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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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|
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package jsr166e; |
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|
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import java.util.concurrent.ThreadLocalRandom; |
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import java.util.concurrent.TimeUnit; |
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import java.util.concurrent.locks.Lock; |
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import java.util.concurrent.locks.Condition; |
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import java.util.concurrent.locks.ReadWriteLock; |
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import java.util.concurrent.locks.LockSupport; |
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|
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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. </li> |
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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. </li> |
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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 |
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* in write mode. Method {@link #validate} returns true if the lock |
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* has not since been acquired in write mode. This mode can be |
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* thought of as an extremely weak version of a read-lock, that can |
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* be broken by a writer at any time. The use of optimistic mode |
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* for short read-only code segments often reduces contention and |
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* improves throughput. However, its use is inherently fragile. |
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* Optimistic read sections should only read fields and hold them in |
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* local variables for later use after validation. Fields read while |
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* in optimistic mode may be wildly inconsistent, so usage applies |
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* only when you are familiar enough with data representations to |
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* check consistency and/or repeatedly invoke method {@code |
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* validate()}. For example, such steps are typically required when |
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* first reading an object or array reference, and then accessing |
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* one of its fields, elements or methods. </li> |
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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 mode and |
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* 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>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>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.<br> |
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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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* void move(double deltaX, double deltaY) { // an exclusively locked method |
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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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* double distanceFromOriginV1() { // A read-only method |
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* long stamp; |
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* if ((stamp = sl.tryOptimisticRead()) != 0L) { // optimistic |
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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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* return Math.sqrt(currentX * currentX + currentY * currentY); |
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* } |
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* stamp = sl.readLock(); // fall back to read lock |
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* try { |
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* double currentX = x; |
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* double currentY = y; |
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* return Math.sqrt(currentX * currentX + currentY * currentY); |
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* } finally { |
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* sl.unlockRead(stamp); |
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* } |
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* } |
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* |
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* double distanceFromOriginV2() { // combines code paths |
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* double currentX = 0.0, currentY = 0.0; |
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* for (long stamp = sl.tryOptimisticRead(); ; stamp = sl.readLock()) { |
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* try { |
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* currentX = x; |
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* currentY = y; |
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* } finally { |
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* if (sl.tryConvertToOptimisticRead(stamp) != 0L) // unlock or validate |
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* break; |
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* } |
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* } |
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* return Math.sqrt(currentX * currentX + currentY * currentY); |
