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

Comparing jsr166/src/jsr166e/StampedLock.java (file contents):
Revision 1.6 by dl, Fri Oct 12 17:35:29 2012 UTC vs.
Revision 1.26 by jsr166, Wed Jan 9 02:51:37 2013 UTC

#	Line 11 | Line 11 | import java.util.concurrent.TimeUnit;
11
12		/**
13		* A capability-based lock with three modes for controlling read/write
14	<	* access.  The state of a StampedLock consists of a version and
15	<	* mode. Lock acquisition methods return a stamp that represents and
14	>	* access.  The state of a StampedLock consists of a version and mode.
15	>	* Lock acquisition methods return a stamp that represents and
16		* controls access with respect to a lock state; "try" versions of
17		* these methods may instead return the special value zero to
18		* represent failure to acquire access. Lock release and conversion
#	Line 55 | Line 55 | import java.util.concurrent.TimeUnit;
55		* <p>This class also supports methods that conditionally provide
56		* conversions across the three modes. For example, method {@link
57		* #tryConvertToWriteLock} attempts to "upgrade" a mode, returning
58	<	* valid write stamp if (1) already in writing mode (2) in reading
58	>	* a valid write stamp if (1) already in writing mode (2) in reading
59		* mode and there are no other readers or (3) in optimistic mode and
60		* the lock is available. The forms of these methods are designed to
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 76 | Line 79 | import java.util.concurrent.TimeUnit;
79		* into initial unlocked state, so they are not useful for remote
80		* locking.
81		*
82	<	* <p>The scheduling policy of StampedLock does not consistently prefer
83	<	* readers over writers or vice versa.
82	>	* <p>The scheduling policy of StampedLock does not consistently
83	>	* prefer readers over writers or vice versa.  A zero return from any
84	>	* "try" method for acquiring or converting locks does not carry any
85	>	* information about the state of the lock; a subsequent invocation
86	>	* may succeed.
87		*
88		* <p><b>Sample Usage.</b> The following illustrates some usage idioms
89		* in a class that maintains simple two-dimensional points. The sample
#	Line 119 | Line 125 | import java.util.concurrent.TimeUnit;
125		*   }
126		*
127		*   double distanceFromOriginV2() { // combines code paths
128	<	*     for (long stamp = sl.optimisticRead(); ; stamp = sl.readLock()) {
129	<	*       double currentX, currentY;
128	>	*     double currentX = 0.0, currentY = 0.0;
129	>	*     for (long stamp = sl.tryOptimisticRead(); ; stamp = sl.readLock()) {
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 136 | 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 149 | Line 156 | import java.util.concurrent.TimeUnit;
156		*         }
157		*       }
158		*     } finally {
159	<	*        sl.unlock(stamp);
159	>	*       sl.unlock(stamp);
160		*     }
161		*   }
162		* }}</pre>
#	Line 177 | 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 213 | Line 220 | public class StampedLock implements java
220		* in-progress spins/signals, and others do not account for
221		* cancellations.
222		*
223	+	* Controlled, randomized spinning is used in the two await
224	+	* methods to reduce (increasingly expensive) context switching
225	+	* while also avoiding sustained memory thrashing among many
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% probability) before enqueuing, 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
233	+	* indicates that some other thread barged and obtained lock), it
234	+	* escalates spins (up to MAX_HEAD_SPINS) to reduce the likelihood
235	+	* of continually losing to barging threads.
236	+	*
237		* As noted in Boehm's paper (above), sequence validation (mainly
238		* method validate()) requires stricter ordering rules than apply
239		* to normal volatile reads (of "state").  In the absence of (but
#	Line 244 | Line 265 | public class StampedLock implements java
265		private static final int OVERFLOW_YIELD_RATE = 7; // must be power 2 - 1
266
267		/** The number of bits to use for reader count before overflowing */
268	<	private static final int  LG_READERS = 7;
268	>	private static final int LG_READERS = 7;
269
270		// Values for lock state and stamp operations
271		private static final long RUNIT = 1L;
#	Line 293 | 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 317 | Line 338 | public class StampedLock implements java
338		* Exclusively acquires the lock if it is immediately available.
339		*
340		* @return a stamp that can be used to unlock or convert mode,
341	<	* or zero if the lock is not available.
341	>	* or zero if the lock is not available
342		*/
343		public long tryWriteLock() {
344		long s, next;
#	Line 494 | 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 522 | Line 544 | public class StampedLock implements java
544		}
545
546		/**
547	<	* If the lock state matches the given stamp, releases
547	>	* If the lock state matches the given stamp, releases the
548		* non-exclusive lock.
549		*
550		* @param stamp a stamp returned by a read-lock operation
#	Line 589 | Line 611 | public class StampedLock implements java
611		/**
612		* If the lock state matches the given stamp then performs one of
613		* the following actions. If the stamp represents holding a write
614	<	* lock, returns it. Or, if a read lock, if the write lock is
615	<	* available, releases the read and returns a write stamp. Or, if
616	<	* an optimistic read, returns a write stamp only if immediately
