8 |
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|
9 |
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import java.util.concurrent.ThreadLocalRandom; |
10 |
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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 |
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|
16 |
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/** |
17 |
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* A capability-based lock with three modes for controlling read/write |
40 |
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* <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 |
|
* |
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 |
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 |
333 |
|
* @return a stamp that can be used to unlock or convert mode |
334 |
|
*/ |
335 |
|
public long writeLock() { |
336 |
< |
long s, next; // bypass acquireWrite in fully onlocked case only |
336 |
> |
long s, next; // bypass acquireWrite in fully unlocked case only |
337 |
|
return ((((s = state) & ABITS) == 0L && |
338 |
|
U.compareAndSwapLong(this, STATE, s, next = s + WBIT)) ? |
339 |
|
next : acquireWrite(false, 0L)); |
368 |
|
long nanos = unit.toNanos(time); |
369 |
|
if (!Thread.interrupted()) { |
370 |
|
long next, deadline; |
371 |
< |
if ((next = tryWriteLock()) != 0) |
371 |
> |
if ((next = tryWriteLock()) != 0L) |
372 |
|
return next; |
373 |
|
if (nanos <= 0L) |
374 |
|
return 0L; |
405 |
|
* @return a stamp that can be used to unlock or convert mode |
406 |
|
*/ |
407 |
|
public long readLock() { |
408 |
< |
long s, next; // bypass acquireRead on fully onlocked case only |
408 |
> |
long s, next; // bypass acquireRead on fully unlocked case only |
409 |
|
return ((((s = state) & ABITS) == 0L && |
410 |
|
U.compareAndSwapLong(this, STATE, s, next = s + RUNIT)) ? |
411 |
|
next : acquireRead(false, 0L)); |
444 |
|
*/ |
445 |
|
public long tryReadLock(long time, TimeUnit unit) |
446 |
|
throws InterruptedException { |
447 |
< |
long next, deadline; |
447 |
> |
long s, m, next, deadline; |
448 |
|
long nanos = unit.toNanos(time); |
449 |
|
if (!Thread.interrupted()) { |
450 |
< |
if ((next = tryReadLock()) != 0) |
451 |
< |
return next; |
450 |
> |
if ((m = (s = state) & ABITS) != WBIT) { |
451 |
> |
if (m < RFULL) { |
452 |
> |
if (U.compareAndSwapLong(this, STATE, s, next = s + RUNIT)) |
453 |
> |
return next; |
454 |
> |
} |
455 |
> |
else if ((next = tryIncReaderOverflow(s)) != 0L) |
456 |
> |
return next; |
457 |
> |
} |
458 |
|
if (nanos <= 0L) |
459 |
|
return 0L; |
460 |
|
if ((deadline = System.nanoTime() + nanos) == 0L) |
498 |
|
* Returns true if the lock has not been exclusively acquired |
499 |
|
* since issuance of the given stamp. Always returns false if the |
500 |
|
* stamp is zero. Always returns true if the stamp represents a |
501 |
< |
* currently held lock. |
501 |
> |
* currently held lock. Invoking this method with a value not |
502 |
> |
* obtained from {@link #tryOptimisticRead} or a locking method |
503 |
> |
* for this lock has no defined effect or result. |
504 |
|
* |
505 |
|
* @return true if the lock has not been exclusively acquired |
506 |
|
* since issuance of the given stamp; else false |
536 |
|
* not match the current state of this lock |
537 |
|
*/ |
538 |
|
public void unlockRead(long stamp) { |
539 |
< |
long s, m; WNode h; |
540 |
< |
if ((stamp & RBITS) != 0L) { |
541 |
< |
while (((s = state) & SBITS) == (stamp & SBITS)) { |
542 |
< |
if ((m = s & ABITS) == 0L) |
539 |
> |
long s, m; WNode h; |
540 |
> |
for (;;) { |
541 |
> |
if (((s = state) & SBITS) != (stamp & SBITS) || |
542 |
> |
(stamp & ABITS) == 0L || (m = s & ABITS) == 0L || m == WBIT) |
543 |
> |
