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

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

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?

>
> +       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.