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

[Bui Quang Minh <minhquangbui99@gmail.com>]

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@redhat.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.

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;
>>   }