Re: [PATCH net v3 1/3] virtio-net: don't schedule delayed refill worker

[Bui Quang Minh]

On 1/10/26 09:12, Jakub Kicinski wrote:
> On Tue,  6 Jan 2026 22:04:36 +0700 Bui Quang Minh wrote:
> > When we fail to refill the receive buffers, we schedule a delayed worker
> > to retry later. However, this worker creates some concurrency issues.
> > For example, when the worker runs concurrently with virtnet_xdp_set,
> > both need to temporarily disable queue's NAPI before enabling again.
> > Without proper synchronization, a deadlock can happen when
> > napi_disable() is called on an already disabled NAPI. That
> > napi_disable() call will be stuck and so will the subsequent
> > napi_enable() call.
> >
> > To simplify the logic and avoid further problems, we will instead retry
> > refilling in the next NAPI poll.
> Happy to see this go FWIW. If it causes issues we should consider
> adding some retry logic in the core (NAPI) rather than locally in
> the driver..
>
> > Fixes: 4bc12818b363 ("virtio-net: disable delayed refill when pausing rx")
> > Reported-by: Paolo Abeni <pabeni@xxxxxxxxxx>
> > Closes: https://netdev-ctrl.bots.linux.dev/logs/vmksft/drv-hw-dbg/results/400961/3-xdp-py/stderr
> The Closes should probably point to Paolo's report. We'll wipe these CI
> logs sooner or later but the lore archive will stick around.

I'll fix it in the next version.

> > @@ -3230,9 +3230,10 @@ static int virtnet_open(struct net_device *dev)
> >   
> >   	for (i = 0; i < vi->max_queue_pairs; i++) {
> >   		if (i < vi->curr_queue_pairs)
> > -			/* Make sure we have some buffers: if oom use wq. */
> > -			if (!try_fill_recv(vi, &vi->rq[i], GFP_KERNEL))
> > -				schedule_delayed_work(&vi->refill, 0);
> > +			/* Pre-fill rq agressively, to make sure we are ready to
> > +			 * get packets immediately.
> > +			 */
> > +			try_fill_recv(vi, &vi->rq[i], GFP_KERNEL);
> We should enforce _some_ minimal fill level at the time of open().
> If the ring is completely empty no traffic will ever flow, right?
> Perhaps I missed scheduling the NAPI somewhere..

The NAPI is enabled and scheduled in virtnet_napi_enable(). The code 
path is like this

virtnet_enable_queue_pair
-> virtnet_napi_enable
  -> virtnet_napi_do_enable
    -> virtqueue_napi_schedule

The same happens in __virtnet_rx_resume().

> >   		err = virtnet_enable_queue_pair(vi, i);
> >   		if (err < 0)
> > @@ -3472,16 +3473,15 @@ static void __virtnet_rx_resume(struct virtnet_info *vi,
> >   				struct receive_queue *rq,
> >   				bool refill)
> >   {
> > -	bool running = netif_running(vi->dev);
> > -	bool schedule_refill = false;
> > +	if (netif_running(vi->dev)) {
> > +		/* Pre-fill rq agressively, to make sure we are ready to get
> > +		 * packets immediately.
> > +		 */
> > +		if (refill)
> > +			try_fill_recv(vi, rq, GFP_KERNEL);
> Similar thing here? Tho not sure we can fail here..
>
> > -	if (refill && !try_fill_recv(vi, rq, GFP_KERNEL))
> > -		schedule_refill = true;
> > -	if (running)
> >   		virtnet_napi_enable(rq);
> > -
> > -	if (schedule_refill)
> > -		schedule_delayed_work(&vi->refill, 0);
> > +	}
> >   }
> >   
> >   static void virtnet_rx_resume_all(struct virtnet_info *vi)
> > @@ -3829,11 +3829,13 @@ static int virtnet_set_queues(struct virtnet_info *vi, u16 queue_pairs)
> >   	}
> >   succ:
> >   	vi->curr_queue_pairs = queue_pairs;
> > -	/* virtnet_open() will refill when device is going to up. */
> > -	spin_lock_bh(&vi->refill_lock);
> > -	if (dev->flags & IFF_UP && vi->refill_enabled)
> > -		schedule_delayed_work(&vi->refill, 0);
> > -	spin_unlock_bh(&vi->refill_lock);
> > +	if (dev->flags & IFF_UP) {
> > +		local_bh_disable();
> > +		for (int i = 0; i < vi->curr_queue_pairs; ++i)
> > +			virtqueue_napi_schedule(&vi->rq[i].napi, vi->rq[i].vq);
> > +
> nit: spurious new line

I'll delete it in the next version.

> > +		local_bh_enable();
> > +	}
> >   
> >   	return 0;
> >   }