/*
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
/*
* This file is available under and governed by the GNU General Public
* License version 2 only, as published by the Free Software Foundation.
* However, the following notice accompanied the original version of this
* file:
*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/
*/
package java.util.concurrent.locks;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;
import java.util.concurrent.TimeUnit;
import jdk.internal.vm.annotation.ReservedStackAccess;
/**
* A capability-based lock with three modes for controlling read/write
* access. The state of a StampedLock consists of a version and mode.
* Lock acquisition methods return a stamp that represents and
* controls access with respect to a lock state; "try" versions of
* these methods may instead return the special value zero to
* represent failure to acquire access. Lock release and conversion
* methods require stamps as arguments, and fail if they do not match
* the state of the lock. The three modes are:
*
* <ul>
*
* <li><b>Writing.</b> Method {@link #writeLock} possibly blocks
* waiting for exclusive access, returning a stamp that can be used
* in method {@link #unlockWrite} to release the lock. Untimed and
* timed versions of {@code tryWriteLock} are also provided. When
* the lock is held in write mode, no read locks may be obtained,
* and all optimistic read validations will fail.
*
* <li><b>Reading.</b> Method {@link #readLock} possibly blocks
* waiting for non-exclusive access, returning a stamp that can be
* used in method {@link #unlockRead} to release the lock. Untimed
* and timed versions of {@code tryReadLock} are also provided.
*
* <li><b>Optimistic Reading.</b> Method {@link #tryOptimisticRead}
* returns a non-zero stamp only if the lock is not currently held
* in write mode. Method {@link #validate} returns true if the lock
* has not been acquired in write mode since obtaining a given
* stamp. This mode can be thought of as an extremely weak version
* of a read-lock, that can be broken by a writer at any time. The
* use of optimistic mode for short read-only code segments often
* reduces contention and improves throughput. However, its use is
* inherently fragile. Optimistic read sections should only read
* fields and hold them in local variables for later use after
* validation. Fields read while in optimistic mode may be wildly
* inconsistent, so usage applies only when you are familiar enough
* with data representations to check consistency and/or repeatedly
* invoke method {@code validate()}. For example, such steps are
* typically required when first reading an object or array
* reference, and then accessing one of its fields, elements or
* methods.
*
* </ul>
*
* <p>This class also supports methods that conditionally provide
* conversions across the three modes. For example, method {@link
* #tryConvertToWriteLock} attempts to "upgrade" a mode, returning
* a valid write stamp if (1) already in writing mode (2) in reading
* mode and there are no other readers or (3) in optimistic mode and
* the lock is available. The forms of these methods are designed to
* help reduce some of the code bloat that otherwise occurs in
* retry-based designs.
*
* <p>StampedLocks are designed for use as internal utilities in the
* development of thread-safe components. Their use relies on
* knowledge of the internal properties of the data, objects, and
* methods they are protecting. They are not reentrant, so locked
* bodies should not call other unknown methods that may try to
* re-acquire locks (although you may pass a stamp to other methods
* that can use or convert it). The use of read lock modes relies on
* the associated code sections being side-effect-free. Unvalidated
* optimistic read sections cannot call methods that are not known to
* tolerate potential inconsistencies. Stamps use finite
* representations, and are not cryptographically secure (i.e., a
* valid stamp may be guessable). Stamp values may recycle after (no
* sooner than) one year of continuous operation. A stamp held without
* use or validation for longer than this period may fail to validate
* correctly. StampedLocks are serializable, but always deserialize
* into initial unlocked state, so they are not useful for remote
* locking.
*
* <p>Like {@link java.util.concurrent.Semaphore Semaphore}, but unlike most
* {@link Lock} implementations, StampedLocks have no notion of ownership.
* Locks acquired in one thread can be released or converted in another.
*
* <p>The scheduling policy of StampedLock does not consistently
* prefer readers over writers or vice versa. All "try" methods are
* best-effort and do not necessarily conform to any scheduling or
* fairness policy. A zero return from any "try" method for acquiring
* or converting locks does not carry any information about the state
* of the lock; a subsequent invocation may succeed.
