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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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|
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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 in a different (and generally |
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* narrower) range of contexts than most other locks: They are not |
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* reentrant, so locked bodies should not call other unknown methods |
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* that may try to re-acquire locks (although you may pass a stamp to |
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* other methods that can use or convert it). Unvalidated optimistic |
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* read sections should further not 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. A zero return from any |
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* "try" method for acquiring or converting locks does not carry any |
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* information about the state of the lock; a subsequent invocation |
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* may succeed. |
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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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* for (long stamp = sl.tryOptimisticRead(); ; stamp = sl.readLock()) { |
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* double currentX, currentY; |
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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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* return Math.sqrt(currentX * currentX + currentY * currentY); |
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* } |
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* } |
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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 = 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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* Ordered RW locks (see Shirako et al |
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* http://dl.acm.org/citation.cfm?id=2312015) |
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* and Phase-Fair locks (see Brandenburg & Anderson, especially |
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* http://www.cs.unc.edu/~bbb/diss/). |
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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 then 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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* Waiting readers and writers use different queues. The writer |
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* queue is a modified form of CLH lock. (For discussion of CLH, |
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* see the internal documentation of AbstractQueuedSynchronizer.) |
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* The reader "queue" is a form of Treiber stack, that supports |
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* simpler/faster operations because order within a queue doesn't |
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* matter and all are signalled at once. However the sequence of |
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* threads within the queue vs the current stamp does matter (see |
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* Shirako et al) so each carries its incoming stamp value. |
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* Waiting writers never need to track sequence values, so they |
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* don't. |
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* |
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* These queue mechanics hardwire the scheduling policy. Ignoring |
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* trylocks, cancellation, and spinning, they implement Phase-Fair |
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* preferences: |
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* 1. Unlocked writers prefer to signal waiting readers |
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* 2. Fully unlocked readers prefer to signal waiting writers |
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* 3. When read-locked and a waiting writer exists, the writer |
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* is preferred to incoming readers |
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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. Additionally, initial |
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* phases of the await* methods (invoked from readLock() and |
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* writeLock()) use controlled spins that have similar effect. |
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* Phase-fair preferences may also be broken on cancellations due |
