Re: hda codec unbind refcount hang

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Fri, Sep 09, 2022 at 05:59:47PM +0200, Takashi Iwai wrote:
> On Fri, 09 Sep 2022 17:45:25 +0200,
> Ville Syrjälä wrote:
> > 
> > Hi Takashi,
> > 
> > commit 7206998f578d ("ALSA: hda: Fix potential deadlock at codec
> > unbinding") introduced a problem on at least one of my older machines.
> > 
> > The problem happens when hda_codec_driver_remove() encounters a
> > codec without any pcms (and thus the refcount is 1) and tries to
> > call refcount_dec(). Turns out refcount_dec() doesn't like to be
> > used for dropping the refcount to 0, and instead if spews a warning
> > and does its saturate thing. The subsequent wait_event() is then
> > permanently stuck waiting on the saturated refcount.
> > 
> > I've definitely seen the same kind of pattern used elsewhere
> > in the kernel as well, so the fact that refcount_t can't be used
> > to implement it is a bit of surprise to me. I guess most other
> > places still use atomic_t instead.
> 
> Does the patch below work around it?  It seem to be a subtle
> difference between refcount_dec() and refcount_dec_and_test().

Aye, this works.

Tested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

> 
> 
> thanks,
> 
> Takashi
> 
> -- 8< --
> --- a/sound/pci/hda/hda_bind.c
> +++ b/sound/pci/hda/hda_bind.c
> @@ -157,10 +157,11 @@ static int hda_codec_driver_remove(struct device *dev)
>  		return codec->bus->core.ext_ops->hdev_detach(&codec->core);
>  	}
>  
> -	refcount_dec(&codec->pcm_ref);
> -	snd_hda_codec_disconnect_pcms(codec);
> -	snd_hda_jack_tbl_disconnect(codec);
> -	wait_event(codec->remove_sleep, !refcount_read(&codec->pcm_ref));
> +	if (!refcount_dec_and_test(&codec->pcm_ref)) {
> +		snd_hda_codec_disconnect_pcms(codec);
> +		snd_hda_jack_tbl_disconnect(codec);
> +		wait_event(codec->remove_sleep, !refcount_read(&codec->pcm_ref));
> +	}
>  	snd_power_sync_ref(codec->bus->card);
>  
>  	if (codec->patch_ops.free)

-- 
Ville Syrjälä
Intel