Re: [PATCH net] virtio-net: enable all napis before scheduling refill work

[Jason Wang <jasowang@redhat.com>]

On Thu, Dec 11, 2025 at 11:04 PM Bui Quang Minh
<minhquangbui99@gmail.com> wrote:
>
> On 12/11/25 14:27, Jason Wang wrote:
> > On Wed, Dec 10, 2025 at 11:33 PM Bui Quang Minh
> > <minhquangbui99@gmail.com> wrote:
> >> On 12/10/25 12:45, Jason Wang wrote:
> >>> On Tue, Dec 9, 2025 at 11:23 PM Bui Quang Minh <minhquangbui99@gmail.com> wrote:
> >>>> On 12/9/25 11:30, Jason Wang wrote:
> >>>>> On Mon, Dec 8, 2025 at 11:35 PM Bui Quang Minh <minhquangbui99@gmail.com> wrote:
> >>>>>> Calling napi_disable() on an already disabled napi can cause the
> >>>>>> deadlock. In commit 4bc12818b363 ("virtio-net: disable delayed refill
> >>>>>> when pausing rx"), to avoid the deadlock, when pausing the RX in
> >>>>>> virtnet_rx_pause[_all](), we disable and cancel the delayed refill work.
> >>>>>> However, in the virtnet_rx_resume_all(), we enable the delayed refill
> >>>>>> work too early before enabling all the receive queue napis.
> >>>>>>
> >>>>>> The deadlock can be reproduced by running
> >>>>>> selftests/drivers/net/hw/xsk_reconfig.py with multiqueue virtio-net
> >>>>>> device and inserting a cond_resched() inside the for loop in
> >>>>>> virtnet_rx_resume_all() to increase the success rate. Because the worker
> >>>>>> processing the delayed refilled work runs on the same CPU as
> >>>>>> virtnet_rx_resume_all(), a reschedule is needed to cause the deadlock.
> >>>>>> In real scenario, the contention on netdev_lock can cause the
> >>>>>> reschedule.
> >>>>>>
> >>>>>> This fixes the deadlock by ensuring all receive queue's napis are
> >>>>>> enabled before we enable the delayed refill work in
> >>>>>> virtnet_rx_resume_all() and virtnet_open().
> >>>>>>
> >>>>>> Fixes: 4bc12818b363 ("virtio-net: disable delayed refill when pausing rx")
> >>>>>> Reported-by: Paolo Abeni <pabeni@redhat.com>
> >>>>>> Closes: https://netdev-ctrl.bots.linux.dev/logs/vmksft/drv-hw-dbg/results/400961/3-xdp-py/stderr
> >>>>>> Signed-off-by: Bui Quang Minh <minhquangbui99@gmail.com>
> >>>>>> ---
> >>>>>>     drivers/net/virtio_net.c | 59 +++++++++++++++++++---------------------
> >>>>>>     1 file changed, 28 insertions(+), 31 deletions(-)
> >>>>>>
> >>>>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> >>>>>> index 8e04adb57f52..f2b1ea65767d 100644
> >>>>>> --- a/drivers/net/virtio_net.c
> >>>>>> +++ b/drivers/net/virtio_net.c
> >>>>>> @@ -2858,6 +2858,20 @@ static bool try_fill_recv(struct virtnet_info *vi, struct receive_queue *rq,
> >>>>>>            return err != -ENOMEM;
> >>>>>>     }
> >>>>>>
> >>>>>> +static void virtnet_rx_refill_all(struct virtnet_info *vi)
> >>>>>> +{
> >>>>>> +       bool schedule_refill = false;
> >>>>>> +       int i;
> >>>>>> +
> >>>>>> +       enable_delayed_refill(vi);
> >>>>> This seems to be still racy?
> >>>>>
> >>>>> For example, in virtnet_open() we had:
> >>>>>
> >>>>> static int virtnet_open(struct net_device *dev)
> >>>>> {
> >>>>>            struct virtnet_info *vi = netdev_priv(dev);
> >>>>>            int i, err;
> >>>>>
> >>>>>            for (i = 0; i < vi->max_queue_pairs; i++) {
> >>>>>                    err = virtnet_enable_queue_pair(vi, i);
> >>>>>                    if (err < 0)
