--- jsr166/src/jsr166e/StampedLock.java 2013/01/24 21:35:03 1.31 +++ jsr166/src/jsr166e/StampedLock.java 2013/07/14 19:55:05 1.37 @@ -5,8 +5,6 @@ */ package jsr166e; - -import java.util.concurrent.ThreadLocalRandom; import java.util.concurrent.TimeUnit; import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.Condition; @@ -40,19 +38,20 @@ import java.util.concurrent.locks.LockSu *
  • Optimistic Reading. Method {@link #tryOptimisticRead} * returns a non-zero stamp only if the lock is not currently held * in write mode. Method {@link #validate} returns true if the lock - * has not since been acquired in write mode. This mode can be - * thought of as an extremely weak version of a read-lock, that can - * be broken by a writer at any time. The use of optimistic mode - * for short read-only code segments often reduces contention and - * improves throughput. However, its use is inherently fragile. - * Optimistic read sections should only read fields and hold them in - * local variables for later use after validation. Fields read while - * in optimistic mode may be wildly inconsistent, so usage applies - * only when you are familiar enough with data representations to - * check consistency and/or repeatedly invoke method {@code - * validate()}. For example, such steps are typically required when - * first reading an object or array reference, and then accessing - * one of its fields, elements or methods.
    • + * has not been acquired in write mode since obtaining a given + * stamp. This mode can be thought of as an extremely weak version + * of a read-lock, that can be broken by a writer at any time. The + * use of optimistic mode for short read-only code segments often + * reduces contention and improves throughput. However, its use is + * inherently fragile. Optimistic read sections should only read + * fields and hold them in local variables for later use after + * validation. Fields read while in optimistic mode may be wildly + * inconsistent, so usage applies only when you are familiar enough + * with data representations to check consistency and/or repeatedly + * invoke method {@code validate()}. For example, such steps are + * typically required when first reading an object or array + * reference, and then accessing one of its fields, elements or + * methods. * * * @@ -118,34 +117,17 @@ import java.util.concurrent.locks.LockSu * } * } * - * double distanceFromOriginV1() { // A read-only method - * long stamp; - * if ((stamp = sl.tryOptimisticRead()) != 0L) { // optimistic - * double currentX = x; - * double currentY = y; - * if (sl.validate(stamp)) - * return Math.sqrt(currentX * currentX + currentY * currentY); - * } - * stamp = sl.readLock(); // fall back to read lock - * try { - * double currentX = x; - * double currentY = y; - * return Math.sqrt(currentX * currentX + currentY * currentY); - * } finally { - * sl.unlockRead(stamp); - * } - * } - * - * double distanceFromOriginV2() { // combines code paths - * double currentX = 0.0, currentY = 0.0; - * for (long stamp = sl.tryOptimisticRead(); ; stamp = sl.readLock()) { - * try { - * currentX = x; - * currentY = y; - * } finally { - * if (sl.tryConvertToOptimisticRead(stamp) != 0L) // unlock or validate - * break; - * } + * double distanceFromOrigin() { // A read-only method + * long stamp = sl.tryOptimisticRead(); + * double currentX = x, currentY = y; + * if (!sl.validate(stamp)) { + * stamp = sl.readLock(); + * try { + * currentX = x; + * currentY = y; + * } finally { + * sl.unlockRead(stamp); + * } * } * return Math.sqrt(currentX * currentX + currentY * currentY); * } @@ -252,6 +234,7 @@ public class StampedLock implements java * motivation to further spread out contended locations, but might * be subject to future improvements. */ + private static final long serialVersionUID = -6001602636862214147L; /** Number of processors, for spin control */ @@ -356,6 +339,8 @@ public class StampedLock implements java * Behavior under timeout and interruption matches that specified * for method {@link Lock#tryLock(long,TimeUnit)}. * + * @param time the maximum time to wait for