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

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Message-ID: <CACGkMEtpARauj6GSZu+iY3Lx=c+rq_C019r4E-eisx2mujB6=A@mail.gmail.com>
Date: Wed, 3 Dec 2025 14:37:35 +0800
From: Jason Wang <jasowang@...hat.com>
To: Bui Quang Minh <minhquangbui99@...il.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 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?

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

Thanks