> >>>>>                            goto err_enable_qp;
> >>>>>            }
> >>>>>
> >>>>>            virtnet_rx_refill_all(vi);
> >>>>>
> >>>>> So NAPI and refill work is enabled in this case, so the refill work
> >>>>> could be scheduled and run at the same time?
> >>>> Yes, that's what we expect. We must ensure that refill work is scheduled
> >>>> only when all NAPIs are enabled. The deadlock happens when refill work
> >>>> is scheduled but there are still disabled RX NAPIs.
> >>> Just to make sure we are on the same page, I meant, after refill work
> >>> is enabled, rq0 is NAPI is enabled, in this case the refill work could
> >>> be triggered by the rq0's NAPI so we may end up in the refill work
> >>> that it tries to disable rq1's NAPI while holding the netdev lock.
> >> I don't quite get your point. The current deadlock scenario is this
> >>
> >> virtnet_rx_resume_all
> >> napi_enable(rq0) (the rq1 napi is still disabled)
> >> enable_refill_work
> >>
> >> refill_work
> >> napi_disable(rq0) -> still okay
> >> napi_enable(rq0) -> still okay
> >> napi_disable(rq1)
> >> -> hold netdev_lock
> >>       -> stuck inside the while loop in napi_disable_locked
> >>               while (val & (NAPIF_STATE_SCHED | NAPIF_STATE_NPSVC)) {
> >>                   usleep_range(20, 200);
> >>                   val = READ_ONCE(n->state);
> >>               }
> >>
> >>
> >> napi_enable(rq1)
> >> -> stuck while trying to acquire the netdev_lock
> >>
> >> The problem is that we must not call napi_disable() on an already
> >> disabled NAPI (rq1's NAPI in the example).
> >>
> >> In the new virtnet_open
> >>
> >> static int virtnet_open(struct net_device *dev)
> >> {
> >>            struct virtnet_info *vi = netdev_priv(dev);
> >>            int i, err;
> >>
> >>            // Note that at this point, refill work is still disabled, vi->refill_enabled == false,
> >>            // so even if virtnet_receive is called, the refill_work will not be scheduled.
> >>            for (i = 0; i < vi->max_queue_pairs; i++) {
> >>                    err = virtnet_enable_queue_pair(vi, i);
> >>                    if (err < 0)
> >>                            goto err_enable_qp;
> >>            }
> >>
> >>            // Here all RX NAPIs are enabled so it's safe to enable refill work again
> >>            virtnet_rx_refill_all(vi);
> >>
> > I meant this part:
> >
> > +static void virtnet_rx_refill_all(struct virtnet_info *vi)
> > +{
> > +       bool schedule_refill = false;
> > +       int i;
> > +
> > +       enable_delayed_refill(vi);
> >
> > refill_work could run here.
>
> I don't see how this can trigger the current deadlock race. However, I
> see that this code is racy, the try_fill_recv function is not safe to
> concurrently executed on the same receive queue. So there is a
> requirement that we need to call try_fill_recv before enabling napi. Is
> it what you mean?

Exactly, I meant it's racy.

>
> >
> > +       for (i = 0; i < vi->curr_queue_pairs; i++)
> > +               if (!try_fill_recv(vi, &vi->rq[i], GFP_KERNEL))
> > +                       schedule_refill = true;
> > +
> >
> > I think it can be fixed by moving enable_delayed_refill() here.
> >
> > +       if (schedule_refill)
> > +               schedule_delayed_work(&vi->refill, 0);
> > +}
>
> Thanks,
> Quang Minh.
>

Thanks

>