 assert(current->thread_state() == _thread_in_vm, "invariant"); 416 417 for (;;) { 418 ExitOnSuspend eos(this); 419 { 420 ThreadBlockInVMPreprocess<ExitOnSuspend> tbivs(current, eos); 421 EnterI(current); 422 current->set_current_pending_monitor(NULL); 423 // We can go to a safepoint at the end of this block. If we 424 // do a thread dump during that safepoint, then this thread will show 425 // as having "-locked" the monitor, but the OS and java.lang.Thread 426 // states will still report that the thread is blocked trying to 427 // acquire it. 428 // If there is a suspend request, ExitOnSuspend will exit the OM 429 // and set the OM as pending. 430 } 431 if (!eos.exited()) { 432 // ExitOnSuspend did not exit the OM 433 assert(owner_raw() == current, "invariant"); 434 break; 435 } 436 } 437 438 // We've just gotten past the enter-check-for-suspend dance and we now own 439 // the monitor free and clear. 440 } 441 442 add_to_contentions(-1); 443 assert(contentions() >= 0, "must not be negative: contentions=%d", contentions()); 444 current->_Stalled = 0; 445 446 // Must either set _recursions = 0 or ASSERT _recursions == 0. 447 assert(_recursions == 0, "invariant"); 448 assert(owner_raw() == current, "invariant"); 449 assert(_succ != current, "invariant"); 450 assert(object()->mark() == markWord::encode(this), "invariant"); 451 452 // The thread -- now the owner -- is back in vm mode. 453 // Report the glorious news via TI,DTrace and jvmstat. 454 // The probe effect is non-trivial. All the reportage occurs 455 // while we hold the monitor, increasing the length of the critical 456 // section. Amdahl's parallel speedup law comes vividly into play. 457 // 458 // Another option might be to aggregate the events (thread local or 459 // per-monitor aggregation) and defer reporting until a more opportune 460 // time -- such as next time some thread encounters contention but has 461 // yet to acquire the lock. While spinning that thread could 462 // spinning we could increment JVMStat counters, etc. 463 464 DTRACE_MONITOR_PROBE(contended__entered, this, object(), current); 465 if (JvmtiExport::should_post_monitor_contended_entered()) { 466 JvmtiExport::post_monitor_contended_entered(current, this); 467 468 // The current thread already owns the monitor and is not going to 469 // call park() for the remainder of the monitor enter protocol. So 470 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED 471 // event handler consumed an unpark() issued by the thread that 472 // just exited the monitor. 473 } 474 if (event.should_commit()) { 475 event.set_previousOwner((uintptr_t)_previous_owner_tid); 476 event.commit(); 477 } 478 OM_PERFDATA_OP(ContendedLockAttempts, inc()); 479 return true; 480 } 481 482 // Caveat: TryLock() is not necessarily serializing if it returns failure. 483 // Callers must compensate as needed. 484 485 int ObjectMonitor::TryLock(JavaThread* current) { 486 void* own = owner_raw(); 487 if (own != NULL) return 0; 488 if (try_set_owner_from(NULL, current) == NULL) { 489 assert(_recursions == 0, "invariant"); 490 return 1; 491 } 492 // The lock had been free momentarily, but we lost the race to the lock. 493 // Interference -- the CAS failed. 494 // We can either return -1 or retry. 495 // Retry doesn't make as much sense because the lock was just acquired. 496 return -1; 497 } 498 499 // Deflate the specified ObjectMonitor if not in-use. Returns true if it 500 // was deflated and false otherwise. 501 // 502 // The async deflation protocol sets owner to DEFLATER_MARKER and 503 // makes contentions negative as signals to contending threads that 504 // an async deflation is in progress. There are a number of checks 505 // as part of the protocol to make sure that the calling thread has 506 // not lost the race to a contending thread. 507 // 508 // The ObjectMonitor has been successfully async deflated when: 509 // (contentions < 0) 510 // Contending threads that see that condition know to retry their operation. 511 // 512 bool ObjectMonitor::deflate_monitor() { 513 if (is_busy()) { 514 // Easy checks are first - the ObjectMonitor is busy so no deflation. 515 return false; 516 } 517 518 if (ObjectSynchronizer::is_final_audit() && owner_is_DEFLATER_MARKER()) { 519 // The final audit can see an already deflated ObjectMonitor on the 520 // in-use list because MonitorList::unlink_deflated() might have 521 // blocked for the final safepoint before unlinking all the deflated 522 // monitors. 523 assert(contentions() < 0, "must be negative: contentions=%d", contentions()); 524 // Already returned 'true' when it was originally deflated. 525 return false; 526 } 527 528 const oop obj = object_peek(); 529 530 if (obj == NULL) { 531 // If the object died, we can recycle the monitor without racing with 532 // Java threads. The GC already broke the association with the object. 533 set_owner_from(NULL, DEFLATER_MARKER); 534 assert(contentions() >= 0, "must be non-negative: contentions=%d", contentions()); 535 _contentions = -max_jint; 536 } else { 537 // Attempt async deflation protocol. 538 539 // Set a NULL owner to DEFLATER_MARKER to force any contending thread 540 // through the slow path. This is just the first part of the async 541 // deflation dance. 542 if (try_set_owner_from(NULL, DEFLATER_MARKER) != NULL) { 543 // The owner field is no longer NULL so we lost the race since the 544 // ObjectMonitor is now busy. 545 return false; 546 } 547 548 if (contentions() > 0 || _waiters != 0) { 549 // Another thread has raced to enter the ObjectMonitor after 550 // is_busy() above or has already entered and waited on 551 // it which makes it busy so no deflation. Restore owner to 552 // NULL if it is still DEFLATER_MARKER. 553 if (try_set_owner_from(DEFLATER_MARKER, NULL) != DEFLATER_MARKER) { 554 // Deferred decrement for the JT EnterI() that cancelled the async deflation. 555 add_to_contentions(-1); 556 } 557 return false; 558 } 559 560 // Make a zero contentions field negative to force any contending threads 561 // to retry. This is the second part of the async deflation dance. 562 if (Atomic::cmpxchg(&_contentions, (jint)0, -max_jint) != 0) { 563 // Contentions was no longer 0 so we lost the race since the 564 // ObjectMonitor is now busy. Restore owner to NULL if it is 565 // still DEFLATER_MARKER: 566 if (try_set_owner_from(DEFLATER_MARKER, NULL) != DEFLATER_MARKER) { 567 // Deferred decrement for the JT EnterI() that cancelled the async deflation. 568 add_to_contentions(-1); 569 } 570 return false; 571 } 572 } 573 574 // Sanity checks for the races: 575 guarantee(owner_is_DEFLATER_MARKER(), "must be deflater marker"); 576 guarantee(contentions() < 0, "must be negative: contentions=%d", 577 contentions()); 578 guarantee(_waiters == 0, "must be 0: waiters=%d", _waiters); 579 guarantee(_cxq == NULL, "must be no contending threads: cxq=" 580 INTPTR_FORMAT, p2i(_cxq)); 581 guarantee(_EntryList == NULL, 582 "must be no entering threads: EntryList=" INTPTR_FORMAT, 583 p2i(_EntryList)); 584 585 if (obj != NULL) { 586 if (log_is_enabled(Trace, monitorinflation)) { 587 ResourceMark rm; 588 log_trace(monitorinflation)("deflate_monitor: object=" INTPTR_FORMAT 589 ", mark=" INTPTR_FORMAT ", type='%s'", 590 p2i(obj), obj->mark().value(), 591 obj->klass()->external_name()); 592 } 593 594 // Install the old mark word if nobody else has already done it. 595 install_displaced_markword_in_object(obj); 596 } 597 598 // We leave owner == DEFLATER_MARKER and contentions < 0 599 // to force any racing threads to retry. 600 return true; // Success, ObjectMonitor has been deflated. 601 } 602 603 // We might access the dead object headers for parsable heap walk, make sure 604 // headers are in correct shape, e.g. monitors deflated. 605 void ObjectMonitor::maybe_deflate_dead(oop* p) { 606 oop obj = *p; 607 assert(obj != NULL, "must not yet been cleared"); 608 markWord mark = obj->mark(); 609 if (mark.has_monitor()) { 610 ObjectMonitor* monitor = mark.monitor(); 611 if (p == monitor->_object.ptr_raw()) { 612 assert(monitor->object_peek() == obj, "lock object must match"); 613 markWord dmw = monitor->header(); 614 obj->set_mark(dmw); 615 } 616 } 617 } 618 619 // Install the displaced mark word (dmw) of a deflating ObjectMonitor 620 // into the header of the object associated with the monitor. This 621 // idempotent method is called by a thread that is deflating a 622 // monitor and by other threads that have detected a race with the 623 // deflation process. 624 void ObjectMonitor::install_displaced_markword_in_object(const oop obj) { 625 // This function must only be called when (owner == DEFLATER_MARKER 626 // && contentions <= 0), but we can't guarantee that here because 627 // those values could change when the ObjectMonitor gets moved from 628 // the global free list to a per-thread free list. 629 630 guarantee(obj != NULL, "must be non-NULL"); 631 632 // Separate loads in is_being_async_deflated(), which is almost always 633 // called before this function, from the load of dmw/header below. 634 635 // _contentions and dmw/header may get written by different threads. 636 // Make sure to observe them in the same order when having several observers. 637 OrderAccess::loadload_for_IRIW(); 638 639 const oop l_object = object_peek(); 640 if (l_object == NULL) { 641 // ObjectMonitor's object ref has already been cleared by async 642 // deflation or GC so we're done here. 643 return; 644 } 645 assert(l_object == obj, "object=" INTPTR_FORMAT " must equal obj=" 646 INTPTR_FORMAT, p2i(l_object), p2i(obj)); 647 648 markWord dmw = header(); 649 // The dmw has to be neutral (not NULL, not locked and not marked). 