1 /*
   2  * Copyright (c) 1998, 2017, 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  *
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  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.
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  23  */
  24 
  25 #include "precompiled.hpp"
  26 #include "runtime/atomic.hpp"
  27 #include "runtime/interfaceSupport.hpp"
  28 #include "runtime/mutex.hpp"
  29 #include "runtime/orderAccess.inline.hpp"
  30 #include "runtime/osThread.hpp"
  31 #include "runtime/thread.inline.hpp"
  32 #include "utilities/events.hpp"
  33 #include "utilities/macros.hpp"
  34 
  35 // o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o
  36 //
  37 // Native Monitor-Mutex locking - theory of operations
  38 //
  39 // * Native Monitors are completely unrelated to Java-level monitors,
  40 //   although the "back-end" slow-path implementations share a common lineage.
  41 //   See objectMonitor:: in synchronizer.cpp.
  42 //   Native Monitors do *not* support nesting or recursion but otherwise
  43 //   they're basically Hoare-flavor monitors.
  44 //
  45 // * A thread acquires ownership of a Monitor/Mutex by CASing the LockByte
  46 //   in the _LockWord from zero to non-zero.  Note that the _Owner field
  47 //   is advisory and is used only to verify that the thread calling unlock()
  48 //   is indeed the last thread to have acquired the lock.
  49 //
  50 // * Contending threads "push" themselves onto the front of the contention
  51 //   queue -- called the cxq -- with CAS and then spin/park.
  52 //   The _LockWord contains the LockByte as well as the pointer to the head
  53 //   of the cxq.  Colocating the LockByte with the cxq precludes certain races.
  54 //
  55 // * Using a separately addressable LockByte allows for CAS:MEMBAR or CAS:0
  56 //   idioms.  We currently use MEMBAR in the uncontended unlock() path, as
  57 //   MEMBAR often has less latency than CAS.  If warranted, we could switch to
  58 //   a CAS:0 mode, using timers to close the resultant race, as is done
  59 //   with Java Monitors in synchronizer.cpp.
  60 //
  61 //   See the following for a discussion of the relative cost of atomics (CAS)
  62 //   MEMBAR, and ways to eliminate such instructions from the common-case paths:
  63 //   -- http://blogs.sun.com/dave/entry/biased_locking_in_hotspot
  64 //   -- http://blogs.sun.com/dave/resource/MustangSync.pdf
  65 //   -- http://blogs.sun.com/dave/resource/synchronization-public2.pdf
  66 //   -- synchronizer.cpp
  67 //
  68 // * Overall goals - desiderata
  69 //   1. Minimize context switching
  70 //   2. Minimize lock migration
  71 //   3. Minimize CPI -- affinity and locality
  72 //   4. Minimize the execution of high-latency instructions such as CAS or MEMBAR
  73 //   5. Minimize outer lock hold times
  74 //   6. Behave gracefully on a loaded system
  75 //
  76 // * Thread flow and list residency:
  77 //
  78 //   Contention queue --> EntryList --> OnDeck --> Owner --> !Owner
  79 //   [..resident on monitor list..]
  80 //   [...........contending..................]
  81 //
  82 //   -- The contention queue (cxq) contains recently-arrived threads (RATs).
  83 //      Threads on the cxq eventually drain into the EntryList.
  84 //   -- Invariant: a thread appears on at most one list -- cxq, EntryList
  85 //      or WaitSet -- at any one time.
  86 //   -- For a given monitor there can be at most one "OnDeck" thread at any
  87 //      given time but if needbe this particular invariant could be relaxed.
  88 //
  89 // * The WaitSet and EntryList linked lists are composed of ParkEvents.
  90 //   I use ParkEvent instead of threads as ParkEvents are immortal and
  91 //   type-stable, meaning we can safely unpark() a possibly stale
  92 //   list element in the unlock()-path.  (That's benign).
  93 //
  94 // * Succession policy - providing for progress:
  95 //
  96 //   As necessary, the unlock()ing thread identifies, unlinks, and unparks
  97 //   an "heir presumptive" tentative successor thread from the EntryList.
  98 //   This becomes the so-called "OnDeck" thread, of which there can be only
  99 //   one at any given time for a given monitor.  The wakee will recontend
 100 //   for ownership of monitor.
 101 //
 102 //   Succession is provided for by a policy of competitive handoff.
 103 //   The exiting thread does _not_ grant or pass ownership to the
 104 //   successor thread.  (This is also referred to as "handoff" succession").
 105 //   Instead the exiting thread releases ownership and possibly wakes
 106 //   a successor, so the successor can (re)compete for ownership of the lock.
 107 //
 108 //   Competitive handoff provides excellent overall throughput at the expense
 109 //   of short-term fairness.  If fairness is a concern then one remedy might
 110 //   be to add an AcquireCounter field to the monitor.  After a thread acquires
 111 //   the lock it will decrement the AcquireCounter field.  When the count
 112 //   reaches 0 the thread would reset the AcquireCounter variable, abdicate
 113 //   the lock directly to some thread on the EntryList, and then move itself to the
 114 //   tail of the EntryList.
 115 //
 116 //   But in practice most threads engage or otherwise participate in resource
 117 //   bounded producer-consumer relationships, so lock domination is not usually
 118 //   a practical concern.  Recall too, that in general it's easier to construct
 119 //   a fair lock from a fast lock, but not vice-versa.
 120 //
 121 // * The cxq can have multiple concurrent "pushers" but only one concurrent
 122 //   detaching thread.  This mechanism is immune from the ABA corruption.
 123 //   More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
 124 //   We use OnDeck as a pseudo-lock to enforce the at-most-one detaching
 125 //   thread constraint.
 126 //
 127 // * Taken together, the cxq and the EntryList constitute or form a
 128 //   single logical queue of threads stalled trying to acquire the lock.
 129 //   We use two distinct lists to reduce heat on the list ends.
 130 //   Threads in lock() enqueue onto cxq while threads in unlock() will
 131 //   dequeue from the EntryList.  (c.f. Michael Scott's "2Q" algorithm).
 132 //   A key desideratum is to minimize queue & monitor metadata manipulation
 133 //   that occurs while holding the "outer" monitor lock -- that is, we want to
 134 //   minimize monitor lock holds times.
 135 //
 136 //   The EntryList is ordered by the prevailing queue discipline and
 137 //   can be organized in any convenient fashion, such as a doubly-linked list or
 138 //   a circular doubly-linked list.  If we need a priority queue then something akin
 139 //   to Solaris' sleepq would work nicely.  Viz.,
 140 //   -- http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
 141 //   -- http://cvs.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/uts/common/os/sleepq.c
 142 //   Queue discipline is enforced at ::unlock() time, when the unlocking thread
 143 //   drains the cxq into the EntryList, and orders or reorders the threads on the
 144 //   EntryList accordingly.
 145 //
 146 //   Barring "lock barging", this mechanism provides fair cyclic ordering,
 147 //   somewhat similar to an elevator-scan.
 148 //
 149 // * OnDeck
 150 //   --  For a given monitor there can be at most one OnDeck thread at any given
 151 //       instant.  The OnDeck thread is contending for the lock, but has been
 152 //       unlinked from the EntryList and cxq by some previous unlock() operations.
 153 //       Once a thread has been designated the OnDeck thread it will remain so
 154 //       until it manages to acquire the lock -- being OnDeck is a stable property.
 155 //   --  Threads on the EntryList or cxq are _not allowed to attempt lock acquisition.
 156 //   --  OnDeck also serves as an "inner lock" as follows.  Threads in unlock() will, after
 157 //       having cleared the LockByte and dropped the outer lock,  attempt to "trylock"
 158 //       OnDeck by CASing the field from null to non-null.  If successful, that thread
 159 //       is then responsible for progress and succession and can use CAS to detach and
 160 //       drain the cxq into the EntryList.  By convention, only this thread, the holder of
 161 //       the OnDeck inner lock, can manipulate the EntryList or detach and drain the
 162 //       RATs on the cxq into the EntryList.  This avoids ABA corruption on the cxq as
 163 //       we allow multiple concurrent "push" operations but restrict detach concurrency
 164 //       to at most one thread.  Having selected and detached a successor, the thread then
 165 //       changes the OnDeck to refer to that successor, and then unparks the successor.
