linux-kernel - Re: Public ridicule due to sound/soc/soc-core.c abuse of the driver model

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Message-ID: <20120106205036.GB13857@n2100.arm.linux.org.uk>
Date:	Fri, 6 Jan 2012 20:50:36 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Mark Brown <broonie@...nsource.wolfsonmicro.com>
Cc:	Greg KH <greg@...ah.com>,
	Frank Mandarino <fmandarino@...relia.com>,
	Liam Girdwood <lrg@...com>, Jaroslav Kysela <perex@...ex.cz>,
	Takashi Iwai <tiwai@...e.de>, alsa-devel@...a-project.org,
	linux-kernel@...r.kernel.org
Subject: Re: Public ridicule due to sound/soc/soc-core.c abuse of the
	driver model

On Fri, Jan 06, 2012 at 12:15:01PM -0800, Mark Brown wrote:
> On Fri, Jan 06, 2012 at 11:40:52AM -0800, Greg KH wrote:
> 
> > It was recently pointed out to me that the sound/soc/soc-core.c is
> > flagrantly abusing the driver model by providing "empty" release
> > functions, just to keep the kernel from complaining:
> 
> > Come on people, do you think that I wrote the code in the kernel that
> > produces those errors just for fun?  It was telling you to fix your code
> > by providing a function to free the structure that is being released,
> > not to try to trick the kernel because you think you know better.  The
> > kernel was trying to help you out here, to get the programming model
> > correct, in a place that was commonly misunderstood.
> 
> The problem is that due to the entertaining nature of AC'97 support in
> Linux we don't actually have anything to free at this point - we'd need
> to redo the whole infrastructure, not just this code.

The fact is that these bugs are oopses waiting to happen.  All you need
is the kernel to hold a reference to the kobject underlying the struct
device, and then release that reference after you've kfree'd the structure
containing the struct device, and the kernel will go bang.

I also disagree with your statement - I think it's quite simple to fix:

1. Change to using a flag to indicate whether the struct device is
   registered rather than ac97->dev.bus.
2. Change snd_soc_new_ac97_codec() to initialize codec->ac97->dev,
   including calling device_initialize() on it and setting the release
   function.
3. Change device_register(&codec->ac97->dev); to
   device_add(&codec->ac97->dev);
4. Make the AC97 device release function do the freeing of 'codec->ac97'
5. Change the current device_unregister(&codec->ac97->dev); to
   device_del(&codec->ac97->dev);
6. Change the various kfree(codec->ac97); in the file to do a
   device_put(&codec->ac97->dev);

This will cause device_put() to check the refcount, when that hits
zero it will call the release function, and that in turn will then
kfree() the ac97 structure.

If someone else is holding a reference to this struct device, kfree
will be called once that reference is dropped - and this is the
exact behaviour the driver model as a whole requires.

At least this gets the AC97 ASoC stuff fixed, and one less abuse of the
driver model in the kernel.
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