1499 lines
55 KiB
Java
1499 lines
55 KiB
Java
/*
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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*
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* This code is free software; you can redistribute it and/or modify it
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* under the terms of the GNU General Public License version 2 only, as
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* published by the Free Software Foundation. Oracle designates this
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* particular file as subject to the "Classpath" exception as provided
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* by Oracle in the LICENSE file that accompanied this code.
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*
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* This code is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* version 2 for more details (a copy is included in the LICENSE file that
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* accompanied this code).
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*
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* You should have received a copy of the GNU General Public License version
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* 2 along with this work; if not, write to the Free Software Foundation,
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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*
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* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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* or visit www.oracle.com if you need additional information or have any
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* questions.
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*/
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/*
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* This file is available under and governed by the GNU General Public
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* License version 2 only, as published by the Free Software Foundation.
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* However, the following notice accompanied the original version of this
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* file:
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*
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* Written by Doug Lea with assistance from members of JCP JSR-166
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* Expert Group and released to the public domain, as explained at
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* http://creativecommons.org/publicdomain/zero/1.0/
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*/
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package java.util.concurrent.locks;
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import java.util.concurrent.TimeUnit;
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import jdk.internal.misc.Unsafe;
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import jdk.internal.vm.annotation.ReservedStackAccess;
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/**
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* A capability-based lock with three modes for controlling read/write
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* access. The state of a StampedLock consists of a version and mode.
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* Lock acquisition methods return a stamp that represents and
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* controls access with respect to a lock state; "try" versions of
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* these methods may instead return the special value zero to
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* represent failure to acquire access. Lock release and conversion
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* methods require stamps as arguments, and fail if they do not match
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* the state of the lock. The three modes are:
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*
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* <ul>
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*
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* <li><b>Writing.</b> Method {@link #writeLock} possibly blocks
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* waiting for exclusive access, returning a stamp that can be used
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* in method {@link #unlockWrite} to release the lock. Untimed and
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* timed versions of {@code tryWriteLock} are also provided. When
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* the lock is held in write mode, no read locks may be obtained,
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* and all optimistic read validations will fail.
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*
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* <li><b>Reading.</b> Method {@link #readLock} possibly blocks
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* waiting for non-exclusive access, returning a stamp that can be
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* used in method {@link #unlockRead} to release the lock. Untimed
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* and timed versions of {@code tryReadLock} are also provided.
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*
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* <li><b>Optimistic Reading.</b> Method {@link #tryOptimisticRead}
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* returns a non-zero stamp only if the lock is not currently held in
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* write mode. Method {@link #validate} returns true if the lock has not
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* been acquired in write mode since obtaining a given stamp, in which
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* case all actions prior to the most recent write lock release
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* happen-before actions following the call to {@code tryOptimisticRead}.
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* This mode can be thought of as an extremely weak version of a
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* read-lock, that can be broken by a writer at any time. The use of
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* optimistic read mode for short read-only code segments often reduces
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* contention and improves throughput. However, its use is inherently
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* fragile. Optimistic read sections should only read fields and hold
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* them in local variables for later use after validation. Fields read
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* while in optimistic read mode may be wildly inconsistent, so usage
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* applies only when you are familiar enough with data representations to
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* check consistency and/or repeatedly invoke method {@code validate()}.
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* For example, such steps are typically required when first reading an
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* object or array reference, and then accessing one of its fields,
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* elements or methods.
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*
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* </ul>
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*
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* <p>This class also supports methods that conditionally provide
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* conversions across the three modes. For example, method {@link
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* #tryConvertToWriteLock} attempts to "upgrade" a mode, returning
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* a valid write stamp if (1) already in writing mode (2) in reading
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* mode and there are no other readers or (3) in optimistic read mode
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* and the lock is available. The forms of these methods are designed to
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* help reduce some of the code bloat that otherwise occurs in
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* retry-based designs.
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*
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* <p>StampedLocks are designed for use as internal utilities in the
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* development of thread-safe components. Their use relies on
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* knowledge of the internal properties of the data, objects, and
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* methods they are protecting. They are not reentrant, so locked
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* bodies should not call other unknown methods that may try to
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* re-acquire locks (although you may pass a stamp to other methods
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* that can use or convert it). The use of read lock modes relies on
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* the associated code sections being side-effect-free. Unvalidated
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* optimistic read sections cannot call methods that are not known to
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* tolerate potential inconsistencies. Stamps use finite
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* representations, and are not cryptographically secure (i.e., a
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* valid stamp may be guessable). Stamp values may recycle after (no
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* sooner than) one year of continuous operation. A stamp held without
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* use or validation for longer than this period may fail to validate
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* correctly. StampedLocks are serializable, but always deserialize
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* into initial unlocked state, so they are not useful for remote
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* locking.
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*
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* <p>Like {@link java.util.concurrent.Semaphore Semaphore}, but unlike most
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* {@link Lock} implementations, StampedLocks have no notion of ownership.
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* Locks acquired in one thread can be released or converted in another.
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*
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* <p>The scheduling policy of StampedLock does not consistently
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* prefer readers over writers or vice versa. All "try" methods are
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* best-effort and do not necessarily conform to any scheduling or
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* fairness policy. A zero return from any "try" method for acquiring
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* or converting locks does not carry any information about the state
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* of the lock; a subsequent invocation may succeed.
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*
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* <p>Because it supports coordinated usage across multiple lock
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* modes, this class does not directly implement the {@link Lock} or
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* {@link ReadWriteLock} interfaces. However, a StampedLock may be
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* viewed {@link #asReadLock()}, {@link #asWriteLock()}, or {@link
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* #asReadWriteLock()} in applications requiring only the associated
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* set of functionality.
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*
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* <p><b>Memory Synchronization.</b> Methods with the effect of
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* successfully locking in any mode have the same memory
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* synchronization effects as a <em>Lock</em> action, as described in
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* Chapter 17 of <cite>The Java Language Specification</cite>.
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* Methods successfully unlocking in write mode have the same memory
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* synchronization effects as an <em>Unlock</em> action. In optimistic
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* read usages, actions prior to the most recent write mode unlock action
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* are guaranteed to happen-before those following a tryOptimisticRead
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* only if a later validate returns true; otherwise there is no guarantee
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* that the reads between tryOptimisticRead and validate obtain a
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* consistent snapshot.
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*
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* <p><b>Sample Usage.</b> The following illustrates some usage idioms
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* in a class that maintains simple two-dimensional points. The sample
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* code illustrates some try/catch conventions even though they are
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* not strictly needed here because no exceptions can occur in their
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* bodies.
