On 2012-11-14 02:02, Lukáš Czerner wrote: > 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 ? It is, yes. But you are checking the condition without the lock, so you could be operating on a stale value. The point is, you have to safely check the condition _after prepare_to_wait() to be completely safe. And you do not. Either lo_bio_count needs to be atomic, or you need to use a variant of wait_event() that holds the appropriate lock before prepare_to_wait() and condition check, then dropping it for the sleep. See wait_even_lock_irq() in drivers/md/md.h. -- Jens Axboe -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html