650 assert(dmw.is_neutral(), "must be neutral: dmw=" INTPTR_FORMAT, dmw.value()); 651 652 // Install displaced mark word if the object's header still points 653 // to this ObjectMonitor. More than one racing caller to this function 654 // can rarely reach this point, but only one can win. 655 markWord res = obj->cas_set_mark(dmw, markWord::encode(this)); 656 if (res != markWord::encode(this)) { 657 // This should be rare so log at the Info level when it happens. 658 log_info(monitorinflation)("install_displaced_markword_in_object: " 659 "failed cas_set_mark: new_mark=" INTPTR_FORMAT 660 ", old_mark=" INTPTR_FORMAT ", res=" INTPTR_FORMAT, 661 dmw.value(), markWord::encode(this).value(), 662 res.value()); 663 } 664 665 // Note: It does not matter which thread restored the header/dmw 666 // into the object's header. The thread deflating the monitor just 667 // wanted the object's header restored and it is. The threads that 668 // detected a race with the deflation process also wanted the 669 // object's header restored before they retry their operation and 670 // because it is restored they will only retry once. 671 } 672 673 // Convert the fields used by is_busy() to a string that can be 674 // used for diagnostic output. 675 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) { 676 ss->print("is_busy: waiters=%d, ", _waiters); 677 if (contentions() > 0) { 678 ss->print("contentions=%d, ", contentions()); 679 } else { 680 ss->print("contentions=0"); 681 } 682 if (!owner_is_DEFLATER_MARKER()) { 683 ss->print("owner=" INTPTR_FORMAT, p2i(owner_raw())); 684 } else { 685 // We report NULL instead of DEFLATER_MARKER here because is_busy() 686 // ignores DEFLATER_MARKER values. 687 ss->print("owner=" INTPTR_FORMAT, NULL_WORD); 688 } 689 ss->print(", cxq=" INTPTR_FORMAT ", EntryList=" INTPTR_FORMAT, p2i(_cxq), 690 p2i(_EntryList)); 691 return ss->base(); 692 } 693 694 #define MAX_RECHECK_INTERVAL 1000 695 696 void ObjectMonitor::EnterI(JavaThread* current) { 697 assert(current->thread_state() == _thread_blocked, "invariant"); 698 699 // Try the lock - TATAS 700 if (TryLock (current) > 0) { 701 assert(_succ != current, "invariant"); 702 assert(owner_raw() == current, "invariant"); 703 assert(_Responsible != current, "invariant"); 704 return; 705 } 706 707 if (try_set_owner_from(DEFLATER_MARKER, current) == DEFLATER_MARKER) { 708 // Cancelled the in-progress async deflation by changing owner from 709 // DEFLATER_MARKER to current. As part of the contended enter protocol, 710 // contentions was incremented to a positive value before EnterI() 711 // was called and that prevents the deflater thread from winning the 712 // last part of the 2-part async deflation protocol. After EnterI() 713 // returns to enter(), contentions is decremented because the caller 714 // now owns the monitor. We bump contentions an extra time here to 715 // prevent the deflater thread from winning the last part of the 716 // 2-part async deflation protocol after the regular decrement 717 // occurs in enter(). The deflater thread will decrement contentions 718 // after it recognizes that the async deflation was cancelled. 719 add_to_contentions(1); 720 assert(_succ != current, "invariant"); 721 assert(_Responsible != current, "invariant"); 722 return; 723 } 724 725 assert(InitDone, "Unexpectedly not initialized"); 726 727 // We try one round of spinning *before* enqueueing current. 728 // 729 // If the _owner is ready but OFFPROC we could use a YieldTo() 730 // operation to donate the remainder of this thread's quantum 731 // to the owner. This has subtle but beneficial affinity 732 // effects. 733 734 if (TrySpin(current) > 0) { 735 assert(owner_raw() == current, "invariant"); 736 assert(_succ != current, "invariant"); 737 assert(_Responsible != current, "invariant"); 738 return; 739 } 740 741 // The Spin failed -- Enqueue and park the thread ... 742 assert(_succ != current, "invariant"); 743 assert(owner_raw() != current, "invariant"); 744 assert(_Responsible != current, "invariant"); 745 746 // Enqueue "current" on ObjectMonitor's _cxq. 747 // 748 // Node acts as a proxy for current. 749 // As an aside, if were to ever rewrite the synchronization code mostly 750 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class 751 // Java objects. This would avoid awkward lifecycle and liveness issues, 752 // as well as eliminate a subset of ABA issues. 753 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events. 754 755 ObjectWaiter node(current); 756 current->_ParkEvent->reset(); 757 node._prev = (ObjectWaiter*) 0xBAD; 758 node.TState = ObjectWaiter::TS_CXQ; 759 760 // Push "current" onto the front of the _cxq. 761 // Once on cxq/EntryList, current stays on-queue until it acquires the lock. 762 // Note that spinning tends to reduce the rate at which threads 763 // enqueue and dequeue on EntryList|cxq. 764 ObjectWaiter* nxt; 765 for (;;) { 766 node._next = nxt = _cxq; 767 if (Atomic::cmpxchg(&_cxq, nxt, &node) == nxt) break; 768 769 // Interference - the CAS failed because _cxq changed. Just retry. 770 // As an optional optimization we retry the lock. 771 if (TryLock (current) > 0) { 772 assert(_succ != current, "invariant"); 773 assert(owner_raw() == current, "invariant"); 774 assert(_Responsible != current, "invariant"); 775 return; 776 } 777 } 778 779 // Check for cxq|EntryList edge transition to non-null. This indicates 780 // the onset of contention. While contention persists exiting threads 781 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit 782 // operations revert to the faster 1-0 mode. This enter operation may interleave 783 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we 784 // arrange for one of the contending thread to use a timed park() operations 785 // to detect and recover from the race. (Stranding is form of progress failure 786 // where the monitor is unlocked but all the contending threads remain parked). 787 // That is, at least one of the contended threads will periodically poll _owner. 788 // One of the contending threads will become the designated "Responsible" thread. 789 // The Responsible thread uses a timed park instead of a normal indefinite park 790 // operation -- it periodically wakes and checks for and recovers from potential 791 // strandings admitted by 1-0 exit operations. We need at most one Responsible 792 // thread per-monitor at any given moment. Only threads on cxq|EntryList may 793 // be responsible for a monitor. 794 // 795 // Currently, one of the contended threads takes on the added role of "Responsible". 796 // A viable alternative would be to use a dedicated "stranding checker" thread 797 // that periodically iterated over all the threads (or active monitors) and unparked 798 // successors where there was risk of stranding. This would help eliminate the 799 // timer scalability issues we see on some platforms as we'd only have one thread 800 // -- the checker -- parked on a timer. 801 802 if (nxt == NULL && _EntryList == NULL) { 803 // Try to assume the role of responsible thread for the monitor. 804 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=current } 805 Atomic::replace_if_null(&_Responsible, current); 806 } 807 808 // The lock might have been released while this thread was occupied queueing 809 // itself onto _cxq. To close the race and avoid "stranding" and 810 // progress-liveness failure we must resample-retry _owner before parking. 811 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner. 812 // In this case the ST-MEMBAR is accomplished with CAS(). 813 // 814 // TODO: Defer all thread state transitions until park-time. 815 // Since state transitions are heavy and inefficient we'd like 816 // to defer the state transitions until absolutely necessary, 817 // and in doing so avoid some transitions ... 818 819 int nWakeups = 0; 820 int recheckInterval = 1; 821 822 for (;;) { 823 824 if (TryLock(current) > 0) break; 825 assert(owner_raw() != current, "invariant"); 826 827 // park self 828 if (_Responsible == current) { 829 current->_ParkEvent->park((jlong) recheckInterval); 830 // Increase the recheckInterval, but clamp the value. 831 recheckInterval *= 8; 832 if (recheckInterval > MAX_RECHECK_INTERVAL) { 833 recheckInterval = MAX_RECHECK_INTERVAL; 834 } 835 } else { 836 current->_ParkEvent->park(); 837 } 838 839 if (TryLock(current) > 0) break; 840 841 if (try_set_owner_from(DEFLATER_MARKER, current) == DEFLATER_MARKER) { 842 // Cancelled the in-progress async deflation by changing owner from 843 // DEFLATER_MARKER to current. As part of the contended enter protocol, 844 // contentions was incremented to a positive value before EnterI() 845 // was called and that prevents the deflater thread from winning the 846 // last part of the 2-part async deflation protocol. After EnterI() 847 // returns to enter(), contentions is decremented because the caller 848 // now owns the monitor. We bump contentions an extra time here to 849 // prevent the deflater thread from winning the last part of the 850 // 2-part async deflation protocol after the regular decrement 851 // occurs in enter(). The deflater thread will decrement contentions 852 // after it recognizes that the async deflation was cancelled. 853 add_to_contentions(1); 854 break; 855 } 856 857 // The lock is still contested. 858 // Keep a tally of the # of futile wakeups. 859 // Note that the counter is not protected by a lock or updated by atomics. 860 // That is by design - we trade "lossy" counters which are exposed to 861 // races during updates for a lower probe effect. 862 863 // This PerfData object can be used in parallel with a safepoint. 864 // See the work around in PerfDataManager::destroy(). 865 OM_PERFDATA_OP(FutileWakeups, inc()); 866 ++nWakeups; 867 868 // Assuming this is not a spurious wakeup we'll normally find _succ == current. 869 // We can defer clearing _succ until after the spin completes 870 // TrySpin() must tolerate being called with _succ == current. 