Subject: Re: [PATCH -next] virtio_net: Avoid loop in virtnet_poll

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 2 Aug 2020 00:40:25 -0400

On Sun, Aug 02, 2020 at 11:41:23AM +0800, Mao Wenan wrote:
> The loop may exist if vq->broken is true,
> virtqueue_get_buf_ctx_packed or virtqueue_get_buf_ctx_split
> will return NULL, so virtnet_poll will reschedule napi to
> receive packet, it will lead cpu usage(si) to 100%.
> 
> call trace as below:
> virtnet_poll
> 	virtnet_receive
> 		virtqueue_get_buf_ctx
> 			virtqueue_get_buf_ctx_packed
> 			virtqueue_get_buf_ctx_split
> 	virtqueue_napi_complete
> 		virtqueue_napi_schedule //it will reschedule napi
> 
> Signed-off-by: Mao Wenan <[email-masked]>

I think it's more a bug in virtqueue_poll : virtqueue_get_buf reports
NULL on broken, so virtqueue_poll should report false for consistency.

> ---
>  drivers/net/virtio_net.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index ba38765..a058da1 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -327,7 +327,8 @@ static void virtqueue_napi_complete(struct napi_struct *napi,
>  
>  	opaque = virtqueue_enable_cb_prepare(vq);
>  	if (napi_complete_done(napi, processed)) {
> -		if (unlikely(virtqueue_poll(vq, opaque)))
> +		if (unlikely(virtqueue_poll(vq, opaque)) &&
> +		    unlikely(!virtqueue_is_broken(vq)))
>  			virtqueue_napi_schedule(napi, vq);
>  	} else {
>  		virtqueue_disable_cb(vq);
> -- 
> 1.8.3.1