Re: blk_congestion_wait racy?

[Nick Piggin]

Andrew Morton wrote:

> Nick Piggin <piggin@xxxxxxxxxxxxxxx> 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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