871 // Try yet another round of adaptive spinning. 872 if (TrySpin(current) > 0) break; 873 874 // We can find that we were unpark()ed and redesignated _succ while 875 // we were spinning. That's harmless. If we iterate and call park(), 876 // park() will consume the event and return immediately and we'll 877 // just spin again. This pattern can repeat, leaving _succ to simply 878 // spin on a CPU. 879 880 if (_succ == current) _succ = NULL; 881 882 // Invariant: after clearing _succ a thread *must* retry _owner before parking. 883 OrderAccess::fence(); 884 } 885 886 // Egress : 887 // current has acquired the lock -- Unlink current from the cxq or EntryList. 888 // Normally we'll find current on the EntryList . 889 // From the perspective of the lock owner (this thread), the 890 // EntryList is stable and cxq is prepend-only. 891 // The head of cxq is volatile but the interior is stable. 892 // In addition, current.TState is stable. 893 894 assert(owner_raw() == current, "invariant"); 895 896 UnlinkAfterAcquire(current, &node); 897 if (_succ == current) _succ = NULL; 898 899 assert(_succ != current, "invariant"); 900 if (_Responsible == current) { 901 _Responsible = NULL; 902 OrderAccess::fence(); // Dekker pivot-point 903 904 // We may leave threads on cxq|EntryList without a designated 905 // "Responsible" thread. This is benign. When this thread subsequently 906 // exits the monitor it can "see" such preexisting "old" threads -- 907 // threads that arrived on the cxq|EntryList before the fence, above -- 908 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads 909 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible 910 // non-null and elect a new "Responsible" timer thread. 911 // 912 // This thread executes: 913 // ST Responsible=null; MEMBAR (in enter epilogue - here) 914 // LD cxq|EntryList (in subsequent exit) 915 // 916 // Entering threads in the slow/contended path execute: 917 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog) 918 // The (ST cxq; MEMBAR) is accomplished with CAS(). 919 // 920 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent 921 // exit operation from floating above the ST Responsible=null. 922 } 923 924 // We've acquired ownership with CAS(). 925 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics. 926 // But since the CAS() this thread may have also stored into _succ, 927 // EntryList, cxq or Responsible. These meta-data updates must be 928 // visible __before this thread subsequently drops the lock. 929 // Consider what could occur if we didn't enforce this constraint -- 930 // STs to monitor meta-data and user-data could reorder with (become 931 // visible after) the ST in exit that drops ownership of the lock. 932 // Some other thread could then acquire the lock, but observe inconsistent 933 // or old monitor meta-data and heap data. That violates the JMM. 934 // To that end, the 1-0 exit() operation must have at least STST|LDST 935 // "release" barrier semantics. Specifically, there must be at least a 936 // STST|LDST barrier in exit() before the ST of null into _owner that drops 937 // the lock. The barrier ensures that changes to monitor meta-data and data 938 // protected by the lock will be visible before we release the lock, and 939 // therefore before some other thread (CPU) has a chance to acquire the lock. 940 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html. 941 // 942 // Critically, any prior STs to _succ or EntryList must be visible before 943 // the ST of null into _owner in the *subsequent* (following) corresponding 944 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily 945 // execute a serializing instruction. 946 947 return; 948 } 949 950 // ReenterI() is a specialized inline form of the latter half of the 951 // contended slow-path from EnterI(). We use ReenterI() only for 952 // monitor reentry in wait(). 953 // 954 // In the future we should reconcile EnterI() and ReenterI(). 955 956 void ObjectMonitor::ReenterI(JavaThread* current, ObjectWaiter* currentNode) { 957 assert(current != NULL, "invariant"); 958 assert(currentNode != NULL, "invariant"); 959 assert(currentNode->_thread == current, "invariant"); 960 assert(_waiters > 0, "invariant"); 961 assert(object()->mark() == markWord::encode(this), "invariant"); 962 963 assert(current->thread_state() != _thread_blocked, "invariant"); 964 965 int nWakeups = 0; 966 for (;;) { 967 ObjectWaiter::TStates v = currentNode->TState; 968 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant"); 969 assert(owner_raw() != current, "invariant"); 970 971 if (TryLock(current) > 0) break; 972 if (TrySpin(current) > 0) break; 973 974 { 975 OSThreadContendState osts(current->osthread()); 976 977 assert(current->thread_state() == _thread_in_vm, "invariant"); 978 979 { 980 ClearSuccOnSuspend csos(this); 981 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos); 982 current->_ParkEvent->park(); 983 } 984 } 985 986 // Try again, but just so we distinguish between futile wakeups and 987 // successful wakeups. The following test isn't algorithmically 988 // necessary, but it helps us maintain sensible statistics. 989 if (TryLock(current) > 0) break; 990 991 // The lock is still contested. 992 // Keep a tally of the # of futile wakeups. 993 // Note that the counter is not protected by a lock or updated by atomics. 994 // That is by design - we trade "lossy" counters which are exposed to 995 // races during updates for a lower probe effect. 996 ++nWakeups; 997 998 // Assuming this is not a spurious wakeup we'll normally 999 // find that _succ == current. 1000 if (_succ == current) _succ = NULL; 1001 1002 // Invariant: after clearing _succ a contending thread 1003 // *must* retry _owner before parking. 1004 OrderAccess::fence(); 1005 1006 // This PerfData object can be used in parallel with a safepoint. 1007 // See the work around in PerfDataManager::destroy(). 1008 OM_PERFDATA_OP(FutileWakeups, inc()); 1009 } 1010 1011 // current has acquired the lock -- Unlink current from the cxq or EntryList . 1012 // Normally we'll find current on the EntryList. 1013 // Unlinking from the EntryList is constant-time and atomic-free. 1014 // From the perspective of the lock owner (this thread), the 1015 // EntryList is stable and cxq is prepend-only. 1016 // The head of cxq is volatile but the interior is stable. 1017 // In addition, current.TState is stable. 1018 1019 assert(owner_raw() == current, "invariant"); 1020 assert(object()->mark() == markWord::encode(this), "invariant"); 1021 UnlinkAfterAcquire(current, currentNode); 1022 if (_succ == current) _succ = NULL; 1023 assert(_succ != current, "invariant"); 1024 currentNode->TState = ObjectWaiter::TS_RUN; 1025 OrderAccess::fence(); // see comments at the end of EnterI() 1026 } 1027 1028 // By convention we unlink a contending thread from EntryList|cxq immediately 1029 // after the thread acquires the lock in ::enter(). Equally, we could defer 1030 // unlinking the thread until ::exit()-time. 1031 1032 void ObjectMonitor::UnlinkAfterAcquire(JavaThread* current, ObjectWaiter* currentNode) { 1033 assert(owner_raw() == current, "invariant"); 1034 assert(currentNode->_thread == current, "invariant"); 1035 1036 if (currentNode->TState == ObjectWaiter::TS_ENTER) { 1037 // Normal case: remove current from the DLL EntryList . 1038 // This is a constant-time operation. 1039 ObjectWaiter* nxt = currentNode->_next; 1040 ObjectWaiter* prv = currentNode->_prev; 1041 if (nxt != NULL) nxt->_prev = prv; 1042 if (prv != NULL) prv->_next = nxt; 1043 if (currentNode == _EntryList) _EntryList = nxt; 1044 assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant"); 1045 assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant"); 1046 } else { 1047 assert(currentNode->TState == ObjectWaiter::TS_CXQ, "invariant"); 1048 // Inopportune interleaving -- current is still on the cxq. 1049 // This usually means the enqueue of self raced an exiting thread. 1050 // Normally we'll find current near the front of the cxq, so 1051 // dequeueing is typically fast. If needbe we can accelerate 1052 // this with some MCS/CHL-like bidirectional list hints and advisory 1053 // back-links so dequeueing from the interior will normally operate 1054 // in constant-time. 1055 // Dequeue current from either the head (with CAS) or from the interior 1056 // with a linear-time scan and normal non-atomic memory operations. 1057 // CONSIDER: if current is on the cxq then simply drain cxq into EntryList 1058 // and then unlink current from EntryList. We have to drain eventually, 1059 // so it might as well be now. 1060 1061 ObjectWaiter* v = _cxq; 1062 assert(v != NULL, "invariant"); 1063 if (v != currentNode || Atomic::cmpxchg(&_cxq, v, currentNode->_next) != v) { 1064 // The CAS above can fail from interference IFF a "RAT" arrived. 1065 // In that case current must be in the interior and can no longer be 1066 // at the head of cxq. 1067 if (v == currentNode) { 1068 assert(_cxq != v, "invariant"); 1069 v = _cxq; // CAS above failed - start scan at head of list 1070 } 1071 ObjectWaiter* p; 1072 ObjectWaiter* q = NULL; 1073 for (p = v; p != NULL && p != currentNode; p = p->_next) { 1074 q = p; 1075 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant"); 1076 } 1077 assert(v != currentNode, "invariant"); 1078 assert(p == currentNode, "Node not found on cxq"); 1079 assert(p != _cxq, "invariant"); 1080 assert(q != NULL, "invariant"); 1081 assert(q->_next == p, "invariant"); 1082 q->_next = p->_next; 1083 } 1084 } 1085 1086 #ifdef ASSERT 1087 // Diagnostic hygiene ... 