Re: [PATCH net-next v2 1/2] virtio_net: xsk: fix race in rx wake up

[Menglong Dong]

On 2026/6/15 10:48 Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx> write:
> On Thu, 11 Jun 2026 10:56:43 +0800, menglong8.dong@xxxxxxxxx wrote:
> > From: Menglong Dong <dongml2@xxxxxxxxxxxxxxx>
> >
> > During packet receiving in virtio-net, the rq can be empty, which means
> > "rq->vq->num_free == virtqueue_get_vring_size(rq->vq)", in
> > virtnet_add_recvbuf_xsk(), if we are using xsk. Meanwhile, the fill ring
> > can be empty too, which means we can't allocate anything from
> > xsk_buff_alloc_batch(). Then, we will set the XDP_RING_NEED_WAKEUP flag.
> >
[...]
> >
> > +	need_wakeup = xsk_uses_need_wakeup(pool);
> >  	xsk_buffs = rq->xsk_buffs;
> >
> > +	/* If both rq->vq and fill ring are empty, and then the user submit
> > +	 * all the chunks to the fill ring and check the wake up flag
> > +	 * after xsk_buff_alloc_batch() and before xsk_set_rx_need_wakeup(),
> > +	 * we will lose the chance to wake up the rx napi, so we have to
> > +	 * set the need_wakeup flag here.
> > +	 */
> > +	if (need_wakeup && virtqueue_get_vring_size(rq->vq) == rq->vq->num_free)
> > +		xsk_set_rx_need_wakeup(pool);
> 
> Is Condition A here too strict? We should trigger the wakeup under a wider range
> of scenarios.

Hi, Xuan. Thinks for your reviewing :)

The logic here is a addition logic to the origin wake up logic, which I planed
to fix a race condition. However, this race condition seems not likely to happen,
as we discussed in this thread:

https://lore.kernel.org/netdev/rHZz5_ylT4WggoZ-Ic2Q4w@xxxxxxxxx/

So this patch is not necessary, and I'll send the 2nd patch standalone.

Thanks!
Menglong Dong

> 
> > +
> >  	num = xsk_buff_alloc_batch(pool, xsk_buffs, rq->vq->num_free);
> >  	if (!num) {
> > -		if (xsk_uses_need_wakeup(pool)) {
> > +		if (need_wakeup) {
> >  			xsk_set_rx_need_wakeup(pool);
> >  			/* Return 0 instead of -ENOMEM so that NAPI is
> >  			 * descheduled.
> > @@ -1341,8 +1352,6 @@ static int virtnet_add_recvbuf_xsk(struct virtnet_info *vi, struct receive_queue
> >  		}
> >
> >  		return -ENOMEM;
> > -	} else {
> > -		xsk_clear_rx_need_wakeup(pool);
> >  	}
> >
> >  	len = xsk_pool_get_rx_frame_size(pool) + vi->hdr_len;
> > @@ -1363,6 +1372,16 @@ static int virtnet_add_recvbuf_xsk(struct virtnet_info *vi, struct receive_queue
> >  			goto err;
> >  	}
> >
> > +	if (need_wakeup) {
> > +		if (rq->vq->num_free)
> > +			/* We have free buffers, so we'd better wake up the
> > +			 * rx napi as soon as possible.
> > +			 */
> > +			xsk_set_rx_need_wakeup(pool);
> 
> Is the purpose of waking up RX NAPI to invoke try_fill_recv? However,
> virtnet_poll does not call try_fill_recv directly. it is done
> conditionally.
> 
> Thanks.
> 
> 
> > +		else
> > +			xsk_clear_rx_need_wakeup(pool);
> > +	}
> > +
> >  	return num;
> >
> >  err:
> > --
> > 2.54.0
> >
> 
>