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* to timeouts and interrupts. Rule #3 (incoming readers when a |
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* waiting writer) is approximated with varying precision in |
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* different contexts -- some checks do not account for |
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* in-progress spins/signals, and others do not account for |
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* cancellations. |
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* |
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* Controlled, randomized spinning is used in the two await |
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* methods to reduce (increasingly expensive) context switching |
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* while also avoiding sustained memory thrashing among many |
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* threads. Both await methods use a similar spin strategy: If |
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* the associated queue appears to be empty, then the thread |
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* spin-waits up to SPINS times (where each iteration decreases |
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* spin count with 50% probablility) before enqueing, and then, if |
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* it is the first thread to be enqueued, spins again up to SPINS |
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* times before blocking. If, upon wakening it fails to obtain |
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* lock, and is still (or becomes) the first waiting thread (which |
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* indicates that some other thread barged and obtained lock), it |
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* escalates spins (up to MAX_HEAD_SPINS) to reduce the likelihood |
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* of continually losing to barging threads. |
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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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|
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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 : 1; |
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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 : 1; |
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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 await 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 writer 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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/** Wait nodes for readers */ |
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static final class RNode { |
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final long seq; // stamp value upon enqueue |
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volatile Thread waiter; // null if no longer waiting |
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volatile RNode next; |
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RNode(long s, Thread w) { seq = s; waiter = w; } |
290 |
} |
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|
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/** Wait nodes for writers */ |
293 |
static final class WNode { |
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volatile int status; // 0, WAITING, or CANCELLED |
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volatile WNode prev; |
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volatile WNode next; |
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volatile Thread thread; |
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WNode(Thread t, WNode p) { thread = t; prev = p; } |
299 |
} |
300 |
|
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/** Head of writer CLH queue */ |
302 |
private transient volatile WNode whead; |
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/** Tail (last) of writer CLH queue */ |
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private transient volatile WNode wtail; |
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/** Head of read queue */ |
306 |
private transient volatile RNode rhead; |
307 |
/** The state of the lock -- high bits hold sequence, low bits read count */ |
308 |
private transient volatile long state; |
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/** extra reader count when state read count saturated */ |
310 |
private transient int readerOverflow; |
311 |
|
312 |
/** |
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* Creates a new lock initially in unlocked state. |
314 |
*/ |
315 |
public StampedLock() { |
316 |
state = ORIGIN; |
317 |
} |
318 |
|
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/** |
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* Exclusively acquires the lock, blocking if necessary |