617	<	* available. This method returns zero in all other cases.
614	>	* lock, returns it.  Or, if a read lock, if the write lock is
615	>	* available, releases the read lock and returns a write stamp.
616	>	* Or, if an optimistic read, returns a write stamp only if
617	>	* immediately available. This method returns zero in all other
618	>	* cases.
619		*
620		* @param stamp a stamp
621		* @return a valid write stamp, or zero on failure
#	Line 611 | 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 649 | 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 683 | 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 757 | 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;
761	<	return (m = state & ABITS) > 0L && m < WBIT;
783	>	return (state & RBITS) != 0L;
784		}
785
786		private void readObject(java.io.ObjectInputStream s)
#	Line 774 | 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 794 | 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 850 | 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) {
857	–	q = null;
879		for (WNode t = wtail; t != null && t != h; t = t.prev)
880		if (t.status <= 0)
881		q = t;
#	Line 869 | 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) {
872	–	q = null;
893		for (WNode t = wtail; t != null && t != h; t = t.prev)
894		if (t.status <= 0)
895		q = t;
#	Line 891 | Line 911 | public class StampedLock implements java
911		/**
912		* RNG for local spins. The first call from await{Read,Write}
913		* produces a thread-local value. Unless zero, subsequent calls
914	<	* use an xorShift to further reduce memory traffic.  Both await
895	<	* methods use a similar spin strategy: If associated queue
896	<	* appears to be empty, then the thread spin-waits up to SPINS
897	<	* times before enqueing, and then, if the first thread to be
898	<	* enqueued, spins again up to SPINS times before blocking. If,
899	<	* upon wakening it fails to obtain lock, and is still (or
900	<	* becomes) the first waiting thread (which indicates that some
901	<	* other thread barged and obtained lock), it escalates spins (up
902	<	* to MAX_HEAD_SPINS) to reduce the likelihood of continually
903	<	* losing to barging threads.
914	>	* use an xorShift to further reduce memory traffic.
915		*/
916		private static int nextRandom(int r) {
917		if (r == 0)
#	Line 948 | 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)
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;;) {
#	Line 971 | 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 980 | 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 & ABITS) != 0L)) // recheck
999		U.park(false, time);
1000	<	if (interruptible && Thread.interrupted())
1001	<	return cancelWriter(node, true);
987	<	}
1000	>	if (interruptible && Thread.interrupted())
1001	>	return cancelWriter(node, true);
1002		}
1003		}
1004		}
#	Line 993 | 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;
1002	<	if (node != null && (pred = node.prev) != null) {
1003	<	WNode pp;
1017	>	if (node != null) {
1018		node.thread = null;
1005	–	while (pred.status == CANCELLED && (pp = pred.prev) != null)
1006	–	pred = node.prev = pp;
1007	–	WNode predNext = pred.next;
1019		node.status = CANCELLED;
1020	<	if (predNext != null) {
1021	<	Thread w = null;
1022	<	WNode succ = node.next;
1023	<	while (succ != null && succ.status == CANCELLED)
1024	<	succ = succ.next;
1025	<	if (succ != null)
1026	<	w = succ.thread;
1027	<	else if (node == wtail)
1028	<	U.compareAndSwapObject(this, WTAIL, node, pred);
1029	<	U.compareAndSwapObject(pred, WNEXT, predNext, succ);
1030	<	if (w != 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 != 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	>	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 1091 | 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);
1098	<	}
1118	>	if (interruptible && Thread.interrupted())
1119	>	return cancelReader(node, true);
1120		}
1121		}
1122		}
#	Line 1183 | Line 1204 | public class StampedLock implements java
1204		private static sun.misc.Unsafe getUnsafe() {
1205		try {
1206		return sun.misc.Unsafe.getUnsafe();
1207	<	} catch (SecurityException se) {
1208	<	try {
1209	<	return java.security.AccessController.doPrivileged
1210	<	(new java.security
1211	<	.PrivilegedExceptionAction<sun.misc.Unsafe>() {
1212	<	public sun.misc.Unsafe run() throws Exception {
1213	<	java.lang.reflect.Field f = sun.misc
1214	<	.Unsafe.class.getDeclaredField("theUnsafe");
1215	<	f.setAccessible(true);
1216	<	return (sun.misc.Unsafe) f.get(null);
1217	<	}});
1218	<	} catch (java.security.PrivilegedActionException e) {
1219	<	throw new RuntimeException("Could not initialize intrinsics",
1220	<	e.getCause());
1221	<	}
1207	>	} catch (SecurityException tryReflectionInstead) {}
1208	>	try {
1209	>	return java.security.AccessController.doPrivileged
1210	>	(new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() {
1211	>	public sun.misc.Unsafe run() throws Exception {
1212	>	Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class;
1213	>	for (java.lang.reflect.Field f : k.getDeclaredFields()) {
1214	>	f.setAccessible(true);
1215	>	Object x = f.get(null);
1216	>	if (k.isInstance(x))
1217	>	return k.cast(x);
1218	>	}
1219	>	throw new NoSuchFieldError("the Unsafe");
1220	>	}});
1221	>	} catch (java.security.PrivilegedActionException e) {
1222	>	throw new RuntimeException("Could not initialize intrinsics",
1223	>	e.getCause());
1224		}
1225		}
1203	–
1226		}

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