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 |
26 |
|
* in method {@link #unlockWrite} to release the lock. Untimed and |
27 |
|
* timed versions of {@code tryWriteLock} are also provided. When |
28 |
|
* the lock is held in write mode, no read locks may be obtained, |
29 |
< |
* and all observer validations will fail. </li> |
29 |
> |
* and all optimistic read validations will fail. </li> |
30 |
|
* |
31 |
|
* <li><b>Reading.</b> Method {@link #readLock} possibly blocks |
32 |
|
* waiting for non-exclusive access, returning a stamp that can be |
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 |
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 |
93 |
|
* |
94 |
|
* <pre>{@code |
95 |
|
* class Point { |
96 |
< |
* private volatile double x, y; |
96 |
> |
* private double x, y; |
97 |
|
* private final StampedLock sl = new StampedLock(); |
98 |
|
* |
99 |
|
* void move(double deltaX, double deltaY) { // an exclusively locked method |
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 |
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; |
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. |
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 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 |
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 |
258 |
|
/** Maximum number of retries before blocking on acquisition */ |
259 |
|
private static final int SPINS = (NCPU > 1) ? 1 << 6 : 1; |
260 |
|
|
261 |
< |
/** Maximum number of retries before re-blocking on write lock */ |
261 |
> |
/** Maximum number of retries before re-blocking */ |
262 |
|
private static final int MAX_HEAD_SPINS = (NCPU > 1) ? 1 << 12 : 1; |
263 |
|
|
264 |
|
/** The period for yielding when waiting for overflow spinlock */ |
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; |
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; |
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; |
350 |
|
|
351 |
|
/** |
352 |
|
* Exclusively acquires the lock if it is available within the |
353 |
< |
* given time and the current thread has not been interrupted |
353 |
> |
* given time and the current thread has not been interrupted. |
354 |
|
* |
355 |
|
* @return a stamp that can be used to unlock or convert mode, |
356 |
|
* or zero if the lock is not available |
440 |
|
|
441 |
|
/** |
442 |
|
* Non-exclusively acquires the lock if it is available within the |
443 |
< |
* given time and the current thread has not been interrupted |
443 |
> |
* given time and the current thread has not been interrupted. |
444 |
|
* |
445 |
|
* @return a stamp that can be used to unlock or convert mode, |
446 |
|
* or zero if the lock is not available |
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 |
|
|
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 |
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 |
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; |
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 |
|
} |
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 |
|
} |
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) |
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) { |
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) { |
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; |
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; |
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) |
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;;) { |
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) |
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 |
|
} |
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(); |
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 |
|
} |
1124 |
|
/** |
1125 |
|
* If node non-null, forces cancel status and unsplices from queue |
1126 |
|
* if possible, by traversing entire queue looking for cancelled |
1127 |
< |
* nodes, cleaning out all at head, but stopping upon first |
1107 |
< |
* encounter otherwise. |
1127 |
> |
* nodes. |
1128 |
|
*/ |
1129 |
|
private long cancelReader(RNode node, boolean interrupted) { |
1130 |
|
Thread w; |
1139 |
|
} |
1140 |
|
else { |
1141 |
|
U.compareAndSwapObject(pred, RNEXT, p, q); |
1142 |
< |
break; |
1142 |
> |
p = pred.next; |
1143 |
|
} |
1144 |
|
} |
1145 |
|
else { |