--- jsr166/src/jsr166e/StampedLock.java 2013/01/24 21:35:03 1.31
+++ jsr166/src/jsr166e/StampedLock.java 2016/06/28 14:52:19 1.42
@@ -6,12 +6,10 @@
package jsr166e;
-import java.util.concurrent.ThreadLocalRandom;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReadWriteLock;
-import java.util.concurrent.locks.LockSupport;
/**
* A capability-based lock with three modes for controlling read/write
@@ -30,29 +28,30 @@ import java.util.concurrent.locks.LockSu
* in method {@link #unlockWrite} to release the lock. Untimed and
* timed versions of {@code tryWriteLock} are also provided. When
* the lock is held in write mode, no read locks may be obtained,
- * and all optimistic read validations will fail.
+ * and all optimistic read validations will fail.
*
* Reading. Method {@link #readLock} possibly blocks
* waiting for non-exclusive access, returning a stamp that can be
* used in method {@link #unlockRead} to release the lock. Untimed
- * and timed versions of {@code tryReadLock} are also provided.
+ * and timed versions of {@code tryReadLock} are also provided.
*
* 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);
* }
@@ -214,7 +196,11 @@ public class StampedLock implements java
* incoming reader arrives while read lock is held but there is a
* queued writer, this incoming reader is queued. (This rule is
* responsible for some of the complexity of method acquireRead,
- * but without it, the lock becomes highly unfair.)
+ * but without it, the lock becomes highly unfair.) Method release
+ * does not (and sometimes cannot) itself wake up cowaiters. This
+ * is done by the primary thread, but helped by any other threads
+ * with nothing better to do in methods acquireRead and
+ * acquireWrite.
*
* These rules apply to threads actually queued. All tryLock forms
* opportunistically try to acquire locks regardless of preference
@@ -252,16 +238,20 @@ 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 */
private static final int NCPU = Runtime.getRuntime().availableProcessors();
- /** Maximum number of retries before blocking on acquisition */
+ /** Maximum number of retries before enqueuing on acquisition */
private static final int SPINS = (NCPU > 1) ? 1 << 6 : 0;
+ /** Maximum number of retries before blocking at head on acquisition */
+ private static final int HEAD_SPINS = (NCPU > 1) ? 1 << 10 : 0;
+
/** Maximum number of retries before re-blocking */
- private static final int MAX_HEAD_SPINS = (NCPU > 1) ? 1 << 12 : 0;
+ private static final int MAX_HEAD_SPINS = (NCPU > 1) ? 1 << 16 : 0;
/** The period for yielding when waiting for overflow spinlock */
private static final int OVERFLOW_YIELD_RATE = 7; // must be power 2 - 1
@@ -356,6 +346,8 @@ 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
@@ -403,8 +395,8 @@ public class StampedLock implements java
* @return a stamp that can be used to unlock or convert mode
*/
public long readLock() {
- long s, next; // bypass acquireRead on fully unlocked case only
- return ((((s = state) & ABITS) == 0L &&
+ long s = state, next; // bypass acquireRead on common uncontended case
+ return ((whead == wtail && (s & ABITS) < RFULL &&
U.compareAndSwapLong(this, STATE, s, next = s + RUNIT)) ?
next : acquireRead(false, 0L));
}
@@ -435,6 +427,8 @@ 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 +436,17 @@ 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 +490,12 @@ 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) {
@@ -670,7 +673,7 @@ public class StampedLock implements java
long a = stamp & ABITS, m, s, next; WNode h;
for (;;) {
s = U.getLongVolatile(this, STATE); // see above
- if ((s & SBITS) != (stamp & SBITS))
+ if (((s = state) & SBITS) != (stamp & SBITS))
break;
if ((m = s & ABITS) == 0L) {
if (a != 0L)
@@ -705,7 +708,7 @@ 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 +726,7 @@ 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;
@@ -741,30 +744,66 @@ public class StampedLock implements java
return false;
}
+ // status monitoring methods
+
/**
- * Returns true if the lock is currently held exclusively.
+ * Returns combined state-held and overflow read count for given
+ * state s.
+ */
+ private int getReadLockCount(long s) {
+ long readers;
+ if ((readers = s & RBITS) >= RFULL)
+ readers = RFULL + readerOverflow;
+ return (int) readers;
+ }
+
+ /**
+ * Returns {@code true} if the lock is currently held exclusively.
