/*
* Quasar: lightweight strands and actors for the JVM.
* Copyright (c) 2013-2014, Parallel Universe Software Co. All rights reserved.
*
* This program and the accompanying materials are dual-licensed under
* either the terms of the Eclipse Public License v1.0 as published by
* the Eclipse Foundation
*
* or (per the licensee's choosing)
*
* under the terms of the GNU Lesser General Public License version 3.0
* as published by the Free Software Foundation.
*/
/*
* Based on code:
*/
/*
* 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 co.paralleluniverse.strands.concurrent;
import co.paralleluniverse.common.util.UtilUnsafe;
import co.paralleluniverse.fibers.SuspendExecution;
import co.paralleluniverse.fibers.Suspendable;
import co.paralleluniverse.strands.Strand;
import java.util.concurrent.ThreadLocalRandom;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReadWriteLock;
/**
* 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>
*
* <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>
*
* <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. </li>
*
* </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 strand-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>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.<br>
*
* <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 distanceFromOriginV1() { // A read-only method
* long stamp;
* if ((stamp = sl.tryOptimisticRead()) != 0L) { // optimistic
* double currentX = x;
* double currentY = y;
* if (sl.validate(stamp))
* return Math.sqrt(currentX * currentX + currentY * currentY);
* }
* stamp = sl.readLock(); // fall back to read lock
* try {
* double currentX = x;
* double currentY = y;
* return Math.sqrt(currentX * currentX + currentY * currentY);
* } finally {
* sl.unlockRead(stamp);
* }
* }
*
* double distanceFromOriginV2() { // combines code paths
* double currentX = 0.0, currentY = 0.0;
* for (long stamp = sl.tryOptimisticRead(); ; stamp = sl.readLock()) {
* try {
* currentX = x;
* currentY = y;
* } finally {
* if (sl.tryConvertToOptimisticRead(stamp) != 0L) // unlock or validate
* break;
* }
* }
* 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.)
*
* These rules apply to strands 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 strands. We limit spins to the head of
* queue. A strand spin-waits up to SPINS times (where each
* iteration decreases spin count with 50% probability) before
* blocking. If, upon wakening it fails to obtain lock, and is
* still (or becomes) the first waiting strand (which indicates
* that some other strand barged and obtained lock), it escalates
* spins (up to MAX_HEAD_SPINS) to reduce the likelihood of
* continually losing to barging strands.
*
* 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"). In the absence of (but
* continual hope for) explicit JVM support of intrinsics with
* double-sided reordering prohibition, or corresponding fence
* intrinsics, we for now uncomfortably rely on the fact that the
* Unsafe.getXVolatile intrinsic must have this property
* (syntactic volatile reads do not) for internal purposes anyway,
* even though it is not documented.
*
* 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 blocking on acquisition
*/
private static final int SPINS = (NCPU > 1) ? 1 << 6 : 0;
/**
* Maximum number of retries before re-blocking
*/
private static final int MAX_HEAD_SPINS = (NCPU > 1) ? 1 << 12 : 0;
/**
* 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
// Initial value for lock state; avoid 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 Strand strand; // 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;
}
/**
* Exclusively acquires the lock, blocking if necessary
* until available.
*
* @return a stamp that can be used to unlock or convert mode
*/
@Suspendable
public long writeLock() {
try {
long s, next; // bypass acquireWrite in fully unlocked case only
return ((((s = state) & ABITS) == 0L
&& U.compareAndSwapLong(this, STATE, s, next = s + WBIT))
? next : acquireWrite(false, 0L));
} catch (SuspendExecution e) {
throw new AssertionError();
}
}
/**
* Exclusively acquires the lock if it is immediately available.
*
* @return a stamp that can be used to unlock or convert mode,
* or zero if the lock is not available
*/
public long tryWriteLock() {
long s, next;
return ((((s = state) & ABITS) == 0L
&& U.compareAndSwapLong(this, STATE, s, next = s + WBIT))
? next : 0L);
}
/**
* Exclusively acquires the lock if it is available within the
* given time and the current strand has not been interrupted.
* Behavior under timeout and interruption matches that specified
* for method {@link Lock#tryLock(long,TimeUnit)}.
*
* @return a stamp that can be used to unlock or convert mode,
* or zero if the lock is not available
* @throws InterruptedException if the current strand is interrupted
* before acquiring the lock
*/
@Suspendable
public long tryWriteLock(long time, TimeUnit unit)
throws InterruptedException {
try {
long nanos = unit.toNanos(time);
if (!Strand.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();
} catch (SuspendExecution e) {
throw new AssertionError();
}
}
/**
* Exclusively acquires the lock, blocking if necessary
* until available or the current strand is interrupted.
