linux-kernel - Re: [alsa-devel] unload Audio drivers while playback stream is active case kernel crash

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Message-ID: <54B63E4F.2010506@metafoo.de>
Date:	Wed, 14 Jan 2015 11:00:47 +0100
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Takashi Iwai <tiwai@...e.de>
CC:	Mark Brown <broonie@...nel.org>,
	"alsa-devel@...a-project.org" <alsa-devel@...a-project.org>,
	"Wang, Jiada (ESD)" <Jiada_Wang@...tor.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"lgirdwood@...il.com" <lgirdwood@...il.com>,
	"Frkuska, Joshua" <Joshua_Frkuska@...tor.com>
Subject: Re: [alsa-devel] unload Audio drivers while playback stream is active
 case kernel crash

On 01/14/2015 09:47 AM, Takashi Iwai wrote:
> At Wed, 14 Jan 2015 09:15:36 +0100,
> Lars-Peter Clausen wrote:
>>
>> On 01/14/2015 08:43 AM, Takashi Iwai wrote:
>>> At Tue, 13 Jan 2015 21:54:12 +0000,
>>> Mark Brown wrote:
>>>>
>>>> On Tue, Jan 13, 2015 at 06:24:44PM +0100, Takashi Iwai wrote:
>>>>> Wang, Jiada (ESD) wrote:
>>>>
>>>>>> I am using i.MX6Q sabreSD board, which have imx_wm892 machine driver, wm8962 codec and SSI CPU DAI,
>>>>
>>>>>> I got Kernel crash when unloading audio drivers (playback stream is active)
>>>>>> modprobe -r snd_soc_imx_wm8962
>>>>>> modprobe -r snd_soc_fsl_ssi
>>>>>> modprobe -r snd_soc_wm8962
>>>>
>>>>> The root problem is that you can unload the module while playing.
>>>>> The corresponding module refcounts should have been increased during
>>>>> used.
>>>>
>>>>> Do we miss [try_]module_get() somewhere in ASoC?
>>>>
>>>> That doesn't help, users can still forcibly unbind the driver at runtime
>>>> without loading the module - and there's always the potential for
>>>> actually hotpluggable hardware.  The teardown paths should be able to
>>>> cope somewhat gracefully.
>>>
>>> The module refcount has to be handled while being used for stopping
>>> module unload.  That's irrelevant from the dynamic unbinding support
>>> itself.  Of course, the module refcount doesn't save the world, but
>>> it's the right fix for this particular scenario.
>>
>> Refcounting won't help in this case. The issue is caused by a delayed work
>> item that gets launched when the PCM stream is stopped. So if you decrease
>> the refcount when the stream is stopped you still have a window where it is
>> possible to remove the module while the work is still being scheduled.
>
> OK, so it's not about active stream.  From the reporter's description,
> I supposed that the module gets unloaded while playing a stream, which
> shouldn't be allowed.

Well one of the ways to trigger this is to remove the module while the 
stream is active. But it is not exclusively a problem module unload problem. 
E.g. the same happens if you hot-unplug the ASoC card.

I don't think that we need to prevent module unload when a stream is active. 
 From a framework point of view is not different from hot-unplug. I don't 
see a reason why we'd jump through hoops to actively forbid removing the 
module once it works just fine.

- Lars
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