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

[Bui Quang Minh <minhquangbui99@gmail.com>]

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.

Thanks,
Quang Minh.