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

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Message-ID: <CACVXFVNnQmJ6J3U4xTmKbBQfuYQBxXA4djNL_s7dD3jC1CGhhg@mail.gmail.com>
Date:	Mon, 19 May 2014 08:26:10 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Jens Axboe <axboe@...nel.dk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] virtio_blk: fix race between start and stop queue

On Fri, May 16, 2014 at 9:59 AM, Rusty Russell <rusty@...tcorp.com.au> wrote:
> Ming Lei <tom.leiming@...il.com> writes:
>> 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.
>
> OK, but why the flag?  Isn't moving the
>         blk_mq_start_stopped_hw_queues(vblk->disk->queue, true);
>
> inside the lock sufficient?

The flag has been removed in v1. I introduced the flag just for
avoiding unnecessary calling blk_mq_start_stopped_hw_queues()
at most of time, but it isn't a big deal.

Thanks,
-- 
Ming Lei
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