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Subject: Re: [PATCH] b43: Fix locking problem when stopping rfkill polling
From: Johannes Berg <johannes@sipsolutions.net>
To: Larry Finger <Larry.Finger@lwfinger.net>
Cc: "John W. Linville" <linville@tuxdriver.com>,
	linux-wireless@vger.kernel.org
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Date: Thu, 08 Oct 2009 00:36:55 +0200
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On Wed, 2009-10-07 at 14:28 -0500, Larry Finger wrote:

> > OK, but why do we start polling under the lock but stop polling without
> > the lock?  Should we start polling without holding the lock too?
>=20
> I'll test that, but I suspect it doesn't matter. Of course, the reason
> I put the stop under the lock was for symmetry, but then I got the
> following when shutting down:
>=20
>  b43-phy0 debug: Removing Interface type 2
>=20
>  =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D
>  [ INFO: possible circular locking dependency detected ]
>  2.6.32-rc3-wl #225
>  -------------------------------------------------------
>  modprobe/25391 is trying to acquire lock:
>   (&(&rfkill->poll_work)->work){+.+...}, at: [<ffffffff81054a7f>]
> __cancel_work_timer+0xd9/0x224

This is because when stopping polling we need to cancel the work and
sync on it, but when starting it's completely async so starting can be
in any context.

johannes

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