Re: hda codec unbind refcount hang

[Takashi Iwai <tiwai@suse.de>]

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().


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)