--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/src/hotspot/share/runtime/mutex.cpp Tue Sep 12 19:03:39 2017 +0200
@@ -0,0 +1,1404 @@
+/*
+ * Copyright (c) 1998, 2017, Oracle and/or its affiliates. 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 Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ *
+ */
+
+#include "precompiled.hpp"
+#include "runtime/atomic.hpp"
+#include "runtime/interfaceSupport.hpp"
+#include "runtime/mutex.hpp"
+#include "runtime/orderAccess.inline.hpp"
+#include "runtime/osThread.hpp"
+#include "runtime/thread.inline.hpp"
+#include "utilities/events.hpp"
+#include "utilities/macros.hpp"
+
+// 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 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(ESelf)) == 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 -- contend for -- the lock.
+ // CONSIDER: use Self->OnDeck instead of m->OnDeck.
+ // Deschedule Self so that others may run.
+ while (OrderAccess::load_ptr_acquire(&_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;
+ // 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
+ // epilogue 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");
+ // Conceptually we need a MEMBAR #storestore|#loadstore barrier or fence immediately
+ // before the store that releases the lock. Crucially, all the stores and loads in the
+ // critical section must be globally visible before the store of 0 into the lock-word
+ // that releases the lock becomes globally visible. That is, memory accesses in the
+ // critical section should not be allowed to bypass or overtake the following ST that
+ // releases the lock. As such, to prevent accesses within the critical section
+ // from "leaking" out, we need a release fence between the critical section and the
+ // store that releases the lock. In practice that release barrier is elided on
+ // platforms with strong memory models such as TSO.
+ //
+ // Note that the OrderAccess::storeload() fence that appears after unlock store
+ // provides for progress conditions and succession and is _not related to exclusion
+ // safety or lock release consistency.
+ OrderAccess::release_store(&_LockWord.Bytes[_LSBINDEX], 0); // drop outer lock
+
+ OrderAccess::storeload();
+ ParkEvent * const w = _OnDeck; // raw load as we will just return if non-NULL
+ 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");
+
+ // Pass OnDeck role to w, ensuring that _EntryList has been set first.
+ // w will clear _OnDeck once it acquires the outer lock.
+ // Note that once we set _OnDeck that thread can acquire the mutex, proceed
+ // with its critical section and then enter this code to unlock the mutex. So
+ // you can have multiple threads active in IUnlock at the same time.
+ OrderAccess::release_store_ptr(&_OnDeck, w);
+
+ // 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 (OrderAccess::load_ptr_acquire(&_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) {
+ // Ensure that the Monitor requires/allows safepoint checks.
+ assert(_safepoint_check_required != Monitor::_safepoint_check_never,
+ "This lock should never have a safepoint check: %s", name());
+
+#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()) {
+ // Horrible 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) {
+ // Ensure that the Monitor does not require or allow safepoint checks.
+ assert(_safepoint_check_required != Monitor::_safepoint_check_always,
+ "This lock should always have a safepoint check: %s", name());
+ 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 succeeds 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 Thread::current_or_null()
+ // 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 = Thread::current_or_null();
+ 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 -- contend for -- the lock.
+ // CONSIDER: use Self->OnDeck instead of m->OnDeck.
+ for (;;) {
+ if (OrderAccess::load_ptr_acquire(&_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) {
+ // Make sure safepoint checking is used properly.
+ assert(!(_safepoint_check_required == Monitor::_safepoint_check_never && no_safepoint_check == false),
+ "This lock should never have a safepoint check: %s", name());
+ assert(!(_safepoint_check_required == Monitor::_safepoint_check_always && no_safepoint_check == true),
+ "This lock should always have a safepoint check: %s", name());
+
+ 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() {
+#ifdef ASSERT
+ uintptr_t owner = UNS(_owner);
+ uintptr_t lockword = UNS(_LockWord.FullWord);
+ uintptr_t entrylist = UNS(_EntryList);
+ uintptr_t waitset = UNS(_WaitSet);
+ uintptr_t ondeck = UNS(_OnDeck);
+ // Print _name with precision limit, in case failure is due to memory
+ // corruption that also trashed _name.
+ assert((owner|lockword|entrylist|waitset|ondeck) == 0,
+ "%.*s: _owner(" INTPTR_FORMAT ")|_LockWord(" INTPTR_FORMAT ")|_EntryList(" INTPTR_FORMAT ")|_WaitSet("
+ INTPTR_FORMAT ")|_OnDeck(" INTPTR_FORMAT ") != 0",
+ MONITOR_NAME_LEN, _name, owner, lockword, entrylist, waitset, ondeck);
+#endif
+}
+
+void Monitor::ClearMonitor(Monitor * m, const char *name) {
+ m->_owner = NULL;
+ m->_snuck = false;
+ if (name == NULL) {
+ strcpy(m->_name, "UNKNOWN");
+ } else {
+ strncpy(m->_name, name, MONITOR_NAME_LEN - 1);
+ m->_name[MONITOR_NAME_LEN - 1] = '\0';
+ }
+ 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,
+ SafepointCheckRequired safepoint_check_required) {
+ ClearMonitor(this, name);
+#ifdef ASSERT
+ _allow_vm_block = allow_vm_block;
+ _rank = Rank;
+ NOT_PRODUCT(_safepoint_check_required = safepoint_check_required;)
+#endif
+}
+
+Mutex::Mutex(int Rank, const char * name, bool allow_vm_block,
+ SafepointCheckRequired safepoint_check_required) {
+ ClearMonitor((Monitor *) this, name);
+#ifdef ASSERT
+ _allow_vm_block = allow_vm_block;
+ _rank = Rank;
+ NOT_PRODUCT(_safepoint_check_required = safepoint_check_required;)
+#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, p2i(this));
+ st->print("] %s", _name);
+ st->print(" - owner thread: " PTR_FORMAT, p2i(_owner));
+}
+
+
+
+
+// ----------------------------------------------------------------------------------
+// Non-product code
+
+#ifndef PRODUCT
+void Monitor::print_on(outputStream* st) const {
+ st->print_cr("Mutex: [" PTR_FORMAT "/" PTR_FORMAT "] %s - owner: " PTR_FORMAT,
+ p2i(this), _LockWord.FullWord, _name, p2i(_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");
+
+ // 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:
+ // . 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 == Safepoint_lock && contains(locks, Terminator_lock) &&
+ SafepointSynchronize::is_synchronizing())) {
+ new_owner->print_owned_locks();
+ fatal("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();
+
+ // 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()) {
+ fatal("VM thread using lock %s (not allowed to block on)", name());
+ }
+ debug_only(if (rank() != Mutex::special) \
+ thread->check_for_valid_safepoint_state(false);)
+ }
+ assert(!os::ThreadCrashProtection::is_crash_protected(thread),
+ "locking not allowed when crash protection is set");
+}
+
+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