truffle: src/share/vm/runtime/mutex.cpp comparison

comparison src/share/vm/runtime/mutex.cpp @ 0:a61af66fc99e jdk7-b24

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author	duke
date	Sat, 01 Dec 2007 00:00:00 +0000
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children	2a8eb116ebbe

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+:a61af66fc99e
+/*
+* Copyright 1998-2005 Sun Microsystems, Inc.  All Rights Reserved.
+* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+*
+* This code is free software; you can redistribute it and/or modify it
+* under the terms of the GNU General Public License version 2 only, as
+* published by the Free Software Foundation.
+*
+* This code is distributed in the hope that it will be useful, but WITHOUT
+* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+* FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+* version 2 for more details (a copy is included in the LICENSE file that
+* accompanied this code).
+*
+* You should have received a copy of the GNU General Public License version
+* 2 along with this work; if not, write to the Free Software Foundation,
+* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+*
+* Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
+* CA 95054 USA or visit www.sun.com if you need additional information or
+* have any questions.
+*
+*/
+# include "incls/_precompiled.incl"
+# include "incls/_mutex.cpp.incl"
+// 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
+//
+// Native Monitor-Mutex locking - theory of operations
+//
+// * Native Monitors are completely unrelated to Java-level monitors,
+//   although the "back-end" slow-path implementations share a common lineage.
+//   See objectMonitor:: in synchronizer.cpp.
+//   Native Monitors do *not* support nesting or recursion but otherwise
+//   they're basically Hoare-flavor monitors.
+//
+// * A thread acquires ownership of a Monitor/Mutex by CASing the LockByte
+//   in the _LockWord from zero to non-zero.  Note that the _Owner field
+//   is advisory and is used only to verify that the thread calling unlock()
+//   is indeed the last thread to have acquired the lock.
+//
+// * Contending threads "push" themselves onto the front of the contention
+//   queue -- called the cxq -- with CAS and then spin/park.
+//   The _LockWord contains the LockByte as well as the pointer to the head
+//   of the cxq.  Colocating the LockByte with the cxq precludes certain races.
+//
+// * Using a separately addressable LockByte allows for CAS:MEMBAR or CAS:0
+//   idioms.  We currently use MEMBAR in the uncontended unlock() path, as
+//   MEMBAR often has less latency than CAS.  If warranted, we could switch to
+//   a CAS:0 mode, using timers to close the resultant race, as is done
+//   with Java Monitors in synchronizer.cpp.
+//
+//   See the following for a discussion of the relative cost of atomics (CAS)
+//   MEMBAR, and ways to eliminate such instructions from the common-case paths:
+//   -- http://blogs.sun.com/dave/entry/biased_locking_in_hotspot
+//   -- http://blogs.sun.com/dave/resource/MustangSync.pdf
+//   -- http://blogs.sun.com/dave/resource/synchronization-public2.pdf
+//   -- synchronizer.cpp
+//
+// * Overall goals - desiderata
+//   1. Minimize context switching
+//   2. Minimize lock migration
+//   3. Minimize CPI -- affinity and locality
+//   4. Minimize the execution of high-latency instructions such as CAS or MEMBAR
+//   5. Minimize outer lock hold times
+//   6. Behave gracefully on a loaded system
+//
+// * Thread flow and list residency:
+//
+//   Contention queue --> EntryList --> OnDeck --> Owner --> !Owner
+//   [..resident on monitor list..]
+//   [...........contending..................]
+//
+//   -- The contention queue (cxq) contains recently-arrived threads (RATs).
+//      Threads on the cxq eventually drain into the EntryList.
+//   -- Invariant: a thread appears on at most one list -- cxq, EntryList
+//      or WaitSet -- at any one time.
+//   -- For a given monitor there can be at most one "OnDeck" thread at any
+//      given time but if needbe this particular invariant could be relaxed.
+//
+// * The WaitSet and EntryList linked lists are composed of ParkEvents.
+//   I use ParkEvent instead of threads as ParkEvents are immortal and
+//   type-stable, meaning we can safely unpark() a possibly stale
+//   list element in the unlock()-path.  (That's benign).
+//
+// * Succession policy - providing for progress:
+//
+//   As necessary, the unlock()ing thread identifies, unlinks, and unparks
+//   an "heir presumptive" tentative successor thread from the EntryList.
+//   This becomes the so-called "OnDeck" thread, of which there can be only
+//   one at any given time for a given monitor.  The wakee will recontend
+//   for ownership of monitor.
+//
+//   Succession is provided for by a policy of competitive handoff.
+//   The exiting thread does _not_ grant or pass ownership to the
+//   successor thread.  (This is also referred to as "handoff" succession").
+//   Instead the exiting thread releases ownership and possibly wakes
+//   a successor, so the successor can (re)compete for ownership of the lock.
+//
+//   Competitive handoff provides excellent overall throughput at the expense
+//   of short-term fairness.  If fairness is a concern then one remedy might
+//   be to add an AcquireCounter field to the monitor.  After a thread acquires
+//   the lock it will decrement the AcquireCounter field.  When the count
+//   reaches 0 the thread would reset the AcquireCounter variable, abdicate
+//   the lock directly to some thread on the EntryList, and then move itself to the
+//   tail of the EntryList.
+//
+//   But in practice most threads engage or otherwise participate in resource
+//   bounded producer-consumer relationships, so lock domination is not usually
+//   a practical concern.  Recall too, that in general it's easier to construct
+//   a fair lock from a fast lock, but not vice-versa.
+//
+// * The cxq can have multiple concurrent "pushers" but only one concurrent
+//   detaching thread.  This mechanism is immune from the ABA corruption.
+//   More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
+//   We use OnDeck as a pseudo-lock to enforce the at-most-one detaching
+//   thread constraint.
+//
+// * Taken together, the cxq and the EntryList constitute or form a
+//   single logical queue of threads stalled trying to acquire the lock.
+//   We use two distinct lists to reduce heat on the list ends.
+//   Threads in lock() enqueue onto cxq while threads in unlock() will
+//   dequeue from the EntryList.  (c.f. Michael Scott's "2Q" algorithm).
+//   A key desideratum is to minimize queue & monitor metadata manipulation
+//   that occurs while holding the "outer" monitor lock -- that is, we want to
+//   minimize monitor lock holds times.
+//
+//   The EntryList is ordered by the prevailing queue discipline and
+//   can be organized in any convenient fashion, such as a doubly-linked list or
+//   a circular doubly-linked list.  If we need a priority queue then something akin
+//   to Solaris' sleepq would work nicely.  Viz.,
+//   -- http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
+//   -- http://cvs.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/uts/common/os/sleepq.c
+//   Queue discipline is enforced at ::unlock() time, when the unlocking thread
+//   drains the cxq into the EntryList, and orders or reorders the threads on the
+//   EntryList accordingly.
+//
+//   Barring "lock barging", this mechanism provides fair cyclic ordering,
+//   somewhat similar to an elevator-scan.
+//
+// * OnDeck
+//   --  For a given monitor there can be at most one OnDeck thread at any given
+//       instant.  The OnDeck thread is contending for the lock, but has been
+//       unlinked from the EntryList and cxq by some previous unlock() operations.
+//       Once a thread has been designated the OnDeck thread it will remain so
+//       until it manages to acquire the lock -- being OnDeck is a stable property.
+//   --  Threads on the EntryList or cxq are _not allowed to attempt lock acquisition.