321 |
* until available. |
322 |
* |
323 |
* @return a stamp that can be used to unlock or convert mode |
324 |
*/ |
325 |
public long writeLock() { |
326 |
long s, next; |
327 |
if (((s = state) & ABITS) == 0L && |
328 |
U.compareAndSwapLong(this, STATE, s, next = s + WBIT)) |
329 |
return next; |
330 |
return awaitWrite(false, 0L); |
331 |
} |
332 |
|
333 |
/** |
334 |
* Exclusively acquires the lock if it is immediately available. |
335 |
* |
336 |
* @return a stamp that can be used to unlock or convert mode, |
337 |
* or zero if the lock is not available. |
338 |
*/ |
339 |
public long tryWriteLock() { |
340 |
long s, next; |
341 |
if (((s = state) & ABITS) == 0L && |
342 |
U.compareAndSwapLong(this, STATE, s, next = s + WBIT)) |
343 |
return next; |
344 |
return 0L; |
345 |
} |
346 |
|
347 |
/** |
348 |
* Exclusively acquires the lock if it is available within the |
349 |
* given time and the current thread has not been interrupted. |
350 |
* |
351 |
* @return a stamp that can be used to unlock or convert mode, |
352 |
* or zero if the lock is not available |
353 |
* @throws InterruptedException if the current thread is interrupted |
354 |
* before acquiring the lock |
355 |
*/ |
356 |
public long tryWriteLock(long time, TimeUnit unit) |
357 |
throws InterruptedException { |
358 |
long nanos = unit.toNanos(time); |
359 |
if (!Thread.interrupted()) { |
360 |
long s, next, deadline; |
361 |
if (((s = state) & ABITS) == 0L && |
362 |
U.compareAndSwapLong(this, STATE, s, next = s + WBIT)) |
363 |
return next; |
364 |
if (nanos <= 0L) |
365 |
return 0L; |
366 |
if ((deadline = System.nanoTime() + nanos) == 0L) |
367 |
deadline = 1L; |
368 |
if ((next = awaitWrite(true, deadline)) != INTERRUPTED) |
369 |
return next; |
370 |
} |
371 |
throw new InterruptedException(); |
372 |
} |
373 |
|
374 |
/** |
375 |
* Exclusively acquires the lock, blocking if necessary |
376 |
* until available or the current thread is interrupted. |
377 |
* |
378 |
* @return a stamp that can be used to unlock or convert mode |
379 |
* @throws InterruptedException if the current thread is interrupted |
380 |
* before acquiring the lock |
381 |
*/ |
382 |
public long writeLockInterruptibly() throws InterruptedException { |
383 |
if (!Thread.interrupted()) { |
384 |
long s, next; |
385 |
if (((s = state) & ABITS) == 0L && |
386 |
U.compareAndSwapLong(this, STATE, s, next = s + WBIT)) |
387 |
return next; |
388 |
if ((next = awaitWrite(true, 0L)) != INTERRUPTED) |
389 |
return next; |
390 |
} |
391 |
throw new InterruptedException(); |
392 |
} |
393 |
|
394 |
/** |
395 |
* Non-exclusively acquires the lock, blocking if necessary |
396 |
* until available. |
397 |
* |
398 |
* @return a stamp that can be used to unlock or convert mode |
399 |
*/ |
400 |
public long readLock() { |
401 |
for (;;) { |
402 |
long s, m, next; |
403 |
if ((m = (s = state) & ABITS) == 0L || |
404 |
(m < WBIT && whead == wtail)) { |
405 |
if (m < RFULL) { |
406 |
if (U.compareAndSwapLong(this, STATE, s, next = s + RUNIT)) |
407 |
return next; |
408 |
} |
409 |
else if ((next = tryIncReaderOverflow(s)) != 0L) |
410 |
return next; |
411 |
} |
412 |
else |
413 |
return awaitRead(s, false, 0L); |
414 |
} |
415 |
} |
416 |
|
417 |
/** |
418 |
* Non-exclusively acquires the lock if it is immediately available. |
419 |
* |
420 |
* @return a stamp that can be used to unlock or convert mode, |
421 |
* or zero if the lock is not available |
422 |
*/ |
423 |
public long tryReadLock() { |
424 |
for (;;) { |
425 |
long s, m, next; |
426 |
if ((m = (s = state) & ABITS) == WBIT) |
427 |
return 0L; |
428 |
else if (m < RFULL) { |
429 |
if (U.compareAndSwapLong(this, STATE, s, next = s + RUNIT)) |
430 |
return next; |
431 |
} |
432 |
else if ((next = tryIncReaderOverflow(s)) != 0L) |
433 |
return next; |
434 |
} |
435 |
} |
436 |
|
437 |
/** |
438 |
* Non-exclusively acquires the lock if it is available within the |
439 |
* given time and the current thread has not been interrupted. |
440 |
* |
441 |
* @return a stamp that can be used to unlock or convert mode, |
442 |
* or zero if the lock is not available |
443 |
* @throws InterruptedException if the current thread is interrupted |
444 |
* before acquiring the lock |
445 |
*/ |
446 |
public long tryReadLock(long time, TimeUnit unit) |
447 |
throws InterruptedException { |
448 |
long nanos = unit.toNanos(time); |
449 |
if (!Thread.interrupted()) { |
450 |
for (;;) { |
451 |
long s, m, next, deadline; |
452 |
if ((m = (s = state) & ABITS) == WBIT || |
453 |
(m != 0L && whead != wtail)) { |
454 |
if (nanos <= 0L) |
455 |
return 0L; |
456 |
if ((deadline = System.nanoTime() + nanos) == 0L) |
457 |
deadline = 1L; |
458 |
if ((next = awaitRead(s, true, deadline)) != INTERRUPTED) |
459 |
return next; |
460 |
break; |
461 |
} |
462 |
else if (m < RFULL) { |
463 |
if (U.compareAndSwapLong(this, STATE, s, next = s + RUNIT)) |
464 |
return next; |
465 |
} |
466 |
else if ((next = tryIncReaderOverflow(s)) != 0L) |
467 |
return next; |
468 |
} |
469 |
} |
470 |
throw new InterruptedException(); |