* Behavior under interruption matches that specified
* for method {@link Lock#lockInterruptibly()}.
*
* @return a stamp that can be used to unlock or convert mode
* @throws InterruptedException if the current strand is interrupted
* before acquiring the lock
*/
@Suspendable
public long writeLockInterruptibly() throws InterruptedException {
try {
long next;
if (!Strand.interrupted()
&& (next = acquireWrite(true, 0L)) != INTERRUPTED)
return next;
throw new InterruptedException();
} catch (SuspendExecution e) {
throw new AssertionError();
}
}
/**
* Non-exclusively acquires the lock, blocking if necessary
* until available.
*
* @return a stamp that can be used to unlock or convert mode
*/
@Suspendable
public long readLock() {
try {
long s, next; // bypass acquireRead on fully unlocked case only
return ((((s = state) & ABITS) == 0L
&& U.compareAndSwapLong(this, STATE, s, next = s + RUNIT))
? next : acquireRead(false, 0L));
} catch (SuspendExecution e) {
throw new AssertionError();
}
}
/**
* Non-exclusively acquires the lock if it is immediately available.
*
* @return a stamp that can be used to unlock or convert mode,
* or zero if the lock is not available
*/
public long tryReadLock() {
for (;;) {
long s, m, next;
if ((m = (s = state) & ABITS) == WBIT)
return 0L;
else if (m < RFULL) {
if (U.compareAndSwapLong(this, STATE, s, next = s + RUNIT))
return next;
} else if ((next = tryIncReaderOverflow(s)) != 0L)
return next;
}
}
/**
* Non-exclusively acquires the lock if it is available within the
* given time and the current strand has not been interrupted.
* Behavior under timeout and interruption matches that specified
* for method {@link Lock#tryLock(long,TimeUnit)}.
*
* @return a stamp that can be used to unlock or convert mode,
* or zero if the lock is not available
* @throws InterruptedException if the current strand is interrupted
* before acquiring the lock
*/
@Suspendable
public long tryReadLock(long time, TimeUnit unit)
throws InterruptedException {
try {
long s, m, next, deadline;
long nanos = unit.toNanos(time);
if (!Strand.interrupted()) {
if ((m = (s = state) & ABITS) != WBIT) {
if (m < RFULL) {
if (U.compareAndSwapLong(this, STATE, 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();
} catch (SuspendExecution e) {
throw new AssertionError();
}
}
/**
* Non-exclusively acquires the lock, blocking if necessary
* until available or the current strand is interrupted.
* Behavior under interruption matches that specified
* for method {@link Lock#lockInterruptibly()}.
*
* @return a stamp that can be used to unlock or convert mode
* @throws InterruptedException if the current strand is interrupted
* before acquiring the lock
*/
@Suspendable
public long readLockInterruptibly() throws InterruptedException {
try {
long next;
if (!Strand.interrupted()
&& (next = acquireRead(true, 0L)) != INTERRUPTED)
return next;
throw new InterruptedException();
} catch (SuspendExecution e) {
throw new AssertionError();
}
}
/**
* Returns a stamp that can later be validated, or zero
* if exclusively locked.
*
* @return a 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.
*
* @return true if the lock has not been exclusively acquired
* since issuance of the given stamp; else false
*/
public boolean validate(long stamp) {
// See above about current use of getLongVolatile here
return (stamp & SBITS) == (U.getLongVolatile(this, 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
*/
public void unlockWrite(long stamp) {
WNode h;
if (state != stamp || (stamp & WBIT) == 0L)
throw new IllegalMonitorStateException();
state = (stamp += WBIT) == 0L ? ORIGIN : stamp;
if ((h = whead) != null && h.status != 0)
release(h);
}
/**
* 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
*/
public void unlockRead(long stamp) {
long s, m;
WNode h;
for (;;) {
if (((s = state) & SBITS) != (stamp & SBITS)
|| (stamp & ABITS) == 0L || (m = s & ABITS) == 0L || m == WBIT)
throw new IllegalMonitorStateException();
if (m < RFULL) {
if (U.compareAndSwapLong(this, STATE, s, s - RUNIT)) {
if (m == RUNIT && (h = whead) != null && h.status != 0)
release(h);
break;
}
} else if (tryDecReaderOverflow(s) != 0L)
break;
}
}
/**
