1 /* 2 * Copyright (c) 1998, 2023, 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/javaThread.inline.hpp" 45 #include "runtime/mutexLocker.hpp" 46 #include "runtime/objectMonitor.hpp" 47 #include "runtime/objectMonitor.inline.hpp" 48 #include "runtime/orderAccess.hpp" 49 #include "runtime/osThread.hpp" 50 #include "runtime/perfData.hpp" 51 #include "runtime/safefetch.hpp" 52 #include "runtime/safepointMechanism.inline.hpp" 53 #include "runtime/sharedRuntime.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 = nullptr; \ 70 int len = 0; \ 71 jlong jtid = SharedRuntime::get_java_tid(thread); \ 72 Symbol* klassname = obj->klass()->name(); \ 73 if (klassname != nullptr) { \ 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 = nullptr; 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 = JavaThread::cast(self); 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(nullptr), 263 _previous_owner_tid(0), 264 _next_om(nullptr), 265 _recursions(0), 266 _EntryList(nullptr), 267 _cxq(nullptr), 268 _succ(nullptr), 269 _Responsible(nullptr), 270 _Spinner(0), 271 _SpinDuration(ObjectMonitor::Knob_SpinLimit), 272 _contentions(0), 273 _WaitSet(nullptr), 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 return _object.resolve(); 285 } 286 287 oop ObjectMonitor::object_peek() const { 288 return _object.peek(); 289 } 290 291 void ObjectMonitor::ExitOnSuspend::operator()(JavaThread* current) { 292 if (current->is_suspended()) { 293 _om->_recursions = 0; 294 _om->_succ = nullptr; 295 // Don't need a full fence after clearing successor here because of the call to exit(). 296 _om->exit(current, false /* not_suspended */); 297 _om_exited = true; 298 299 current->set_current_pending_monitor(_om); 300 } 301 } 302 303 void ObjectMonitor::ClearSuccOnSuspend::operator()(JavaThread* current) { 304 if (current->is_suspended()) { 305 if (_om->_succ == current) { 306 _om->_succ = nullptr; 307 OrderAccess::fence(); // always do a full fence when successor is cleared 308 } 309 } 310 } 311 312 // ----------------------------------------------------------------------------- 313 // Enter support 314 315 bool ObjectMonitor::enter(JavaThread* current) { 316 // The following code is ordered to check the most common cases first 317 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors. 318 319 void* cur = try_set_owner_from(nullptr, current); 320 if (cur == nullptr) { 321 assert(_recursions == 0, "invariant"); 322 return true; 323 } 324 325 if (cur == current) { 326 // TODO-FIXME: check for integer overflow! BUGID 6557169. 327 _recursions++; 328 return true; 329 } 330 331 if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) { 332 assert(_recursions == 0, "internal state error"); 333 _recursions = 1; 334 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*. 335 return true; 336 } 337 338 // We've encountered genuine contention. 339 assert(current->_Stalled == 0, "invariant"); 340 current->_Stalled = intptr_t(this); 341 342 // Try one round of spinning *before* enqueueing current 343 // and before going through the awkward and expensive state 344 // transitions. The following spin is strictly optional ... 345 // Note that if we acquire the monitor from an initial spin 346 // we forgo posting JVMTI events and firing DTRACE probes. 347 if (TrySpin(current) > 0) { 348 assert(owner_raw() == current, "must be current: owner=" INTPTR_FORMAT, p2i(owner_raw())); 349 assert(_recursions == 0, "must be 0: recursions=" INTX_FORMAT, _recursions); 350 assert(object()->mark() == markWord::encode(this), 351 "object mark must match encoded this: mark=" INTPTR_FORMAT 352 ", encoded this=" INTPTR_FORMAT, object()->mark().value(), 353 markWord::encode(this).value()); 354 current->_Stalled = 0; 355 return true; 356 } 357 358 assert(owner_raw() != current, "invariant"); 359 assert(_succ != current, "invariant"); 360 assert(!SafepointSynchronize::is_at_safepoint(), "invariant"); 361 assert(current->thread_state() != _thread_blocked, "invariant"); 362 363 // Keep track of contention for JVM/TI and M&M queries. 364 add_to_contentions(1); 365 if (is_being_async_deflated()) { 366 // Async deflation is in progress and our contentions increment 367 // above lost the race to async deflation. Undo the work and 368 // force the caller to retry. 369 const oop l_object = object(); 370 if (l_object != nullptr) { 371 // Attempt to restore the header/dmw to the object's header so that 372 // we only retry once if the deflater thread happens to be slow. 373 install_displaced_markword_in_object(l_object); 374 } 375 current->_Stalled = 0; 376 add_to_contentions(-1); 377 return false; 378 } 379 380 JFR_ONLY(JfrConditionalFlush<EventJavaMonitorEnter> flush(current);) 381 EventJavaMonitorEnter event; 382 if (event.is_started()) { 383 event.set_monitorClass(object()->klass()); 384 // Set an address that is 'unique enough', such that events close in 385 // time and with the same address are likely (but not guaranteed) to 386 // belong to the same object. 387 event.set_address((uintptr_t)this); 388 } 389 390 { // Change java thread status to indicate blocked on monitor enter. 391 JavaThreadBlockedOnMonitorEnterState jtbmes(current, this); 392 393 assert(current->current_pending_monitor() == nullptr, "invariant"); 394 current->set_current_pending_monitor(this); 395 396 DTRACE_MONITOR_PROBE(contended__enter, this, object(), current); 397 if (JvmtiExport::should_post_monitor_contended_enter()) { 398 JvmtiExport::post_monitor_contended_enter(current, this); 399 400 // The current thread does not yet own the monitor and does not 401 // yet appear on any queues that would get it made the successor. 402 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event 403 // handler cannot accidentally consume an unpark() meant for the 404 // ParkEvent associated with this ObjectMonitor. 405 } 406 407 OSThreadContendState osts(current->osthread()); 408 409 assert(current->thread_state() == _thread_in_vm, "invariant"); 410 411 for (;;) { 412 ExitOnSuspend eos(this); 413 { 414 ThreadBlockInVMPreprocess<ExitOnSuspend> tbivs(current, eos, true /* allow_suspend */); 415 EnterI(current); 416 current->set_current_pending_monitor(nullptr); 417 // We can go to a safepoint at the end of this block. If we 418 // do a thread dump during that safepoint, then this thread will show 419 // as having "-locked" the monitor, but the OS and java.lang.Thread 420 // states will still report that the thread is blocked trying to 421 // acquire it. 422 // If there is a suspend request, ExitOnSuspend will exit the OM 423 // and set the OM as pending. 424 } 425 if (!eos.exited()) { 426 // ExitOnSuspend did not exit the OM 427 assert(owner_raw() == current, "invariant"); 428 break; 429 } 430 } 431 432 // We've just gotten past the enter-check-for-suspend dance and we now own 433 // the monitor free and clear. 434 } 435 436 add_to_contentions(-1); 437 assert(contentions() >= 0, "must not be negative: contentions=%d", contentions()); 438 current->_Stalled = 0; 439 440 // Must either set _recursions = 0 or ASSERT _recursions == 0. 441 assert(_recursions == 0, "invariant"); 442 assert(owner_raw() == current, "invariant"); 443 assert(_succ != current, "invariant"); 444 assert(object()->mark() == markWord::encode(this), "invariant"); 445 446 // The thread -- now the owner -- is back in vm mode. 447 // Report the glorious news via TI,DTrace and jvmstat. 448 // The probe effect is non-trivial. All the reportage occurs 449 // while we hold the monitor, increasing the length of the critical 450 // section. Amdahl's parallel speedup law comes vividly into play. 451 // 452 // Another option might be to aggregate the events (thread local or 453 // per-monitor aggregation) and defer reporting until a more opportune 454 // time -- such as next time some thread encounters contention but has 455 // yet to acquire the lock. While spinning that thread could 456 // spinning we could increment JVMStat counters, etc. 457 458 DTRACE_MONITOR_PROBE(contended__entered, this, object(), current); 459 if (JvmtiExport::should_post_monitor_contended_entered()) { 460 JvmtiExport::post_monitor_contended_entered(current, this); 461 462 // The current thread already owns the monitor and is not going to 463 // call park() for the remainder of the monitor enter protocol. So 464 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED 465 // event handler consumed an unpark() issued by the thread that 466 // just exited the monitor. 467 } 468 if (event.should_commit()) { 469 event.set_previousOwner(_previous_owner_tid); 470 event.commit(); 471 } 472 OM_PERFDATA_OP(ContendedLockAttempts, inc()); 473 return true; 474 } 475 476 // Caveat: TryLock() is not necessarily serializing if it returns failure. 477 // Callers must compensate as needed. 478 479 int ObjectMonitor::TryLock(JavaThread* current) { 480 void* own = owner_raw(); 481 if (own != nullptr) return 0; 482 if (try_set_owner_from(nullptr, current) == nullptr) { 483 assert(_recursions == 0, "invariant"); 484 return 1; 485 } 486 // The lock had been free momentarily, but we lost the race to the lock. 487 // Interference -- the CAS failed. 488 // We can either return -1 or retry. 489 // Retry doesn't make as much sense because the lock was just acquired. 490 return -1; 491 } 492 493 // Deflate the specified ObjectMonitor if not in-use. Returns true if it 494 // was deflated and false otherwise. 495 // 496 // The async deflation protocol sets owner to DEFLATER_MARKER and 497 // makes contentions negative as signals to contending threads that 498 // an async deflation is in progress. There are a number of checks 499 // as part of the protocol to make sure that the calling thread has 500 // not lost the race to a contending thread. 501 // 502 // The ObjectMonitor has been successfully async deflated when: 503 // (contentions < 0) 504 // Contending threads that see that condition know to retry their operation. 505 // 506 bool ObjectMonitor::deflate_monitor() { 507 if (is_busy()) { 508 // Easy checks are first - the ObjectMonitor is busy so no deflation. 509 return false; 510 } 511 512 if (ObjectSynchronizer::is_final_audit() && owner_is_DEFLATER_MARKER()) { 513 // The final audit can see an already deflated ObjectMonitor on the 514 // in-use list because MonitorList::unlink_deflated() might have 515 // blocked for the final safepoint before unlinking all the deflated 516 // monitors. 