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* } |
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* |
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* void moveIfAtOrigin(double newX, double newY) { // upgrade |
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* // Could instead start with optimistic, not read mode |
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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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* @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 (see its internal documentation for |
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* a fuller account), where each node is tagged (field mode) as |
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* either a reader or writer. Sets of waiting readers are grouped |
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* (linked) under a common node (field cowait) so act as a single |
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* node with respect to most CLH mechanics. By virtue of the |
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* queue structure, wait nodes need not actually carry sequence |
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* numbers; we know each is greater than its predecessor. This |
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* simplifies the scheduling policy to a mainly-FIFO scheme that |
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* incorporates elements of Phase-Fair locks (see Brandenburg & |
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* Anderson, especially http://www.cs.unc.edu/~bbb/diss/). In |
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* particular, we use the phase-fair anti-barging rule: If an |
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* incoming reader arrives while read lock is held but there is a |
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* queued writer, this incoming reader is queued. (This rule is |
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* responsible for some of the complexity of method acquireRead, |
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* but without it, the lock becomes highly unfair.) |
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* |
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* These rules apply to threads actually queued. All tryLock forms |
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* opportunistically try to acquire locks regardless of preference |
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* rules, and so may "barge" their way in. Randomized spinning is |
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* used in the acquire methods to reduce (increasingly expensive) |
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* context switching while also avoiding sustained memory |
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* thrashing among many threads. We limit spins to the head of |
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* queue. A thread spin-waits up to SPINS times (where each |
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* iteration decreases spin count with 50% probability) before |
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* blocking. If, upon wakening it fails to obtain lock, and is |
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* still (or becomes) the first waiting thread (which indicates |
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* that some other thread barged and obtained lock), it escalates |
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* spins (up to MAX_HEAD_SPINS) to reduce the likelihood of |
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* continually losing to barging threads. |
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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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* As noted in Boehm's paper (above), sequence validation (mainly |
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* method validate()) requires stricter ordering rules than apply |
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* to normal volatile reads (of "state"). In the absence of (but |
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* continual hope for) explicit JVM support of intrinsics with |
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* double-sided reordering prohibition, or corresponding fence |
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* intrinsics, we for now uncomfortably rely on the fact that the |
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* Unsafe.getXVolatile intrinsic must have this property |
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* (syntactic volatile reads do not) for internal purposes anyway, |
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* even though it is not documented. |
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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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* adaptive-spin CLH locks to reduce memory contention lessens |
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* motivation to further spread out contended locations, but might |
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* be subject to future improvements. |
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*/ |
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private static final long serialVersionUID = -6001602636862214147L; |
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|
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/** Number of processors, for spin control */ |
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private static final int NCPU = Runtime.getRuntime().availableProcessors(); |
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|
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/** Maximum number of retries before blocking on acquisition */ |