*
- * @return 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},
@@ -879,6 +918,12 @@ 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 +931,11 @@ 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 +952,11 @@ 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 +976,7 @@ 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
@@ -945,17 +992,8 @@ public class StampedLock implements java
if (t.status <= 0)
q = t;
}
- if (q != null) {
- for (WNode r = q;;) { // release co-waiters too
- if ((w = r.thread) != null) {
- r.thread = null;
- U.unpark(w);
- }
- if ((r = q.cowait) == null)
- break;
- U.compareAndSwapObject(q, WCOWAIT, r, r.cowait);
- }
- }
+ if (q != null && (w = q.thread) != null)
+ U.unpark(w);
}
}
@@ -971,22 +1009,22 @@ public class StampedLock implements java
private long acquireWrite(boolean interruptible, long deadline) {
WNode node = null, p;
for (int spins = -1;;) { // spin while enqueuing
- long s, ns;
- if (((s = state) & ABITS) == 0L) {
+ long m, s, ns;
+ if ((m = (s = state) & ABITS) == 0L) {
if (U.compareAndSwapLong(this, STATE, s, ns = s + WBIT))
return ns;
}
+ else if (spins < 0)
+ spins = (m == WBIT && wtail == whead) ? SPINS : 0;
else if (spins > 0) {
if (ThreadLocalRandom.current().nextInt() >= 0)
--spins;
}
else if ((p = wtail) == null) { // initialize queue
- WNode h = new WNode(WMODE, null);
- if (U.compareAndSwapObject(this, WHEAD, null, h))
- wtail = h;
+ WNode hd = new WNode(WMODE, null);
+ if (U.compareAndSwapObject(this, WHEAD, null, hd))
+ wtail = hd;
}
- else if (spins < 0)
- spins = (p == whead) ? SPINS : 0;
else if (node == null)
node = new WNode(WMODE, p);
else if (node.prev != p)
@@ -997,14 +1035,18 @@ public class StampedLock implements java
}
}
- for (int spins = SPINS;;) {
- WNode np, pp; int ps; long s, ns; Thread w;
- while ((np = node.prev) != p && np != null)
- (p = np).next = node; // stale
- if (whead == p) {
+ for (int spins = -1;;) {
+ WNode h, np, pp; int ps;
+ if ((h = whead) == p) {
+ if (spins < 0)
+ spins = HEAD_SPINS;
+ else if (spins < MAX_HEAD_SPINS)
+ spins <<= 1;
for (int k = spins;;) { // spin at head
+ long s, ns;
if (((s = state) & ABITS) == 0L) {
- if (U.compareAndSwapLong(this, STATE, s, ns = s+WBIT)) {
+ if (U.compareAndSwapLong(this, STATE, s,
+ ns = s + WBIT)) {
whead = node;
node.prev = null;
return ns;
@@ -1014,30 +1056,45 @@ public class StampedLock implements java
--k <= 0)
break;
}
- if (spins < MAX_HEAD_SPINS)
- spins <<= 1;
}
- if ((ps = p.status) == 0)
- U.compareAndSwapInt(p, WSTATUS, 0, WAITING);
- else if (ps == CANCELLED) {
- if ((pp = p.prev) != null) {
- node.prev = pp;
- pp.next = node;
+ else if (h != null) { // help release stale waiters
+ WNode c; Thread w;
+ while ((c = h.cowait) != null) {
+ if (U.compareAndSwapObject(h, WCOWAIT, c, c.cowait) &&
+ (w = c.thread) != null)
+ U.unpark(w);
}
}
- else {
- long time; // 0 argument to park means no timeout
- if (deadline == 0L)
- time = 0L;
- else if ((time = deadline - System.nanoTime()) <= 0L)
- return cancelWaiter(node, null, false);
- node.thread = Thread.currentThread();
- if (node.prev == p && p.status == WAITING && // recheck
- (p != whead || (state & ABITS) != 0L))
- U.park(false, time);
- node.thread = null;
- if (interruptible && Thread.interrupted())
- return cancelWaiter(node, null, true);
+ if (whead == h) {
+ if ((np = node.prev) != p) {
+ if (np != null)
+ (p = np).next = node; // stale
+ }
+ else if ((ps = p.status) == 0)
+ U.compareAndSwapInt(p, WSTATUS, 0, WAITING);
+ else if (ps == CANCELLED) {
