Messages in this thread | | | Date | Thu, 06 Jan 2005 19:53:37 +1100 | From | Nick Piggin <> | Subject | Re: memory barrier in ll_rw_blk.c (was Re: [PATCH][5/?] count writeback pages in nr_scanned) |
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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. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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