throw new IllegalMonitorStateException(); |
544 |
> |
if (m < RFULL) { |
545 |
> |
if (U.compareAndSwapLong(this, STATE, s, s - RUNIT)) { |
546 |
> |
if (m == RUNIT && (h = whead) != null && h.status != 0) |
547 |
> |
release(h); |
548 |
|
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 |
– |
} |
549 |
|
} |
539 |
– |
else if (m >= WBIT) |
540 |
– |
break; |
541 |
– |
else if (tryDecReaderOverflow(s) != 0L) |
542 |
– |
return; |
550 |
|
} |
551 |
+ |
else if (tryDecReaderOverflow(s) != 0L) |
552 |
+ |
break; |
553 |
|
} |
545 |
– |
throw new IllegalMonitorStateException(); |
554 |
|
} |
555 |
|
|
556 |
|
/** |
833 |
|
throws InterruptedException { |
834 |
|
return tryReadLock(time, unit) != 0L; |
835 |
|
} |
836 |
< |
// note that we give up ability to check mode so just use current state |
829 |
< |
public void unlock() { unlockRead(state); } |
836 |
> |
public void unlock() { unstampedUnlockRead(); } |
837 |
|
public Condition newCondition() { |
838 |
|
throw new UnsupportedOperationException(); |
839 |
|
} |
849 |
|
throws InterruptedException { |
850 |
|
return tryWriteLock(time, unit) != 0L; |
851 |
|
} |
852 |
< |
public void unlock() { unlockWrite(state); } |
852 |
> |
public void unlock() { unstampedUnlockWrite(); } |
853 |
|
public Condition newCondition() { |
854 |
|
throw new UnsupportedOperationException(); |
855 |
|
} |
860 |
|
public Lock writeLock() { return asWriteLock(); } |
861 |
|
} |
862 |
|
|
863 |
+ |
// Unlock methods without stamp argument checks for view classes. |
864 |
+ |
// Needed because view-class lock methods throw away stamps. |
865 |
+ |
|
866 |
+ |
final void unstampedUnlockWrite() { |
867 |
+ |
WNode h; long s; |
868 |
+ |
if (((s = state) & WBIT) == 0L) |
869 |
+ |
throw new IllegalMonitorStateException(); |
870 |
+ |
state = (s += WBIT) == 0L ? ORIGIN : s; |
871 |
+ |
if ((h = whead) != null && h.status != 0) |
872 |
+ |
release(h); |
873 |
+ |
} |
874 |
+ |
|
875 |
+ |
final void unstampedUnlockRead() { |
876 |
+ |
for (;;) { |
877 |
+ |
long s, m; WNode h; |
878 |
+ |
if ((m = (s = state) & ABITS) == 0L || m >= WBIT) |
879 |
+ |
throw new IllegalMonitorStateException(); |
880 |
+ |
else if (m < RFULL) { |
881 |
+ |
if (U.compareAndSwapLong(this, STATE, s, s - RUNIT)) { |
882 |
+ |
if (m == RUNIT && (h = whead) != null && h.status != 0) |
883 |
+ |
release(h); |
884 |
+ |
break; |
885 |
+ |
} |
886 |
+ |
} |
887 |
+ |
else if (tryDecReaderOverflow(s) != 0L) |
888 |
+ |
break; |
889 |
+ |
} |
890 |
+ |
} |
891 |
+ |
|
892 |
|
// internals |
893 |
|
|
894 |
|
/** |
939 |
|
return 0L; |
940 |
|
} |
941 |
|
|
942 |
< |
/* |
942 |
> |
/** |
943 |
|
* Wakes up the successor of h (normally whead). This is normally |
944 |
|
* just h.next, but may require traversal from wtail if next |
945 |
|
* pointers are lagging. This may fail to wake up an acquiring |
1013 |
|
(p = np).next = node; // stale |
1014 |
|
if (whead == p) { |
1015 |
|
for (int k = spins;;) { // spin at head |
1016 |
< |
if (((s = state) & ABITS) == 0L && |
1017 |
< |
U.compareAndSwapLong(this, STATE, s, ns = s + WBIT)) { |
1018 |
< |
whead = node; |
1019 |
< |
node.prev = null; |
1020 |
< |
return ns; |
1016 |
> |
if (((s = state) & ABITS) == 0L) { |
1017 |
> |
if (U.compareAndSwapLong(this, STATE, s, ns = s+WBIT)) { |
1018 |
> |
whead = node; |
1019 |
> |
node.prev = null; |
1020 |
> |
return ns; |
1021 |
> |
} |
1022 |
|
} |
1023 |
|
else if (ThreadLocalRandom.current().nextInt() >= 0 && |
1024 |
|
--k <= 0) |
1040 |
|
if (deadline == 0L) |
1041 |
|
time = 0L; |
1042 |
|
else if ((time = deadline - System.nanoTime()) <= 0L) |
1043 |
< |
return cancelWaiter(node, null, false); |
1043 |
> |
return cancelWaiter(node, node, false); |
1044 |
|
node.thread = Thread.currentThread(); |
1045 |
|
