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root/jsr166/jsr166/src/jsr166e/StampedLock.java

Comparing jsr166/src/jsr166e/StampedLock.java (file contents):
Revision 1.14 by jsr166, Sun Oct 14 16:23:02 2012 UTC vs.
Revision 1.22 by jsr166, Wed Oct 17 00:02:47 2012 UTC

#	Line 61 | Line 61 | import java.util.concurrent.TimeUnit;
61		* help reduce some of the code bloat that otherwise occurs in
62		* retry-based designs.
63		*
64	<	* <p>StampedLocks are designed for use in a different (and generally
65	<	* narrower) range of contexts than most other locks: They are not
66	<	* reentrant, so locked bodies should not call other unknown methods
67	<	* that may try to re-acquire locks (although you may pass a stamp to
68	<	* other methods that can use or convert it). Unvalidated optimistic
69	<	* read sections should further not call methods that are not known to
64	>	* <p>StampedLocks are designed for use as internal utilities in the
65	>	* development of thread-safe components. Their use relies on
66	>	* knowledge of the internal properties of the data, objects, and
67	>	* methods they are protecting.  They are not reentrant, so locked
68	>	* bodies should not call other unknown methods that may try to
69	>	* re-acquire locks (although you may pass a stamp to other methods
70	>	* that can use or convert it).  The use of read lock modes relies on
71	>	* the associated code sections being side-effect-free.  Unvalidated
72	>	* optimistic read sections cannot call methods that are not known to
73		* tolerate potential inconsistencies.  Stamps use finite
74		* representations, and are not cryptographically secure (i.e., a
75		* valid stamp may be guessable). Stamp values may recycle after (no
#	Line 122 | Line 125 | import java.util.concurrent.TimeUnit;
125		*   }
126		*
127		*   double distanceFromOriginV2() { // combines code paths
128	+	*     double currentX = 0.0, currentY = 0.0;
129		*     for (long stamp = sl.tryOptimisticRead(); ; stamp = sl.readLock()) {
126	–	*       double currentX, currentY;
130		*       try {
131		*         currentX = x;
132		*         currentY = y;
133		*       } finally {
134		*         if (sl.tryConvertToOptimisticRead(stamp) != 0L) // unlock or validate
135	<	*           return Math.sqrt(currentX * currentX + currentY * currentY);
135	>	*           break;
136		*       }
137		*     }
138	+	*     return Math.sqrt(currentX * currentX + currentY * currentY);
139		*   }
140		*
141		*   void moveIfAtOrigin(double newX, double newY) { // upgrade
#	Line 139 | Line 143 | import java.util.concurrent.TimeUnit;
143		*     long stamp = sl.readLock();
144		*     try {
145		*       while (x == 0.0 && y == 0.0) {
146	<	*         long ws = tryConvertToWriteLock(stamp);
146	>	*         long ws = sl.tryConvertToWriteLock(stamp);
147		*         if (ws != 0L) {
148		*           stamp = ws;
149		*           x = newX;
#	Line 180 | Line 184 | public class StampedLock implements java
184		* read-locked.  The read count is ignored when validating
185		* "optimistic" seqlock-reader-style stamps.  Because we must use
186		* a small finite number of bits (currently 7) for readers, a
187	<	* supplementary reader overflow word is used when then number of
187	>	* supplementary reader overflow word is used when the number of
188		* readers exceeds the count field. We do this by treating the max
189		* reader count value (RBITS) as a spinlock protecting overflow
190		* updates.
#	Line 222 | Line 226 | public class StampedLock implements java
226		* threads.  Both await methods use a similar spin strategy: If
227		* the associated queue appears to be empty, then the thread
228		* spin-waits up to SPINS times (where each iteration decreases
229	<	* spin count with 50% probablility) before enqueing, and then, if
229	>	* spin count with 50% probability) before enqueing, and then, if
230		* it is the first thread to be enqueued, spins again up to SPINS
231		* times before blocking. If, upon wakening it fails to obtain
232		* lock, and is still (or becomes) the first waiting thread (which
#	Line 310 | Line 314 | public class StampedLock implements java
314		private transient int readerOverflow;
315
316		/**
317	<	* Creates a new lock initially in unlocked state.
317	>	* Creates a new lock, initially in unlocked state.
318		*/
319		public StampedLock() {
320		state = ORIGIN;
#	Line 511 | Line 515 | public class StampedLock implements java
515		}
516
517		/**
518	<	* Returns true if the lock has not been exclusively held since
519	<	* issuance of the given stamp. Always returns false if the stamp
520	<	* is zero. Always returns true if the stamp represents a
518	>	* Returns true if the lock has not been exclusively acquired
519	>	* since issuance of the given stamp. Always returns false if the
520	>	* stamp is zero. Always returns true if the stamp represents a
521		* currently held lock.