* 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) {
long a = stamp & ABITS, m, s;
WNode h;
while (((s = state) & SBITS) == (stamp & SBITS)) {
if ((m = s & ABITS) == 0L)
break;
else if (m == WBIT) {
if (a != m)
break;
state = (s += WBIT) == 0L ? ORIGIN : s;
if ((h = whead) != null && h.status != 0)
release(h);
return;
} else if (a == 0L || a >= WBIT)
break;
else if (m < RFULL) {
if (U.compareAndSwapLong(this, STATE, 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, 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 (U.compareAndSwapLong(this, STATE, s, next = s + WBIT))
return next;
} else if (m == WBIT) {
if (a != m)
break;
return stamp;
} else if (m == RUNIT && a != 0L) {
if (U.compareAndSwapLong(this, STATE, s,
next = s - RUNIT + WBIT))
return next;
} else
break;
}
return 0L;
}
/**
* If the lock state matches the given stamp, 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 = stamp & ABITS, m, s, next;
WNode h;
while (((s = state) & SBITS) == (stamp & SBITS)) {
if ((m = s & ABITS) == 0L) {
if (a != 0L)
break;
else if (m < RFULL) {
if (U.compareAndSwapLong(this, STATE, s, next = s + RUNIT))
return next;
} else if ((next = tryIncReaderOverflow(s)) != 0L)
return next;
} else if (m == WBIT) {
if (a != m)
break;
state = next = s + (WBIT + RUNIT);
if ((h = whead) != null && h.status != 0)
release(h);
return next;
} else if (a != 0L && a < WBIT)
return stamp;
else
break;
}
return 0L;
}
/**
* If the lock state matches the given stamp then, 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 = stamp & ABITS, m, s, next;
WNode h;
for (;;) {
s = U.getLongVolatile(this, STATE); // see above
if ((s & SBITS) != (stamp & SBITS))
break;
if ((m = s & ABITS) == 0L) {
if (a != 0L)
break;
return s;
} else if (m == WBIT) {
if (a != m)
break;
state = next = (s += WBIT) == 0L ? ORIGIN : s;
if ((h = whead) != null && h.status != 0)
release(h);
return next;
} else if (a == 0L || a >= WBIT)
break;
else if (m < RFULL) {
if (U.compareAndSwapLong(this, STATE, 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 true if the lock was held, else false
*/
public boolean tryUnlockWrite() {
long s;
WNode h;
if (((s = state) & WBIT) != 0L) {
state = (s += WBIT) == 0L ? ORIGIN : s;
if ((h = whead) != null && h.status != 0)
release(h);
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 true if the read lock was held, else false
*/
public boolean tryUnlockRead() {
long s, m;
WNode h;
while ((m = (s = state) & ABITS) != 0L && m < WBIT) {
if (m < RFULL) {
if (U.compareAndSwapLong(this, STATE, 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;
}
/**
* Returns true if the lock is currently held exclusively.
*
* @return true if the lock is currently held exclusively
*/
public boolean isWriteLocked() {
return (state & WBIT) != 0L;
}
/**
* Returns true if the lock is currently held non-exclusively.
*
* @return true if the lock is currently held non-exclusively
*/
public boolean isReadLocked() {
return (state & RBITS) != 0L;
}
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
s.defaultReadObject();
state = ORIGIN; // reset to unlocked state
}
/**
* 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;
return ((v = readLockView) != null ? v
: (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;
return ((v = writeLockView) != null ? v
: (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;
return ((v = readWriteLockView) != null ? v
: (readWriteLockView = new ReadWriteLockView()));
}
// view classes
final class ReadLockView implements Lock {
@Suspendable
public void lock() {
readLock();
}
@Suspendable
public void lockInterruptibly() throws InterruptedException {
readLockInterruptibly();
}
public boolean tryLock() {
return tryReadLock() != 0L;
}
@Suspendable
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 {
@Suspendable
public void lock() {
writeLock();
}
@Suspendable
public void lockInterruptibly() throws InterruptedException {
writeLockInterruptibly();
}
public boolean tryLock() {
return tryWriteLock() != 0L;
}
@Suspendable
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() {
WNode h;
long s;
if (((s = state) & WBIT) == 0L)
throw new IllegalMonitorStateException();
state = (s += WBIT) == 0L ? ORIGIN : s;
