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

[Bui Quang Minh]

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@xxxxxxxxx>
> > ---
> >   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