+ if ((pp = p.prev) != null) {
+ node.prev = pp;
+ pp.next = node;
+ }
+ }
+ else {
+ long time; // 0 argument to park means no timeout
+ if (deadline == 0L)
+ time = 0L;
+ else if ((time = deadline - System.nanoTime()) <= 0L)
+ return cancelWaiter(node, node, false);
+ Thread wt = Thread.currentThread();
+ U.putObject(wt, PARKBLOCKER, this);
+ node.thread = wt;
+ if (p.status < 0 && (p != h || (state & ABITS) != 0L) &&
+ whead == h && node.prev == p)
+ U.park(false, time); // emulate LockSupport.park
+ node.thread = null;
+ U.putObject(wt, PARKBLOCKER, null);
+ if (interruptible && Thread.interrupted())
+ return cancelWaiter(node, node, true);
+ }
}
}
}
@@ -1052,134 +1109,280 @@ public class StampedLock implements java
* @return next state, or INTERRUPTED
*/
private long acquireRead(boolean interruptible, long deadline) {
- WNode node = null, group = null, p;
+ WNode node = null, p;
for (int spins = -1;;) {
- for (;;) {
- long s, m, ns; WNode h, q; Thread w; // anti-barging guard
- if (group == null && (h = whead) != null &&
- (q = h.next) != null && q.mode != RMODE)
- break;
- if ((m = (s = state) & ABITS) < RFULL ?
- U.compareAndSwapLong(this, STATE, s, ns = s + RUNIT) :
- (m < WBIT && (ns = tryIncReaderOverflow(s)) != 0L)) {
- if (group != null) { // help release others
- for (WNode r = group;;) {
- if ((w = r.thread) != null) {
- r.thread = null;
- U.unpark(w);
+ WNode h;
+ if ((h = whead) == (p = wtail)) {
+ for (long m, s, ns;;) {
+ if ((m = (s = state) & ABITS) < RFULL ?
+ U.compareAndSwapLong(this, STATE, s, ns = s + RUNIT) :
+ (m < WBIT && (ns = tryIncReaderOverflow(s)) != 0L))
+ return ns;
+ else if (m >= WBIT) {
+ if (spins > 0) {
+ if (ThreadLocalRandom.current().nextInt() >= 0)
+ --spins;
+ }
+ else {
+ if (spins == 0) {
+ WNode nh = whead, np = wtail;
+ if ((nh == h && np == p) || (h = nh) != (p = np))
+ break;
}
- if ((r = group.cowait) == null)
- break;
- U.compareAndSwapObject(group, WCOWAIT, r, r.cowait);
+ spins = SPINS;
}
}
- return ns;
}
- if (m >= WBIT)
- break;
}
- if (spins > 0) {
- if (ThreadLocalRandom.current().nextInt() >= 0)
- --spins;
+ if (p == null) { // initialize queue
+ WNode hd = new WNode(WMODE, null);
+ if (U.compareAndSwapObject(this, WHEAD, null, hd))
+ wtail = hd;
}
- else if ((p = wtail) == null) {
- WNode h = new WNode(WMODE, null);
- if (U.compareAndSwapObject(this, WHEAD, null, h))
- wtail = h;
- }
- else if (spins < 0)
- spins = (p == whead) ? SPINS : 0;
else if (node == null)
- node = new WNode(WMODE, p);
- else if (node.prev != p)
- node.prev = p;
- else if (p.mode == RMODE && p != whead) {
- WNode pp = p.prev; // become co-waiter with group p
- if (pp != null && p == wtail &&
- U.compareAndSwapObject(p, WCOWAIT,
- node.cowait = p.cowait, node)) {
- node.thread = Thread.currentThread();
- for (long time;;) {
- if (interruptible && Thread.interrupted())
- return cancelWaiter(node, p, true);
+ node = new WNode(RMODE, p);
+ else if (h == p || p.mode != RMODE) {
+ if (node.prev != p)
+ node.prev = p;
+ else if (U.compareAndSwapObject(this, WTAIL, p, node)) {
+ p.next = node;
+ break;
+ }
+ }
+ else if (!U.compareAndSwapObject(p, WCOWAIT,
+ node.cowait = p.cowait, node))
+ node.cowait = null;
+ else {
+ for (;;) {
+ WNode pp, c; Thread w;
+ if ((h = whead) != null && (c = h.cowait) != null &&
+ U.compareAndSwapObject(h, WCOWAIT, c, c.cowait) &&
+ (w = c.thread) != null) // help release
+ U.unpark(w);
+ if (h == (pp = p.prev) || h == p || pp == null) {
+ long m, s, ns;
+ do {
+ if ((m = (s = state) & ABITS) < RFULL ?
