Il 17/06/2014 22:36, Konrad Rzeszutek Wilk ha scritto:
+ /* One more attempt - but if we fail mark it as pending. */
+ if (val == _Q_LOCKED_VAL) {
+ new = Q_LOCKED_VAL |_Q_PENDING_VAL;
+
+ old = atomic_cmpxchg(&lock->val, val, new);
+ if (old == _Q_LOCKED_VAL) /* YEEY! */
+ return;
+ val = old;
+ }
Note that Peter's code is in a for(;;) loop:
+ for (;;) {
+ /*
+ * If we observe any contention; queue.
+ */
+ if (val & ~_Q_LOCKED_MASK)
+ goto queue;
+
+ new = _Q_LOCKED_VAL;
+ if (val == new)
+ new |= _Q_PENDING_VAL;
+
+ old = atomic_cmpxchg(&lock->val, val, new);
+ if (old == val)
+ break;
+
+ val = old;
+ }
+
+ /*
+ * we won the trylock
+ */
+ if (new == _Q_LOCKED_VAL)
+ return;
So what you'd have is basically:
/*
* One more attempt if no one is already in queue. Perhaps
* they have unlocked the spinlock already.
*/
if (val == _Q_LOCKED_VAL && atomic_read(&lock->val) == 0) {
old = atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL);
if (old == 0) /* YEEY! */
return;
val = old;
}
But I agree with Waiman that this is unlikely to trigger often enough.
It does have to be handled in the slowpath for correctness, but the most
likely path is (0,0,1)->(0,1,1).
Paolo
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