Subject: Re: [PATCH net v2 1/3] virtio-net: don't schedule delayed refill worker

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 4 Jan 2026 10:12:10 -0500

On Sun, Jan 04, 2026 at 09:54:30PM +0700, Bui Quang Minh wrote:
> On 1/4/26 21:03, Michael S. Tsirkin wrote:
> > On Sun, Jan 04, 2026 at 03:34:52PM +0700, Bui Quang Minh wrote:
> > > On 1/4/26 13:09, Jason Wang wrote:
> > > > On Fri, Jan 2, 2026 at 11:20 PM Bui Quang Minh <[email-masked]> wrote:
> > > > > When we fail to refill the receive buffers, we schedule a delayed worker
> > > > > to retry later. However, this worker creates some concurrency issues
> > > > > such as races and deadlocks. To simplify the logic and avoid further
> > > > > problems, we will instead retry refilling in the next NAPI poll.
> > > > > 
> > > > > Fixes: 4bc12818b363 ("virtio-net: disable delayed refill when pausing rx")
> > > > > Reported-by: Paolo Abeni <[email-masked]>
> > > > > Closes: https://netdev-ctrl.bots.linux.dev/logs/vmksft/drv-hw-dbg/results/400961/3-xdp-py/stderr
> > > > > Cc: [email-masked]
> > > > > Suggested-by: Xuan Zhuo <[email-masked]>
> > > > > Signed-off-by: Bui Quang Minh <[email-masked]>
> > > > > ---
> > > > >    drivers/net/virtio_net.c | 55 ++++++++++++++++++++++------------------
> > > > >    1 file changed, 30 insertions(+), 25 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > > > index 1bb3aeca66c6..ac514c9383ae 100644
> > > > > --- a/drivers/net/virtio_net.c
> > > > > +++ b/drivers/net/virtio_net.c
> > > > > @@ -3035,7 +3035,7 @@ static int virtnet_receive_packets(struct virtnet_info *vi,
> > > > >    }
> > > > > 
> > > > >    static int virtnet_receive(struct receive_queue *rq, int budget,
> > > > > -                          unsigned int *xdp_xmit)
> > > > > +                          unsigned int *xdp_xmit, bool *retry_refill)
> > > > >    {
> > > > >           struct virtnet_info *vi = rq->vq->vdev->priv;
> > > > >           struct virtnet_rq_stats stats = {};
> > > > > @@ -3047,12 +3047,8 @@ static int virtnet_receive(struct receive_queue *rq, int budget,
> > > > >                   packets = virtnet_receive_packets(vi, rq, budget, xdp_xmit, &stats);
> > > > > 
> > > > >           if (rq->vq->num_free > min((unsigned int)budget, virtqueue_get_vring_size(rq->vq)) / 2) {
> > > > > -               if (!try_fill_recv(vi, rq, GFP_ATOMIC)) {
> > > > > -                       spin_lock(&vi->refill_lock);
> > > > > -                       if (vi->refill_enabled)
> > > > > -                               schedule_delayed_work(&vi->refill, 0);
> > > > > -                       spin_unlock(&vi->refill_lock);
> > > > > -               }
> > > > > +               if (!try_fill_recv(vi, rq, GFP_ATOMIC))
> > > > > +                       *retry_refill = true;
> > > > >           }
> > > > > 
> > > > >           u64_stats_set(&stats.packets, packets);
> > > > > @@ -3129,18 +3125,18 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
> > > > >           struct send_queue *sq;
> > > > >           unsigned int received;
> > > > >           unsigned int xdp_xmit = 0;
> > > > > -       bool napi_complete;
> > > > > +       bool napi_complete, retry_refill = false;
> > > > > 
> > > > >           virtnet_poll_cleantx(rq, budget);
> > > > > 
> > > > > -       received = virtnet_receive(rq, budget, &xdp_xmit);
> > > > > +       received = virtnet_receive(rq, budget, &xdp_xmit, &retry_refill);
> > > > I think we can simply let virtnet_receive() to return the budget when
> > > > reill fails.
> > > That makes sense, I'll change it.
> > > 
> > > > >           rq->packets_in_napi += received;
> > > > > 
> > > > >           if (xdp_xmit & VIRTIO_XDP_REDIR)
> > > > >                   xdp_do_flush();
> > > > > 
> > > > >           /* Out of packets? */
> > > > > -       if (received < budget) {
> > > > > +       if (received < budget && !retry_refill) {
> > > > >                   napi_complete = virtqueue_napi_complete(napi, rq->vq, received);
> > > > >                   /* Intentionally not taking dim_lock here. This may result in a
> > > > >                    * spurious net_dim call. But if that happens virtnet_rx_dim_work
> > > > > @@ -3160,7 +3156,7 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
> > > > >                   virtnet_xdp_put_sq(vi, sq);
> > > > >           }
> > > > > 
> > > > > -       return received;
> > > > > +       return retry_refill ? budget : received;
> > > > >    }
> > > > > 
> > > > >    static void virtnet_disable_queue_pair(struct virtnet_info *vi, int qp_index)
> > > > > @@ -3230,9 +3226,11 @@ 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);
> > > > > +                       /* If this fails, we will retry later in
> > > > > +                        * NAPI poll, which is scheduled in the below
> > > > > +                        * virtnet_enable_queue_pair
> > > > > +                        */
> > > > > +                       try_fill_recv(vi, &vi->rq[i], GFP_KERNEL);
> > > > Consider NAPI will be eventually scheduled, I wonder if it's still
> > > > worth to refill here.
> > > With GFP_KERNEL here, I think it's more likely to succeed than GFP_ATOMIC in
> > > NAPI poll. Another small benefit is that the actual packet can happen
> > > earlier. In case the receive buffer is empty and we don't refill here, the
> > > #1 NAPI poll refill the buffer and the #2 NAPI poll can receive packets. The
> > > #2 NAPI poll is scheduled in the interrupt handler because the #1 NAPI poll
> > > will deschedule the NAPI and enable the device interrupt. In case we
> > > successfully refill here, the #1 NAPI poll can receive packets right away.
> > > 
> > Right. But I think this is a part that needs elucidating, not
> > error handling.
> > 
> > /* Pre-fill rq agressively, to make sure we are ready to get packets
> >   * immediately.
> >   * */
> > 
> > > > >                   err = virtnet_enable_queue_pair(vi, i);
> > > > >                   if (err < 0)
> > > > > @@ -3473,15 +3471,15 @@ static void __virtnet_rx_resume(struct virtnet_info *vi,
> > > > >                                   bool refill)
> > > > >    {
> > > > >           bool running = netif_running(vi->dev);
> > > > > -       bool schedule_refill = false;
> > > > > 
> > > > > -       if (refill && !try_fill_recv(vi, rq, GFP_KERNEL))
> > > > > -               schedule_refill = true;
> > > > > +       if (refill)
> > > > > +               /* If this fails, we will retry later in NAPI poll, which is
> > > > > +                * scheduled in the below virtnet_napi_enable
> > > > > +                */
> > > > > +               try_fill_recv(vi, rq, GFP_KERNEL);
> > > > and here.
> > > > 
> > > > > +
> > > > >           if (running)
> > > > >                   virtnet_napi_enable(rq);
> > here the part that isn't clear is why are we refilling if !running
> > and what handles failures in that case.
> 
> You are right, we should not refill when !running. I'll move the if (refill)
> inside the if (running).

