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

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Date:	Mon, 04 Apr 2011 04:12:48 +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

I added some printk entries and finally got suspicious output after

rmmod cx88_blackbird ; modprobe cx88_blackbird debug=true

[...]
cx88[0]/2: registered device video3 [mpeg]
cx88[0]/2-bb: mpeg_open
[cx88-blackbird.c,mpeg_open(),line 1059] mutex_lock(&dev->core->lock);
cx88[0]/2-bb: Initialize codec
[...]
cx88[0]/2-bb: open dev=video3
[cx88-blackbird.c,mpeg_open(),line 1103] mutex_unlock(&dev->core->lock); 
// normal exit
[...]
cx88[1]/2: subsystem: 0070:9601, board: Hauppauge WinTV-HVR1300 
DVB-T/Hybrid MPEG Encoder [card=56]
[cx88-blackbird.c,mpeg_release(),line 1122] mutex_lock(&dev->core->lock);
cx88[0] core->active_ref=0
[cx88-mpeg.c,cx8802_request_release(),line 655]                         
   // BANG !!!!!!!!!!!!!!! core->active_ref=-1
[cx88-blackbird.c,mpeg_release(),line 1129] drv->request_release(drv); 
// drv->core->active_ref=(0->0)
[cx88-blackbird.c,mpeg_release(),line 1133] 
mutex_unlock(&dev->core->lock); // normal exit
cx88[1]/2-bb: cx8802_blackbird_probe
[...]

Analyzing this lead me to the conclusion, that a call to mpeg_open() in 
cx88-blackbird.c
returns with 0 (= success)
while not actually having increased the active_ref count.

Here's the relevant code fragment ...

static int mpeg_open(struct file *file)
{
[...]
     /* Make sure we can acquire the hardware */
     drv = cx8802_get_driver(dev, CX88_MPEG_BLACKBIRD);      // HOW TO 
DEAL WITH NULL  ??????
     if (drv) {
         err = drv->request_acquire(drv);
         if(err != 0) {
             dprintk(1,"%s: Unable to acquire hardware, %d\n", __func__, 
err);
             mutex_unlock(&dev->core->lock);
             return err;
         }
     }
[...]
     return 0;
}

I suspect, that

drv = cx8802_get_driver(dev, CX88_MPEG_BLACKBIRD);

in my case might evaluates to NULL (not normally but during driver 
initialization!?)
which then leads to

1) mpeg_open() returns with success

and

2) active_ref count has not been increased

which results in a negative active_ref count later on.

But I might as well be totally wrong.

Andi

On 02.04.2011 22:13, Andreas Huber wrote:
> 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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