517 assert(contentions() < 0, "must be negative: contentions=%d", contentions()); 518 // Already returned 'true' when it was originally deflated. 519 return false; 520 } 521 522 const oop obj = object_peek(); 523 524 if (obj == nullptr) { 525 // If the object died, we can recycle the monitor without racing with 526 // Java threads. The GC already broke the association with the object. 527 set_owner_from(nullptr, DEFLATER_MARKER); 528 assert(contentions() >= 0, "must be non-negative: contentions=%d", contentions()); 529 _contentions = INT_MIN; // minimum negative int 530 } else { 531 // Attempt async deflation protocol. 532 533 // Set a null owner to DEFLATER_MARKER to force any contending thread 534 // through the slow path. This is just the first part of the async 535 // deflation dance. 536 if (try_set_owner_from(nullptr, DEFLATER_MARKER) != nullptr) { 537 // The owner field is no longer null so we lost the race since the 538 // ObjectMonitor is now busy. 539 return false; 540 } 541 542 if (contentions() > 0 || _waiters != 0) { 543 // Another thread has raced to enter the ObjectMonitor after 544 // is_busy() above or has already entered and waited on 545 // it which makes it busy so no deflation. Restore owner to 546 // null if it is still DEFLATER_MARKER. 547 if (try_set_owner_from(DEFLATER_MARKER, nullptr) != DEFLATER_MARKER) { 548 // Deferred decrement for the JT EnterI() that cancelled the async deflation. 549 add_to_contentions(-1); 550 } 551 return false; 552 } 553 554 // Make a zero contentions field negative to force any contending threads 555 // to retry. This is the second part of the async deflation dance. 556 if (Atomic::cmpxchg(&_contentions, 0, INT_MIN) != 0) { 557 // Contentions was no longer 0 so we lost the race since the 558 // ObjectMonitor is now busy. Restore owner to null if it is 559 // still DEFLATER_MARKER: 560 if (try_set_owner_from(DEFLATER_MARKER, nullptr) != DEFLATER_MARKER) { 561 // Deferred decrement for the JT EnterI() that cancelled the async deflation. 562 add_to_contentions(-1); 563 } 564 return false; 565 } 566 } 567 568 // Sanity checks for the races: 569 guarantee(owner_is_DEFLATER_MARKER(), "must be deflater marker"); 570 guarantee(contentions() < 0, "must be negative: contentions=%d", 571 contentions()); 572 guarantee(_waiters == 0, "must be 0: waiters=%d", _waiters); 573 guarantee(_cxq == nullptr, "must be no contending threads: cxq=" 574 INTPTR_FORMAT, p2i(_cxq)); 575 guarantee(_EntryList == nullptr, 576 "must be no entering threads: EntryList=" INTPTR_FORMAT, 577 p2i(_EntryList)); 578 579 if (obj != nullptr) { 580 if (log_is_enabled(Trace, monitorinflation)) { 581 ResourceMark rm; 582 log_trace(monitorinflation)("deflate_monitor: object=" INTPTR_FORMAT 583 ", mark=" INTPTR_FORMAT ", type='%s'", 584 p2i(obj), obj->mark().value(), 585 obj->klass()->external_name()); 586 } 587 588 // Install the old mark word if nobody else has already done it. 589 install_displaced_markword_in_object(obj); 590 } 591 592 // We leave owner == DEFLATER_MARKER and contentions < 0 593 // to force any racing threads to retry. 594 return true; // Success, ObjectMonitor has been deflated. 595 } 596 597 // Install the displaced mark word (dmw) of a deflating ObjectMonitor 598 // into the header of the object associated with the monitor. This 599 // idempotent method is called by a thread that is deflating a 600 // monitor and by other threads that have detected a race with the 601 // deflation process. 602 void ObjectMonitor::install_displaced_markword_in_object(const oop obj) { 603 // This function must only be called when (owner == DEFLATER_MARKER 604 // && contentions <= 0), but we can't guarantee that here because 605 // those values could change when the ObjectMonitor gets moved from 606 // the global free list to a per-thread free list. 607 608 guarantee(obj != nullptr, "must be non-null"); 609 610 // Separate loads in is_being_async_deflated(), which is almost always 611 // called before this function, from the load of dmw/header below. 612 613 // _contentions and dmw/header may get written by different threads. 614 // Make sure to observe them in the same order when having several observers. 615 OrderAccess::loadload_for_IRIW(); 616 617 const oop l_object = object_peek(); 618 if (l_object == nullptr) { 619 // ObjectMonitor's object ref has already been cleared by async 620 // deflation or GC so we're done here. 621 return; 622 } 623 assert(l_object == obj, "object=" INTPTR_FORMAT " must equal obj=" 624 INTPTR_FORMAT, p2i(l_object), p2i(obj)); 625 626 markWord dmw = header(); 627 // The dmw has to be neutral (not null, not locked and not marked). 628 assert(dmw.is_neutral(), "must be neutral: dmw=" INTPTR_FORMAT, dmw.value()); 629 630 // Install displaced mark word if the object's header still points 631 // to this ObjectMonitor. More than one racing caller to this function 632 // can rarely reach this point, but only one can win. 633 markWord res = obj->cas_set_mark(dmw, markWord::encode(this)); 634 if (res != markWord::encode(this)) { 635 // This should be rare so log at the Info level when it happens. 636 log_info(monitorinflation)("install_displaced_markword_in_object: " 637 "failed cas_set_mark: new_mark=" INTPTR_FORMAT 638 ", old_mark=" INTPTR_FORMAT ", res=" INTPTR_FORMAT, 639 dmw.value(), markWord::encode(this).value(), 640 res.value()); 641 } 642 643 // Note: It does not matter which thread restored the header/dmw 644 // into the object's header. The thread deflating the monitor just 645 // wanted the object's header restored and it is. The threads that 646 // detected a race with the deflation process also wanted the 647 // object's header restored before they retry their operation and 648 // because it is restored they will only retry once. 649 } 650 651 // Convert the fields used by is_busy() to a string that can be 652 // used for diagnostic output. 653 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) { 654 ss->print("is_busy: waiters=%d, ", _waiters); 655 if (contentions() > 0) { 656 ss->print("contentions=%d, ", contentions()); 657 } else { 658 ss->print("contentions=0"); 659 } 660 if (!owner_is_DEFLATER_MARKER()) { 661 ss->print("owner=" INTPTR_FORMAT, p2i(owner_raw())); 662 } else { 663 // We report null instead of DEFLATER_MARKER here because is_busy() 664 // ignores DEFLATER_MARKER values. 665 ss->print("owner=" INTPTR_FORMAT, NULL_WORD); 666 } 667 ss->print(", cxq=" INTPTR_FORMAT ", EntryList=" INTPTR_FORMAT, p2i(_cxq), 668 p2i(_EntryList)); 669 return ss->base(); 670 } 671 672 #define MAX_RECHECK_INTERVAL 1000 673 674 void ObjectMonitor::EnterI(JavaThread* current) { 675 assert(current->thread_state() == _thread_blocked, "invariant"); 676 677 // Try the lock - TATAS 678 if (TryLock (current) > 0) { 679 assert(_succ != current, "invariant"); 680 assert(owner_raw() == current, "invariant"); 681 assert(_Responsible != current, "invariant"); 682 return; 683 } 684 685 if (try_set_owner_from(DEFLATER_MARKER, current) == DEFLATER_MARKER) { 686 // Cancelled the in-progress async deflation by changing owner from 687 // DEFLATER_MARKER to current. As part of the contended enter protocol, 688 // contentions was incremented to a positive value before EnterI() 689 // was called and that prevents the deflater thread from winning the 690 // last part of the 2-part async deflation protocol. After EnterI() 691 // returns to enter(), contentions is decremented because the caller 692 // now owns the monitor. We bump contentions an extra time here to 693 // prevent the deflater thread from winning the last part of the 694 // 2-part async deflation protocol after the regular decrement 695 // occurs in enter(). The deflater thread will decrement contentions 696 // after it recognizes that the async deflation was cancelled. 697 add_to_contentions(1); 698 assert(_succ != current, "invariant"); 699 assert(_Responsible != current, "invariant"); 700 return; 701 } 702 703 assert(InitDone, "Unexpectedly not initialized"); 704 705 // We try one round of spinning *before* enqueueing current. 706 // 707 // If the _owner is ready but OFFPROC we could use a YieldTo() 708 // operation to donate the remainder of this thread's quantum 709 // to the owner. This has subtle but beneficial affinity 710 // effects. 711 712 if (TrySpin(current) > 0) { 713 assert(owner_raw() == current, "invariant"); 714 assert(_succ != current, "invariant"); 715 assert(_Responsible != current, "invariant"); 716 return; 717 } 718 719 // The Spin failed -- Enqueue and park the thread ... 720 assert(_succ != current, "invariant"); 721 assert(owner_raw() != current, "invariant"); 722 assert(_Responsible != current, "invariant"); 723 724 // Enqueue "current" on ObjectMonitor's _cxq. 725 // 726 // Node acts as a proxy for current. 727 // As an aside, if were to ever rewrite the synchronization code mostly 728 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class 729 // Java objects. This would avoid awkward lifecycle and liveness issues, 730 // as well as eliminate a subset of ABA issues. 731 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events. 732 733 ObjectWaiter node(current); 734 current->_ParkEvent->reset(); 735 node._prev = (ObjectWaiter*) 0xBAD; 736 node.TState = ObjectWaiter::TS_CXQ; 737 738 // Push "current" onto the front of the _cxq. 739 // Once on cxq/EntryList, current stays on-queue until it acquires the lock. 740 // Note that spinning tends to reduce the rate at which threads 741 // enqueue and dequeue on EntryList|cxq. 742 ObjectWaiter* nxt; 743 for (;;) { 744 node._next = nxt = _cxq; 745 if (Atomic::cmpxchg(&_cxq, nxt, &node) == nxt) break; 746 747 // Interference - the CAS failed because _cxq changed. Just retry. 748 // As an optional optimization we retry the lock. 749 if (TryLock (current) > 0) { 750 assert(_succ != current, "invariant"); 751 assert(owner_raw() == current, "invariant"); 752 assert(_Responsible != current, "invariant"); 753 return; 754 } 755 } 756 757 // Check for cxq|EntryList edge transition to non-null. This indicates 758 // the onset of contention. While contention persists exiting threads 759 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit 760 // operations revert to the faster 1-0 mode. This enter operation may interleave 761 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we 762 // arrange for one of the contending thread to use a timed park() operations 763 // to detect and recover from the race. (Stranding is form of progress failure 764 // where the monitor is unlocked but all the contending threads remain parked). 