if ((h = whead) != null && h.status != 0)
release(h);
}
final void unstampedUnlockRead() {
for (;;) {
long s, m;
WNode h;
if ((m = (s = state) & ABITS) == 0L || m >= WBIT)
throw new IllegalMonitorStateException();
else if (m < RFULL) {
if (U.compareAndSwapLong(this, STATE, s, s - RUNIT)) {
if (m == RUNIT && (h = whead) != null && h.status != 0)
release(h);
break;
}
} else if (tryDecReaderOverflow(s) != 0L)
break;
}
}
// 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 (U.compareAndSwapLong(this, STATE, s, s | RBITS)) {
++readerOverflow;
state = s;
return s;
}
} else if ((ThreadLocalRandom.current().nextInt()
& OVERFLOW_YIELD_RATE) == 0)
if (!Strand.isCurrentFiber())
Thread.yield();//Strand.yield();
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 (U.compareAndSwapLong(this, STATE, s, s | RBITS)) {
int r;
long next;
if ((r = readerOverflow) > 0) {
readerOverflow = r - 1;
next = s;
} else
next = s - RUNIT;
state = next;
return next;
}
} else if ((ThreadLocalRandom.current().nextInt()
& OVERFLOW_YIELD_RATE) == 0)
if (!Strand.isCurrentFiber())
Thread.yield();//Strand.yield();
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
* strand 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;
Strand w;
U.compareAndSwapInt(h, WSTATUS, 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) {
for (WNode r = q;;) { // release co-waiters too
if ((w = r.strand) != null) {
r.strand = null;
w.unpark();
}
if ((r = q.cowait) == null)
break;
U.compareAndSwapObject(q, WCOWAIT, r, r.cowait);
}
}
}
}
/**
* 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) throws SuspendExecution {
WNode node = null, p;
for (int spins = -1;;) { // spin while enqueuing
long s, ns;
if (((s = state) & ABITS) == 0L) {
if (U.compareAndSwapLong(this, STATE, s, ns = s + WBIT))
return ns;
} else if (spins > 0) {
if (ThreadLocalRandom.current().nextInt() >= 0)
--spins;
} else if ((p = wtail) == null) { // initialize queue
WNode h = new WNode(WMODE, null);
if (U.compareAndSwapObject(this, WHEAD, null, h))
wtail = h;
} else if (spins < 0)
spins = (p == whead) ? SPINS : 0;
else if (node == null)
node = new WNode(WMODE, p);
else if (node.prev != p)
node.prev = p;
else if (U.compareAndSwapObject(this, WTAIL, p, node)) {
p.next = node;
break;
}
}
for (int spins = SPINS;;) {
WNode np, pp;
int ps;
long s, ns;
Strand w;
while ((np = node.prev) != p && np != null)
(p = np).next = node; // stale
if (whead == p) {
for (int k = spins;;) { // spin at head
if (((s = state) & ABITS) == 0L) {
if (U.compareAndSwapLong(this, STATE, s, ns = s + WBIT)) {
whead = node;
node.prev = null;
return ns;
}
} else if (ThreadLocalRandom.current().nextInt() >= 0
&& --k <= 0)
break;
}
if (spins < MAX_HEAD_SPINS)
spins <<= 1;
}
if ((ps = p.status) == 0)
U.compareAndSwapInt(p, WSTATUS, 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);
node.strand = Strand.currentStrand();
if (node.prev == p && p.status == WAITING && // recheck
(p != whead || (state & ABITS) != 0L))
park(time);
node.strand = null;
if (interruptible && Strand.interrupted())
return cancelWaiter(node, node, 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) throws SuspendExecution {
WNode node = null, group = null, p;
for (int spins = -1;;) {
for (;;) {
long s, m, ns;
WNode h, q;
Strand w; // anti-barging guard
if (group == null && (h = whead) != null
&& (q = h.next) != null && q.mode != RMODE)
break;
if ((m = (s = state) & ABITS) < RFULL
? U.compareAndSwapLong(this, STATE, s, ns = s + RUNIT)
: (m < WBIT && (ns = tryIncReaderOverflow(s)) != 0L)) {
if (group != null) { // help release others
for (WNode r = group;;) {
if ((w = r.strand) != null) {
r.strand = null;
w.unpark();
}
if ((r = group.cowait) == null)
break;
U.compareAndSwapObject(group, WCOWAIT, r, r.cowait);
}
}
return ns;
}
if (m >= WBIT)
break;
}
if (spins > 0) {
if (ThreadLocalRandom.current().nextInt() >= 0)
--spins;
} else if ((p = wtail) == null) {
WNode h = new WNode(WMODE, null);
if (U.compareAndSwapObject(this, WHEAD, null, h))
wtail = h;
} else if (spins < 0)
spins = (p == whead) ? SPINS : 0;
else if (node == null)
node = new WNode(WMODE, p);
else if (node.prev != p)
node.prev = p;
