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

[Ming Lei]

On Fri, May 16, 2014 at 10:57 PM, Jens Axboe <axboe@xxxxxxxxx> 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.


Thanks,
-- 
Ming Lei
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