471 |
} |
472 |
|
473 |
/** |
474 |
* Non-exclusively acquires the lock, blocking if necessary |
475 |
* until available or the current thread is interrupted. |
476 |
* |
477 |
* @return a stamp that can be used to unlock or convert mode |
478 |
* @throws InterruptedException if the current thread is interrupted |
479 |
* before acquiring the lock |
480 |
*/ |
481 |
public long readLockInterruptibly() throws InterruptedException { |
482 |
if (!Thread.interrupted()) { |
483 |
for (;;) { |
484 |
long s, next, m; |
485 |
if ((m = (s = state) & ABITS) == WBIT || |
486 |
(m != 0L && whead != wtail)) { |
487 |
if ((next = awaitRead(s, true, 0L)) != INTERRUPTED) |
488 |
return next; |
489 |
break; |
490 |
} |
491 |
else if (m < RFULL) { |
492 |
if (U.compareAndSwapLong(this, STATE, s, next = s + RUNIT)) |
493 |
return next; |
494 |
} |
495 |
else if ((next = tryIncReaderOverflow(s)) != 0L) |
496 |
return next; |
497 |
} |
498 |
} |
499 |
throw new InterruptedException(); |
500 |
} |
501 |
|
502 |
/** |
503 |
* Returns a stamp that can later be validated, or zero |
504 |
* if exclusively locked. |
505 |
* |
506 |
* @return a stamp, or zero if exclusively locked |
507 |
*/ |
508 |
public long tryOptimisticRead() { |
509 |
long s; |
510 |
return (((s = state) & WBIT) == 0L) ? (s & SBITS) : 0L; |
511 |
} |
512 |
|
513 |
/** |
514 |
* Returns true if the lock has not been exclusively held since |
515 |
* issuance of the given stamp. Always returns false if the stamp |
516 |
* is zero. Always returns true if the stamp represents a |
517 |
* currently held lock. |
518 |
* |
519 |
* @return true if the lock has not been exclusively held since |
520 |
* issuance of the given stamp; else false |
521 |
*/ |
522 |
public boolean validate(long stamp) { |
523 |
return (stamp & SBITS) == (U.getLongVolatile(this, STATE) & SBITS); |
524 |
} |
525 |
|
526 |
/** |
527 |
* If the lock state matches the given stamp, releases the |
528 |
* exclusive lock. |
529 |
* |
530 |
* @param stamp a stamp returned by a write-lock operation |
531 |
* @throws IllegalMonitorStateException if the stamp does |
532 |
* not match the current state of this lock |
533 |
*/ |
534 |
public void unlockWrite(long stamp) { |
535 |
if (state != stamp || (stamp & WBIT) == 0L) |
536 |
throw new IllegalMonitorStateException(); |
537 |
state = (stamp += WBIT) == 0L ? ORIGIN : stamp; |
538 |
readerPrefSignal(); |
539 |
} |
540 |
|
541 |
/** |
542 |
* If the lock state matches the given stamp, releases the |
543 |
* non-exclusive lock. |
544 |
* |
545 |
* @param stamp a stamp returned by a read-lock operation |
546 |
* @throws IllegalMonitorStateException if the stamp does |
547 |
* not match the current state of this lock |
548 |
*/ |
549 |
public void unlockRead(long stamp) { |
550 |
long s, m; |
551 |
if ((stamp & RBITS) != 0L) { |
552 |
while (((s = state) & SBITS) == (stamp & SBITS)) { |
553 |
if ((m = s & ABITS) == 0L) |
554 |
break; |
555 |
else if (m < RFULL) { |
556 |
if (U.compareAndSwapLong(this, STATE, s, s - RUNIT)) { |
557 |
if (m == RUNIT) |
558 |
writerPrefSignal(); |
559 |
return; |
560 |
} |
561 |
} |
562 |
else if (m >= WBIT) |
563 |
break; |
564 |
else if (tryDecReaderOverflow(s) != 0L) |
565 |
return; |
566 |
} |
567 |
} |
568 |
throw new IllegalMonitorStateException(); |
569 |
} |
570 |
|
571 |
/** |
572 |
* If the lock state matches the given stamp, releases the |
573 |
* corresponding mode of the lock. |
574 |
* |
575 |
* @param stamp a stamp returned by a lock operation |
576 |
* @throws IllegalMonitorStateException if the stamp does |
577 |
* not match the current state of this lock |
578 |
*/ |
579 |
public void unlock(long stamp) { |
580 |
long a = stamp & ABITS, m, s; |
581 |
while (((s = state) & SBITS) == (stamp & SBITS)) { |
582 |
if ((m = s & ABITS) == 0L) |
583 |
break; |
584 |
else if (m == WBIT) { |
585 |
if (a != m) |
586 |
break; |
587 |
state = (s += WBIT) == 0L ? ORIGIN : s; |
588 |
readerPrefSignal(); |
589 |
return; |
590 |
} |
591 |
else if (a == 0L || a >= WBIT) |
592 |
break; |
593 |
else if (m < RFULL) { |
594 |
if (U.compareAndSwapLong(this, STATE, s, s - RUNIT)) { |
595 |
if (m == RUNIT) |
596 |
writerPrefSignal(); |
597 |
return; |
598 |
} |
599 |
} |
600 |
else if (tryDecReaderOverflow(s) != 0L) |
601 |
return; |
602 |
} |
603 |
throw new IllegalMonitorStateException(); |
604 |
} |
605 |
|
606 |
/** |
607 |
* If the lock state matches the given stamp then performs one of |
608 |
* the following actions. If the stamp represents holding a write |
609 |
* lock, returns it. Or, if a read lock, if the write lock is |
610 |
* available, releases the read and returns a write stamp. Or, if |
611 |
* an optimistic read, returns a write stamp only if immediately |
612 |
* available. This method returns zero in all other cases. |
613 |
* |
614 |
* @param stamp a stamp |
615 |
* @return a valid write stamp, or zero on failure |
616 |
*/ |
617 |
public long tryConvertToWriteLock(long stamp) { |
618 |
long a = stamp & ABITS, m, s, next; |
619 |
while (((s = state) & SBITS) == (stamp & SBITS)) { |