765 // That is, at least one of the contended threads will periodically poll _owner. 766 // One of the contending threads will become the designated "Responsible" thread. 767 // The Responsible thread uses a timed park instead of a normal indefinite park 768 // operation -- it periodically wakes and checks for and recovers from potential 769 // strandings admitted by 1-0 exit operations. We need at most one Responsible 770 // thread per-monitor at any given moment. Only threads on cxq|EntryList may 771 // be responsible for a monitor. 772 // 773 // Currently, one of the contended threads takes on the added role of "Responsible". 774 // A viable alternative would be to use a dedicated "stranding checker" thread 775 // that periodically iterated over all the threads (or active monitors) and unparked 776 // successors where there was risk of stranding. This would help eliminate the 777 // timer scalability issues we see on some platforms as we'd only have one thread 778 // -- the checker -- parked on a timer. 779 780 if (nxt == nullptr && _EntryList == nullptr) { 781 // Try to assume the role of responsible thread for the monitor. 782 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=current } 783 Atomic::replace_if_null(&_Responsible, current); 784 } 785 786 // The lock might have been released while this thread was occupied queueing 787 // itself onto _cxq. To close the race and avoid "stranding" and 788 // progress-liveness failure we must resample-retry _owner before parking. 789 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner. 790 // In this case the ST-MEMBAR is accomplished with CAS(). 791 // 792 // TODO: Defer all thread state transitions until park-time. 793 // Since state transitions are heavy and inefficient we'd like 794 // to defer the state transitions until absolutely necessary, 795 // and in doing so avoid some transitions ... 796 797 int nWakeups = 0; 798 int recheckInterval = 1; 799 800 for (;;) { 801 802 if (TryLock(current) > 0) break; 803 assert(owner_raw() != current, "invariant"); 804 805 // park self 806 if (_Responsible == current) { 807 current->_ParkEvent->park((jlong) recheckInterval); 808 // Increase the recheckInterval, but clamp the value. 809 recheckInterval *= 8; 810 if (recheckInterval > MAX_RECHECK_INTERVAL) { 811 recheckInterval = MAX_RECHECK_INTERVAL; 812 } 813 } else { 814 current->_ParkEvent->park(); 815 } 816 817 if (TryLock(current) > 0) break; 818 819 if (try_set_owner_from(DEFLATER_MARKER, current) == DEFLATER_MARKER) { 820 // Cancelled the in-progress async deflation by changing owner from 821 // DEFLATER_MARKER to current. As part of the contended enter protocol, 822 // contentions was incremented to a positive value before EnterI() 823 // was called and that prevents the deflater thread from winning the 824 // last part of the 2-part async deflation protocol. After EnterI() 825 // returns to enter(), contentions is decremented because the caller 826 // now owns the monitor. We bump contentions an extra time here to 827 // prevent the deflater thread from winning the last part of the 828 // 2-part async deflation protocol after the regular decrement 829 // occurs in enter(). The deflater thread will decrement contentions 830 // after it recognizes that the async deflation was cancelled. 831 add_to_contentions(1); 832 break; 833 } 834 835 // The lock is still contested. 836 // Keep a tally of the # of futile wakeups. 837 // Note that the counter is not protected by a lock or updated by atomics. 838 // That is by design - we trade "lossy" counters which are exposed to 839 // races during updates for a lower probe effect. 840 841 // This PerfData object can be used in parallel with a safepoint. 842 // See the work around in PerfDataManager::destroy(). 843 OM_PERFDATA_OP(FutileWakeups, inc()); 844 ++nWakeups; 845 846 // Assuming this is not a spurious wakeup we'll normally find _succ == current. 847 // We can defer clearing _succ until after the spin completes 848 // TrySpin() must tolerate being called with _succ == current. 849 // Try yet another round of adaptive spinning. 850 if (TrySpin(current) > 0) break; 851 852 // We can find that we were unpark()ed and redesignated _succ while 853 // we were spinning. That's harmless. If we iterate and call park(), 854 // park() will consume the event and return immediately and we'll 855 // just spin again. This pattern can repeat, leaving _succ to simply 856 // spin on a CPU. 857 858 if (_succ == current) _succ = nullptr; 859 860 // Invariant: after clearing _succ a thread *must* retry _owner before parking. 861 OrderAccess::fence(); 862 } 863 864 // Egress : 865 // current has acquired the lock -- Unlink current from the cxq or EntryList. 866 // Normally we'll find current on the EntryList . 867 // From the perspective of the lock owner (this thread), the 868 // EntryList is stable and cxq is prepend-only. 869 // The head of cxq is volatile but the interior is stable. 870 // In addition, current.TState is stable. 871 872 assert(owner_raw() == current, "invariant"); 873 874 UnlinkAfterAcquire(current, &node); 875 if (_succ == current) _succ = nullptr; 876 877 assert(_succ != current, "invariant"); 878 if (_Responsible == current) { 879 _Responsible = nullptr; 880 OrderAccess::fence(); // Dekker pivot-point 881 882 // We may leave threads on cxq|EntryList without a designated 883 // "Responsible" thread. This is benign. When this thread subsequently 884 // exits the monitor it can "see" such preexisting "old" threads -- 885 // threads that arrived on the cxq|EntryList before the fence, above -- 886 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads 887 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible 888 // non-null and elect a new "Responsible" timer thread. 889 // 890 // This thread executes: 891 // ST Responsible=null; MEMBAR (in enter epilogue - here) 892 // LD cxq|EntryList (in subsequent exit) 893 // 894 // Entering threads in the slow/contended path execute: 895 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog) 896 // The (ST cxq; MEMBAR) is accomplished with CAS(). 897 // 898 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent 899 // exit operation from floating above the ST Responsible=null. 900 } 901 902 // We've acquired ownership with CAS(). 903 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics. 904 // But since the CAS() this thread may have also stored into _succ, 905 // EntryList, cxq or Responsible. These meta-data updates must be 906 // visible __before this thread subsequently drops the lock. 907 // Consider what could occur if we didn't enforce this constraint -- 908 // STs to monitor meta-data and user-data could reorder with (become 909 // visible after) the ST in exit that drops ownership of the lock. 910 // Some other thread could then acquire the lock, but observe inconsistent 911 // or old monitor meta-data and heap data. That violates the JMM. 912 // To that end, the 1-0 exit() operation must have at least STST|LDST 913 // "release" barrier semantics. Specifically, there must be at least a 914 // STST|LDST barrier in exit() before the ST of null into _owner that drops 915 // the lock. The barrier ensures that changes to monitor meta-data and data 916 // protected by the lock will be visible before we release the lock, and 917 // therefore before some other thread (CPU) has a chance to acquire the lock. 918 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html. 919 // 920 // Critically, any prior STs to _succ or EntryList must be visible before 921 // the ST of null into _owner in the *subsequent* (following) corresponding 922 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily 923 // execute a serializing instruction. 924 925 return; 926 } 927 928 // ReenterI() is a specialized inline form of the latter half of the 929 // contended slow-path from EnterI(). We use ReenterI() only for 930 // monitor reentry in wait(). 931 // 932 // In the future we should reconcile EnterI() and ReenterI(). 933 934 void ObjectMonitor::ReenterI(JavaThread* current, ObjectWaiter* currentNode) { 935 assert(current != nullptr, "invariant"); 936 assert(currentNode != nullptr, "invariant"); 937 assert(currentNode->_thread == current, "invariant"); 938 assert(_waiters > 0, "invariant"); 939 assert(object()->mark() == markWord::encode(this), "invariant"); 940 941 assert(current->thread_state() != _thread_blocked, "invariant"); 942 943 int nWakeups = 0; 944 for (;;) { 945 ObjectWaiter::TStates v = currentNode->TState; 946 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant"); 947 assert(owner_raw() != current, "invariant"); 948 949 if (TryLock(current) > 0) break; 950 if (TrySpin(current) > 0) break; 951 952 { 953 OSThreadContendState osts(current->osthread()); 954 955 assert(current->thread_state() == _thread_in_vm, "invariant"); 956 957 { 958 ClearSuccOnSuspend csos(this); 959 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */); 960 current->_ParkEvent->park(); 961 } 962 } 963 964 // Try again, but just so we distinguish between futile wakeups and 965 // successful wakeups. The following test isn't algorithmically 966 // necessary, but it helps us maintain sensible statistics. 967 if (TryLock(current) > 0) break; 968 969 // The lock is still contested. 970 // Keep a tally of the # of futile wakeups. 971 // Note that the counter is not protected by a lock or updated by atomics. 972 // That is by design - we trade "lossy" counters which are exposed to 973 // races during updates for a lower probe effect. 974 ++nWakeups; 975 976 // Assuming this is not a spurious wakeup we'll normally 977 // find that _succ == current. 978 if (_succ == current) _succ = nullptr; 979 980 // Invariant: after clearing _succ a contending thread 981 // *must* retry _owner before parking. 982 OrderAccess::fence(); 983 984 // This PerfData object can be used in parallel with a safepoint. 985 // See the work around in PerfDataManager::destroy(). 986 OM_PERFDATA_OP(FutileWakeups, inc()); 987 } 988 989 // current has acquired the lock -- Unlink current from the cxq or EntryList . 990 // Normally we'll find current on the EntryList. 991 // Unlinking from the EntryList is constant-time and atomic-free. 992 // From the perspective of the lock owner (this thread), the 993 // EntryList is stable and cxq is prepend-only. 994 // The head of cxq is volatile but the interior is stable. 