Re: [PATCH v3] loop: Limit the number of requests in the bio list

[Jens Axboe]

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

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