*
* <p>Because it supports coordinated usage across multiple lock
* modes, this class does not directly implement the {@link Lock} or
* {@link ReadWriteLock} interfaces. However, a StampedLock may be
* viewed {@link #asReadLock()}, {@link #asWriteLock()}, or {@link
* #asReadWriteLock()} in applications requiring only the associated
* set of functionality.
*
* <p><b>Sample Usage.</b> The following illustrates some usage idioms
* in a class that maintains simple two-dimensional points. The sample
* code illustrates some try/catch conventions even though they are
* not strictly needed here because no exceptions can occur in their
* bodies.
*
* <pre> {@code
* class Point {
* private double x, y;
* private final StampedLock sl = new StampedLock();
*
* void move(double deltaX, double deltaY) { // an exclusively locked method
* long stamp = sl.writeLock();
* try {
* x += deltaX;
* y += deltaY;
* } finally {
* sl.unlockWrite(stamp);
* }
* }
*
* double distanceFromOrigin() { // A read-only method
* long stamp = sl.tryOptimisticRead();
* double currentX = x, currentY = y;
* if (!sl.validate(stamp)) {
* stamp = sl.readLock();
* try {
* currentX = x;
* currentY = y;
* } finally {
* sl.unlockRead(stamp);
* }
* }
* return Math.sqrt(currentX * currentX + currentY * currentY);
* }
*
* void moveIfAtOrigin(double newX, double newY) { // upgrade
* // Could instead start with optimistic, not read mode
* long stamp = sl.readLock();
* try {
* while (x == 0.0 && y == 0.0) {
* long ws = sl.tryConvertToWriteLock(stamp);
* if (ws != 0L) {
* stamp = ws;
* x = newX;
* y = newY;
* break;
* }
* else {
* sl.unlockRead(stamp);
* stamp = sl.writeLock();
* }
* }
* } finally {
* sl.unlock(stamp);
* }
* }
* }}</pre>
*
* @since 1.8
* @author Doug Lea
*/
public class StampedLock implements java.io.Serializable {
/*
* Algorithmic notes:
*
* The design employs elements of Sequence locks
* (as used in linux kernels; see Lameter's
* http://www.lameter.com/gelato2005.pdf
* and elsewhere; see
* Boehm's http://www.hpl.hp.com/techreports/2012/HPL-2012-68.html)
* and Ordered RW locks (see Shirako et al
* http://dl.acm.org/citation.cfm?id=2312015)
*
* Conceptually, the primary state of the lock includes a sequence
* number that is odd when write-locked and even otherwise.
* However, this is offset by a reader count that is non-zero when
* read-locked. The read count is ignored when validating
* "optimistic" seqlock-reader-style stamps. Because we must use
* a small finite number of bits (currently 7) for readers, a
* supplementary reader overflow word is used when the number of
* readers exceeds the count field. We do this by treating the max
* reader count value (RBITS) as a spinlock protecting overflow
* updates.
*
* Waiters use a modified form of CLH lock used in
* AbstractQueuedSynchronizer (see its internal documentation for
* a fuller account), where each node is tagged (field mode) as
* either a reader or writer. Sets of waiting readers are grouped
* (linked) under a common node (field cowait) so act as a single
* node with respect to most CLH mechanics. By virtue of the
* queue structure, wait nodes need not actually carry sequence
* numbers; we know each is greater than its predecessor. This
* simplifies the scheduling policy to a mainly-FIFO scheme that
* incorporates elements of Phase-Fair locks (see Brandenburg &
* Anderson, especially http://www.cs.unc.edu/~bbb/diss/). In
* particular, we use the phase-fair anti-barging rule: If an
* incoming reader arrives while read lock is held but there is a
* queued writer, this incoming reader is queued. (This rule is
* responsible for some of the complexity of method acquireRead,
* but without it, the lock becomes highly unfair.) Method release
* does not (and sometimes cannot) itself wake up cowaiters. This
* is done by the primary thread, but helped by any other threads
* with nothing better to do in methods acquireRead and
* acquireWrite.
*
* These rules apply to threads actually queued. All tryLock forms
* opportunistically try to acquire locks regardless of preference
* rules, and so may "barge" their way in. Randomized spinning is
* used in the acquire methods to reduce (increasingly expensive)
* context switching while also avoiding sustained memory
* thrashing among many threads. We limit spins to the head of
* queue. If, upon wakening, a thread fails to obtain lock, and is
* still (or becomes) the first waiting thread (which indicates
* that some other thread barged and obtained lock), it escalates
* spins (up to MAX_HEAD_SPINS) to reduce the likelihood of
* continually losing to barging threads.