995 // In addition, current.TState is stable. 996 997 assert(owner_raw() == current, "invariant"); 998 assert(object()->mark() == markWord::encode(this), "invariant"); 999 UnlinkAfterAcquire(current, currentNode); 1000 if (_succ == current) _succ = nullptr; 1001 assert(_succ != current, "invariant"); 1002 currentNode->TState = ObjectWaiter::TS_RUN; 1003 OrderAccess::fence(); // see comments at the end of EnterI() 1004 } 1005 1006 // By convention we unlink a contending thread from EntryList|cxq immediately 1007 // after the thread acquires the lock in ::enter(). Equally, we could defer 1008 // unlinking the thread until ::exit()-time. 1009 1010 void ObjectMonitor::UnlinkAfterAcquire(JavaThread* current, ObjectWaiter* currentNode) { 1011 assert(owner_raw() == current, "invariant"); 1012 assert(currentNode->_thread == current, "invariant"); 1013 1014 if (currentNode->TState == ObjectWaiter::TS_ENTER) { 1015 // Normal case: remove current from the DLL EntryList . 1016 // This is a constant-time operation. 1017 ObjectWaiter* nxt = currentNode->_next; 1018 ObjectWaiter* prv = currentNode->_prev; 1019 if (nxt != nullptr) nxt->_prev = prv; 1020 if (prv != nullptr) prv->_next = nxt; 1021 if (currentNode == _EntryList) _EntryList = nxt; 1022 assert(nxt == nullptr || nxt->TState == ObjectWaiter::TS_ENTER, "invariant"); 1023 assert(prv == nullptr || prv->TState == ObjectWaiter::TS_ENTER, "invariant"); 1024 } else { 1025 assert(currentNode->TState == ObjectWaiter::TS_CXQ, "invariant"); 1026 // Inopportune interleaving -- current is still on the cxq. 1027 // This usually means the enqueue of self raced an exiting thread. 1028 // Normally we'll find current near the front of the cxq, so 1029 // dequeueing is typically fast. If needbe we can accelerate 1030 // this with some MCS/CHL-like bidirectional list hints and advisory 1031 // back-links so dequeueing from the interior will normally operate 1032 // in constant-time. 1033 // Dequeue current from either the head (with CAS) or from the interior 1034 // with a linear-time scan and normal non-atomic memory operations. 1035 // CONSIDER: if current is on the cxq then simply drain cxq into EntryList 1036 // and then unlink current from EntryList. We have to drain eventually, 1037 // so it might as well be now. 1038 1039 ObjectWaiter* v = _cxq; 1040 assert(v != nullptr, "invariant"); 1041 if (v != currentNode || Atomic::cmpxchg(&_cxq, v, currentNode->_next) != v) { 1042 // The CAS above can fail from interference IFF a "RAT" arrived. 1043 // In that case current must be in the interior and can no longer be 1044 // at the head of cxq. 1045 if (v == currentNode) { 1046 assert(_cxq != v, "invariant"); 1047 v = _cxq; // CAS above failed - start scan at head of list 1048 } 1049 ObjectWaiter* p; 1050 ObjectWaiter* q = nullptr; 1051 for (p = v; p != nullptr && p != currentNode; p = p->_next) { 1052 q = p; 1053 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant"); 1054 } 1055 assert(v != currentNode, "invariant"); 1056 assert(p == currentNode, "Node not found on cxq"); 1057 assert(p != _cxq, "invariant"); 1058 assert(q != nullptr, "invariant"); 1059 assert(q->_next == p, "invariant"); 1060 q->_next = p->_next; 1061 } 1062 } 1063 1064 #ifdef ASSERT 1065 // Diagnostic hygiene ... 1066 currentNode->_prev = (ObjectWaiter*) 0xBAD; 1067 currentNode->_next = (ObjectWaiter*) 0xBAD; 1068 currentNode->TState = ObjectWaiter::TS_RUN; 1069 #endif 1070 } 1071 1072 // ----------------------------------------------------------------------------- 1073 // Exit support 1074 // 1075 // exit() 1076 // ~~~~~~ 1077 // Note that the collector can't reclaim the objectMonitor or deflate 1078 // the object out from underneath the thread calling ::exit() as the 1079 // thread calling ::exit() never transitions to a stable state. 1080 // This inhibits GC, which in turn inhibits asynchronous (and 1081 // inopportune) reclamation of "this". 1082 // 1083 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ; 1084 // There's one exception to the claim above, however. EnterI() can call 1085 // exit() to drop a lock if the acquirer has been externally suspended. 1086 // In that case exit() is called with _thread_state == _thread_blocked, 1087 // but the monitor's _contentions field is > 0, which inhibits reclamation. 1088 // 1089 // 1-0 exit 1090 // ~~~~~~~~ 1091 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of 1092 // the fast-path operators have been optimized so the common ::exit() 1093 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock(). 1094 // The code emitted by fast_unlock() elides the usual MEMBAR. This 1095 // greatly improves latency -- MEMBAR and CAS having considerable local 1096 // latency on modern processors -- but at the cost of "stranding". Absent the 1097 // MEMBAR, a thread in fast_unlock() can race a thread in the slow 1098 // ::enter() path, resulting in the entering thread being stranding 1099 // and a progress-liveness failure. Stranding is extremely rare. 1100 // We use timers (timed park operations) & periodic polling to detect 1101 // and recover from stranding. Potentially stranded threads periodically 1102 // wake up and poll the lock. See the usage of the _Responsible variable. 1103 // 1104 // The CAS() in enter provides for safety and exclusion, while the CAS or 1105 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking 1106 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding. 1107 // We detect and recover from stranding with timers. 1108 // 1109 // If a thread transiently strands it'll park until (a) another 1110 // thread acquires the lock and then drops the lock, at which time the 1111 // exiting thread will notice and unpark the stranded thread, or, (b) 1112 // the timer expires. If the lock is high traffic then the stranding latency 1113 // will be low due to (a). If the lock is low traffic then the odds of 1114 // stranding are lower, although the worst-case stranding latency 1115 // is longer. Critically, we don't want to put excessive load in the 1116 // platform's timer subsystem. We want to minimize both the timer injection 1117 // rate (timers created/sec) as well as the number of timers active at 1118 // any one time. (more precisely, we want to minimize timer-seconds, which is 1119 // the integral of the # of active timers at any instant over time). 1120 // Both impinge on OS scalability. Given that, at most one thread parked on 1121 // a monitor will use a timer. 1122 // 1123 // There is also the risk of a futile wake-up. If we drop the lock 1124 // another thread can reacquire the lock immediately, and we can 1125 // then wake a thread unnecessarily. This is benign, and we've 1126 // structured the code so the windows are short and the frequency 1127 // of such futile wakups is low. 1128 1129 void ObjectMonitor::exit(JavaThread* current, bool not_suspended) { 1130 void* cur = owner_raw(); 1131 if (current != cur) { 1132 if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) { 1133 assert(_recursions == 0, "invariant"); 1134 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*. 1135 _recursions = 0; 1136 } else { 1137 // Apparent unbalanced locking ... 1138 // Naively we'd like to throw IllegalMonitorStateException. 1139 // As a practical matter we can neither allocate nor throw an 1140 // exception as ::exit() can be called from leaf routines. 1141 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties. 1142 // Upon deeper reflection, however, in a properly run JVM the only 1143 // way we should encounter this situation is in the presence of 1144 // unbalanced JNI locking. TODO: CheckJNICalls. 1145 // See also: CR4414101 1146 #ifdef ASSERT 1147 LogStreamHandle(Error, monitorinflation) lsh; 1148 lsh.print_cr("ERROR: ObjectMonitor::exit(): thread=" INTPTR_FORMAT 1149 " is exiting an ObjectMonitor it does not own.", p2i(current)); 1150 lsh.print_cr("The imbalance is possibly caused by JNI locking."); 1151 print_debug_style_on(&lsh); 1152 assert(false, "Non-balanced monitor enter/exit!"); 1153 #endif 1154 return; 1155 } 1156 } 1157 1158 if (_recursions != 0) { 1159 _recursions--; // this is simple recursive enter 1160 return; 1161 } 1162 1163 // Invariant: after setting Responsible=null an thread must execute 1164 // a MEMBAR or other serializing instruction before fetching EntryList|cxq. 1165 _Responsible = nullptr; 1166 1167 #if INCLUDE_JFR 1168 // get the owner's thread id for the MonitorEnter event 1169 // if it is enabled and the thread isn't suspended 1170 if (not_suspended && EventJavaMonitorEnter::is_enabled()) { 1171 _previous_owner_tid = JFR_THREAD_ID(current); 1172 } 1173 #endif 1174 1175 for (;;) { 1176 assert(current == owner_raw(), "invariant"); 1177 1178 // Drop the lock. 1179 // release semantics: prior loads and stores from within the critical section 1180 // must not float (reorder) past the following store that drops the lock. 1181 // Uses a storeload to separate release_store(owner) from the 1182 // successor check. The try_set_owner() below uses cmpxchg() so 1183 // we get the fence down there. 1184 release_clear_owner(current); 1185 OrderAccess::storeload(); 1186 1187 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != nullptr) { 1188 return; 1189 } 1190 // Other threads are blocked trying to acquire the lock. 1191 1192 // Normally the exiting thread is responsible for ensuring succession, 1193 // but if other successors are ready or other entering threads are spinning 1194 // then this thread can simply store null into _owner and exit without 1195 // waking a successor. The existence of spinners or ready successors 1196 // guarantees proper succession (liveness). Responsibility passes to the 1197 // ready or running successors. The exiting thread delegates the duty. 1198 // More precisely, if a successor already exists this thread is absolved 1199 // of the responsibility of waking (unparking) one. 1200 // 1201 // The _succ variable is critical to reducing futile wakeup frequency. 1202 // _succ identifies the "heir presumptive" thread that has been made 1203 // ready (unparked) but that has not yet run. We need only one such 1204 // successor thread to guarantee progress. 1205 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf 1206 // section 3.3 "Futile Wakeup Throttling" for details. 1207 // 1208 // Note that spinners in Enter() also set _succ non-null. 1209 // In the current implementation spinners opportunistically set 1210 // _succ so that exiting threads might avoid waking a successor. 