+//   --  OnDeck also serves as an "inner lock" as follows.  Threads in unlock() will, after
+//       having cleared the LockByte and dropped the outer lock,  attempt to "trylock"
+//       OnDeck by CASing the field from null to non-null.  If successful, that thread
+//       is then responsible for progress and succession and can use CAS to detach and
+//       drain the cxq into the EntryList.  By convention, only this thread, the holder of
+//       the OnDeck inner lock, can manipulate the EntryList or detach and drain the
+//       RATs on the cxq into the EntryList.  This avoids ABA corruption on the cxq as
+//       we allow multiple concurrent "push" operations but restrict detach concurrency
+//       to at most one thread.  Having selected and detached a successor, the thread then
+//       changes the OnDeck to refer to that successor, and then unparks the successor.
+//       That successor will eventually acquire the lock and clear OnDeck.  Beware
+//       that the OnDeck usage as a lock is asymmetric.  A thread in unlock() transiently
+//       "acquires" OnDeck, performs queue manipulations, passes OnDeck to some successor,
+//       and then the successor eventually "drops" OnDeck.  Note that there's never
+//       any sense of contention on the inner lock, however.  Threads never contend
+//       or wait for the inner lock.
+//   --  OnDeck provides for futile wakeup throttling a described in section 3.3 of
+//       See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
+//       In a sense, OnDeck subsumes the ObjectMonitor _Succ and ObjectWaiter
+//       TState fields found in Java-level objectMonitors.  (See synchronizer.cpp).
+//
+// * Waiting threads reside on the WaitSet list -- wait() puts
+//   the caller onto the WaitSet.  Notify() or notifyAll() simply
+//   transfers threads from the WaitSet to either the EntryList or cxq.
+//   Subsequent unlock() operations will eventually unpark the notifyee.
+//   Unparking a notifee in notify() proper is inefficient - if we were to do so
+//   it's likely the notifyee would simply impale itself on the lock held
+//   by the notifier.
+//
+// * The mechanism is obstruction-free in that if the holder of the transient
+//   OnDeck lock in unlock() is preempted or otherwise stalls, other threads
+//   can still acquire and release the outer lock and continue to make progress.
+//   At worst, waking of already blocked contending threads may be delayed,
+//   but nothing worse.  (We only use "trylock" operations on the inner OnDeck
+//   lock).
+//
+// * Note that thread-local storage must be initialized before a thread
+//   uses Native monitors or mutexes.  The native monitor-mutex subsystem
+//   depends on Thread::current().
+//
+// * The monitor synchronization subsystem avoids the use of native
+//   synchronization primitives except for the narrow platform-specific
+//   park-unpark abstraction.  See the comments in os_solaris.cpp regarding
+//   the semantics of park-unpark.  Put another way, this monitor implementation
+//   depends only on atomic operations and park-unpark.  The monitor subsystem
+//   manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
+//   underlying OS manages the READY<->RUN transitions.
+//
+// * The memory consistency model provide by lock()-unlock() is at least as
+//   strong or stronger than the Java Memory model defined by JSR-133.
+//   That is, we guarantee at least entry consistency, if not stronger.
+//   See http://g.oswego.edu/dl/jmm/cookbook.html.
+//
+// * Thread:: currently contains a set of purpose-specific ParkEvents:
+//   _MutexEvent, _ParkEvent, etc.  A better approach might be to do away with
+//   the purpose-specific ParkEvents and instead implement a general per-thread
+//   stack of available ParkEvents which we could provision on-demand.  The
+//   stack acts as a local cache to avoid excessive calls to ParkEvent::Allocate()
+//   and ::Release().  A thread would simply pop an element from the local stack before it
+//   enqueued or park()ed.  When the contention was over the thread would
+//   push the no-longer-needed ParkEvent back onto its stack.
+//
+// * A slightly reduced form of ILock() and IUnlock() have been partially
+//   model-checked (Murphi) for safety and progress at T=1,2,3 and 4.
+//   It'd be interesting to see if TLA/TLC could be useful as well.
+//
+// * Mutex-Monitor is a low-level "leaf" subsystem.  That is, the monitor
+//   code should never call other code in the JVM that might itself need to
+//   acquire monitors or mutexes.  That's true *except* in the case of the
+//   ThreadBlockInVM state transition wrappers.  The ThreadBlockInVM DTOR handles
+//   mutator reentry (ingress) by checking for a pending safepoint in which case it will
+//   call SafepointSynchronize::block(), which in turn may call Safepoint_lock->lock(), etc.
+//   In that particular case a call to lock() for a given Monitor can end up recursively
+//   calling lock() on another monitor.   While distasteful, this is largely benign
+//   as the calls come from jacket that wraps lock(), and not from deep within lock() itself.
+//
+//   It's unfortunate that native mutexes and thread state transitions were convolved.
+//   They're really separate concerns and should have remained that way.  Melding
+//   them together was facile -- a bit too facile.   The current implementation badly
+//   conflates the two concerns.
+//
+// * TODO-FIXME:
+//
+//   -- Add DTRACE probes for contended acquire, contended acquired, contended unlock
+//      We should also add DTRACE probes in the ParkEvent subsystem for
+//      Park-entry, Park-exit, and Unpark.
+//
+//   -- We have an excess of mutex-like constructs in the JVM, namely:
+//      1. objectMonitors for Java-level synchronization (synchronizer.cpp)
+//      2. low-level muxAcquire and muxRelease
+//      3. low-level spinAcquire and spinRelease
+//      4. native Mutex:: and Monitor::
+//      5. jvm_raw_lock() and _unlock()
+//      6. JVMTI raw monitors -- distinct from (5) despite having a confusingly
+//         similar name.
+//
+// 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
+// CASPTR() uses the canonical argument order that dominates in the literature.
+// Our internal cmpxchg_ptr() uses a bastardized ordering to accommodate Sun .il templates.
+#define CASPTR(a,c,s) intptr_t(Atomic::cmpxchg_ptr ((void *)(s),(void *)(a),(void *)(c)))
+#define UNS(x) (uintptr_t(x))
+#define TRACE(m) { static volatile int ctr = 0 ; int x = ++ctr ; if ((x & (x-1))==0) { ::printf ("%d:%s\n", x, #m); ::fflush(stdout); }}
+// Simplistic low-quality Marsaglia SHIFT-XOR RNG.
+// Bijective except for the trailing mask operation.
+// Useful for spin loops as the compiler can't optimize it away.
+static inline jint MarsagliaXORV (jint x) {
+if (x == 0) x = 1|os::random() ;
+x ^= x << 6;
+x ^= ((unsigned)x) >> 21;
+x ^= x << 7 ;
+return x & 0x7FFFFFFF ;
+}
+static inline jint MarsagliaXOR (jint * const a) {
+jint x = *a ;
+if (x == 0) x = UNS(a)|1 ;
+x ^= x << 6;
+x ^= ((unsigned)x) >> 21;
+x ^= x << 7 ;
+*a = x ;
+return x & 0x7FFFFFFF ;
+}
+static int Stall (int its) {
+static volatile jint rv = 1 ;
+volatile int OnFrame = 0 ;
+jint v = rv ^ UNS(OnFrame) ;
+while (--its >= 0) {
+v = MarsagliaXORV (v) ;
+}
+// Make this impossible for the compiler to optimize away,
+// but (mostly) avoid W coherency sharing on MP systems.