 166 //       That successor will eventually acquire the lock and clear OnDeck.  Beware
 167 //       that the OnDeck usage as a lock is asymmetric.  A thread in unlock() transiently
 168 //       "acquires" OnDeck, performs queue manipulations, passes OnDeck to some successor,
 169 //       and then the successor eventually "drops" OnDeck.  Note that there's never
 170 //       any sense of contention on the inner lock, however.  Threads never contend
 171 //       or wait for the inner lock.
 172 //   --  OnDeck provides for futile wakeup throttling a described in section 3.3 of
 173 //       See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
 174 //       In a sense, OnDeck subsumes the ObjectMonitor _Succ and ObjectWaiter
 175 //       TState fields found in Java-level objectMonitors.  (See synchronizer.cpp).
 176 //
 177 // * Waiting threads reside on the WaitSet list -- wait() puts
 178 //   the caller onto the WaitSet.  Notify() or notifyAll() simply
 179 //   transfers threads from the WaitSet to either the EntryList or cxq.
 180 //   Subsequent unlock() operations will eventually unpark the notifyee.
 181 //   Unparking a notifee in notify() proper is inefficient - if we were to do so
 182 //   it's likely the notifyee would simply impale itself on the lock held
 183 //   by the notifier.
 184 //
 185 // * The mechanism is obstruction-free in that if the holder of the transient
 186 //   OnDeck lock in unlock() is preempted or otherwise stalls, other threads
 187 //   can still acquire and release the outer lock and continue to make progress.
 188 //   At worst, waking of already blocked contending threads may be delayed,
 189 //   but nothing worse.  (We only use "trylock" operations on the inner OnDeck
 190 //   lock).
 191 //
 192 // * Note that thread-local storage must be initialized before a thread
 193 //   uses Native monitors or mutexes.  The native monitor-mutex subsystem
 194 //   depends on Thread::current().
 195 //
 196 // * The monitor synchronization subsystem avoids the use of native
 197 //   synchronization primitives except for the narrow platform-specific
 198 //   park-unpark abstraction.  See the comments in os_solaris.cpp regarding
 199 //   the semantics of park-unpark.  Put another way, this monitor implementation
 200 //   depends only on atomic operations and park-unpark.  The monitor subsystem
 201 //   manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
 202 //   underlying OS manages the READY<->RUN transitions.
 203 //
 204 // * The memory consistency model provide by lock()-unlock() is at least as
 205 //   strong or stronger than the Java Memory model defined by JSR-133.
 206 //   That is, we guarantee at least entry consistency, if not stronger.
 207 //   See http://g.oswego.edu/dl/jmm/cookbook.html.
 208 //
 209 // * Thread:: currently contains a set of purpose-specific ParkEvents:
 210 //   _MutexEvent, _ParkEvent, etc.  A better approach might be to do away with
 211 //   the purpose-specific ParkEvents and instead implement a general per-thread
 212 //   stack of available ParkEvents which we could provision on-demand.  The
 213 //   stack acts as a local cache to avoid excessive calls to ParkEvent::Allocate()
 214 //   and ::Release().  A thread would simply pop an element from the local stack before it
 215 //   enqueued or park()ed.  When the contention was over the thread would
 216 //   push the no-longer-needed ParkEvent back onto its stack.
 217 //
 218 // * A slightly reduced form of ILock() and IUnlock() have been partially
 219 //   model-checked (Murphi) for safety and progress at T=1,2,3 and 4.
 220 //   It'd be interesting to see if TLA/TLC could be useful as well.
 221 //
 222 // * Mutex-Monitor is a low-level "leaf" subsystem.  That is, the monitor
 223 //   code should never call other code in the JVM that might itself need to
 224 //   acquire monitors or mutexes.  That's true *except* in the case of the
 225 //   ThreadBlockInVM state transition wrappers.  The ThreadBlockInVM DTOR handles
 226 //   mutator reentry (ingress) by checking for a pending safepoint in which case it will
 227 //   call SafepointSynchronize::block(), which in turn may call Safepoint_lock->lock(), etc.
 228 //   In that particular case a call to lock() for a given Monitor can end up recursively
 229 //   calling lock() on another monitor.   While distasteful, this is largely benign
 230 //   as the calls come from jacket that wraps lock(), and not from deep within lock() itself.
 231 //
 232 //   It's unfortunate that native mutexes and thread state transitions were convolved.
 233 //   They're really separate concerns and should have remained that way.  Melding
 234 //   them together was facile -- a bit too facile.   The current implementation badly
 235 //   conflates the two concerns.
 236 //
 237 // * TODO-FIXME:
 238 //
 239 //   -- Add DTRACE probes for contended acquire, contended acquired, contended unlock
 240 //      We should also add DTRACE probes in the ParkEvent subsystem for
 241 //      Park-entry, Park-exit, and Unpark.
 242 //
 243 //   -- We have an excess of mutex-like constructs in the JVM, namely:
 244 //      1. objectMonitors for Java-level synchronization (synchronizer.cpp)
 245 //      2. low-level muxAcquire and muxRelease
 246 //      3. low-level spinAcquire and spinRelease
 247 //      4. native Mutex:: and Monitor::
 248 //      5. jvm_raw_lock() and _unlock()
 249 //      6. JVMTI raw monitors -- distinct from (5) despite having a confusingly
 250 //         similar name.
 251 //
 252 // o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o
 253 






 254 #define UNS(x) (uintptr_t(x))
 255 #define TRACE(m)                   \
 256   {                                \
 257     static volatile int ctr = 0;   \
 258     int x = ++ctr;                 \
 259     if ((x & (x - 1)) == 0) {      \
 260       ::printf("%d:%s\n", x, #m);  \
 261       ::fflush(stdout);            \
 262     }                              \
 263   }
 264 
 265 const intptr_t _LBIT = 1;
 266 
 267 // Endian-ness ... index of least-significant byte in SplitWord.Bytes[]
 268 #ifdef VM_LITTLE_ENDIAN
 269  #define _LSBINDEX 0
 270 #else
 271  #define _LSBINDEX (sizeof(intptr_t)-1)
 272 #endif
 273 
 274 // Simplistic low-quality Marsaglia SHIFT-XOR RNG.
 275 // Bijective except for the trailing mask operation.
 276 // Useful for spin loops as the compiler can't optimize it away.
 277 
 278 static inline jint MarsagliaXORV(jint x) {
 279   if (x == 0) x = 1|os::random();
 280   x ^= x << 6;
 281   x ^= ((unsigned)x) >> 21;
 282   x ^= x << 7;
 283   return x & 0x7FFFFFFF;
 284 }
 285 
 286 static int Stall(int its) {
 287   static volatile jint rv = 1;
 288   volatile int OnFrame = 0;
 289   jint v = rv ^ UNS(OnFrame);
 290   while (--its >= 0) {
 291     v = MarsagliaXORV(v);
 292   }
 293   // Make this impossible for the compiler to optimize away,
 294   // but (mostly) avoid W coherency sharing on MP systems.
 295   if (v == 0x12345) rv = v;
 296   return v;
 297 }
 298 
 299 int Monitor::TryLock() {
 300   intptr_t v = _LockWord.FullWord;
 301   for (;;) {
 302     if ((v & _LBIT) != 0) return 0;
 303     const intptr_t u = Atomic::cmpxchg(v|_LBIT, &_LockWord.FullWord, v);
 304     if (v == u) return 1;
 305     v = u;
 306   }
 307 }
 308 
 309 int Monitor::TryFast() {
 310   // Optimistic fast-path form ...
 311   // Fast-path attempt for the common uncontended case.
 312   // Avoid RTS->RTO $ coherence upgrade on typical SMP systems.
 313   intptr_t v = Atomic::cmpxchg(_LBIT, &_LockWord.FullWord, (intptr_t)0);  // agro ...
 314   if (v == 0) return 1;
 315 
 316   for (;;) {
 317     if ((v & _LBIT) != 0) return 0;
 318     const intptr_t u = Atomic::cmpxchg(v|_LBIT, &_LockWord.FullWord, v);
 319     if (v == u) return 1;
 320     v = u;
 321   }
 322 }
 323 
 324 int Monitor::ILocked() {
 325   const intptr_t w = _LockWord.FullWord & 0xFF;
 326   assert(w == 0 || w == _LBIT, "invariant");
 327   return w == _LBIT;
 328 }
 329 
 330 // Polite TATAS spinlock with exponential backoff - bounded spin.
 331 // Ideally we'd use processor cycles, time or vtime to control
 332 // the loop, but we currently use iterations.
 333 // All the constants within were derived empirically but work over
 334 // over the spectrum of J2SE reference platforms.