*
* Nearly all of these mechanics are carried out in methods
* acquireWrite and acquireRead, that, as typical of such code,
* sprawl out because actions and retries rely on consistent sets
* of locally cached reads.
*
* As noted in Boehm's paper (above), sequence validation (mainly
* method validate()) requires stricter ordering rules than apply
* to normal volatile reads (of "state"). To force orderings of
* reads before a validation and the validation itself in those
* cases where this is not already forced, we use acquireFence.
* Unlike in that paper, we allow writers to use plain writes.
* One would not expect reorderings of such writes with the lock
* acquisition CAS because there is a "control dependency", but it
* is theoretically possible, so we additionally add a
* storeStoreFence after lock acquisition CAS.
*
* ----------------------------------------------------------------
* Here's an informal proof that plain reads by _successful_
* readers see plain writes from preceding but not following
* writers (following Boehm and the C++ standard [atomics.fences]):
*
* Because of the total synchronization order of accesses to
* volatile long state containing the sequence number, writers and
* _successful_ readers can be globally sequenced.
*
* int x, y;
*
* Writer 1:
* inc sequence (odd - "locked")
* storeStoreFence();
* x = 1; y = 2;
* inc sequence (even - "unlocked")
*
* Successful Reader:
* read sequence (even)
* // must see writes from Writer 1 but not Writer 2
* r1 = x; r2 = y;
* acquireFence();
* read sequence (even - validated unchanged)
* // use r1 and r2
*
* Writer 2:
* inc sequence (odd - "locked")
* storeStoreFence();
* x = 3; y = 4;
* inc sequence (even - "unlocked")
*
* Visibility of writer 1's stores is normal - reader's initial
* read of state synchronizes with writer 1's final write to state.
* Lack of visibility of writer 2's plain writes is less obvious.
* If reader's read of x or y saw writer 2's write, then (assuming
* semantics of C++ fences) the storeStoreFence would "synchronize"
* with reader's acquireFence and reader's validation read must see
* writer 2's initial write to state and so validation must fail.
* But making this "proof" formal and rigorous is an open problem!
* ----------------------------------------------------------------
*
* The memory layout keeps lock state and queue pointers together
* (normally on the same cache line). This usually works well for
* read-mostly loads. In most other cases, the natural tendency of
* adaptive-spin CLH locks to reduce memory contention lessens
* motivation to further spread out contended locations, but might
* be subject to future improvements.
*/
private static final long serialVersionUID = -6001602636862214147L;
/** Number of processors, for spin control */
private static final int NCPU = Runtime.getRuntime().availableProcessors();
/** Maximum number of retries before enqueuing on acquisition; at least 1 */
private static final int SPINS = (NCPU > 1) ? 1 << 6 : 1;
/** Maximum number of tries before blocking at head on acquisition */
private static final int HEAD_SPINS = (NCPU > 1) ? 1 << 10 : 1;
/** Maximum number of retries before re-blocking */
private static final int MAX_HEAD_SPINS = (NCPU > 1) ? 1 << 16 : 1;
/** The period for yielding when waiting for overflow spinlock */
private static final int OVERFLOW_YIELD_RATE = 7; // must be power 2 - 1
/** The number of bits to use for reader count before overflowing */
private static final int LG_READERS = 7;
// Values for lock state and stamp operations
private static final long RUNIT = 1L;
private static final long WBIT = 1L << LG_READERS;
private static final long RBITS = WBIT - 1L;
private static final long RFULL = RBITS - 1L;
private static final long ABITS = RBITS | WBIT;
private static final long SBITS = ~RBITS; // note overlap with ABITS
/*
* 3 stamp modes can be distinguished by examining (m = stamp & ABITS):
* write mode: m == WBIT
* optimistic read mode: m == 0L (even when read lock is held)
* read mode: m > 0L && m <= RFULL (the stamp is a copy of state, but the
* read hold count in the stamp is unused other than to determine mode)
*
* This differs slightly from the encoding of state:
* (state & ABITS) == 0L indicates the lock is currently unlocked.