620 |
if ((m = s & ABITS) == 0L) { |
621 |
if (a != 0L) |
622 |
break; |
623 |
if (U.compareAndSwapLong(this, STATE, s, next = s + WBIT)) |
624 |
return next; |
625 |
} |
626 |
else if (m == WBIT) { |
627 |
if (a != m) |
628 |
break; |
629 |
return stamp; |
630 |
} |
631 |
else if (m == RUNIT && a != 0L && a < WBIT) { |
632 |
if (U.compareAndSwapLong(this, STATE, s, |
633 |
next = s - RUNIT + WBIT)) |
634 |
return next; |
635 |
} |
636 |
else |
637 |
break; |
638 |
} |
639 |
return 0L; |
640 |
} |
641 |
|
642 |
/** |
643 |
* If the lock state matches the given stamp then performs one of |
644 |
* the following actions. If the stamp represents holding a write |
645 |
* lock, releases it and obtains a read lock. Or, if a read lock, |
646 |
* returns it. Or, if an optimistic read, acquires a read lock and |
647 |
* returns a read stamp only if immediately available. This method |
648 |
* returns zero in all other cases. |
649 |
* |
650 |
* @param stamp a stamp |
651 |
* @return a valid read stamp, or zero on failure |
652 |
*/ |
653 |
public long tryConvertToReadLock(long stamp) { |
654 |
long a = stamp & ABITS, m, s, next; |
655 |
while (((s = state) & SBITS) == (stamp & SBITS)) { |
656 |
if ((m = s & ABITS) == 0L) { |
657 |
if (a != 0L) |
658 |
break; |
659 |
else if (m < RFULL) { |
660 |
if (U.compareAndSwapLong(this, STATE, s, next = s + RUNIT)) |
661 |
return next; |
662 |
} |
663 |
else if ((next = tryIncReaderOverflow(s)) != 0L) |
664 |
return next; |
665 |
} |
666 |
else if (m == WBIT) { |
667 |
if (a != m) |
668 |
break; |
669 |
next = state = s + (WBIT + RUNIT); |
670 |
readerPrefSignal(); |
671 |
return next; |
672 |
} |
673 |
else if (a != 0L && a < WBIT) |
674 |
return stamp; |
675 |
else |
676 |
break; |
677 |
} |
678 |
return 0L; |
679 |
} |
680 |
|
681 |
/** |
682 |
* If the lock state matches the given stamp then, if the stamp |
683 |
* represents holding a lock, releases it and returns an |
684 |
* observation stamp. Or, if an optimistic read, returns it if |
685 |
* validated. This method returns zero in all other cases, and so |
686 |
* may be useful as a form of "tryUnlock". |
687 |
* |
688 |
* @param stamp a stamp |
689 |
* @return a valid optimistic read stamp, or zero on failure |
690 |
*/ |
691 |
public long tryConvertToOptimisticRead(long stamp) { |
692 |
long a = stamp & ABITS, m, s, next; |
693 |
while (((s = U.getLongVolatile(this, STATE)) & |
694 |
SBITS) == (stamp & SBITS)) { |
695 |
if ((m = s & ABITS) == 0L) { |
696 |
if (a != 0L) |
697 |
break; |
698 |
return s; |
699 |
} |
700 |
else if (m == WBIT) { |
701 |
if (a != m) |
702 |
break; |
703 |
next = state = (s += WBIT) == 0L ? ORIGIN : s; |
704 |
readerPrefSignal(); |
705 |
return next; |
706 |
} |
707 |
else if (a == 0L || a >= WBIT) |
708 |
break; |
709 |
else if (m < RFULL) { |
710 |
if (U.compareAndSwapLong(this, STATE, s, next = s - RUNIT)) { |
711 |
if (m == RUNIT) |
712 |
writerPrefSignal(); |
713 |
return next & SBITS; |
714 |
} |
715 |
} |
716 |
else if ((next = tryDecReaderOverflow(s)) != 0L) |
717 |
return next & SBITS; |
718 |
} |
719 |
return 0L; |
720 |
} |
721 |
|
722 |
/** |
723 |
* Releases the write lock if it is held, without requiring a |
724 |
* stamp value. This method may be useful for recovery after |
725 |
* errors. |
726 |
* |
727 |
* @return true if the lock was held, else false |
728 |
*/ |
729 |
public boolean tryUnlockWrite() { |
730 |
long s; |
731 |
if (((s = state) & WBIT) != 0L) { |
732 |
state = (s += WBIT) == 0L ? ORIGIN : s; |
733 |
readerPrefSignal(); |
734 |
return true; |
735 |
} |
736 |
return false; |
737 |
} |
738 |
|
739 |
/** |
740 |
* Releases one hold of the read lock if it is held, without |
741 |
* requiring a stamp value. This method may be useful for recovery |
742 |
* after errors. |
743 |
* |
744 |
* @return true if the read lock was held, else false |
745 |
*/ |
746 |
public boolean tryUnlockRead() { |
747 |
long s, m; |
748 |
while ((m = (s = state) & ABITS) != 0L && m < WBIT) { |
749 |
if (m < RFULL) { |
750 |
if (U.compareAndSwapLong(this, STATE, s, s - RUNIT)) { |
751 |
if (m == RUNIT) |
752 |
writerPrefSignal(); |
753 |
return true; |
754 |
} |
755 |
} |
756 |
else if (tryDecReaderOverflow(s) != 0L) |
757 |
return true; |
758 |
} |
759 |
return false; |
760 |
} |
761 |
|
762 |
/** |
763 |
* Returns true if the lock is currently held exclusively. |
764 |
* |
765 |
* @return true if the lock is currently held exclusively |
766 |
*/ |
767 |
public boolean isWriteLocked() { |
768 |
return (state & WBIT) != 0L; |
769 |
} |
770 |
|
771 |
/** |
772 |
* Returns true if the lock is currently held non-exclusively. |
773 |
* |
774 |
* @return true if the lock is currently held non-exclusively |
775 |
*/ |
776 |
public boolean isReadLocked() { |
777 |
long m; |
778 |
return (m = state & ABITS) > 0L && m < WBIT; |
779 |
} |
780 |
|
781 |
private void readObject(java.io.ObjectInputStream s) |
782 |
throws java.io.IOException, ClassNotFoundException { |
783 |
s.defaultReadObject(); |
784 |
state = ORIGIN; // reset to unlocked state |
