Re: memory barrier in ll_rw_blk.c (was Re: [PATCH][5/?] count writebackpages in nr_scanned)

[Nick Piggin]

Jens Axboe wrote:
> On Thu, Jan 06 2005, Nick Piggin wrote:

> > No that's right... but between the prepare_to_wait and the io_schedule,
> > get_request takes the lock and checks nr_requests. I think we are safe?
>
>
> It looks like it, yes you are right. But it looks to be needed a few
> lines further down instead, though :-)
>
> ===== drivers/block/ll_rw_blk.c 1.281 vs edited =====
> --- 1.281/drivers/block/ll_rw_blk.c     2004-12-01 09:13:57 +01:00
> +++ edited/drivers/block/ll_rw_blk.c    2005-01-06 09:32:19 +01:00
> @@ -1630,11 +1630,11 @@
>         if (rl->count[rw] < queue_congestion_off_threshold(q))
>                 clear_queue_congested(q, rw);
>         if (rl->count[rw]+1 <= q->nr_requests) {
> -               smp_mb();
>                 if (waitqueue_active(&rl->wait[rw]))
>                         wake_up(&rl->wait[rw]);
>                 blk_clear_queue_full(q, rw);
>         }
> +       smp_mb();
>         if (unlikely(waitqueue_active(&rl->drain)) &&
>             !rl->count[READ] && !rl->count[WRITE])
>                 wake_up(&rl->drain);

Yes, looks like you're right there.

Any point in doing it like this

	if (!rl->count[READ] && !rl->count[WRITE]) {
		smb_mb();
		if (unlikely(waitqueue_active(...)))
			wake_up()
	}

I wonder? I don't have any feeling of how memory barriers impact performance
on a very parallel system with CPUs that do lots of memory reordering like
POWER5.
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