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*
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* <pre> {@code
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* class Point {
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* private double x, y;
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* private final StampedLock sl = new StampedLock();
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*
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* // an exclusively locked method
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* void move(double deltaX, double deltaY) {
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* long stamp = sl.writeLock();
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* try {
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* x += deltaX;
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* y += deltaY;
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* } finally {
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* sl.unlockWrite(stamp);
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* }
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* }
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*
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* // a read-only method
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* // upgrade from optimistic read to read lock
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* double distanceFromOrigin() {
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* long stamp = sl.tryOptimisticRead();
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* try {
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* retryHoldingLock: for (;; stamp = sl.readLock()) {
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* if (stamp == 0L)
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* continue retryHoldingLock;
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* // possibly racy reads
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* double currentX = x;
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* double currentY = y;
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* if (!sl.validate(stamp))
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* continue retryHoldingLock;
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* return Math.hypot(currentX, currentY);
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* }
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* } finally {
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* if (StampedLock.isReadLockStamp(stamp))
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* sl.unlockRead(stamp);
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* }
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* }
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*
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* // upgrade from optimistic read to write lock
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* void moveIfAtOrigin(double newX, double newY) {
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* long stamp = sl.tryOptimisticRead();
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* try {
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* retryHoldingLock: for (;; stamp = sl.writeLock()) {
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* if (stamp == 0L)
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* continue retryHoldingLock;
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* // possibly racy reads
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* double currentX = x;
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* double currentY = y;
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* if (!sl.validate(stamp))
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* continue retryHoldingLock;
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* if (currentX != 0.0 || currentY != 0.0)
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* break;
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* stamp = sl.tryConvertToWriteLock(stamp);
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* if (stamp == 0L)
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* continue retryHoldingLock;
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* // exclusive access
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* x = newX;
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* y = newY;
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* return;
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* }
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* } finally {
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* if (StampedLock.isWriteLockStamp(stamp))
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* sl.unlockWrite(stamp);
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* }
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* }
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*
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* // upgrade read lock to write lock
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* void moveIfAtOrigin2(double newX, double newY) {
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* long stamp = sl.readLock();
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* try {
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* while (x == 0.0 && y == 0.0) {
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* long ws = sl.tryConvertToWriteLock(stamp);
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* if (ws != 0L) {
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* stamp = ws;
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* x = newX;
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* y = newY;
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* break;
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* }
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* else {
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* sl.unlockRead(stamp);
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* stamp = sl.writeLock();
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* }
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* }
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* } finally {
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* sl.unlock(stamp);
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* }
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* }
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* }}</pre>
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*
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* @jls 17.4 Memory Model
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* @since 1.8
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* @author Doug Lea
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*/
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public class StampedLock implements java.io.Serializable {
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/*
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* Algorithmic notes:
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*
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* The design employs elements of Sequence locks
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* (as used in linux kernels; see Lameter's
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* http://www.lameter.com/gelato2005.pdf
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* and elsewhere; see
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* Boehm's http://www.hpl.hp.com/techreports/2012/HPL-2012-68.html)
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* and Ordered RW locks (see Shirako et al
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* http://dl.acm.org/citation.cfm?id=2312015)
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*
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* Conceptually, the primary state of the lock includes a sequence
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* number that is odd when write-locked and even otherwise.
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* However, this is offset by a reader count that is non-zero when
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* read-locked. The read count is ignored when validating
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* "optimistic" seqlock-reader-style stamps. Because we must use
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* a small finite number of bits (currently 7) for readers, a
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* supplementary reader overflow word is used when the number of
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* readers exceeds the count field. We do this by treating the max
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* reader count value (RBITS) as a spinlock protecting overflow
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* updates.
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*
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* Waiters use a modified form of CLH lock used in
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* AbstractQueuedSynchronizer (AQS; see its internal documentation
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* for a fuller account), where each node is either a ReaderNode
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* or WriterNode. Implementation of queued Writer mode is
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* identical to AQS except for lock-state operations. Sets of
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* waiting readers are grouped (linked) under a common node (field
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* cowaiters) so act as a single node with respect to most CLH
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* mechanics. This simplifies the scheduling policy to a
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* mainly-FIFO scheme that incorporates elements of Phase-Fair
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* locks (see Brandenburg & Anderson, especially
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* http://www.cs.unc.edu/~bbb/diss/). Method release does not
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* itself wake up cowaiters. This is done by the primary thread,
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* but helped by other cowaiters as they awaken.
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*
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* These rules apply to threads actually queued. Threads may also
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* try to acquire locks before or in the process of enqueueing
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* regardless of preference rules, and so may "barge" their way
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* in. Methods writeLock and readLock (but not the other variants
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* of each) first unconditionally try to CAS state, falling back
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* to test-and-test-and-set retries on failure, slightly shrinking
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* race windows on initial attempts, thus making success more
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* likely. Also, when some threads cancel (via interrupt or
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* timeout), phase-fairness is at best roughly approximated.
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*
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* Nearly all of these mechanics are carried out in methods
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* acquireWrite and acquireRead, that, as typical of such code,
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* sprawl out because actions and retries rely on consistent sets
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* of locally cached reads.
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*
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* For an explanation of the use of acquireFence, see
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* http://gee.cs.oswego.edu/dl/html/j9mm.html as well as Boehm's
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* paper (above). Note that sequence validation (mainly method
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* validate()) requires stricter ordering rules than apply to
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* normal volatile reads (of "state"). To ensure that writeLock
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* acquisitions strictly precede subsequent writes in cases where
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* this is not already forced, we use a storeStoreFence.
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*
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* The memory layout keeps lock state and queue pointers together
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* (normally on the same cache line). This usually works well for
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* read-mostly loads. In most other cases, the natural tendency of
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* CLH locks to reduce memory contention lessens motivation to
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* further spread out contended locations, but might be subject to
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* future improvements.
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*/
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private static final long serialVersionUID = -6001602636862214147L;
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/** The number of bits to use for reader count before overflowing */
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private static final int LG_READERS = 7; // 127 readers
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// Values for lock state and stamp operations
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private static final long RUNIT = 1L;
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private static final long WBIT = 1L << LG_READERS;
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private static final long RBITS = WBIT - 1L;
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private static final long RFULL = RBITS - 1L;
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private static final long ABITS = RBITS | WBIT;
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private static final long SBITS = ~RBITS; // note overlap with ABITS
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// not writing and conservatively non-overflowing
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private static final long RSAFE = ~(3L << (LG_READERS - 1));
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/*
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* 3 stamp modes can be distinguished by examining (m = stamp & ABITS):
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* write mode: m == WBIT
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* optimistic read mode: m == 0L (even when read lock is held)
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* read mode: m > 0L && m <= RFULL (the stamp is a copy of state, but the
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* read hold count in the stamp is unused other than to determine mode)
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*
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* This differs slightly from the encoding of state:
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* (state & ABITS) == 0L indicates the lock is currently unlocked.
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* (state & ABITS) == RBITS is a special transient value
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* indicating spin-locked to manipulate reader bits overflow.
