linux-kernel - Re: [PATCH net 2/3] virtio-net: ensure rx NAPI is enabled before enabling refill work

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Message-ID: <3acaaca3-37b3-4104-ac5a-441f3d4243c6@gmail.com>
Date: Thu, 25 Dec 2025 00:49:49 +0700
From: Bui Quang Minh <minhquangbui99@...il.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: netdev@...r.kernel.org, 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>,
 Jakub Kicinski <kuba@...nel.org>, 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
Subject: Re: [PATCH net 2/3] virtio-net: ensure rx NAPI is enabled before
 enabling refill work

On 12/24/25 08:45, Michael S. Tsirkin wrote:
> On Tue, Dec 23, 2025 at 10:25:32PM +0700, Bui Quang Minh wrote:
>> Calling napi_disable() on an already disabled napi can cause the
>> deadlock. Because the delayed refill work will call napi_disable(), we
>> must ensure that refill work is only enabled and scheduled after we have
>> enabled the rx queue's NAPI.
>>
>> Signed-off-by: Bui Quang Minh <minhquangbui99@...il.com>
>> ---
>>   drivers/net/virtio_net.c | 31 ++++++++++++++++++++++++-------
>>   1 file changed, 24 insertions(+), 7 deletions(-)
>>
>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>> index 63126e490bda..8016d2b378cf 100644
>> --- a/drivers/net/virtio_net.c
>> +++ b/drivers/net/virtio_net.c
>> @@ -3208,16 +3208,31 @@ static int virtnet_open(struct net_device *dev)
>>   	int i, err;
>>   
>>   	for (i = 0; i < vi->max_queue_pairs; i++) {
>> +		bool schedule_refill = false;
>
>
>> +
>> +		/* - We must call try_fill_recv before enabling napi of the same
>> +		 * receive queue so that it doesn't race with the call in
>> +		 * virtnet_receive.
>> +		 * - We must enable and schedule delayed refill work only when
>> +		 * we have enabled all the receive queue's napi. Otherwise, in
>> +		 * refill_work, we have a deadlock when calling napi_disable on
>> +		 * an already disabled napi.
>> +		 */
>
> I would do:
>
> 	bool refill = i < vi->curr_queue_pairs;
>
> in fact this is almost the same as resume with
> one small difference. pass a flag so we do not duplicate code?

I'll fix it in next version.

>
>>   		if (i < vi->curr_queue_pairs) {
>> -			enable_delayed_refill(&vi->rq[i]);
>>   			/* Make sure we have some buffers: if oom use wq. */
>>   			if (!try_fill_recv(vi, &vi->rq[i], GFP_KERNEL))
>> -				schedule_delayed_work(&vi->rq[i].refill, 0);
>> +				schedule_refill = true;
>>   		}
>>   
>>   		err = virtnet_enable_queue_pair(vi, i);
>>   		if (err < 0)
>>   			goto err_enable_qp;
>> +
>> +		if (i < vi->curr_queue_pairs) {
>> +			enable_delayed_refill(&vi->rq[i]);
>> +			if (schedule_refill)
>> +				schedule_delayed_work(&vi->rq[i].refill, 0);
>
> hmm. should not schedule be under the lock?

I see that schedule is safe to be called concurrently.

     struct work_struct {
         atomic_long_t data;
         struct list_head entry;
         work_func_t func;
     #ifdef CONFIG_LOCKDEP
         struct lockdep_map lockdep_map;
     #endif
     };

The atomic_long_t field to set pending bit and worker pool's lock help 
with the synchronization.


>
>> +		}
>>   	}
>>   
>>   	if (virtio_has_feature(vi->vdev, VIRTIO_NET_F_STATUS)) {
>> @@ -3456,11 +3471,16 @@ static void __virtnet_rx_resume(struct virtnet_info *vi,
>>   	bool running = netif_running(vi->dev);
>>   	bool schedule_refill = false;
>>   
>> +	/* See the comment in virtnet_open for the ordering rule
>> +	 * of try_fill_recv, receive queue napi_enable and delayed
>> +	 * refill enable/schedule.
>> +	 */
> so maybe common code?
>
>>   	if (refill && !try_fill_recv(vi, rq, GFP_KERNEL))
>>   		schedule_refill = true;
>>   	if (running)
>>   		virtnet_napi_enable(rq);
>>   
>> +	enable_delayed_refill(rq);
>>   	if (schedule_refill)
>>   		schedule_delayed_work(&rq->refill, 0);
> hmm. should not schedule be under the lock?
>
>>   }
>> @@ -3470,18 +3490,15 @@ static void virtnet_rx_resume_all(struct virtnet_info *vi)
>>   	int i;
>>   
>>   	for (i = 0; i < vi->max_queue_pairs; i++) {
>> -		if (i < vi->curr_queue_pairs) {
>> -			enable_delayed_refill(&vi->rq[i]);
>> +		if (i < vi->curr_queue_pairs)
>>   			__virtnet_rx_resume(vi, &vi->rq[i], true);
>> -		} else {
>> +		else
>>   			__virtnet_rx_resume(vi, &vi->rq[i], false);
>> -		}
>>   	}
>>   }
>>   
>>   static void virtnet_rx_resume(struct virtnet_info *vi, struct receive_queue *rq)
>>   {
>> -	enable_delayed_refill(rq);
>>   	__virtnet_rx_resume(vi, rq, true);
>>   }
> so I would add bool to virtnet_rx_resume and call it everywhere,
> removing __virtnet_rx_resume. can be a patch on top.

I'll create another patch after this patch to clean up the 
__virtnet_rx_resume in the next version.

Thanks,
Quang Minh.

>
>>   
>> -- 
>> 2.43.0