Re: [PATCH net] virtio-net: don't re-enable refill work too early when NAPI is disabled

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Apr 30, 2025 at 11:49:15AM +0800, Jason Wang wrote:
> On Tue, Apr 29, 2025 at 10:31 PM Jakub Kicinski <kuba@kernel.org> wrote:
> >
> > Commit 4bc12818b363 ("virtio-net: disable delayed refill when pausing rx")
> > fixed a deadlock between reconfig paths and refill work trying to disable
> > the same NAPI instance. The refill work can't run in parallel with reconfig
> > because trying to double-disable a NAPI instance causes a stall under the
> > instance lock, which the reconfig path needs to re-enable the NAPI and
> > therefore unblock the stalled thread.
> >
> > There are two cases where we re-enable refill too early. One is in the
> > virtnet_set_queues() handler. We call it when installing XDP:
> >
> >    virtnet_rx_pause_all(vi);
> >    ...
> >    virtnet_napi_tx_disable(..);
> >    ...
> >    virtnet_set_queues(..);
> >    ...
> >    virtnet_rx_resume_all(..);
> >
> > We want the work to be disabled until we call virtnet_rx_resume_all(),
> > but virtnet_set_queues() kicks it before NAPIs were re-enabled.
> >
> > The other case is a more trivial case of mis-ordering in
> > __virtnet_rx_resume() found by code inspection.
> >
> > Fixes: 4bc12818b363 ("virtio-net: disable delayed refill when pausing rx")
> > Fixes: 413f0271f396 ("net: protect NAPI enablement with netdev_lock()")
> > Signed-off-by: Jakub Kicinski <kuba@kernel.org>
> > ---
> > CC: mst@redhat.com
> > CC: jasowang@redhat.com
> > CC: xuanzhuo@linux.alibaba.com
> > CC: eperezma@redhat.com
> > CC: minhquangbui99@gmail.com
> > CC: romieu@fr.zoreil.com
> > CC: kuniyu@amazon.com
> > CC: virtualization@lists.linux.dev
> > ---
> >  drivers/net/virtio_net.c | 9 ++++++---
> >  1 file changed, 6 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 848fab51dfa1..4c904e176495 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -3383,12 +3383,15 @@ 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_delayed_work(&vi->refill, 0);
> > -
> > +               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)
> > @@ -3728,7 +3731,7 @@ 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. */
> > -       if (dev->flags & IFF_UP)
> > +       if (dev->flags & IFF_UP && vi->refill_enabled)
> >                 schedule_delayed_work(&vi->refill, 0);
> 
> This has the assumption that the toggle of the refill_enabled is under
> RTNL. Though it's true now but it looks to me it's better to protect
> it against refill_lock.
> 
> Thanks

Good point.

> >
> >         return 0;
> > --
> > 2.49.0
> >