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

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Date:	Fri, 16 May 2014 09:45:24 -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 v1] virtio_blk: fix race between start and stop queue

On 2014-05-16 09:43, Ming Lei wrote:
> 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.

I agree, it can definitely be setup so that it would not be hard to 
trigger. But I don't recall seeing any hang bugs since 3.13 was 
released, which would seem to indicate that it doesn't happen a lot in 
the wild with default settings.

-- 
Jens Axboe

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