Re: [PATCH] virtio_net: Fix recursive call to cpus_read_lock()

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Dec 21, 2020 at 10:36:48PM -0500, Jeff Dike wrote:
> virtnet_set_channels can recursively call cpus_read_lock if CONFIG_XPS
> and CONFIG_HOTPLUG are enabled.
> 
> The path is:
>     virtnet_set_channels - calls get_online_cpus(), which is a trivial
> wrapper around cpus_read_lock()
>     netif_set_real_num_tx_queues
>     netif_reset_xps_queues_gt
>     netif_reset_xps_queues - calls cpus_read_lock()
> 
> This call chain and potential deadlock happens when the number of TX
> queues is reduced.
> 
> This commit the removes netif_set_real_num_[tr]x_queues calls from
> inside the get/put_online_cpus section, as they don't require that it
> be held.
> 
> Signed-off-by: Jeff Dike <jdike@akamai.com>

Acked-by: Michael S. Tsirkin <mst@redhat.com>

> ---
>  drivers/net/virtio_net.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 052975ea0af4..e02c7e0f1cf9 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -2093,14 +2093,16 @@ static int virtnet_set_channels(struct net_device *dev,
>  
>  	get_online_cpus();
>  	err = _virtnet_set_queues(vi, queue_pairs);
> -	if (!err) {
> -		netif_set_real_num_tx_queues(dev, queue_pairs);
> -		netif_set_real_num_rx_queues(dev, queue_pairs);
> -
> -		virtnet_set_affinity(vi);
> +	if (err){
> +		put_online_cpus();
> +		goto err;
>  	}
> +	virtnet_set_affinity(vi);
>  	put_online_cpus();
>  
> +	netif_set_real_num_tx_queues(dev, queue_pairs);
> +	netif_set_real_num_rx_queues(dev, queue_pairs);
> + err:
>  	return err;
>  }
>  
> -- 
> 2.17.1