ViewVC Help

View File | Revision Log | Show Annotations | Download File | Root Listing

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

Comparing jsr166/src/jsr166e/StampedLock.java (file contents):
Revision 1.28 by dl, Tue Jan 22 15:42:28 2013 UTC vs.
Revision 1.32 by dl, Sat Jan 26 01:22:37 2013 UTC

#	Line 8 | Line 8 | package jsr166e;
8
9		import java.util.concurrent.ThreadLocalRandom;
10		import java.util.concurrent.TimeUnit;
11	<	import java.util.concurrent.locks.*;
11	>	import java.util.concurrent.locks.Lock;
12	>	import java.util.concurrent.locks.Condition;
13	>	import java.util.concurrent.locks.ReadWriteLock;
14	>	import java.util.concurrent.locks.LockSupport;
15
16		/**
17		* A capability-based lock with three modes for controlling read/write
#	Line 37 | Line 40 | import java.util.concurrent.locks.*;
40		*  <li><b>Optimistic Reading.</b> Method {@link #tryOptimisticRead}
41		*   returns a non-zero stamp only if the lock is not currently held
42		*   in write mode. Method {@link #validate} returns true if the lock
43	<	*   has not since been acquired in write mode. This mode can be
44	<	*   thought of as an extremely weak version of a read-lock, that can
45	<	*   be broken by a writer at any time.  The use of optimistic mode
46	<	*   for short read-only code segments often reduces contention and
47	<	*   improves throughput.  However, its use is inherently fragile.
48	<	*   Optimistic read sections should only read fields and hold them in
49	<	*   local variables for later use after validation. Fields read while
50	<	*   in optimistic mode may be wildly inconsistent, so usage applies
51	<	*   only when you are familiar enough with data representations to
52	<	*   check consistency and/or repeatedly invoke method {@code
53	<	*   validate()}.  For example, such steps are typically required when
54	<	*   first reading an object or array reference, and then accessing
55	<	*   one of its fields, elements or methods. </li>
43	>	*   has not been acquired in write mode since obtaining a given
44	>	*   stamp.  This mode can be thought of as an extremely weak version
45	>	*   of a read-lock, that can be broken by a writer at any time.  The
46	>	*   use of optimistic mode for short read-only code segments often
47	>	*   reduces contention and improves throughput.  However, its use is
48	>	*   inherently fragile.  Optimistic read sections should only read
49	>	*   fields and hold them in local variables for later use after
50	>	*   validation. Fields read while in optimistic mode may be wildly
51	>	*   inconsistent, so usage applies only when you are familiar enough
52	>	*   with data representations to check consistency and/or repeatedly
53	>	*   invoke method {@code validate()}.  For example, such steps are
54	>	*   typically required when first reading an object or array
55	>	*   reference, and then accessing one of its fields, elements or
56	>	*   methods. </li>
57		*
58		* </ul>
59		*
#	Line 198 | Line 202 | public class StampedLock implements java
202		*
203		* Waiters use a modified form of CLH lock used in
204		* AbstractQueuedSynchronizer (see its internal documentation for
205	<	* a fuller account), where each node it tagged (field mode) as
205	>	* a fuller account), where each node is tagged (field mode) as
206		* either a reader or writer. Sets of waiting readers are grouped
207		* (linked) under a common node (field cowait) so act as a single
208	<	* node with respect to most CLH mechanics.  By virtue of its
209	<	* structure, wait nodes need not actually carry sequence numbers;
210	<	* we know each is >= its predecessor.  These queue mechanics
211	<	* simplify the scheduling policy to a mainly-FIFO scheme that
208	>	* node with respect to most CLH mechanics.  By virtue of the
209	>	* queue structure, wait nodes need not actually carry sequence
210	>	* numbers; we know each is greater than its predecessor.  This
211	>	* simplifies the scheduling policy to a mainly-FIFO scheme that
212		* incorporates elements of Phase-Fair locks (see Brandenburg &
213		* Anderson, especially http://www.cs.unc.edu/~bbb/diss/).  In
214		* particular, we use the phase-fair anti-barging rule: If an
#	Line 229 | Line 233 | public class StampedLock implements java
233		*
234		* Nearly all of these mechanics are carried out in methods
235		* acquireWrite and acquireRead, that, as typical of such code,
236	<	* sprawl out because actions and retries rely on consitent sets
237	<	* of locally cahced reads.
236	>	* sprawl out because actions and retries rely on consistent sets
237	>	* of locally cached reads.
238		*
239		* As noted in Boehm's paper (above), sequence validation (mainly
240		* method validate()) requires stricter ordering rules than apply
#	Line 249 | Line 253 | public class StampedLock implements java
253		* motivation to further spread out contended locations, but might
254		* be subject to future improvements.