else if (p.mode == RMODE && p != whead) {
WNode pp = p.prev; // become co-waiter with group p
if (pp != null && p == wtail
&& U.compareAndSwapObject(p, WCOWAIT,
node.cowait = p.cowait, node)) {
node.strand = Strand.currentStrand();
for (long time;;) {
if (interruptible && Strand.interrupted())
return cancelWaiter(node, p, true);
if (deadline == 0L)
time = 0L;
else if ((time = deadline - System.nanoTime()) <= 0L)
return cancelWaiter(node, p, false);
if (node.strand == null)
break;
if (p.prev != pp || p.status == CANCELLED
|| p == whead || p.prev != pp) {
node.strand = null;
break;
}
if (node.strand == null) // must recheck
break;
park(time);
}
group = p;
}
node = null; // throw away
} else if (U.compareAndSwapObject(this, WTAIL, p, node)) {
p.next = node;
break;
}
}
for (int spins = SPINS;;) {
WNode np, pp, r;
int ps;
long m, s, ns;
Strand w;
while ((np = node.prev) != p && np != null)
(p = np).next = node;
if (whead == p) {
for (int k = spins;;) {
if ((m = (s = state) & ABITS) != WBIT) {
if (m < RFULL
? U.compareAndSwapLong(this, STATE, s, ns = s + RUNIT)
: (ns = tryIncReaderOverflow(s)) != 0L) {
whead = node;
node.prev = null;
while ((r = node.cowait) != null) {
if (U.compareAndSwapObject(node, WCOWAIT,
r, r.cowait)
&& (w = r.strand) != null) {
r.strand = null;
w.unpark(); // release co-waiter
}
}
return ns;
}
} else if (ThreadLocalRandom.current().nextInt() >= 0
&& --k <= 0)
break;
}
if (spins < MAX_HEAD_SPINS)
spins <<= 1;
}
if ((ps = p.status) == 0)
U.compareAndSwapInt(p, WSTATUS, 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);
node.strand = Strand.currentStrand();
if (node.prev == p && p.status == WAITING
&& (p != whead || (state & ABITS) != WBIT))
park(time);
node.strand = null;
if (interruptible && Strand.interrupted())
return cancelWaiter(node, node, 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 nonnull, the waiter
* @param group either node or the group node is cowaiting with
* @param interrupted if already interrupted
* @return INTERRUPTED if interrupted or Strand.interrupted, else zero
*/
private long cancelWaiter(WNode node, WNode group, boolean interrupted) {
if (node != null && group != null) {
Strand w;
node.status = CANCELLED;
node.strand = null;
// unsplice cancelled nodes from group
for (WNode p = group, q; (q = p.cowait) != null;) {
if (q.status == CANCELLED)
U.compareAndSwapObject(p, WNEXT, q, q.next);
else
p = q;
}
if (group == node) {
WNode r; // detach and wake up uncancelled co-waiters
while ((r = node.cowait) != null) {
if (U.compareAndSwapObject(node, WCOWAIT, r, r.cowait)
&& (w = r.strand) != null) {
r.strand = null;
w.unpark();
}
}
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
U.compareAndSwapObject(node, WNEXT,
succ, succ = q)) {
if (succ == null && node == wtail)
U.compareAndSwapObject(this, WTAIL, node, pred);
break;
}
}
if (pred.next == node) // unsplice pred link
U.compareAndSwapObject(pred, WNEXT, node, succ);
if (succ != null && (w = succ.strand) != null) {
succ.strand = null;
w.unpark(); // wake up succ to observe new pred
}
if (pred.status != CANCELLED || (pp = pred.prev) == null)
break;
node.prev = pp; // repeat if new pred wrong/cancelled
U.compareAndSwapObject(pp, WNEXT, 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 || Strand.interrupted()) ? INTERRUPTED : 0L;
}
// Unsafe mechanics
private static final sun.misc.Unsafe U;
private static final long STATE;
private static final long WHEAD;
private static final long WTAIL;
private static final long WNEXT;
private static final long WSTATUS;
private static final long WCOWAIT;
static {
try {
U = UtilUnsafe.getUnsafe();
Class<?> k = StampedLock.class;
Class<?> wk = WNode.class;
STATE = U.objectFieldOffset(k.getDeclaredField("state"));
WHEAD = U.objectFieldOffset(k.getDeclaredField("whead"));
WTAIL = U.objectFieldOffset(k.getDeclaredField("wtail"));
WSTATUS = U.objectFieldOffset(wk.getDeclaredField("status"));
WNEXT = U.objectFieldOffset(wk.getDeclaredField("next"));
WCOWAIT = U.objectFieldOffset(wk.getDeclaredField("cowait"));
} catch (Exception e) {
throw new Error(e);
}
}
private static void park(long time) throws SuspendExecution {
if (time != 0)
Strand.parkNanos(time);
else
Strand.park();
}
}