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*/
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/** Initial value for lock state; avoids failure value zero. */
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private static final long ORIGIN = WBIT << 1;
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// Special value from cancelled acquire methods so caller can throw IE
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private static final long INTERRUPTED = 1L;
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// Bits for Node.status
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static final int WAITING = 1;
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static final int CANCELLED = 0x80000000; // must be negative
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/** CLH nodes */
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abstract static class Node {
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volatile Node prev; // initially attached via casTail
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volatile Node next; // visibly nonnull when signallable
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Thread waiter; // visibly nonnull when enqueued
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volatile int status; // written by owner, atomic bit ops by others
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// methods for atomic operations
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final boolean casPrev(Node c, Node v) { // for cleanQueue
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return U.weakCompareAndSetReference(this, PREV, c, v);
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}
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final boolean casNext(Node c, Node v) { // for cleanQueue
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return U.weakCompareAndSetReference(this, NEXT, c, v);
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}
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final int getAndUnsetStatus(int v) { // for signalling
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return U.getAndBitwiseAndInt(this, STATUS, ~v);
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}
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final void setPrevRelaxed(Node p) { // for off-queue assignment
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U.putReference(this, PREV, p);
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}
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final void setStatusRelaxed(int s) { // for off-queue assignment
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U.putInt(this, STATUS, s);
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}
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final void clearStatus() { // for reducing unneeded signals
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U.putIntOpaque(this, STATUS, 0);
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}
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private static final long STATUS
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= U.objectFieldOffset(Node.class, "status");
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private static final long NEXT
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= U.objectFieldOffset(Node.class, "next");
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private static final long PREV
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= U.objectFieldOffset(Node.class, "prev");
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}
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static final class WriterNode extends Node { // node for writers
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}
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static final class ReaderNode extends Node { // node for readers
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volatile ReaderNode cowaiters; // list of linked readers
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final boolean casCowaiters(ReaderNode c, ReaderNode v) {
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return U.weakCompareAndSetReference(this, COWAITERS, c, v);
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}
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final void setCowaitersRelaxed(ReaderNode p) {
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U.putReference(this, COWAITERS, p);
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}
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private static final long COWAITERS
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= U.objectFieldOffset(ReaderNode.class, "cowaiters");
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}
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/** Head of CLH queue */
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private transient volatile Node head;
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/** Tail (last) of CLH queue */
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private transient volatile Node tail;
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// views
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transient ReadLockView readLockView;
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transient WriteLockView writeLockView;
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transient ReadWriteLockView readWriteLockView;
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/** Lock sequence/state */
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private transient volatile long state;
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/** extra reader count when state read count saturated */
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private transient int readerOverflow;
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/**
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* Creates a new lock, initially in unlocked state.
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*/
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public StampedLock() {
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state = ORIGIN;
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}
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// internal lock methods
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private boolean casState(long expect, long update) {
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return U.compareAndSetLong(this, STATE, expect, update);
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}
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@ReservedStackAccess
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private long tryAcquireWrite() {
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long s, nextState;
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if (((s = state) & ABITS) == 0L && casState(s, nextState = s | WBIT)) {
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U.storeStoreFence();
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return nextState;
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}
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return 0L;
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}
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@ReservedStackAccess
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private long tryAcquireRead() {
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for (long s, m, nextState;;) {
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if ((m = (s = state) & ABITS) < RFULL) {
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if (casState(s, nextState = s + RUNIT))
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return nextState;
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}
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else if (m == WBIT)
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return 0L;
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else if ((nextState = tryIncReaderOverflow(s)) != 0L)
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return nextState;
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}
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}
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|
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/**
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* Returns an unlocked state, incrementing the version and
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* avoiding special failure value 0L.
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*
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* @param s a write-locked state (or stamp)
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*/
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private static long unlockWriteState(long s) {
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return ((s += WBIT) == 0L) ? ORIGIN : s;
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}
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private long releaseWrite(long s) {
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long nextState = state = unlockWriteState(s);
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signalNext(head);
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return nextState;
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}
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|
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/**
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* Exclusively acquires the lock, blocking if necessary
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|
* until available.
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*
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* @return a write stamp that can be used to unlock or convert mode
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*/
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@ReservedStackAccess
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public long writeLock() {
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|
// try unconditional CAS confirming weak read
|
|
long s = U.getLongOpaque(this, STATE) & ~ABITS, nextState;
|
|
if (casState(s, nextState = s | WBIT)) {
|
|
U.storeStoreFence();
|
|
return nextState;
|
|
}
|
|
return acquireWrite(false, false, 0L);
|
|
}
|
|
|
|
/**
|
|
* Exclusively acquires the lock if it is immediately available.
|
|
*
|
|
* @return a write stamp that can be used to unlock or convert mode,
|
|
* or zero if the lock is not available
|
|
*/
|
|
public long tryWriteLock() {
|
|
return tryAcquireWrite();
|
|
}
|
|
|
|
/**
|
|
* Exclusively acquires the lock if it is available within the
|
|
* given time and the current thread has not been interrupted.
|
|
* Behavior under timeout and interruption matches that specified
|
|
* for method {@link Lock#tryLock(long,TimeUnit)}.
|
|
*
|
|
* @param time the maximum time to wait for the lock
|
|
* @param unit the time unit of the {@code time} argument
|
|
* @return a write stamp that can be used to unlock or convert mode,
|
|
* or zero if the lock is not available
|
|
* @throws InterruptedException if the current thread is interrupted
|
|
* before acquiring the lock
|
|
*/
|
|
public long tryWriteLock(long time, TimeUnit unit)
|
|
throws InterruptedException {
|
|
long nanos = unit.toNanos(time);
|
|
if (!Thread.interrupted()) {
|
|
long nextState;
|
|
if ((nextState = tryAcquireWrite()) != 0L)
|
|
return nextState;
|
|
if (nanos <= 0L)
|
|
return 0L;
|
|
nextState = acquireWrite(true, true, System.nanoTime() + nanos);
|
|
if (nextState != INTERRUPTED)
|
|
return nextState;
|
|
}
|
|
throw new InterruptedException();
|
|
}
|
|
|
|
/**
|
|
* Exclusively acquires the lock, blocking if necessary
|
|
* until available or the current thread is interrupted.
|
|
* Behavior under interruption matches that specified
|
|
* for method {@link Lock#lockInterruptibly()}.
|
|
*
|
|
* @return a write stamp that can be used to unlock or convert mode
|
|
* @throws InterruptedException if the current thread is interrupted
|
|
* before acquiring the lock
|
|
*/
|
|
public long writeLockInterruptibly() throws InterruptedException {
|
|
long nextState;
|
|
if (!Thread.interrupted() &&
|
|
((nextState = tryAcquireWrite()) != 0L ||
|
|
(nextState = acquireWrite(true, false, 0L)) != INTERRUPTED))
|
|
return nextState;
|
|
throw new InterruptedException();
|
|
}
|
|
|
|
/**
|
|
* Non-exclusively acquires the lock, blocking if necessary
|
|
* until available.
|
|
*
|
|
* @return a read stamp that can be used to unlock or convert mode
|
|
*/
|
|
@ReservedStackAccess
|
|
public long readLock() {
|
|
// unconditionally optimistically try non-overflow case once
|
|
long s = U.getLongOpaque(this, STATE) & RSAFE, nextState;
|
|
if (casState(s, nextState = s + RUNIT))
|
|
return nextState;
|
|
else
|
|
return acquireRead(false, false, 0L);
|
|
}
|
|
|
|
/**
|
|
* Non-exclusively acquires the lock if it is immediately available.
|
|
*
|
|
* @return a read stamp that can be used to unlock or convert mode,
|
|
* or zero if the lock is not available
|
|
*/
|
|
public long tryReadLock() {
|
|
return tryAcquireRead();
|
|
}
|
|
|
|
/**
|
|
* Non-exclusively acquires the lock if it is available within the
|
|
* given time and the current thread has not been interrupted.