1088 currentNode->_prev = (ObjectWaiter*) 0xBAD; 1089 currentNode->_next = (ObjectWaiter*) 0xBAD; 1090 currentNode->TState = ObjectWaiter::TS_RUN; 1091 #endif 1092 } 1093 1094 // ----------------------------------------------------------------------------- 1095 // Exit support 1096 // 1097 // exit() 1098 // ~~~~~~ 1099 // Note that the collector can't reclaim the objectMonitor or deflate 1100 // the object out from underneath the thread calling ::exit() as the 1101 // thread calling ::exit() never transitions to a stable state. 1102 // This inhibits GC, which in turn inhibits asynchronous (and 1103 // inopportune) reclamation of "this". 1104 // 1105 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ; 1106 // There's one exception to the claim above, however. EnterI() can call 1107 // exit() to drop a lock if the acquirer has been externally suspended. 1108 // In that case exit() is called with _thread_state == _thread_blocked, 1109 // but the monitor's _contentions field is > 0, which inhibits reclamation. 1110 // 1111 // 1-0 exit 1112 // ~~~~~~~~ 1113 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of 1114 // the fast-path operators have been optimized so the common ::exit() 1115 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock(). 1116 // The code emitted by fast_unlock() elides the usual MEMBAR. This 1117 // greatly improves latency -- MEMBAR and CAS having considerable local 1118 // latency on modern processors -- but at the cost of "stranding". Absent the 1119 // MEMBAR, a thread in fast_unlock() can race a thread in the slow 1120 // ::enter() path, resulting in the entering thread being stranding 1121 // and a progress-liveness failure. Stranding is extremely rare. 1122 // We use timers (timed park operations) & periodic polling to detect 1123 // and recover from stranding. Potentially stranded threads periodically 1124 // wake up and poll the lock. See the usage of the _Responsible variable. 1125 // 1126 // The CAS() in enter provides for safety and exclusion, while the CAS or 1127 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking 1128 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding. 1129 // We detect and recover from stranding with timers. 1130 // 1131 // If a thread transiently strands it'll park until (a) another 1132 // thread acquires the lock and then drops the lock, at which time the 1133 // exiting thread will notice and unpark the stranded thread, or, (b) 1134 // the timer expires. If the lock is high traffic then the stranding latency 1135 // will be low due to (a). If the lock is low traffic then the odds of 1136 // stranding are lower, although the worst-case stranding latency 1137 // is longer. Critically, we don't want to put excessive load in the 1138 // platform's timer subsystem. We want to minimize both the timer injection 1139 // rate (timers created/sec) as well as the number of timers active at 1140 // any one time. (more precisely, we want to minimize timer-seconds, which is 1141 // the integral of the # of active timers at any instant over time). 1142 // Both impinge on OS scalability. Given that, at most one thread parked on 1143 // a monitor will use a timer. 1144 // 1145 // There is also the risk of a futile wake-up. If we drop the lock 1146 // another thread can reacquire the lock immediately, and we can 1147 // then wake a thread unnecessarily. This is benign, and we've 1148 // structured the code so the windows are short and the frequency 1149 // of such futile wakups is low. 1150 1151 void ObjectMonitor::exit(JavaThread* current, bool not_suspended) { 1152 void* cur = owner_raw(); 1153 if (current != cur) { 1154 if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) { 1155 assert(_recursions == 0, "invariant"); 1156 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*. 1157 _recursions = 0; 1158 } else { 1159 // Apparent unbalanced locking ... 1160 // Naively we'd like to throw IllegalMonitorStateException. 1161 // As a practical matter we can neither allocate nor throw an 1162 // exception as ::exit() can be called from leaf routines. 1163 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties. 1164 // Upon deeper reflection, however, in a properly run JVM the only 1165 // way we should encounter this situation is in the presence of 1166 // unbalanced JNI locking. TODO: CheckJNICalls. 1167 // See also: CR4414101 1168 #ifdef ASSERT 1169 LogStreamHandle(Error, monitorinflation) lsh; 1170 lsh.print_cr("ERROR: ObjectMonitor::exit(): thread=" INTPTR_FORMAT 1171 " is exiting an ObjectMonitor it does not own.", p2i(current)); 1172 lsh.print_cr("The imbalance is possibly caused by JNI locking."); 1173 print_debug_style_on(&lsh); 1174 assert(false, "Non-balanced monitor enter/exit!"); 1175 #endif 1176 return; 1177 } 1178 } 1179 1180 if (_recursions != 0) { 1181 _recursions--; // this is simple recursive enter 1182 return; 1183 } 1184 1185 // Invariant: after setting Responsible=null an thread must execute 1186 // a MEMBAR or other serializing instruction before fetching EntryList|cxq. 1187 _Responsible = NULL; 1188 1189 #if INCLUDE_JFR 1190 // get the owner's thread id for the MonitorEnter event 1191 // if it is enabled and the thread isn't suspended 1192 if (not_suspended && EventJavaMonitorEnter::is_enabled()) { 1193 _previous_owner_tid = JFR_THREAD_ID(current); 1194 } 1195 #endif 1196 1197 for (;;) { 1198 assert(current == owner_raw(), "invariant"); 1199 1200 // Drop the lock. 1201 // release semantics: prior loads and stores from within the critical section 1202 // must not float (reorder) past the following store that drops the lock. 1203 // Uses a storeload to separate release_store(owner) from the 1204 // successor check. The try_set_owner() below uses cmpxchg() so 1205 // we get the fence down there. 1206 release_clear_owner(current); 1207 OrderAccess::storeload(); 1208 1209 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) { 1210 return; 1211 } 1212 // Other threads are blocked trying to acquire the lock. 1213 1214 // Normally the exiting thread is responsible for ensuring succession, 1215 // but if other successors are ready or other entering threads are spinning 1216 // then this thread can simply store NULL into _owner and exit without 1217 // waking a successor. The existence of spinners or ready successors 1218 // guarantees proper succession (liveness). Responsibility passes to the 1219 // ready or running successors. The exiting thread delegates the duty. 1220 // More precisely, if a successor already exists this thread is absolved 1221 // of the responsibility of waking (unparking) one. 1222 // 1223 // The _succ variable is critical to reducing futile wakeup frequency. 1224 // _succ identifies the "heir presumptive" thread that has been made 1225 // ready (unparked) but that has not yet run. We need only one such 1226 // successor thread to guarantee progress. 1227 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf 1228 // section 3.3 "Futile Wakeup Throttling" for details. 1229 // 1230 // Note that spinners in Enter() also set _succ non-null. 1231 // In the current implementation spinners opportunistically set 1232 // _succ so that exiting threads might avoid waking a successor. 1233 // Another less appealing alternative would be for the exiting thread 1234 // to drop the lock and then spin briefly to see if a spinner managed 1235 // to acquire the lock. If so, the exiting thread could exit 1236 // immediately without waking a successor, otherwise the exiting 1237 // thread would need to dequeue and wake a successor. 1238 // (Note that we'd need to make the post-drop spin short, but no 1239 // shorter than the worst-case round-trip cache-line migration time. 1240 // The dropped lock needs to become visible to the spinner, and then 1241 // the acquisition of the lock by the spinner must become visible to 1242 // the exiting thread). 1243 1244 // It appears that an heir-presumptive (successor) must be made ready. 1245 // Only the current lock owner can manipulate the EntryList or 1246 // drain _cxq, so we need to reacquire the lock. If we fail 1247 // to reacquire the lock the responsibility for ensuring succession 1248 // falls to the new owner. 1249 // 1250 if (try_set_owner_from(NULL, current) != NULL) { 1251 return; 1252 } 1253 1254 guarantee(owner_raw() == current, "invariant"); 1255 1256 ObjectWaiter* w = NULL; 1257 1258 w = _EntryList; 1259 if (w != NULL) { 1260 // I'd like to write: guarantee (w->_thread != current). 1261 // But in practice an exiting thread may find itself on the EntryList. 1262 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and 1263 // then calls exit(). Exit release the lock by setting O._owner to NULL. 1264 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The 1265 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then 1266 // release the lock "O". T2 resumes immediately after the ST of null into 1267 // _owner, above. T2 notices that the EntryList is populated, so it 1268 // reacquires the lock and then finds itself on the EntryList. 1269 // Given all that, we have to tolerate the circumstance where "w" is 1270 // associated with current. 1271 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant"); 1272 ExitEpilog(current, w); 1273 return; 1274 } 1275 1276 // If we find that both _cxq and EntryList are null then just 1277 // re-run the exit protocol from the top. 1278 w = _cxq; 1279 if (w == NULL) continue; 1280 1281 // Drain _cxq into EntryList - bulk transfer. 1282 // First, detach _cxq. 1283 // The following loop is tantamount to: w = swap(&cxq, NULL) 1284 for (;;) { 1285 assert(w != NULL, "Invariant"); 1286 ObjectWaiter* u = Atomic::cmpxchg(&_cxq, w, (ObjectWaiter*)NULL); 1287 if (u == w) break; 1288 w = u; 1289 } 1290 1291 assert(w != NULL, "invariant"); 1292 assert(_EntryList == NULL, "invariant"); 1293 1294 // Convert the LIFO SLL anchored by _cxq into a DLL. 1295 // The list reorganization step operates in O(LENGTH(w)) time. 1296 // It's critical that this step operate quickly as 1297 // "current" still holds the outer-lock, restricting parallelism 1298 // and effectively lengthening the critical section. 