+if (v == 0x12345) rv = v ;
+return v ;
+}
+int Monitor::TryLock () {
+intptr_t v = _LockWord.FullWord ;
+for (;;) {
+if ((v & _LBIT) != 0) return 0 ;
+const intptr_t u = CASPTR (&_LockWord, v, v|_LBIT) ;
+if (v == u) return 1 ;
+v = u ;
+}
+}
+int Monitor::TryFast () {
+// Optimistic fast-path form ...
+// Fast-path attempt for the common uncontended case.
+// Avoid RTS->RTO $ coherence upgrade on typical SMP systems.
+intptr_t v = CASPTR (&_LockWord, 0, _LBIT) ;  // agro ...
+if (v == 0) return 1 ;
+for (;;) {
+if ((v & _LBIT) != 0) return 0 ;
+const intptr_t u = CASPTR (&_LockWord, v, v|_LBIT) ;
+if (v == u) return 1 ;
+v = u ;
+}
+}
+int Monitor::ILocked () {
+const intptr_t w = _LockWord.FullWord & 0xFF ;
+assert (w == 0 || w == _LBIT, "invariant") ;
+return w == _LBIT ;
+}
+// Polite TATAS spinlock with exponential backoff - bounded spin.
+// Ideally we'd use processor cycles, time or vtime to control
+// the loop, but we currently use iterations.
+// All the constants within were derived empirically but work over
+// over the spectrum of J2SE reference platforms.
+// On Niagara-class systems the back-off is unnecessary but
+// is relatively harmless.  (At worst it'll slightly retard
+// acquisition times).  The back-off is critical for older SMP systems
+// where constant fetching of the LockWord would otherwise impair
+// scalability.
+//
+// Clamp spinning at approximately 1/2 of a context-switch round-trip.
+// See synchronizer.cpp for details and rationale.
+int Monitor::TrySpin (Thread * const Self) {
+if (TryLock())    return 1 ;
+if (!os::is_MP()) return 0 ;
+int Probes  = 0 ;
+int Delay   = 0 ;
+int Steps   = 0 ;
+int SpinMax = NativeMonitorSpinLimit ;
+int flgs    = NativeMonitorFlags ;
+for (;;) {
+intptr_t v = _LockWord.FullWord;
+if ((v & _LBIT) == 0) {
+if (CASPTR (&_LockWord, v, v|_LBIT) == v) {
+return 1 ;
+}
+continue ;
+}
+if ((flgs & 8) == 0) {
+SpinPause () ;
+}
+// Periodically increase Delay -- variable Delay form
+// conceptually: delay *= 1 + 1/Exponent
+++ Probes;
+if (Probes > SpinMax) return 0 ;
+if ((Probes & 0x7) == 0) {
+Delay = ((Delay << 1)|1) & 0x7FF ;
+// CONSIDER: Delay += 1 + (Delay/4); Delay &= 0x7FF ;
+}
+if (flgs & 2) continue ;
+// Consider checking _owner's schedctl state, if OFFPROC abort spin.
+// If the owner is OFFPROC then it's unlike that the lock will be dropped
+// in a timely fashion, which suggests that spinning would not be fruitful
+// or profitable.
+// Stall for "Delay" time units - iterations in the current implementation.
+// Avoid generating coherency traffic while stalled.
+// Possible ways to delay:
+//   PAUSE, SLEEP, MEMBAR #sync, MEMBAR #halt,
+//   wr %g0,%asi, gethrtime, rdstick, rdtick, rdtsc, etc. ...
+// Note that on Niagara-class systems we want to minimize STs in the
+// spin loop.  N1 and brethren write-around the L1$ over the xbar into the L2$.
+// Furthermore, they don't have a W$ like traditional SPARC processors.
+// We currently use a Marsaglia Shift-Xor RNG loop.
+Steps += Delay ;
+if (Self != NULL) {
+jint rv = Self->rng[0] ;
+for (int k = Delay ; --k >= 0; ) {
+rv = MarsagliaXORV (rv) ;
+if ((flgs & 4) == 0 && SafepointSynchronize::do_call_back()) return 0 ;
+}
+Self->rng[0] = rv ;
+} else {
+Stall (Delay) ;
+}
+}
+}
+static int ParkCommon (ParkEvent * ev, jlong timo) {
+// Diagnostic support - periodically unwedge blocked threads
+intx nmt = NativeMonitorTimeout ;
+if (nmt > 0 && (nmt < timo || timo <= 0)) {
+timo = nmt ;
+}
+int err = OS_OK ;
+if (0 == timo) {
+ev->park() ;
+} else {
+err = ev->park(timo) ;
+}
+return err ;
+}
+inline int Monitor::AcquireOrPush (ParkEvent * ESelf) {
+intptr_t v = _LockWord.FullWord ;
+for (;;) {
+if ((v & _LBIT) == 0) {
+const intptr_t u = CASPTR (&_LockWord, v, v|_LBIT) ;
+if (u == v) return 1 ;        // indicate acquired
+v = u ;
+} else {
+// Anticipate success ...
+ESelf->ListNext = (ParkEvent *) (v & ~_LBIT) ;
+const intptr_t u = CASPTR (&_LockWord, v, intptr_t(ESelf)|_LBIT) ;
+if (u == v) return 0 ;        // indicate pushed onto cxq
+v = u ;
+}
+// Interference - LockWord change - just retry
+}
+}
+// ILock and IWait are the lowest level primitive internal blocking
+// synchronization functions.  The callers of IWait and ILock must have
+// performed any needed state transitions beforehand.
+// IWait and ILock may directly call park() without any concern for thread state.
+// Note that ILock and IWait do *not* access _owner.
+// _owner is a higher-level logical concept.
+void Monitor::ILock (Thread * Self) {
+assert (_OnDeck != Self->_MutexEvent, "invariant") ;
+if (TryFast()) {
+Exeunt:
+assert (ILocked(), "invariant") ;
+return ;
+}
+ParkEvent * const ESelf = Self->_MutexEvent ;
+assert (_OnDeck != ESelf, "invariant") ;
+// As an optimization, spinners could conditionally try to set ONDECK to _LBIT
+// Synchronizer.cpp uses a similar optimization.
+if (TrySpin (Self)) goto Exeunt ;
+// Slow-path - the lock is contended.
+// Either Enqueue Self on cxq or acquire the outer lock.
+// LockWord encoding = (cxq,LOCKBYTE)
+ESelf->reset() ;
+OrderAccess::fence() ;
+// Optional optimization ... try barging on the inner lock
+if ((NativeMonitorFlags & 32) && CASPTR (&_OnDeck, NULL, UNS(Self)) == 0) {
+goto OnDeck_LOOP ;
+}
+if (AcquireOrPush (ESelf)) goto Exeunt ;
+// At any given time there is at most one ondeck thread.
+// ondeck implies not resident on cxq and not resident on EntryList
+// Only the OnDeck thread can try to acquire -- contended for -- the lock.
+// CONSIDER: use Self->OnDeck instead of m->OnDeck.
+// Deschedule Self so that others may run.
+while (_OnDeck != ESelf) {
+ParkCommon (ESelf, 0) ;
+}
+// Self is now in the ONDECK position and will remain so until it
+// manages to acquire the lock.
+OnDeck_LOOP:
+for (;;) {
+assert (_OnDeck == ESelf, "invariant") ;
+if (TrySpin (Self)) break ;
+// CONSIDER: if ESelf->TryPark() && TryLock() break ...
