Re: [PATCH] ASoC: cs42l43: Move shutter IRQ handling into a worker thread

[Charles Keepax <ckeepax@xxxxxxxxxxxxxxxxxxxxx>]

On Mon, Jul 29, 2024 at 09:30:00PM +0200, Takashi Iwai wrote:
> On Mon, 29 Jul 2024 18:44:59 +0200,
> Charles Keepax wrote:
> > 
> > On Mon, Jul 29, 2024 at 06:18:50PM +0200, Takashi Iwai wrote:
> > > On Mon, 29 Jul 2024 17:59:32 +0200,
> > > Charles Keepax wrote:
> > > > 
> > > > The microphone/speaker privacy shutter ALSA control handlers need to
> > > > call pm_runtime_resume, since the hardware needs to be powered up to
> > > > check the hardware state of the shutter. The IRQ handler for the
> > > > shutters also needs to notify the ALSA control to inform user-space
> > > > the shutters updated. However this leads to a mutex inversion, between
> > > > the sdw_dev_lock and the controls_rwsem.
> > > 
> > > That's bad, how does the mutex inversion look like?  Generally
> > > speaking, a call of snd_ctl_notify() from an irq handler is a very
> > > standard procedure, and it should work without too much workaround.
> > > 
> > 
> > SoundWire IRQs are called under the sdw_dev_lock, and then in the
> > IRQ handler we call snd_ctl_notify which takes controls_rwsem.
> 
> snd_ctl_notify() doesn't take rwsem but ctl_files_rwlock.
> And rwlock isn't taken except for two cases, at a list traverse at
> enumerating from user-space and at the device disconnection, so it
> shouldn't race.  Anything missing there...?
> 

The trouble here isn't the snd_ctl_notify, the handler uses
snd_soc_component_notify_control which also does a
snd_soc_card_get_kcontrol, which eventually calls snd_ctl_find_id
which does take the controls_rwsem.

Thanks,
Charles