On Tue, 13 Nov 2012, Jens Axboe wrote:
> Date: Tue, 13 Nov 2012 09:42:58 -0700
> From: Jens Axboe <axboe@xxxxxxxxx>
> To: Lukas Czerner <lczerner@xxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx,
> jmoyer@xxxxxxxxxx, akpm@xxxxxxxxxxxxxxxxxxxx
> Subject: Re: [PATCH v3] loop: Limit the number of requests in the bio list
>
> > @@ -489,6 +491,12 @@ static void loop_make_request(struct request_queue *q, struct bio *old_bio)
> > goto out;
> > if (unlikely(rw == WRITE && (lo->lo_flags & LO_FLAGS_READ_ONLY)))
> > goto out;
> > + if (lo->lo_bio_count >= q->nr_congestion_on) {
> > + spin_unlock_irq(&lo->lo_lock);
> > + wait_event(lo->lo_req_wait, lo->lo_bio_count <
> > + q->nr_congestion_off);
> > + spin_lock_irq(&lo->lo_lock);
> > + }
>
> This makes me nervous. You are reading lo_bio_count outside the lock. If
> you race with the prepare_to_wait() and condition check in
> __wait_event(), then you will sleep forever.
Hi Jens,
I am sorry for being dense, but I do not see how this would be
possible. The only place we increase the lo_bio_count is after that
piece of code (possibly after the wait). Moreover every time we're
decreasing the lo_bio_count and it is smaller than nr_congestion_off
we will wake_up().
That's how wait_event/wake_up is supposed to be used, right ?
Thanks!
-Lukas
>
> md has private helpers for this, seems it would be a good idea to move
> these into the regular wait includes and use them here too.
>
>
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