Subject: Re: [PATCH net V2] virtio-net: correctly enable callback during start_xmit

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 15 Dec 2022 04:02:49 -0500

On Thu, Dec 15, 2022 at 11:27:19AM +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)));
> 
> When NAPI is used and kick is false, the callback won't be enabled
> here. And when the virtqueue is about to be full, the tx will be
> disabled, but we still don't enable tx interrupt which will cause a TX
> hang. This could be observed when using pktgen with burst enabled.
> 
> Fixing this by trying to enable tx interrupt after we disable TX when
> we're not using napi or kick is false.
> 
> Fixes: a7766ef18b33 ("virtio_net: disable cb aggressively")
> Signed-off-by: Jason Wang <[email-masked]>
> ---
> The patch is needed for -stable.
> Changes since V1:
> - enable tx interrupt after we disable tx
> ---
>  drivers/net/virtio_net.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 86e52454b5b5..dcf3a536d78a 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1873,7 +1873,7 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
>  	 */
>  	if (sq->vq->num_free < 2+MAX_SKB_FRAGS) {
>  		netif_stop_subqueue(dev, qnum);
> -		if (!use_napi &&
> +		if ((!use_napi || !kick) &&
>  		    unlikely(!virtqueue_enable_cb_delayed(sq->vq))) {
>  			/* More just got used, free them then recheck. */
>  			free_old_xmit_skbs(sq, false);

This will work but the following lines are:

                       if (sq->vq->num_free >= 2+MAX_SKB_FRAGS) {
                                netif_start_subqueue(dev, qnum);
                                virtqueue_disable_cb(sq->vq);
                        }

and I thought we are supposed to keep callbacks enabled with napi?
One of the ideas of napi is to free on napi callback, not here
immediately.

I think it is easier to just do a separate branch here. Along the
lines of:

		if (use_napi) {
			if (unlikely(!virtqueue_enable_cb_delayed(sq->vq)))
				virtqueue_napi_schedule(napi, vq);
		} else {
			... old code ...
		}

also reduces chances of regressions on !napi (which is not well tested)
and keeps callbacks off while we free skbs.

No?

> -- 
> 2.25.1