1299 // Invariant: s chases t chases u. 1300 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so 1301 // we have faster access to the tail. 1302 1303 _EntryList = w; 1304 ObjectWaiter* q = NULL; 1305 ObjectWaiter* p; 1306 for (p = w; p != NULL; p = p->_next) { 1307 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant"); 1308 p->TState = ObjectWaiter::TS_ENTER; 1309 p->_prev = q; 1310 q = p; 1311 } 1312 1313 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL 1314 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog(). 1315 1316 // See if we can abdicate to a spinner instead of waking a thread. 1317 // A primary goal of the implementation is to reduce the 1318 // context-switch rate. 1319 if (_succ != NULL) continue; 1320 1321 w = _EntryList; 1322 if (w != NULL) { 1323 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant"); 1324 ExitEpilog(current, w); 1325 return; 1326 } 1327 } 1328 } 1329 1330 void ObjectMonitor::ExitEpilog(JavaThread* current, ObjectWaiter* Wakee) { 1331 assert(owner_raw() == current, "invariant"); 1332 1333 // Exit protocol: 1334 // 1. ST _succ = wakee 1335 // 2. membar #loadstore|#storestore; 1336 // 2. ST _owner = NULL 1337 // 3. unpark(wakee) 1338 1339 _succ = Wakee->_thread; 1340 ParkEvent * Trigger = Wakee->_event; 1341 1342 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again. 1343 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be 1344 // out-of-scope (non-extant). 1345 Wakee = NULL; 1346 1347 // Drop the lock. 1348 // Uses a fence to separate release_store(owner) from the LD in unpark(). 1349 release_clear_owner(current); 1350 OrderAccess::fence(); 1351 1352 DTRACE_MONITOR_PROBE(contended__exit, this, object(), current); 1353 Trigger->unpark(); 1354 1355 // Maintain stats and report events to JVMTI 1356 OM_PERFDATA_OP(Parks, inc()); 1357 } 1358 1359 1360 // ----------------------------------------------------------------------------- 1361 // Class Loader deadlock handling. 1362 // 1363 // complete_exit exits a lock returning recursion count 1364 // complete_exit/reenter operate as a wait without waiting 1365 // complete_exit requires an inflated monitor 1366 // The _owner field is not always the Thread addr even with an 1367 // inflated monitor, e.g. the monitor can be inflated by a non-owning 1368 // thread due to contention. 1369 intx ObjectMonitor::complete_exit(JavaThread* current) { 1370 assert(InitDone, "Unexpectedly not initialized"); 1371 1372 void* cur = owner_raw(); 1373 if (current != cur) { 1374 if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) { 1375 assert(_recursions == 0, "internal state error"); 1376 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*. 1377 _recursions = 0; 1378 } 1379 } 1380 1381 guarantee(current == owner_raw(), "complete_exit not owner"); 1382 intx save = _recursions; // record the old recursion count 1383 _recursions = 0; // set the recursion level to be 0 1384 exit(current); // exit the monitor 1385 guarantee(owner_raw() != current, "invariant"); 1386 return save; 1387 } 1388 1389 // reenter() enters a lock and sets recursion count 1390 // complete_exit/reenter operate as a wait without waiting 1391 bool ObjectMonitor::reenter(intx recursions, JavaThread* current) { 1392 1393 guarantee(owner_raw() != current, "reenter already owner"); 1394 if (!enter(current)) { 1395 return false; 1396 } 1397 // Entered the monitor. 1398 guarantee(_recursions == 0, "reenter recursion"); 1399 _recursions = recursions; 1400 return true; 1401 } 1402 1403 // Checks that the current THREAD owns this monitor and causes an 1404 // immediate return if it doesn't. We don't use the CHECK macro 1405 // because we want the IMSE to be the only exception that is thrown 1406 // from the call site when false is returned. Any other pending 1407 // exception is ignored. 1408 #define CHECK_OWNER() \ 1409 do { \ 1410 if (!check_owner(THREAD)) { \ 1411 assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \ 1412 return; \ 1413 } \ 1414 } while (false) 1415 1416 // Returns true if the specified thread owns the ObjectMonitor. 1417 // Otherwise returns false and throws IllegalMonitorStateException 1418 // (IMSE). If there is a pending exception and the specified thread 1419 // is not the owner, that exception will be replaced by the IMSE. 1420 bool ObjectMonitor::check_owner(TRAPS) { 1421 JavaThread* current = THREAD; 1422 void* cur = owner_raw(); 1423 assert(cur != anon_owner_ptr(), "no anon owner here"); 1424 if (cur == current) { 1425 return true; 1426 } 1427 if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) { 1428 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*. 1429 _recursions = 0; 1430 return true; 1431 } 1432 THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(), 1433 "current thread is not owner", false); 1434 } 1435 1436 static inline bool is_excluded(const Klass* monitor_klass) { 1437 assert(monitor_klass != nullptr, "invariant"); 1438 NOT_JFR_RETURN_(false); 1439 JFR_ONLY(return vmSymbols::jfr_chunk_rotation_monitor() == monitor_klass->name()); 1440 } 1441 1442 static void post_monitor_wait_event(EventJavaMonitorWait* event, 1443 ObjectMonitor* monitor, 1444 jlong notifier_tid, 1445 jlong timeout, 1446 bool timedout) { 1447 assert(event != NULL, "invariant"); 1448 assert(monitor != NULL, "invariant"); 1449 const Klass* monitor_klass = monitor->object()->klass(); 1450 if (is_excluded(monitor_klass)) { 1451 return; 1452 } 1453 event->set_monitorClass(monitor_klass); 1454 event->set_timeout(timeout); 1455 // Set an address that is 'unique enough', such that events close in 1456 // time and with the same address are likely (but not guaranteed) to 1457 // belong to the same object. 1458 event->set_address((uintptr_t)monitor); 1459 event->set_notifier(notifier_tid); 1460 event->set_timedOut(timedout); 1461 event->commit(); 1462 } 1463 1464 // ----------------------------------------------------------------------------- 1465 // Wait/Notify/NotifyAll 1466 // 1467 // Note: a subset of changes to ObjectMonitor::wait() 1468 // will need to be replicated in complete_exit 1469 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) { 1470 JavaThread* current = THREAD; 1471 1472 assert(InitDone, "Unexpectedly not initialized"); 1473 1474 CHECK_OWNER(); // Throws IMSE if not owner. 1475 1476 EventJavaMonitorWait event; 1477 1478 // check for a pending interrupt 1479 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) { 1480 // post monitor waited event. Note that this is past-tense, we are done waiting. 1481 if (JvmtiExport::should_post_monitor_waited()) { 1482 // Note: 'false' parameter is passed here because the 1483 // wait was not timed out due to thread interrupt. 1484 JvmtiExport::post_monitor_waited(current, this, false); 1485 1486 // In this short circuit of the monitor wait protocol, the 1487 // current thread never drops ownership of the monitor and 1488 // never gets added to the wait queue so the current thread 1489 // cannot be made the successor. This means that the 1490 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally 1491 // consume an unpark() meant for the ParkEvent associated with 1492 // this ObjectMonitor. 1493 } 1494 if (event.should_commit()) { 1495 post_monitor_wait_event(&event, this, 0, millis, false); 1496 } 1497 THROW(vmSymbols::java_lang_InterruptedException()); 1498 return; 1499 } 1500 1501 assert(current->_Stalled == 0, "invariant"); 1502 current->_Stalled = intptr_t(this); 1503 current->set_current_waiting_monitor(this); 1504 1505 // create a node to be put into the queue 1506 // Critically, after we reset() the event but prior to park(), we must check 1507 // for a pending interrupt. 1508 ObjectWaiter node(current); 1509 node.TState = ObjectWaiter::TS_WAIT; 1510 current->_ParkEvent->reset(); 1511 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag 1512 1513 // Enter the waiting queue, which is a circular doubly linked list in this case 1514 // but it could be a priority queue or any data structure. 1515 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only 1516 // by the the owner of the monitor *except* in the case where park() 1517 // returns because of a timeout of interrupt. Contention is exceptionally rare 1518 // so we use a simple spin-lock instead of a heavier-weight blocking lock. 1519 1520 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add"); 1521 AddWaiter(&node); 1522 Thread::SpinRelease(&_WaitSetLock); 1523 1524 _Responsible = NULL; 1525 1526 intx save = _recursions; // record the old recursion count 1527 _waiters++; // increment the number of waiters 1528 _recursions = 0; // set the recursion level to be 1 1529 exit(current); // exit the monitor 1530 guarantee(owner_raw() != current, "invariant"); 1531 1532 // The thread is on the WaitSet list - now park() it. 1533 // On MP systems it's conceivable that a brief spin before we park 1534 // could be profitable. 1535 // 1536 // TODO-FIXME: change the following logic to a loop of the form 1537 // while (!timeout && !interrupted && _notified == 0) park() 1538 1539 int ret = OS_OK; 1540 int WasNotified = 0; 1541 1542 // Need to check interrupt state whilst still _thread_in_vm 1543 bool interrupted = interruptible && current->is_interrupted(false); 1544 1545 { // State transition wrappers 1546 OSThread* osthread = current->osthread(); 1547 OSThreadWaitState osts(osthread, true); 1548 1549 assert(current->thread_state() == _thread_in_vm, "invariant"); 1550 1551 { 1552 ClearSuccOnSuspend csos(this); 1553 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos); 1554 if (interrupted || HAS_PENDING_EXCEPTION) { 1555 // Intentionally empty 1556 } else if (node._notified == 0) { 1557 if (millis <= 0) { 1558 current->_ParkEvent->park(); 1559 } else { 1560 ret = current->_ParkEvent->park(millis); 1561 } 1562 } 1563 } 1564 1565 // Node may be on the WaitSet, the EntryList (or cxq), or in transition 1566 // from the WaitSet to the EntryList. 