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private static final int SPINS = (NCPU > 1) ? 1 << 6 : 0; |
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|
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/** Maximum number of retries before re-blocking */ |
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private static final int MAX_HEAD_SPINS = (NCPU > 1) ? 1 << 12 : 0; |
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|
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/** The period for yielding when waiting for overflow spinlock */ |
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private static final int OVERFLOW_YIELD_RATE = 7; // must be power 2 - 1 |
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|
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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; |
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|
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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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|
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// Initial value for lock state; avoid failure value zero |
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private static final long ORIGIN = WBIT << 1; |
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|
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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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|
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// Values for node status; order matters |
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private static final int WAITING = -1; |
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private static final int CANCELLED = 1; |
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|
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// Modes for nodes (int not boolean to allow arithmetic) |
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private static final int RMODE = 0; |
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private static final int WMODE = 1; |
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|
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/** Wait nodes */ |
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static final class WNode { |
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volatile WNode prev; |
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volatile WNode next; |
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volatile WNode cowait; // list of linked readers |
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volatile Thread thread; // non-null while possibly parked |
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volatile int status; // 0, WAITING, or CANCELLED |
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final int mode; // RMODE or WMODE |
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WNode(int m, WNode p) { mode = m; prev = p; } |
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} |
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|
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/** Head of CLH queue */ |
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private transient volatile WNode whead; |
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/** Tail (last) of CLH queue */ |
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private transient volatile WNode wtail; |
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|
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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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|
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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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/** |
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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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|
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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 stamp that can be used to unlock or convert mode |
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*/ |
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public long writeLock() { |
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long s, next; // bypass acquireWrite in fully unlocked case only |
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return ((((s = state) & ABITS) == 0L && |
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U.compareAndSwapLong(this, STATE, s, next = s + WBIT)) ? |
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next : acquireWrite(false, 0L)); |
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} |
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|
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/** |
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* Exclusively acquires the lock if it is immediately available. |
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* |
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* @return a stamp that can be used to unlock or convert mode, |
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* or zero if the lock is not available |
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*/ |
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public long tryWriteLock() { |
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long s, next; |
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return ((((s = state) & ABITS) == 0L && |
349 |
U.compareAndSwapLong(this, STATE, s, next = s + WBIT)) ? |
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next : 0L); |
351 |
} |
352 |
|
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/** |
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* Exclusively acquires the lock if it is available within the |