Sounds like a helper that does refill+virtnet_napi_enable
would be in order then?  fill_recv_aggressively(vi, rq) ?

> > 
> > > > > -
> > > > > -       if (schedule_refill)
> > > > > -               schedule_delayed_work(&vi->refill, 0);
> > > > >    }
> > > > > 
> > > > >    static void virtnet_rx_resume_all(struct virtnet_info *vi)
> > > > > @@ -3777,6 +3775,7 @@ static int virtnet_set_queues(struct virtnet_info *vi, u16 queue_pairs)
> > > > >           struct virtio_net_rss_config_trailer old_rss_trailer;
> > > > >           struct net_device *dev = vi->dev;
> > > > >           struct scatterlist sg;
> > > > > +       int i;
> > > > > 
> > > > >           if (!vi->has_cvq || !virtio_has_feature(vi->vdev, VIRTIO_NET_F_MQ))
> > > > >                   return 0;
> > > > > @@ -3829,11 +3828,17 @@ 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) {
> > > > > +               /* Let the NAPI poll refill the receive buffer for us. We can't
> > > > > +                * safely call try_fill_recv() here because the NAPI might be
> > > > > +                * enabled already.
> > > > > +                */
> > > > > +               local_bh_disable();
> > > > > +               for (i = 0; i < vi->curr_queue_pairs; i++)
> > > > > +                       virtqueue_napi_schedule(&vi->rq[i].napi, vi->rq[i].vq);
> > > > > +
> > > > > +               local_bh_enable();
> > > > > +       }
> > > > > 
> > > > >           return 0;
> > > > >    }
> > > > > --
> > > > > 2.43.0
> > > > > 
> > > > Thanks
> > > > 
> > > Thanks,
> > > Quang Minh.