1567 // See if we need to remove Node from the WaitSet. 1568 // We use double-checked locking to avoid grabbing _WaitSetLock 1569 // if the thread is not on the wait queue. 1570 // 1571 // Note that we don't need a fence before the fetch of TState. 1572 // In the worst case we'll fetch a old-stale value of TS_WAIT previously 1573 // written by the is thread. (perhaps the fetch might even be satisfied 1574 // by a look-aside into the processor's own store buffer, although given 1575 // the length of the code path between the prior ST and this load that's 1576 // highly unlikely). If the following LD fetches a stale TS_WAIT value 1577 // then we'll acquire the lock and then re-fetch a fresh TState value. 1578 // That is, we fail toward safety. 1579 1580 if (node.TState == ObjectWaiter::TS_WAIT) { 1581 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink"); 1582 if (node.TState == ObjectWaiter::TS_WAIT) { 1583 DequeueSpecificWaiter(&node); // unlink from WaitSet 1584 assert(node._notified == 0, "invariant"); 1585 node.TState = ObjectWaiter::TS_RUN; 1586 } 1587 Thread::SpinRelease(&_WaitSetLock); 1588 } 1589 1590 // The thread is now either on off-list (TS_RUN), 1591 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ). 1592 // The Node's TState variable is stable from the perspective of this thread. 1593 // No other threads will asynchronously modify TState. 1594 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant"); 1595 OrderAccess::loadload(); 1596 if (_succ == current) _succ = NULL; 1597 WasNotified = node._notified; 1598 1599 // Reentry phase -- reacquire the monitor. 1600 // re-enter contended monitor after object.wait(). 1601 // retain OBJECT_WAIT state until re-enter successfully completes 1602 // Thread state is thread_in_vm and oop access is again safe, 1603 // although the raw address of the object may have changed. 1604 // (Don't cache naked oops over safepoints, of course). 1605 1606 // post monitor waited event. Note that this is past-tense, we are done waiting. 1607 if (JvmtiExport::should_post_monitor_waited()) { 1608 JvmtiExport::post_monitor_waited(current, this, ret == OS_TIMEOUT); 1609 1610 if (node._notified != 0 && _succ == current) { 1611 // In this part of the monitor wait-notify-reenter protocol it 1612 // is possible (and normal) for another thread to do a fastpath 1613 // monitor enter-exit while this thread is still trying to get 1614 // to the reenter portion of the protocol. 1615 // 1616 // The ObjectMonitor was notified and the current thread is 1617 // the successor which also means that an unpark() has already 1618 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can 1619 // consume the unpark() that was done when the successor was 1620 // set because the same ParkEvent is shared between Java 1621 // monitors and JVM/TI RawMonitors (for now). 1622 // 1623 // We redo the unpark() to ensure forward progress, i.e., we 1624 // don't want all pending threads hanging (parked) with none 1625 // entering the unlocked monitor. 1626 node._event->unpark(); 1627 } 1628 } 1629 1630 if (event.should_commit()) { 1631 post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT); 1632 } 1633 1634 OrderAccess::fence(); 1635 1636 assert(current->_Stalled != 0, "invariant"); 1637 current->_Stalled = 0; 1638 1639 assert(owner_raw() != current, "invariant"); 1640 ObjectWaiter::TStates v = node.TState; 1641 if (v == ObjectWaiter::TS_RUN) { 1642 enter(current); 1643 } else { 1644 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant"); 1645 ReenterI(current, &node); 1646 node.wait_reenter_end(this); 1647 } 1648 1649 // current has reacquired the lock. 1650 // Lifecycle - the node representing current must not appear on any queues. 1651 // Node is about to go out-of-scope, but even if it were immortal we wouldn't 1652 // want residual elements associated with this thread left on any lists. 1653 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant"); 1654 assert(owner_raw() == current, "invariant"); 1655 assert(_succ != current, "invariant"); 1656 } // OSThreadWaitState() 1657 1658 current->set_current_waiting_monitor(NULL); 1659 1660 guarantee(_recursions == 0, "invariant"); 1661 _recursions = save // restore the old recursion count 1662 + JvmtiDeferredUpdates::get_and_reset_relock_count_after_wait(current); // increased by the deferred relock count 1663 _waiters--; // decrement the number of waiters 1664 1665 // Verify a few postconditions 1666 assert(owner_raw() == current, "invariant"); 1667 assert(_succ != current, "invariant"); 1668 assert(object()->mark() == markWord::encode(this), "invariant"); 1669 1670 // check if the notification happened 1671 if (!WasNotified) { 1672 // no, it could be timeout or Thread.interrupt() or both 1673 // check for interrupt event, otherwise it is timeout 1674 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) { 1675 THROW(vmSymbols::java_lang_InterruptedException()); 1676 } 1677 } 1678 1679 // NOTE: Spurious wake up will be consider as timeout. 1680 // Monitor notify has precedence over thread interrupt. 1681 } 1682 1683 1684 // Consider: 1685 // If the lock is cool (cxq == null && succ == null) and we're on an MP system 1686 // then instead of transferring a thread from the WaitSet to the EntryList 1687 // we might just dequeue a thread from the WaitSet and directly unpark() it. 1688 1689 void ObjectMonitor::INotify(JavaThread* current) { 1690 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify"); 1691 ObjectWaiter* iterator = DequeueWaiter(); 1692 if (iterator != NULL) { 1693 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant"); 1694 guarantee(iterator->_notified == 0, "invariant"); 1695 // Disposition - what might we do with iterator ? 1696 // a. add it directly to the EntryList - either tail (policy == 1) 1697 // or head (policy == 0). 1698 // b. push it onto the front of the _cxq (policy == 2). 1699 // For now we use (b). 1700 1701 iterator->TState = ObjectWaiter::TS_ENTER; 1702 1703 iterator->_notified = 1; 1704 iterator->_notifier_tid = JFR_THREAD_ID(current); 1705 1706 ObjectWaiter* list = _EntryList; 1707 if (list != NULL) { 1708 assert(list->_prev == NULL, "invariant"); 1709 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant"); 1710 assert(list != iterator, "invariant"); 1711 } 1712 1713 // prepend to cxq 1714 if (list == NULL) { 1715 iterator->_next = iterator->_prev = NULL; 1716 _EntryList = iterator; 1717 } else { 1718 iterator->TState = ObjectWaiter::TS_CXQ; 1719 for (;;) { 1720 ObjectWaiter* front = _cxq; 1721 iterator->_next = front; 1722 if (Atomic::cmpxchg(&_cxq, front, iterator) == front) { 1723 break; 1724 } 1725 } 1726 } 1727 1728 // _WaitSetLock protects the wait queue, not the EntryList. We could 1729 // move the add-to-EntryList operation, above, outside the critical section 1730 // protected by _WaitSetLock. In practice that's not useful. With the 1731 // exception of wait() timeouts and interrupts the monitor owner 1732 // is the only thread that grabs _WaitSetLock. There's almost no contention 1733 // on _WaitSetLock so it's not profitable to reduce the length of the 1734 // critical section. 1735 1736 iterator->wait_reenter_begin(this); 1737 } 1738 Thread::SpinRelease(&_WaitSetLock); 1739 } 1740 1741 // Consider: a not-uncommon synchronization bug is to use notify() when 1742 // notifyAll() is more appropriate, potentially resulting in stranded 1743 // threads; this is one example of a lost wakeup. A useful diagnostic 1744 // option is to force all notify() operations to behave as notifyAll(). 1745 // 1746 // Note: We can also detect many such problems with a "minimum wait". 1747 // When the "minimum wait" is set to a small non-zero timeout value 1748 // and the program does not hang whereas it did absent "minimum wait", 1749 // that suggests a lost wakeup bug. 1750 1751 void ObjectMonitor::notify(TRAPS) { 1752 JavaThread* current = THREAD; 1753 CHECK_OWNER(); // Throws IMSE if not owner. 1754 if (_WaitSet == NULL) { 1755 return; 1756 } 1757 DTRACE_MONITOR_PROBE(notify, this, object(), current); 1758 INotify(current); 1759 OM_PERFDATA_OP(Notifications, inc(1)); 1760 } 1761 1762 1763 // The current implementation of notifyAll() transfers the waiters one-at-a-time 1764 // from the waitset to the EntryList. This could be done more efficiently with a 1765 // single bulk transfer but in practice it's not time-critical. Beware too, 1766 // that in prepend-mode we invert the order of the waiters. Let's say that the 1767 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend 1768 // mode the waitset will be empty and the EntryList will be "DCBAXYZ". 1769 1770 void ObjectMonitor::notifyAll(TRAPS) { 1771 JavaThread* current = THREAD; 1772 CHECK_OWNER(); // Throws IMSE if not owner. 1773 if (_WaitSet == NULL) { 1774 return; 1775 } 1776 1777 DTRACE_MONITOR_PROBE(notifyAll, this, object(), current); 1778 int tally = 0; 1779 while (_WaitSet != NULL) { 1780 tally++; 1781 INotify(current); 1782 } 1783 1784 OM_PERFDATA_OP(Notifications, inc(tally)); 1785 } 1786 1787 // ----------------------------------------------------------------------------- 1788 // Adaptive Spinning Support 1789 // 1790 // Adaptive spin-then-block - rational spinning 1791 // 1792 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS 1793 // algorithm. On high order SMP systems it would be better to start with 1794 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH, 1795 // a contending thread could enqueue itself on the cxq and then spin locally 1796 // on a thread-specific variable such as its ParkEvent._Event flag. 1797 // That's left as an exercise for the reader. Note that global spinning is 1798 // not problematic on Niagara, as the L2 cache serves the interconnect and 1799 // has both low latency and massive bandwidth. 