+// It's probably wise to spin only if we *actually* blocked
+// CONSIDER: check the lockbyte, if it remains set then
+// preemptively drain the cxq into the EntryList.
+// The best place and time to perform queue operations -- lock metadata --
+// is _before having acquired the outer lock, while waiting for the lock to drop.
+ParkCommon (ESelf, 0) ;
+}
+assert (_OnDeck == ESelf, "invariant") ;
+_OnDeck = NULL ;
+// Note that we current drop the inner lock (clear OnDeck) in the slow-path
+// epilog immediately after having acquired the outer lock.
+// But instead we could consider the following optimizations:
+// A. Shift or defer dropping the inner lock until the subsequent IUnlock() operation.
+//    This might avoid potential reacquisition of the inner lock in IUlock().
+// B. While still holding the inner lock, attempt to opportunistically select
+//    and unlink the next ONDECK thread from the EntryList.
+//    If successful, set ONDECK to refer to that thread, otherwise clear ONDECK.
+//    It's critical that the select-and-unlink operation run in constant-time as
+//    it executes when holding the outer lock and may artificially increase the
+//    effective length of the critical section.
+// Note that (A) and (B) are tantamount to succession by direct handoff for
+// the inner lock.
+goto Exeunt ;
+}
+void Monitor::IUnlock (bool RelaxAssert) {
+assert (ILocked(), "invariant") ;
+_LockWord.Bytes[_LSBINDEX] = 0 ;       // drop outer lock
+OrderAccess::storeload ();
+ParkEvent * const w = _OnDeck ;
+assert (RelaxAssert || w != Thread::current()->_MutexEvent, "invariant") ;
+if (w != NULL) {
+// Either we have a valid ondeck thread or ondeck is transiently "locked"
+// by some exiting thread as it arranges for succession.  The LSBit of
+// OnDeck allows us to discriminate two cases.  If the latter, the
+// responsibility for progress and succession lies with that other thread.
+// For good performance, we also depend on the fact that redundant unpark()
+// operations are cheap.  That is, repeated Unpark()ing of the ONDECK thread
+// is inexpensive.  This approach provides implicit futile wakeup throttling.
+// Note that the referent "w" might be stale with respect to the lock.
+// In that case the following unpark() is harmless and the worst that'll happen
+// is a spurious return from a park() operation.  Critically, if "w" _is stale,
+// then progress is known to have occurred as that means the thread associated
+// with "w" acquired the lock.  In that case this thread need take no further
+// action to guarantee progress.
+if ((UNS(w) & _LBIT) == 0) w->unpark() ;
+return ;
+}
+intptr_t cxq = _LockWord.FullWord ;
+if (((cxq & ~_LBIT)|UNS(_EntryList)) == 0) {
+return ;      // normal fast-path exit - cxq and EntryList both empty
+}
+if (cxq & _LBIT) {
+// Optional optimization ...
+// Some other thread acquired the lock in the window since this
+// thread released it.  Succession is now that thread's responsibility.
+return ;
+}
+Succession:
+// Slow-path exit - this thread must ensure succession and progress.
+// OnDeck serves as lock to protect cxq and EntryList.
+// Only the holder of OnDeck can manipulate EntryList or detach the RATs from cxq.
+// Avoid ABA - allow multiple concurrent producers (enqueue via push-CAS)
+// but only one concurrent consumer (detacher of RATs).
+// Consider protecting this critical section with schedctl on Solaris.
+// Unlike a normal lock, however, the exiting thread "locks" OnDeck,
+// picks a successor and marks that thread as OnDeck.  That successor
+// thread will then clear OnDeck once it eventually acquires the outer lock.
+if (CASPTR (&_OnDeck, NULL, _LBIT) != UNS(NULL)) {
+return ;
+}
+ParkEvent * List = _EntryList ;
+if (List != NULL) {
+// Transfer the head of the EntryList to the OnDeck position.
+// Once OnDeck, a thread stays OnDeck until it acquires the lock.
+// For a given lock there is at most OnDeck thread at any one instant.
+WakeOne:
+assert (List == _EntryList, "invariant") ;
+ParkEvent * const w = List ;
+assert (RelaxAssert || w != Thread::current()->_MutexEvent, "invariant") ;
+_EntryList = w->ListNext ;
+// as a diagnostic measure consider setting w->_ListNext = BAD
+assert (UNS(_OnDeck) == _LBIT, "invariant") ;
+_OnDeck = w ;           // pass OnDeck to w.
+// w will clear OnDeck once it acquires the outer lock
+// Another optional optimization ...
+// For heavily contended locks it's not uncommon that some other
+// thread acquired the lock while this thread was arranging succession.
+// Try to defer the unpark() operation - Delegate the responsibility
+// for unpark()ing the OnDeck thread to the current or subsequent owners
+// That is, the new owner is responsible for unparking the OnDeck thread.
+OrderAccess::storeload() ;
+cxq = _LockWord.FullWord ;
+if (cxq & _LBIT) return ;
+w->unpark() ;
+return ;
+}
+cxq = _LockWord.FullWord ;
+if ((cxq & ~_LBIT) != 0) {
+// The EntryList is empty but the cxq is populated.
+// drain RATs from cxq into EntryList
+// Detach RATs segment with CAS and then merge into EntryList
+for (;;) {
+// optional optimization - if locked, the owner is responsible for succession
+if (cxq & _LBIT) goto Punt ;
+const intptr_t vfy = CASPTR (&_LockWord, cxq, cxq & _LBIT) ;
+if (vfy == cxq) break ;
+cxq = vfy ;
+// Interference - LockWord changed - Just retry
+// We can see concurrent interference from contending threads
+// pushing themselves onto the cxq or from lock-unlock operations.
+// From the perspective of this thread, EntryList is stable and
+// the cxq is prepend-only -- the head is volatile but the interior
+// of the cxq is stable.  In theory if we encounter interference from threads
+// pushing onto cxq we could simply break off the original cxq suffix and
+// move that segment to the EntryList, avoiding a 2nd or multiple CAS attempts
+// on the high-traffic LockWord variable.   For instance lets say the cxq is "ABCD"
+// when we first fetch cxq above.  Between the fetch -- where we observed "A"
+// -- and CAS -- where we attempt to CAS null over A -- "PQR" arrive,
+// yielding cxq = "PQRABCD".  In this case we could simply set A.ListNext
+// null, leaving cxq = "PQRA" and transfer the "BCD" segment to the EntryList.
+// Note too, that it's safe for this thread to traverse the cxq
+// without taking any special concurrency precautions.
+}
+// We don't currently reorder the cxq segment as we move it onto
+// the EntryList, but it might make sense to reverse the order
+// or perhaps sort by thread priority.  See the comments in
+// synchronizer.cpp objectMonitor::exit().
+assert (_EntryList == NULL, "invariant") ;
+_EntryList = List = (ParkEvent *)(cxq & ~_LBIT) ;
+assert (List != NULL, "invariant") ;
+goto WakeOne ;
+}
+// cxq|EntryList is empty.
+// w == NULL implies that cxq|EntryList == NULL in the past.
+// Possible race - rare inopportune interleaving.
+// A thread could have added itself to cxq since this thread previously checked.
+// Detect and recover by refetching cxq.
+Punt:
+assert (UNS(_OnDeck) == _LBIT, "invariant") ;
+_OnDeck = NULL ;            // Release inner lock.
