Re: [PATCH 2/2] ASoC: max98090: fix lockdep warning

[Tzung-Bi Shih <tzungbi@google.com>]

On Thu, Jan 9, 2020 at 1:36 PM Tzung-Bi Shih <tzungbi@google.com> wrote:
> On Wed, Jan 8, 2020 at 7:50 PM Marek Szyprowski
> <m.szyprowski@samsung.com> wrote:
> > This fixes the following lockdep warning observed on Exynos4412-based
> > Odroid U3 board:
> > ======================================================
> > -> #1 (&card->controls_rwsem){++++}:
> >        snd_ctl_add_replace+0x3c/0x84
> >        dapm_create_or_share_kcontrol+0x24c/0x2e0
> >        snd_soc_dapm_new_widgets+0x308/0x594
> >        snd_soc_bind_card+0x80c/0xad4
> >        devm_snd_soc_register_card+0x34/0x6c
> >        odroid_audio_probe+0x288/0x34c
> >        platform_drv_probe+0x6c/0xa4

I noticed the stack is a little different than the last time
(odroid_audio_probe vs. asoc_simple_probe).  Did you use the same
machine to test?
>        asoc_simple_probe+0x244/0x4a0
>        platform_drv_probe+0x6c/0xa4
(https://mailman.alsa-project.org/pipermail/alsa-devel/2019-December/160142.html)

> I would like to spend some time to find the root cause.  It would be a
> little challenging though (I have no real runtime to test...).

After a few hours of study, I failed to find the reason to cause the
possible circular locking.  And would need more of your input.

Followed the information provided in the message
(https://mailman.alsa-project.org/pipermail/alsa-devel/2019-December/160535.html).
As the message said "snd_soc_of_get_dai_link_codecs() return
-EPROBE_DEFER".  The snd_soc_of_get_dai_link_codecs( ) is before
devm_snd_soc_register_card( ), and I didn't find any side effects in
odroid_audio_probe( ).

Only a very minor issue: snd_soc_of_put_dai_link_codecs(codec_link)
will be called twice.  One in snd_soc_of_get_dai_link_codecs( ) when
return -EPROBE_DEFER; another one is under the label
"err_put_cpu_dai".
(https://elixir.bootlin.com/linux/v5.5-rc5/source/sound/soc/samsung/odroid.c#L328)
 The code doesn't generate any side effects because of
snd_soc_of_put_dai_link_codecs( )'s robustness.

Another minor thing: odroid_card_dais is not immutable but
odroid_audio_probe( ) would try to modify it
(https://elixir.bootlin.com/linux/v5.5-rc5/source/sound/soc/samsung/odroid.c#L295).
Again, I don't think it would produce any troubles.  I guess no
machine would have multiple sound cards, share the same machine
driver, and unbind/bind in runtime.

> It is weird: userspace should not see things (e.g. no controlC0) until
> snd_card_register( ).

(based on today's broonie/sound.git/for-next) I would like to provide
you more information about this statement to help you find further
information.
When userspace can see the control device?  Ideally,
snd_soc_bind_card( ) -> snd_card_register( ) ->
snd_device_register_all( ) -> __snd_device_register( ) ->
snd_ctl_dev_register( ) -> snd_register_device( ).
If you look at the calling stack of possible circular locking,
snd_soc_dapm_new_widgets( ) is before snd_card_register( ).  That's
why we think userspace should not see control devices (i.e. controlC0,
controlC1, ...) and should not be able to set mixer control via ioctl(
).


As this may not directly be related to the issue, could you share the
init script of alsactl in your system?  Does it follow the convention?
 (i.e. sound card is ready when receives controlC* changed event in
udev rule 78-sound-card.rules)
> 6. when userspace init scripts (alsactl) enumerates devices
(https://mailman.alsa-project.org/pipermail/alsa-devel/2019-December/160535.html)