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

[jiwang]

Hi

On 01/14/2015 05:15 PM, 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.
>
> And while we do flush the scheduled work when we remove the ASoC card 
> this is done before snd_card_free() is called. So when snd_card_free() 
> is called it gets re-scheduled again. I think the correct fix is to 
> add a snd_card_disconnect() at the very top of 
> soc_cleanup_card_resources().
when stream is active, snd_card_disconnect() will trigger pcm_close() be 
executed by another thread,
we can't ensure the pcm_close() is executed before the rest of 
soc_cleanup_card_resources().

- Jiada

> - Lars

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