Subject: Re: [PATCH net V2 2/4] vhost_net: switch to use mutex_trylock() in vhost_net_busy_poll()

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 12 Dec 2018 09:20:57 -0500

On Wed, Dec 12, 2018 at 06:08:17PM +0800, Jason Wang wrote:
> We used to hold the mutex of paired virtqueue in
> vhost_net_busy_poll(). But this will results an inconsistent lock
> order which may cause deadlock if we try to bring back the protection
> of device IOTLB with vq mutex that requires to hold mutex of all
> virtqueues at the same time.
> 
> Fix this simply by switching to use mutex_trylock(), when fail just
> skip the busy polling. This can happen when device IOTLB is under
> updating which should be rare.
> 
> Fixes: commit 78139c94dc8c ("net: vhost: lock the vqs one by one")
> Cc: Tonghao Zhang <[email-masked]>
> Signed-off-by: Jason Wang <[email-masked]>

Acked-by: Michael S. Tsirkin <[email-masked]>

and I think we should try to put this fix in 4.20 too.

> ---
>  drivers/vhost/net.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index ab11b2bee273..ad7a6f475a44 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -513,7 +513,13 @@ static void vhost_net_busy_poll(struct vhost_net *net,
>  	struct socket *sock;
>  	struct vhost_virtqueue *vq = poll_rx ? tvq : rvq;
>  
> -	mutex_lock_nested(&vq->mutex, poll_rx ? VHOST_NET_VQ_TX: VHOST_NET_VQ_RX);
> +	/* Try to hold the vq mutex of the paired virtqueue. We can't
> +	 * use mutex_lock() here since we could not guarantee a
> +	 * consistenet lock ordering.
> +	 */
> +	if (!mutex_trylock(&vq->mutex))
> +		return;
> +
>  	vhost_disable_notify(&net->dev, vq);
>  	sock = rvq->private_data;
>  
> -- 
> 2.17.1