+OrderAccess::storeload();   // Dekker duality - pivot point
+// Resample LockWord/cxq to recover from possible race.
+// For instance, while this thread T1 held OnDeck, some other thread T2 might
+// acquire the outer lock.  Another thread T3 might try to acquire the outer
+// lock, but encounter contention and enqueue itself on cxq.  T2 then drops the
+// outer lock, but skips succession as this thread T1 still holds OnDeck.
+// T1 is and remains responsible for ensuring succession of T3.
+//
+// Note that we don't need to recheck EntryList, just cxq.
+// If threads moved onto EntryList since we dropped OnDeck
+// that implies some other thread forced succession.
+cxq = _LockWord.FullWord ;
+if ((cxq & ~_LBIT) != 0 && (cxq & _LBIT) == 0) {
+goto Succession ;         // potential race -- re-run succession
+}
+return ;
+}
+bool Monitor::notify() {
+assert (_owner == Thread::current(), "invariant") ;
+assert (ILocked(), "invariant") ;
+if (_WaitSet == NULL) return true ;
+NotifyCount ++ ;
+// Transfer one thread from the WaitSet to the EntryList or cxq.
+// Currently we just unlink the head of the WaitSet and prepend to the cxq.
+// And of course we could just unlink it and unpark it, too, but
+// in that case it'd likely impale itself on the reentry.
+Thread::muxAcquire (_WaitLock, "notify:WaitLock") ;
+ParkEvent * nfy = _WaitSet ;
+if (nfy != NULL) {                  // DCL idiom
+_WaitSet = nfy->ListNext ;
+assert (nfy->Notified == 0, "invariant") ;
+// push nfy onto the cxq
+for (;;) {
+const intptr_t v = _LockWord.FullWord ;
+assert ((v & 0xFF) == _LBIT, "invariant") ;
+nfy->ListNext = (ParkEvent *)(v & ~_LBIT);
+if (CASPTR (&_LockWord, v, UNS(nfy)|_LBIT) == v) break;
+// interference - _LockWord changed -- just retry
+}
+// Note that setting Notified before pushing nfy onto the cxq is
+// also legal and safe, but the safety properties are much more
+// subtle, so for the sake of code stewardship ...
+OrderAccess::fence() ;
+nfy->Notified = 1;
+}
+Thread::muxRelease (_WaitLock) ;
+if (nfy != NULL && (NativeMonitorFlags & 16)) {
+// Experimental code ... light up the wakee in the hope that this thread (the owner)
+// will drop the lock just about the time the wakee comes ONPROC.
+nfy->unpark() ;
+}
+assert (ILocked(), "invariant") ;
+return true ;
+}
+// Currently notifyAll() transfers the waiters one-at-a-time from the waitset
+// to the cxq.  This could be done more efficiently with a single bulk en-mass transfer,
+// but in practice notifyAll() for large #s of threads is rare and not time-critical.
+// Beware too, that we invert the order of the waiters.  Lets say that the
+// waitset is "ABCD" and the cxq is "XYZ".  After a notifyAll() the waitset
+// will be empty and the cxq will be "DCBAXYZ".  This is benign, of course.
+bool Monitor::notify_all() {
+assert (_owner == Thread::current(), "invariant") ;
+assert (ILocked(), "invariant") ;
+while (_WaitSet != NULL) notify() ;
+return true ;
+}
+int Monitor::IWait (Thread * Self, jlong timo) {
+assert (ILocked(), "invariant") ;
+// Phases:
+// 1. Enqueue Self on WaitSet - currently prepend
+// 2. unlock - drop the outer lock
+// 3. wait for either notification or timeout
+// 4. lock - reentry - reacquire the outer lock
+ParkEvent * const ESelf = Self->_MutexEvent ;
+ESelf->Notified = 0 ;
+ESelf->reset() ;
+OrderAccess::fence() ;
+// Add Self to WaitSet
+// Ideally only the holder of the outer lock would manipulate the WaitSet -
+// That is, the outer lock would implicitly protect the WaitSet.
+// But if a thread in wait() encounters a timeout it will need to dequeue itself
+// from the WaitSet _before it becomes the owner of the lock.  We need to dequeue
+// as the ParkEvent -- which serves as a proxy for the thread -- can't reside
+// on both the WaitSet and the EntryList|cxq at the same time..  That is, a thread
+// on the WaitSet can't be allowed to compete for the lock until it has managed to
+// unlink its ParkEvent from WaitSet.  Thus the need for WaitLock.
+// Contention on the WaitLock is minimal.
+//
+// Another viable approach would be add another ParkEvent, "WaitEvent" to the
+// thread class.  The WaitSet would be composed of WaitEvents.  Only the
+// owner of the outer lock would manipulate the WaitSet.  A thread in wait()
+// could then compete for the outer lock, and then, if necessary, unlink itself
+// from the WaitSet only after having acquired the outer lock.  More precisely,
+// there would be no WaitLock.  A thread in in wait() would enqueue its WaitEvent
+// on the WaitSet; release the outer lock; wait for either notification or timeout;
+// reacquire the inner lock; and then, if needed, unlink itself from the WaitSet.
+//
+// Alternatively, a 2nd set of list link fields in the ParkEvent might suffice.
+// One set would be for the WaitSet and one for the EntryList.
+// We could also deconstruct the ParkEvent into a "pure" event and add a
+// new immortal/TSM "ListElement" class that referred to ParkEvents.
+// In that case we could have one ListElement on the WaitSet and another
+// on the EntryList, with both referring to the same pure Event.
+Thread::muxAcquire (_WaitLock, "wait:WaitLock:Add") ;
+ESelf->ListNext = _WaitSet ;
+_WaitSet = ESelf ;
+Thread::muxRelease (_WaitLock) ;
+// Release the outer lock
+// We call IUnlock (RelaxAssert=true) as a thread T1 might
+// enqueue itself on the WaitSet, call IUnlock(), drop the lock,
+// and then stall before it can attempt to wake a successor.
+// Some other thread T2 acquires the lock, and calls notify(), moving
+// T1 from the WaitSet to the cxq.  T2 then drops the lock.  T1 resumes,
+// and then finds *itself* on the cxq.  During the course of a normal
+// IUnlock() call a thread should _never find itself on the EntryList
+// or cxq, but in the case of wait() it's possible.
+// See synchronizer.cpp objectMonitor::wait().
+IUnlock (true) ;
+// Wait for either notification or timeout
+// Beware that in some circumstances we might propagate
+// spurious wakeups back to the caller.
+for (;;) {
+if (ESelf->Notified) break ;
+int err = ParkCommon (ESelf, timo) ;
+if (err == OS_TIMEOUT || (NativeMonitorFlags & 1)) break ;
+}
+// Prepare for reentry - if necessary, remove ESelf from WaitSet
+// ESelf can be:
+// 1. Still on the WaitSet.  This can happen if we exited the loop by timeout.
+// 2. On the cxq or EntryList
+// 3. Not resident on cxq, EntryList or WaitSet, but in the OnDeck position.
+OrderAccess::fence() ;
+int WasOnWaitSet = 0 ;
+if (ESelf->Notified == 0) {
+Thread::muxAcquire (_WaitLock, "wait:WaitLock:remove") ;
+if (ESelf->Notified == 0) {     // DCL idiom
+assert (_OnDeck != ESelf, "invariant") ;   // can't be both OnDeck and on WaitSet
+// ESelf is resident on the WaitSet -- unlink it.