1211 // Another less appealing alternative would be for the exiting thread 1212 // to drop the lock and then spin briefly to see if a spinner managed 1213 // to acquire the lock. If so, the exiting thread could exit 1214 // immediately without waking a successor, otherwise the exiting 1215 // thread would need to dequeue and wake a successor. 1216 // (Note that we'd need to make the post-drop spin short, but no 1217 // shorter than the worst-case round-trip cache-line migration time. 1218 // The dropped lock needs to become visible to the spinner, and then 1219 // the acquisition of the lock by the spinner must become visible to 1220 // the exiting thread). 1221 1222 // It appears that an heir-presumptive (successor) must be made ready. 1223 // Only the current lock owner can manipulate the EntryList or 1224 // drain _cxq, so we need to reacquire the lock. If we fail 1225 // to reacquire the lock the responsibility for ensuring succession 1226 // falls to the new owner. 1227 // 1228 if (try_set_owner_from(nullptr, current) != nullptr) { 1229 return; 1230 } 1231 1232 guarantee(owner_raw() == current, "invariant"); 1233 1234 ObjectWaiter* w = nullptr; 1235 1236 w = _EntryList; 1237 if (w != nullptr) { 1238 // I'd like to write: guarantee (w->_thread != current). 1239 // But in practice an exiting thread may find itself on the EntryList. 1240 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and 1241 // then calls exit(). Exit release the lock by setting O._owner to null. 1242 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The 1243 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then 1244 // release the lock "O". T2 resumes immediately after the ST of null into 1245 // _owner, above. T2 notices that the EntryList is populated, so it 1246 // reacquires the lock and then finds itself on the EntryList. 1247 // Given all that, we have to tolerate the circumstance where "w" is 1248 // associated with current. 1249 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant"); 1250 ExitEpilog(current, w); 1251 return; 1252 } 1253 1254 // If we find that both _cxq and EntryList are null then just 1255 // re-run the exit protocol from the top. 1256 w = _cxq; 1257 if (w == nullptr) continue; 1258 1259 // Drain _cxq into EntryList - bulk transfer. 1260 // First, detach _cxq. 1261 // The following loop is tantamount to: w = swap(&cxq, nullptr) 1262 for (;;) { 1263 assert(w != nullptr, "Invariant"); 1264 ObjectWaiter* u = Atomic::cmpxchg(&_cxq, w, (ObjectWaiter*)nullptr); 1265 if (u == w) break; 1266 w = u; 1267 } 1268 1269 assert(w != nullptr, "invariant"); 1270 assert(_EntryList == nullptr, "invariant"); 1271 1272 // Convert the LIFO SLL anchored by _cxq into a DLL. 1273 // The list reorganization step operates in O(LENGTH(w)) time. 1274 // It's critical that this step operate quickly as 1275 // "current" still holds the outer-lock, restricting parallelism 1276 // and effectively lengthening the critical section. 1277 // Invariant: s chases t chases u. 1278 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so 1279 // we have faster access to the tail. 1280 1281 _EntryList = w; 1282 ObjectWaiter* q = nullptr; 1283 ObjectWaiter* p; 1284 for (p = w; p != nullptr; p = p->_next) { 1285 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant"); 1286 p->TState = ObjectWaiter::TS_ENTER; 1287 p->_prev = q; 1288 q = p; 1289 } 1290 1291 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = nullptr 1292 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog(). 1293 1294 // See if we can abdicate to a spinner instead of waking a thread. 1295 // A primary goal of the implementation is to reduce the 1296 // context-switch rate. 1297 if (_succ != nullptr) continue; 1298 1299 w = _EntryList; 1300 if (w != nullptr) { 1301 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant"); 1302 ExitEpilog(current, w); 1303 return; 1304 } 1305 } 1306 } 1307 1308 void ObjectMonitor::ExitEpilog(JavaThread* current, ObjectWaiter* Wakee) { 1309 assert(owner_raw() == current, "invariant"); 1310 1311 // Exit protocol: 1312 // 1. ST _succ = wakee 1313 // 2. membar #loadstore|#storestore; 1314 // 2. ST _owner = nullptr 1315 // 3. unpark(wakee) 1316 1317 _succ = Wakee->_thread; 1318 ParkEvent * Trigger = Wakee->_event; 1319 1320 // Hygiene -- once we've set _owner = nullptr we can't safely dereference Wakee again. 1321 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be 1322 // out-of-scope (non-extant). 1323 Wakee = nullptr; 1324 1325 // Drop the lock. 1326 // Uses a fence to separate release_store(owner) from the LD in unpark(). 1327 release_clear_owner(current); 1328 OrderAccess::fence(); 1329 1330 DTRACE_MONITOR_PROBE(contended__exit, this, object(), current); 1331 Trigger->unpark(); 1332 1333 // Maintain stats and report events to JVMTI 1334 OM_PERFDATA_OP(Parks, inc()); 1335 } 1336 1337 // complete_exit exits a lock returning recursion count 1338 // complete_exit requires an inflated monitor 1339 // The _owner field is not always the Thread addr even with an 1340 // inflated monitor, e.g. the monitor can be inflated by a non-owning 1341 // thread due to contention. 1342 intx ObjectMonitor::complete_exit(JavaThread* current) { 1343 assert(InitDone, "Unexpectedly not initialized"); 1344 1345 void* cur = owner_raw(); 1346 if (current != cur) { 1347 if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) { 1348 assert(_recursions == 0, "internal state error"); 1349 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*. 1350 _recursions = 0; 1351 } 1352 } 1353 1354 guarantee(current == owner_raw(), "complete_exit not owner"); 1355 intx save = _recursions; // record the old recursion count 1356 _recursions = 0; // set the recursion level to be 0 1357 exit(current); // exit the monitor 1358 guarantee(owner_raw() != current, "invariant"); 1359 return save; 1360 } 1361 1362 // Checks that the current THREAD owns this monitor and causes an 1363 // immediate return if it doesn't. We don't use the CHECK macro 1364 // because we want the IMSE to be the only exception that is thrown 1365 // from the call site when false is returned. Any other pending 1366 // exception is ignored. 1367 #define CHECK_OWNER() \ 1368 do { \ 1369 if (!check_owner(THREAD)) { \ 1370 assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \ 1371 return; \ 1372 } \ 1373 } while (false) 1374 1375 // Returns true if the specified thread owns the ObjectMonitor. 1376 // Otherwise returns false and throws IllegalMonitorStateException 1377 // (IMSE). If there is a pending exception and the specified thread 1378 // is not the owner, that exception will be replaced by the IMSE. 1379 bool ObjectMonitor::check_owner(TRAPS) { 1380 JavaThread* current = THREAD; 1381 void* cur = owner_raw(); 1382 assert(cur != anon_owner_ptr(), "no anon owner here"); 1383 if (cur == current) { 1384 return true; 1385 } 1386 if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) { 1387 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*. 1388 _recursions = 0; 1389 return true; 1390 } 1391 THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(), 1392 "current thread is not owner", false); 1393 } 1394 1395 static inline bool is_excluded(const Klass* monitor_klass) { 1396 assert(monitor_klass != nullptr, "invariant"); 1397 NOT_JFR_RETURN_(false); 1398 JFR_ONLY(return vmSymbols::jfr_chunk_rotation_monitor() == monitor_klass->name()); 1399 } 1400 1401 static void post_monitor_wait_event(EventJavaMonitorWait* event, 1402 ObjectMonitor* monitor, 1403 uint64_t notifier_tid, 1404 jlong timeout, 1405 bool timedout) { 1406 assert(event != nullptr, "invariant"); 1407 assert(monitor != nullptr, "invariant"); 1408 const Klass* monitor_klass = monitor->object()->klass(); 1409 if (is_excluded(monitor_klass)) { 1410 return; 1411 } 1412 event->set_monitorClass(monitor_klass); 1413 event->set_timeout(timeout); 1414 // Set an address that is 'unique enough', such that events close in 1415 // time and with the same address are likely (but not guaranteed) to 1416 // belong to the same object. 1417 event->set_address((uintptr_t)monitor); 1418 event->set_notifier(notifier_tid); 1419 event->set_timedOut(timedout); 1420 event->commit(); 1421 } 1422 1423 // ----------------------------------------------------------------------------- 1424 // Wait/Notify/NotifyAll 1425 // 1426 // Note: a subset of changes to ObjectMonitor::wait() 1427 // will need to be replicated in complete_exit 1428 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) { 1429 JavaThread* current = THREAD; 1430 1431 assert(InitDone, "Unexpectedly not initialized"); 1432 1433 CHECK_OWNER(); // Throws IMSE if not owner. 1434 1435 EventJavaMonitorWait event; 1436 1437 // check for a pending interrupt 1438 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) { 1439 // post monitor waited event. Note that this is past-tense, we are done waiting. 1440 if (JvmtiExport::should_post_monitor_waited()) { 1441 // Note: 'false' parameter is passed here because the 1442 // wait was not timed out due to thread interrupt. 1443 JvmtiExport::post_monitor_waited(current, this, false); 1444 1445 // In this short circuit of the monitor wait protocol, the 1446 // current thread never drops ownership of the monitor and 1447 // never gets added to the wait queue so the current thread 1448 // cannot be made the successor. This means that the 1449 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally 1450 // consume an unpark() meant for the ParkEvent associated with 1451 // this ObjectMonitor. 1452 } 1453 if (event.should_commit()) { 1454 post_monitor_wait_event(&event, this, 0, millis, false); 1455 } 1456 THROW(vmSymbols::java_lang_InterruptedException()); 1457 return; 1458 } 1459 1460 assert(current->_Stalled == 0, "invariant"); 1461 current->_Stalled = intptr_t(this); 1462 current->set_current_waiting_monitor(this); 1463 1464 // create a node to be put into the queue 1465 // Critically, after we reset() the event but prior to park(), we must check 1466 // for a pending interrupt. 1467 ObjectWaiter node(current); 1468 node.TState = ObjectWaiter::TS_WAIT; 1469 current->_ParkEvent->reset(); 1470 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag 1471 1472 // Enter the waiting queue, which is a circular doubly linked list in this case 1473 // but it could be a priority queue or any data structure. 