+ U.compareAndSwapLong(this, STATE, s,
+ ns = s + RUNIT) :
+ (m < WBIT &&
+ (ns = tryIncReaderOverflow(s)) != 0L))
+ return ns;
+ } while (m < WBIT);
+ }
+ if (whead == h && p.prev == pp) {
+ long time;
+ if (pp == null || h == p || p.status > 0) {
+ node = null; // throw away
+ break;
+ }
if (deadline == 0L)
time = 0L;
else if ((time = deadline - System.nanoTime()) <= 0L)
return cancelWaiter(node, p, false);
- if (node.thread == null)
- break;
- if (p.prev != pp || p.status == CANCELLED ||
- p == whead || p.prev != pp) {
- node.thread = null;
- break;
+ Thread wt = Thread.currentThread();
+ U.putObject(wt, PARKBLOCKER, this);
+ node.thread = wt;
+ if ((h != pp || (state & ABITS) == WBIT) &&
+ whead == h && p.prev == pp)
+ U.park(false, time);
+ node.thread = null;
+ U.putObject(wt, PARKBLOCKER, null);
+ if (interruptible && Thread.interrupted())
+ return cancelWaiter(node, p, true);
+ }
+ }
+ }
+ }
+
+ for (int spins = -1;;) {
+ WNode h, np, pp; int ps;
+ if ((h = whead) == p) {
+ if (spins < 0)
+ spins = HEAD_SPINS;
+ else if (spins < MAX_HEAD_SPINS)
+ spins <<= 1;
+ for (int k = spins;;) { // spin at head
+ long m, s, ns;
+ if ((m = (s = state) & ABITS) < RFULL ?
+ U.compareAndSwapLong(this, STATE, s, ns = s + RUNIT) :
+ (m < WBIT && (ns = tryIncReaderOverflow(s)) != 0L)) {
+ WNode c; Thread w;
+ whead = node;
+ node.prev = null;
+ while ((c = node.cowait) != null) {
+ if (U.compareAndSwapObject(node, WCOWAIT,
+ c, c.cowait) &&
+ (w = c.thread) != null)
+ U.unpark(w);
}
- if (node.thread == null) // must recheck
- break;
- U.park(false, time);
+ return ns;
}
- group = p;
+ else if (m >= WBIT &&
+ ThreadLocalRandom.current().nextInt() >= 0 && --k <= 0)
+ break;
}
- node = null; // throw away
}
- else if (U.compareAndSwapObject(this, WTAIL, p, node)) {
- p.next = node;
- break;
+ else if (h != null) {
+ WNode c; Thread w;
+ while ((c = h.cowait) != null) {
+ if (U.compareAndSwapObject(h, WCOWAIT, c, c.cowait) &&
+ (w = c.thread) != null)
+ U.unpark(w);
+ }
+ }
+ if (whead == h) {
+ if ((np = node.prev) != p) {
+ if (np != null)
+ (p = np).next = node; // stale
+ }
+ else if ((ps = p.status) == 0)
+ U.compareAndSwapInt(p, WSTATUS, 0, WAITING);
+ else if (ps == CANCELLED) {
+ if ((pp = p.prev) != null) {
+ node.prev = pp;
+ pp.next = node;
+ }
+ }
+ else {
+ long time;
+ if (deadline == 0L)
+ time = 0L;
+ else if ((time = deadline - System.nanoTime()) <= 0L)
+ return cancelWaiter(node, node, false);
+ Thread wt = Thread.currentThread();
+ U.putObject(wt, PARKBLOCKER, this);
+ node.thread = wt;
+ if (p.status < 0 &&
+ (p != h || (state & ABITS) == WBIT) &&
+ whead == h && node.prev == p)
+ U.park(false, time);
+ node.thread = null;
+ U.putObject(wt, PARKBLOCKER, null);
+ if (interruptible && Thread.interrupted())
+ return cancelWaiter(node, node, true);
+ }
}
}
+ }
- for (int spins = SPINS;;) {
- WNode np, pp, r; int ps; long m, s, ns; Thread w;
- while ((np = node.prev) != p && np != null)
- (p = np).next = node;
- if (whead == p) {
- for (int k = spins;;) {
- if ((m = (s = state) & ABITS) != WBIT) {
- if (m < RFULL ?