 335 // On Niagara-class systems the back-off is unnecessary but
 336 // is relatively harmless.  (At worst it'll slightly retard
 337 // acquisition times).  The back-off is critical for older SMP systems
 338 // where constant fetching of the LockWord would otherwise impair
 339 // scalability.
 340 //
 341 // Clamp spinning at approximately 1/2 of a context-switch round-trip.
 342 // See synchronizer.cpp for details and rationale.
 343 
 344 int Monitor::TrySpin(Thread * const Self) {
 345   if (TryLock())    return 1;
 346   if (!os::is_MP()) return 0;
 347 
 348   int Probes  = 0;
 349   int Delay   = 0;
 350   int Steps   = 0;
 351   int SpinMax = NativeMonitorSpinLimit;
 352   int flgs    = NativeMonitorFlags;
 353   for (;;) {
 354     intptr_t v = _LockWord.FullWord;
 355     if ((v & _LBIT) == 0) {
 356       if (Atomic::cmpxchg (v|_LBIT, &_LockWord.FullWord, v) == v) {
 357         return 1;
 358       }
 359       continue;
 360     }
 361 
 362     if ((flgs & 8) == 0) {
 363       SpinPause();
 364     }
 365 
 366     // Periodically increase Delay -- variable Delay form
 367     // conceptually: delay *= 1 + 1/Exponent
 368     ++Probes;
 369     if (Probes > SpinMax) return 0;
 370 
 371     if ((Probes & 0x7) == 0) {
 372       Delay = ((Delay << 1)|1) & 0x7FF;
 373       // CONSIDER: Delay += 1 + (Delay/4); Delay &= 0x7FF ;
 374     }
 375 
 376     if (flgs & 2) continue;
 377 
 378     // Consider checking _owner's schedctl state, if OFFPROC abort spin.
 379     // If the owner is OFFPROC then it's unlike that the lock will be dropped
 380     // in a timely fashion, which suggests that spinning would not be fruitful
 381     // or profitable.
 382 
 383     // Stall for "Delay" time units - iterations in the current implementation.
 384     // Avoid generating coherency traffic while stalled.
 385     // Possible ways to delay:
 386     //   PAUSE, SLEEP, MEMBAR #sync, MEMBAR #halt,
 387     //   wr %g0,%asi, gethrtime, rdstick, rdtick, rdtsc, etc. ...
 388     // Note that on Niagara-class systems we want to minimize STs in the
 389     // spin loop.  N1 and brethren write-around the L1$ over the xbar into the L2$.
 390     // Furthermore, they don't have a W$ like traditional SPARC processors.
 391     // We currently use a Marsaglia Shift-Xor RNG loop.
 392     Steps += Delay;
 393     if (Self != NULL) {
 394       jint rv = Self->rng[0];
 395       for (int k = Delay; --k >= 0;) {
 396         rv = MarsagliaXORV(rv);
 397         if ((flgs & 4) == 0 && SafepointSynchronize::do_call_back()) return 0;
 398       }
 399       Self->rng[0] = rv;
 400     } else {
 401       Stall(Delay);
 402     }
 403   }
 404 }
 405 
 406 static int ParkCommon(ParkEvent * ev, jlong timo) {
 407   // Diagnostic support - periodically unwedge blocked threads
 408   intx nmt = NativeMonitorTimeout;
 409   if (nmt > 0 && (nmt < timo || timo <= 0)) {
 410     timo = nmt;
 411   }
 412   int err = OS_OK;
 413   if (0 == timo) {
 414     ev->park();
 415   } else {
 416     err = ev->park(timo);
 417   }
 418   return err;
 419 }
 420 
 421 inline int Monitor::AcquireOrPush(ParkEvent * ESelf) {
 422   intptr_t v = _LockWord.FullWord;
 423   for (;;) {
 424     if ((v & _LBIT) == 0) {
 425       const intptr_t u = Atomic::cmpxchg(v|_LBIT, &_LockWord.FullWord, v);
 426       if (u == v) return 1;        // indicate acquired
 427       v = u;
 428     } else {
 429       // Anticipate success ...
 430       ESelf->ListNext = (ParkEvent *)(v & ~_LBIT);
 431       const intptr_t u = Atomic::cmpxchg(intptr_t(ESelf)|_LBIT, &_LockWord.FullWord, v);
 432       if (u == v) return 0;        // indicate pushed onto cxq
 433       v = u;
 434     }
 435     // Interference - LockWord change - just retry
 436   }
 437 }
 438 
 439 // ILock and IWait are the lowest level primitive internal blocking
 440 // synchronization functions.  The callers of IWait and ILock must have
 441 // performed any needed state transitions beforehand.
 442 // IWait and ILock may directly call park() without any concern for thread state.
 443 // Note that ILock and IWait do *not* access _owner.
 444 // _owner is a higher-level logical concept.
 445 
 446 void Monitor::ILock(Thread * Self) {
 447   assert(_OnDeck != Self->_MutexEvent, "invariant");
 448 
 449   if (TryFast()) {
 450  Exeunt:
 451     assert(ILocked(), "invariant");
 452     return;
 453   }
 454 
 455   ParkEvent * const ESelf = Self->_MutexEvent;
 456   assert(_OnDeck != ESelf, "invariant");
 457 
 458   // As an optimization, spinners could conditionally try to set _OnDeck to _LBIT
 459   // Synchronizer.cpp uses a similar optimization.
 460   if (TrySpin(Self)) goto Exeunt;
 461 
 462   // Slow-path - the lock is contended.
 463   // Either Enqueue Self on cxq or acquire the outer lock.
 464   // LockWord encoding = (cxq,LOCKBYTE)
 465   ESelf->reset();
 466   OrderAccess::fence();
 467 
 468   // Optional optimization ... try barging on the inner lock
 469   if ((NativeMonitorFlags & 32) && Atomic::cmpxchg(ESelf, &_OnDeck, (ParkEvent*)NULL) == NULL) {
 470     goto OnDeck_LOOP;
 471   }
 472 
 473   if (AcquireOrPush(ESelf)) goto Exeunt;
 474 
 475   // At any given time there is at most one ondeck thread.
 476   // ondeck implies not resident on cxq and not resident on EntryList
 477   // Only the OnDeck thread can try to acquire -- contend for -- the lock.
 478   // CONSIDER: use Self->OnDeck instead of m->OnDeck.
 479   // Deschedule Self so that others may run.
 480   while (OrderAccess::load_acquire(&_OnDeck) != ESelf) {
 481     ParkCommon(ESelf, 0);
 482   }
 483 
 484   // Self is now in the OnDeck position and will remain so until it
 485   // manages to acquire the lock.
 486  OnDeck_LOOP:
 487   for (;;) {
 488     assert(_OnDeck == ESelf, "invariant");
 489     if (TrySpin(Self)) break;
 490     // It's probably wise to spin only if we *actually* blocked
 491     // CONSIDER: check the lockbyte, if it remains set then
 492     // preemptively drain the cxq into the EntryList.
 493     // The best place and time to perform queue operations -- lock metadata --
 494     // is _before having acquired the outer lock, while waiting for the lock to drop.
 495     ParkCommon(ESelf, 0);
 496   }
 497 
 498   assert(_OnDeck == ESelf, "invariant");
 499   _OnDeck = NULL;
 500 
 501   // Note that we current drop the inner lock (clear OnDeck) in the slow-path
 502   // epilogue immediately after having acquired the outer lock.
 503   // But instead we could consider the following optimizations:
 504   // A. Shift or defer dropping the inner lock until the subsequent IUnlock() operation.
 505   //    This might avoid potential reacquisition of the inner lock in IUlock().
 506   // B. While still holding the inner lock, attempt to opportunistically select
 507   //    and unlink the next OnDeck thread from the EntryList.
 508   //    If successful, set OnDeck to refer to that thread, otherwise clear OnDeck.
 509   //    It's critical that the select-and-unlink operation run in constant-time as
 510   //    it executes when holding the outer lock and may artificially increase the
 511   //    effective length of the critical section.
 512   // Note that (A) and (B) are tantamount to succession by direct handoff for
 513   // the inner lock.