the lock + * @param unit the time unit of the {@code time} argument * @return a stamp that can be used to unlock or convert mode, * or zero if the lock is not available * @throws InterruptedException if the current thread is interrupted @@ -435,6 +420,8 @@ public class StampedLock implements java * Behavior under timeout and interruption matches that specified * for method {@link Lock#tryLock(long,TimeUnit)}. * + * @param time the maximum time to wait for the lock + * @param unit the time unit of the {@code time} argument * @return a stamp that can be used to unlock or convert mode, * or zero if the lock is not available * @throws InterruptedException if the current thread is interrupted @@ -442,11 +429,17 @@ public class StampedLock implements java */ public long tryReadLock(long time, TimeUnit unit) throws InterruptedException { - long next, deadline; + long s, m, next, deadline; long nanos = unit.toNanos(time); if (!Thread.interrupted()) { - if ((next = tryReadLock()) != 0L) - return next; + if ((m = (s = state) & ABITS) != WBIT) { + if (m < RFULL) { + if (U.compareAndSwapLong(this, STATE, s, next = s + RUNIT)) + return next; + } + else if ((next = tryIncReaderOverflow(s)) != 0L) + return next; + } if (nanos <= 0L) return 0L; if ((deadline = System.nanoTime() + nanos) == 0L) @@ -490,9 +483,12 @@ public class StampedLock implements java * Returns true if the lock has not been exclusively acquired * since issuance of the given stamp. Always returns false if the * stamp is zero. Always returns true if the stamp represents a - * currently held lock. + * currently held lock. Invoking this method with a value not + * obtained from {@link #tryOptimisticRead} or a locking method + * for this lock has no defined effect or result. * - * @return true if the lock has not been exclusively acquired + * @param stamp a stamp + * @return {@code true} if the lock has not been exclusively acquired * since issuance of the given stamp; else false */ public boolean validate(long stamp) { @@ -705,7 +701,7 @@ public class StampedLock implements java * stamp value. This method may be useful for recovery after * errors. * - * @return true if the lock was held, else false + * @return {@code true} if the lock was held, else false */ public boolean tryUnlockWrite() { long s; WNode h; @@ -723,7 +719,7 @@ public class StampedLock implements java * requiring a stamp value. This method may be useful for recovery * after errors. * - * @return true if the read lock was held, else false + * @return {@code true} if the read lock was held, else false */ public boolean tryUnlockRead() { long s, m; WNode h; @@ -741,31 +737,67 @@ public class StampedLock implements java return false; } + // status monitoring methods + + /** + * Returns combined state-held and overflow read count for given + * state s. + */ + private int getReadLockCount(long s) { + long readers; + if ((readers = s & RBITS) >= RFULL) + readers = RFULL + readerOverflow; + return (int) readers; + } + /** - * Returns true if the lock is currently held exclusively. + * Returns {@code true} if the lock is currently held exclusively. * - * @return true if the lock is currently held exclusively + * @return {@code true} if the lock is currently held exclusively */ public boolean isWriteLocked() { return (state & WBIT) != 0L; } /** - * Returns true if the lock is currently held non-exclusively. + * Returns {@code true} if the lock is currently held non-exclusively. * - * @return true if the lock is currently held non-exclusively + * @return {@code true} if the lock is currently held non-exclusively */ public boolean isReadLocked() { return (state & RBITS) != 0L; } - private void readObject(java.io.ObjectInputStream s) - throws java.io.IOException, ClassNotFoundException { - s.defaultReadObject(); - state = ORIGIN; // reset to unlocked state + /** + * Queries the number of read locks held for this lock. This + * method is designed for use in monitoring system state, not for + * synchronization control. + * @return the number of read locks held + */ + public int getReadLockCount() { + return