785 |
} |
786 |
|
787 |
// internals |
788 |
|
789 |
/** |
790 |
* Tries to increment readerOverflow by first setting state |
791 |
* access bits value to RBITS, indicating hold of spinlock, |
792 |
* then updating, then releasing. |
793 |
* |
794 |
* @param stamp, assumed that (stamp & ABITS) >= RFULL |
795 |
* @return new stamp on success, else zero |
796 |
*/ |
797 |
private long tryIncReaderOverflow(long s) { |
798 |
if ((s & ABITS) == RFULL) { |
799 |
if (U.compareAndSwapLong(this, STATE, s, s | RBITS)) { |
800 |
++readerOverflow; |
801 |
state = s; |
802 |
return s; |
803 |
} |
804 |
} |
805 |
else if ((ThreadLocalRandom.current().nextInt() & |
806 |
OVERFLOW_YIELD_RATE) == 0) |
807 |
Thread.yield(); |
808 |
return 0L; |
809 |
} |
810 |
|
811 |
/** |
812 |
* Tries to decrement readerOverflow. |
813 |
* |
814 |
* @param stamp, assumed that (stamp & ABITS) >= RFULL |
815 |
* @return new stamp on success, else zero |
816 |
*/ |
817 |
private long tryDecReaderOverflow(long s) { |
818 |
if ((s & ABITS) == RFULL) { |
819 |
if (U.compareAndSwapLong(this, STATE, s, s | RBITS)) { |
820 |
int r; long next; |
821 |
if ((r = readerOverflow) > 0) { |
822 |
readerOverflow = r - 1; |
823 |
next = s; |
824 |
} |
825 |
else |
826 |
next = s - RUNIT; |
827 |
state = next; |
828 |
return next; |
829 |
} |
830 |
} |
831 |
else if ((ThreadLocalRandom.current().nextInt() & |
832 |
OVERFLOW_YIELD_RATE) == 0) |
833 |
Thread.yield(); |
834 |
return 0L; |
835 |
} |
836 |
|
837 |
/* |
838 |
* The two versions of signal implement the phase-fair policy. |
839 |
* They include almost the same code, but repacked in different |
840 |
* ways. Integrating the policy with the mechanics eliminates |
841 |
* state rechecks that would be needed with separate reader and |
842 |
* writer signal methods. Both methods assume that they are |
843 |
* called when the lock is last known to be available, and |
844 |
* continue until the lock is unavailable, or at least one thread |
845 |
* is signalled, or there are no more waiting threads. Signalling |
846 |
* a reader entails popping (CASing) from rhead and unparking |
847 |
* unless the thread already cancelled (indicated by a null waiter |
848 |
* field). Signalling a writer requires finding the first node, |
849 |
* i.e., the successor of whead. This is normally just head.next, |
850 |
* but may require traversal from wtail if next pointers are |
851 |
* lagging. These methods may fail to wake up an acquiring thread |
852 |
* when one or more have been cancelled, but the cancel methods |
853 |
* themselves provide extra safeguards to ensure liveness. |
854 |
*/ |
855 |
|
856 |
private void readerPrefSignal() { |
857 |
boolean readers = false; |
858 |
RNode p; WNode h, q; long s; Thread w; |
859 |
while ((p = rhead) != null) { |
860 |
if (((s = state) & WBIT) != 0L) |
861 |
return; |
862 |
if (p.seq == (s & SBITS)) |
863 |
break; |
864 |
readers = true; |
865 |
if (U.compareAndSwapObject(this, RHEAD, p, p.next) && |
866 |
(w = p.waiter) != null && |
867 |
U.compareAndSwapObject(p, WAITER, w, null)) |
868 |
U.unpark(w); |
869 |
} |
870 |
if (!readers && (state & ABITS) == 0L && |
871 |
(h = whead) != null && h.status != 0) { |
872 |
U.compareAndSwapInt(h, STATUS, WAITING, 0); |
873 |
if ((q = h.next) == null || q.status == CANCELLED) { |
874 |
q = null; |
875 |
for (WNode t = wtail; t != null && t != h; t = t.prev) |
876 |
if (t.status <= 0) |
877 |
q = t; |
878 |
} |
879 |
if (q != null && (w = q.thread) != null) |
880 |
U.unpark(w); |
881 |
} |
882 |
} |
883 |
|
884 |
private void writerPrefSignal() { |
885 |
RNode p; WNode h, q; long s; Thread w; |
886 |
if ((h = whead) != null && h.status != 0) { |
887 |
U.compareAndSwapInt(h, STATUS, WAITING, 0); |
888 |
if ((q = h.next) == null || q.status == CANCELLED) { |
889 |
q = null; |
890 |
for (WNode t = wtail; t != null && t != h; t = t.prev) |
891 |
if (t.status <= 0) |
892 |
q = t; |
893 |
} |
894 |
if (q != null && (w = q.thread) != null) |
895 |
U.unpark(w); |
896 |
} |
897 |
else { |
898 |
while ((p = rhead) != null && ((s = state) & WBIT) == 0L && |
899 |
p.seq != (s & SBITS)) { |
900 |
if (U.compareAndSwapObject(this, RHEAD, p, p.next) && |
901 |
(w = p.waiter) != null && |
902 |
U.compareAndSwapObject(p, WAITER, w, null)) |
903 |
U.unpark(w); |
904 |
} |
905 |
} |
906 |
} |
907 |
|
908 |
/** |
909 |
* RNG for local spins. The first call from await{Read,Write} |
910 |
* produces a thread-local value. Unless zero, subsequent calls |
911 |
* use an xorShift to further reduce memory traffic. |
912 |
*/ |
913 |
private static int nextRandom(int r) { |
914 |
if (r == 0) |
915 |
return ThreadLocalRandom.current().nextInt(); |
916 |
r ^= r << 1; // xorshift |
917 |
r ^= r >>> 3; |
918 |
r ^= r << 10; |
919 |
return r; |
920 |
} |
921 |
|
922 |
/** |
923 |
* Possibly spins trying to obtain write lock, then enqueues and |
924 |
* blocks while not head of write queue or cannot acquire lock, |
925 |
* possibly spinning when at head; cancelling on timeout or |
926 |
* interrupt. |