1474 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only 1475 // by the owner of the monitor *except* in the case where park() 1476 // returns because of a timeout of interrupt. Contention is exceptionally rare 1477 // so we use a simple spin-lock instead of a heavier-weight blocking lock. 1478 1479 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add"); 1480 AddWaiter(&node); 1481 Thread::SpinRelease(&_WaitSetLock); 1482 1483 _Responsible = nullptr; 1484 1485 intx save = _recursions; // record the old recursion count 1486 _waiters++; // increment the number of waiters 1487 _recursions = 0; // set the recursion level to be 1 1488 exit(current); // exit the monitor 1489 guarantee(owner_raw() != current, "invariant"); 1490 1491 // The thread is on the WaitSet list - now park() it. 1492 // On MP systems it's conceivable that a brief spin before we park 1493 // could be profitable. 1494 // 1495 // TODO-FIXME: change the following logic to a loop of the form 1496 // while (!timeout && !interrupted && _notified == 0) park() 1497 1498 int ret = OS_OK; 1499 int WasNotified = 0; 1500 1501 // Need to check interrupt state whilst still _thread_in_vm 1502 bool interrupted = interruptible && current->is_interrupted(false); 1503 1504 { // State transition wrappers 1505 OSThread* osthread = current->osthread(); 1506 OSThreadWaitState osts(osthread, true); 1507 1508 assert(current->thread_state() == _thread_in_vm, "invariant"); 1509 1510 { 1511 ClearSuccOnSuspend csos(this); 1512 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */); 1513 if (interrupted || HAS_PENDING_EXCEPTION) { 1514 // Intentionally empty 1515 } else if (node._notified == 0) { 1516 if (millis <= 0) { 1517 current->_ParkEvent->park(); 1518 } else { 1519 ret = current->_ParkEvent->park(millis); 1520 } 1521 } 1522 } 1523 1524 // Node may be on the WaitSet, the EntryList (or cxq), or in transition 1525 // from the WaitSet to the EntryList. 1526 // See if we need to remove Node from the WaitSet. 1527 // We use double-checked locking to avoid grabbing _WaitSetLock 1528 // if the thread is not on the wait queue. 1529 // 1530 // Note that we don't need a fence before the fetch of TState. 1531 // In the worst case we'll fetch a old-stale value of TS_WAIT previously 1532 // written by the is thread. (perhaps the fetch might even be satisfied 1533 // by a look-aside into the processor's own store buffer, although given 1534 // the length of the code path between the prior ST and this load that's 1535 // highly unlikely). If the following LD fetches a stale TS_WAIT value 1536 // then we'll acquire the lock and then re-fetch a fresh TState value. 1537 // That is, we fail toward safety. 1538 1539 if (node.TState == ObjectWaiter::TS_WAIT) { 1540 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink"); 1541 if (node.TState == ObjectWaiter::TS_WAIT) { 1542 DequeueSpecificWaiter(&node); // unlink from WaitSet 1543 assert(node._notified == 0, "invariant"); 1544 node.TState = ObjectWaiter::TS_RUN; 1545 } 1546 Thread::SpinRelease(&_WaitSetLock); 1547 } 1548 1549 // The thread is now either on off-list (TS_RUN), 1550 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ). 1551 // The Node's TState variable is stable from the perspective of this thread. 1552 // No other threads will asynchronously modify TState. 1553 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant"); 1554 OrderAccess::loadload(); 1555 if (_succ == current) _succ = nullptr; 1556 WasNotified = node._notified; 1557 1558 // Reentry phase -- reacquire the monitor. 1559 // re-enter contended monitor after object.wait(). 1560 // retain OBJECT_WAIT state until re-enter successfully completes 1561 // Thread state is thread_in_vm and oop access is again safe, 1562 // although the raw address of the object may have changed. 1563 // (Don't cache naked oops over safepoints, of course). 1564 1565 // post monitor waited event. Note that this is past-tense, we are done waiting. 1566 if (JvmtiExport::should_post_monitor_waited()) { 1567 JvmtiExport::post_monitor_waited(current, this, ret == OS_TIMEOUT); 1568 1569 if (node._notified != 0 && _succ == current) { 1570 // In this part of the monitor wait-notify-reenter protocol it 1571 // is possible (and normal) for another thread to do a fastpath 1572 // monitor enter-exit while this thread is still trying to get 1573 // to the reenter portion of the protocol. 1574 // 1575 // The ObjectMonitor was notified and the current thread is 1576 // the successor which also means that an unpark() has already 1577 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can 1578 // consume the unpark() that was done when the successor was 1579 // set because the same ParkEvent is shared between Java 1580 // monitors and JVM/TI RawMonitors (for now). 1581 // 1582 // We redo the unpark() to ensure forward progress, i.e., we 1583 // don't want all pending threads hanging (parked) with none 1584 // entering the unlocked monitor. 1585 node._event->unpark(); 1586 } 1587 } 1588 1589 if (event.should_commit()) { 1590 post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT); 1591 } 1592 1593 OrderAccess::fence(); 1594 1595 assert(current->_Stalled != 0, "invariant"); 1596 current->_Stalled = 0; 1597 1598 assert(owner_raw() != current, "invariant"); 1599 ObjectWaiter::TStates v = node.TState; 1600 if (v == ObjectWaiter::TS_RUN) { 1601 enter(current); 1602 } else { 1603 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant"); 1604 ReenterI(current, &node); 1605 node.wait_reenter_end(this); 1606 } 1607 1608 // current has reacquired the lock. 1609 // Lifecycle - the node representing current must not appear on any queues. 1610 // Node is about to go out-of-scope, but even if it were immortal we wouldn't 1611 // want residual elements associated with this thread left on any lists. 1612 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant"); 1613 assert(owner_raw() == current, "invariant"); 1614 assert(_succ != current, "invariant"); 1615 } // OSThreadWaitState() 1616 1617 current->set_current_waiting_monitor(nullptr); 1618 1619 guarantee(_recursions == 0, "invariant"); 1620 int relock_count = JvmtiDeferredUpdates::get_and_reset_relock_count_after_wait(current); 1621 _recursions = save // restore the old recursion count 1622 + relock_count; // increased by the deferred relock count 1623 current->inc_held_monitor_count(relock_count); // Deopt never entered these counts. 1624 _waiters--; // decrement the number of waiters 1625 1626 // Verify a few postconditions 1627 assert(owner_raw() == current, "invariant"); 1628 assert(_succ != current, "invariant"); 1629 assert(object()->mark() == markWord::encode(this), "invariant"); 1630 1631 // check if the notification happened 1632 if (!WasNotified) { 1633 // no, it could be timeout or Thread.interrupt() or both 1634 // check for interrupt event, otherwise it is timeout 1635 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) { 1636 THROW(vmSymbols::java_lang_InterruptedException()); 1637 } 1638 } 1639 1640 // NOTE: Spurious wake up will be consider as timeout. 1641 // Monitor notify has precedence over thread interrupt. 1642 } 1643 1644 1645 // Consider: 1646 // If the lock is cool (cxq == null && succ == null) and we're on an MP system 1647 // then instead of transferring a thread from the WaitSet to the EntryList 1648 // we might just dequeue a thread from the WaitSet and directly unpark() it. 1649 1650 void ObjectMonitor::INotify(JavaThread* current) { 1651 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify"); 1652 ObjectWaiter* iterator = DequeueWaiter(); 1653 if (iterator != nullptr) { 1654 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant"); 1655 guarantee(iterator->_notified == 0, "invariant"); 1656 // Disposition - what might we do with iterator ? 1657 // a. add it directly to the EntryList - either tail (policy == 1) 1658 // or head (policy == 0). 1659 // b. push it onto the front of the _cxq (policy == 2). 1660 // For now we use (b). 1661 1662 iterator->TState = ObjectWaiter::TS_ENTER; 1663 1664 iterator->_notified = 1; 1665 iterator->_notifier_tid = JFR_THREAD_ID(current); 1666 1667 ObjectWaiter* list = _EntryList; 1668 if (list != nullptr) { 1669 assert(list->_prev == nullptr, "invariant"); 1670 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant"); 1671 assert(list != iterator, "invariant"); 1672 } 1673 1674 // prepend to cxq 1675 if (list == nullptr) { 1676 iterator->_next = iterator->_prev = nullptr; 1677 _EntryList = iterator; 1678 } else { 1679 iterator->TState = ObjectWaiter::TS_CXQ; 1680 for (;;) { 1681 ObjectWaiter* front = _cxq; 1682 iterator->_next = front; 1683 if (Atomic::cmpxchg(&_cxq, front, iterator) == front) { 1684 break; 1685 } 1686 } 1687 } 1688 1689 // _WaitSetLock protects the wait queue, not the EntryList. We could 1690 // move the add-to-EntryList operation, above, outside the critical section 1691 // protected by _WaitSetLock. In practice that's not useful. With the 1692 // exception of wait() timeouts and interrupts the monitor owner 1693 // is the only thread that grabs _WaitSetLock. There's almost no contention 1694 // on _WaitSetLock so it's not profitable to reduce the length of the 1695 // critical section. 1696 1697 iterator->wait_reenter_begin(this); 1698 } 1699 Thread::SpinRelease(&_WaitSetLock); 1700 } 1701 1702 // Consider: a not-uncommon synchronization bug is to use notify() when 1703 // notifyAll() is more appropriate, potentially resulting in stranded 1704 // threads; this is one example of a lost wakeup. A useful diagnostic 1705 // option is to force all notify() operations to behave as notifyAll(). 1706 // 1707 // Note: We can also detect many such problems with a "minimum wait". 1708 // When the "minimum wait" is set to a small non-zero timeout value 1709 // and the program does not hang whereas it did absent "minimum wait", 1710 // that suggests a lost wakeup bug. 1711 1712 void ObjectMonitor::notify(TRAPS) { 1713 JavaThread* current = THREAD; 1714 CHECK_OWNER(); // Throws IMSE if not owner. 1715 if (_WaitSet == nullptr) { 1716 return; 1717 } 1718 DTRACE_MONITOR_PROBE(notify, this, object(), current); 1719 INotify(current); 1720 OM_PERFDATA_OP(Notifications, inc(1)); 1721 } 1722 1723 1724 // The current implementation of notifyAll() transfers the waiters one-at-a-time 1725 // from the waitset to the EntryList. This could be done more efficiently with a 1726 // single bulk transfer but in practice it's not time-critical. Beware too, 1727 // that in prepend-mode we invert the order of the waiters. Let's say that the 1728 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend 1729 // mode the waitset will be empty and the EntryList will be "DCBAXYZ". 