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

[Lars-Peter Clausen]

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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