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* given time and the current thread has not been interrupted. |
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* Behavior under timeout and interruption matches that specified |
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* for method {@link Lock#tryLock(long,TimeUnit)}. |
358 |
* |
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* @return a stamp that can be used to unlock or convert mode, |
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* or zero if the lock is not available |
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* @throws InterruptedException if the current thread is interrupted |
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* before acquiring the lock |
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*/ |
364 |
public long tryWriteLock(long time, TimeUnit unit) |
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throws InterruptedException { |
366 |
long nanos = unit.toNanos(time); |
367 |
if (!Thread.interrupted()) { |
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long next, deadline; |
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if ((next = tryWriteLock()) != 0L) |
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return next; |
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if (nanos <= 0L) |
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return 0L; |
373 |
if ((deadline = System.nanoTime() + nanos) == 0L) |
374 |
deadline = 1L; |
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if ((next = acquireWrite(true, deadline)) != INTERRUPTED) |
376 |
return next; |
377 |
} |
378 |
throw new InterruptedException(); |
379 |
} |
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|
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/** |
382 |
* Exclusively acquires the lock, blocking if necessary |
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* until available or the current thread is interrupted. |
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* Behavior under interruption matches that specified |
385 |
* for method {@link Lock#lockInterruptibly()}. |
386 |
* |
387 |
* @return a stamp that can be used to unlock or convert mode |
388 |
* @throws InterruptedException if the current thread is interrupted |
389 |
* before acquiring the lock |
390 |
*/ |
391 |
public long writeLockInterruptibly() throws InterruptedException { |
392 |
long next; |
393 |
if (!Thread.interrupted() && |
394 |
(next = acquireWrite(true, 0L)) != INTERRUPTED) |
395 |
return next; |
396 |
throw new InterruptedException(); |
397 |
} |
398 |
|
399 |
/** |
400 |
* Non-exclusively acquires the lock, blocking if necessary |
401 |
* until available. |
402 |
* |
403 |
* @return a stamp that can be used to unlock or convert mode |
404 |
*/ |
405 |
public long readLock() { |
406 |
long s, next; // bypass acquireRead on fully unlocked case only |
407 |
return ((((s = state) & ABITS) == 0L && |
408 |
U.compareAndSwapLong(this, STATE, s, next = s + RUNIT)) ? |
409 |
next : acquireRead(false, 0L)); |
410 |
} |
411 |
|
412 |
/** |
413 |
* Non-exclusively acquires the lock if it is immediately available. |
414 |
* |
415 |
* @return a stamp that can be used to unlock or convert mode, |
416 |
* or zero if the lock is not available |
417 |
*/ |
418 |
public long tryReadLock() { |
419 |
for (;;) { |
420 |
long s, m, next; |
421 |
if ((m = (s = state) & ABITS) == WBIT) |
422 |
return 0L; |
423 |
else if (m < RFULL) { |
424 |
if (U.compareAndSwapLong(this, STATE, s, next = s + RUNIT)) |
425 |
return next; |
426 |
} |
427 |
else if ((next = tryIncReaderOverflow(s)) != 0L) |
428 |
return next; |
429 |
} |
430 |
} |
431 |
|
432 |
/** |
433 |
* Non-exclusively acquires the lock if it is available within the |
434 |
* given time and the current thread has not been interrupted. |
435 |
* Behavior under timeout and interruption matches that specified |
436 |
* for method {@link Lock#tryLock(long,TimeUnit)}. |
437 |
* |
438 |
* @return a stamp that can be used to unlock or convert mode, |
439 |
* or zero if the lock is not available |
440 |
* @throws InterruptedException if the current thread is interrupted |
441 |
* before acquiring the lock |
442 |
*/ |
443 |
public long tryReadLock(long time, TimeUnit unit) |
444 |
throws InterruptedException { |
445 |
long next, deadline; |
446 |
long nanos = unit.toNanos(time); |
447 |
if (!Thread.interrupted()) { |
448 |
if ((next = tryReadLock()) != 0L) |
449 |
return next; |
450 |
if (nanos <= 0L) |
451 |
return 0L; |
452 |
if ((deadline = System.nanoTime() + nanos) == 0L) |
453 |
deadline = 1L; |
454 |
if ((next = acquireRead(true, deadline)) != INTERRUPTED) |
455 |
return next; |
456 |
} |
457 |
throw new InterruptedException(); |
458 |
} |
459 |
|
460 |
/** |
461 |
* Non-exclusively acquires the lock, blocking if necessary |
462 |
* until available or the current thread is interrupted. |
463 |
* Behavior under interruption matches that specified |
464 |
* for method {@link Lock#lockInterruptibly()}. |
465 |
* |
466 |
* @return a stamp that can be used to unlock or convert mode |
467 |
* @throws InterruptedException if the current thread is interrupted |
468 |
* before acquiring the lock |
469 |
*/ |
470 |
public long readLockInterruptibly() throws InterruptedException { |
471 |
long next; |
472 |
if (!Thread.interrupted() && |
473 |
(next = acquireRead(true, 0L)) != INTERRUPTED) |
474 |
return next; |
475 |
throw new InterruptedException(); |
476 |
} |
477 |
|
478 |
/** |
479 |
* Returns a stamp that can later be validated, or zero |
480 |
* if exclusively locked. |
481 |
* |
482 |
* @return a stamp, or zero if exclusively locked |
483 |
*/ |
484 |
public long tryOptimisticRead() { |
485 |
long s; |
486 |
return (((s = state) & WBIT) == 0L) ? (s & SBITS) : 0L; |
487 |
} |
488 |
|
489 |
/** |
490 |
* Returns true if the lock has not been exclusively acquired |
491 |
* since issuance of the given stamp. Always returns false if the |
492 |
* stamp is zero. Always returns true if the stamp represents a |
493 |
