Messages in this thread Patch in this message |  | | | Date | Wed, 10 Mar 2004 16:47:01 +1100 | | From | Nick Piggin <> | | Subject | Re: blk_congestion_wait racy? |
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Andrew Morton wrote:
>Nick Piggin <piggin@cyberone.com.au> wrote: > >>But I'm guessing that you have no requests in flight by the time >> blk_congestion_wait gets called, so nothing ever gets kicked. >> > >That's why blk_congestion_wait() in -mm propagates the schedule_timeout() >return value. You can do: > > if (blk_congestion_wait(...)) > printk("ouch\n"); > >If your kernel says ouch much, we have a problem. > >
Martin, have you tried adding this printk?
Andrew, could you take the following patch (even though it didn't fix the problem).
I think the smp_mb isn't needed because the rl waitqueue stuff is serialised by the queue spinlocks.
The addition of the smp_mb and the other change is to try to close the window for races a bit. Obviously they can still happen, it's a racy interface and it doesn't matter much.
linux-2.6-npiggin/drivers/block/ll_rw_blk.c | 6 ++++-- 1 files changed, 4 insertions(+), 2 deletions(-) diff -puN drivers/block/ll_rw_blk.c~blk-congestion-races drivers/block/ll_rw_blk.c --- linux-2.6/drivers/block/ll_rw_blk.c~blk-congestion-races 2004-03-10 16:38:33.000000000 +1100 +++ linux-2.6-npiggin/drivers/block/ll_rw_blk.c 2004-03-10 16:41:29.000000000 +1100 @@ -110,6 +110,9 @@ static void clear_queue_congested(reques bit = (rw == WRITE) ? BDI_write_congested : BDI_read_congested; clear_bit(bit, &q->backing_dev_info.state); + + smp_mb(); /* congestion_wqh is not synchronised. This is still racy, + * but better. It isn't a big deal */ if (waitqueue_active(wqh)) wake_up(wqh); } @@ -1543,7 +1546,6 @@ static void freed_request(request_queue_ 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]); if (!waitqueue_active(&rl->wait[rw])) @@ -2036,8 +2038,8 @@ long blk_congestion_wait(int rw, long ti DEFINE_WAIT(wait); wait_queue_head_t *wqh = &congestion_wqh[rw]; - blk_run_queues(); prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE); + blk_run_queues(); ret = io_schedule_timeout(timeout); finish_wait(wqh, &wait); return ret; _
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