1800 // 1801 // Broadly, we can fix the spin frequency -- that is, the % of contended lock 1802 // acquisition attempts where we opt to spin -- at 100% and vary the spin count 1803 // (duration) or we can fix the count at approximately the duration of 1804 // a context switch and vary the frequency. Of course we could also 1805 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor. 1806 // For a description of 'Adaptive spin-then-block mutual exclusion in 1807 // multi-threaded processing,' see U.S. Pat. No. 8046758. 1808 // 1809 // This implementation varies the duration "D", where D varies with 1810 // the success rate of recent spin attempts. (D is capped at approximately 1811 // length of a round-trip context switch). The success rate for recent 1812 // spin attempts is a good predictor of the success rate of future spin 1813 // attempts. The mechanism adapts automatically to varying critical 1814 // section length (lock modality), system load and degree of parallelism. 1815 // D is maintained per-monitor in _SpinDuration and is initialized 1816 // optimistically. Spin frequency is fixed at 100%. 1817 // 1818 // Note that _SpinDuration is volatile, but we update it without locks 1819 // or atomics. The code is designed so that _SpinDuration stays within 1820 // a reasonable range even in the presence of races. The arithmetic 1821 // operations on _SpinDuration are closed over the domain of legal values, 1822 // so at worst a race will install and older but still legal value. 1823 // At the very worst this introduces some apparent non-determinism. 1824 // We might spin when we shouldn't or vice-versa, but since the spin 1825 // count are relatively short, even in the worst case, the effect is harmless. 1826 // 1827 // Care must be taken that a low "D" value does not become an 1828 // an absorbing state. Transient spinning failures -- when spinning 1829 // is overall profitable -- should not cause the system to converge 1830 // on low "D" values. We want spinning to be stable and predictable 1831 // and fairly responsive to change and at the same time we don't want 1832 // it to oscillate, become metastable, be "too" non-deterministic, 1833 // or converge on or enter undesirable stable absorbing states. 1834 // 1835 // We implement a feedback-based control system -- using past behavior 1836 // to predict future behavior. We face two issues: (a) if the 1837 // input signal is random then the spin predictor won't provide optimal 1838 // results, and (b) if the signal frequency is too high then the control 1839 // system, which has some natural response lag, will "chase" the signal. 1840 // (b) can arise from multimodal lock hold times. Transient preemption 1841 // can also result in apparent bimodal lock hold times. 1842 // Although sub-optimal, neither condition is particularly harmful, as 1843 // in the worst-case we'll spin when we shouldn't or vice-versa. 1844 // The maximum spin duration is rather short so the failure modes aren't bad. 1845 // To be conservative, I've tuned the gain in system to bias toward 1846 // _not spinning. Relatedly, the system can sometimes enter a mode where it 1847 // "rings" or oscillates between spinning and not spinning. This happens 1848 // when spinning is just on the cusp of profitability, however, so the 1849 // situation is not dire. The state is benign -- there's no need to add 1850 // hysteresis control to damp the transition rate between spinning and 1851 // not spinning. 1852 1853 // Spinning: Fixed frequency (100%), vary duration 1854 int ObjectMonitor::TrySpin(JavaThread* current) { 1855 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning. 1856 int ctr = Knob_FixedSpin; 1857 if (ctr != 0) { 1858 while (--ctr >= 0) { 1859 if (TryLock(current) > 0) return 1; 1860 SpinPause(); 1861 } 1862 return 0; 1863 } 1864 1865 for (ctr = Knob_PreSpin + 1; --ctr >= 0;) { 1866 if (TryLock(current) > 0) { 1867 // Increase _SpinDuration ... 1868 // Note that we don't clamp SpinDuration precisely at SpinLimit. 1869 // Raising _SpurDuration to the poverty line is key. 1870 int x = _SpinDuration; 1871 if (x < Knob_SpinLimit) { 1872 if (x < Knob_Poverty) x = Knob_Poverty; 1873 _SpinDuration = x + Knob_BonusB; 1874 } 1875 return 1; 1876 } 1877 SpinPause(); 1878 } 1879 1880 // Admission control - verify preconditions for spinning 1881 // 1882 // We always spin a little bit, just to prevent _SpinDuration == 0 from 1883 // becoming an absorbing state. Put another way, we spin briefly to 1884 // sample, just in case the system load, parallelism, contention, or lock 1885 // modality changed. 1886 // 1887 // Consider the following alternative: 1888 // Periodically set _SpinDuration = _SpinLimit and try a long/full 1889 // spin attempt. "Periodically" might mean after a tally of 1890 // the # of failed spin attempts (or iterations) reaches some threshold. 1891 // This takes us into the realm of 1-out-of-N spinning, where we 1892 // hold the duration constant but vary the frequency. 1893 1894 ctr = _SpinDuration; 1895 if (ctr <= 0) return 0; 1896 1897 if (NotRunnable(current, (JavaThread*) owner_raw())) { 1898 return 0; 1899 } 1900 1901 // We're good to spin ... spin ingress. 1902 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades 1903 // when preparing to LD...CAS _owner, etc and the CAS is likely 1904 // to succeed. 1905 if (_succ == NULL) { 1906 _succ = current; 1907 } 1908 Thread* prv = NULL; 1909 1910 // There are three ways to exit the following loop: 1911 // 1. A successful spin where this thread has acquired the lock. 1912 // 2. Spin failure with prejudice 1913 // 3. Spin failure without prejudice 1914 1915 while (--ctr >= 0) { 1916 1917 // Periodic polling -- Check for pending GC 1918 // Threads may spin while they're unsafe. 1919 // We don't want spinning threads to delay the JVM from reaching 1920 // a stop-the-world safepoint or to steal cycles from GC. 1921 // If we detect a pending safepoint we abort in order that 1922 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b) 1923 // this thread, if safe, doesn't steal cycles from GC. 1924 // This is in keeping with the "no loitering in runtime" rule. 1925 // We periodically check to see if there's a safepoint pending. 1926 if ((ctr & 0xFF) == 0) { 1927 if (SafepointMechanism::should_process(current)) { 1928 goto Abort; // abrupt spin egress 1929 } 1930 SpinPause(); 1931 } 1932 1933 // Probe _owner with TATAS 1934 // If this thread observes the monitor transition or flicker 1935 // from locked to unlocked to locked, then the odds that this 1936 // thread will acquire the lock in this spin attempt go down 1937 // considerably. The same argument applies if the CAS fails 1938 // or if we observe _owner change from one non-null value to 1939 // another non-null value. In such cases we might abort 1940 // the spin without prejudice or apply a "penalty" to the 1941 // spin count-down variable "ctr", reducing it by 100, say. 1942 1943 JavaThread* ox = (JavaThread*) owner_raw(); 1944 if (ox == NULL) { 1945 ox = (JavaThread*)try_set_owner_from(NULL, current); 1946 if (ox == NULL) { 1947 // The CAS succeeded -- this thread acquired ownership 1948 // Take care of some bookkeeping to exit spin state. 1949 if (_succ == current) { 1950 _succ = NULL; 1951 } 1952 1953 // Increase _SpinDuration : 1954 // The spin was successful (profitable) so we tend toward 1955 // longer spin attempts in the future. 1956 // CONSIDER: factor "ctr" into the _SpinDuration adjustment. 1957 // If we acquired the lock early in the spin cycle it 1958 // makes sense to increase _SpinDuration proportionally. 1959 // Note that we don't clamp SpinDuration precisely at SpinLimit. 1960 int x = _SpinDuration; 1961 if (x < Knob_SpinLimit) { 1962 if (x < Knob_Poverty) x = Knob_Poverty; 1963 _SpinDuration = x + Knob_Bonus; 1964 } 1965 return 1; 1966 } 1967 1968 // The CAS failed ... we can take any of the following actions: 1969 // * penalize: ctr -= CASPenalty 1970 // * exit spin with prejudice -- goto Abort; 1971 // * exit spin without prejudice. 1972 // * Since CAS is high-latency, retry again immediately. 1973 prv = ox; 1974 goto Abort; 1975 } 1976 1977 // Did lock ownership change hands ? 1978 if (ox != prv && prv != NULL) { 1979 goto Abort; 1980 } 1981 prv = ox; 1982 1983 // Abort the spin if the owner is not executing. 1984 // The owner must be executing in order to drop the lock. 1985 // Spinning while the owner is OFFPROC is idiocy. 1986 // Consider: ctr -= RunnablePenalty ; 1987 if (NotRunnable(current, ox)) { 1988 goto Abort; 1989 } 1990 if (_succ == NULL) { 1991 _succ = current; 1992 } 1993 } 1994 1995 // Spin failed with prejudice -- reduce _SpinDuration. 1996 // TODO: Use an AIMD-like policy to adjust _SpinDuration. 1997 // AIMD is globally stable. 1998 { 1999 int x = _SpinDuration; 2000 if (x > 0) { 2001 // Consider an AIMD scheme like: x -= (x >> 3) + 100 2002 // This is globally sample and tends to damp the response. 2003 x -= Knob_Penalty; 2004 if (x < 0) x = 0; 2005 _SpinDuration = x; 2006 } 2007 } 2008 2009 Abort: 2010 if (_succ == current) { 2011 _succ = NULL; 2012 // Invariant: after setting succ=null a contending thread 2013 // must recheck-retry _owner before parking. This usually happens 2014 // in the normal usage of TrySpin(), but it's safest 2015 // to make TrySpin() as foolproof as possible. 