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

[Peter Zijlstra]

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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