Re: [PATCH net] virtio-net: correctly enable callback during start_xmit

Michael S. Tsirkin
2022-12-12T09:27:17Z

On Mon, Dec 12, 2022 at 05:10:29PM +0800, Jason Wang wrote:
> Commit a7766ef18b33("virtio_net: disable cb aggressively") enables
> virtqueue callback via the following statement:
>
> do {
> ......
> } while (use_napi && kick &&
> unlikely(!virtqueue_enable_cb_delayed(sq->vq)));
>
> This will cause a missing call to virtqueue_enable_cb_delayed() when
> kick is false. Fixing this by removing the checking of the kick from
> the condition to make sure callback is enabled correctly.
>
> Fixes: a7766ef18b33 ("virtio_net: disable cb aggressively")
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
> The patch is needed for -stable.

stable rules don't allow for theoretical fixes. Was a problem observed?

> ---
> drivers/net/virtio_net.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 86e52454b5b5..44d7daf0267b 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1834,8 +1834,8 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
>
> free_old_xmit_skbs(sq, false);
>
> - } while (use_napi && kick &&
> - unlikely(!virtqueue_enable_cb_delayed(sq->vq)));
> + } while (use_napi &&
> + unlikely(!virtqueue_enable_cb_delayed(sq->vq)));
>

A bit more explanation pls. kick simply means !netdev_xmit_more -
if it's false we know there will be another packet, then transmissing
that packet will invoke virtqueue_enable_cb_delayed. No?

> /* timestamp packet in software */
> skb_tx_timestamp(skb);
> --
> 2.25.1