| 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 |
|
* |
| 253 |
|
* motivation to further spread out contended locations, but might |
| 254 |
|
* be subject to future improvements. |
| 255 |
|
*/ |
| 256 |
+ |
|
| 257 |
|
private static final long serialVersionUID = -6001602636862214147L; |
| 258 |
|
|
| 259 |
|
/** Number of processors, for spin control */ |
| 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()) != 0L) |
| 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 |
| 896 |
|
* access bits value to RBITS, indicating hold of spinlock, |
| 897 |
|
* then updating, then releasing. |
| 898 |
|
* |
| 899 |
< |
* @param s, assumed that (s & ABITS) >= RFULL |
| 899 |
> |
* @param s a reader overflow stamp: (s & ABITS) >= RFULL |
| 900 |
|
* @return new stamp on success, else zero |
| 901 |
|
*/ |
| 902 |
|
private long tryIncReaderOverflow(long s) { |
| 903 |
+ |
// assert (s & ABITS) >= RFULL |
| 904 |
|
if ((s & ABITS) == RFULL) { |
| 905 |
|
if (U.compareAndSwapLong(this, STATE, s, s | RBITS)) { |
| 906 |
|
++readerOverflow; |
| 917 |
|
/** |
| 918 |
|
* Tries to decrement readerOverflow. |
| 919 |
|
* |
| 920 |
< |
* @param s, assumed that (s & ABITS) >= RFULL |
| 920 |
> |
* @param s a reader overflow stamp: (s & ABITS) >= RFULL |
| 921 |
|
* @return new stamp on success, else zero |
| 922 |
|
*/ |
| 923 |
|
private long tryDecReaderOverflow(long s) { |
| 924 |
+ |
// assert (s & ABITS) >= RFULL |
| 925 |
|
if ((s & ABITS) == RFULL) { |
| 926 |
|
if (U.compareAndSwapLong(this, STATE, s, s | RBITS)) { |
| 927 |
|
int r; long next; |
| 941 |
|
return 0L; |
| 942 |
|
} |
| 943 |
|
|
| 944 |
< |
/* |
| 944 |
> |
/** |
| 945 |
|
* Wakes up the successor of h (normally whead). This is normally |
| 946 |
|
* just h.next, but may require traversal from wtail if next |
| 947 |
|
* pointers are lagging. This may fail to wake up an acquiring |
| 1042 |
|
if (deadline == 0L) |
| 1043 |
|
time = 0L; |
| 1044 |
|
else if ((time = deadline - System.nanoTime()) <= 0L) |
| 1045 |
< |
return cancelWaiter(node, null, false); |
| 1045 |
> |
return cancelWaiter(node, node, false); |
| 1046 |
|
node.thread = Thread.currentThread(); |
| 1047 |
|
if (node.prev == p && p.status == WAITING && // recheck |
| 1048 |
|
(p != whead || (state & ABITS) != 0L)) |
| 1049 |
|
U.park(false, time); |
| 1050 |
|
node.thread = null; |
| 1051 |
|
if (interruptible && Thread.interrupted()) |
| 1052 |
< |
return cancelWaiter(node, null, true); |
| 1052 |
> |
return cancelWaiter(node, node, true); |
| 1053 |
|
} |
| 1054 |
|
} |
| 1055 |
|
} |
| 1182 |
|
if (deadline == 0L) |
| 1183 |
|
time = 0L; |
| 1184 |
|
else if ((time = deadline - System.nanoTime()) <= 0L) |
| 1185 |
< |
return cancelWaiter(node, null, false); |
| 1185 |
> |
return cancelWaiter(node, node, false); |
| 1186 |
|
node.thread = Thread.currentThread(); |
| 1187 |
|
if (node.prev == p && p.status == WAITING && |
| 1188 |
|
(p != whead || (state & ABITS) != WBIT)) |
| 1189 |
|
U.park(false, time); |
| 1190 |
|
node.thread = null; |
| 1191 |
|
if (interruptible && Thread.interrupted()) |
| 1192 |
< |
return cancelWaiter(node, null, true); |
| 1192 |
> |
return cancelWaiter(node, node, true); |
| 1193 |
> |
} |
| 1194 |
> |
} |
| 1195 |
> |
} |
| 1196 |
> |
|
| 1197 |
> |
/** |
| 1198 |
> |
* If node non-null, forces cancel status and unsplices it from |
| 1199 |
> |
* queue if possible and wakes up any cowaiters (of the node, or |
| 1200 |
> |
* group, as applicable), and in any case helps release current |
| 1201 |
> |
* first waiter if lock is free. (Calling with null arguments |
| 1202 |
> |
* serves as a conditional form of release, which is not currently |
| 1203 |
> |
* needed but may be needed under possible future cancellation |
| 1204 |
> |
* policies). This is a variant of cancellation methods in |
| 1205 |
> |
* AbstractQueuedSynchronizer (see its detailed explanation in AQS |
| 1206 |
> |
* internal documentation). |
| 1207 |
> |
* |
| 1208 |
> |
* @param node if nonnull, the waiter |
| 1209 |
> |
* @param group either node or the group node is cowaiting with |
| 1210 |
> |
* @param interrupted if already interrupted |
| 1211 |
> |
* @return INTERRUPTED if interrupted or Thread.interrupted, else zero |
| 1212 |
> |
*/ |
| 1213 |
> |
