[patch 4/9] blk: no memory barrier

Nick Piggin
2005-04-12T13:19:38Z

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4/9

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SUSE Labs, Novell Inc.

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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
waiter.

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 <nickpiggin@yahoo.com.au>

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]))
wake_up(&rl->wait[rw]);