 514   goto Exeunt;
 515 }
 516 
 517 void Monitor::IUnlock(bool RelaxAssert) {
 518   assert(ILocked(), "invariant");
 519   // Conceptually we need a MEMBAR #storestore|#loadstore barrier or fence immediately
 520   // before the store that releases the lock.  Crucially, all the stores and loads in the
 521   // critical section must be globally visible before the store of 0 into the lock-word
 522   // that releases the lock becomes globally visible.  That is, memory accesses in the
 523   // critical section should not be allowed to bypass or overtake the following ST that
 524   // releases the lock.  As such, to prevent accesses within the critical section
 525   // from "leaking" out, we need a release fence between the critical section and the
 526   // store that releases the lock.  In practice that release barrier is elided on
 527   // platforms with strong memory models such as TSO.
 528   //
 529   // Note that the OrderAccess::storeload() fence that appears after unlock store
 530   // provides for progress conditions and succession and is _not related to exclusion
 531   // safety or lock release consistency.
 532   OrderAccess::release_store(&_LockWord.Bytes[_LSBINDEX], jbyte(0)); // drop outer lock
 533 
 534   OrderAccess::storeload();
 535   ParkEvent * const w = _OnDeck; // raw load as we will just return if non-NULL
 536   assert(RelaxAssert || w != Thread::current()->_MutexEvent, "invariant");
 537   if (w != NULL) {
 538     // Either we have a valid ondeck thread or ondeck is transiently "locked"
 539     // by some exiting thread as it arranges for succession.  The LSBit of
 540     // OnDeck allows us to discriminate two cases.  If the latter, the
 541     // responsibility for progress and succession lies with that other thread.
 542     // For good performance, we also depend on the fact that redundant unpark()
 543     // operations are cheap.  That is, repeated Unpark()ing of the OnDeck thread
 544     // is inexpensive.  This approach provides implicit futile wakeup throttling.
 545     // Note that the referent "w" might be stale with respect to the lock.
 546     // In that case the following unpark() is harmless and the worst that'll happen
 547     // is a spurious return from a park() operation.  Critically, if "w" _is stale,
 548     // then progress is known to have occurred as that means the thread associated
 549     // with "w" acquired the lock.  In that case this thread need take no further
 550     // action to guarantee progress.
 551     if ((UNS(w) & _LBIT) == 0) w->unpark();
 552     return;
 553   }
 554 
 555   intptr_t cxq = _LockWord.FullWord;
 556   if (((cxq & ~_LBIT)|UNS(_EntryList)) == 0) {
 557     return;      // normal fast-path exit - cxq and EntryList both empty
 558   }
 559   if (cxq & _LBIT) {
 560     // Optional optimization ...
 561     // Some other thread acquired the lock in the window since this
 562     // thread released it.  Succession is now that thread's responsibility.
 563     return;
 564   }
 565 
 566  Succession:
 567   // Slow-path exit - this thread must ensure succession and progress.
 568   // OnDeck serves as lock to protect cxq and EntryList.
 569   // Only the holder of OnDeck can manipulate EntryList or detach the RATs from cxq.
 570   // Avoid ABA - allow multiple concurrent producers (enqueue via push-CAS)
 571   // but only one concurrent consumer (detacher of RATs).
 572   // Consider protecting this critical section with schedctl on Solaris.
 573   // Unlike a normal lock, however, the exiting thread "locks" OnDeck,
 574   // picks a successor and marks that thread as OnDeck.  That successor
 575   // thread will then clear OnDeck once it eventually acquires the outer lock.
 576   if (Atomic::cmpxchg((ParkEvent*)_LBIT, &_OnDeck, (ParkEvent*)NULL) != NULL) {
 577     return;
 578   }
 579 
 580   ParkEvent * List = _EntryList;
 581   if (List != NULL) {
 582     // Transfer the head of the EntryList to the OnDeck position.
 583     // Once OnDeck, a thread stays OnDeck until it acquires the lock.
 584     // For a given lock there is at most OnDeck thread at any one instant.
 585    WakeOne:
 586     assert(List == _EntryList, "invariant");
 587     ParkEvent * const w = List;
 588     assert(RelaxAssert || w != Thread::current()->_MutexEvent, "invariant");
 589     _EntryList = w->ListNext;
 590     // as a diagnostic measure consider setting w->_ListNext = BAD
 591     assert(intptr_t(_OnDeck) == _LBIT, "invariant");
 592 
 593     // Pass OnDeck role to w, ensuring that _EntryList has been set first.
 594     // w will clear _OnDeck once it acquires the outer lock.
 595     // Note that once we set _OnDeck that thread can acquire the mutex, proceed
 596     // with its critical section and then enter this code to unlock the mutex. So
 597     // you can have multiple threads active in IUnlock at the same time.
 598     OrderAccess::release_store(&_OnDeck, w);
 599 
 600     // Another optional optimization ...
 601     // For heavily contended locks it's not uncommon that some other
 602     // thread acquired the lock while this thread was arranging succession.
 603     // Try to defer the unpark() operation - Delegate the responsibility
 604     // for unpark()ing the OnDeck thread to the current or subsequent owners
 605     // That is, the new owner is responsible for unparking the OnDeck thread.
 606     OrderAccess::storeload();
 607     cxq = _LockWord.FullWord;
 608     if (cxq & _LBIT) return;
 609 
 610     w->unpark();
 611     return;
 612   }
 613 
 614   cxq = _LockWord.FullWord;
 615   if ((cxq & ~_LBIT) != 0) {
 616     // The EntryList is empty but the cxq is populated.
 617     // drain RATs from cxq into EntryList
 618     // Detach RATs segment with CAS and then merge into EntryList
 619     for (;;) {
 620       // optional optimization - if locked, the owner is responsible for succession
 621       if (cxq & _LBIT) goto Punt;
 622       const intptr_t vfy = Atomic::cmpxchg(cxq & _LBIT, &_LockWord.FullWord, cxq);
 623       if (vfy == cxq) break;
 624       cxq = vfy;
 625       // Interference - LockWord changed - Just retry
 626       // We can see concurrent interference from contending threads
 627       // pushing themselves onto the cxq or from lock-unlock operations.
 628       // From the perspective of this thread, EntryList is stable and
 629       // the cxq is prepend-only -- the head is volatile but the interior
 630       // of the cxq is stable.  In theory if we encounter interference from threads
 631       // pushing onto cxq we could simply break off the original cxq suffix and
 632       // move that segment to the EntryList, avoiding a 2nd or multiple CAS attempts
 633       // on the high-traffic LockWord variable.   For instance lets say the cxq is "ABCD"
 634       // when we first fetch cxq above.  Between the fetch -- where we observed "A"
 635       // -- and CAS -- where we attempt to CAS null over A -- "PQR" arrive,
 636       // yielding cxq = "PQRABCD".  In this case we could simply set A.ListNext
 637       // null, leaving cxq = "PQRA" and transfer the "BCD" segment to the EntryList.
 638       // Note too, that it's safe for this thread to traverse the cxq
 639       // without taking any special concurrency precautions.
 640     }
 641 
 642     // We don't currently reorder the cxq segment as we move it onto
 643     // the EntryList, but it might make sense to reverse the order
 644     // or perhaps sort by thread priority.  See the comments in
 645     // synchronizer.cpp objectMonitor::exit().
 646     assert(_EntryList == NULL, "invariant");
 647     _EntryList = List = (ParkEvent *)(cxq & ~_LBIT);
 648     assert(List != NULL, "invariant");
 649     goto WakeOne;
 650   }
 651 
 652   // cxq|EntryList is empty.
 653   // w == NULL implies that cxq|EntryList == NULL in the past.
 654   // Possible race - rare inopportune interleaving.
 655   // A thread could have added itself to cxq since this thread previously checked.
 656   // Detect and recover by refetching cxq.
 657  Punt:
 658   assert(intptr_t(_OnDeck) == _LBIT, "invariant");
 659   _OnDeck = NULL;            // Release inner lock.
 660   OrderAccess::storeload();   // Dekker duality - pivot point
 661 
 662   // Resample LockWord/cxq to recover from possible race.
 663   // For instance, while this thread T1 held OnDeck, some other thread T2 might
 664   // acquire the outer lock.  Another thread T3 might try to acquire the outer
 665   // lock, but encounter contention and enqueue itself on cxq.  T2 then drops the
 666   // outer lock, but skips succession as this thread T1 still holds OnDeck.
 667   // T1 is and remains responsible for ensuring succession of T3.
 668   //
 669   // Note that we don't need to recheck EntryList, just cxq.
 670   // If threads moved onto EntryList since we dropped OnDeck
 671   // that implies some other thread forced succession.
