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

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Message-ID: <CACGkMEvFzYiRNxMdJ9xNPcZmotY-9pD+bfF4BD5z+HnaAt1zug@mail.gmail.com>
Date: Tue, 2 Dec 2025 14:03:32 +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 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.

Thanks