if (node.prev == p && p.status == WAITING && // recheck |
1046 |
< |
(p != whead || (state & ABITS) != 0L)) { |
1046 |
> |
(p != whead || (state & ABITS) != 0L)) |
1047 |
|
U.park(false, time); |
1011 |
– |
if (interruptible && Thread.interrupted()) |
1012 |
– |
return cancelWaiter(node, null, true); |
1013 |
– |
} |
1048 |
|
node.thread = null; |
1049 |
+ |
if (interruptible && Thread.interrupted()) |
1050 |
+ |
return cancelWaiter(node, node, true); |
1051 |
|
} |
1052 |
|
} |
1053 |
|
} |
1069 |
|
if (group == null && (h = whead) != null && |
1070 |
|
(q = h.next) != null && q.mode != RMODE) |
1071 |
|
break; |
1072 |
< |
if ((m = (s = state) & ABITS) == WBIT) |
1037 |
< |
break; |
1038 |
< |
if (m < RFULL ? |
1072 |
> |
if ((m = (s = state) & ABITS) < RFULL ? |
1073 |
|
U.compareAndSwapLong(this, STATE, s, ns = s + RUNIT) : |
1074 |
< |
(ns = tryIncReaderOverflow(s)) != 0L) { |
1074 |
> |
(m < WBIT && (ns = tryIncReaderOverflow(s)) != 0L)) { |
1075 |
|
if (group != null) { // help release others |
1076 |
|
for (WNode r = group;;) { |
1077 |
|
if ((w = r.thread) != null) { |
1085 |
|
} |
1086 |
|
return ns; |
1087 |
|
} |
1088 |
+ |
if (m >= WBIT) |
1089 |
+ |
break; |
1090 |
|
} |
1091 |
|
if (spins > 0) { |
1092 |
|
if (ThreadLocalRandom.current().nextInt() >= 0) |
1110 |
|
node.cowait = p.cowait, node)) { |
1111 |
|
node.thread = Thread.currentThread(); |
1112 |
|
for (long time;;) { |
1113 |
+ |
if (interruptible && Thread.interrupted()) |
1114 |
+ |
return cancelWaiter(node, p, true); |
1115 |
|
if (deadline == 0L) |
1116 |
|
time = 0L; |
1117 |
|
else if ((time = deadline - System.nanoTime()) <= 0L) |
1126 |
|
if (node.thread == null) // must recheck |
1127 |
|
break; |
1128 |
|
U.park(false, time); |
1091 |
– |
if (interruptible && Thread.interrupted()) |
1092 |
– |
return cancelWaiter(node, p, true); |
1129 |
|
} |
1130 |
|
group = p; |
1131 |
|
} |
1180 |
|
if (deadline == 0L) |
1181 |
|
time = 0L; |
1182 |
|
else if ((time = deadline - System.nanoTime()) <= 0L) |
1183 |
< |
return cancelWaiter(node, null, false); |
1183 |
> |
return cancelWaiter(node, node, false); |
1184 |
|
node.thread = Thread.currentThread(); |
1185 |
|
if (node.prev == p && p.status == WAITING && |
1186 |
< |
(p != whead || (state & ABITS) != WBIT)) { |
1186 |
> |
(p != whead || (state & ABITS) != WBIT)) |
1187 |
|
U.park(false, time); |
1152 |
– |
if (interruptible && Thread.interrupted()) |
1153 |
– |
return cancelWaiter(node, null, true); |
1154 |
– |
} |
1188 |
|
node.thread = null; |
1189 |
+ |
if (interruptible && Thread.interrupted()) |
1190 |
+ |
return cancelWaiter(node, node, true); |
1191 |
|
} |
1192 |
|
} |
1193 |
|
} |
1194 |
|
|
1195 |
|
/** |
1196 |
< |
* If node non-null, forces cancel status and unsplices from queue |
1197 |
< |
* if possible. This is a variant of cancellation methods in |
1196 |
> |
* If node non-null, forces cancel status and unsplices it from |
1197 |
> |
* queue if possible and wakes up any cowaiters (of the node, or |
1198 |
> |
* group, as applicable), and in any case helps release current |
1199 |
> |
* first waiter if lock is free. (Calling with null arguments |
1200 |
> |
* serves as a conditional form of release, which is not currently |
1201 |
> |
* needed but may be needed under possible future cancellation |
1202 |
> |
* policies). This is a variant of cancellation methods in |
1203 |
|
* AbstractQueuedSynchronizer (see its detailed explanation in AQS |
1204 |
< |
* internal documentation) that more conservatively wakes up other |
1205 |
< |
* threads that may have had their links changed, so as to preserve |
1206 |
< |
* liveness in the main signalling methods. |
1204 |
> |
* internal documentation). |
1205 |
> |
* |
1206 |
> |
* @param node if nonnull, the waiter |
