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    Subject[patch 4/9] blk: no memory barrier

    SUSE Labs, Novell Inc.
    This memory barrier is not needed because the waitqueue will only get
    waiters on it in the following situations:

    rq->count has exceeded the threshold - however all manipulations of ->count
    are performed under the runqueue lock, and so we will correctly pick up any

    Memory allocation for the request fails. In this case, there is no additional
    help provided by the memory barrier. We are guaranteed to eventually wake
    up waiters because the request allocation mempool guarantees that if the mem
    allocation for a request fails, there must be some requests in flight. They
    will wake up waiters when they are retired.

    Signed-off-by: Nick Piggin <>

    Index: linux-2.6/drivers/block/ll_rw_blk.c
    --- linux-2.6.orig/drivers/block/ll_rw_blk.c 2005-04-12 22:05:44.000000000 +1000
    +++ linux-2.6/drivers/block/ll_rw_blk.c 2005-04-12 22:26:13.000000000 +1000
    @@ -1828,7 +1828,6 @@ static void __freed_request(request_queu
    clear_queue_congested(q, rw);

    if (rl->count[rw] + 1 <= q->nr_requests) {
    - smp_mb();
    if (waitqueue_active(&rl->wait[rw]))
     \ /
      Last update: 2005-04-12 15:30    [W:0.020 / U:1.136 seconds]
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