linux-kernel - Re: [PATCH net] netrom: fix possible deadlock in nr_rt_device

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Message-ID: <CAEoi9W57D-BfpYUAe5M3zjJvTUQUL4UUB+iWkpRO_o8JWfS7FQ@mail.gmail.com>
Date: Tue, 10 Jun 2025 09:36:44 -0400
From: Dan Cross <crossd@...il.com>
To: David Ranch <linux-hams@...nnet.net>
Cc: Jakub Kicinski <kuba@...nel.org>, Denis Arefev <arefev@...mel.ru>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	Nikita Marushkin <hfggklm@...il.com>, Ilya Shchipletsov <rabbelkin@...l.ru>, Hongbo Li <lihongbo22@...wei.com>, 
	linux-hams@...r.kernel.org, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, lvc-project@...uxtesting.org, 
	stable@...r.kernel.org, syzbot+ccdfb85a561b973219c7@...kaller.appspotmail.com
Subject: Re: [PATCH net] netrom: fix possible deadlock in nr_rt_device_down

On Mon, Jun 9, 2025 at 7:31 PM David Ranch <linux-hams@...nnet.net> wrote:
> That's unclear to me but maybe someone else knowing the code better than
> myself can chime in.  I have to assume having these locks present
> are for a reason.

The suggestion was not to remove locking, but rather, to fold multiple
separate locks into one. That is, have a single lock that covers both
the neighbor list and the node list. Naturally, there would be more
contention around a single lock in contrast to multiple, more granular
locks. But given that NETROM has very low performance requirements,
and that the data that these locks protect doesn't change that often,
that's probably fine and would eliminate the possibility of deadlock
due to lock ordering issues.

        - Dan C.

> On 06/09/2025 04:26 PM, Jakub Kicinski wrote:
> > On Mon, 9 Jun 2025 16:16:32 -0700 David Ranch wrote:
> >> I'm not sure what you mean by "the only user of this code".  There are
> >> many people using the Linux AX.25 + NETROM stack but we unfortunately
> >> don't have a active kernel maintainer for this code today.
> >
> > Alright, sorry. Either way - these locks are not performance critical
> > for you, right?
> >
>