+// A doubly-linked list would be better here so we can unlink in constant-time.
+// We have to unlink before we potentially recontend as ESelf might otherwise
+// end up on the cxq|EntryList -- it can't be on two lists at once.
+ParkEvent * p = _WaitSet ;
+ParkEvent * q = NULL ;            // classic q chases p
+while (p != NULL && p != ESelf) {
+q = p ;
+p = p->ListNext ;
+}
+assert (p == ESelf, "invariant") ;
+if (p == _WaitSet) {      // found at head
+assert (q == NULL, "invariant") ;
+_WaitSet = p->ListNext ;
+} else {                  // found in interior
+assert (q->ListNext == p, "invariant") ;
+q->ListNext = p->ListNext ;
+}
+WasOnWaitSet = 1 ;        // We were *not* notified but instead encountered timeout
+}
+Thread::muxRelease (_WaitLock) ;
+}
+// Reentry phase - reacquire the lock
+if (WasOnWaitSet) {
+// ESelf was previously on the WaitSet but we just unlinked it above
+// because of a timeout.  ESelf is not resident on any list and is not OnDeck
+assert (_OnDeck != ESelf, "invariant") ;
+ILock (Self) ;
+} else {
+// A prior notify() operation moved ESelf from the WaitSet to the cxq.
+// ESelf is now on the cxq, EntryList or at the OnDeck position.
+// The following fragment is extracted from Monitor::ILock()
+for (;;) {
+if (_OnDeck == ESelf && TrySpin(Self)) break ;
+ParkCommon (ESelf, 0) ;
+}
+assert (_OnDeck == ESelf, "invariant") ;
+_OnDeck = NULL ;
+}
+assert (ILocked(), "invariant") ;
+return WasOnWaitSet != 0 ;        // return true IFF timeout
+}
+// ON THE VMTHREAD SNEAKING PAST HELD LOCKS:
+// In particular, there are certain types of global lock that may be held
+// by a Java thread while it is blocked at a safepoint but before it has
+// written the _owner field. These locks may be sneakily acquired by the
+// VM thread during a safepoint to avoid deadlocks. Alternatively, one should
+// identify all such locks, and ensure that Java threads never block at
+// safepoints while holding them (_no_safepoint_check_flag). While it
+// seems as though this could increase the time to reach a safepoint
+// (or at least increase the mean, if not the variance), the latter
+// approach might make for a cleaner, more maintainable JVM design.
+//
+// Sneaking is vile and reprehensible and should be excised at the 1st
+// opportunity.  It's possible that the need for sneaking could be obviated
+// as follows.  Currently, a thread might (a) while TBIVM, call pthread_mutex_lock
+// or ILock() thus acquiring the "physical" lock underlying Monitor/Mutex.
+// (b) stall at the TBIVM exit point as a safepoint is in effect.  Critically,
+// it'll stall at the TBIVM reentry state transition after having acquired the
+// underlying lock, but before having set _owner and having entered the actual
+// critical section.  The lock-sneaking facility leverages that fact and allowed the
+// VM thread to logically acquire locks that had already be physically locked by mutators
+// but where mutators were known blocked by the reentry thread state transition.
+//
+// If we were to modify the Monitor-Mutex so that TBIVM state transitions tightly
+// wrapped calls to park(), then we could likely do away with sneaking.  We'd
+// decouple lock acquisition and parking.  The critical invariant  to eliminating
+// sneaking is to ensure that we never "physically" acquire the lock while TBIVM.
+// An easy way to accomplish this is to wrap the park calls in a narrow TBIVM jacket.
+// One difficulty with this approach is that the TBIVM wrapper could recurse and
+// call lock() deep from within a lock() call, while the MutexEvent was already enqueued.
+// Using a stack (N=2 at minimum) of ParkEvents would take care of that problem.
+//
+// But of course the proper ultimate approach is to avoid schemes that require explicit
+// sneaking or dependence on any any clever invariants or subtle implementation properties
+// of Mutex-Monitor and instead directly address the underlying design flaw.
+void Monitor::lock (Thread * Self) {
+#ifdef CHECK_UNHANDLED_OOPS
+// Clear unhandled oops so we get a crash right away.  Only clear for non-vm
+// or GC threads.
+if (Self->is_Java_thread()) {
+Self->clear_unhandled_oops();
+}
+#endif // CHECK_UNHANDLED_OOPS
+debug_only(check_prelock_state(Self));
+assert (_owner != Self              , "invariant") ;
+assert (_OnDeck != Self->_MutexEvent, "invariant") ;
+if (TryFast()) {
+Exeunt:
+assert (ILocked(), "invariant") ;
+assert (owner() == NULL, "invariant");
+set_owner (Self);
+return ;
+}
+// The lock is contended ...
+bool can_sneak = Self->is_VM_thread() && SafepointSynchronize::is_at_safepoint();
+if (can_sneak && _owner == NULL) {
+// a java thread has locked the lock but has not entered the
+// critical region -- let's just pretend we've locked the lock
+// and go on.  we note this with _snuck so we can also
+// pretend to unlock when the time comes.
+_snuck = true;
+goto Exeunt ;
+}
+// Try a brief spin to avoid passing thru thread state transition ...
+if (TrySpin (Self)) goto Exeunt ;
+check_block_state(Self);
+if (Self->is_Java_thread()) {
+// Horribile dictu - we suffer through a state transition
+assert(rank() > Mutex::special, "Potential deadlock with special or lesser rank mutex");
+ThreadBlockInVM tbivm ((JavaThread *) Self) ;
+ILock (Self) ;
+} else {
+// Mirabile dictu
+ILock (Self) ;
+}
+goto Exeunt ;
+}
+void Monitor::lock() {
+this->lock(Thread::current());
+}
+// Lock without safepoint check - a degenerate variant of lock().
+// Should ONLY be used by safepoint code and other code
+// that is guaranteed not to block while running inside the VM. If this is called with
+// thread state set to be in VM, the safepoint synchronization code will deadlock!
+void Monitor::lock_without_safepoint_check (Thread * Self) {
+assert (_owner != Self, "invariant") ;
+ILock (Self) ;
+assert (_owner == NULL, "invariant");
+set_owner (Self);
+}
+void Monitor::lock_without_safepoint_check () {
+lock_without_safepoint_check (Thread::current()) ;
+}
+// Returns true if thread succeceed [sic] in grabbing the lock, otherwise false.
+bool Monitor::try_lock() {
+Thread * const Self = Thread::current();
+debug_only(check_prelock_state(Self));
+// assert(!thread->is_inside_signal_handler(), "don't lock inside signal handler");
+// Special case, where all Java threads are stopped.
+// The lock may have been acquired but _owner is not yet set.
+// In that case the VM thread can safely grab the lock.
+// It strikes me this should appear _after the TryLock() fails, below.
+bool can_sneak = Self->is_VM_thread() && SafepointSynchronize::is_at_safepoint();
+if (can_sneak && _owner == NULL) {
+set_owner(Self); // Do not need to be atomic, since we are at a safepoint
+_snuck = true;
+return true;
+}
+if (TryLock()) {
+// We got the lock
+assert (_owner == NULL, "invariant");
+set_owner (Self);
+return true;
+}
+return false;
+}
+void Monitor::unlock() {
+assert (_owner  == Thread::current(), "invariant") ;
+assert (_OnDeck != Thread::current()->_MutexEvent , "invariant") ;
+set_owner (NULL) ;
+if (_snuck) {
+assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak");
+_snuck = false;
+return ;
+}
+IUnlock (false) ;
+}
+// Yet another degenerate version of Monitor::lock() or lock_without_safepoint_check()
+// jvm_raw_lock() and _unlock() can be called by non-Java threads via JVM_RawMonitorEnter.
