[alsa-devel] [PATCH] ASoC: hda: increment codec device refcount when it is added to the card

[Ranjani Sridharan]

On Fri, 2019-05-31 at 08:11 +0200, Takashi Iwai wrote:
> On Thu, 30 May 2019 23:00:10 +0200,
> Pierre-Louis Bossart wrote:
> > 
> > 
> > 
> > On 5/30/19 3:18 PM, Ranjani Sridharan wrote:
> > > Calling snd_device_new() makes the codec devices managed by the
> > > card.
> > > So, when the card is removed, the refcount for the codec
> > > device is decremented and results in the codec device's kobject
> > > being cleaned up if the refcount is 0. But, this leads to a NULL
> > > pointer exception while attempting to remove the symlinks when
> > > the
> > > codec driver is released later on. Therefore, increment the codec
> > > device's refcount before adding it to the card to prevent this.
> > 
> > Ranjani, you should add a bit of context for the rest of the
> > list...
> > 
> > This patch suggest a solution to a set of sightings occurring when
> > removing/adding modules in a loop, and the current analysis points
> > to
> > a difference between the way the HDMI and HDaudio codecs are
> > handled.
> > 
> > https://github.com/thesofproject/linux/issues/981
> > https://github.com/thesofproject/linux/issues/966
> > https://github.com/thesofproject/linux/pull/988
> > 
> > Since it's not SOF specific it's better to get feedback directly
> > from
> > the large ALSA community/maintainers. We probably want to focus on
> > the
> > platform-specific/vendor-specific stuff on GitHub and use the
> > mailing
> > list for such framework-level changes.
> 
> Hm, I still wonder why this doens't happen with the HDA legacy.
> 
> What is the shortest way to trigger the bug manually without a
> script?
Hi Takashi,

With SOF, I can reproduce the issue if I just unload the sof_pci_dev
module with rmmod. 

Basically, the remove routine for the SOF pci device, unregisters the
machine driver and then removes the codec device. So the first step of
unregistering the machine driver frees the card which decrements the
refcount for the HDA codec's kobject. In the case of HDMI codec, since
it is not managed by the card, the refcount is not decremented when the
card is removed. 

Thanks,
Ranjani
> 
> 
> thanks,
> 
> Takashi
> 
>