522		*
523	<	* @return true if the lock has not been exclusively held since
524	<	* issuance of the given stamp; else false
523	>	* @return true if the lock has not been exclusively acquired
524	>	* since issuance of the given stamp; else false
525		*/
526		public boolean validate(long stamp) {
527	+	// See above about current use of getLongVolatile here
528		return (stamp & SBITS) == (U.getLongVolatile(this, STATE) & SBITS);
529		}
530
#	Line 629 | Line 634 | public class StampedLock implements java
634		break;
635		return stamp;
636		}
637	<	else if (m == RUNIT && a != 0L && a < WBIT) {
637	>	else if (m == RUNIT && a != 0L) {
638		if (U.compareAndSwapLong(this, STATE, s,
639		next = s - RUNIT + WBIT))
640		return next;
#	Line 667 | Line 672 | public class StampedLock implements java
672		else if (m == WBIT) {
673		if (a != m)
674		break;
675	<	next = state = s + (WBIT + RUNIT);
675	>	state = next = s + (WBIT + RUNIT);
676		readerPrefSignal();
677		return next;
678		}
#	Line 701 | Line 706 | public class StampedLock implements java
706		else if (m == WBIT) {
707		if (a != m)
708		break;
709	<	next = state = (s += WBIT) == 0L ? ORIGIN : s;
709	>	state = next = (s += WBIT) == 0L ? ORIGIN : s;
710		readerPrefSignal();
711		return next;
712		}
#	Line 775 | Line 780 | public class StampedLock implements java
780		* @return true if the lock is currently held non-exclusively
781		*/
782		public boolean isReadLocked() {
783	<	long m;
779	<	return (m = state & ABITS) > 0L && m < WBIT;
783	>	return (state & RBITS) != 0L;
784		}
785
786		private void readObject(java.io.ObjectInputStream s)
#	Line 792 | Line 796 | public class StampedLock implements java
796		* access bits value to RBITS, indicating hold of spinlock,
797		* then updating, then releasing.
798		*
799	<	* @param stamp, assumed that (stamp & ABITS) >= RFULL
799	>	* @param s, assumed that (s & ABITS) >= RFULL
800		* @return new stamp on success, else zero
801		*/
802		private long tryIncReaderOverflow(long s) {
#	Line 812 | Line 816 | public class StampedLock implements java
816		/**
817		* Tries to decrement readerOverflow.
818		*
819	<	* @param stamp, assumed that (stamp & ABITS) >= RFULL
819	>	* @param s, assumed that (s & ABITS) >= RFULL
820		* @return new stamp on success, else zero
821		*/
822		private long tryDecReaderOverflow(long s) {
#	Line 868 | Line 872 | public class StampedLock implements java
872		U.compareAndSwapObject(p, WAITER, w, null))
873		U.unpark(w);
874		}
875	<	if (!readers && (state & ABITS) == 0L &&
876	<	(h = whead) != null && h.status != 0) {
875	>	if (!readers && (h = whead) != null && h.status != 0 &&
876	>	(state & ABITS) == 0L) {
877		U.compareAndSwapInt(h, STATUS, WAITING, 0);
878		if ((q = h.next) == null || q.status == CANCELLED) {
875	–	q = null;
879		for (WNode t = wtail; t != null && t != h; t = t.prev)
880		if (t.status <= 0)
881		q = t;
#	Line 887 | Line 890 | public class StampedLock implements java
890		if ((h = whead) != null && h.status != 0) {
891		U.compareAndSwapInt(h, STATUS, WAITING, 0);
892		if ((q = h.next) == null || q.status == CANCELLED) {
890	–	q = null;
893		for (WNode t = wtail; t != null && t != h; t = t.prev)
894		if (t.status <= 0)
895		q = t;
#	Line 957 | Line 959 | public class StampedLock implements java
959		else if (U.compareAndSwapObject(this, WTAIL, p, node)) {
960		p.next = node;
961		for (int headSpins = SPINS;;) {
962	<	WNode np; int ps;
963	<	if ((np = node.prev) != p && np != null &&
964	<	(p = np).next != node)
963	<	p.next = node; // stale
962	>	WNode np, pp; int ps;
963	>	while ((np = node.prev) != p && np != null)
964	>	(p = np).next = node; // stale
965		if (p == whead) {
966		for (int k = headSpins;;) {
967		if (((s = state) & ABITS) == 0L) {
#	Line 981 | Line 982 | public class StampedLock implements java
982		}