255		*/
252	–
256		private static final long serialVersionUID = -6001602636862214147L;
257
258		/** Number of processors, for spin control */
#	Line 329 | Line 332 | public class StampedLock implements java
332		* @return a stamp that can be used to unlock or convert mode
333		*/
334		public long writeLock() {
335	<	long s, next;  // bypass acquireWrite in fully onlocked case only
335	>	long s, next;  // bypass acquireWrite in fully unlocked case only
336		return ((((s = state) & ABITS) == 0L &&
337		U.compareAndSwapLong(this, STATE, s, next = s + WBIT)) ?
338		next : acquireWrite(false, 0L));
#	Line 364 | Line 367 | public class StampedLock implements java
367		long nanos = unit.toNanos(time);
368		if (!Thread.interrupted()) {
369		long next, deadline;
370	<	if ((next = tryWriteLock()) != 0)
370	>	if ((next = tryWriteLock()) != 0L)
371		return next;
372		if (nanos <= 0L)
373		return 0L;
#	Line 401 | Line 404 | public class StampedLock implements java
404		* @return a stamp that can be used to unlock or convert mode
405		*/
406		public long readLock() {
407	<	long s, next;  // bypass acquireRead on fully onlocked case only
407	>	long s, next;  // bypass acquireRead on fully unlocked case only
408		return ((((s = state) & ABITS) == 0L &&
409		U.compareAndSwapLong(this, STATE, s, next = s + RUNIT)) ?
410		next : acquireRead(false, 0L));
#	Line 443 | Line 446 | public class StampedLock implements java
446		long next, deadline;
447		long nanos = unit.toNanos(time);
448		if (!Thread.interrupted()) {
449	<	if ((next = tryReadLock()) != 0)
449	>	if ((next = tryReadLock()) != 0L)
450		return next;
451		if (nanos <= 0L)
452		return 0L;
#	Line 488 | Line 491 | public class StampedLock implements java
491		* Returns true if the lock has not been exclusively acquired
492		* since issuance of the given stamp. Always returns false if the
493		* stamp is zero. Always returns true if the stamp represents a
494	<	* currently held lock.
494	>	* currently held lock. Invoking this method with a value not
495	>	* obtained from {@link #tryOptimisticRead} or a locking method
496	>	* for this lock has no defined effect or result.
497		*
498		* @return true if the lock has not been exclusively acquired
499		* since issuance of the given stamp; else false
#	Line 524 | Line 529 | public class StampedLock implements java
529		* not match the current state of this lock
530		*/
531		public void unlockRead(long stamp) {
532	<	long s, m;  WNode h;
533	<	if ((stamp & RBITS) != 0L) {
534	<	while (((s = state) & SBITS) == (stamp & SBITS)) {
535	<	if ((m = s & ABITS) == 0L)
532	>	long s, m; WNode h;
533	>	for (;;) {
534	>	if (((s = state) & SBITS) != (stamp & SBITS) ||
535	>	(stamp & ABITS) == 0L || (m = s & ABITS) == 0L || m == WBIT)
536	>	throw new IllegalMonitorStateException();
537	>	if (m < RFULL) {
538	>	if (U.compareAndSwapLong(this, STATE, s, s - RUNIT)) {
539	>	if (m == RUNIT && (h = whead) != null && h.status != 0)
540	>	release(h);
541		break;
532	–	else if (m < RFULL) {
533	–	if (U.compareAndSwapLong(this, STATE, s, s - RUNIT)) {
534	–	if (m == RUNIT && (h = whead) != null && h.status != 0)
535	–	release(h);
536	–	return;
537	–	}
542		}
539	–	else if (m >= WBIT)
540	–	break;
541	–	else if (tryDecReaderOverflow(s) != 0L)
542	–	return;
543		}
544	+	else if (tryDecReaderOverflow(s) != 0L)
545	+	break;
546		}
545	–	throw new IllegalMonitorStateException();
547		}
548
549		/**
#	Line 825 | Line 826 | public class StampedLock implements java
826		throws InterruptedException {
827		return tryReadLock(time, unit) != 0L;
828		}
829	<	// note that we give up ability to check mode so just use current state
829	<	public void unlock() { unlockRead(state); }
829	>	public void unlock() { unstampedUnlockRead(); }
830		public Condition newCondition() {
831		throw new UnsupportedOperationException();
832		}
#	Line 842 | Line 842 | public class StampedLock implements java
842		throws InterruptedException {
843		return tryWriteLock(time, unit) != 0L;
844		}
845	<	public void unlock() { unlockWrite(state); }
845	>	public void unlock() { unstampedUnlockWrite(); }
846		public Condition newCondition() {
847		throw new UnsupportedOperationException();
848		}
#	Line 853 | Line 853 | public class StampedLock implements java
853		public Lock writeLock() { return asWriteLock(); }
854		}
855
856	+	// Unlock methods without stamp argument checks for view classes.