|
|
* Behavior under timeout and interruption matches that specified
|
|
* for method {@link Lock#tryLock(long,TimeUnit)}.
|
|
*
|
|
* @param time the maximum time to wait for the lock
|
|
* @param unit the time unit of the {@code time} argument
|
|
* @return a read stamp that can be used to unlock or convert mode,
|
|
* or zero if the lock is not available
|
|
* @throws InterruptedException if the current thread is interrupted
|
|
* before acquiring the lock
|
|
*/
|
|
public long tryReadLock(long time, TimeUnit unit)
|
|
throws InterruptedException {
|
|
long nanos = unit.toNanos(time);
|
|
if (!Thread.interrupted()) {
|
|
long nextState;
|
|
if (tail == head && (nextState = tryAcquireRead()) != 0L)
|
|
return nextState;
|
|
if (nanos <= 0L)
|
|
return 0L;
|
|
nextState = acquireRead(true, true, System.nanoTime() + nanos);
|
|
if (nextState != INTERRUPTED)
|
|
return nextState;
|
|
}
|
|
throw new InterruptedException();
|
|
}
|
|
|
|
/**
|
|
* Non-exclusively acquires the lock, blocking if necessary
|
|
* until available or the current thread is interrupted.
|
|
* Behavior under interruption matches that specified
|
|
* for method {@link Lock#lockInterruptibly()}.
|
|
*
|
|
* @return a read stamp that can be used to unlock or convert mode
|
|
* @throws InterruptedException if the current thread is interrupted
|
|
* before acquiring the lock
|
|
*/
|
|
public long readLockInterruptibly() throws InterruptedException {
|
|
long nextState;
|
|
if (!Thread.interrupted() &&
|
|
((nextState = tryAcquireRead()) != 0L ||
|
|
(nextState = acquireRead(true, false, 0L)) != INTERRUPTED))
|
|
return nextState;
|
|
throw new InterruptedException();
|
|
}
|
|
|
|
/**
|
|
* Returns a stamp that can later be validated, or zero
|
|
* if exclusively locked.
|
|
*
|
|
* @return a valid optimistic read stamp, or zero if exclusively locked
|
|
*/
|
|
public long tryOptimisticRead() {
|
|
long s;
|
|
return (((s = state) & WBIT) == 0L) ? (s & SBITS) : 0L;
|
|
}
|
|
|
|
/**
|
|
* Returns true if the lock has not been exclusively acquired
|
|
* since issuance of the given stamp. Always returns false if the
|
|
* stamp is zero. Always returns true if the stamp represents a
|
|
* currently held lock. Invoking this method with a value not
|
|
* obtained from {@link #tryOptimisticRead} or a locking method
|
|
* for this lock has no defined effect or result.
|
|
*
|
|
* @param stamp a stamp
|
|
* @return {@code true} if the lock has not been exclusively acquired
|
|
* since issuance of the given stamp; else false
|
|
*/
|
|
public boolean validate(long stamp) {
|
|
U.loadFence();
|
|
return (stamp & SBITS) == (state & SBITS);
|
|
}
|
|
|
|
/**
|
|
* If the lock state matches the given stamp, releases the
|
|
* exclusive lock.
|
|
*
|
|
* @param stamp a stamp returned by a write-lock operation
|
|
* @throws IllegalMonitorStateException if the stamp does
|
|
* not match the current state of this lock
|
|
*/
|
|
@ReservedStackAccess
|
|
public void unlockWrite(long stamp) {
|
|
if (state != stamp || (stamp & WBIT) == 0L)
|
|
throw new IllegalMonitorStateException();
|
|
releaseWrite(stamp);
|
|
}
|
|
|
|
/**
|
|
* If the lock state matches the given stamp, releases the
|
|
* non-exclusive lock.
|
|
*
|
|
* @param stamp a stamp returned by a read-lock operation
|
|
* @throws IllegalMonitorStateException if the stamp does
|
|
* not match the current state of this lock
|
|
*/
|
|
@ReservedStackAccess
|
|
public void unlockRead(long stamp) {
|
|
long s, m;
|
|
if ((stamp & RBITS) != 0L) {
|
|
while (((s = state) & SBITS) == (stamp & SBITS) &&
|
|
((m = s & RBITS) != 0L)) {
|
|
if (m < RFULL) {
|
|
if (casState(s, s - RUNIT)) {
|
|
if (m == RUNIT)
|
|
signalNext(head);
|
|
return;
|
|
}
|
|
}
|
|
else if (tryDecReaderOverflow(s) != 0L)
|
|
return;
|
|
}
|
|
}
|
|
throw new IllegalMonitorStateException();
|
|
}
|
|
|
|
/**
|
|
* If the lock state matches the given stamp, releases the
|
|
* corresponding mode of the lock.
|
|
*
|
|
* @param stamp a stamp returned by a lock operation
|
|
* @throws IllegalMonitorStateException if the stamp does
|
|
* not match the current state of this lock
|
|
*/
|
|
public void unlock(long stamp) {
|
|
if ((stamp & WBIT) != 0L)
|
|
unlockWrite(stamp);
|
|
else
|
|
unlockRead(stamp);
|
|
}
|
|
|
|
/**
|
|
* If the lock state matches the given stamp, atomically performs one of
|
|
* the following actions. If the stamp represents holding a write
|
|
* lock, returns it. Or, if a read lock, if the write lock is
|
|
* available, releases the read lock and returns a write stamp.
|
|
* Or, if an optimistic read, returns a write stamp only if
|
|
* immediately available. This method returns zero in all other
|
|
* cases.
|
|
*
|
|
* @param stamp a stamp
|
|
* @return a valid write stamp, or zero on failure
|
|
*/
|
|
public long tryConvertToWriteLock(long stamp) {
|
|
long a = stamp & ABITS, m, s, nextState;
|
|
while (((s = state) & SBITS) == (stamp & SBITS)) {
|
|
if ((m = s & ABITS) == 0L) {
|
|
if (a != 0L)
|
|
break;
|
|
if (casState(s, nextState = s | WBIT)) {
|
|
U.storeStoreFence();
|
|
return nextState;
|
|
}
|
|
} else if (m == WBIT) {
|
|
if (a != m)
|
|
break;
|
|
return stamp;
|
|
} else if (m == RUNIT && a != 0L) {
|
|
if (casState(s, nextState = s - RUNIT + WBIT))
|
|
return nextState;
|
|
} else
|
|
break;
|
|
}
|
|
return 0L;
|
|
}
|
|
|
|
/**
|
|
* If the lock state matches the given stamp, atomically performs one of
|
|
* the following actions. If the stamp represents holding a write
|
|
* lock, releases it and obtains a read lock. Or, if a read lock,
|
|
* returns it. Or, if an optimistic read, acquires a read lock and
|
|
* returns a read stamp only if immediately available. This method
|
|
* returns zero in all other cases.
