[PATCH 2/2] ALSA: pcm: fix snd_pcm_link() lockdep splat

[Michał Mirosław]

Add and use snd_pcm_stream_lock_nested() in snd_pcm_link/unlink
implementation.  The code is fine, but generates a lockdep complaint:

============================================
WARNING: possible recursive locking detected
5.7.1mq+ #381 Tainted: G           O
--------------------------------------------
pulseaudio/4180 is trying to acquire lock:
ffff888402d6f508 (&group->lock){-...}-{2:2}, at: snd_pcm_common_ioctl+0xda8/0xee0 [snd_pcm]

but task is already holding lock:
ffff8883f7a8cf18 (&group->lock){-...}-{2:2}, at: snd_pcm_common_ioctl+0xe4e/0xee0 [snd_pcm]

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(&group->lock);
  lock(&group->lock);

 *** DEADLOCK ***

 May be due to missing lock nesting notation

2 locks held by pulseaudio/4180:
 #0: ffffffffa1a05190 (snd_pcm_link_rwsem){++++}-{3:3}, at: snd_pcm_common_ioctl+0xca0/0xee0 [snd_pcm]
 #1: ffff8883f7a8cf18 (&group->lock){-...}-{2:2}, at: snd_pcm_common_ioctl+0xe4e/0xee0 [snd_pcm]
[...]

Cc: stable at vger.kernel.org
Fixes: f57f3df03a8e ("ALSA: pcm: More fine-grained PCM link locking")
Signed-off-by: Michał Mirosław <mirq-linux at rere.qmqm.pl>
---
 sound/core/pcm_native.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
index 3ad399cb6f30..5b36881f7095 100644
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -138,6 +138,16 @@ void snd_pcm_stream_lock_irq(struct snd_pcm_substream *substream)
 }
 EXPORT_SYMBOL_GPL(snd_pcm_stream_lock_irq);
 
+static void snd_pcm_stream_lock_nested(struct snd_pcm_substream *substream)
+{
+	struct snd_pcm_group *group = &substream->self_group;
+
+	if (substream->pcm->nonatomic)
+		mutex_lock_nested(&group->mutex, SINGLE_DEPTH_NESTING);
+	else
+		spin_lock_nested(&group->lock, SINGLE_DEPTH_NESTING);
+}
+
 /**
  * snd_pcm_stream_unlock_irq - Unlock the PCM stream
  * @substream: PCM substream
@@ -2200,7 +2210,7 @@ static int snd_pcm_link(struct snd_pcm_substream *substream, int fd)
 	snd_pcm_stream_unlock_irq(substream);
 
 	snd_pcm_group_lock_irq(target_group, nonatomic);
-	snd_pcm_stream_lock(substream1);
+	snd_pcm_stream_lock_nested(substream1);
 	snd_pcm_group_assign(substream1, target_group);
 	refcount_inc(&target_group->refs);
 	snd_pcm_stream_unlock(substream1);
@@ -2216,7 +2226,7 @@ static int snd_pcm_link(struct snd_pcm_substream *substream, int fd)
 
 static void relink_to_local(struct snd_pcm_substream *substream)
 {
-	snd_pcm_stream_lock(substream);
+	snd_pcm_stream_lock_nested(substream);
 	snd_pcm_group_assign(substream, &substream->self_group);
 	snd_pcm_stream_unlock(substream);
 }
-- 
2.20.1