* currently held lock. |
494 |
* |
495 |
* @return true if the lock has not been exclusively acquired |
496 |
* since issuance of the given stamp; else false |
497 |
*/ |
498 |
public boolean validate(long stamp) { |
499 |
// See above about current use of getLongVolatile here |
500 |
return (stamp & SBITS) == (U.getLongVolatile(this, STATE) & SBITS); |
501 |
} |
502 |
|
503 |
/** |
504 |
* If the lock state matches the given stamp, releases the |
505 |
* exclusive lock. |
506 |
* |
507 |
* @param stamp a stamp returned by a write-lock operation |
508 |
* @throws IllegalMonitorStateException if the stamp does |
509 |
* not match the current state of this lock |
510 |
*/ |
511 |
public void unlockWrite(long stamp) { |
512 |
WNode h; |
513 |
if (state != stamp || (stamp & WBIT) == 0L) |
514 |
throw new IllegalMonitorStateException(); |
515 |
state = (stamp += WBIT) == 0L ? ORIGIN : stamp; |
516 |
if ((h = whead) != null && h.status != 0) |
517 |
release(h); |
518 |
} |
519 |
|
520 |
/** |
521 |
* If the lock state matches the given stamp, releases the |
522 |
* non-exclusive lock. |
523 |
* |
524 |
* @param stamp a stamp returned by a read-lock operation |
525 |
* @throws IllegalMonitorStateException if the stamp does |
526 |
* not match the current state of this lock |
527 |
*/ |
528 |
public void unlockRead(long stamp) { |
529 |
long s, m; WNode h; |
530 |
for (;;) { |
531 |
if (((s = state) & SBITS) != (stamp & SBITS) || |
532 |
(stamp & ABITS) == 0L || (m = s & ABITS) == 0L || m == WBIT) |
533 |
throw new IllegalMonitorStateException(); |
534 |
if (m < RFULL) { |
535 |
if (U.compareAndSwapLong(this, STATE, s, s - RUNIT)) { |
536 |
if (m == RUNIT && (h = whead) != null && h.status != 0) |
537 |
release(h); |
538 |
break; |
539 |
} |
540 |
} |
541 |
else if (tryDecReaderOverflow(s) != 0L) |
542 |
break; |
543 |
} |
544 |
} |
545 |
|
546 |
/** |
547 |
* If the lock state matches the given stamp, releases the |
548 |
* corresponding mode of the lock. |
549 |
* |
550 |
* @param stamp a stamp returned by a lock operation |
551 |
* @throws IllegalMonitorStateException if the stamp does |
552 |
* not match the current state of this lock |
553 |
*/ |
554 |
public void unlock(long stamp) { |
555 |
long a = stamp & ABITS, m, s; WNode h; |
556 |
while (((s = state) & SBITS) == (stamp & SBITS)) { |
557 |
if ((m = s & ABITS) == 0L) |
558 |
break; |
559 |
else if (m == WBIT) { |
560 |
if (a != m) |
561 |
break; |
562 |
state = (s += WBIT) == 0L ? ORIGIN : s; |
563 |
if ((h = whead) != null && h.status != 0) |
564 |
release(h); |
565 |
return; |
566 |
} |
567 |
else if (a == 0L || a >= WBIT) |
568 |
break; |
569 |
else if (m < RFULL) { |
570 |
if (U.compareAndSwapLong(this, STATE, s, s - RUNIT)) { |
571 |
if (m == RUNIT && (h = whead) != null && h.status != 0) |
572 |
release(h); |
573 |
return; |
574 |
} |
575 |
} |
576 |
else if (tryDecReaderOverflow(s) != 0L) |
577 |
return; |
578 |
} |
579 |
throw new IllegalMonitorStateException(); |
580 |
} |
581 |
|
582 |
/** |
583 |
* If the lock state matches the given stamp, performs one of |
584 |
* the following actions. If the stamp represents holding a write |
585 |
* lock, returns it. Or, if a read lock, if the write lock is |
586 |
* available, releases the read lock and returns a write stamp. |
587 |
* Or, if an optimistic read, returns a write stamp only if |
588 |
* immediately available. This method returns zero in all other |
589 |
* cases. |
590 |
* |
591 |
* @param stamp a stamp |
592 |
* @return a valid write stamp, or zero on failure |
593 |
*/ |
594 |
public long tryConvertToWriteLock(long stamp) { |
595 |
long a = stamp & ABITS, m, s, next; |
596 |
while (((s = state) & SBITS) == (stamp & SBITS)) { |
597 |
if ((m = s & ABITS) == 0L) { |
598 |
if (a != 0L) |
599 |
break; |
600 |
if (U.compareAndSwapLong(this, STATE, s, next = s + WBIT)) |
601 |
return next; |
602 |
} |
603 |
else if (m == WBIT) { |
604 |
if (a != m) |
605 |
break; |
606 |
return stamp; |
607 |
} |
608 |
else if (m == RUNIT && a != 0L) { |
609 |
if (U.compareAndSwapLong(this, STATE, s, |
610 |
next = s - RUNIT + WBIT)) |
611 |
return next; |
612 |
} |
613 |
else |
614 |
break; |
615 |
} |
616 |
return 0L; |
617 |
} |
618 |
|
619 |
/** |
620 |
* If the lock state matches the given stamp, performs one of |
621 |
* the following actions. If the stamp represents holding a write |
622 |
* lock, releases it and obtains a read lock. Or, if a read lock, |
623 |
* returns it. Or, if an optimistic read, acquires a read lock and |
624 |
* returns a read stamp only if immediately available. This method |
625 |
* returns zero in all other cases. |
626 |
* |
627 |
* @param stamp a stamp |
628 |
* @return a valid read stamp, or zero on failure |
629 |
*/ |
630 |
public long tryConvertToReadLock(long stamp) { |
631 |
long a = stamp & ABITS, m, s, next; WNode h; |
632 |
while (((s = state) & SBITS) == (stamp & SBITS)) { |
633 |
if ((m = s & ABITS) == 0L) { |
634 |
if (a != 0L) |
635 |
break; |
636 |
else if (m < RFULL) { |
637 |
if (U.compareAndSwapLong(this, STATE, s, next = s + RUNIT)) |
638 |
return next; |
639 |
} |
640 |
else if ((next = tryIncReaderOverflow(s)) != 0L) |
641 |
return next; |
642 |
} |
643 |
else if (m == WBIT) { |
644 |
if (a != m) |
645 |
break; |
646 |
state = next = s + (WBIT + RUNIT); |
647 |
if ((h = whead) != null && h.status != 0) |
648 |
release(h); |
649 |
return next; |
650 |
} |
651 |
else if (a != 0L && a < WBIT) |
652 |
return stamp; |
653 |
else |
654 |
break; |
655 |
} |
656 |
return 0L; |
657 |
} |
658 |
|
659 |
/** |
660 |
* If the lock state matches the given stamp then, if the stamp |
661 |
* represents holding a lock, releases it and returns an |
662 |
* observation stamp. Or, if an optimistic read, returns it if |
663 |
* validated. This method returns zero in all other cases, and so |
664 |
* may be useful as a form of "tryUnlock". |
665 |
* |
666 |
* @param stamp a stamp |
667 |
* @return a valid optimistic read stamp, or zero on failure |
668 |
*/ |
669 |
