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@xxxxxxxxxxxxxxx>
>
>
> 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
