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

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Message-ID: <faad67c7-8b25-4516-ab37-3b154ee4d0cf@gmail.com>
Date: Tue, 2 Dec 2025 22:29:32 +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 12/2/25 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 
rtnl_lock(). But in the refill_work, we don't acquire the rtnl_lock(), 
so it seems like we will have race condition before 413f0271f3966 ("net: 
protect NAPI enablement with netdev_lock()").

Thanks,
Quang Minh.