--- a/src/hotspot/share/runtime/mutex.cpp Tue Feb 05 13:21:59 2019 -0500
+++ b/src/hotspot/share/runtime/mutex.cpp Tue Feb 05 15:12:13 2019 -0500
@@ -1,5 +1,5 @@
/*
- * Copyright (c) 1998, 2018, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1998, 2019, 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
@@ -23,915 +23,81 @@
*/
#include "precompiled.hpp"
-#include "runtime/atomic.hpp"
+#include "logging/log.hpp"
#include "runtime/interfaceSupport.inline.hpp"
#include "runtime/mutex.hpp"
-#include "runtime/orderAccess.hpp"
#include "runtime/osThread.hpp"
#include "runtime/safepointMechanism.inline.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
-#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); \
- } \
- }
-
-const intptr_t _LBIT = 1;
-
-// Endian-ness ... index of least-significant byte in SplitWord.Bytes[]
-#ifdef VM_LITTLE_ENDIAN
- #define _LSBINDEX 0
-#else
- #define _LSBINDEX (sizeof(intptr_t)-1)
-#endif
-
-// 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 = Atomic::cmpxchg(v|_LBIT, &_LockWord.FullWord, v);
- 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 = Atomic::cmpxchg(_LBIT, &_LockWord.FullWord, (intptr_t)0); // agro ...
- if (v == 0) return 1;
-
- for (;;) {
- if ((v & _LBIT) != 0) return 0;
- const intptr_t u = Atomic::cmpxchg(v|_LBIT, &_LockWord.FullWord, v);
- 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 SpinMax = 20;
- for (;;) {
- intptr_t v = _LockWord.FullWord;
- if ((v & _LBIT) == 0) {
- if (Atomic::cmpxchg (v|_LBIT, &_LockWord.FullWord, v) == v) {
- return 1;
- }
- continue;
- }
-
- 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 ;
- }
-
- // 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.
- if (Self != NULL) {
- jint rv = Self->rng[0];
- for (int k = Delay; --k >= 0;) {
- rv = MarsagliaXORV(rv);
- if (SafepointMechanism::should_block(Self)) return 0;
- }
- Self->rng[0] = rv;
- } else {
- Stall(Delay);
- }
- }
-}
-
-static int ParkCommon(ParkEvent * ev, jlong timo) {
- // Diagnostic support - periodically unwedge blocked threads
- 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 = Atomic::cmpxchg(v|_LBIT, &_LockWord.FullWord, v);
- if (u == v) return 1; // indicate acquired
- v = u;
- } else {
- // Anticipate success ...
- ESelf->ListNext = (ParkEvent *)(v & ~_LBIT);
- const intptr_t u = Atomic::cmpxchg(intptr_t(ESelf)|_LBIT, &_LockWord.FullWord, v);
- 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();
-
- 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_acquire(&_OnDeck) != ESelf) {
- ParkCommon(ESelf, 0);
- }
-
- // Self is now in the OnDeck position and will remain so until it
- // manages to acquire the lock.
- 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], jbyte(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).
- // 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 (!Atomic::replace_if_null((ParkEvent*)_LBIT, &_OnDeck)) {
- 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(intptr_t(_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(&_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 = Atomic::cmpxchg(cxq & _LBIT, &_LockWord.FullWord, cxq);
- 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(intptr_t(_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;
-
- // 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 (Atomic::cmpxchg(intptr_t(nfy)|_LBIT, &_LockWord.FullWord, v) == 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);
- 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) 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_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) {
+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();
+ // Clear unhandled oops in JavaThreads so we get a crash right away.
+ if (self->is_Java_thread()) {
+ self->clear_unhandled_oops();
}
#endif // CHECK_UNHANDLED_OOPS
- DEBUG_ONLY(check_prelock_state(Self, StrictSafepointChecks);)
- assert(_owner != Self, "invariant");
- assert(_OnDeck != Self->_MutexEvent, "invariant");
+ DEBUG_ONLY(check_prelock_state(self, StrictSafepointChecks));
+ assert(_owner != self, "invariant");
+
+ Monitor* in_flight_monitor = NULL;
+ DEBUG_ONLY(int retry_cnt = 0;)
+ while (!_lock.try_lock()) {
+ // The lock is contended
+
+ #ifdef ASSERT
+ check_block_state(self);
+ if (retry_cnt++ > 3) {
+ log_trace(vmmonitor)("JavaThread " INTPTR_FORMAT " on %d attempt trying to acquire vmmonitor %s", p2i(self), retry_cnt, _name);
+ }
+ #endif // ASSERT
- if (TryFast()) {
- Exeunt:
- assert(ILocked(), "invariant");
- assert(owner() == NULL, "invariant");
- set_owner(Self);
- return;
+ if (self->is_Java_thread()) {
+ assert(rank() > Mutex::special, "Potential deadlock with special or lesser rank mutex");
+ { ThreadBlockInVMWithDeadlockCheck tbivmdc((JavaThread *) self, &in_flight_monitor);
+ in_flight_monitor = this; // save for ~ThreadBlockInVMWithDeadlockCheck
+ _lock.lock();
+ }
+ if (in_flight_monitor != NULL) {
+ // Not unlocked by ~ThreadBlockInVMWithDeadlockCheck
+ break;
+ }
+ } else {
+ _lock.lock();
+ break;
+ }
}
- // 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;
-
- DEBUG_ONLY(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;
+ assert_owner(NULL);
+ set_owner(self);
}
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!