2016 OrderAccess::fence(); 2017 if (TryLock(current) > 0) return 1; 2018 } 2019 return 0; 2020 } 2021 2022 // NotRunnable() -- informed spinning 2023 // 2024 // Don't bother spinning if the owner is not eligible to drop the lock. 2025 // Spin only if the owner thread is _thread_in_Java or _thread_in_vm. 2026 // The thread must be runnable in order to drop the lock in timely fashion. 2027 // If the _owner is not runnable then spinning will not likely be 2028 // successful (profitable). 2029 // 2030 // Beware -- the thread referenced by _owner could have died 2031 // so a simply fetch from _owner->_thread_state might trap. 2032 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state. 2033 // Because of the lifecycle issues, the _thread_state values 2034 // observed by NotRunnable() might be garbage. NotRunnable must 2035 // tolerate this and consider the observed _thread_state value 2036 // as advisory. 2037 // 2038 // Beware too, that _owner is sometimes a BasicLock address and sometimes 2039 // a thread pointer. 2040 // Alternately, we might tag the type (thread pointer vs basiclock pointer) 2041 // with the LSB of _owner. Another option would be to probabilistically probe 2042 // the putative _owner->TypeTag value. 2043 // 2044 // Checking _thread_state isn't perfect. Even if the thread is 2045 // in_java it might be blocked on a page-fault or have been preempted 2046 // and sitting on a ready/dispatch queue. 2047 // 2048 // The return value from NotRunnable() is *advisory* -- the 2049 // result is based on sampling and is not necessarily coherent. 2050 // The caller must tolerate false-negative and false-positive errors. 2051 // Spinning, in general, is probabilistic anyway. 2052 2053 2054 int ObjectMonitor::NotRunnable(JavaThread* current, JavaThread* ox) { 2055 // Check ox->TypeTag == 2BAD. 2056 if (ox == NULL) return 0; 2057 2058 // Avoid transitive spinning ... 2059 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L. 2060 // Immediately after T1 acquires L it's possible that T2, also 2061 // spinning on L, will see L.Owner=T1 and T1._Stalled=L. 2062 // This occurs transiently after T1 acquired L but before 2063 // T1 managed to clear T1.Stalled. T2 does not need to abort 2064 // its spin in this circumstance. 2065 intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1)); 2066 2067 if (BlockedOn == 1) return 1; 2068 if (BlockedOn != 0) { 2069 return BlockedOn != intptr_t(this) && owner_raw() == ox; 2070 } 2071 2072 assert(sizeof(ox->_thread_state == sizeof(int)), "invariant"); 2073 int jst = SafeFetch32((int *) &ox->_thread_state, -1);; 2074 // consider also: jst != _thread_in_Java -- but that's overspecific. 2075 return jst == _thread_blocked || jst == _thread_in_native; 2076 } 2077 2078 2079 // ----------------------------------------------------------------------------- 2080 // WaitSet management ... 2081 2082 ObjectWaiter::ObjectWaiter(JavaThread* current) { 2083 _next = NULL; 2084 _prev = NULL; 2085 _notified = 0; 2086 _notifier_tid = 0; 2087 TState = TS_RUN; 2088 _thread = current; 2089 _event = _thread->_ParkEvent; 2090 _active = false; 2091 assert(_event != NULL, "invariant"); 2092 } 2093 2094 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) { 2095 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(_thread, mon); 2096 } 2097 2098 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) { 2099 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(_thread, _active); 2100 } 2101 2102 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) { 2103 assert(node != NULL, "should not add NULL node"); 2104 assert(node->_prev == NULL, "node already in list"); 2105 assert(node->_next == NULL, "node already in list"); 2106 // put node at end of queue (circular doubly linked list) 2107 if (_WaitSet == NULL) { 2108 _WaitSet = node; 2109 node->_prev = node; 2110 node->_next = node; 2111 } else { 2112 ObjectWaiter* head = _WaitSet; 2113 ObjectWaiter* tail = head->_prev; 2114 assert(tail->_next == head, "invariant check"); 2115 tail->_next = node; 2116 head->_prev = node; 2117 node->_next = head; 2118 node->_prev = tail; 2119 } 2120 } 2121 2122 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() { 2123 // dequeue the very first waiter 2124 ObjectWaiter* waiter = _WaitSet; 2125 if (waiter) { 2126 DequeueSpecificWaiter(waiter); 2127 } 2128 return waiter; 2129 } 2130 2131 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) { 2132 assert(node != NULL, "should not dequeue NULL node"); 2133 assert(node->_prev != NULL, "node already removed from list"); 2134 assert(node->_next != NULL, "node already removed from list"); 2135 // when the waiter has woken up because of interrupt, 2136 // timeout or other spurious wake-up, dequeue the 2137 // waiter from waiting list 2138 ObjectWaiter* next = node->_next; 2139 if (next == node) { 2140 assert(node->_prev == node, "invariant check"); 2141 _WaitSet = NULL; 2142 } else { 2143 ObjectWaiter* prev = node->_prev; 2144 assert(prev->_next == node, "invariant check"); 2145 assert(next->_prev == node, "invariant check"); 2146 next->_prev = prev; 2147 prev->_next = next; 2148 if (_WaitSet == node) { 2149 _WaitSet = next; 2150 } 2151 } 2152 node->_next = NULL; 2153 node->_prev = NULL; 2154 } 2155 2156 // ----------------------------------------------------------------------------- 2157 // PerfData support 2158 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL; 2159 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL; 2160 PerfCounter * ObjectMonitor::_sync_Parks = NULL; 2161 PerfCounter * ObjectMonitor::_sync_Notifications = NULL; 2162 PerfCounter * ObjectMonitor::_sync_Inflations = NULL; 2163 PerfCounter * ObjectMonitor::_sync_Deflations = NULL; 2164 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL; 2165 2166 // One-shot global initialization for the sync subsystem. 2167 // We could also defer initialization and initialize on-demand 2168 // the first time we call ObjectSynchronizer::inflate(). 2169 // Initialization would be protected - like so many things - by 2170 // the MonitorCache_lock. 2171 2172 void ObjectMonitor::Initialize() { 2173 assert(!InitDone, "invariant"); 2174 2175 if (!os::is_MP()) { 2176 Knob_SpinLimit = 0; 2177 Knob_PreSpin = 0; 2178 Knob_FixedSpin = -1; 2179 } 2180 2181 if (UsePerfData) { 2182 EXCEPTION_MARK; 2183 #define NEWPERFCOUNTER(n) \ 2184 { \ 2185 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \ 2186 CHECK); \ 2187 } 2188 #define NEWPERFVARIABLE(n) \ 2189 { \ 2190 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \ 2191 CHECK); \ 2192 } 2193 NEWPERFCOUNTER(_sync_Inflations); 2194 NEWPERFCOUNTER(_sync_Deflations); 2195 NEWPERFCOUNTER(_sync_ContendedLockAttempts); 2196 NEWPERFCOUNTER(_sync_FutileWakeups); 2197 NEWPERFCOUNTER(_sync_Parks); 2198 NEWPERFCOUNTER(_sync_Notifications); 2199 NEWPERFVARIABLE(_sync_MonExtant); 2200 #undef NEWPERFCOUNTER 2201 #undef NEWPERFVARIABLE 2202 } 2203 2204 _oop_storage = OopStorageSet::create_weak("ObjectSynchronizer Weak", mtSynchronizer); 2205 2206 DEBUG_ONLY(InitDone = true;) 2207 } 2208 2209 void ObjectMonitor::print_on(outputStream* st) const { 2210 // The minimal things to print for markWord printing, more can be added for debugging and logging. 2211 st->print("{contentions=0x%08x,waiters=0x%08x" 2212 ",recursions=" INTX_FORMAT ",owner=" INTPTR_FORMAT "}", 2213 contentions(), waiters(), recursions(), 2214 p2i(owner())); 2215 } 2216 void ObjectMonitor::print() const { print_on(tty); } 2217 2218 #ifdef ASSERT 2219 // Print the ObjectMonitor like a debugger would: 2220 // 2221 // (ObjectMonitor) 0x00007fdfb6012e40 = { 2222 // _header = 0x0000000000000001 2223 // _object = 0x000000070ff45fd0 2224 // _pad_buf0 = { 2225 // [0] = '\0' 2226 // ... 2227 // [43] = '\0' 2228 // } 2229 // _owner = 0x0000000000000000 2230 // _previous_owner_tid = 0 2231 // _pad_buf1 = { 2232 // [0] = '\0' 2233 // ... 2234 // [47] = '\0' 2235 // } 2236 // _next_om = 0x0000000000000000 2237 // _recursions = 0 2238 // _EntryList = 0x0000000000000000 2239 // _cxq = 0x0000000000000000 2240 // _succ = 0x0000000000000000 2241 // _Responsible = 0x0000000000000000 2242 // _Spinner = 0 2243 // _SpinDuration = 5000 2244 // _contentions = 0 2245 // _WaitSet = 0x0000700009756248 2246 // _waiters = 1 2247 // _WaitSetLock = 0 2248 // } 2249 // 2250 void ObjectMonitor::print_debug_style_on(outputStream* st) const { 2251 st->print_cr("(ObjectMonitor*) " INTPTR_FORMAT " = {", p2i(this)); 2252 st->print_cr(" _header = " INTPTR_FORMAT, header().value()); 2253 st->print_cr(" _object = " INTPTR_FORMAT, p2i(object_peek())); 2254 st->print_cr(" _pad_buf0 = {"); 2255 st->print_cr(" [0] = '\\0'"); 2256 st->print_cr(" ..."); 2257 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf0) - 1); 2258 st->print_cr(" }"); 2259 st->print_cr(" _owner = " INTPTR_FORMAT, p2i(owner_raw())); 2260 st->print_cr(" _previous_owner_tid = " JLONG_FORMAT, _previous_owner_tid); 2261 st->print_cr(" _pad_buf1 = {"); 2262 st->print_cr(" [0] = '\\0'"); 2263 st->print_cr(" ..."); 2264 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf1) - 1); 2265 st->print_cr(" }"); 2266 st->print_cr(" _next_om = " INTPTR_FORMAT, p2i(next_om())); 2267 st->print_cr(" _recursions = " INTX_FORMAT, _recursions); 2268 st->print_cr(" _EntryList = " INTPTR_FORMAT, p2i(_EntryList)); 2269 st->print_cr(" _cxq = " INTPTR_FORMAT, p2i(_cxq)); 2270 st->print_cr(" _succ = " INTPTR_FORMAT, p2i(_succ)); 2271 st->print_cr(" _Responsible = " INTPTR_FORMAT, p2i(_Responsible)); 2272 st->print_cr(" _Spinner = %d", _Spinner); 2273 st->print_cr(" _SpinDuration = %d", _SpinDuration); 2274 st->print_cr(" _contentions = %d", contentions()); 2275 st->print_cr(" _WaitSet = " INTPTR_FORMAT, p2i(_WaitSet)); 2276 st->print_cr(" _waiters = %d", _waiters); 2277 st->print_cr(" _WaitSetLock = %d", _WaitSetLock); 2278 st->print_cr("}"); 2279 } 2280 #endif