|
|
*
|
|
* @param stamp a stamp
|
|
* @return a valid read stamp, or zero on failure
|
|
*/
|
|
public long tryConvertToReadLock(long stamp) {
|
|
long a, s, nextState;
|
|
while (((s = state) & SBITS) == (stamp & SBITS)) {
|
|
if ((a = stamp & ABITS) >= WBIT) {
|
|
if (s != stamp) // write stamp
|
|
break;
|
|
nextState = state = unlockWriteState(s) + RUNIT;
|
|
signalNext(head);
|
|
return nextState;
|
|
} else if (a == 0L) { // optimistic read stamp
|
|
if ((s & ABITS) < RFULL) {
|
|
if (casState(s, nextState = s + RUNIT))
|
|
return nextState;
|
|
} else if ((nextState = tryIncReaderOverflow(s)) != 0L)
|
|
return nextState;
|
|
} else { // already a read stamp
|
|
if ((s & ABITS) == 0L)
|
|
break;
|
|
return stamp;
|
|
}
|
|
}
|
|
return 0L;
|
|
}
|
|
|
|
/**
|
|
* If the lock state matches the given stamp then, atomically, if the stamp
|
|
* represents holding a lock, releases it and returns an
|
|
* observation stamp. Or, if an optimistic read, returns it if
|
|
* validated. This method returns zero in all other cases, and so
|
|
* may be useful as a form of "tryUnlock".
|
|
*
|
|
* @param stamp a stamp
|
|
* @return a valid optimistic read stamp, or zero on failure
|
|
*/
|
|
public long tryConvertToOptimisticRead(long stamp) {
|
|
long a, m, s, nextState;
|
|
U.loadFence();
|
|
while (((s = state) & SBITS) == (stamp & SBITS)) {
|
|
if ((a = stamp & ABITS) >= WBIT) {
|
|
if (s != stamp) // write stamp
|
|
break;
|
|
return releaseWrite(s);
|
|
} else if (a == 0L) { // already an optimistic read stamp
|
|
return stamp;
|
|
} else if ((m = s & ABITS) == 0L) { // invalid read stamp
|
|
break;
|
|
} else if (m < RFULL) {
|
|
if (casState(s, nextState = s - RUNIT)) {
|
|
if (m == RUNIT)
|
|
signalNext(head);
|
|
return nextState & SBITS;
|
|
}
|
|
} else if ((nextState = tryDecReaderOverflow(s)) != 0L)
|
|
return nextState & SBITS;
|
|
}
|
|
return 0L;
|
|
}
|
|
|
|
/**
|
|
* Releases the write lock if it is held, without requiring a
|
|
* stamp value. This method may be useful for recovery after
|
|
* errors.
|
|
*
|
|
* @return {@code true} if the lock was held, else false
|
|
*/
|
|
@ReservedStackAccess
|
|
public boolean tryUnlockWrite() {
|
|
long s;
|
|
if (((s = state) & WBIT) != 0L) {
|
|
releaseWrite(s);
|
|
return true;
|
|
}
|
|
return false;
|
|
}
|
|
|
|
/**
|
|
* Releases one hold of the read lock if it is held, without
|
|
* requiring a stamp value. This method may be useful for recovery
|
|
* after errors.
|
|
*
|
|
* @return {@code true} if the read lock was held, else false
|
|
*/
|
|
@ReservedStackAccess
|
|
public boolean tryUnlockRead() {
|
|
long s, m;
|
|
while ((m = (s = state) & ABITS) != 0L && m < WBIT) {
|
|
if (m < RFULL) {
|
|
if (casState(s, s - RUNIT)) {
|
|
if (m == RUNIT)
|
|
signalNext(head);
|
|
return true;
|
|
}
|
|
}
|
|
else if (tryDecReaderOverflow(s) != 0L)
|
|
return true;
|
|
}
|
|
return false;
|
|
}
|
|
|
|
// status monitoring methods
|
|
|
|
/**
|
|
* Returns combined state-held and overflow read count for given
|
|
* state s.
|
|
*/
|
|
private int getReadLockCount(long s) {
|
|
long readers;
|
|
if ((readers = s & RBITS) >= RFULL)
|
|
readers = RFULL + readerOverflow;
|
|
return (int) readers;
|
|
}
|
|
|
|
/**
|
|
* Returns {@code true} if the lock is currently held exclusively.
|
|
*
|
|
* @return {@code true} if the lock is currently held exclusively
|
|
*/
|
|
public boolean isWriteLocked() {
|
|
return (state & WBIT) != 0L;
|
|
}
|
|
|
|
/**
|
|
* Returns {@code true} if the lock is currently held non-exclusively.
|
|
*
|
|
* @return {@code true} if the lock is currently held non-exclusively
|
|
*/
|
|
public boolean isReadLocked() {
|
|
return (state & RBITS) != 0L;
|
|
}
|
|
|
|
/**
|
|
* Tells whether a stamp represents holding a lock exclusively.
|
|
* This method may be useful in conjunction with
|
|
* {@link #tryConvertToWriteLock}, for example: <pre> {@code
|
|
* long stamp = sl.tryOptimisticRead();
|
|
* try {
|
|
* ...
|
|
* stamp = sl.tryConvertToWriteLock(stamp);
|
|
* ...
|
|
* } finally {
|
|
* if (StampedLock.isWriteLockStamp(stamp))
|
|
* sl.unlockWrite(stamp);
|
|
* }}</pre>
|
|
*
|
|
* @param stamp a stamp returned by a previous StampedLock operation
|
|
* @return {@code true} if the stamp was returned by a successful
|
|
* write-lock operation
|
|
* @since 10
|
|
*/
|
|
public static boolean isWriteLockStamp(long stamp) {
|
|
return (stamp & ABITS) == WBIT;
|
|
}
|
|
|
|
/**
|
|
* Tells whether a stamp represents holding a lock non-exclusively.
|
|
* This method may be useful in conjunction with
|
|
* {@link #tryConvertToReadLock}, for example: <pre> {@code
|
|
* long stamp = sl.tryOptimisticRead();
|
|
* try {
|
|
* ...
|
|
* stamp = sl.tryConvertToReadLock(stamp);
|
|
* ...
|
|
* } finally {
|
|
* if (StampedLock.isReadLockStamp(stamp))
|
|
* sl.unlockRead(stamp);
|
|
* }}</pre>
|
|
*
|
|
* @param stamp a stamp returned by a previous StampedLock operation
|
|
* @return {@code true} if the stamp was returned by a successful
|
|
* read-lock operation
|
|
* @since 10
|
|
*/
|
|
public static boolean isReadLockStamp(long stamp) {
|
|
return (stamp & RBITS) != 0L;
|
|
}
|
|
|
|
/**
|
|
* Tells whether a stamp represents holding a lock.
|
|
* This method may be useful in conjunction with
|
|
* {@link #tryConvertToReadLock} and {@link #tryConvertToWriteLock},
|
|
* for example: <pre> {@code
|
|
* long stamp = sl.tryOptimisticRead();
|
|
* try {
|
|
* ...
|
|
* stamp = sl.tryConvertToReadLock(stamp);
|
|
* ...
|
|
* stamp = sl.tryConvertToWriteLock(stamp);
|
|
* ...