 672   cxq = _LockWord.FullWord;
 673   if ((cxq & ~_LBIT) != 0 && (cxq & _LBIT) == 0) {
 674     goto Succession;         // potential race -- re-run succession
 675   }
 676   return;
 677 }
 678 
 679 bool Monitor::notify() {
 680   assert(_owner == Thread::current(), "invariant");
 681   assert(ILocked(), "invariant");
 682   if (_WaitSet == NULL) return true;
 683   NotifyCount++;
 684 
 685   // Transfer one thread from the WaitSet to the EntryList or cxq.
 686   // Currently we just unlink the head of the WaitSet and prepend to the cxq.
 687   // And of course we could just unlink it and unpark it, too, but
 688   // in that case it'd likely impale itself on the reentry.
 689   Thread::muxAcquire(_WaitLock, "notify:WaitLock");
 690   ParkEvent * nfy = _WaitSet;
 691   if (nfy != NULL) {                  // DCL idiom
 692     _WaitSet = nfy->ListNext;
 693     assert(nfy->Notified == 0, "invariant");
 694     // push nfy onto the cxq
 695     for (;;) {
 696       const intptr_t v = _LockWord.FullWord;
 697       assert((v & 0xFF) == _LBIT, "invariant");
 698       nfy->ListNext = (ParkEvent *)(v & ~_LBIT);
 699       if (Atomic::cmpxchg(intptr_t(nfy)|_LBIT, &_LockWord.FullWord, v) == v) break;
 700       // interference - _LockWord changed -- just retry
 701     }
 702     // Note that setting Notified before pushing nfy onto the cxq is
 703     // also legal and safe, but the safety properties are much more
 704     // subtle, so for the sake of code stewardship ...
 705     OrderAccess::fence();
 706     nfy->Notified = 1;
 707   }
 708   Thread::muxRelease(_WaitLock);
 709   if (nfy != NULL && (NativeMonitorFlags & 16)) {
 710     // Experimental code ... light up the wakee in the hope that this thread (the owner)
 711     // will drop the lock just about the time the wakee comes ONPROC.
 712     nfy->unpark();
 713   }
 714   assert(ILocked(), "invariant");
 715   return true;
 716 }
 717 
 718 // Currently notifyAll() transfers the waiters one-at-a-time from the waitset
 719 // to the cxq.  This could be done more efficiently with a single bulk en-mass transfer,
 720 // but in practice notifyAll() for large #s of threads is rare and not time-critical.
 721 // Beware too, that we invert the order of the waiters.  Lets say that the
 722 // waitset is "ABCD" and the cxq is "XYZ".  After a notifyAll() the waitset
 723 // will be empty and the cxq will be "DCBAXYZ".  This is benign, of course.
 724 
 725 bool Monitor::notify_all() {
 726   assert(_owner == Thread::current(), "invariant");
 727   assert(ILocked(), "invariant");
 728   while (_WaitSet != NULL) notify();
 729   return true;
 730 }
 731 
 732 int Monitor::IWait(Thread * Self, jlong timo) {
 733   assert(ILocked(), "invariant");
 734 
 735   // Phases:
 736   // 1. Enqueue Self on WaitSet - currently prepend
 737   // 2. unlock - drop the outer lock
 738   // 3. wait for either notification or timeout
 739   // 4. lock - reentry - reacquire the outer lock
 740 
 741   ParkEvent * const ESelf = Self->_MutexEvent;
 742   ESelf->Notified = 0;
 743   ESelf->reset();
 744   OrderAccess::fence();
 745 
 746   // Add Self to WaitSet
 747   // Ideally only the holder of the outer lock would manipulate the WaitSet -
 748   // That is, the outer lock would implicitly protect the WaitSet.
 749   // But if a thread in wait() encounters a timeout it will need to dequeue itself
 750   // from the WaitSet _before it becomes the owner of the lock.  We need to dequeue
 751   // as the ParkEvent -- which serves as a proxy for the thread -- can't reside
 752   // on both the WaitSet and the EntryList|cxq at the same time..  That is, a thread
 753   // on the WaitSet can't be allowed to compete for the lock until it has managed to
 754   // unlink its ParkEvent from WaitSet.  Thus the need for WaitLock.
 755   // Contention on the WaitLock is minimal.
 756   //
 757   // Another viable approach would be add another ParkEvent, "WaitEvent" to the
 758   // thread class.  The WaitSet would be composed of WaitEvents.  Only the
 759   // owner of the outer lock would manipulate the WaitSet.  A thread in wait()
 760   // could then compete for the outer lock, and then, if necessary, unlink itself
 761   // from the WaitSet only after having acquired the outer lock.  More precisely,
 762   // there would be no WaitLock.  A thread in in wait() would enqueue its WaitEvent
 763   // on the WaitSet; release the outer lock; wait for either notification or timeout;
 764   // reacquire the inner lock; and then, if needed, unlink itself from the WaitSet.
 765   //
 766   // Alternatively, a 2nd set of list link fields in the ParkEvent might suffice.
 767   // One set would be for the WaitSet and one for the EntryList.
 768   // We could also deconstruct the ParkEvent into a "pure" event and add a
 769   // new immortal/TSM "ListElement" class that referred to ParkEvents.
 770   // In that case we could have one ListElement on the WaitSet and another
 771   // on the EntryList, with both referring to the same pure Event.
 772 
 773   Thread::muxAcquire(_WaitLock, "wait:WaitLock:Add");
 774   ESelf->ListNext = _WaitSet;
 775   _WaitSet = ESelf;
 776   Thread::muxRelease(_WaitLock);
 777 
 778   // Release the outer lock
 779   // We call IUnlock (RelaxAssert=true) as a thread T1 might
 780   // enqueue itself on the WaitSet, call IUnlock(), drop the lock,
 781   // and then stall before it can attempt to wake a successor.
 782   // Some other thread T2 acquires the lock, and calls notify(), moving
 783   // T1 from the WaitSet to the cxq.  T2 then drops the lock.  T1 resumes,
 784   // and then finds *itself* on the cxq.  During the course of a normal
 785   // IUnlock() call a thread should _never find itself on the EntryList
 786   // or cxq, but in the case of wait() it's possible.
 787   // See synchronizer.cpp objectMonitor::wait().
 788   IUnlock(true);
 789 
 790   // Wait for either notification or timeout
 791   // Beware that in some circumstances we might propagate
 792   // spurious wakeups back to the caller.
 793 
 794   for (;;) {
 795     if (ESelf->Notified) break;
 796     int err = ParkCommon(ESelf, timo);
 797     if (err == OS_TIMEOUT || (NativeMonitorFlags & 1)) break;
 798   }
 799 
 800   // Prepare for reentry - if necessary, remove ESelf from WaitSet
 801   // ESelf can be:
 802   // 1. Still on the WaitSet.  This can happen if we exited the loop by timeout.
 803   // 2. On the cxq or EntryList
 804   // 3. Not resident on cxq, EntryList or WaitSet, but in the OnDeck position.
 805 
 806   OrderAccess::fence();
 807   int WasOnWaitSet = 0;
 808   if (ESelf->Notified == 0) {
 809     Thread::muxAcquire(_WaitLock, "wait:WaitLock:remove");
 810     if (ESelf->Notified == 0) {     // DCL idiom
 811       assert(_OnDeck != ESelf, "invariant");   // can't be both OnDeck and on WaitSet
 812       // ESelf is resident on the WaitSet -- unlink it.
 813       // A doubly-linked list would be better here so we can unlink in constant-time.
 814       // We have to unlink before we potentially recontend as ESelf might otherwise
 815       // end up on the cxq|EntryList -- it can't be on two lists at once.
 816       ParkEvent * p = _WaitSet;
 817       ParkEvent * q = NULL;            // classic q chases p
 818       while (p != NULL && p != ESelf) {
 819         q = p;
 820         p = p->ListNext;
 821       }
 822       assert(p == ESelf, "invariant");
 823       if (p == _WaitSet) {      // found at head
 824         assert(q == NULL, "invariant");
 825         _WaitSet = p->ListNext;
 826       } else {                  // found in interior
 827         assert(q->ListNext == p, "invariant");
 828         q->ListNext = p->ListNext;
 829       }
 830       WasOnWaitSet = 1;        // We were *not* notified but instead encountered timeout
 831     }
 832     Thread::muxRelease(_WaitLock);
 833   }
 834 
 835   // Reentry phase - reacquire the lock
 836   if (WasOnWaitSet) {
 837     // ESelf was previously on the WaitSet but we just unlinked it above
 838     // because of a timeout.  ESelf is not resident on any list and is not OnDeck
 839     assert(_OnDeck != ESelf, "invariant");
 840     ILock(Self);
 841   } else {
 842     // A prior notify() operation moved ESelf from the WaitSet to the cxq.
