linux-kernel - Re: [PATCH net] virtio-net: correctly enable callback during start

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Message-ID: <20221212042144-mutt-send-email-mst@kernel.org>
Date:   Mon, 12 Dec 2022 04:25:22 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
        pabeni@...hat.com, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] virtio-net: correctly enable callback during
 start_xmit

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@...hat.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