|
|
* } finally {
|
|
* if (StampedLock.isLockStamp(stamp))
|
|
* sl.unlock(stamp);
|
|
* }}</pre>
|
|
*
|
|
* @param stamp a stamp returned by a previous StampedLock operation
|
|
* @return {@code true} if the stamp was returned by a successful
|
|
* read-lock or write-lock operation
|
|
* @since 10
|
|
*/
|
|
public static boolean isLockStamp(long stamp) {
|
|
return (stamp & ABITS) != 0L;
|
|
}
|
|
|
|
/**
|
|
* Tells whether a stamp represents a successful optimistic read.
|
|
*
|
|
* @param stamp a stamp returned by a previous StampedLock operation
|
|
* @return {@code true} if the stamp was returned by a successful
|
|
* optimistic read operation, that is, a non-zero return from
|
|
* {@link #tryOptimisticRead()} or
|
|
* {@link #tryConvertToOptimisticRead(long)}
|
|
* @since 10
|
|
*/
|
|
public static boolean isOptimisticReadStamp(long stamp) {
|
|
return (stamp & ABITS) == 0L && stamp != 0L;
|
|
}
|
|
|
|
/**
|
|
* Queries the number of read locks held for this lock. This
|
|
* method is designed for use in monitoring system state, not for
|
|
* synchronization control.
|
|
* @return the number of read locks held
|
|
*/
|
|
public int getReadLockCount() {
|
|
return getReadLockCount(state);
|
|
}
|
|
|
|
/**
|
|
* Returns a string identifying this lock, as well as its lock
|
|
* state. The state, in brackets, includes the String {@code
|
|
* "Unlocked"} or the String {@code "Write-locked"} or the String
|
|
* {@code "Read-locks:"} followed by the current number of
|
|
* read-locks held.
|
|
*
|
|
* @return a string identifying this lock, as well as its lock state
|
|
*/
|
|
public String toString() {
|
|
long s = state;
|
|
return super.toString() +
|
|
((s & ABITS) == 0L ? "[Unlocked]" :
|
|
(s & WBIT) != 0L ? "[Write-locked]" :
|
|
"[Read-locks:" + getReadLockCount(s) + "]");
|
|
}
|
|
|
|
// views
|
|
|
|
/**
|
|
* Returns a plain {@link Lock} view of this StampedLock in which
|
|
* the {@link Lock#lock} method is mapped to {@link #readLock},
|
|
* and similarly for other methods. The returned Lock does not
|
|
* support a {@link Condition}; method {@link Lock#newCondition()}
|
|
* throws {@code UnsupportedOperationException}.
|
|
*
|
|
* @return the lock
|
|
*/
|
|
public Lock asReadLock() {
|
|
ReadLockView v;
|
|
if ((v = readLockView) != null) return v;
|
|
return readLockView = new ReadLockView();
|
|
}
|
|
|
|
/**
|
|
* Returns a plain {@link Lock} view of this StampedLock in which
|
|
* the {@link Lock#lock} method is mapped to {@link #writeLock},
|
|
* and similarly for other methods. The returned Lock does not
|
|
* support a {@link Condition}; method {@link Lock#newCondition()}
|
|
* throws {@code UnsupportedOperationException}.
|
|
*
|
|
* @return the lock
|
|
*/
|
|
public Lock asWriteLock() {
|
|
WriteLockView v;
|
|
if ((v = writeLockView) != null) return v;
|
|
return writeLockView = new WriteLockView();
|
|
}
|
|
|
|
/**
|
|
* Returns a {@link ReadWriteLock} view of this StampedLock in
|
|
* which the {@link ReadWriteLock#readLock()} method is mapped to
|
|
* {@link #asReadLock()}, and {@link ReadWriteLock#writeLock()} to
|
|
* {@link #asWriteLock()}.
|
|
*
|
|
* @return the lock
|
|
*/
|
|
public ReadWriteLock asReadWriteLock() {
|
|
ReadWriteLockView v;
|
|
if ((v = readWriteLockView) != null) return v;
|
|
return readWriteLockView = new ReadWriteLockView();
|
|
}
|
|
|
|
// view classes
|
|
|
|
final class ReadLockView implements Lock {
|
|
public void lock() { readLock(); }
|
|
public void lockInterruptibly() throws InterruptedException {
|
|
readLockInterruptibly();
|
|
}
|
|
public boolean tryLock() { return tryReadLock() != 0L; }
|
|
public boolean tryLock(long time, TimeUnit unit)
|
|
throws InterruptedException {
|
|
return tryReadLock(time, unit) != 0L;
|
|
}
|
|
public void unlock() { unstampedUnlockRead(); }
|
|
public Condition newCondition() {
|
|
throw new UnsupportedOperationException();
|
|
}
|
|
}
|
|
|
|
final class WriteLockView implements Lock {
|
|
public void lock() { writeLock(); }
|
|
public void lockInterruptibly() throws InterruptedException {
|
|
writeLockInterruptibly();
|
|
}
|
|
public boolean tryLock() { return tryWriteLock() != 0L; }
|
|
public boolean tryLock(long time, TimeUnit unit)
|
|
throws InterruptedException {
|
|
return tryWriteLock(time, unit) != 0L;
|
|
}
|
|
public void unlock() { unstampedUnlockWrite(); }
|
|
public Condition newCondition() {
|
|
throw new UnsupportedOperationException();
|
|
}
|
|
}
|
|
|
|
final class ReadWriteLockView implements ReadWriteLock {
|
|
public Lock readLock() { return asReadLock(); }
|
|
public Lock writeLock() { return asWriteLock(); }
|
|
}
|
|
|
|
// Unlock methods without stamp argument checks for view classes.
|
|
// Needed because view-class lock methods throw away stamps.
|
|
|
|
final void unstampedUnlockWrite() {
|
|
long s;
|
|
if (((s = state) & WBIT) == 0L)
|
|
throw new IllegalMonitorStateException();
|
|
releaseWrite(s);
|
|
}
|
|
|
|
final void unstampedUnlockRead() {
|
|
long s, m;
|
|
while ((m = (s = state) & RBITS) > 0L) {
|
|
if (m < RFULL) {
|
|
if (casState(s, s - RUNIT)) {
|
|
if (m == RUNIT)
|
|
signalNext(head);
|
|
return;
|
|
}
|
|
}
|
|
else if (tryDecReaderOverflow(s) != 0L)
|
|
return;
|
|
}
|
|
throw new IllegalMonitorStateException();
|
|
}
|
|
|
|
private void readObject(java.io.ObjectInputStream s)
|
|
throws java.io.IOException, ClassNotFoundException {
|
|
s.defaultReadObject();
|
|
state = ORIGIN; // reset to unlocked state
|
|
}
|
|
|
|
// overflow handling methods
|
|
|
|
/**
|
|
* Tries to increment readerOverflow by first setting state
|
|
* access bits value to RBITS, indicating hold of spinlock,
|
|
* then updating, then releasing.
|
|
*
|
|
* @param s a reader overflow stamp: (s & ABITS) >= RFULL
|
|
* @return new stamp on success, else zero
|
|
*/
|
|
private long tryIncReaderOverflow(long s) {
|
|
// assert (s & ABITS) >= RFULL;
|
|
if ((s & ABITS) != RFULL)
|
|
Thread.onSpinWait();
|
|
else if (casState(s, s | RBITS)) {
|
|
++readerOverflow;
|
|
return state = s;
|
|
}
|
|
return 0L;
|
|
}
|
|
|
|
/**
|
|
* Tries to decrement readerOverflow.