 843     // ESelf is now on the cxq, EntryList or at the OnDeck position.
 844     // The following fragment is extracted from Monitor::ILock()
 845     for (;;) {
 846       if (OrderAccess::load_acquire(&_OnDeck) == ESelf && TrySpin(Self)) break;
 847       ParkCommon(ESelf, 0);
 848     }
 849     assert(_OnDeck == ESelf, "invariant");
 850     _OnDeck = NULL;
 851   }
 852 
 853   assert(ILocked(), "invariant");
 854   return WasOnWaitSet != 0;        // return true IFF timeout
 855 }
 856 
 857 
 858 // ON THE VMTHREAD SNEAKING PAST HELD LOCKS:
 859 // In particular, there are certain types of global lock that may be held
 860 // by a Java thread while it is blocked at a safepoint but before it has
 861 // written the _owner field. These locks may be sneakily acquired by the
 862 // VM thread during a safepoint to avoid deadlocks. Alternatively, one should
 863 // identify all such locks, and ensure that Java threads never block at
 864 // safepoints while holding them (_no_safepoint_check_flag). While it
 865 // seems as though this could increase the time to reach a safepoint
 866 // (or at least increase the mean, if not the variance), the latter
 867 // approach might make for a cleaner, more maintainable JVM design.
 868 //
 869 // Sneaking is vile and reprehensible and should be excised at the 1st
 870 // opportunity.  It's possible that the need for sneaking could be obviated
 871 // as follows.  Currently, a thread might (a) while TBIVM, call pthread_mutex_lock
 872 // or ILock() thus acquiring the "physical" lock underlying Monitor/Mutex.
 873 // (b) stall at the TBIVM exit point as a safepoint is in effect.  Critically,
 874 // it'll stall at the TBIVM reentry state transition after having acquired the
 875 // underlying lock, but before having set _owner and having entered the actual
 876 // critical section.  The lock-sneaking facility leverages that fact and allowed the
 877 // VM thread to logically acquire locks that had already be physically locked by mutators
 878 // but where mutators were known blocked by the reentry thread state transition.
 879 //
 880 // If we were to modify the Monitor-Mutex so that TBIVM state transitions tightly
 881 // wrapped calls to park(), then we could likely do away with sneaking.  We'd
 882 // decouple lock acquisition and parking.  The critical invariant  to eliminating
 883 // sneaking is to ensure that we never "physically" acquire the lock while TBIVM.
 884 // An easy way to accomplish this is to wrap the park calls in a narrow TBIVM jacket.
 885 // One difficulty with this approach is that the TBIVM wrapper could recurse and
 886 // call lock() deep from within a lock() call, while the MutexEvent was already enqueued.
 887 // Using a stack (N=2 at minimum) of ParkEvents would take care of that problem.
 888 //
 889 // But of course the proper ultimate approach is to avoid schemes that require explicit
 890 // sneaking or dependence on any any clever invariants or subtle implementation properties
 891 // of Mutex-Monitor and instead directly address the underlying design flaw.
 892 
 893 void Monitor::lock(Thread * Self) {
 894   // Ensure that the Monitor requires/allows safepoint checks.
 895   assert(_safepoint_check_required != Monitor::_safepoint_check_never,
 896          "This lock should never have a safepoint check: %s", name());
 897 
 898 #ifdef CHECK_UNHANDLED_OOPS
 899   // Clear unhandled oops so we get a crash right away.  Only clear for non-vm
 900   // or GC threads.
 901   if (Self->is_Java_thread()) {
 902     Self->clear_unhandled_oops();
 903   }
 904 #endif // CHECK_UNHANDLED_OOPS
 905 
 906   debug_only(check_prelock_state(Self));
 907   assert(_owner != Self, "invariant");
 908   assert(_OnDeck != Self->_MutexEvent, "invariant");
 909 
 910   if (TryFast()) {
 911  Exeunt:
 912     assert(ILocked(), "invariant");
 913     assert(owner() == NULL, "invariant");
 914     set_owner(Self);
 915     return;
 916   }
 917 
 918   // The lock is contended ...
 919 
 920   bool can_sneak = Self->is_VM_thread() && SafepointSynchronize::is_at_safepoint();
 921   if (can_sneak && _owner == NULL) {
 922     // a java thread has locked the lock but has not entered the
 923     // critical region -- let's just pretend we've locked the lock
 924     // and go on.  we note this with _snuck so we can also
 925     // pretend to unlock when the time comes.
 926     _snuck = true;
 927     goto Exeunt;
 928   }
 929 
 930   // Try a brief spin to avoid passing thru thread state transition ...
 931   if (TrySpin(Self)) goto Exeunt;
 932 
 933   check_block_state(Self);
 934   if (Self->is_Java_thread()) {
 935     // Horrible dictu - we suffer through a state transition
 936     assert(rank() > Mutex::special, "Potential deadlock with special or lesser rank mutex");
 937     ThreadBlockInVM tbivm((JavaThread *) Self);
 938     ILock(Self);
 939   } else {
 940     // Mirabile dictu
 941     ILock(Self);
 942   }
 943   goto Exeunt;
 944 }
 945 
 946 void Monitor::lock() {
 947   this->lock(Thread::current());
 948 }
 949 
 950 // Lock without safepoint check - a degenerate variant of lock().
 951 // Should ONLY be used by safepoint code and other code
 952 // that is guaranteed not to block while running inside the VM. If this is called with
 953 // thread state set to be in VM, the safepoint synchronization code will deadlock!
 954 
 955 void Monitor::lock_without_safepoint_check(Thread * Self) {
 956   // Ensure that the Monitor does not require or allow safepoint checks.
 957   assert(_safepoint_check_required != Monitor::_safepoint_check_always,
 958          "This lock should always have a safepoint check: %s", name());
 959   assert(_owner != Self, "invariant");
 960   ILock(Self);
 961   assert(_owner == NULL, "invariant");
 962   set_owner(Self);
 963 }
 964 
 965 void Monitor::lock_without_safepoint_check() {
 966   lock_without_safepoint_check(Thread::current());
 967 }
 968 
 969 
 970 // Returns true if thread succeeds in grabbing the lock, otherwise false.
 971 
 972 bool Monitor::try_lock() {
 973   Thread * const Self = Thread::current();
 974   debug_only(check_prelock_state(Self));
 975   // assert(!thread->is_inside_signal_handler(), "don't lock inside signal handler");
 976 
 977   // Special case, where all Java threads are stopped.
 978   // The lock may have been acquired but _owner is not yet set.
 979   // In that case the VM thread can safely grab the lock.
 980   // It strikes me this should appear _after the TryLock() fails, below.
 981   bool can_sneak = Self->is_VM_thread() && SafepointSynchronize::is_at_safepoint();
 982   if (can_sneak && _owner == NULL) {
 983     set_owner(Self); // Do not need to be atomic, since we are at a safepoint
 984     _snuck = true;
 985     return true;
 986   }
 987 
 988   if (TryLock()) {
 989     // We got the lock
 990     assert(_owner == NULL, "invariant");
 991     set_owner(Self);
 992     return true;
 993   }
 994   return false;
 995 }
 996 
 997 void Monitor::unlock() {
 998   assert(_owner == Thread::current(), "invariant");
 999   assert(_OnDeck != Thread::current()->_MutexEvent, "invariant");
1000   set_owner(NULL);
1001   if (_snuck) {
1002     assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak");
1003     _snuck = false;
1004     return;
1005   }
1006   IUnlock(false);
1007 }
1008 
1009 // Yet another degenerate version of Monitor::lock() or lock_without_safepoint_check()
1010 // jvm_raw_lock() and _unlock() can be called by non-Java threads via JVM_RawMonitorEnter.
1011 //
1012 // There's no expectation that JVM_RawMonitors will interoperate properly with the native
1013 // Mutex-Monitor constructs.  We happen to implement JVM_RawMonitors in terms of
1014 // native Mutex-Monitors simply as a matter of convenience.  A simple abstraction layer
1015 // over a pthread_mutex_t would work equally as well, but require more platform-specific
1016 // code -- a "PlatformMutex".  Alternatively, a simply layer over muxAcquire-muxRelease
1017 // would work too.