927 |
* |
928 |
* @param interruptible true if should check interrupts and if so |
929 |
* return INTERRUPTED |
930 |
* @param deadline if nonzero, the System.nanoTime value to timeout |
931 |
* at (and return zero) |
932 |
*/ |
933 |
private long awaitWrite(boolean interruptible, long deadline) { |
934 |
WNode node = null; |
935 |
for (int r = 0, spins = -1;;) { |
936 |
WNode p; long s, next; |
937 |
if (((s = state) & ABITS) == 0L) { |
938 |
if (U.compareAndSwapLong(this, STATE, s, next = s + WBIT)) |
939 |
return next; |
940 |
} |
941 |
else if (spins < 0) |
942 |
spins = whead == wtail ? SPINS : 0; |
943 |
else if (spins > 0) { |
944 |
if ((r = nextRandom(r)) >= 0) |
945 |
--spins; |
946 |
} |
947 |
else if ((p = wtail) == null) { // initialize queue |
948 |
if (U.compareAndSwapObject(this, WHEAD, null, |
949 |
new WNode(null, null))) |
950 |
wtail = whead; |
951 |
} |
952 |
else if (node == null) |
953 |
node = new WNode(Thread.currentThread(), p); |
954 |
else if (node.prev != p) |
955 |
node.prev = p; |
956 |
else if (U.compareAndSwapObject(this, WTAIL, p, node)) { |
957 |
p.next = node; |
958 |
for (int headSpins = SPINS;;) { |
959 |
WNode np; int ps; |
960 |
if ((np = node.prev) != p && np != null && |
961 |
(p = np).next != node) |
962 |
p.next = node; // stale |
963 |
if (p == whead) { |
964 |
for (int k = headSpins;;) { |
965 |
if (((s = state) & ABITS) == 0L) { |
966 |
if (U.compareAndSwapLong(this, STATE, |
967 |
s, next = s + WBIT)) { |
968 |
whead = node; |
969 |
node.thread = null; |
970 |
node.prev = null; |
971 |
return next; |
972 |
} |
973 |
break; |
974 |
} |
975 |
if ((r = nextRandom(r)) >= 0 && --k <= 0) |
976 |
break; |
977 |
} |
978 |
if (headSpins < MAX_HEAD_SPINS) |
979 |
headSpins <<= 1; |
980 |
} |
981 |
if ((ps = p.status) == 0) |
982 |
U.compareAndSwapInt(p, STATUS, 0, WAITING); |
983 |
else if (ps == CANCELLED) |
984 |
node.prev = p.prev; |
985 |
else { |
986 |
long time; // 0 argument to park means no timeout |
987 |
if (deadline == 0L) |
988 |
time = 0L; |
989 |
else if ((time = deadline - System.nanoTime()) <= 0L) |
990 |
return cancelWriter(node, false); |
991 |
if (node.prev == p && p.status == WAITING && |
992 |
(p != whead || (state & WBIT) != 0L)) { // recheck |
993 |
U.park(false, time); |
994 |
if (interruptible && Thread.interrupted()) |
995 |
return cancelWriter(node, true); |
996 |
} |
997 |
} |
998 |
} |
999 |
} |
1000 |
} |
1001 |
} |
1002 |
|
1003 |
/** |
1004 |
* If node non-null, forces cancel status and unsplices from queue |
1005 |
* if possible. This is a streamlined variant of cancellation |
1006 |
* methods in AbstractQueuedSynchronizer that includes a detailed |
1007 |
* explanation. |
1008 |
*/ |
1009 |
private long cancelWriter(WNode node, boolean interrupted) { |
1010 |
WNode pred; |
1011 |
if (node != null && (pred = node.prev) != null) { |
1012 |
WNode pp; |
1013 |
node.thread = null; |
1014 |
while (pred.status == CANCELLED && (pp = pred.prev) != null) |
1015 |
pred = node.prev = pp; |
1016 |
WNode predNext = pred.next; |
1017 |
node.status = CANCELLED; |
1018 |
if (predNext != null) { |
1019 |
Thread w; |
1020 |
WNode succ = node.next; |
1021 |
if (succ == null || succ.status == CANCELLED) { |
1022 |
succ = null; |
1023 |
for (WNode t = wtail; t != null && t != node; t = t.prev) |
1024 |
if (t.status <= 0) |
1025 |
succ = t; |
1026 |
if (succ == null && node == wtail) |
1027 |
U.compareAndSwapObject(this, WTAIL, node, pred); |
1028 |
} |
1029 |
U.compareAndSwapObject(pred, WNEXT, predNext, succ); |
1030 |
if (succ != null && (w = succ.thread) != null) |
1031 |
U.unpark(w); |
1032 |
} |
1033 |
} |
1034 |
writerPrefSignal(); |
1035 |
return (interrupted || Thread.interrupted()) ? INTERRUPTED : 0L; |
1036 |
} |
1037 |
|
1038 |
/** |
1039 |
* Waits for read lock or timeout or interrupt. The form of |
1040 |
* awaitRead differs from awaitWrite mainly because it must |
1041 |
* restart (with a new wait node) if the thread was unqueued and |
1042 |
* unparked but could not the obtain lock. We also need to help |
1043 |
* with preference rules by not trying to acquire the lock before |
1044 |
* enqueuing if there is a known waiting writer, but also helping |
1045 |
* to release those threads that are still queued from the last |
1046 |
* release. |
1047 |
*/ |
1048 |
private long awaitRead(long stamp, boolean interruptible, long deadline) { |
1049 |
long seq = stamp & SBITS; |
1050 |
RNode node = null; |
1051 |
boolean queued = false; |
1052 |
for (int r = 0, headSpins = SPINS, spins = -1;;) { |
1053 |
long s, m, next; RNode p; WNode wh; Thread w; |
1054 |
if ((m = (s = state) & ABITS) != WBIT && |
1055 |
((s & SBITS) != seq || (wh = whead) == null || |
1056 |
wh.status == 0)) { |
1057 |
if (m < RFULL ? |
1058 |
U.compareAndSwapLong(this, STATE, s, next = s + RUNIT) : |
1059 |
(next = tryIncReaderOverflow(s)) != 0L) { |
1060 |
if (node != null && (w = node.waiter) != null) |
1061 |
U.compareAndSwapObject(node, WAITER, w, null); |
1062 |
if ((p = rhead) != null && (s & SBITS) != p.seq && |
1063 |
