Messages in this thread Patch in this message | | | Date | Thu, 6 Jan 2005 09:32:52 +0100 | From | Jens Axboe <> | Subject | Re: memory barrier in ll_rw_blk.c (was Re: [PATCH][5/?] count writeback pages in nr_scanned) |
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On Thu, Jan 06 2005, Nick Piggin wrote: > Jens Axboe wrote: > >On Thu, Jan 06 2005, Nick Piggin wrote: > > >> > >>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. > > > > > >Not sure I agree completely. Yes it will work, but only because it tests > ><= q->nr_requests and I don't think that 'eventually' is good enough :-) > > > >The actual waitqueue manipulation doesn't happen under the queue lock, > >so the memory barrier is needed to pickup the change on SMP. So I'd like > >to keep the barrier. > > > > 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); > >I'd prefer to add smp_mb() to waitqueue_active() actually! > > > > That may be a good idea (I haven't really taken much notice of how other > code uses it). > > I'm not worried about any possible performance advantages of removing it, > rather just having a memory barrier without comments can be perplexing.
I fully agree, subtle things like that should always be commented.
-- Jens Axboe
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