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

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Message-ID: <20260109181239.1c272f88@kernel.org>
Date: Fri, 9 Jan 2026 18:12:39 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: Bui Quang Minh <minhquangbui99@...il.com>
Cc: netdev@...r.kernel.org, "Michael S. Tsirkin" <mst@...hat.com>, Jason
 Wang <jasowang@...hat.com>, Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Eugenio
 Pérez <eperezma@...hat.com>, Andrew Lunn
 <andrew+netdev@...n.ch>, "David S. Miller" <davem@...emloft.net>, Eric
 Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, Alexei
 Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Jesper Dangaard Brouer <hawk@...nel.org>, John Fastabend
 <john.fastabend@...il.com>, Stanislav Fomichev <sdf@...ichev.me>,
 virtualization@...ts.linux.dev, linux-kernel@...r.kernel.org,
 bpf@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH net v3 1/3] virtio-net: don't schedule delayed refill
 worker

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@...hat.com>
> 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.

> @@ -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..

>  		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

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