1730 1731 void ObjectMonitor::notifyAll(TRAPS) { 1732 JavaThread* current = THREAD; 1733 CHECK_OWNER(); // Throws IMSE if not owner. 1734 if (_WaitSet == nullptr) { 1735 return; 1736 } 1737 1738 DTRACE_MONITOR_PROBE(notifyAll, this, object(), current); 1739 int tally = 0; 1740 while (_WaitSet != nullptr) { 1741 tally++; 1742 INotify(current); 1743 } 1744 1745 OM_PERFDATA_OP(Notifications, inc(tally)); 1746 } 1747 1748 // ----------------------------------------------------------------------------- 1749 // Adaptive Spinning Support 1750 // 1751 // Adaptive spin-then-block - rational spinning 1752 // 1753 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS 1754 // algorithm. On high order SMP systems it would be better to start with 1755 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH, 1756 // a contending thread could enqueue itself on the cxq and then spin locally 1757 // on a thread-specific variable such as its ParkEvent._Event flag. 1758 // That's left as an exercise for the reader. Note that global spinning is 1759 // not problematic on Niagara, as the L2 cache serves the interconnect and 1760 // has both low latency and massive bandwidth. 1761 // 1762 // Broadly, we can fix the spin frequency -- that is, the % of contended lock 1763 // acquisition attempts where we opt to spin -- at 100% and vary the spin count 1764 // (duration) or we can fix the count at approximately the duration of 1765 // a context switch and vary the frequency. Of course we could also 1766 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor. 1767 // For a description of 'Adaptive spin-then-block mutual exclusion in 1768 // multi-threaded processing,' see U.S. Pat. No. 8046758. 1769 // 1770 // This implementation varies the duration "D", where D varies with 1771 // the success rate of recent spin attempts. (D is capped at approximately 1772 // length of a round-trip context switch). The success rate for recent 1773 // spin attempts is a good predictor of the success rate of future spin 1774 // attempts. The mechanism adapts automatically to varying critical 1775 // section length (lock modality), system load and degree of parallelism. 1776 // D is maintained per-monitor in _SpinDuration and is initialized 1777 // optimistically. Spin frequency is fixed at 100%. 1778 // 1779 // Note that _SpinDuration is volatile, but we update it without locks 1780 // or atomics. The code is designed so that _SpinDuration stays within 1781 // a reasonable range even in the presence of races. The arithmetic 1782 // operations on _SpinDuration are closed over the domain of legal values, 1783 // so at worst a race will install and older but still legal value. 1784 // At the very worst this introduces some apparent non-determinism. 1785 // We might spin when we shouldn't or vice-versa, but since the spin 1786 // count are relatively short, even in the worst case, the effect is harmless. 1787 // 1788 // Care must be taken that a low "D" value does not become an 1789 // an absorbing state. Transient spinning failures -- when spinning 1790 // is overall profitable -- should not cause the system to converge 1791 // on low "D" values. We want spinning to be stable and predictable 1792 // and fairly responsive to change and at the same time we don't want 1793 // it to oscillate, become metastable, be "too" non-deterministic, 1794 // or converge on or enter undesirable stable absorbing states. 1795 // 1796 // We implement a feedback-based control system -- using past behavior 1797 // to predict future behavior. We face two issues: (a) if the 1798 // input signal is random then the spin predictor won't provide optimal 1799 // results, and (b) if the signal frequency is too high then the control 1800 // system, which has some natural response lag, will "chase" the signal. 1801 // (b) can arise from multimodal lock hold times. Transient preemption 1802 // can also result in apparent bimodal lock hold times. 1803 // Although sub-optimal, neither condition is particularly harmful, as 1804 // in the worst-case we'll spin when we shouldn't or vice-versa. 1805 // The maximum spin duration is rather short so the failure modes aren't bad. 1806 // To be conservative, I've tuned the gain in system to bias toward 1807 // _not spinning. Relatedly, the system can sometimes enter a mode where it 1808 // "rings" or oscillates between spinning and not spinning. This happens 1809 // when spinning is just on the cusp of profitability, however, so the 1810 // situation is not dire. The state is benign -- there's no need to add 1811 // hysteresis control to damp the transition rate between spinning and 1812 // not spinning. 1813 1814 // Spinning: Fixed frequency (100%), vary duration 1815 int ObjectMonitor::TrySpin(JavaThread* current) { 1816 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning. 1817 int ctr = Knob_FixedSpin; 1818 if (ctr != 0) { 1819 while (--ctr >= 0) { 1820 if (TryLock(current) > 0) return 1; 1821 SpinPause(); 1822 } 1823 return 0; 1824 } 1825 1826 for (ctr = Knob_PreSpin + 1; --ctr >= 0;) { 1827 if (TryLock(current) > 0) { 1828 // Increase _SpinDuration ... 1829 // Note that we don't clamp SpinDuration precisely at SpinLimit. 1830 // Raising _SpurDuration to the poverty line is key. 1831 int x = _SpinDuration; 1832 if (x < Knob_SpinLimit) { 1833 if (x < Knob_Poverty) x = Knob_Poverty; 1834 _SpinDuration = x + Knob_BonusB; 1835 } 1836 return 1; 1837 } 1838 SpinPause(); 1839 } 1840 1841 // Admission control - verify preconditions for spinning 1842 // 1843 // We always spin a little bit, just to prevent _SpinDuration == 0 from 1844 // becoming an absorbing state. Put another way, we spin briefly to 1845 // sample, just in case the system load, parallelism, contention, or lock 1846 // modality changed. 1847 // 1848 // Consider the following alternative: 1849 // Periodically set _SpinDuration = _SpinLimit and try a long/full 1850 // spin attempt. "Periodically" might mean after a tally of 1851 // the # of failed spin attempts (or iterations) reaches some threshold. 1852 // This takes us into the realm of 1-out-of-N spinning, where we 1853 // hold the duration constant but vary the frequency. 1854 1855 ctr = _SpinDuration; 1856 if (ctr <= 0) return 0; 1857 1858 if (NotRunnable(current, static_cast<JavaThread*>(owner_raw()))) { 1859 return 0; 1860 } 1861 1862 // We're good to spin ... spin ingress. 1863 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades 1864 // when preparing to LD...CAS _owner, etc and the CAS is likely 1865 // to succeed. 1866 if (_succ == nullptr) { 1867 _succ = current; 1868 } 1869 Thread* prv = nullptr; 1870 1871 // There are three ways to exit the following loop: 1872 // 1. A successful spin where this thread has acquired the lock. 1873 // 2. Spin failure with prejudice 1874 // 3. Spin failure without prejudice 1875 1876 while (--ctr >= 0) { 1877 1878 // Periodic polling -- Check for pending GC 1879 // Threads may spin while they're unsafe. 1880 // We don't want spinning threads to delay the JVM from reaching 1881 // a stop-the-world safepoint or to steal cycles from GC. 1882 // If we detect a pending safepoint we abort in order that 1883 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b) 1884 // this thread, if safe, doesn't steal cycles from GC. 1885 // This is in keeping with the "no loitering in runtime" rule. 1886 // We periodically check to see if there's a safepoint pending. 1887 if ((ctr & 0xFF) == 0) { 1888 // Can't call SafepointMechanism::should_process() since that 1889 // might update the poll values and we could be in a thread_blocked 1890 // state here which is not allowed so just check the poll. 1891 if (SafepointMechanism::local_poll_armed(current)) { 1892 goto Abort; // abrupt spin egress 1893 } 1894 SpinPause(); 1895 } 1896 1897 // Probe _owner with TATAS 1898 // If this thread observes the monitor transition or flicker 1899 // from locked to unlocked to locked, then the odds that this 1900 // thread will acquire the lock in this spin attempt go down 1901 // considerably. The same argument applies if the CAS fails 1902 // or if we observe _owner change from one non-null value to 1903 // another non-null value. In such cases we might abort 1904 // the spin without prejudice or apply a "penalty" to the 1905 // spin count-down variable "ctr", reducing it by 100, say. 1906 1907 JavaThread* ox = static_cast<JavaThread*>(owner_raw()); 1908 if (ox == nullptr) { 1909 ox = static_cast<JavaThread*>(try_set_owner_from(nullptr, current)); 1910 if (ox == nullptr) { 1911 // The CAS succeeded -- this thread acquired ownership 1912 // Take care of some bookkeeping to exit spin state. 1913 if (_succ == current) { 1914 _succ = nullptr; 1915 } 1916 1917 // Increase _SpinDuration : 1918 // The spin was successful (profitable) so we tend toward 1919 // longer spin attempts in the future. 1920 // CONSIDER: factor "ctr" into the _SpinDuration adjustment. 1921 // If we acquired the lock early in the spin cycle it 1922 // makes sense to increase _SpinDuration proportionally. 1923 // Note that we don't clamp SpinDuration precisely at SpinLimit. 1924 int x = _SpinDuration; 1925 if (x < Knob_SpinLimit) { 1926 if (x < Knob_Poverty) x = Knob_Poverty; 1927 _SpinDuration = x + Knob_Bonus; 1928 } 1929 return 1; 1930 } 1931 1932 // The CAS failed ... we can take any of the following actions: 1933 // * penalize: ctr -= CASPenalty 1934 // * exit spin with prejudice -- goto Abort; 1935 // * exit spin without prejudice. 1936 // * Since CAS is high-latency, retry again immediately. 1937 prv = ox; 1938 goto Abort; 1939 } 1940 1941 // Did lock ownership change hands ? 1942 if (ox != prv && prv != nullptr) { 1943 goto Abort; 1944 } 1945 prv = ox; 1946 1947 // Abort the spin if the owner is not executing. 1948 // The owner must be executing in order to drop the lock. 1949 // Spinning while the owner is OFFPROC is idiocy. 1950 // Consider: ctr -= RunnablePenalty ; 1951 if (NotRunnable(current, ox)) { 1952 goto Abort; 1953 } 1954 if (_succ == nullptr) { 1955 _succ = current; 1956 } 1957 } 1958 1959 // Spin failed with prejudice -- reduce _SpinDuration. 1960 // TODO: Use an AIMD-like policy to adjust _SpinDuration. 1961 // AIMD is globally stable. 