linux-kernel - Re: [PATCH net v2] virtio-net: enable all napis before scheduling refill work

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Message-ID: <3f5613e9-ccd0-4096-afc3-67ee94f6f660@gmail.com>
Date: Tue, 16 Dec 2025 23:23:04 +0700
From: Bui Quang Minh <minhquangbui99@...il.com>
To: Jason Wang <jasowang@...hat.com>
Cc: netdev@...r.kernel.org, "Michael S. Tsirkin" <mst@...hat.com>,
 Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Eugenio Pérez
 <eperezma@...hat.com>, Andrew Lunn <andrew+netdev@...n.ch>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Jesper Dangaard Brouer <hawk@...nel.org>,
 John Fastabend <john.fastabend@...il.com>,
 Stanislav Fomichev <sdf@...ichev.me>, virtualization@...ts.linux.dev,
 linux-kernel@...r.kernel.org, bpf@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH net v2] virtio-net: enable all napis before scheduling
 refill work

On 12/16/25 11:16, Jason Wang wrote:
> On Fri, Dec 12, 2025 at 11:28 PM Bui Quang Minh
> <minhquangbui99@...il.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@...hat.com>
>> Closes: https://netdev-ctrl.bots.linux.dev/logs/vmksft/drv-hw-dbg/results/400961/3-xdp-py/stderr
>> Cc: stable@...r.kernel.org
>> Signed-off-by: Bui Quang Minh <minhquangbui99@...il.com>
>> ---
>> Changes in v2:
>> - Move try_fill_recv() before rx napi_enable()
>> - Link to v1: https://lore.kernel.org/netdev/20251208153419.18196-1-minhquangbui99@gmail.com/
>> ---
>>   drivers/net/virtio_net.c | 71 +++++++++++++++++++++++++---------------
>>   1 file changed, 45 insertions(+), 26 deletions(-)
>>
>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>> index 8e04adb57f52..4e08880a9467 100644
>> --- a/drivers/net/virtio_net.c
>> +++ b/drivers/net/virtio_net.c
>> @@ -3214,21 +3214,31 @@ static void virtnet_update_settings(struct virtnet_info *vi)
>>   static int virtnet_open(struct net_device *dev)
>>   {
>>          struct virtnet_info *vi = netdev_priv(dev);
>> +       bool schedule_refill = false;
>>          int i, err;
>>
>> -       enable_delayed_refill(vi);
>> -
>> +       /* - We must call try_fill_recv before enabling napi of the same receive
>> +        * queue so that it doesn't race with the call in virtnet_receive.
>> +        * - We must enable and schedule delayed refill work only when we have
>> +        * enabled all the receive queue's napi. Otherwise, in refill_work, we
>> +        * have a deadlock when calling napi_disable on an already disabled
>> +        * napi.
>> +        */
>>          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);
>> +                               schedule_refill = true;
>>
>>                  err = virtnet_enable_queue_pair(vi, i);
>>                  if (err < 0)
>>                          goto err_enable_qp;
>>          }
> So NAPI could be scheduled and it may want to refill but since refill
> is not enabled, there would be no refill work.
>
> Is this a problem?

You are right. It is indeed a problem.

I think we can unconditionally schedule the delayed refill after 
enabling all the RX NAPIs (don't check the boolean schedule_refill 
anymore) to ensure that we will have refill work. We can still keep the 
try_fill_recv here to fill the receive buffer earlier in normal case. 
What do you think?

>
>
>> +       enable_delayed_refill(vi);
>> +       if (schedule_refill)
>> +               schedule_delayed_work(&vi->refill, 0);
>> +
>>          if (virtio_has_feature(vi->vdev, VIRTIO_NET_F_STATUS)) {
>>                  if (vi->status & VIRTIO_NET_S_LINK_UP)
>>                          netif_carrier_on(vi->dev);
>> @@ -3463,39 +3473,48 @@ static void virtnet_rx_pause(struct virtnet_info *vi, struct receive_queue *rq)
>>          __virtnet_rx_pause(vi, rq);
>>   }
>>
> Thanks
>

Thanks,
Quang Minh.