1207 |
> |
* @param group, either node or the group node is cowaiting with |
1208 |
> |
* @param interrupted if already interrupted |
1209 |
> |
* @return INTERRUPTED if interrupted or Thread.interrupted, else zero |
1210 |
|
*/ |
1211 |
|
private long cancelWaiter(WNode node, WNode group, boolean interrupted) { |
1212 |
< |
if (node != null) { |
1213 |
< |
node.thread = null; |
1212 |
> |
if (node != null && group != null) { |
1213 |
> |
Thread w; |
1214 |
|
node.status = CANCELLED; |
1215 |
< |
if (group != null) { |
1216 |
< |
for (WNode p = group, q; p != null; p = q) { |
1217 |
< |
if ((q = p.cowait) != null && q.status == CANCELLED) { |
1218 |
< |
U.compareAndSwapObject(p, WCOWAIT, q, q.cowait); |
1219 |
< |
break; |
1215 |
> |
node.thread = null; |
1216 |
> |
// unsplice cancelled nodes from group |
1217 |
> |
for (WNode p = group, q; (q = p.cowait) != null;) { |
1218 |
> |
if (q.status == CANCELLED) |
1219 |
> |
U.compareAndSwapObject(p, WNEXT, q, q.next); |
1220 |
> |
else |
1221 |
> |
p = q; |
1222 |
> |
} |
1223 |
> |
if (group == node) { |
1224 |
> |
WNode r; // detach and wake up uncancelled co-waiters |
1225 |
> |
while ((r = node.cowait) != null) { |
1226 |
> |
if (U.compareAndSwapObject(node, WCOWAIT, r, r.cowait) && |
1227 |
> |
(w = r.thread) != null) { |
1228 |
> |
r.thread = null; |
1229 |
> |
U.unpark(w); |
1230 |
|
} |
1231 |
|
} |
1232 |
< |
} |
1233 |
< |
else { |
1181 |
< |
for (WNode pred = node.prev; pred != null; ) { |
1182 |
< |
WNode succ, pp; Thread w; |
1232 |
> |
for (WNode pred = node.prev; pred != null; ) { // unsplice |
1233 |
> |
WNode succ, pp; // find valid successor |
1234 |
|
while ((succ = node.next) == null || |
1235 |
|
succ.status == CANCELLED) { |
1236 |
< |
WNode q = null; |
1236 |
> |
WNode q = null; // find successor the slow way |
1237 |
|
for (WNode t = wtail; t != null && t != node; t = t.prev) |
1238 |
|
if (t.status != CANCELLED) |
1239 |
< |
q = t; |
1240 |
< |
if (succ == q || |
1239 |
> |
q = t; // don't link if succ cancelled |
1240 |
> |
if (succ == q || // ensure accurate successor |
1241 |
|
U.compareAndSwapObject(node, WNEXT, |
1242 |
|
succ, succ = q)) { |
1243 |
|
if (succ == null && node == wtail) |
1245 |
|
break; |
1246 |
|
} |
1247 |
|
} |
1248 |
< |
if (pred.next == node) |
1248 |
> |
if (pred.next == node) // unsplice pred link |
1249 |
|
U.compareAndSwapObject(pred, WNEXT, node, succ); |
1250 |
< |
if (succ != null && (w = succ.thread) != null) |
1251 |
< |
U.unpark(w); |
1250 |
> |
if (succ != null && (w = succ.thread) != null) { |
1251 |
> |
succ.thread = null; |
1252 |
> |
U.unpark(w); // wake up succ to observe new pred |
1253 |
> |
} |
1254 |
|
if (pred.status != CANCELLED || (pp = pred.prev) == null) |
1255 |
|
break; |
1256 |
< |
node.prev = pp; // repeat for new pred |
1256 |
> |
node.prev = pp; // repeat if new pred wrong/cancelled |
1257 |
|
U.compareAndSwapObject(pp, WNEXT, pred, succ); |
1258 |
|
pred = pp; |
1259 |
|
} |
1260 |
|
} |
1261 |
|
} |
1262 |
< |
release(whead); |
1262 |
> |
WNode h; // Possibly release first waiter |
1263 |
> |
while ((h = whead) != null) { |
1264 |
> |
long s; WNode q; // similar to release() but check eligibility |
1265 |
> |
if ((q = h.next) == null || q.status == CANCELLED) { |
1266 |
> |
for (WNode t = wtail; t != null && t != h; t = t.prev) |
1267 |
> |
if (t.status <= 0) |
1268 |
> |
q = t; |
1269 |
> |
} |
1270 |
> |
if (h == whead) { |
1271 |
> |
if (q != null && h.status == 0 && |
1272 |
> |
((s = state) & ABITS) != WBIT && // waiter is eligible |
1273 |
> |
(s == 0L || q.mode == RMODE)) |
1274 |
> |
release(h); |
1275 |
> |
break; |
1276 |
> |
} |
1277 |
> |
} |
1278 |
|
return (interrupted || Thread.interrupted()) ? INTERRUPTED : 0L; |
1279 |
|
} |
1280 |
|
|