U.compareAndSwapObject(this, RHEAD, p, p.next) && |
1064 |
(w = p.waiter) != null && |
1065 |
U.compareAndSwapObject(p, WAITER, w, null)) |
1066 |
U.unpark(w); // help signal other waiters |
1067 |
return next; |
1068 |
} |
1069 |
} |
1070 |
else if (m != WBIT && (p = rhead) != null && |
1071 |
(s & SBITS) != p.seq) { // help release old readers |
1072 |
if (U.compareAndSwapObject(this, RHEAD, p, p.next) && |
1073 |
(w = p.waiter) != null && |
1074 |
U.compareAndSwapObject(p, WAITER, w, null)) |
1075 |
U.unpark(w); |
1076 |
} |
1077 |
else if (queued && node != null && node.waiter == null) { |
1078 |
node = null; // restart |
1079 |
queued = false; |
1080 |
spins = -1; |
1081 |
} |
1082 |
else if (spins < 0) { |
1083 |
if (rhead != node) |
1084 |
spins = 0; |
1085 |
else if ((spins = headSpins) < MAX_HEAD_SPINS && node != null) |
1086 |
headSpins <<= 1; |
1087 |
} |
1088 |
else if (spins > 0) { |
1089 |
if ((r = nextRandom(r)) >= 0) |
1090 |
--spins; |
1091 |
} |
1092 |
else if (node == null) |
1093 |
node = new RNode(seq, Thread.currentThread()); |
1094 |
else if (!queued) { |
1095 |
if (queued = U.compareAndSwapObject(this, RHEAD, |
1096 |
node.next = rhead, node)) |
1097 |
spins = -1; |
1098 |
} |
1099 |
else { |
1100 |
long time; |
1101 |
if (deadline == 0L) |
1102 |
time = 0L; |
1103 |
else if ((time = deadline - System.nanoTime()) <= 0L) |
1104 |
return cancelReader(node, false); |
1105 |
if ((state & WBIT) != 0L && node.waiter != null) { // recheck |
1106 |
U.park(false, time); |
1107 |
if (interruptible && Thread.interrupted()) |
1108 |
return cancelReader(node, true); |
1109 |
} |
1110 |
} |
1111 |
} |
1112 |
} |
1113 |
|
1114 |
/** |
1115 |
* If node non-null, forces cancel status and unsplices from queue |
1116 |
* if possible, by traversing entire queue looking for cancelled |
1117 |
* nodes. |
1118 |
*/ |
1119 |
private long cancelReader(RNode node, boolean interrupted) { |
1120 |
Thread w; |
1121 |
if (node != null && (w = node.waiter) != null && |
1122 |
U.compareAndSwapObject(node, WAITER, w, null)) { |
1123 |
for (RNode pred = null, p = rhead; p != null;) { |
1124 |
RNode q = p.next; |
1125 |
if (p.waiter == null) { |
1126 |
if (pred == null) { |
1127 |
U.compareAndSwapObject(this, RHEAD, p, q); |
1128 |
p = rhead; |
1129 |
} |
1130 |
else { |
1131 |
U.compareAndSwapObject(pred, RNEXT, p, q); |
1132 |
p = pred.next; |
1133 |
} |
1134 |
} |
1135 |
else { |
1136 |
pred = p; |
1137 |
p = q; |
1138 |
} |
1139 |
} |
1140 |
} |
1141 |
readerPrefSignal(); |
1142 |
return (interrupted || Thread.interrupted()) ? INTERRUPTED : 0L; |
1143 |
} |
1144 |
|
1145 |
// Unsafe mechanics |
1146 |
private static final sun.misc.Unsafe U; |
1147 |
private static final long STATE; |
1148 |
private static final long RHEAD; |
1149 |
private static final long WHEAD; |
1150 |
private static final long WTAIL; |
1151 |
private static final long RNEXT; |
1152 |
private static final long WNEXT; |
1153 |
private static final long WPREV; |
1154 |
private static final long WAITER; |
1155 |
private static final long STATUS; |
1156 |
|
1157 |
static { |
1158 |
try { |
1159 |
U = getUnsafe(); |
1160 |
Class<?> k = StampedLock.class; |
1161 |
Class<?> rk = RNode.class; |
1162 |
Class<?> wk = WNode.class; |
1163 |
STATE = U.objectFieldOffset |
1164 |
(k.getDeclaredField("state")); |
1165 |
RHEAD = U.objectFieldOffset |
1166 |
(k.getDeclaredField("rhead")); |
1167 |
WHEAD = U.objectFieldOffset |
1168 |
(k.getDeclaredField("whead")); |
1169 |
WTAIL = U.objectFieldOffset |
1170 |
(k.getDeclaredField("wtail")); |
1171 |
RNEXT = U.objectFieldOffset |
1172 |
(rk.getDeclaredField("next")); |
1173 |
WAITER = U.objectFieldOffset |
1174 |
(rk.getDeclaredField("waiter")); |
1175 |
STATUS = U.objectFieldOffset |
1176 |
(wk.getDeclaredField("status")); |
1177 |
WNEXT = U.objectFieldOffset |
1178 |
(wk.getDeclaredField("next")); |
1179 |
WPREV = U.objectFieldOffset |
1180 |
(wk.getDeclaredField("prev")); |
1181 |
|
1182 |
} catch (Exception e) { |
1183 |
throw new Error(e); |
1184 |
} |
1185 |
} |
1186 |
|
1187 |
/** |
1188 |
* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package. |
1189 |
* Replace with a simple call to Unsafe.getUnsafe when integrating |
1190 |
* into a jdk. |
1191 |
* |
1192 |
* @return a sun.misc.Unsafe |
1193 |
*/ |
1194 |
private static sun.misc.Unsafe getUnsafe() { |
1195 |
try { |
1196 |
return sun.misc.Unsafe.getUnsafe(); |
1197 |
} catch (SecurityException se) { |
1198 |
try { |
1199 |
return java.security.AccessController.doPrivileged |
1200 |
(new java.security |
1201 |
.PrivilegedExceptionAction<sun.misc.Unsafe>() { |
1202 |
public sun.misc.Unsafe run() throws Exception { |
1203 |
java.lang.reflect.Field f = sun.misc |
1204 |
.Unsafe.class.getDeclaredField("theUnsafe"); |
1205 |
f.setAccessible(true); |
1206 |
return (sun.misc.Unsafe) f.get(null); |
1207 |
}}); |
1208 |
} catch (java.security.PrivilegedActionException e) { |
1209 |
throw new RuntimeException("Could not initialize intrinsics", |
1210 |
e.getCause()); |
1211 |
} |
1212 |
} |
1213 |
} |
1214 |
|
1215 |
} |