+// Lock without safepoint check - a degenerate variant of lock() for use by
+// JavaThreads when it is known to be safe to not check for a safepoint when
+// acquiring this lock. If the thread blocks acquiring the lock it is not
+// safepoint-safe and so will prevent a safepoint from being reached. If used
+// in the wrong way this can lead to a deadlock with the safepoint code.
-void Monitor::lock_without_safepoint_check(Thread * Self) {
- // Ensure that the Monitor does not require or allow safepoint checks.
+void Monitor::lock_without_safepoint_check(Thread * self) {
+ // Ensure that the Monitor does not require 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);
+ assert(_owner != self, "invariant");
+ _lock.lock();
+ assert_owner(NULL);
+ set_owner(self);
}
void Monitor::lock_without_safepoint_check() {
@@ -942,117 +108,36 @@
// 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, false);)
- // assert(!thread->is_inside_signal_handler(), "don't lock inside signal handler");
+ Thread * const self = Thread::current();
+ DEBUG_ONLY(check_prelock_state(self, false);)
- // 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);
+ if (_lock.try_lock()) {
+ assert_owner(NULL);
+ 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);
+void Monitor::release_for_safepoint() {
+ assert_owner(NULL);
+ _lock.unlock();
}
-// 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_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::unlock() {
+ assert_owner(Thread::current());
+ set_owner(NULL);
+ _lock.unlock();
}
-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);
+void Monitor::notify() {
+ assert_owner(Thread::current());
+ _lock.notify();
+}
+
+void Monitor::notify_all() {
+ assert_owner(Thread::current());
+ _lock.notify_all();
}
bool Monitor::wait(bool no_safepoint_check, long timeout,
@@ -1063,22 +148,24 @@
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");
+ // timeout is in milliseconds - with zero meaning never timeout
+ assert(timeout >= 0, "negative timeout");
+
+ Thread * const self = Thread::current();
+ assert_owner(self);
// 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");
+ guarantee(no_safepoint_check || self->is_Java_thread(), "invariant");
#ifdef ASSERT
- Monitor * least = get_least_ranked_lock_besides_this(Self->owned_locks());
+ 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());
+ " 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
@@ -1088,75 +175,79 @@
// abdicating the lock in wait
set_owner(NULL);
if (no_safepoint_check) {
- wait_status = IWait(Self, timeout);
+ wait_status = _lock.wait(timeout);
+ set_owner(self);
} else {
- assert(Self->is_Java_thread(), "invariant");
- JavaThread *jt = (JavaThread *)Self;
+ assert(self->is_Java_thread(), "invariant");
+ JavaThread *jt = (JavaThread *)self;
+ Monitor* in_flight_monitor = NULL;
- // Enter safepoint region - ornate and Rococo ...
- ThreadBlockInVM tbivm(jt);
- OSThreadWaitState osts(Self->osthread(), false /* not Object.wait() */);
+ {
+ ThreadBlockInVMWithDeadlockCheck tbivmdc(jt, &in_flight_monitor);
+ 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()
+ }
- if (as_suspend_equivalent) {
- jt->set_suspend_equivalent();
- // cleared by handle_special_suspend_equivalent_condition() or
- // java_suspend_self()
+ wait_status = _lock.wait(timeout);
+ in_flight_monitor = this; // save for ~ThreadBlockInVMWithDeadlockCheck
+
+ // 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.
+ _lock.unlock();
+ jt->java_suspend_self();
+ _lock.lock();
+ }
}
- 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");
+ if (in_flight_monitor != NULL) {
+ // Not unlocked by ~ThreadBlockInVMWithDeadlockCheck
+ assert_owner(NULL);
+ // Conceptually reestablish ownership of the lock.
+ set_owner(self);
+ } else {
+ lock(self);
}
}
-
- // 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
}
+
+// Temporary JVM_RawMonitor* support.
+// 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.
+
+void Monitor::jvm_raw_lock() {
+ _lock.lock();
+ assert_owner(NULL);
+}
+
+void Monitor::jvm_raw_unlock() {
+ assert_owner(NULL);
+ _lock.unlock();
+}
+
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
+ assert_owner(NULL);
}
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() {
@@ -1186,9 +277,7 @@
}
bool Monitor::owned_by_self() const {
- bool ret = _owner == Thread::current();
- assert(!ret || _LockWord.Bytes[_LSBINDEX] != 0, "invariant");
- return ret;
+ return _owner == Thread::current();
}
void Monitor::print_on_error(outputStream* st) const {
@@ -1197,21 +286,32 @@
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));
+ st->print_cr("Mutex: [" PTR_FORMAT "] %s - owner: " PTR_FORMAT,
+ p2i(this), _name, p2i(_owner));
}
#endif
#ifndef PRODUCT
#ifdef ASSERT
+
+void Monitor::assert_owner(Thread * expected) {
+ const char* msg = "invalid owner";
+ if (expected == NULL) {
+ msg = "should be un-owned";
+ }
+ else if (expected == Thread::current()) {
+ msg = "should be owned by current thread";
+ }
+ assert(_owner == expected,
+ "%s: owner=" INTPTR_FORMAT ", should be=" INTPTR_FORMAT,
+ msg, p2i(_owner), p2i(expected));
+}
+
Monitor * Monitor::get_least_ranked_lock(Monitor * locks) {
Monitor *res, *tmp;
for (res = tmp = locks; tmp != NULL; tmp = tmp->next()) {
@@ -1297,8 +397,8 @@
// 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.
+ // 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: