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 <jd...@akamai.com>

Acked-by: Michael S. Tsirkin <m...@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

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