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

[Toshiaki Makita <toshiaki.makita1@gmail.com>]

On 2025/10/28 5:05, Jesper Dangaard Brouer wrote:

> (1) In veth_xmit(), the racy conditional wake-up logic and its memory barrier
> are removed. Instead, after stopping the queue, we unconditionally call
> __veth_xdp_flush(rq). This guarantees that the NAPI consumer is scheduled,
> making it solely responsible for re-waking the TXQ.

Maybe another option is to use !ptr_ring_full() instead of ptr_ring_empty()?
I'm not sure which is better. Anyway I'm ok with your approach.

...

> (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)))) {

Not sure if this is necessary.
 From commitlog, your intention seems to be making sure to wake up the queue,
but you wake up the queue immediately after this hunk in the same function,
so isn't it guaranteed without scheduling another napi?

>   			if (napi_schedule_prep(&rq->xdp_napi)) {
>   				WRITE_ONCE(rq->rx_notify_masked, true);
>   				__napi_schedule(&rq->xdp_napi);
> @@ -998,6 +992,13 @@ static int veth_poll(struct napi_struct *napi, int budget)
>   		veth_xdp_flush(rq, &bq);
>   	xdp_clear_return_frame_no_direct();
>   
> +	/* Release backpressure per NAPI poll */
> +	smp_rmb(); /* Paired with netif_tx_stop_queue set_bit */
> +	if (peer_txq && netif_tx_queue_stopped(peer_txq)) {
> +		txq_trans_cond_update(peer_txq);
> +		netif_tx_wake_queue(peer_txq);
> +	}
> +
>   	return done;
>   }

--
Toshiaki Makita