Re: [PATCH net V2 2/2] veth: more robust handing of race to avoid txq getting stuck

[Paolo Abeni <pabeni@redhat.com>]

On 10/27/25 9:05 PM, Jesper Dangaard Brouer wrote:
> (3) Finally, the NAPI completion check in veth_poll() is updated. If NAPI is
> about to complete (napi_complete_done), it now also checks if the peer TXQ
> is stopped. If the ring is empty but the peer TXQ is stopped, NAPI will
> reschedule itself. This prevents a new race where the producer stops the
> queue just as the consumer is finishing its poll, ensuring the wakeup is not
> missed.

[...]

> @@ -986,7 +979,8 @@ static int veth_poll(struct napi_struct *napi, int budget)
>  	if (done < budget && napi_complete_done(napi, done)) {
>  		/* Write rx_notify_masked before reading ptr_ring */
>  		smp_store_mb(rq->rx_notify_masked, false);
> -		if (unlikely(!__ptr_ring_empty(&rq->xdp_ring))) {
> +		if (unlikely(!__ptr_ring_empty(&rq->xdp_ring) ||
> +			     (peer_txq && netif_tx_queue_stopped(peer_txq)))) {
>  			if (napi_schedule_prep(&rq->xdp_napi)) {
>  				WRITE_ONCE(rq->rx_notify_masked, true);
>  				__napi_schedule(&rq->xdp_napi);

Double checking I'm read the code correctly. The above is supposed to
trigger when something alike the following happens

[producer]				[consumer]
					veth_poll()
					[ring empty]
veth_xmit
  veth_forward_skb
  [NETDEV_TX_BUSY]		
					napi_complete_done()
					
  netif_tx_stop_queue
  __veth_xdp_flush()
  rq->rx_notify_masked == true
					WRITE_ONCE(rq->rx_notify_masked,
						   false);

?

I think the above can't happen, the producer should need to fill the
whole ring in-between the ring check and napi_complete_done().

Am I misreading it?

/P