linux-kernel - Re: [PATCH 3/3] [media] cx88: use a mutex to protect cx8802

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Message-ID: <4D978378.7060106@gmx.at>
Date:	Sat, 02 Apr 2011 22:13:44 +0200
From:	Andreas Huber <hobrom@....at>
To:	Jonathan Nieder <jrnieder@...il.com>
CC:	linux-media@...r.kernel.org, Huber Andreas <hobrom@...ax.at>,
	Mauro Carvalho Chehab <mchehab@...hat.com>,
	Hans Verkuil <hverkuil@...all.nl>,
	linux-kernel@...r.kernel.org, andrew.walker27@...world.com,
	Ben Hutchings <ben@...adent.org.uk>,
	Trent Piepho <xyzzy@...akeasy.org>,
	Roland Stoll <dvb.rs@...dex.de>
Subject: Re: [PATCH 3/3] [media] cx88: use a mutex to protect cx8802_devlist

Hi Jonathan, thanks for locking into it.
I'll try to debug more deeply what's going wrong and keep you up to date.
Andi.

On 02.04.2011 21:29, Jonathan Nieder wrote:
> Hi Andreas,
>
> (please turn off HTML mail.)
> Andreas Huber wrote:
>
>> There is a reference count bug in the driver code. The driver's
>> active_ref count may become negative which leads to unpredictable
>> behavior. (mpeg video device inaccessible, etc ...)
> Hmm, the patchset didn't touch active_ref handling.
>
> active_ref was added by v2.6.25-rc3~132^2~7 (V4L/DVB (7194):
> cx88-mpeg: Allow concurrent access to cx88-mpeg devices, 2008-02-11)
> and relies on three assumptions:
>
>   * (successful) calls to cx8802_driver::request_acquire are balanced
>     with calls to cx8802_driver::request_release;
>
>   * cx8802_driver::advise_acquire is non-null if and only if
>     cx8802_driver::advise_release is (since both are NULL for
>     blackbird, non-NULL for dvb);
>
>   * no data races.
>
> I suppose it would be more idiomatic to use an atomic_t, but access to
> active_ref was previously protected by the BKL and now it is protected
> by core->lock.  So it's not clear to me why this doesn't work.
>
> Any hints?  (e.g., a detailed reproduction recipe, or a log after
> adding a printk to find out when exactly active_ref becomes negative)
>
> Thanks for reporting.
> Jonathan

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