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

[Takashi Iwai <tiwai@xxxxxxx>]

On Tue, 30 Jul 2024 10:43:01 +0200,
Charles Keepax wrote:
> 
> 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.

OK, now it's clearer.  I think we can change snd_ctl_find_*() rather
to take rwlock instead of rwsem.  It's only a quick look-up, hence
rwlock can work well.

Totally untested patch set is below.


Takashi

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