Message-ID: <5434a67e-dd6e-4cd1-870b-fdd32ad34a28@gmail.com>
Date: Fri, 19 Dec 2025 12:03:29 +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 12/17/25 09:58, Jason Wang wrote:
> On Wed, Dec 17, 2025 at 12:23 AM Bui Quang Minh
> <minhquangbui99@...il.com> wrote:
>> 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?
> Or we can have a reill_pending

Okay, let me implement this in the next version.

> but basically I think we need something
> that is much more simple. That is, using a per rq work instead of a
> global one?

I think we can leave this in a net-next patch later.

Thanks,
Quang Minh

>
> Thanks
>
>>>
>>>> +       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.
>>

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