diff -r 000000000000 -r f90c822e73f8 src/share/vm/runtime/mutex.cpp --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/src/share/vm/runtime/mutex.cpp Wed Apr 27 01:25:04 2016 +0800 @@ -0,0 +1,1390 @@ + +/* + * Copyright (c) 1998, 2014, 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/mutex.hpp" +#include "runtime/osThread.hpp" +#include "runtime/thread.inline.hpp" +#include "utilities/events.hpp" +#ifdef TARGET_OS_FAMILY_linux +# include "mutex_linux.inline.hpp" +#endif +#ifdef TARGET_OS_FAMILY_solaris +# include "mutex_solaris.inline.hpp" +#endif +#ifdef TARGET_OS_FAMILY_windows +# include "mutex_windows.inline.hpp" +#endif +#ifdef TARGET_OS_FAMILY_bsd +# include "mutex_bsd.inline.hpp" +#endif + +PRAGMA_FORMAT_MUTE_WARNINGS_FOR_GCC + +// 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") ; + // 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 ; + 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, 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) { + ClearMonitor (this, name) ; +#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, name) ; +#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"); + + // 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(); + fatal(err_msg("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(err_msg("VM thread using lock %s (not allowed to block on)", + name())); + } + debug_only(if (rank() != Mutex::special) \ + thread->check_for_valid_safepoint_state(false);) + } + if (thread->is_Watcher_thread()) { + assert(!WatcherThread::watcher_thread()->has_crash_protection(), + "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