public long tryConvertToOptimisticRead(long stamp) { |
670 |
long a = stamp & ABITS, m, s, next; WNode h; |
671 |
for (;;) { |
672 |
s = U.getLongVolatile(this, STATE); // see above |
673 |
if ((s & SBITS) != (stamp & SBITS)) |
674 |
break; |
675 |
if ((m = s & ABITS) == 0L) { |
676 |
if (a != 0L) |
677 |
break; |
678 |
return s; |
679 |
} |
680 |
else if (m == WBIT) { |
681 |
if (a != m) |
682 |
break; |
683 |
state = next = (s += WBIT) == 0L ? ORIGIN : s; |
684 |
if ((h = whead) != null && h.status != 0) |
685 |
release(h); |
686 |
return next; |
687 |
} |
688 |
else if (a == 0L || a >= WBIT) |
689 |
break; |
690 |
else if (m < RFULL) { |
691 |
if (U.compareAndSwapLong(this, STATE, s, next = s - RUNIT)) { |
692 |
if (m == RUNIT && (h = whead) != null && h.status != 0) |
693 |
release(h); |
694 |
return next & SBITS; |
695 |
} |
696 |
} |
697 |
else if ((next = tryDecReaderOverflow(s)) != 0L) |
698 |
return next & SBITS; |
699 |
} |
700 |
return 0L; |
701 |
} |
702 |
|
703 |
/** |
704 |
* Releases the write lock if it is held, without requiring a |
705 |
* stamp value. This method may be useful for recovery after |
706 |
* errors. |
707 |
* |
708 |
* @return true if the lock was held, else false |
709 |
*/ |
710 |
public boolean tryUnlockWrite() { |
711 |
long s; WNode h; |
712 |
if (((s = state) & WBIT) != 0L) { |
713 |
state = (s += WBIT) == 0L ? ORIGIN : s; |
714 |
if ((h = whead) != null && h.status != 0) |
715 |
release(h); |
716 |
return true; |
717 |
} |
718 |
return false; |
719 |
} |
720 |
|
721 |
/** |
722 |
* Releases one hold of the read lock if it is held, without |
723 |
* requiring a stamp value. This method may be useful for recovery |
724 |
* after errors. |
725 |
* |
726 |
* @return true if the read lock was held, else false |
727 |
*/ |
728 |
public boolean tryUnlockRead() { |
729 |
long s, m; WNode h; |
730 |
while ((m = (s = state) & ABITS) != 0L && m < WBIT) { |
731 |
if (m < RFULL) { |
732 |
if (U.compareAndSwapLong(this, STATE, s, s - RUNIT)) { |
733 |
if (m == RUNIT && (h = whead) != null && h.status != 0) |
734 |
release(h); |
735 |
return true; |
736 |
} |
737 |
} |
738 |
else if (tryDecReaderOverflow(s) != 0L) |
739 |
return true; |
740 |
} |
741 |
return false; |
742 |
} |
743 |
|
744 |
/** |
745 |
* Returns true if the lock is currently held exclusively. |
746 |
* |
747 |
* @return true if the lock is currently held exclusively |
748 |
*/ |
749 |
public boolean isWriteLocked() { |
750 |
return (state & WBIT) != 0L; |
751 |
} |
752 |
|
753 |
/** |
754 |
* Returns true if the lock is currently held non-exclusively. |
755 |
* |
756 |
* @return true if the lock is currently held non-exclusively |
757 |
*/ |
758 |
public boolean isReadLocked() { |
759 |
return (state & RBITS) != 0L; |
760 |
} |
761 |
|
762 |
private void readObject(java.io.ObjectInputStream s) |
763 |
throws java.io.IOException, ClassNotFoundException { |
764 |
s.defaultReadObject(); |
765 |
state = ORIGIN; // reset to unlocked state |
766 |
} |
767 |
|
768 |
/** |
769 |
* Returns a plain {@link Lock} view of this StampedLock in which |
770 |
* the {@link Lock#lock} method is mapped to {@link #readLock}, |
771 |
* and similarly for other methods. The returned Lock does not |
772 |
* support a {@link Condition}; method {@link |
773 |
* Lock#newCondition()} throws {@code |
774 |
* UnsupportedOperationException}. |
775 |
* |
776 |
* @return the lock |
777 |
*/ |
778 |
public Lock asReadLock() { |
779 |
ReadLockView v; |
780 |
return ((v = readLockView) != null ? v : |
781 |
(readLockView = new ReadLockView())); |
782 |
} |
783 |
|
784 |
/** |
785 |
* Returns a plain {@link Lock} view of this StampedLock in which |
786 |
* the {@link Lock#lock} method is mapped to {@link #writeLock}, |
787 |
* and similarly for other methods. The returned Lock does not |
788 |
* support a {@link Condition}; method {@link |
789 |
* Lock#newCondition()} throws {@code |
790 |
* UnsupportedOperationException}. |
791 |
* |
792 |
* @return the lock |
793 |
*/ |
794 |
public Lock asWriteLock() { |
795 |
WriteLockView v; |
796 |
return ((v = writeLockView) != null ? v : |
797 |
(writeLockView = new WriteLockView())); |
798 |
} |
799 |
|
800 |
/** |
801 |
* Returns a {@link ReadWriteLock} view of this StampedLock in |
802 |
* which the {@link ReadWriteLock#readLock()} method is mapped to |
803 |
* {@link #asReadLock()}, and {@link ReadWriteLock#writeLock()} to |
804 |
* {@link #asWriteLock()}. |
805 |
* |
806 |
* @return the lock |
807 |
*/ |
808 |
public ReadWriteLock asReadWriteLock() { |
809 |
ReadWriteLockView v; |
810 |
return ((v = readWriteLockView) != null ? v : |
811 |
(readWriteLockView = new ReadWriteLockView())); |
812 |
} |
813 |
|
814 |
// view classes |
815 |
|
816 |
final class ReadLockView implements Lock { |
817 |
public void lock() { readLock(); } |
818 |
public void lockInterruptibly() throws InterruptedException { |
819 |
readLockInterruptibly(); |
820 |
} |
821 |
public boolean tryLock() { return tryReadLock() != 0L; } |
822 |
public boolean tryLock(long time, TimeUnit unit) |
823 |
throws InterruptedException { |
824 |
return tryReadLock(time, unit) != 0L; |
825 |
} |
826 |
public void unlock() { unstampedUnlockRead(); } |
827 |
public Condition newCondition() { |
828 |
throw new UnsupportedOperationException(); |
829 |
} |
830 |
} |
831 |
|
832 |
final class WriteLockView implements Lock { |
833 |
public void lock() { writeLock(); } |
834 |
public void lockInterruptibly() throws InterruptedException { |
835 |
writeLockInterruptibly(); |
836 |
} |
837 |
public boolean tryLock() { return tryWriteLock() != 0L; } |
838 |
public boolean tryLock(long time, TimeUnit unit) |
839 |
throws InterruptedException { |
840 |
return tryWriteLock(time, unit) != 0L; |
841 |
} |
842 |
public void unlock() { unstampedUnlockWrite(); } |
843 |
public Condition newCondition() { |
844 |
throw new UnsupportedOperationException(); |
845 |
} |
846 |
} |
847 |
|
848 |
final class ReadWriteLockView implements ReadWriteLock { |
849 |
