linux-kernel - [PATCH 5.15 02/83] ALSA: hda: Fix hang at HD-audio codec unbinding due to refcount saturation

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Message-Id: <20221003070722.038362006@linuxfoundation.org>
Date:   Mon,  3 Oct 2022 09:10:27 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Ville Syrjälä 
        <ville.syrjala@...ux.intel.com>, Takashi Iwai <tiwai@...e.de>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.15 02/83] ALSA: hda: Fix hang at HD-audio codec unbinding due to refcount saturation

From: Takashi Iwai <tiwai@...e.de>

[ Upstream commit ead3d3c5b54f76da79c079e61bacb4279ec56965 ]

We fixed the potential deadlock at dynamic unbinding the HD-audio
codec at the commit 7206998f578d ("ALSA: hda: Fix potential deadlock
at codec unbinding"), but ironically, this caused another potential
deadlock.  The current code uses refcount_dec() and waits for the
pending task with wait_event for dropping the refcount to 0.  This
works fine when PCMs are assigned and actually waiting for the
refcount drop.

Meanwhile, when there was no PCM assigned, the refcount_dec() call
itself was supposed to drop to zero -- alas, it doesn't in reality;
refcount_dec() complains, spews kernel warning and it saturates
instead of dropping to 0, due to the nature of refcount_dec()
implementation.  This eventually blocks the wait_event() wakeup and
the code get stuck there.

For avoiding the problem, we call refcount_dec_and_test() and skips
the sync-wait if it already reaches to zero.

The patch does a slight code reshuffling to make sure to invoke other
disconnect calls before the sync-wait, too.

Fixes: 7206998f578d ("ALSA: hda: Fix potential deadlock at codec unbinding")
Reported-by: Ville Syrjälä <ville.syrjala@...ux.intel.com>
Tested-by: Ville Syrjälä <ville.syrjala@...ux.intel.com>
Cc: <stable@...r.kernel.org>
Link: https://lore.kernel.org/r/YxtflWQnslMHVlU7@intel.com
Link: https://lore.kernel.org/r/20220910142550.28494-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@...e.de>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 sound/pci/hda/hda_bind.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/sound/pci/hda/hda_bind.c b/sound/pci/hda/hda_bind.c
index c572fb5886d5..7af251573595 100644
--- a/sound/pci/hda/hda_bind.c
+++ b/sound/pci/hda/hda_bind.c
@@ -157,10 +157,10 @@ 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))
+		wait_event(codec->remove_sleep, !refcount_read(&codec->pcm_ref));
 	snd_power_sync_ref(codec->bus->card);

 	if (codec->patch_ops.free)
-- 
2.35.1