* (state & ABITS) == RBITS is a special transient value
* indicating spin-locked to manipulate reader bits overflow.
*/
/** Initial value for lock state; avoids failure value zero. */
private static final long ORIGIN = WBIT << 1;
// Special value from cancelled acquire methods so caller can throw IE
private static final long INTERRUPTED = 1L;
// Values for node status; order matters
private static final int WAITING = -1;
private static final int CANCELLED = 1;
// Modes for nodes (int not boolean to allow arithmetic)
private static final int RMODE = 0;
private static final int WMODE = 1;
/** Wait nodes */
static final class WNode {
volatile WNode prev;
volatile WNode next;
volatile WNode cowait; // list of linked readers
volatile Thread thread; // non-null while possibly parked
volatile int status; // 0, WAITING, or CANCELLED
final int mode; // RMODE or WMODE
WNode(int m, WNode p) { mode = m; prev = p; }
}
/** Head of CLH queue */
private transient volatile WNode whead;
/** Tail (last) of CLH queue */
private transient volatile WNode wtail;
// views
transient ReadLockView readLockView;
transient WriteLockView writeLockView;
transient ReadWriteLockView readWriteLockView;
/** Lock sequence/state */
private transient volatile long state;
/** extra reader count when state read count saturated */
private transient int readerOverflow;
/**
* Creates a new lock, initially in unlocked state.
*/
public StampedLock() {
state = ORIGIN;
}
private boolean casState(long expectedValue, long newValue) {
return STATE.compareAndSet(this, expectedValue, newValue);
}
private long tryWriteLock(long s) {
// assert (s & ABITS) == 0L;
long next;
if (casState(s, next = s | WBIT)) {
VarHandle.storeStoreFence();
return next;
}
return 0L;
}
/**
* Exclusively acquires the lock, blocking if necessary
* until available.
*
* @return a write stamp that can be used to unlock or convert mode
*/
@ReservedStackAccess
public long writeLock() {
long next;
return ((next = tryWriteLock()) != 0L) ? next : acquireWrite(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
*/
@ReservedStackAccess
public long tryWriteLock() {
long s;
return (((s = state) & ABITS) == 0L) ? tryWriteLock(s) : 0L;
}
/**
* 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 next, deadline;
if ((next = tryWriteLock()) != 0L)
return next;
if (nanos <= 0L)
return 0L;
if ((deadline = System.nanoTime() + nanos) == 0L)
deadline = 1L;
if ((next = acquireWrite(true, deadline)) != INTERRUPTED)
return next;
}
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
*/
@ReservedStackAccess
public long writeLockInterruptibly() throws InterruptedException {
long next;
if (!Thread.interrupted() &&
(next = acquireWrite(true, 0L)) != INTERRUPTED)
return next;
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() {
long s, next;
// bypass acquireRead on common uncontended case
return (whead == wtail
&& ((s = state) & ABITS) < RFULL
&& casState(s, next = s + RUNIT))
? next
: acquireRead(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
*/
@ReservedStackAccess
public long tryReadLock() {
long s, m, next;
while ((m = (s = state) & ABITS) != WBIT) {
if (m < RFULL) {
if (casState(s, next = s + RUNIT))
return next;
}
else if ((next = tryIncReaderOverflow(s)) != 0L)
return next;
}
return 0L;
}
/**
* 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
*/
@ReservedStackAccess
public long tryReadLock(long time, TimeUnit unit)
throws InterruptedException {
long s, m, next, deadline;
long nanos = unit.toNanos(time);
if (!Thread.interrupted()) {
if ((m = (s = state) & ABITS) != WBIT) {
if (m < RFULL) {
if (casState(s, next = s + RUNIT))
return next;
}
else if ((next = tryIncReaderOverflow(s)) != 0L)
return next;
}
if (nanos <= 0L)
return 0L;
if ((deadline = System.nanoTime() + nanos) == 0L)
deadline = 1L;
if ((next = acquireRead(true, deadline)) != INTERRUPTED)
return next;
}
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
*/
@ReservedStackAccess
public long readLockInterruptibly() throws InterruptedException {
long s, next;
if (!Thread.interrupted()
// bypass acquireRead on common uncontended case
&& ((whead == wtail
&& ((s = state) & ABITS) < RFULL
&& casState(s, next = s + RUNIT))
||
(next = acquireRead(true, 0L)) != INTERRUPTED))
return next;
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) {
VarHandle.acquireFence();
return (stamp & SBITS) == (state & SBITS);
}
/**
* Returns an unlocked state, incrementing the version and
* avoiding special failure value 0L.