public Lock readLock() { return asReadLock(); } |
850 |
public Lock writeLock() { return asWriteLock(); } |
851 |
} |
852 |
|
853 |
// Unlock methods without stamp argument checks for view classes. |
854 |
// Needed because view-class lock methods throw away stamps. |
855 |
|
856 |
final void unstampedUnlockWrite() { |
857 |
WNode h; long s; |
858 |
if (((s = state) & WBIT) == 0L) |
859 |
throw new IllegalMonitorStateException(); |
860 |
state = (s += WBIT) == 0L ? ORIGIN : s; |
861 |
if ((h = whead) != null && h.status != 0) |
862 |
release(h); |
863 |
} |
864 |
|
865 |
final void unstampedUnlockRead() { |
866 |
for (;;) { |
867 |
long s, m; WNode h; |
868 |
if ((m = (s = state) & ABITS) == 0L || m >= WBIT) |
869 |
throw new IllegalMonitorStateException(); |
870 |
else if (m < RFULL) { |
871 |
if (U.compareAndSwapLong(this, STATE, s, s - RUNIT)) { |
872 |
if (m == RUNIT && (h = whead) != null && h.status != 0) |
873 |
release(h); |
874 |
break; |
875 |
} |
876 |
} |
877 |
else if (tryDecReaderOverflow(s) != 0L) |
878 |
break; |
879 |
} |
880 |
} |
881 |
|
882 |
// internals |
883 |
|
884 |
/** |
885 |
* Tries to increment readerOverflow by first setting state |
886 |
* access bits value to RBITS, indicating hold of spinlock, |
887 |
* then updating, then releasing. |
888 |
* |
889 |
* @param s, assumed that (s & ABITS) >= RFULL |
890 |
* @return new stamp on success, else zero |
891 |
*/ |
892 |
private long tryIncReaderOverflow(long s) { |
893 |
if ((s & ABITS) == RFULL) { |
894 |
if (U.compareAndSwapLong(this, STATE, s, s | RBITS)) { |
895 |
++readerOverflow; |
896 |
state = s; |
897 |
return s; |
898 |
} |
899 |
} |
900 |
else if ((ThreadLocalRandom.current().nextInt() & |
901 |
OVERFLOW_YIELD_RATE) == 0) |
902 |
Thread.yield(); |
903 |
return 0L; |
904 |
} |
905 |
|
906 |
/** |
907 |
* Tries to decrement readerOverflow. |
908 |
* |
909 |
* @param s, assumed that (s & ABITS) >= RFULL |
910 |
* @return new stamp on success, else zero |
911 |
*/ |
912 |
private long tryDecReaderOverflow(long s) { |
913 |
if ((s & ABITS) == RFULL) { |
914 |
if (U.compareAndSwapLong(this, STATE, s, s | RBITS)) { |
915 |
int r; long next; |
916 |
if ((r = readerOverflow) > 0) { |
917 |
readerOverflow = r - 1; |
918 |
next = s; |
919 |
} |
920 |
else |
921 |
next = s - RUNIT; |
922 |
state = next; |
923 |
return next; |
924 |
} |
925 |
} |
926 |
else if ((ThreadLocalRandom.current().nextInt() & |
927 |
OVERFLOW_YIELD_RATE) == 0) |
928 |
Thread.yield(); |
929 |
return 0L; |
930 |
} |
931 |
|
932 |
/* |
933 |
* Wakes up the successor of h (normally whead). This is normally |
934 |
* just h.next, but may require traversal from wtail if next |
935 |
* pointers are lagging. This may fail to wake up an acquiring |
936 |
* thread when one or more have been cancelled, but the cancel |
937 |
* methods themselves provide extra safeguards to ensure liveness. |
938 |
*/ |
939 |
private void release(WNode h) { |
940 |
if (h != null) { |
941 |
WNode q; Thread w; |
942 |
U.compareAndSwapInt(h, WSTATUS, WAITING, 0); |
943 |
if ((q = h.next) == null || q.status == CANCELLED) { |
944 |
for (WNode t = wtail; t != null && t != h; t = t.prev) |
945 |
if (t.status <= 0) |
946 |
q = t; |
947 |
} |
948 |
if (q != null) { |
949 |
for (WNode r = q;;) { // release co-waiters too |
950 |
if ((w = r.thread) != null) { |
951 |
r.thread = null; |
952 |
U.unpark(w); |
953 |
} |
954 |
if ((r = q.cowait) == null) |
955 |
break; |
956 |
U.compareAndSwapObject(q, WCOWAIT, r, r.cowait); |
957 |
} |
958 |
} |
959 |
} |
960 |
} |
961 |
|
962 |
/** |
963 |
* See above for explanation. |
964 |
* |
965 |
* @param interruptible true if should check interrupts and if so |
966 |
* return INTERRUPTED |
967 |
* @param deadline if nonzero, the System.nanoTime value to timeout |
968 |
* at (and return zero) |
969 |
* @return next state, or INTERRUPTED |
970 |
*/ |
971 |
private long acquireWrite(boolean interruptible, long deadline) { |
972 |
WNode node = null, p; |
973 |
for (int spins = -1;;) { // spin while enqueuing |
974 |
long s, ns; |
975 |
if (((s = state) & ABITS) == 0L) { |
976 |
if (U.compareAndSwapLong(this, STATE, s, ns = s + WBIT)) |
977 |
return ns; |
978 |
} |
979 |
else if (spins > 0) { |
980 |
if (ThreadLocalRandom.current().nextInt() >= 0) |
981 |
--spins; |
982 |
} |
983 |
else if ((p = wtail) == null) { // initialize queue |
984 |
WNode h = new WNode(WMODE, null); |
985 |
if (U.compareAndSwapObject(this, WHEAD, null, h)) |
986 |
wtail = h; |
987 |
} |
988 |
else if (spins < 0) |
989 |
spins = (p == whead) ? SPINS : 0; |
990 |
else if (node == null) |
991 |
node = new WNode(WMODE, p); |
992 |
else if (node.prev != p) |
993 |
node.prev = p; |
994 |
else if (U.compareAndSwapObject(this, WTAIL, p, node)) { |
995 |
p.next = node; |
996 |
break; |
997 |
} |
998 |
} |
999 |
|
1000 |
for (int spins = SPINS;;) { |
1001 |
WNode np, pp; int ps; long s, ns; Thread w; |
1002 |
while ((np = node.prev) != p && np != null) |
1003 |
(p = np).next = node; // stale |
1004 |
if (whead == p) { |
1005 |
for (int k = spins;;) { // spin at head |
1006 |
if (((s = state) & ABITS) == 0L) { |
1007 |
if (U.compareAndSwapLong(this, STATE, s, ns = s+WBIT)) { |
1008 |
whead = node; |
1009 |
node.prev = null; |
1010 |
return ns; |
1011 |
} |
1012 |
} |
1013 |
else if (ThreadLocalRandom.current().nextInt() >= 0 && |
1014 |
--k <= 0) |
1015 |
break; |
1016 |
} |
1017 |
if (spins < MAX_HEAD_SPINS) |
1018 |
spins <<= 1; |
1019 |
} |
1020 |
if ((ps = p.status) == 0) |
1021 |
U.compareAndSwapInt(p, WSTATUS, 0, WAITING); |
1022 |
else if (ps == CANCELLED) { |
1023 |
if ((pp = p.prev) != null) { |
1024 |
node.prev = pp; |
1025 |
pp.next = node; |
1026 |
} |
1027 |
} |
1028 |
else { |
1029 |
long time; // 0 argument to park means no timeout |
1030 |
if (deadline == 0L) |
1031 |
time = 0L; |
1032 |
else if ((time = deadline - System.nanoTime()) <= 0L) |
1033 |
return cancelWaiter(node, null, false); |
1034 |
node.thread = Thread.currentThread(); |
1035 |
