On Wed, Jun 18, 2014 at 01:29:48PM +0200, Paolo Bonzini wrote:
> 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).
<nods>
>
> Paolo
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