*
* @param s a write-locked state (or stamp)
*/
private static long unlockWriteState(long s) {
return ((s += WBIT) == 0L) ? ORIGIN : s;
}
private long unlockWriteInternal(long s) {
long next; WNode h;
STATE.setVolatile(this, next = unlockWriteState(s));
if ((h = whead) != null && h.status != 0)
release(h);
return next;
}
/**
* 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();
unlockWriteInternal(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; WNode h;
while (((s = state) & SBITS) == (stamp & SBITS)
&& (stamp & RBITS) > 0L
&& ((m = s & RBITS) > 0L)) {
if (m < RFULL) {
if (casState(s, s - RUNIT)) {
if (m == RUNIT && (h = whead) != null && h.status != 0)
release(h);
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
*/
@ReservedStackAccess
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, next;
while (((s = state) & SBITS) == (stamp & SBITS)) {
if ((m = s & ABITS) == 0L) {
if (a != 0L)
break;
if ((next = tryWriteLock(s)) != 0L)
return next;
}
else if (m == WBIT) {
if (a != m)
break;
return stamp;
}
else if (m == RUNIT && a != 0L) {
if (casState(s, next = s - RUNIT + WBIT)) {
VarHandle.storeStoreFence();
return next;
}
}
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, next; WNode h;
while (((s = state) & SBITS) == (stamp & SBITS)) {
if ((a = stamp & ABITS) >= WBIT) {
// write stamp
if (s != stamp)
break;
STATE.setVolatile(this, next = unlockWriteState(s) + RUNIT);
if ((h = whead) != null && h.status != 0)
release(h);
return next;
}
else if (a == 0L) {
// optimistic read stamp
if ((s & ABITS) < RFULL) {
if (casState(s, next = s + RUNIT))
return next;
}
else if ((next = tryIncReaderOverflow(s)) != 0L)
return next;
}
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, next; WNode h;
VarHandle.acquireFence();
while (((s = state) & SBITS) == (stamp & SBITS)) {
if ((a = stamp & ABITS) >= WBIT) {
// write stamp
if (s != stamp)
break;
return unlockWriteInternal(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, next = s - RUNIT)) {
if (m == RUNIT && (h = whead) != null && h.status != 0)
release(h);
return next & SBITS;
}
}
else if ((next = tryDecReaderOverflow(s)) != 0L)
return next & 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) {
unlockWriteInternal(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; WNode h;
while ((m = (s = state) & ABITS) != 0L && m < WBIT) {
if (m < RFULL) {
if (casState(s, s - RUNIT)) {
if (m == RUNIT && (h = whead) != null && h.status != 0)
release(h);
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;
}
/**
* 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();
unlockWriteInternal(s);
}
final void unstampedUnlockRead() {
long s, m; WNode h;
while ((m = (s = state) & RBITS) > 0L) {
if (m < RFULL) {
if (casState(s, s - RUNIT)) {
if (m == RUNIT && (h = whead) != null && h.status != 0)
release(h);
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.setVolatile(this, ORIGIN); // reset to unlocked state
}
// internals
/**
* 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) {
if (casState(s, s | RBITS)) {
++readerOverflow;
STATE.setVolatile(this, s);
return s;
}
}
else if ((LockSupport.nextSecondarySeed() & OVERFLOW_YIELD_RATE) == 0)
Thread.yield();
else
Thread.onSpinWait();
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) {
if (casState(s, s | RBITS)) {
int r; long next;
if ((r = readerOverflow) > 0) {
readerOverflow = r - 1;
next = s;
}
else
next = s - RUNIT;
STATE.setVolatile(this, next);
return next;
}
}
else if ((LockSupport.nextSecondarySeed() & OVERFLOW_YIELD_RATE) == 0)
Thread.yield();
else
Thread.onSpinWait();
return 0L;
}
/**
* Wakes up the successor of h (normally whead). This is normally
* just h.next, but may require traversal from wtail if next
* pointers are lagging. This may fail to wake up an acquiring
* thread when one or more have been cancelled, but the cancel
* methods themselves provide extra safeguards to ensure liveness.