+//
+// There's no expectation that JVM_RawMonitors will interoperate properly with the native
+// Mutex-Monitor constructs.  We happen to implement JVM_RawMonitors in terms of
+// native Mutex-Monitors simply as a matter of convenience.  A simple abstraction layer
+// over a pthread_mutex_t would work equally as well, but require more platform-specific
+// code -- a "PlatformMutex".  Alternatively, a simply layer over muxAcquire-muxRelease
+// would work too.
+//
+// Since the caller might be a foreign thread, we don't necessarily have a Thread.MutexEvent
+// instance available.  Instead, we transiently allocate a ParkEvent on-demand if
+// we encounter contention.  That ParkEvent remains associated with the thread
+// until it manages to acquire the lock, at which time we return the ParkEvent
+// to the global ParkEvent free list.  This is correct and suffices for our purposes.
+//
+// Beware that the original jvm_raw_unlock() had a "_snuck" test but that
+// jvm_raw_lock() didn't have the corresponding test.  I suspect that's an
+// oversight, but I've replicated the original suspect logic in the new code ...
+void Monitor::jvm_raw_lock() {
+assert(rank() == native, "invariant");
+if (TryLock()) {
+Exeunt:
+assert (ILocked(), "invariant") ;
+assert (_owner == NULL, "invariant");
+// This can potentially be called by non-java Threads. Thus, the ThreadLocalStorage
+// might return NULL. Don't call set_owner since it will break on an NULL owner
+// Consider installing a non-null "ANON" distinguished value instead of just NULL.
+_owner = ThreadLocalStorage::thread();
+return ;
+}
+if (TrySpin(NULL)) goto Exeunt ;
+// slow-path - apparent contention
+// Allocate a ParkEvent for transient use.
+// The ParkEvent remains associated with this thread until
+// the time the thread manages to acquire the lock.
+ParkEvent * const ESelf = ParkEvent::Allocate(NULL) ;
+ESelf->reset() ;
+OrderAccess::storeload() ;
+// Either Enqueue Self on cxq or acquire the outer lock.
+if (AcquireOrPush (ESelf)) {
+ParkEvent::Release (ESelf) ;      // surrender the ParkEvent
+goto Exeunt ;
+}
+// At any given time there is at most one ondeck thread.
+// ondeck implies not resident on cxq and not resident on EntryList
+// Only the OnDeck thread can try to acquire -- contended for -- the lock.
+// CONSIDER: use Self->OnDeck instead of m->OnDeck.
+for (;;) {
+if (_OnDeck == ESelf && TrySpin(NULL)) break ;
+ParkCommon (ESelf, 0) ;
+}
+assert (_OnDeck == ESelf, "invariant") ;
+_OnDeck = NULL ;
+ParkEvent::Release (ESelf) ;      // surrender the ParkEvent
+goto Exeunt ;
+}
+void Monitor::jvm_raw_unlock() {
+// Nearly the same as Monitor::unlock() ...
+// directly set _owner instead of using set_owner(null)
+_owner = NULL ;
+if (_snuck) {         // ???
+assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak");
+_snuck = false;
+return ;
+}
+IUnlock(false) ;
+}
+bool Monitor::wait(bool no_safepoint_check, long timeout, bool as_suspend_equivalent) {
+Thread * const Self = Thread::current() ;
+assert (_owner == Self, "invariant") ;
+assert (ILocked(), "invariant") ;
+// as_suspend_equivalent logically implies !no_safepoint_check
+guarantee (!as_suspend_equivalent || !no_safepoint_check, "invariant") ;
+// !no_safepoint_check logically implies java_thread
+guarantee (no_safepoint_check || Self->is_Java_thread(), "invariant") ;
+#ifdef ASSERT
+Monitor * least = get_least_ranked_lock_besides_this(Self->owned_locks());
+assert(least != this, "Specification of get_least_... call above");
+if (least != NULL && least->rank() <= special) {
+tty->print("Attempting to wait on monitor %s/%d while holding"
+" lock %s/%d -- possible deadlock",
+name(), rank(), least->name(), least->rank());
+assert(false, "Shouldn't block(wait) while holding a lock of rank special");
+}
+#endif // ASSERT
+int wait_status ;
+// conceptually set the owner to NULL in anticipation of
+// abdicating the lock in wait
+set_owner(NULL);
+if (no_safepoint_check) {
+wait_status = IWait (Self, timeout) ;
+} else {
+assert (Self->is_Java_thread(), "invariant") ;
+JavaThread *jt = (JavaThread *)Self;
+// Enter safepoint region - ornate and Rococo ...
+ThreadBlockInVM tbivm(jt);
+OSThreadWaitState osts(Self->osthread(), false /* not Object.wait() */);
+if (as_suspend_equivalent) {
+jt->set_suspend_equivalent();
+// cleared by handle_special_suspend_equivalent_condition() or
+// java_suspend_self()
+}
+wait_status = IWait (Self, timeout) ;
+// were we externally suspended while we were waiting?
+if (as_suspend_equivalent && jt->handle_special_suspend_equivalent_condition()) {
+// Our event wait has finished and we own the lock, but
+// while we were waiting another thread suspended us. We don't
+// want to hold the lock while suspended because that
+// would surprise the thread that suspended us.
+assert (ILocked(), "invariant") ;
+IUnlock (true) ;
+jt->java_suspend_self();
+ILock (Self) ;
+assert (ILocked(), "invariant") ;
+}
+}
+// Conceptually reestablish ownership of the lock.
+// The "real" lock -- the LockByte -- was reacquired by IWait().