1018 //
1019 // Since the caller might be a foreign thread, we don't necessarily have a Thread.MutexEvent
1020 // instance available.  Instead, we transiently allocate a ParkEvent on-demand if
1021 // we encounter contention.  That ParkEvent remains associated with the thread
1022 // until it manages to acquire the lock, at which time we return the ParkEvent
1023 // to the global ParkEvent free list.  This is correct and suffices for our purposes.
1024 //
1025 // Beware that the original jvm_raw_unlock() had a "_snuck" test but that
1026 // jvm_raw_lock() didn't have the corresponding test.  I suspect that's an
1027 // oversight, but I've replicated the original suspect logic in the new code ...
1028 
1029 void Monitor::jvm_raw_lock() {
1030   assert(rank() == native, "invariant");
1031 
1032   if (TryLock()) {
1033  Exeunt:
1034     assert(ILocked(), "invariant");
1035     assert(_owner == NULL, "invariant");
1036     // This can potentially be called by non-java Threads. Thus, the Thread::current_or_null()
1037     // might return NULL. Don't call set_owner since it will break on an NULL owner
1038     // Consider installing a non-null "ANON" distinguished value instead of just NULL.
1039     _owner = Thread::current_or_null();
1040     return;
1041   }
1042 
1043   if (TrySpin(NULL)) goto Exeunt;
1044 
1045   // slow-path - apparent contention
1046   // Allocate a ParkEvent for transient use.
1047   // The ParkEvent remains associated with this thread until
1048   // the time the thread manages to acquire the lock.
1049   ParkEvent * const ESelf = ParkEvent::Allocate(NULL);
1050   ESelf->reset();
1051   OrderAccess::storeload();
1052 
1053   // Either Enqueue Self on cxq or acquire the outer lock.
1054   if (AcquireOrPush (ESelf)) {
1055     ParkEvent::Release(ESelf);      // surrender the ParkEvent
1056     goto Exeunt;
1057   }
1058 
1059   // At any given time there is at most one ondeck thread.
1060   // ondeck implies not resident on cxq and not resident on EntryList
1061   // Only the OnDeck thread can try to acquire -- contend for -- the lock.
1062   // CONSIDER: use Self->OnDeck instead of m->OnDeck.
1063   for (;;) {
1064     if (OrderAccess::load_acquire(&_OnDeck) == ESelf && TrySpin(NULL)) break;
1065     ParkCommon(ESelf, 0);
1066   }
1067 
1068   assert(_OnDeck == ESelf, "invariant");
1069   _OnDeck = NULL;
1070   ParkEvent::Release(ESelf);      // surrender the ParkEvent
1071   goto Exeunt;
1072 }
1073 
1074 void Monitor::jvm_raw_unlock() {
1075   // Nearly the same as Monitor::unlock() ...
1076   // directly set _owner instead of using set_owner(null)
1077   _owner = NULL;
1078   if (_snuck) {         // ???
1079     assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak");
1080     _snuck = false;
1081     return;
1082   }
1083   IUnlock(false);
1084 }
1085 
1086 bool Monitor::wait(bool no_safepoint_check, long timeout,
1087                    bool as_suspend_equivalent) {
1088   // Make sure safepoint checking is used properly.
1089   assert(!(_safepoint_check_required == Monitor::_safepoint_check_never && no_safepoint_check == false),
1090          "This lock should never have a safepoint check: %s", name());
1091   assert(!(_safepoint_check_required == Monitor::_safepoint_check_always && no_safepoint_check == true),
1092          "This lock should always have a safepoint check: %s", name());
1093 
1094   Thread * const Self = Thread::current();
1095   assert(_owner == Self, "invariant");
1096   assert(ILocked(), "invariant");
1097 
1098   // as_suspend_equivalent logically implies !no_safepoint_check
1099   guarantee(!as_suspend_equivalent || !no_safepoint_check, "invariant");
1100   // !no_safepoint_check logically implies java_thread
1101   guarantee(no_safepoint_check || Self->is_Java_thread(), "invariant");
1102 
1103   #ifdef ASSERT
1104   Monitor * least = get_least_ranked_lock_besides_this(Self->owned_locks());
1105   assert(least != this, "Specification of get_least_... call above");
1106   if (least != NULL && least->rank() <= special) {
1107     tty->print("Attempting to wait on monitor %s/%d while holding"
1108                " lock %s/%d -- possible deadlock",
1109                name(), rank(), least->name(), least->rank());
1110     assert(false, "Shouldn't block(wait) while holding a lock of rank special");
1111   }
1112   #endif // ASSERT
1113 
1114   int wait_status;
1115   // conceptually set the owner to NULL in anticipation of
1116   // abdicating the lock in wait
1117   set_owner(NULL);
1118   if (no_safepoint_check) {
1119     wait_status = IWait(Self, timeout);
1120   } else {
1121     assert(Self->is_Java_thread(), "invariant");
1122     JavaThread *jt = (JavaThread *)Self;
1123 
1124     // Enter safepoint region - ornate and Rococo ...
1125     ThreadBlockInVM tbivm(jt);
1126     OSThreadWaitState osts(Self->osthread(), false /* not Object.wait() */);
1127 
1128     if (as_suspend_equivalent) {
1129       jt->set_suspend_equivalent();
1130       // cleared by handle_special_suspend_equivalent_condition() or
1131       // java_suspend_self()
1132     }
1133 
1134     wait_status = IWait(Self, timeout);
1135 
1136     // were we externally suspended while we were waiting?
1137     if (as_suspend_equivalent && jt->handle_special_suspend_equivalent_condition()) {
1138       // Our event wait has finished and we own the lock, but
1139       // while we were waiting another thread suspended us. We don't
1140       // want to hold the lock while suspended because that
1141       // would surprise the thread that suspended us.
1142       assert(ILocked(), "invariant");
1143       IUnlock(true);
1144       jt->java_suspend_self();
1145       ILock(Self);
1146       assert(ILocked(), "invariant");
1147     }
1148   }
1149 
1150   // Conceptually reestablish ownership of the lock.
1151   // The "real" lock -- the LockByte -- was reacquired by IWait().
1152   assert(ILocked(), "invariant");
1153   assert(_owner == NULL, "invariant");
1154   set_owner(Self);
1155   return wait_status != 0;          // return true IFF timeout
1156 }
1157 
1158 Monitor::~Monitor() {
1159 #ifdef ASSERT
1160   uintptr_t owner = UNS(_owner);
1161   uintptr_t lockword = UNS(_LockWord.FullWord);
1162   uintptr_t entrylist = UNS(_EntryList);
1163   uintptr_t waitset = UNS(_WaitSet);
1164   uintptr_t ondeck = UNS(_OnDeck);
1165   // Print _name with precision limit, in case failure is due to memory
1166   // corruption that also trashed _name.