*/
private void release(WNode h) {
if (h != null) {
WNode q; Thread w;
WSTATUS.compareAndSet(h, WAITING, 0);
if ((q = h.next) == null || q.status == CANCELLED) {
for (WNode t = wtail; t != null && t != h; t = t.prev)
if (t.status <= 0)
q = t;
}
if (q != null && (w = q.thread) != null)
LockSupport.unpark(w);
}
}
/**
* See above for explanation.
*
* @param interruptible true if should check interrupts and if so
* return INTERRUPTED
* @param deadline if nonzero, the System.nanoTime value to timeout
* at (and return zero)
* @return next state, or INTERRUPTED
*/
private long acquireWrite(boolean interruptible, long deadline) {
WNode node = null, p;
for (int spins = -1;;) { // spin while enqueuing
long m, s, ns;
if ((m = (s = state) & ABITS) == 0L) {
if ((ns = tryWriteLock(s)) != 0L)
return ns;
}
else if (spins < 0)
spins = (m == WBIT && wtail == whead) ? SPINS : 0;
else if (spins > 0) {
--spins;
Thread.onSpinWait();
}
else if ((p = wtail) == null) { // initialize queue
WNode hd = new WNode(WMODE, null);
if (WHEAD.weakCompareAndSet(this, null, hd))
wtail = hd;
}
else if (node == null)
node = new WNode(WMODE, p);
else if (node.prev != p)
node.prev = p;
else if (WTAIL.weakCompareAndSet(this, p, node)) {
p.next = node;
break;
}
}
boolean wasInterrupted = false;
for (int spins = -1;;) {
WNode h, np, pp; int ps;
if ((h = whead) == p) {
if (spins < 0)
spins = HEAD_SPINS;
else if (spins < MAX_HEAD_SPINS)
spins <<= 1;
for (int k = spins; k > 0; --k) { // spin at head
long s, ns;
if (((s = state) & ABITS) == 0L) {
if ((ns = tryWriteLock(s)) != 0L) {
whead = node;
node.prev = null;
if (wasInterrupted)
Thread.currentThread().interrupt();
return ns;
}
}
else
Thread.onSpinWait();
}
}
else if (h != null) { // help release stale waiters
WNode c; Thread w;
while ((c = h.cowait) != null) {
if (WCOWAIT.weakCompareAndSet(h, c, c.cowait) &&
(w = c.thread) != null)
LockSupport.unpark(w);
}
}
if (whead == h) {
if ((np = node.prev) != p) {
if (np != null)
(p = np).next = node; // stale
}
else if ((ps = p.status) == 0)
WSTATUS.compareAndSet(p, 0, WAITING);
else if (ps == CANCELLED) {
if ((pp = p.prev) != null) {
node.prev = pp;
pp.next = node;
}
}
else {
long time; // 0 argument to park means no timeout
if (deadline == 0L)
time = 0L;
else if ((time = deadline - System.nanoTime()) <= 0L)
return cancelWaiter(node, node, false);
Thread wt = Thread.currentThread();
node.thread = wt;
if (p.status < 0 && (p != h || (state & ABITS) != 0L) &&
whead == h && node.prev == p) {
if (time == 0L)
LockSupport.park(this);
else
LockSupport.parkNanos(this, time);
}
node.thread = null;
if (Thread.interrupted()) {
if (interruptible)
return cancelWaiter(node, node, true);
wasInterrupted = true;
}
}
}
}
}
/**
* See above for explanation.
