linux-kernel - Re: [PATCH] rfkill: fix deadlock in rfkill_send

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Message-ID: <fec9cfe4436786f6f9b61ee6e9bb7766efb29d34.camel@sipsolutions.net>
Date:   Sat, 14 Oct 2023 22:01:45 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Simon Horman <horms@...nel.org>, Edward AD <twuufnxlz@...il.com>
Cc:     davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
        linux-kernel@...r.kernel.org, linux-wireless@...r.kernel.org,
        netdev@...r.kernel.org, pabeni@...hat.com,
        syzbot+509238e523e032442b80@...kaller.appspotmail.com,
        syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH] rfkill: fix deadlock in rfkill_send_events

On Sat, 2023-10-14 at 09:29 +0200, Simon Horman wrote:
> On Sat, Oct 14, 2023 at 10:43:22AM +0800, Edward AD wrote:
> > Hi Simon Horman,
> > On Fri, 13 Oct 2023 13:06:38 +0200, Simon Horman wrote:
> > > I am wondering if you considered moving the rfkill_sync() calls
> > > to before &data->mtx is taken, to avoid the need to drop and
> > > retake it?
> > If you move rfkill_sync() before calling &data->mtx, more code will be added 
> > because rfkill_sync() is in the loop body.
> 
> Maybe that is true. And maybe that is a good argument for
> not taking the approach that I suggested. But I do think it
> is simpler from a locking perspective, and that has some merit.
> 

FWIW, I missed this patch and discussion until now, but I already fixed
the issue differently:

https://git.kernel.org/pub/scm/linux/kernel/git/wireless/wireless.git/commit/?id=f2ac54ebf85615a6d78f5eb213a8bbeeb17ebe5d

There was never any need to hold the data->mtx for anything but the list
manipulation, and even that isn't _really_ needed since the 'data' is
completely fresh and not seen anywhere else yet.

(I'll also note that the subject of this thread is wrong since this was
never an *actual* deadlock, just a *possible* one reported by lockdep.)

johannes