Re: [Xen-devel] [PATCH v10 06/19] qspinlock: prolong the stay in the pending bit path

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Wed, May 07, 2014 at 11:01:34AM -0400, Waiman Long wrote:
> @@ -221,11 +222,37 @@ static inline int trylock_pending(struct qspinlock 
> *lock, u32 *pval)
>        */
>       for (;;) {
>               /*
> -              * If we observe any contention; queue.
> +              * If we observe that the queue is not empty,
> +              * return and be queued.
>                */
> -             if (val & ~_Q_LOCKED_MASK)
> +             if (val & _Q_TAIL_MASK)
>                       return 0;
>  
> +             if (val == (_Q_LOCKED_VAL|_Q_PENDING_VAL)) {
> +                     /*
> +                      * If both the lock and pending bits are set, we wait
> +                      * a while to see if that either bit will be cleared.
> +                      * If that is no change, we return and be queued.
> +                      */
> +                     if (!retry)
> +                             return 0;
> +                     retry--;
> +                     cpu_relax();
> +                     cpu_relax();
> +                     *pval = val = atomic_read(&lock->val);
> +                     continue;
> +             } else if (val == _Q_PENDING_VAL) {
> +                     /*
> +                      * Pending bit is set, but not the lock bit.
> +                      * Assuming that the pending bit holder is going to
> +                      * set the lock bit and clear the pending bit soon,
> +                      * it is better to wait than to exit at this point.
> +                      */
> +                     cpu_relax();
> +                     *pval = val = atomic_read(&lock->val);
> +                     continue;
> +             }

Didn't I give a much saner alternative to this mess last time?

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