|
|
*
|
|
* @param s a reader overflow stamp: (s & ABITS) >= RFULL
|
|
* @return new stamp on success, else zero
|
|
*/
|
|
private long tryDecReaderOverflow(long s) {
|
|
// assert (s & ABITS) >= RFULL;
|
|
if ((s & ABITS) != RFULL)
|
|
Thread.onSpinWait();
|
|
else if (casState(s, s | RBITS)) {
|
|
int r; long nextState;
|
|
if ((r = readerOverflow) > 0) {
|
|
readerOverflow = r - 1;
|
|
nextState = s;
|
|
}
|
|
else
|
|
nextState = s - RUNIT;
|
|
return state = nextState;
|
|
}
|
|
return 0L;
|
|
}
|
|
|
|
// release methods
|
|
|
|
/**
|
|
* Wakes up the successor of given node, if one exists, and unsets its
|
|
* WAITING status to avoid park race. This may fail to wake up an
|
|
* eligible thread when one or more have been cancelled, but
|
|
* cancelAcquire ensures liveness.
|
|
*/
|
|
static final void signalNext(Node h) {
|
|
Node s;
|
|
if (h != null && (s = h.next) != null && s.status > 0) {
|
|
s.getAndUnsetStatus(WAITING);
|
|
LockSupport.unpark(s.waiter);
|
|
}
|
|
}
|
|
|
|
/**
|
|
* Removes and unparks all cowaiters of node, if it exists.
|
|
*/
|
|
private static void signalCowaiters(ReaderNode node) {
|
|
if (node != null) {
|
|
for (ReaderNode c; (c = node.cowaiters) != null; ) {
|
|
if (node.casCowaiters(c, c.cowaiters))
|
|
LockSupport.unpark(c.waiter);
|
|
}
|
|
}
|
|
}
|
|
|
|
// queue link methods
|
|
private boolean casTail(Node c, Node v) {
|
|
return U.compareAndSetReference(this, TAIL, c, v);
|
|
}
|
|
|
|
/** tries once to CAS a new dummy node for head */
|
|
private void tryInitializeHead() {
|
|
Node h = new WriterNode();
|
|
if (U.compareAndSetReference(this, HEAD, null, h))
|
|
tail = h;
|
|
}
|
|
|
|
/**
|
|
* For explanation, see above and AbstractQueuedSynchronizer
|
|
* internal documentation.
|
|
*
|
|
* @param interruptible true if should check interrupts and if so
|
|
* return INTERRUPTED
|
|
* @param timed if true use timed waits
|
|
* @param time the System.nanoTime value to timeout at (and return zero)
|
|
* @return next state, or INTERRUPTED
|
|
*/
|
|
private long acquireWrite(boolean interruptible, boolean timed, long time) {
|
|
byte spins = 0, postSpins = 0; // retries upon unpark of first thread
|
|
boolean interrupted = false, first = false;
|
|
WriterNode node = null;
|
|
Node pred = null;
|
|
for (long s, nextState;;) {
|
|
if (!first && (pred = (node == null) ? null : node.prev) != null &&
|
|
!(first = (head == pred))) {
|
|
if (pred.status < 0) {
|
|
cleanQueue(); // predecessor cancelled
|
|
continue;
|
|
} else if (pred.prev == null) {
|
|
Thread.onSpinWait(); // ensure serialization
|
|
continue;
|
|
}
|
|
}
|
|
if ((first || pred == null) && ((s = state) & ABITS) == 0L &&
|
|
casState(s, nextState = s | WBIT)) {
|
|
U.storeStoreFence();
|
|
if (first) {
|
|
node.prev = null;
|
|
head = node;
|
|
pred.next = null;
|
|
node.waiter = null;
|
|
if (interrupted)
|
|
Thread.currentThread().interrupt();
|
|
}
|
|
return nextState;
|
|
} else if (node == null) { // retry before enqueuing
|
|
node = new WriterNode();
|
|
} else if (pred == null) { // try to enqueue
|
|
Node t = tail;
|
|
node.setPrevRelaxed(t);
|
|
if (t == null)
|
|
tryInitializeHead();
|
|
else if (!casTail(t, node))
|
|
node.setPrevRelaxed(null); // back out
|
|
else
|
|
t.next = node;
|
|
} else if (first && spins != 0) { // reduce unfairness
|
|
--spins;
|
|
Thread.onSpinWait();
|
|
} else if (node.status == 0) { // enable signal
|
|
if (node.waiter == null)
|
|
node.waiter = Thread.currentThread();
|
|
node.status = WAITING;
|
|
} else {
|
|
long nanos;
|
|
spins = postSpins = (byte)((postSpins << 1) | 1);
|
|
if (!timed)
|
|
LockSupport.park(this);
|
|
else if ((nanos = time - System.nanoTime()) > 0L)
|
|
LockSupport.parkNanos(this, nanos);
|
|
else
|
|
break;
|
|
node.clearStatus();
|
|
if ((interrupted |= Thread.interrupted()) && interruptible)
|
|
break;
|
|
}
|
|
}
|
|
return cancelAcquire(node, interrupted);
|
|
}
|
|
|
|
/**
|
|
* See above for explanation.