+assert (ILocked(), "invariant") ;
+assert (_owner == NULL, "invariant") ;
+set_owner (Self) ;
+return wait_status != 0 ;          // return true IFF timeout
+}
+Monitor::~Monitor() {
+assert ((UNS(_owner)|UNS(_LockWord.FullWord)|UNS(_EntryList)|UNS(_WaitSet)|UNS(_OnDeck)) == 0, "") ;
+}
+void Monitor::ClearMonitor (Monitor * m) {
+m->_owner             = NULL ;
+m->_snuck             = false ;
+m->_name              = "UNKNOWN" ;
+m->_LockWord.FullWord = 0 ;
+m->_EntryList         = NULL ;
+m->_OnDeck            = NULL ;
+m->_WaitSet           = NULL ;
+m->_WaitLock[0]       = 0 ;
+}
+Monitor::Monitor() { ClearMonitor(this); }
+Monitor::Monitor (int Rank, const char * name, bool allow_vm_block) {
+ClearMonitor (this) ;
+#ifdef ASSERT
+_allow_vm_block  = allow_vm_block;
+_rank            = Rank ;
+#endif
+}
+Mutex::~Mutex() {
+assert ((UNS(_owner)|UNS(_LockWord.FullWord)|UNS(_EntryList)|UNS(_WaitSet)|UNS(_OnDeck)) == 0, "") ;
+}
+Mutex::Mutex (int Rank, const char * name, bool allow_vm_block) {
+ClearMonitor ((Monitor *) this) ;
+#ifdef ASSERT
+_allow_vm_block   = allow_vm_block;
+_rank             = Rank ;
+#endif
+}
+bool Monitor::owned_by_self() const {
+bool ret = _owner == Thread::current();
+assert (!ret || _LockWord.Bytes[_LSBINDEX] != 0, "invariant") ;
+return ret;
+}
+void Monitor::print_on_error(outputStream* st) const {
+st->print("[" PTR_FORMAT, this);
+st->print("] %s", _name);
+st->print(" - owner thread: " PTR_FORMAT, _owner);
+}
+// ----------------------------------------------------------------------------------
+// Non-product code
+#ifndef PRODUCT
+void Monitor::print_on(outputStream* st) const {
+st->print_cr("Mutex: [0x%lx/0x%lx] %s - owner: 0x%lx", this, _LockWord.FullWord, _name, _owner);
+}
+#endif
+#ifndef PRODUCT
+#ifdef ASSERT
+Monitor * Monitor::get_least_ranked_lock(Monitor * locks) {
+Monitor *res, *tmp;
+for (res = tmp = locks; tmp != NULL; tmp = tmp->next()) {
+if (tmp->rank() < res->rank()) {
+res = tmp;
+}
+}
+if (!SafepointSynchronize::is_at_safepoint()) {
+// In this case, we expect the held locks to be
+// in increasing rank order (modulo any native ranks)
+for (tmp = locks; tmp != NULL; tmp = tmp->next()) {
+if (tmp->next() != NULL) {
+assert(tmp->rank() == Mutex::native ||
+tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?");
+}
+}
+}
+return res;
+}
+Monitor* Monitor::get_least_ranked_lock_besides_this(Monitor* locks) {
+Monitor *res, *tmp;
+for (res = NULL, tmp = locks; tmp != NULL; tmp = tmp->next()) {
+if (tmp != this && (res == NULL || tmp->rank() < res->rank())) {
+res = tmp;
+}
+}
+if (!SafepointSynchronize::is_at_safepoint()) {
+// In this case, we expect the held locks to be
+// in increasing rank order (modulo any native ranks)
+for (tmp = locks; tmp != NULL; tmp = tmp->next()) {
+if (tmp->next() != NULL) {
+assert(tmp->rank() == Mutex::native ||
+tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?");
+}
+}
+}
+return res;
+}
+bool Monitor::contains(Monitor* locks, Monitor * lock) {
+for (; locks != NULL; locks = locks->next()) {
+if (locks == lock)
+return true;
+}
+return false;
+}
+#endif
+// Called immediately after lock acquisition or release as a diagnostic
+// to track the lock-set of the thread and test for rank violations that
+// might indicate exposure to deadlock.
+// Rather like an EventListener for _owner (:>).
+void Monitor::set_owner_implementation(Thread *new_owner) {
+// This function is solely responsible for maintaining
+// and checking the invariant that threads and locks
+// are in a 1/N relation, with some some locks unowned.
+// It uses the Mutex::_owner, Mutex::_next, and
+// Thread::_owned_locks fields, and no other function
+// changes those fields.
+// It is illegal to set the mutex from one non-NULL
+// owner to another--it must be owned by NULL as an
+// intermediate state.
+if (new_owner != NULL) {
+// the thread is acquiring this lock
+assert(new_owner == Thread::current(), "Should I be doing this?");
+assert(_owner == NULL, "setting the owner thread of an already owned mutex");
+_owner = new_owner; // set the owner
+// link "this" into the owned locks list
+#ifdef ASSERT  // Thread::_owned_locks is under the same ifdef
+Monitor* locks = get_least_ranked_lock(new_owner->owned_locks());
+// Mutex::set_owner_implementation is a friend of Thread
+assert(this->rank() >= 0, "bad lock rank");
+if (LogMultipleMutexLocking && locks != NULL) {
+Events::log("thread " INTPTR_FORMAT " locks %s, already owns %s", new_owner, name(), locks->name());
+}
+// Deadlock avoidance rules require us to acquire Mutexes only in
+// a global total order. For example m1 is the lowest ranked mutex
+// that the thread holds and m2 is the mutex the thread is trying
+// to acquire, then  deadlock avoidance rules require that the rank
+// of m2 be less  than the rank of m1.
+// The rank Mutex::native  is an exception in that it is not subject
+// to the verification rules.
+// Here are some further notes relating to mutex acquisition anomalies:
+// . under Solaris, the interrupt lock gets acquired when doing
+//   profiling, so any lock could be held.
+// . it is also ok to acquire Safepoint_lock at the very end while we
+//   already hold Terminator_lock - may happen because of periodic safepoints
+if (this->rank() != Mutex::native &&
+this->rank() != Mutex::suspend_resume &&
+locks != NULL && locks->rank() <= this->rank() &&
+!SafepointSynchronize::is_at_safepoint() &&
+this != Interrupt_lock && this != ProfileVM_lock &&
+!(this == Safepoint_lock && contains(locks, Terminator_lock) &&
+SafepointSynchronize::is_synchronizing())) {
+new_owner->print_owned_locks();
+fatal4("acquiring lock %s/%d out of order with lock %s/%d -- possible deadlock",
+this->name(), this->rank(), locks->name(), locks->rank());
+}
+this->_next = new_owner->_owned_locks;
+new_owner->_owned_locks = this;
+#endif
+} else {
+// the thread is releasing this lock
+Thread* old_owner = _owner;
+debug_only(_last_owner = old_owner);
+assert(old_owner != NULL, "removing the owner thread of an unowned mutex");
+assert(old_owner == Thread::current(), "removing the owner thread of an unowned mutex");
+_owner = NULL; // set the owner
+#ifdef ASSERT
+Monitor *locks = old_owner->owned_locks();
+if (LogMultipleMutexLocking && locks != this) {
+Events::log("thread " INTPTR_FORMAT " unlocks %s, still owns %s", old_owner, this->name(), locks->name());
+}
+// remove "this" from the owned locks list
+Monitor *prev = NULL;
+bool found = false;
+for (; locks != NULL; prev = locks, locks = locks->next()) {
+if (locks == this) {
+found = true;
+break;
+}
+}
+assert(found, "Removing a lock not owned");
+if (prev == NULL) {
+old_owner->_owned_locks = _next;
+} else {
+prev->_next = _next;
+}
+_next = NULL;
+#endif
+}
+}
+// Factored out common sanity checks for locking mutex'es. Used by lock() and try_lock()
+void Monitor::check_prelock_state(Thread *thread) {
+assert((!thread->is_Java_thread() || ((JavaThread *)thread)->thread_state() == _thread_in_vm)
+|| rank() == Mutex::special, "wrong thread state for using locks");
+if (StrictSafepointChecks) {
+if (thread->is_VM_thread() && !allow_vm_block()) {
+fatal1("VM thread using lock %s (not allowed to block on)", name());
+}
+debug_only(if (rank() != Mutex::special) \
+thread->check_for_valid_safepoint_state(false);)
+}
+}
+void Monitor::check_block_state(Thread *thread) {
+if (!_allow_vm_block && thread->is_VM_thread()) {
+warning("VM thread blocked on lock");
+print();
+BREAKPOINT;
+}
+assert(_owner != thread, "deadlock: blocking on monitor owned by current thread");
+}
+#endif // PRODUCT

Mercurial > hg > truffle

comparison src/share/vm/runtime/mutex.cpp @ 0:a61af66fc99e jdk7-b24