getReadLockCount(state); } /** + * Returns a string identifying this lock, as well as its lock + * state. The state, in brackets, includes the String {@code + * "Unlocked"} or the String {@code "Write-locked"} or the String + * {@code "Read-locks:"} followed by the current number of + * read-locks held. + * + * @return a string identifying this lock, as well as its lock state + */ + public String toString() { + long s = state; + return super.toString() + + ((s & ABITS) == 0L ? "[Unlocked]" : + (s & WBIT) != 0L ? "[Write-locked]" : + "[Read-locks:" + getReadLockCount(s) + "]"); + } + + // views + + /** * Returns a plain {@link Lock} view of this StampedLock in which * the {@link Lock#lock} method is mapped to {@link #readLock}, * and similarly for other methods. The returned Lock does not @@ -879,6 +911,12 @@ public class StampedLock implements java } } + private void readObject(java.io.ObjectInputStream s) + throws java.io.IOException, ClassNotFoundException { + s.defaultReadObject(); + state = ORIGIN; // reset to unlocked state + } + // internals /** @@ -886,10 +924,11 @@ public class StampedLock implements java * access bits value to RBITS, indicating hold of spinlock, * then updating, then releasing. * - * @param s, assumed that (s & ABITS) >= RFULL + * @param s a reader overflow stamp: (s & ABITS) >= RFULL * @return new stamp on success, else zero */ private long tryIncReaderOverflow(long s) { + // assert (s & ABITS) >= RFULL; if ((s & ABITS) == RFULL) { if (U.compareAndSwapLong(this, STATE, s, s | RBITS)) { ++readerOverflow; @@ -906,10 +945,11 @@ public class StampedLock implements java /** * Tries to decrement readerOverflow. * - * @param s, assumed that (s & ABITS) >= RFULL + * @param s a reader overflow stamp: (s & ABITS) >= RFULL * @return new stamp on success, else zero */ private long tryDecReaderOverflow(long s) { + // assert (s & ABITS) >= RFULL; if ((s & ABITS) == RFULL) { if (U.compareAndSwapLong(this, STATE, s, s | RBITS)) { int r; long next; @@ -929,7 +969,7 @@ public class StampedLock implements java return 0L; } - /* + /** * Wakes up the successor of h (normally whead). This is normally * just h.next, but may require traversal from wtail if next * pointers are lagging. This may fail to wake up an acquiring @@ -1030,14 +1070,17 @@ public class StampedLock implements java if (deadline == 0L) time = 0L; else if ((time = deadline - System.nanoTime()) <= 0L) - return cancelWaiter(node, null, false); - node.thread = Thread.currentThread(); + return cancelWaiter(node, node, false); + Thread wt = Thread.currentThread(); + U.putObject(wt, PARKBLOCKER, this); // emulate LockSupport.park + node.thread = wt; if (node.prev == p && p.status == WAITING && // recheck (p != whead || (state & ABITS) != 0L)) U.park(false, time); node.thread = null; + U.putObject(wt, PARKBLOCKER, null); if (interruptible && Thread.interrupted()) - return cancelWaiter(node, null, true); + return cancelWaiter(node, node, true); } } } @@ -1100,8 +1143,6 @@ public class StampedLock implements java node.cowait = p.cowait, node)) { node.thread = Thread.currentThread(); for (long time;;) { - if (interruptible && Thread.interrupted()) - return cancelWaiter(node, p, true); if (deadline == 0L) time = 0L; else if ((time = deadline - System.nanoTime()) <= 0L) @@ -1113,9 +1154,14 @@ public class StampedLock implements java node.thread = null; break; } + Thread wt = Thread.currentThread(); + U.putObject(wt, PARKBLOCKER, this); if (node.thread == null) // must recheck break; U.park(false, time); + U.putObject(wt, PARKBLOCKER, null); + if (interruptible && Thread.interrupted()) + return cancelWaiter(node, p, true); } group = p; } @@ -1170,16 +1216,141 @@ public class StampedLock implements java if (deadline == 0L) time = 0L; else if ((time = deadline - System.nanoTime()) <= 0L) - return cancelWaiter(node, null, false); - node.thread = Thread.currentThread(); + return cancelWaiter(node, node, false); + Thread wt = Thread.currentThread(); + U.putObject(wt, PARKBLOCKER, this); + node.thread = wt; if (node.prev == p && p.status == WAITING && (p != whead || (state & ABITS) != WBIT)) U.park(false, time); node.thread = null; + U.putObject(wt, PARKBLOCKER, null); if (interruptible && Thread.interrupted()) - return cancelWaiter(node, null, true); + return cancelWaiter(node, node, true); + } + } + } + + /** + * If node non-null, forces cancel status and unsplices it from + * queue if possible and wakes up any cowaiters (of the node, or + * group, as applicable), and in any case helps release current + * first waiter if lock is free. (Calling with null arguments + * serves as a conditional form of release, which is not currently + * needed but may be needed under possible future cancellation + * policies). This is a variant of cancellation methods in + * AbstractQueuedSynchronizer (see its detailed explanation in AQS + * internal documentation). + * + * @param node if nonnull, the waiter + * @param group either node or the group node is cowaiting with + * @param interrupted if already interrupted + * @return INTERRUPTED if interrupted or Thread.interrupted, else zero + */ + private long cancelWaiter(WNode node, WNode group, boolean interrupted) { + if (node != null && group != null) { + Thread w; + node.status = CANCELLED; + node.thread = null; + // unsplice cancelled nodes from group + for (WNode p = group, q; (q = p.cowait) != null;) { + if (q.status == CANCELLED) + U.compareAndSwapObject(p, WNEXT, q, q.next); + else + p = q; + } + if (group == node) { + WNode r; // detach and wake up uncancelled co-waiters + while ((r = node.cowait) != null) { + if (U.compareAndSwapObject(node, WCOWAIT, r, r.cowait) && + (w = r.thread) != null) { + r.thread = null; + U.unpark(w); + } + } + for (WNode pred = node.prev; pred != null; ) { // unsplice + WNode succ, pp; // find valid successor + while ((succ = node.next) == null || + succ.status == CANCELLED) { + WNode q = null; // find successor the slow way + for (WNode t = wtail; t != null && t != node; t = t.prev) + if (t.status != CANCELLED) + q = t; // don't link if succ cancelled + if (succ == q || // ensure accurate successor + U.compareAndSwapObject(node, WNEXT, + succ, succ = q)) { + if (succ == null && node == wtail) + U.compareAndSwapObject(this, WTAIL, node, pred); + break; + } + } + if (pred.next == node) // unsplice pred link + U.compareAndSwapObject(pred, WNEXT, node, succ); + if (succ != null && (w = succ.thread) != null) { + succ.thread = null; + U.unpark(w); // wake up succ to observe new pred + } + if (pred.status != CANCELLED || (pp = pred.prev) == null) + break; + node.prev = pp; // repeat if new pred wrong/cancelled + U.compareAndSwapObject(pp, WNEXT, pred, succ); + pred = pp; + } } } + WNode h; // Possibly release first waiter + while ((h = whead) != null) { + long s; WNode q; // similar to release() but check eligibility + if ((q = h.next) == null || q.status == CANCELLED) { + for (WNode t = wtail; t != null && t != h; t = t.prev) + if (t.status <= 0) + q = t; + } + if (h == whead) { + if (q != null && h.status == 0 && + ((s = state) & ABITS) != WBIT && // waiter is eligible + (s == 0L || q.mode == RMODE)) + release(h); + break; + } + } + return (interrupted || Thread.interrupted()) ? INTERRUPTED : 0L; + } + + // Unsafe mechanics + private static final sun.misc.Unsafe U; + private static final long STATE; + private static final long WHEAD; + private static final long WTAIL; + private static final long WNEXT; + private static final long WSTATUS; + private static final long WCOWAIT; + private static final long PARKBLOCKER; + + static { + try { + U = getUnsafe(); + Class k = StampedLock.class; + Class wk = WNode.class; + STATE = U.objectFieldOffset + (k.getDeclaredField("state")); + WHEAD = U.objectFieldOffset + (k.getDeclaredField("whead")); + WTAIL = U.objectFieldOffset + (k.getDeclaredField("wtail")); + WSTATUS = U.objectFieldOffset + (wk.getDeclaredField("status")); + WNEXT = U.objectFieldOffset + (wk.getDeclaredField("next")); + WCOWAIT = U.objectFieldOffset + (wk.getDeclaredField("cowait")); + Class tk = Thread.class; + PARKBLOCKER = U.objectFieldOffset + (tk.getDeclaredField("parkBlocker")); + + } catch (Exception e) { + throw new Error(e); + } } /**