1167   assert((owner|lockword|entrylist|waitset|ondeck) == 0,
1168          "%.*s: _owner(" INTPTR_FORMAT ")|_LockWord(" INTPTR_FORMAT ")|_EntryList(" INTPTR_FORMAT ")|_WaitSet("
1169          INTPTR_FORMAT ")|_OnDeck(" INTPTR_FORMAT ") != 0",
1170          MONITOR_NAME_LEN, _name, owner, lockword, entrylist, waitset, ondeck);
1171 #endif
1172 }
1173 
1174 void Monitor::ClearMonitor(Monitor * m, const char *name) {
1175   m->_owner             = NULL;
1176   m->_snuck             = false;
1177   if (name == NULL) {
1178     strcpy(m->_name, "UNKNOWN");
1179   } else {
1180     strncpy(m->_name, name, MONITOR_NAME_LEN - 1);
1181     m->_name[MONITOR_NAME_LEN - 1] = '\0';
1182   }
1183   m->_LockWord.FullWord = 0;
1184   m->_EntryList         = NULL;
1185   m->_OnDeck            = NULL;
1186   m->_WaitSet           = NULL;
1187   m->_WaitLock[0]       = 0;
1188 }
1189 
1190 Monitor::Monitor() { ClearMonitor(this); }
1191 
1192 Monitor::Monitor(int Rank, const char * name, bool allow_vm_block,
1193                  SafepointCheckRequired safepoint_check_required) {
1194   ClearMonitor(this, name);
1195 #ifdef ASSERT
1196   _allow_vm_block  = allow_vm_block;
1197   _rank            = Rank;
1198   NOT_PRODUCT(_safepoint_check_required = safepoint_check_required;)
1199 #endif
1200 }
1201 
1202 Mutex::Mutex(int Rank, const char * name, bool allow_vm_block,
1203              SafepointCheckRequired safepoint_check_required) {
1204   ClearMonitor((Monitor *) this, name);
1205 #ifdef ASSERT
1206   _allow_vm_block   = allow_vm_block;
1207   _rank             = Rank;
1208   NOT_PRODUCT(_safepoint_check_required = safepoint_check_required;)
1209 #endif
1210 }
1211 
1212 bool Monitor::owned_by_self() const {
1213   bool ret = _owner == Thread::current();
1214   assert(!ret || _LockWord.Bytes[_LSBINDEX] != 0, "invariant");
1215   return ret;
1216 }
1217 
1218 void Monitor::print_on_error(outputStream* st) const {
1219   st->print("[" PTR_FORMAT, p2i(this));
1220   st->print("] %s", _name);
1221   st->print(" - owner thread: " PTR_FORMAT, p2i(_owner));
1222 }
1223 
1224 
1225 
1226 
1227 // ----------------------------------------------------------------------------------
1228 // Non-product code
1229 
1230 #ifndef PRODUCT
1231 void Monitor::print_on(outputStream* st) const {
1232   st->print_cr("Mutex: [" PTR_FORMAT "/" PTR_FORMAT "] %s - owner: " PTR_FORMAT,
1233                p2i(this), _LockWord.FullWord, _name, p2i(_owner));
1234 }
1235 #endif
1236 
1237 #ifndef PRODUCT
1238 #ifdef ASSERT
1239 Monitor * Monitor::get_least_ranked_lock(Monitor * locks) {
1240   Monitor *res, *tmp;
1241   for (res = tmp = locks; tmp != NULL; tmp = tmp->next()) {
1242     if (tmp->rank() < res->rank()) {
1243       res = tmp;
1244     }
1245   }
1246   if (!SafepointSynchronize::is_at_safepoint()) {
1247     // In this case, we expect the held locks to be
1248     // in increasing rank order (modulo any native ranks)
1249     for (tmp = locks; tmp != NULL; tmp = tmp->next()) {
1250       if (tmp->next() != NULL) {
1251         assert(tmp->rank() == Mutex::native ||
1252                tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?");
1253       }
1254     }
1255   }
1256   return res;
1257 }
1258 
1259 Monitor* Monitor::get_least_ranked_lock_besides_this(Monitor* locks) {
1260   Monitor *res, *tmp;
1261   for (res = NULL, tmp = locks; tmp != NULL; tmp = tmp->next()) {
1262     if (tmp != this && (res == NULL || tmp->rank() < res->rank())) {
1263       res = tmp;
1264     }
1265   }
1266   if (!SafepointSynchronize::is_at_safepoint()) {
1267     // In this case, we expect the held locks to be
1268     // in increasing rank order (modulo any native ranks)
1269     for (tmp = locks; tmp != NULL; tmp = tmp->next()) {
1270       if (tmp->next() != NULL) {
1271         assert(tmp->rank() == Mutex::native ||
1272                tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?");
1273       }
1274     }
1275   }
1276   return res;
1277 }
1278 
1279 
1280 bool Monitor::contains(Monitor* locks, Monitor * lock) {
1281   for (; locks != NULL; locks = locks->next()) {
1282     if (locks == lock) {
1283       return true;
1284     }
1285   }
1286   return false;
1287 }
1288 #endif
1289 
1290 // Called immediately after lock acquisition or release as a diagnostic
1291 // to track the lock-set of the thread and test for rank violations that
1292 // might indicate exposure to deadlock.
1293 // Rather like an EventListener for _owner (:>).
1294 
1295 void Monitor::set_owner_implementation(Thread *new_owner) {
1296   // This function is solely responsible for maintaining
1297   // and checking the invariant that threads and locks
1298   // are in a 1/N relation, with some some locks unowned.
1299   // It uses the Mutex::_owner, Mutex::_next, and
1300   // Thread::_owned_locks fields, and no other function
1301   // changes those fields.
1302   // It is illegal to set the mutex from one non-NULL
1303   // owner to another--it must be owned by NULL as an
1304   // intermediate state.
1305 
1306   if (new_owner != NULL) {
1307     // the thread is acquiring this lock
1308 
1309     assert(new_owner == Thread::current(), "Should I be doing this?");
1310     assert(_owner == NULL, "setting the owner thread of an already owned mutex");
1311     _owner = new_owner; // set the owner
1312 
1313     // link "this" into the owned locks list
1314 
1315 #ifdef ASSERT  // Thread::_owned_locks is under the same ifdef
1316     Monitor* locks = get_least_ranked_lock(new_owner->owned_locks());
1317     // Mutex::set_owner_implementation is a friend of Thread
1318 
1319     assert(this->rank() >= 0, "bad lock rank");
1320 
1321     // Deadlock avoidance rules require us to acquire Mutexes only in
1322     // a global total order. For example m1 is the lowest ranked mutex
1323     // that the thread holds and m2 is the mutex the thread is trying
1324     // to acquire, then  deadlock avoidance rules require that the rank
1325     // of m2 be less  than the rank of m1.
1326     // The rank Mutex::native  is an exception in that it is not subject
1327     // to the verification rules.
1328     // Here are some further notes relating to mutex acquisition anomalies:
1329     // . it is also ok to acquire Safepoint_lock at the very end while we
1330     //   already hold Terminator_lock - may happen because of periodic safepoints
1331     if (this->rank() != Mutex::native &&
1332         this->rank() != Mutex::suspend_resume &&
1333         locks != NULL && locks->rank() <= this->rank() &&
1334         !SafepointSynchronize::is_at_safepoint() &&
1335         !(this == Safepoint_lock && contains(locks, Terminator_lock) &&
1336         SafepointSynchronize::is_synchronizing())) {
1337       new_owner->print_owned_locks();
1338       fatal("acquiring lock %s/%d out of order with lock %s/%d -- "
1339             "possible deadlock", this->name(), this->rank(),
1340             locks->name(), locks->rank());
1341     }
1342 
1343     this->_next = new_owner->_owned_locks;
1344     new_owner->_owned_locks = this;
1345 #endif
1346 
1347   } else {
1348     // the thread is releasing this lock
1349 
1350     Thread* old_owner = _owner;
1351     debug_only(_last_owner = old_owner);
1352 
1353     assert(old_owner != NULL, "removing the owner thread of an unowned mutex");
1354     assert(old_owner == Thread::current(), "removing the owner thread of an unowned mutex");
1355 
1356     _owner = NULL; // set the owner
1357 
1358 #ifdef ASSERT
1359     Monitor *locks = old_owner->owned_locks();
1360 
1361     // remove "this" from the owned locks list
1362 
1363     Monitor *prev = NULL;
1364     bool found = false;
1365     for (; locks != NULL; prev = locks, locks = locks->next()) {
1366       if (locks == this) {
1367         found = true;
1368         break;
1369       }
1370     }
1371     assert(found, "Removing a lock not owned");
1372     if (prev == NULL) {
1373       old_owner->_owned_locks = _next;
1374     } else {
1375       prev->_next = _next;
1376     }
1377     _next = NULL;
1378 #endif
1379   }
1380 }
1381 
1382 
1383 // Factored out common sanity checks for locking mutex'es. Used by lock() and try_lock()
1384 void Monitor::check_prelock_state(Thread *thread) {
1385   assert((!thread->is_Java_thread() || ((JavaThread *)thread)->thread_state() == _thread_in_vm)
1386          || rank() == Mutex::special, "wrong thread state for using locks");
1387   if (StrictSafepointChecks) {
1388     if (thread->is_VM_thread() && !allow_vm_block()) {
1389       fatal("VM thread using lock %s (not allowed to block on)", name());
1390     }
1391     debug_only(if (rank() != Mutex::special) \
1392                thread->check_for_valid_safepoint_state(false);)
1393   }
1394   assert(!os::ThreadCrashProtection::is_crash_protected(thread),
1395          "locking not allowed when crash protection is set");
1396 }
1397 
1398 void Monitor::check_block_state(Thread *thread) {
1399   if (!_allow_vm_block && thread->is_VM_thread()) {
1400     warning("VM thread blocked on lock");
1401     print();
1402     BREAKPOINT;
1403   }
1404   assert(_owner != thread, "deadlock: blocking on monitor owned by current thread");
1405 }
1406 
1407 #endif // PRODUCT
--- EOF ---