857	+	// Needed because view-class lock methods throw away stamps.
858	+
859	+	final void unstampedUnlockWrite() {
860	+	WNode h; long s;
861	+	if (((s = state) & WBIT) == 0L)
862	+	throw new IllegalMonitorStateException();
863	+	state = (s += WBIT) == 0L ? ORIGIN : s;
864	+	if ((h = whead) != null && h.status != 0)
865	+	release(h);
866	+	}
867	+
868	+	final void unstampedUnlockRead() {
869	+	for (;;) {
870	+	long s, m; WNode h;
871	+	if ((m = (s = state) & ABITS) == 0L || m >= WBIT)
872	+	throw new IllegalMonitorStateException();
873	+	else if (m < RFULL) {
874	+	if (U.compareAndSwapLong(this, STATE, s, s - RUNIT)) {
875	+	if (m == RUNIT && (h = whead) != null && h.status != 0)
876	+	release(h);
877	+	break;
878	+	}
879	+	}
880	+	else if (tryDecReaderOverflow(s) != 0L)
881	+	break;
882	+	}
883	+	}
884	+
885		// internals
886
887		/**
#	Line 977 | Line 1006 | public class StampedLock implements java
1006		(p = np).next = node;   // stale
1007		if (whead == p) {
1008		for (int k = spins;;) { // spin at head
1009	<	if (((s = state) & ABITS) == 0L &&
1010	<	U.compareAndSwapLong(this, STATE, s, ns = s + WBIT)) {
1011	<	whead = node;
1012	<	node.prev = null;
1013	<	return ns;
1009	>	if (((s = state) & ABITS) == 0L) {
1010	>	if (U.compareAndSwapLong(this, STATE, s, ns = s+WBIT)) {
1011	>	whead = node;
1012	>	node.prev = null;
1013	>	return ns;
1014	>	}
1015		}
1016		else if (ThreadLocalRandom.current().nextInt() >= 0 &&
1017		--k <= 0)
#	Line 1006 | Line 1036 | public class StampedLock implements java
1036		return cancelWaiter(node, null, false);
1037		node.thread = Thread.currentThread();
1038		if (node.prev == p && p.status == WAITING && // recheck
1039	<	(p != whead || (state & ABITS) != 0L)) {
1039	>	(p != whead || (state & ABITS) != 0L))
1040		U.park(false, time);
1011	–	if (interruptible && Thread.interrupted())
1012	–	return cancelWaiter(node, null, true);
1013	–	}
1041		node.thread = null;
1042	+	if (interruptible && Thread.interrupted())
1043	+	return cancelWaiter(node, null, true);
1044		}
1045		}
1046		}
#	Line 1033 | Line 1062 | public class StampedLock implements java
1062		if (group == null && (h = whead) != null &&
1063		(q = h.next) != null && q.mode != RMODE)
1064		break;
1065	<	if ((m = (s = state) & ABITS) == WBIT)
1037	<	break;
1038	<	if (m < RFULL ?
1065	>	if ((m = (s = state) & ABITS) < RFULL ?
1066		U.compareAndSwapLong(this, STATE, s, ns = s + RUNIT) :
1067	<	(ns = tryIncReaderOverflow(s)) != 0L) {
1067	>	(m < WBIT && (ns = tryIncReaderOverflow(s)) != 0L)) {
1068		if (group != null) {  // help release others
1069		for (WNode r = group;;) {
1070		if ((w = r.thread) != null) {
#	Line 1051 | Line 1078 | public class StampedLock implements java
1078		}
1079		return ns;
1080		}
1081	+	if (m >= WBIT)
1082	+	break;
1083		}
1084		if (spins > 0) {
1085		if (ThreadLocalRandom.current().nextInt() >= 0)
#	Line 1074 | Line 1103 | public class StampedLock implements java
1103		node.cowait = p.cowait, node)) {
1104		node.thread = Thread.currentThread();
1105		for (long time;;) {
1106	+	if (interruptible && Thread.interrupted())
1107	+	return cancelWaiter(node, p, true);
1108		if (deadline == 0L)
1109		time = 0L;
1110		else if ((time = deadline - System.nanoTime()) <= 0L)
#	Line 1088 | Line 1119 | public class StampedLock implements java
1119		if (node.thread == null) // must recheck
1120		break;
1121		U.park(false, time);
1091	–	if (interruptible && Thread.interrupted())
1092	–	return cancelWaiter(node, p, true);
1122		}
1123		group = p;
1124		}
#	Line 1147 | Line 1176 | public class StampedLock implements java
1176		return cancelWaiter(node, null, false);
1177		node.thread = Thread.currentThread();
1178		if (node.prev == p && p.status == WAITING &&
1179	<	(p != whead || (state & ABITS) != WBIT)) {
1179	>	(p != whead || (state & ABITS) != WBIT))
1180		U.park(false, time);
1152	–	if (interruptible && Thread.interrupted())
1153	–	return cancelWaiter(node, null, true);
1154	–	}
1181		node.thread = null;
1182	+	if (interruptible && Thread.interrupted())
1183	+	return cancelWaiter(node, null, true);
1184		}
1185		}
1186		}
1187
1188		/**
1189	<	* If node non-null, forces cancel status and unsplices from queue
1190	<	* if possible. This is a variant of cancellation methods in
1191	<	* AbstractQueuedSynchronizer (see its detailed explanation in AQS
1192	<	* internal documentation) that more conservatively wakes up other
1193	<	* threads that may have had their links changed, so as to preserve
1194	<	* liveness in the main signalling methods.
