linux-kernel - Re: [PATCH RFC] virtio_net: gate delayed refill scheduling

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Message-ID: <eabd665c-b14d-4281-9307-2348791d3a77@gmail.com>
Date: Thu, 4 Dec 2025 22:08:25 +0700
From: Bui Quang Minh <minhquangbui99@...il.com>
To: Jason Wang <jasowang@...hat.com>
Cc: "Michael S. Tsirkin" <mst@...hat.com>, linux-kernel@...r.kernel.org,
 Paolo Abeni <pabeni@...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>, 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,
 netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH RFC] virtio_net: gate delayed refill scheduling

On 12/3/25 13:37, Jason Wang wrote:
> On Tue, Dec 2, 2025 at 11:29 PM Bui Quang Minh <minhquangbui99@...il.com> wrote:
>> On 12/2/25 13:03, Jason Wang wrote:
>>> On Mon, Dec 1, 2025 at 11:04 PM Bui Quang Minh <minhquangbui99@...il.com> wrote:
>>>> On 11/28/25 09:20, Jason Wang wrote:
>>>>> On Fri, Nov 28, 2025 at 1:47 AM Bui Quang Minh <minhquangbui99@...il.com> wrote:
>>>>>> I think the the requeue in refill_work is not the problem here. In
>>>>>> virtnet_rx_pause[_all](), we use cancel_work_sync() which is safe to
>>>>>> use "even if the work re-queues itself". AFAICS, cancel_work_sync()
>>>>>> will disable work -> flush work -> enable again. So if the work requeue
>>>>>> itself in flush work, the requeue will fail because the work is already
>>>>>> disabled.
>>>>> Right.
>>>>>
>>>>>> I think what triggers the deadlock here is a bug in
>>>>>> virtnet_rx_resume_all(). virtnet_rx_resume_all() calls to
>>>>>> __virtnet_rx_resume() which calls napi_enable() and may schedule
>>>>>> refill. It schedules the refill work right after napi_enable the first
>>>>>> receive queue. The correct way must be napi_enable all receive queues
>>>>>> before scheduling refill work.
>>>>> So what you meant is that the napi_disable() is called for a queue
>>>>> whose NAPI has been disabled?
>>>>>
>>>>> cpu0] enable_delayed_refill()
>>>>> cpu0] napi_enable(queue0)
>>>>> cpu0] schedule_delayed_work(&vi->refill)
>>>>> cpu1] napi_disable(queue0)
>>>>> cpu1] napi_enable(queue0)
>>>>> cpu1] napi_disable(queue1)
>>>>>
>>>>> In this case cpu1 waits forever while holding the netdev lock. This
>>>>> looks like a bug since the netdev_lock 413f0271f3966 ("net: protect
>>>>> NAPI enablement with netdev_lock()")?
>>>> Yes, I've tried to fix it in 4bc12818b363 ("virtio-net: disable delayed
>>>> refill when pausing rx"), but it has flaws.
>>> I wonder if a simplified version is just restoring the behaviour
>>> before 413f0271f3966 by using napi_enable_locked() but maybe I miss
>>> something.
>> As far as I understand, before 413f0271f3966 ("net: protect NAPI
>> enablement with netdev_lock()"), the napi is protected by the
> I guess you meant napi enable/disable actually.
>
>> rtnl_lock(). But in the refill_work, we don't acquire the rtnl_lock(),
> Any reason we need to hold rtnl_lock() there?

Correct me if I'm wrong here. Before 413f0271f3966 ("net: protect NAPI 
enablement with netdev_lock()"), napi_disable and napi_enable are not 
safe to be called concurrently.

The example race is

napi_disable -> napi_save_config -> write to n->config->defer_hard_irqs
napi_enable -> napi_restore_config -> read n->config->defer_hard_irqs

In refill_work, we don't hold any locks so the race scenario can happen.

Maybe I misunderstand what you mean by restoring the behavior before 
413f0271f3966. Do you mean that we use this pattern

     In virtnet_xdp_se;

     netdev_lock(dev);
     virtnet_rx_pause_all()
         -> napi_disable_locked

     virtnet_rx_resume_all()
         -> napi_disable_locked
     netdev_unlock(dev);

And in other places where we pause the rx too. It will hold the 
netdev_lock during the time napi is disabled so that even when 
refill_work happens concurrently, napi_disable cannot acquire the 
netdev_lock and gets stuck inside.

>
>> so it seems like we will have race condition before 413f0271f3966 ("net:
>> protect NAPI enablement with netdev_lock()").
>>
>> Thanks,
>> Quang Minh.
>>
> Thanks
>

Thanks,
Quang Minh.