*
* @param interruptible true if should check interrupts and if so
* return INTERRUPTED
* @param deadline if nonzero, the System.nanoTime value to timeout
* at (and return zero)
* @return next state, or INTERRUPTED
*/
private long acquireRead(boolean interruptible, long deadline) {
boolean wasInterrupted = false;
WNode node = null, p;
for (int spins = -1;;) {
WNode h;
if ((h = whead) == (p = wtail)) {
for (long m, s, ns;;) {
if ((m = (s = state) & ABITS) < RFULL ?
casState(s, ns = s + RUNIT) :
(m < WBIT && (ns = tryIncReaderOverflow(s)) != 0L)) {
if (wasInterrupted)
Thread.currentThread().interrupt();
return ns;
}
else if (m >= WBIT) {
if (spins > 0) {
--spins;
Thread.onSpinWait();
}
else {
if (spins == 0) {
WNode nh = whead, np = wtail;
if ((nh == h && np == p) || (h = nh) != (p = np))
break;
}
spins = SPINS;
}
}
}
}
if (p == null) { // initialize queue
WNode hd = new WNode(WMODE, null);
if (WHEAD.weakCompareAndSet(this, null, hd))
wtail = hd;
}
else if (node == null)
node = new WNode(RMODE, p);
else if (h == p || p.mode != RMODE) {
if (node.prev != p)
node.prev = p;
else if (WTAIL.weakCompareAndSet(this, p, node)) {
p.next = node;
break;
}
}
else if (!WCOWAIT.compareAndSet(p, node.cowait = p.cowait, node))
node.cowait = null;
else {
for (;;) {
WNode pp, c; Thread w;
if ((h = whead) != null && (c = h.cowait) != null &&
WCOWAIT.compareAndSet(h, c, c.cowait) &&
(w = c.thread) != null) // help release
LockSupport.unpark(w);
if (Thread.interrupted()) {
if (interruptible)
return cancelWaiter(node, p, true);
wasInterrupted = true;
}
if (h == (pp = p.prev) || h == p || pp == null) {
long m, s, ns;
do {
if ((m = (s = state) & ABITS) < RFULL ?
casState(s, ns = s + RUNIT) :
(m < WBIT &&
(ns = tryIncReaderOverflow(s)) != 0L)) {
if (wasInterrupted)
Thread.currentThread().interrupt();
return ns;
}
} while (m < WBIT);
}
if (whead == h && p.prev == pp) {
long time;
if (pp == null || h == p || p.status > 0) {
node = null; // throw away
break;
}
if (deadline == 0L)
time = 0L;
else if ((time = deadline - System.nanoTime()) <= 0L) {
if (wasInterrupted)
Thread.currentThread().interrupt();
return cancelWaiter(node, p, false);
}
Thread wt = Thread.currentThread();
node.thread = wt;
if ((h != pp || (state & ABITS) == WBIT) &&
whead == h && p.prev == pp) {
if (time == 0L)
LockSupport.park(this);
else
LockSupport.parkNanos(this, time);
}
node.thread = null;
}
}
}
}
for (int spins = -1;;) {
WNode h, np, pp; int ps;
if ((h = whead) == p) {
if (spins < 0)
spins = HEAD_SPINS;
else if (spins < MAX_HEAD_SPINS)
spins <<= 1;
for (int k = spins;;) { // spin at head
long m, s, ns;
if ((m = (s = state) & ABITS) < RFULL ?
casState(s, ns = s + RUNIT) :
(m < WBIT && (ns = tryIncReaderOverflow(s)) != 0L)) {
WNode c; Thread w;
whead = node;
node.prev = null;
while ((c = node.cowait) != null) {
if (WCOWAIT.compareAndSet(node, c, c.cowait) &&
(w = c.thread) != null)
LockSupport.unpark(w);
}
if (wasInterrupted)
Thread.currentThread().interrupt();
return ns;
}
else if (m >= WBIT && --k <= 0)
break;
else
Thread.onSpinWait();
}
}
else if (h != null) {
WNode c; Thread w;
while ((c = h.cowait) != null) {
if (WCOWAIT.compareAndSet(h, c, c.cowait) &&
(w = c.thread) != null)
LockSupport.unpark(w);
}
}
if (whead == h) {
if ((np = node.prev) != p) {
if (np != null)
(p = np).next = node; // stale
}
else if ((ps = p.status) == 0)
WSTATUS.compareAndSet(p, 0, WAITING);
else if (ps == CANCELLED) {
if ((pp = p.prev) != null) {
node.prev = pp;
pp.next = node;
}
}
else {
long time;
if (deadline == 0L)
time = 0L;
else if ((time = deadline - System.nanoTime()) <= 0L)
return cancelWaiter(node, node, false);
Thread wt = Thread.currentThread();
node.thread = wt;
if (p.status < 0 &&
(p != h || (state & ABITS) == WBIT) &&
whead == h && node.prev == p) {
if (time == 0L)
LockSupport.park(this);
else
LockSupport.parkNanos(this, time);
}
node.thread = null;
if (Thread.interrupted()) {
if (interruptible)
return cancelWaiter(node, node, true);
wasInterrupted = true;
}
}
}
}
}
/**
* If node non-null, forces cancel status and unsplices it from
* queue if possible and wakes up any cowaiters (of the node, or
* group, as applicable), and in any case helps release current
* first waiter if lock is free. (Calling with null arguments
* serves as a conditional form of release, which is not currently
* needed but may be needed under possible future cancellation
* policies). This is a variant of cancellation methods in
* AbstractQueuedSynchronizer (see its detailed explanation in AQS
* internal documentation).