1962 { 1963 int x = _SpinDuration; 1964 if (x > 0) { 1965 // Consider an AIMD scheme like: x -= (x >> 3) + 100 1966 // This is globally sample and tends to damp the response. 1967 x -= Knob_Penalty; 1968 if (x < 0) x = 0; 1969 _SpinDuration = x; 1970 } 1971 } 1972 1973 Abort: 1974 if (_succ == current) { 1975 _succ = nullptr; 1976 // Invariant: after setting succ=null a contending thread 1977 // must recheck-retry _owner before parking. This usually happens 1978 // in the normal usage of TrySpin(), but it's safest 1979 // to make TrySpin() as foolproof as possible. 1980 OrderAccess::fence(); 1981 if (TryLock(current) > 0) return 1; 1982 } 1983 return 0; 1984 } 1985 1986 // NotRunnable() -- informed spinning 1987 // 1988 // Don't bother spinning if the owner is not eligible to drop the lock. 1989 // Spin only if the owner thread is _thread_in_Java or _thread_in_vm. 1990 // The thread must be runnable in order to drop the lock in timely fashion. 1991 // If the _owner is not runnable then spinning will not likely be 1992 // successful (profitable). 1993 // 1994 // Beware -- the thread referenced by _owner could have died 1995 // so a simply fetch from _owner->_thread_state might trap. 1996 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state. 1997 // Because of the lifecycle issues, the _thread_state values 1998 // observed by NotRunnable() might be garbage. NotRunnable must 1999 // tolerate this and consider the observed _thread_state value 2000 // as advisory. 2001 // 2002 // Beware too, that _owner is sometimes a BasicLock address and sometimes 2003 // a thread pointer. 2004 // Alternately, we might tag the type (thread pointer vs basiclock pointer) 2005 // with the LSB of _owner. Another option would be to probabilistically probe 2006 // the putative _owner->TypeTag value. 2007 // 2008 // Checking _thread_state isn't perfect. Even if the thread is 2009 // in_java it might be blocked on a page-fault or have been preempted 2010 // and sitting on a ready/dispatch queue. 2011 // 2012 // The return value from NotRunnable() is *advisory* -- the 2013 // result is based on sampling and is not necessarily coherent. 2014 // The caller must tolerate false-negative and false-positive errors. 2015 // Spinning, in general, is probabilistic anyway. 2016 2017 2018 int ObjectMonitor::NotRunnable(JavaThread* current, JavaThread* ox) { 2019 // Check ox->TypeTag == 2BAD. 2020 if (ox == nullptr) return 0; 2021 2022 // Avoid transitive spinning ... 2023 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L. 2024 // Immediately after T1 acquires L it's possible that T2, also 2025 // spinning on L, will see L.Owner=T1 and T1._Stalled=L. 2026 // This occurs transiently after T1 acquired L but before 2027 // T1 managed to clear T1.Stalled. T2 does not need to abort 2028 // its spin in this circumstance. 2029 intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1)); 2030 2031 if (BlockedOn == 1) return 1; 2032 if (BlockedOn != 0) { 2033 return BlockedOn != intptr_t(this) && owner_raw() == ox; 2034 } 2035 2036 assert(sizeof(ox->_thread_state == sizeof(int)), "invariant"); 2037 int jst = SafeFetch32((int *) &ox->_thread_state, -1);; 2038 // consider also: jst != _thread_in_Java -- but that's overspecific. 2039 return jst == _thread_blocked || jst == _thread_in_native; 2040 } 2041 2042 2043 // ----------------------------------------------------------------------------- 2044 // WaitSet management ... 2045 2046 ObjectWaiter::ObjectWaiter(JavaThread* current) { 2047 _next = nullptr; 2048 _prev = nullptr; 2049 _notified = 0; 2050 _notifier_tid = 0; 2051 TState = TS_RUN; 2052 _thread = current; 2053 _event = _thread->_ParkEvent; 2054 _active = false; 2055 assert(_event != nullptr, "invariant"); 2056 } 2057 2058 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) { 2059 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(_thread, mon); 2060 } 2061 2062 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) { 2063 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(_thread, _active); 2064 } 2065 2066 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) { 2067 assert(node != nullptr, "should not add null node"); 2068 assert(node->_prev == nullptr, "node already in list"); 2069 assert(node->_next == nullptr, "node already in list"); 2070 // put node at end of queue (circular doubly linked list) 2071 if (_WaitSet == nullptr) { 2072 _WaitSet = node; 2073 node->_prev = node; 2074 node->_next = node; 2075 } else { 2076 ObjectWaiter* head = _WaitSet; 2077 ObjectWaiter* tail = head->_prev; 2078 assert(tail->_next == head, "invariant check"); 2079 tail->_next = node; 2080 head->_prev = node; 2081 node->_next = head; 2082 node->_prev = tail; 2083 } 2084 } 2085 2086 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() { 2087 // dequeue the very first waiter 2088 ObjectWaiter* waiter = _WaitSet; 2089 if (waiter) { 2090 DequeueSpecificWaiter(waiter); 2091 } 2092 return waiter; 2093 } 2094 2095 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) { 2096 assert(node != nullptr, "should not dequeue nullptr node"); 2097 assert(node->_prev != nullptr, "node already removed from list"); 2098 assert(node->_next != nullptr, "node already removed from list"); 2099 // when the waiter has woken up because of interrupt, 2100 // timeout or other spurious wake-up, dequeue the 2101 // waiter from waiting list 2102 ObjectWaiter* next = node->_next; 2103 if (next == node) { 2104 assert(node->_prev == node, "invariant check"); 2105 _WaitSet = nullptr; 2106 } else { 2107 ObjectWaiter* prev = node->_prev; 2108 assert(prev->_next == node, "invariant check"); 2109 assert(next->_prev == node, "invariant check"); 2110 next->_prev = prev; 2111 prev->_next = next; 2112 if (_WaitSet == node) { 2113 _WaitSet = next; 2114 } 2115 } 2116 node->_next = nullptr; 2117 node->_prev = nullptr; 2118 } 2119 2120 // ----------------------------------------------------------------------------- 2121 // PerfData support 2122 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = nullptr; 2123 PerfCounter * ObjectMonitor::_sync_FutileWakeups = nullptr; 2124 PerfCounter * ObjectMonitor::_sync_Parks = nullptr; 2125 PerfCounter * ObjectMonitor::_sync_Notifications = nullptr; 2126 PerfCounter * ObjectMonitor::_sync_Inflations = nullptr; 2127 PerfCounter * ObjectMonitor::_sync_Deflations = nullptr; 2128 PerfLongVariable * ObjectMonitor::_sync_MonExtant = nullptr; 2129 2130 // One-shot global initialization for the sync subsystem. 2131 // We could also defer initialization and initialize on-demand 2132 // the first time we call ObjectSynchronizer::inflate(). 2133 // Initialization would be protected - like so many things - by 2134 // the MonitorCache_lock. 2135 2136 void ObjectMonitor::Initialize() { 2137 assert(!InitDone, "invariant"); 2138 2139 if (!os::is_MP()) { 2140 Knob_SpinLimit = 0; 2141 Knob_PreSpin = 0; 2142 Knob_FixedSpin = -1; 2143 } 2144 2145 if (UsePerfData) { 2146 EXCEPTION_MARK; 2147 #define NEWPERFCOUNTER(n) \ 2148 { \ 2149 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \ 2150 CHECK); \ 2151 } 2152 #define NEWPERFVARIABLE(n) \ 2153 { \ 2154 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \ 2155 CHECK); \ 2156 } 2157 NEWPERFCOUNTER(_sync_Inflations); 2158 NEWPERFCOUNTER(_sync_Deflations); 2159 NEWPERFCOUNTER(_sync_ContendedLockAttempts); 2160 NEWPERFCOUNTER(_sync_FutileWakeups); 2161 NEWPERFCOUNTER(_sync_Parks); 2162 NEWPERFCOUNTER(_sync_Notifications); 2163 NEWPERFVARIABLE(_sync_MonExtant); 2164 #undef NEWPERFCOUNTER 2165 #undef NEWPERFVARIABLE 2166 } 2167 2168 _oop_storage = OopStorageSet::create_weak("ObjectSynchronizer Weak", mtSynchronizer); 2169 2170 DEBUG_ONLY(InitDone = true;) 2171 } 2172 2173 void ObjectMonitor::print_on(outputStream* st) const { 2174 // The minimal things to print for markWord printing, more can be added for debugging and logging. 2175 st->print("{contentions=0x%08x,waiters=0x%08x" 2176 ",recursions=" INTX_FORMAT ",owner=" INTPTR_FORMAT "}", 2177 contentions(), waiters(), recursions(), 2178 p2i(owner())); 2179 } 2180 void ObjectMonitor::print() const { print_on(tty); } 2181 2182 #ifdef ASSERT 2183 // Print the ObjectMonitor like a debugger would: 2184 // 2185 // (ObjectMonitor) 0x00007fdfb6012e40 = { 2186 // _header = 0x0000000000000001 2187 // _object = 0x000000070ff45fd0 2188 // _pad_buf0 = { 2189 // [0] = '\0' 2190 // ... 2191 // [43] = '\0' 2192 // } 2193 // _owner = 0x0000000000000000 2194 // _previous_owner_tid = 0 2195 // _pad_buf1 = { 2196 // [0] = '\0' 2197 // ... 2198 // [47] = '\0' 2199 // } 2200 // _next_om = 0x0000000000000000 2201 // _recursions = 0 2202 // _EntryList = 0x0000000000000000 2203 // _cxq = 0x0000000000000000 2204 // _succ = 0x0000000000000000 2205 // _Responsible = 0x0000000000000000 2206 // _Spinner = 0 2207 // _SpinDuration = 5000 2208 // _contentions = 0 2209 // _WaitSet = 0x0000700009756248 2210 // _waiters = 1 2211 // _WaitSetLock = 0 2212 // } 2213 // 2214 void ObjectMonitor::print_debug_style_on(outputStream* st) const { 2215 st->print_cr("(ObjectMonitor*) " INTPTR_FORMAT " = {", p2i(this)); 2216 st->print_cr(" _header = " INTPTR_FORMAT, header().value()); 2217 st->print_cr(" _object = " INTPTR_FORMAT, p2i(object_peek())); 2218 st->print_cr(" _pad_buf0 = {"); 2219 st->print_cr(" [0] = '\\0'"); 2220 st->print_cr(" ..."); 2221 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf0) - 1); 2222 st->print_cr(" }"); 2223 st->print_cr(" _owner = " INTPTR_FORMAT, p2i(owner_raw())); 2224 st->print_cr(" _previous_owner_tid = " UINT64_FORMAT, _previous_owner_tid); 2225 st->print_cr(" _pad_buf1 = {"); 2226 st->print_cr(" [0] = '\\0'"); 2227 st->print_cr(" ..."); 2228 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf1) - 1); 2229 st->print_cr(" }"); 2230 st->print_cr(" _next_om = " INTPTR_FORMAT, p2i(next_om())); 2231 st->print_cr(" _recursions = " INTX_FORMAT, _recursions); 2232 st->print_cr(" _EntryList = " INTPTR_FORMAT, p2i(_EntryList)); 2233 st->print_cr(" _cxq = " INTPTR_FORMAT, p2i(_cxq)); 2234 st->print_cr(" _succ = " INTPTR_FORMAT, p2i(_succ)); 2235 st->print_cr(" _Responsible = " INTPTR_FORMAT, p2i(_Responsible)); 2236 st->print_cr(" _Spinner = %d", _Spinner); 2237 st->print_cr(" _SpinDuration = %d", _SpinDuration); 2238 st->print_cr(" _contentions = %d", contentions()); 2239 st->print_cr(" _WaitSet = " INTPTR_FORMAT, p2i(_WaitSet)); 2240 st->print_cr(" _waiters = %d", _waiters); 2241 st->print_cr(" _WaitSetLock = %d", _WaitSetLock); 2242 st->print_cr("}"); 2243 } 2244 #endif