Re: [Regression x2, 3.13-git] virtio block mq hang, iostat busted on virtio devices

[Jens Axboe <axboe@kernel.dk>]

On Tue, Nov 19 2013, Jens Axboe wrote:
> > Looks like a race condition, below works for me, please try.
> > 
> > 
> > Subject: virtio_blk: fix race condition
> > 
> > virtqueue_kick() isn't multi-thread safe.
> > 
> > Signed-off-by: Shaohua Li <shli@fusionio.com>
> > 
> > diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> > index 588479d..f353959 100644
> > --- a/drivers/block/virtio_blk.c
> > +++ b/drivers/block/virtio_blk.c
> > @@ -204,10 +204,11 @@ static int virtio_queue_rq(struct blk_mq_hw_ctx *hctx, struct request *req)
> >  		virtqueue_kick(vblk->vq);
> >  		return BLK_MQ_RQ_QUEUE_BUSY;
> >  	}
> > -	spin_unlock_irqrestore(&vblk->vq_lock, flags);
> >  
> >  	if (last)
> >  		virtqueue_kick(vblk->vq);
> > +	spin_unlock_irqrestore(&vblk->vq_lock, flags);
> > +
> >  	return BLK_MQ_RQ_QUEUE_OK;
> >  }
> 
> Just stumbled on that too. You need one more, btw, for the sg failure
> case:
> 
> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 588479d58f52..6a680d4de7f1 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -199,15 +199,16 @@ static int virtio_queue_rq(struct blk_mq_hw_ctx *hctx, struct request *req)
>  
>  	spin_lock_irqsave(&vblk->vq_lock, flags);
>  	if (__virtblk_add_req(vblk->vq, vbr, vbr->sg, num) < 0) {
> +		virtqueue_kick(vblk->vq);
>  		spin_unlock_irqrestore(&vblk->vq_lock, flags);
>  		blk_mq_stop_hw_queue(hctx);
> -		virtqueue_kick(vblk->vq);
>  		return BLK_MQ_RQ_QUEUE_BUSY;
>  	}
> -	spin_unlock_irqrestore(&vblk->vq_lock, flags);
>  
>  	if (last)
>  		virtqueue_kick(vblk->vq);
> +
> +	spin_unlock_irqrestore(&vblk->vq_lock, flags);
>  	return BLK_MQ_RQ_QUEUE_OK;
>  }

Tested successfully here too. Dave, please give it a go, looks like this
should fix it up for you. Committed here:

http://git.kernel.dk/?p=linux-block.git;a=commit;h=f02b9ac35a47dff745c7637fbc095f01cc03646e

and will send it upstream today, so we can avoid having an -rc1 with a
broken virtio-blk.

-- 
Jens Axboe