983		if ((ps = p.status) == 0)
984		U.compareAndSwapInt(p, STATUS, 0, WAITING);
985	<	else if (ps == CANCELLED)
986	<	node.prev = p.prev;
985	>	else if (ps == CANCELLED) {
986	>	if ((pp = p.prev) != null) {
987	>	node.prev = pp;
988	>	pp.next = node;
989	>	}
990	>	}
991		else {
992		long time; // 0 argument to park means no timeout
993		if (deadline == 0L)
#	Line 990 | Line 995 | public class StampedLock implements java
995		else if ((time = deadline - System.nanoTime()) <= 0L)
996		return cancelWriter(node, false);
997		if (node.prev == p && p.status == WAITING &&
998	<	(p != whead || (state & WBIT) != 0L)) { // recheck
998	>	(p != whead || (state & WBIT) != 0L)) // recheck
999		U.park(false, time);
1000	<	if (interruptible && Thread.interrupted())
1001	<	return cancelWriter(node, true);
997	<	}
1000	>	if (interruptible && Thread.interrupted())
1001	>	return cancelWriter(node, true);
1002		}
1003		}
1004		}
#	Line 1003 | Line 1007 | public class StampedLock implements java
1007
1008		/**
1009		* If node non-null, forces cancel status and unsplices from queue
1010	<	* if possible. This is a streamlined variant of cancellation
1011	<	* methods in AbstractQueuedSynchronizer that includes a detailed
1012	<	* explanation.
1010	>	* if possible. This is a variant of cancellation methods in
1011	>	* AbstractQueuedSynchronizer (see its detailed explanation in AQS
1012	>	* internal documentation) that more conservatively wakes up other
1013	>	* threads that may have had their links changed, so as to preserve
1014	>	* liveness in the main signalling methods.
1015		*/
1016		private long cancelWriter(WNode node, boolean interrupted) {
1017	<	WNode pred;
1012	<	if (node != null && (pred = node.prev) != null) {
1013	<	WNode pp;
1017	>	if (node != null) {
1018		node.thread = null;
1015	–	while (pred.status == CANCELLED && (pp = pred.prev) != null)
1016	–	pred = node.prev = pp;
1017	–	WNode predNext = pred.next;
1019		node.status = CANCELLED;
1020	<	if (predNext != null) {
1021	<	Thread w;
1022	<	WNode succ = node.next;
1023	<	if (succ == null || succ.status == CANCELLED) {
1023	<	succ = null;
1020	>	for (WNode pred = node.prev; pred != null; ) {
1021	>	WNode succ, pp; Thread w;
1022	>	while ((succ = node.next) == null || succ.status == CANCELLED) {
1023	>	WNode q = null;
1024		for (WNode t = wtail; t != null && t != node; t = t.prev)
1025	<	if (t.status <= 0)
1026	<	succ = t;
1027	<	if (succ == null && node == wtail)
1028	<	U.compareAndSwapObject(this, WTAIL, node, pred);
1025	>	if (t.status != CANCELLED)
1026	>	q = t;
1027	>	if (succ == q ||
1028	>	U.compareAndSwapObject(node, WNEXT, succ, succ = q)) {
1029	>	if (succ == null && node == wtail)
1030	>	U.compareAndSwapObject(this, WTAIL, node, pred);
1031	>	break;
1032	>	}
1033		}
1034	<	U.compareAndSwapObject(pred, WNEXT, predNext, succ);
1034	>	if (pred.next == node)
1035	>	U.compareAndSwapObject(pred, WNEXT, node, succ);
1036		if (succ != null && (w = succ.thread) != null)
1037		U.unpark(w);
1038	+	if (pred.status != CANCELLED || (pp = pred.prev) == null)
1039	+	break;
1040	+	node.prev = pp; // repeat for new pred
1041	+	U.compareAndSwapObject(pp, WNEXT, pred, succ);
1042	+	pred = pp;
1043		}
1044		}
1045		writerPrefSignal();
#	Line 1103 | Line 1113 | public class StampedLock implements java
1113		time = 0L;
1114		else if ((time = deadline - System.nanoTime()) <= 0L)
1115		return cancelReader(node, false);
1116	<	if ((state & WBIT) != 0L && node.waiter != null) { // recheck
1116	>	if ((state & WBIT) != 0L && node.waiter != null) // recheck
1117		U.park(false, time);
1118	<	if (interruptible && Thread.interrupted())
1119	<	return cancelReader(node, true);
1110	<	}
1118	>	if (interruptible && Thread.interrupted())
1119	>	return cancelReader(node, true);
1120		}
1121		}
1122		}

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