private long cancelWaiter(WNode node, WNode group, boolean interrupted) { |
| 1214 |
> |
if (node != null && group != null) { |
| 1215 |
> |
Thread w; |
| 1216 |
> |
node.status = CANCELLED; |
| 1217 |
> |
node.thread = null; |
| 1218 |
> |
// unsplice cancelled nodes from group |
| 1219 |
> |
for (WNode p = group, q; (q = p.cowait) != null;) { |
| 1220 |
> |
if (q.status == CANCELLED) |
| 1221 |
> |
U.compareAndSwapObject(p, WNEXT, q, q.next); |
| 1222 |
> |
else |
| 1223 |
> |
p = q; |
| 1224 |
> |
} |
| 1225 |
> |
if (group == node) { |
| 1226 |
> |
WNode r; // detach and wake up uncancelled co-waiters |
| 1227 |
> |
while ((r = node.cowait) != null) { |
| 1228 |
> |
if (U.compareAndSwapObject(node, WCOWAIT, r, r.cowait) && |
| 1229 |
> |
(w = r.thread) != null) { |
| 1230 |
> |
r.thread = null; |
| 1231 |
> |
U.unpark(w); |
| 1232 |
> |
} |
| 1233 |
> |
} |
| 1234 |
> |
for (WNode pred = node.prev; pred != null; ) { // unsplice |
| 1235 |
> |
WNode succ, pp; // find valid successor |
| 1236 |
> |
while ((succ = node.next) == null || |
| 1237 |
> |
succ.status == CANCELLED) { |
| 1238 |
> |
WNode q = null; // find successor the slow way |
| 1239 |
> |
for (WNode t = wtail; t != null && t != node; t = t.prev) |
| 1240 |
> |
if (t.status != CANCELLED) |
| 1241 |
> |
q = t; // don't link if succ cancelled |
| 1242 |
> |
if (succ == q || // ensure accurate successor |
| 1243 |
> |
U.compareAndSwapObject(node, WNEXT, |
| 1244 |
> |
succ, succ = q)) { |
| 1245 |
> |
if (succ == null && node == wtail) |
| 1246 |
> |
U.compareAndSwapObject(this, WTAIL, node, pred); |
| 1247 |
> |
break; |
| 1248 |
> |
} |
| 1249 |
> |
} |
| 1250 |
> |
if (pred.next == node) // unsplice pred link |
| 1251 |
> |
U.compareAndSwapObject(pred, WNEXT, node, succ); |
| 1252 |
> |
if (succ != null && (w = succ.thread) != null) { |
| 1253 |
> |
succ.thread = null; |
| 1254 |
> |
U.unpark(w); // wake up succ to observe new pred |
| 1255 |
> |
} |
| 1256 |
> |
if (pred.status != CANCELLED || (pp = pred.prev) == null) |
| 1257 |
> |
break; |
| 1258 |
> |
node.prev = pp; // repeat if new pred wrong/cancelled |
| 1259 |
> |
U.compareAndSwapObject(pp, WNEXT, pred, succ); |
| 1260 |
> |
pred = pp; |
| 1261 |
> |
} |
| 1262 |
> |
} |
| 1263 |
> |
} |
| 1264 |
> |
WNode h; // Possibly release first waiter |
| 1265 |
> |
while ((h = whead) != null) { |
| 1266 |
> |
long s; WNode q; // similar to release() but check eligibility |
| 1267 |
> |
if ((q = h.next) == null || q.status == CANCELLED) { |
| 1268 |
> |
for (WNode t = wtail; t != null && t != h; t = t.prev) |
| 1269 |
> |
if (t.status <= 0) |
| 1270 |
> |
q = t; |
| 1271 |
> |
} |
| 1272 |
> |
if (h == whead) { |
| 1273 |
> |
if (q != null && h.status == 0 && |
| 1274 |
> |
((s = state) & ABITS) != WBIT && // waiter is eligible |
| 1275 |
> |
(s == 0L || q.mode == RMODE)) |
| 1276 |
> |
release(h); |
| 1277 |
> |
break; |
| 1278 |
|
} |
| 1279 |
|
} |
| 1280 |
+ |
return (interrupted || Thread.interrupted()) ? INTERRUPTED : 0L; |
| 1281 |
+ |
} |
| 1282 |
+ |
|
| 1283 |
+ |
// Unsafe mechanics |
| 1284 |
+ |
private static final sun.misc.Unsafe U; |
| 1285 |
+ |
private static final long STATE; |
| 1286 |
+ |
private static final long WHEAD; |
| 1287 |
+ |
private static final long WTAIL; |
| 1288 |
+ |
private static final long WNEXT; |
| 1289 |
+ |
private static final long WSTATUS; |
| 1290 |
+ |
private static final long WCOWAIT; |
| 1291 |
+ |
|
| 1292 |
+ |
static { |
| 1293 |
+ |
try { |
| 1294 |
+ |
U = getUnsafe(); |
| 1295 |
+ |
Class<?> k = StampedLock.class; |
| 1296 |
+ |
Class<?> wk = WNode.class; |
| 1297 |
+ |
STATE = U.objectFieldOffset |
| 1298 |
+ |
(k.getDeclaredField("state")); |
| 1299 |
+ |
WHEAD = U.objectFieldOffset |
| 1300 |
+ |
(k.getDeclaredField("whead")); |
| 1301 |
+ |
WTAIL = U.objectFieldOffset |
| 1302 |
+ |
(k.getDeclaredField("wtail")); |
| 1303 |
+ |
WSTATUS = U.objectFieldOffset |
| 1304 |
+ |
(wk.getDeclaredField("status")); |
| 1305 |
+ |
WNEXT = U.objectFieldOffset |
| 1306 |
+ |
(wk.getDeclaredField("next")); |
| 1307 |
+ |
WCOWAIT = U.objectFieldOffset |
| 1308 |
+ |
(wk.getDeclaredField("cowait")); |
| 1309 |
+ |
|
| 1310 |
+ |
} catch (Exception e) { |
| 1311 |
+ |
throw new Error(e); |
| 1312 |
+ |
} |
| 1313 |
|
} |
| 1314 |
|
|
| 1315 |
|
/** |
