/**
* If the lock state matches the given stamp then, if the stamp represents holding a lock,
* releases it and returns an observation stamp. Or, if an optimistic read, returns it if
* validated. This method returns zero in all other cases, and so may be useful as a form of
* "tryUnlock".
*
* @param stamp a stamp
* @return a valid optimistic read stamp, or zero on failure
*/
public long tryConvertToOptimisticRead(long stamp) {
long a = stamp & ABITS, m, s, next;
WNode h;
U.loadFence();
for (; ; ) {
if (((s = state) & SBITS) != (stamp & SBITS)) break;
if ((m = s & ABITS) == 0L) {
if (a != 0L) break;
return s;
} else if (m == WBIT) {
if (a != m) break;
state = next = (s += WBIT) == 0L ? ORIGIN : s;
if ((h = whead) != null && h.status != 0) release(h);
return next;
} else if (a == 0L || a >= WBIT) break;
else if (m < RFULL) {
if (U.compareAndSwapLong(this, STATE, s, next = s - RUNIT)) {
if (m == RUNIT && (h = whead) != null && h.status != 0) release(h);
return next & SBITS;
}
} else if ((next = tryDecReaderOverflow(s)) != 0L) return next & SBITS;
}
return 0L;
}
/**
* Returns true if the lock has not been exclusively acquired since issuance of the given stamp.
* Always returns false if the stamp is zero. Always returns true if the stamp represents a
* currently held lock. Invoking this method with a value not obtained from {@link
* #tryOptimisticRead} or a locking method for this lock has no defined effect or result.
*
* @param stamp a stamp
* @return {@code true} if the lock has not been exclusively acquired since issuance of the given
* stamp; else false
*/
public boolean validate(long stamp) {
U.loadFence();
return (stamp & SBITS) == (state & SBITS);
}