*
* @param node if non-null, the waiter
* @param group either node or the group node is cowaiting with
* @param interrupted if already interrupted
* @return INTERRUPTED if interrupted or Thread.interrupted, else zero
*/
private long cancelWaiter(WNode node, WNode group, boolean interrupted) {
if (node != null && group != null) {
Thread w;
node.status = CANCELLED;
// unsplice cancelled nodes from group
for (WNode p = group, q; (q = p.cowait) != null;) {
if (q.status == CANCELLED) {
WCOWAIT.compareAndSet(p, q, q.cowait);
p = group; // restart
}
else
p = q;
}
if (group == node) {
for (WNode r = group.cowait; r != null; r = r.cowait) {
if ((w = r.thread) != null)
LockSupport.unpark(w); // wake up uncancelled co-waiters
}
for (WNode pred = node.prev; pred != null; ) { // unsplice
WNode succ, pp; // find valid successor
while ((succ = node.next) == null ||
succ.status == CANCELLED) {
WNode q = null; // find successor the slow way
for (WNode t = wtail; t != null && t != node; t = t.prev)
if (t.status != CANCELLED)
q = t; // don't link if succ cancelled
if (succ == q || // ensure accurate successor
WNEXT.compareAndSet(node, succ, succ = q)) {
if (succ == null && node == wtail)
WTAIL.compareAndSet(this, node, pred);
break;
}
}
if (pred.next == node) // unsplice pred link
WNEXT.compareAndSet(pred, node, succ);
if (succ != null && (w = succ.thread) != null) {
// wake up succ to observe new pred
succ.thread = null;
LockSupport.unpark(w);
}
if (pred.status != CANCELLED || (pp = pred.prev) == null)
break;
node.prev = pp; // repeat if new pred wrong/cancelled
WNEXT.compareAndSet(pp, pred, succ);
pred = pp;
}
}
}
WNode h; // Possibly release first waiter
while ((h = whead) != null) {
long s; WNode q; // similar to release() but check eligibility
if ((q = h.next) == null || q.status == CANCELLED) {
for (WNode t = wtail; t != null && t != h; t = t.prev)
if (t.status <= 0)
q = t;
}
if (h == whead) {
if (q != null && h.status == 0 &&
((s = state) & ABITS) != WBIT && // waiter is eligible
(s == 0L || q.mode == RMODE))
release(h);
break;
}
}
return (interrupted || Thread.interrupted()) ? INTERRUPTED : 0L;
}
// VarHandle mechanics
private static final VarHandle STATE;
private static final VarHandle WHEAD;
private static final VarHandle WTAIL;
private static final VarHandle WNEXT;
private static final VarHandle WSTATUS;
private static final VarHandle WCOWAIT;
static {
try {
MethodHandles.Lookup l = MethodHandles.lookup();
STATE = l.findVarHandle(StampedLock.class, "state", long.class);
WHEAD = l.findVarHandle(StampedLock.class, "whead", WNode.class);
WTAIL = l.findVarHandle(StampedLock.class, "wtail", WNode.class);
WSTATUS = l.findVarHandle(WNode.class, "status", int.class);
WNEXT = l.findVarHandle(WNode.class, "next", WNode.class);
WCOWAIT = l.findVarHandle(WNode.class, "cowait", WNode.class);
} catch (ReflectiveOperationException e) {
throw new Error(e);
}
}
}