if (node.prev == p && p.status == WAITING && // recheck |
1036 |
(p != whead || (state & ABITS) != 0L)) |
1037 |
U.park(false, time); |
1038 |
node.thread = null; |
1039 |
if (interruptible && Thread.interrupted()) |
1040 |
return cancelWaiter(node, null, true); |
1041 |
} |
1042 |
} |
1043 |
} |
1044 |
|
1045 |
/** |
1046 |
* See above for explanation. |
1047 |
* |
1048 |
* @param interruptible true if should check interrupts and if so |
1049 |
* return INTERRUPTED |
1050 |
* @param deadline if nonzero, the System.nanoTime value to timeout |
1051 |
* at (and return zero) |
1052 |
* @return next state, or INTERRUPTED |
1053 |
*/ |
1054 |
private long acquireRead(boolean interruptible, long deadline) { |
1055 |
WNode node = null, group = null, p; |
1056 |
for (int spins = -1;;) { |
1057 |
for (;;) { |
1058 |
long s, m, ns; WNode h, q; Thread w; // anti-barging guard |
1059 |
if (group == null && (h = whead) != null && |
1060 |
(q = h.next) != null && q.mode != RMODE) |
1061 |
break; |
1062 |
if ((m = (s = state) & ABITS) < RFULL ? |
1063 |
U.compareAndSwapLong(this, STATE, s, ns = s + RUNIT) : |
1064 |
(m < WBIT && (ns = tryIncReaderOverflow(s)) != 0L)) { |
1065 |
if (group != null) { // help release others |
1066 |
for (WNode r = group;;) { |
1067 |
if ((w = r.thread) != null) { |
1068 |
r.thread = null; |
1069 |
U.unpark(w); |
1070 |
} |
1071 |
if ((r = group.cowait) == null) |
1072 |
break; |
1073 |
U.compareAndSwapObject(group, WCOWAIT, r, r.cowait); |
1074 |
} |
1075 |
} |
1076 |
return ns; |
1077 |
} |
1078 |
if (m >= WBIT) |
1079 |
break; |
1080 |
} |
1081 |
if (spins > 0) { |
1082 |
if (ThreadLocalRandom.current().nextInt() >= 0) |
1083 |
--spins; |
1084 |
} |
1085 |
else if ((p = wtail) == null) { |
1086 |
WNode h = new WNode(WMODE, null); |
1087 |
if (U.compareAndSwapObject(this, WHEAD, null, h)) |
1088 |
wtail = h; |
1089 |
} |
1090 |
else if (spins < 0) |
1091 |
spins = (p == whead) ? SPINS : 0; |
1092 |
else if (node == null) |
1093 |
node = new WNode(WMODE, p); |
1094 |
else if (node.prev != p) |
1095 |
node.prev = p; |
1096 |
else if (p.mode == RMODE && p != whead) { |
1097 |
WNode pp = p.prev; // become co-waiter with group p |
1098 |
if (pp != null && p == wtail && |
1099 |
U.compareAndSwapObject(p, WCOWAIT, |
1100 |
node.cowait = p.cowait, node)) { |
1101 |
node.thread = Thread.currentThread(); |
1102 |
for (long time;;) { |
1103 |
if (interruptible && Thread.interrupted()) |
1104 |
return cancelWaiter(node, p, true); |
1105 |
if (deadline == 0L) |
1106 |
time = 0L; |
1107 |
else if ((time = deadline - System.nanoTime()) <= 0L) |
1108 |
return cancelWaiter(node, p, false); |
1109 |
if (node.thread == null) |
1110 |
break; |
1111 |
if (p.prev != pp || p.status == CANCELLED || |
1112 |
p == whead || p.prev != pp) { |
1113 |
node.thread = null; |
1114 |
break; |
1115 |
} |
1116 |
if (node.thread == null) // must recheck |
1117 |
break; |
1118 |
U.park(false, time); |
1119 |
} |
1120 |
group = p; |
1121 |
} |
1122 |
node = null; // throw away |
1123 |
} |
1124 |
else if (U.compareAndSwapObject(this, WTAIL, p, node)) { |
1125 |
p.next = node; |
1126 |
break; |
1127 |
} |
1128 |
} |
1129 |
|
1130 |
for (int spins = SPINS;;) { |
1131 |
WNode np, pp, r; int ps; long m, s, ns; Thread w; |
1132 |
while ((np = node.prev) != p && np != null) |
1133 |
(p = np).next = node; |
1134 |
if (whead == p) { |
1135 |
for (int k = spins;;) { |
1136 |
if ((m = (s = state) & ABITS) != WBIT) { |
1137 |
if (m < RFULL ? |
1138 |
U.compareAndSwapLong(this, STATE, s, ns = s + RUNIT): |
1139 |
(ns = tryIncReaderOverflow(s)) != 0L) { |
1140 |
whead = node; |
1141 |
node.prev = null; |
1142 |
while ((r = node.cowait) != null) { |
1143 |
if (U.compareAndSwapObject(node, WCOWAIT, |
1144 |
r, r.cowait) && |
1145 |
(w = r.thread) != null) { |
1146 |
r.thread = null; |
1147 |
U.unpark(w); // release co-waiter |
1148 |
} |
1149 |
} |
1150 |
return ns; |
1151 |
} |
1152 |
} |
1153 |
else if (ThreadLocalRandom.current().nextInt() >= 0 && |
1154 |
--k <= 0) |
1155 |
break; |
1156 |
} |
1157 |
if (spins < MAX_HEAD_SPINS) |
1158 |
spins <<= 1; |
1159 |
} |
1160 |
if ((ps = p.status) == 0) |
1161 |
U.compareAndSwapInt(p, WSTATUS, 0, WAITING); |
1162 |
else if (ps == CANCELLED) { |
1163 |
if ((pp = p.prev) != null) { |
1164 |
node.prev = pp; |
1165 |
pp.next = node; |
1166 |
} |
1167 |
} |
1168 |
else { |
1169 |
long time; |
1170 |
if (deadline == 0L) |
1171 |
time = 0L; |
1172 |
else if ((time = deadline - System.nanoTime()) <= 0L) |
1173 |
return cancelWaiter(node, null, false); |
1174 |
node.thread = Thread.currentThread(); |
1175 |
if (node.prev == p && p.status == WAITING && |
1176 |
(p != whead || (state & ABITS) != WBIT)) |
1177 |
U.park(false, time); |
1178 |
node.thread = null; |
1179 |
if (interruptible && Thread.interrupted()) |
1180 |
return cancelWaiter(node, null, true); |
1181 |
} |
1182 |
} |
1183 |
} |
1184 |
|
1185 |
/** |
1186 |
* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package. |
1187 |
* Replace with a simple call to Unsafe.getUnsafe when integrating |
1188 |
* into a jdk. |
1189 |
* |
1190 |
* @return a sun.misc.Unsafe |
1191 |
*/ |
1192 |
private static sun.misc.Unsafe getUnsafe() { |
1193 |
try { |
1194 |
return sun.misc.Unsafe.getUnsafe(); |
1195 |
} catch (SecurityException tryReflectionInstead) {} |
1196 |
try { |
1197 |
return java.security.AccessController.doPrivileged |
1198 |
(new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() { |
1199 |
public sun.misc.Unsafe run() throws Exception { |
1200 |
Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class; |
1201 |
for (java.lang.reflect.Field f : k.getDeclaredFields()) { |
1202 |
f.setAccessible(true); |
1203 |
Object x = f.get(null); |
1204 |
if (k.isInstance(x)) |
1205 |
return k.cast(x); |
1206 |
} |
1207 |
throw new NoSuchFieldError("the Unsafe"); |
1208 |
}}); |
1209 |
} catch (java.security.PrivilegedActionException e) { |
1210 |
throw new RuntimeException("Could not initialize intrinsics", |
1211 |
e.getCause()); |
1212 |
} |
1213 |
} |
1214 |
} |