Subject: Re: [PATCH net-next] virtio_net: Fix napi_skb_cache_put warning

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 14 Jul 2024 03:38:42 -0400

On Fri, Jul 12, 2024 at 04:53:25AM -0700, Breno Leitao wrote:
> After the commit bdacf3e34945 ("net: Use nested-BH locking for
> napi_alloc_cache.") was merged, the following warning began to appear:
> 
> 	 WARNING: CPU: 5 PID: 1 at net/core/skbuff.c:1451 napi_skb_cache_put+0x82/0x4b0
> 
> 	  __warn+0x12f/0x340
> 	  napi_skb_cache_put+0x82/0x4b0
> 	  napi_skb_cache_put+0x82/0x4b0
> 	  report_bug+0x165/0x370
> 	  handle_bug+0x3d/0x80
> 	  exc_invalid_op+0x1a/0x50
> 	  asm_exc_invalid_op+0x1a/0x20
> 	  __free_old_xmit+0x1c8/0x510
> 	  napi_skb_cache_put+0x82/0x4b0
> 	  __free_old_xmit+0x1c8/0x510
> 	  __free_old_xmit+0x1c8/0x510
> 	  __pfx___free_old_xmit+0x10/0x10
> 
> The issue arises because virtio is assuming it's running in NAPI context
> even when it's not, such as in the netpoll case.
> 
> To resolve this, modify virtnet_poll_tx() to only set NAPI when budget
> is available. Same for virtnet_poll_cleantx(), which always assumed that
> it was in a NAPI context.
> 
> Fixes: df133f3f9625 ("virtio_net: bulk free tx skbs")
> Suggested-by: Jakub Kicinski <[email-masked]>
> Signed-off-by: Breno Leitao <[email-masked]>

Acked-by: Michael S. Tsirkin <[email-masked]>

though I'm not sure I understand the connection with bdacf3e34945.

> ---
>  drivers/net/virtio_net.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 0b4747e81464..fb1331827308 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -2341,7 +2341,7 @@ static int virtnet_receive(struct receive_queue *rq, int budget,
>  	return packets;
>  }
>  
> -static void virtnet_poll_cleantx(struct receive_queue *rq)
> +static void virtnet_poll_cleantx(struct receive_queue *rq, int budget)
>  {
>  	struct virtnet_info *vi = rq->vq->vdev->priv;
>  	unsigned int index = vq2rxq(rq->vq);
> @@ -2359,7 +2359,7 @@ static void virtnet_poll_cleantx(struct receive_queue *rq)
>  
>  		do {
>  			virtqueue_disable_cb(sq->vq);
> -			free_old_xmit(sq, txq, true);
> +			free_old_xmit(sq, txq, !!budget);
>  		} while (unlikely(!virtqueue_enable_cb_delayed(sq->vq)));
>  
>  		if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS) {
> @@ -2404,7 +2404,7 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
>  	unsigned int xdp_xmit = 0;
>  	bool napi_complete;
>  
> -	virtnet_poll_cleantx(rq);
> +	virtnet_poll_cleantx(rq, budget);
>  
>  	received = virtnet_receive(rq, budget, &xdp_xmit);
>  	rq->packets_in_napi += received;
> @@ -2526,7 +2526,7 @@ static int virtnet_poll_tx(struct napi_struct *napi, int budget)
>  	txq = netdev_get_tx_queue(vi->dev, index);
>  	__netif_tx_lock(txq, raw_smp_processor_id());
>  	virtqueue_disable_cb(sq->vq);
> -	free_old_xmit(sq, txq, true);
> +	free_old_xmit(sq, txq, !!budget);
>  
>  	if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS) {
>  		if (netif_tx_queue_stopped(txq)) {
> -- 
> 2.43.0