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

[Takashi Iwai]

On Fri, 31 May 2019 15:18:03 +0200,
Ranjani Sridharan wrote:
> 
> 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. 

So it's only about hdac_hdmi codec, or only about hdac_hda codec?

And why HDMI codec isn't managed by the card...?  IOW, isn't it
dangerous -- it means the codec being always removable after bound to
the card?


thanks,

Takashi