[PATCH v3] xfs: re-enable xfsaild idle mode and fix associated races

[Christoph Hellwig]

>  			__set_current_state(TASK_KILLABLE);
>  		else
>  			__set_current_state(TASK_INTERRUPTIBLE);
> -		schedule_timeout(tout ?
> -				 msecs_to_jiffies(tout) : MAX_SCHEDULE_TIMEOUT);
> +
> +		spin_lock(&ailp->xa_lock);
> +
> +		/*
> +		 * Idle if the AIL is empty and we are not racing with a target
> +		 * update. We check the AIL after we set the task to a sleep
> +		 * state to guarantee that we either catch an xa_target update
> +		 * or that a wake_up resets the state to TASK_RUNNING.
> +		 * Otherwise, we run the risk of sleeping indefinitely.
> +		 *
> +		 * The barrier matches the xa_target update in xfs_ail_push().
> +		 */
> +		smp_rmb();
> +		if (!xfs_ail_min(ailp) &&
> +		    ailp->xa_target == ailp->xa_target_prev) {
> +			spin_unlock(&ailp->xa_lock);
> +			schedule();
> +			tout = 0;
> +			continue;
> +		}

I still don't like this at all all - we have one place to do all the
timeout decisions, and that is and then end of xfsaild_push.  Splitting
this decision over two functions makes the code a lot harder to
understand and maintain over the long run.

That doesn't mean I don't like the algorithm behind this patch, it just
needs to move into the right place.