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Date: Wed, 20 Nov 2013 00:07:02 -0800
From: Christoph Hellwig <hch@infradead.org>
To: Jens Axboe <axboe@kernel.dk>
Cc: Shaohua Li <shli@kernel.org>, Dave Chinner <david@fromorbit.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [Regression x2, 3.13-git] virtio block mq hang, iostat busted on
 virtio devices
Message-ID: <20131120080702.GB13147@infradead.org>
References: <20131119212042.GB4094@kernel.dk>
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> 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);

Note that virtqueue_kick can be split into a virtqueue_prepare_split
that needs the lock, and a virtqueue_notify that doesn't, but it might
not be nessecary to do that optimization right now.

Btw, how much do I need to bribe you to send mails where I don't have 
step over 500 lines of dead quotes to find the actual patch?