linux-kernel - Re: [PATCH v1] virtio_blk: fix race between start and stop queue

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Date:	Fri, 16 May 2014 23:43:17 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH v1] virtio_blk: fix race between start and stop queue

On Fri, May 16, 2014 at 11:32 PM, Jens Axboe <axboe@...nel.dk> wrote:
> On 2014-05-16 09:31, 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.
>
>
> Thanks, this looks good, I'll apply it for 3.16 (with a stable marker, even
> if it is an unlikely event).

Thanks.

It shouldn't be very difficult to happen in case of
non-indirect descriptor, and it is easy to reproduce
when module parameter of 'virtblk_queue_depth'
is bigger than vq->num_free for non-indirect case.


Thanks,
-- 
Ming Lei
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