netdev - Re: [PATCH] virtio-net: disable delayed refill when pausing rx

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Message-ID: <CACGkMEs7O7D5sztwJVn45c+1pap20Oi5f=02Sy_qxFjbeHuYiQ@mail.gmail.com>
Date: Tue, 8 Apr 2025 15:34:33 +0800
From: Jason Wang <jasowang@...hat.com>
To: Bui Quang Minh <minhquangbui99@...il.com>
Cc: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, "Michael S . Tsirkin" <mst@...hat.com>, 
	Andrew Lunn <andrew+netdev@...n.ch>, 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>, Eugenio Pérez <eperezma@...hat.com>, 
	"David S . Miller" <davem@...emloft.net>, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
	bpf@...r.kernel.org, virtualization@...ts.linux.dev
Subject: Re: [PATCH] virtio-net: disable delayed refill when pausing rx

On Mon, Apr 7, 2025 at 10:27 AM Bui Quang Minh <minhquangbui99@...il.com> wrote:
>
> On 4/7/25 08:03, Xuan Zhuo wrote:
> > On Fri,  4 Apr 2025 16:39:03 +0700, Bui Quang Minh <minhquangbui99@...il.com> wrote:
> >> When pausing rx (e.g. set up xdp, xsk pool, rx resize), we call
> >> napi_disable() on the receive queue's napi. In delayed refill_work, it
> >> also calls napi_disable() on the receive queue's napi. This can leads to
> >> deadlock when napi_disable() is called on an already disabled napi. This
> >> scenario can be reproducible by binding a XDP socket to virtio-net
> >> interface without setting up the fill ring. As a result, try_fill_recv
> >> will fail until the fill ring is set up and refill_work is scheduled.
> >
> > So, what is the problem? The refill_work is waiting? As I know, that thread
> > will sleep some time, so the cpu can do other work.
>
> When napi_disable is called on an already disabled napi, it will sleep
> in napi_disable_locked while still holding the netdev_lock. As a result,
> later napi_enable gets stuck too as it cannot acquire the netdev_lock.
> This leads to refill_work and the pause-then-resume tx are stuck altogether.

This needs to be added to the chagelog. And it looks like this is a fix for

commit 413f0271f3966e0c73d4937963f19335af19e628
Author: Jakub Kicinski <kuba@...nel.org>
Date:   Tue Jan 14 19:53:14 2025 -0800

    net: protect NAPI enablement with netdev_lock()

?

I wonder if it's simpler to just hold the netdev lock in resize or xsk
binding instead of this.

Thanks

>
> Thanks,
> Quang Minh.
>