1189	>	* If node non-null, forces cancel status and unsplices it from
1190	>	* queue if possible and wakes up any cowaiters. This is a variant
1191	>	* of cancellation methods in AbstractQueuedSynchronizer (see its
1192	>	* detailed explanation in AQS internal documentation) that more
1193	>	* conservatively wakes up other threads that may have had their
1194	>	* links changed, so as to preserve liveness in the main
1195	>	* signalling methods.
1196	>	*
1197	>	* @param node if nonnull, the waiter
1198	>	* @param group, if nonnull, the group current thread is cowaiting with
1199	>	* @param interrupted if already interrupted
1200	>	* @return INTERRUPTED if interrupted or Thread.interrupted, else zero
1201		*/
1202		private long cancelWaiter(WNode node, WNode group, boolean interrupted) {
1203		if (node != null) {
1204		node.thread = null;
1205		node.status = CANCELLED;
1206	<	if (group != null) {
1207	<	for (WNode p = group, q; p != null; p = q) {
1208	<	if ((q = p.cowait) != null && q.status == CANCELLED) {
1209	<	U.compareAndSwapObject(p, WCOWAIT, q, q.cowait);
1210	<	break;
1211	<	}
1206	>	Thread w; // wake up co-waiters; unsplice cancelled ones
1207	>	for (WNode q, p = (group != null) ? group : node; p != null; ) {
1208	>	if ((q = p.cowait) == null)
1209	>	break;
1210	>	if ((w = q.thread) != null) {
1211	>	q.thread = null;
1212	>	U.unpark(w);
1213		}
1214	+	if (q.status == CANCELLED)
1215	+	U.compareAndSwapObject(p, WCOWAIT, q, q.cowait);
1216	+	else
1217	+	p = q;
1218		}
1219	<	else {
1219	>	if (group == null)  {        // unsplice both prev and next links
1220		for (WNode pred = node.prev; pred != null; ) {
1221	<	WNode succ, pp; Thread w;
1221	>	WNode succ, pp;      // first unsplice next
1222		while ((succ = node.next) == null ||
1223		succ.status == CANCELLED) {
1224	<	WNode q = null;
1224	>	WNode q = null;  // find successor the slow way
1225		for (WNode t = wtail; t != null && t != node; t = t.prev)
1226		if (t.status != CANCELLED)
1227	<	q = t;
1228	<	if (succ == q ||
1227	>	q = t;   // don't link if succ cancelled
1228	>	if (succ == q || // ensure accurate successor
1229		U.compareAndSwapObject(node, WNEXT,
1230		succ, succ = q)) {
1231		if (succ == null && node == wtail)
#	Line 1194 | Line 1233 | public class StampedLock implements java
1233		break;
1234		}
1235		}
1236	<	if (pred.next == node)
1236	>	if (pred.next == node) // unsplice pred link
1237		U.compareAndSwapObject(pred, WNEXT, node, succ);
1238	<	if (succ != null && (w = succ.thread) != null)
1239	<	U.unpark(w);
1238	>	if (succ != null && (w = succ.thread) != null) {
1239	>	succ.thread = null;
1240	>	U.unpark(w);      // conservatively wake up new succ
1241	>	}
1242		if (pred.status != CANCELLED || (pp = pred.prev) == null)
1243		break;
1244	<	node.prev = pp; // repeat for new pred
1244	>	node.prev = pp; // repeat in case new pred wrong/cancelled
1245		U.compareAndSwapObject(pp, WNEXT, pred, succ);
1246		pred = pp;
1247		}

Diff Legend

–	Removed lines
+	Added lines
<	Changed lines
>	Changed lines