linux-kernel - Re: [PATCH] virtio_blk: fix race between start and stop queue

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Date:	Fri, 16 May 2014 09:17:08 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Ming Lei <tom.leiming@...il.com>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH] virtio_blk: fix race between start and stop queue

On 2014-05-16 09:15, Ming Lei wrote:
> On Fri, May 16, 2014 at 10:57 PM, Jens Axboe <axboe@...nel.dk> wrote:
>> On 2014-05-16 08:53, Jens Axboe wrote:
>>>
>>> On 2014-05-15 06:33, Ming Lei wrote:
>>>>
>>>> When there isn't enough vring descriptor for adding to vq,
>>>> blk-mq will be put as stopped state until some of pending
>>>> descriptors are completed & freed.
>>>>
>>>> Unfortunately, the vq's interrupt may come just before
>>>> blk-mq's BLK_MQ_S_STOPPED flag is set, so the blk-mq will
>>>> still be kept as stopped even though lots of descriptors
>>>> are completed and freed in the interrupt handler. The worst
>>>> case is that all pending descriptors are freed in the
>>>> interrupt handler, and the queue is kept as stopped forever.
>>>>
>>>> This patch fixes the problem by starting/stopping blk-mq
>>>> with holding vq_lock.
>>>
>>>
>>> Why not just use blk_mq_start_hw_queues()?
>>
>>
>> Or, if you want to maintain current heuristics, just move the start and stop
>> under the vq_lock. That should prevent the race, as far as I can tell. Not
>> sure what that extra queue_stopped would buy you, seems a lot cleaner to
>> just maintain this state exclusively in the queue.
>
> Yes.
>
> But the flag can avoid to call blk_mq_start_stopped_hw_queues()
> unnecessarily, which needn't at most of times.  Considered that
> the interrupt may happen with very high frequency, I suggest to
> introduce the extra flag.

virtio-blk just has one queue, so the flag is at least pointless for 
now. And since the other code stops all of them anyway, I don't see any 
reason not to just rely on that.

-- 
Jens Axboe

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