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 09:03:06 -0500

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.

> > > -
> > > -       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.