- U.compareAndSwapLong(this, STATE, s, ns = s + RUNIT):
- (ns = tryIncReaderOverflow(s)) != 0L) {
- whead = node;
- node.prev = null;
- while ((r = node.cowait) != null) {
- if (U.compareAndSwapObject(node, WCOWAIT,
- r, r.cowait) &&
- (w = r.thread) != null) {
- r.thread = null;
- U.unpark(w); // release co-waiter
- }
- }
- return ns;
+ /**
+ * If node non-null, forces cancel status and unsplices it from
+ * queue if possible and wakes up any cowaiters (of the node, or
+ * group, as applicable), and in any case helps release current
+ * first waiter if lock is free. (Calling with null arguments
+ * serves as a conditional form of release, which is not currently
+ * needed but may be needed under possible future cancellation
+ * policies). This is a variant of cancellation methods in
+ * AbstractQueuedSynchronizer (see its detailed explanation in AQS
+ * internal documentation).
+ *
+ * @param node if non-null, the waiter
+ * @param group either node or the group node is cowaiting with
+ * @param interrupted if already interrupted
+ * @return INTERRUPTED if interrupted or Thread.interrupted, else zero
+ */
+ private long cancelWaiter(WNode node, WNode group, boolean interrupted) {
+ if (node != null && group != null) {
+ Thread w;
+ node.status = CANCELLED;
+ // unsplice cancelled nodes from group
+ for (WNode p = group, q; (q = p.cowait) != null;) {
+ if (q.status == CANCELLED) {
+ U.compareAndSwapObject(p, WCOWAIT, q, q.cowait);
+ p = group; // restart
+ }
+ else
+ p = q;
+ }
+ if (group == node) {
+ for (WNode r = group.cowait; r != null; r = r.cowait) {
+ if ((w = r.thread) != null)
+ U.unpark(w); // wake up uncancelled co-waiters
+ }
+ for (WNode pred = node.prev; pred != null; ) { // unsplice
+ WNode succ, pp; // find valid successor
+ while ((succ = node.next) == null ||
+ succ.status == CANCELLED) {
+ WNode q = null; // find successor the slow way
+ for (WNode t = wtail; t != null && t != node; t = t.prev)
+ if (t.status != CANCELLED)
+ q = t; // don't link if succ cancelled
+ if (succ == q || // ensure accurate successor
+ U.compareAndSwapObject(node, WNEXT,
+ succ, succ = q)) {
+ if (succ == null && node == wtail)
+ U.compareAndSwapObject(this, WTAIL, node, pred);
+ break;
}
}
- else if (ThreadLocalRandom.current().nextInt() >= 0 &&
- --k <= 0)
+ 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;
}
- if (spins < MAX_HEAD_SPINS)
- spins <<= 1;
}
- if ((ps = p.status) == 0)
- U.compareAndSwapInt(p, WSTATUS, 0, WAITING);
- else if (ps == CANCELLED) {
- if ((pp = p.prev) != null) {
- node.prev = pp;
- pp.next = node;
- }
+ }
+ 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;
}
- else {
- long time;
- if (deadline == 0L)
- time = 0L;
- else if ((time = deadline - System.nanoTime()) <= 0L)
- return cancelWaiter(node, null, false);
- node.thread = Thread.currentThread();
- if (node.prev == p && p.status == WAITING &&
- (p != whead || (state & ABITS) != WBIT))
- U.park(false, time);
- node.thread = null;
- if (interruptible && Thread.interrupted())
- return cancelWaiter(node, null, true);
+ 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);
+ }
}
/**