|
|
*
|
|
* @param interruptible true if should check interrupts and if so
|
|
* return INTERRUPTED
|
|
* @param timed if true use timed waits
|
|
* @param time the System.nanoTime value to timeout at (and return zero)
|
|
* @return next state, or INTERRUPTED
|
|
*/
|
|
private long acquireRead(boolean interruptible, boolean timed, long time) {
|
|
boolean interrupted = false;
|
|
ReaderNode node = null;
|
|
/*
|
|
* Loop:
|
|
* if empty, try to acquire
|
|
* if tail is Reader, try to cowait; restart if leader stale or cancels
|
|
* else try to create and enqueue node, and wait in 2nd loop below
|
|
*/
|
|
for (;;) {
|
|
ReaderNode leader; long nextState;
|
|
Node tailPred = null, t = tail;
|
|
if ((t == null || (tailPred = t.prev) == null) &&
|
|
(nextState = tryAcquireRead()) != 0L) // try now if empty
|
|
return nextState;
|
|
else if (t == null)
|
|
tryInitializeHead();
|
|
else if (tailPred == null || !(t instanceof ReaderNode)) {
|
|
if (node == null)
|
|
node = new ReaderNode();
|
|
if (tail == t) {
|
|
node.setPrevRelaxed(t);
|
|
if (casTail(t, node)) {
|
|
t.next = node;
|
|
break; // node is leader; wait in loop below
|
|
}
|
|
node.setPrevRelaxed(null);
|
|
}
|
|
} else if ((leader = (ReaderNode)t) == tail) { // try to cowait
|
|
for (boolean attached = false;;) {
|
|
if (leader.status < 0 || leader.prev == null)
|
|
break;
|
|
else if (node == null)
|
|
node = new ReaderNode();
|
|
else if (node.waiter == null)
|
|
node.waiter = Thread.currentThread();
|
|
else if (!attached) {
|
|
ReaderNode c = leader.cowaiters;
|
|
node.setCowaitersRelaxed(c);
|
|
attached = leader.casCowaiters(c, node);
|
|
if (!attached)
|
|
node.setCowaitersRelaxed(null);
|
|
} else {
|
|
long nanos = 0L;
|
|
if (!timed)
|
|
LockSupport.park(this);
|
|
else if ((nanos = time - System.nanoTime()) > 0L)
|
|
LockSupport.parkNanos(this, nanos);
|
|
interrupted |= Thread.interrupted();
|
|
if ((interrupted && interruptible) ||
|
|
(timed && nanos <= 0L))
|
|
return cancelCowaiter(node, leader, interrupted);
|
|
}
|
|
}
|
|
if (node != null)
|
|
node.waiter = null;
|
|
long ns = tryAcquireRead();
|
|
signalCowaiters(leader);
|
|
if (interrupted)
|
|
Thread.currentThread().interrupt();
|
|
if (ns != 0L)
|
|
return ns;
|
|
else
|
|
node = null; // restart if stale, missed, or leader cancelled
|
|
}
|
|
}
|
|
|
|
// node is leader of a cowait group; almost same as acquireWrite
|
|
byte spins = 0, postSpins = 0; // retries upon unpark of first thread
|
|
boolean first = false;
|
|
Node pred = null;
|
|
for (long nextState;;) {
|
|
if (!first && (pred = node.prev) != null &&
|
|
!(first = (head == pred))) {
|
|
if (pred.status < 0) {
|
|
cleanQueue(); // predecessor cancelled
|
|
continue;
|
|
} else if (pred.prev == null) {
|
|
Thread.onSpinWait(); // ensure serialization
|
|
continue;
|
|
}
|
|
}
|
|
if ((first || pred == null) &&
|
|
(nextState = tryAcquireRead()) != 0L) {
|
|
if (first) {
|
|
node.prev = null;
|
|
head = node;
|
|
pred.next = null;
|
|
node.waiter = null;
|
|
}
|
|
signalCowaiters(node);
|
|
if (interrupted)
|
|
Thread.currentThread().interrupt();
|
|
return nextState;
|
|
} else if (first && spins != 0) {
|
|
--spins;
|
|
Thread.onSpinWait();
|
|
} else if (node.status == 0) {
|
|
if (node.waiter == null)
|
|
node.waiter = Thread.currentThread();
|
|
node.status = WAITING;
|
|
} else {
|
|
long nanos;
|
|
spins = postSpins = (byte)((postSpins << 1) | 1);
|
|
if (!timed)
|
|
LockSupport.park(this);
|
|
else if ((nanos = time - System.nanoTime()) > 0L)
|
|
LockSupport.parkNanos(this, nanos);
|
|
else
|
|
break;
|
|
node.clearStatus();
|
|
if ((interrupted |= Thread.interrupted()) && interruptible)
|
|
break;
|
|
}
|
|
}
|
|
return cancelAcquire(node, interrupted);
|
|
}
|
|
|
|
// Cancellation support
|
|
|
|
/**
|
|
* Possibly repeatedly traverses from tail, unsplicing cancelled
|
|
* nodes until none are found. Unparks nodes that may have been
|
|
* relinked to be next eligible acquirer.
|
|
*/
|
|
private void cleanQueue() {
|
|
for (;;) { // restart point
|
|
for (Node q = tail, s = null, p, n;;) { // (p, q, s) triples
|
|
if (q == null || (p = q.prev) == null)
|
|
return; // end of list
|
|
if (s == null ? tail != q : (s.prev != q || s.status < 0))
|
|
break; // inconsistent
|
|
if (q.status < 0) { // cancelled
|
|
if ((s == null ? casTail(q, p) : s.casPrev(q, p)) &&
|
|
q.prev == p) {
|
|
p.casNext(q, s); // OK if fails
|
|
if (p.prev == null)
|
|
signalNext(p);
|
|
}
|
|
break;
|
|
}
|
|
if ((n = p.next) != q) { // help finish
|
|
if (n != null && q.prev == p && q.status >= 0) {
|
|
p.casNext(n, q);
|
|
if (p.prev == null)
|
|
signalNext(p);
|
|
}
|
|
break;
|
|
}
|
|
s = q;
|
|
q = q.prev;
|
|
}
|
|
}
|
|
}
|
|
|
|
/**
|
|
* If leader exists, possibly repeatedly traverses cowaiters,
|
|
* unsplicing the given cancelled node until not found.
|
|
*/
|
|
private void unlinkCowaiter(ReaderNode node, ReaderNode leader) {
|
|
if (leader != null) {
|
|
while (leader.prev != null && leader.status >= 0) {
|
|
for (ReaderNode p = leader, q; ; p = q) {
|
|
if ((q = p.cowaiters) == null)
|
|
return;
|
|
if (q == node) {
|
|
p.casCowaiters(q, q.cowaiters);
|
|
break; // recheck even if succeeded
|
|
}
|
|
}
|
|
}
|
|
}
|
|
}
|
|
|
|
/**
|
|
* If node non-null, forces cancel status and unsplices it from
|
|
* queue, wakes up any cowaiters, and possibly wakes up successor
|
|
* to recheck status.
|
|
*
|
|
* @param node the waiter (may be null if not yet enqueued)
|
|
* @param interrupted if already interrupted
|
|
* @return INTERRUPTED if interrupted or Thread.interrupted, else zero
|
|
*/
|
|
private long cancelAcquire(Node node, boolean interrupted) {
|
|
if (node != null) {
|
|
node.waiter = null;
|
|
node.status = CANCELLED;
|
|
cleanQueue();
|
|
if (node instanceof ReaderNode)
|
|
signalCowaiters((ReaderNode)node);
|
|
}
|
|
return (interrupted || Thread.interrupted()) ? INTERRUPTED : 0L;
|
|
}
|
|
|
|
/**
|
|
* If node non-null, forces cancel status and unsplices from
|
|
* leader's cowaiters list unless/until it is also cancelled.
|
|
*
|
|
* @param node if non-null, the waiter
|
|
* @param leader if non-null, the node heading cowaiters list
|
|
* @param interrupted if already interrupted
|
|
* @return INTERRUPTED if interrupted or Thread.interrupted, else zero
|
|
*/
|
|
private long cancelCowaiter(ReaderNode node, ReaderNode leader,
|
|
boolean interrupted) {
|
|
if (node != null) {
|
|
node.waiter = null;
|
|
node.status = CANCELLED;
|
|
unlinkCowaiter(node, leader);
|
|
}
|
|
return (interrupted || Thread.interrupted()) ? INTERRUPTED : 0L;
|
|
}
|
|
|
|
// Unsafe
|
|
private static final Unsafe U = Unsafe.getUnsafe();
|
|
private static final long STATE
|
|
= U.objectFieldOffset(StampedLock.class, "state");
|
|
private static final long HEAD
|
|
= U.objectFieldOffset(StampedLock.class, "head");
|
|
private static final long TAIL
|
|
= U.objectFieldOffset(StampedLock.class, "tail");
|
|
|
|
static {
|
|
Class<?> ensureLoaded = LockSupport.class;
|
|
}
|
|
}
|