[PATCH 4.9 27/51] ALSA: pcm: Call snd_pcm_unlink() conditionally at closing

[Greg Kroah-Hartman]

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Takashi Iwai <tiwai@xxxxxxx>

commit b51abed8355e5556886623b2772fa6b7598d2282 upstream.

Currently the PCM core calls snd_pcm_unlink() always unconditionally
at closing a stream.  However, since snd_pcm_unlink() invokes the
global rwsem down, the lock can be easily contended.  More badly, when
a thread runs in a high priority RT-FIFO, it may stall at spinning.

Basically the call of snd_pcm_unlink() is required only for the linked
streams that are already rare occasion.  For normal use cases, this
code path is fairly superfluous.

As an optimization (and also as a workaround for the RT problem
above in normal situations without linked streams), this patch adds a
check before calling snd_pcm_unlink() and calls it only when needed.

Reported-by: Chanho Min <chanho.min@xxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 sound/core/pcm_native.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -2225,7 +2225,8 @@ int snd_pcm_hw_constraints_complete(stru
 
 static void pcm_release_private(struct snd_pcm_substream *substream)
 {
-	snd_pcm_unlink(substream);
+	if (snd_pcm_stream_linked(substream))
+		snd_pcm_unlink(substream);
 }
 
 void snd_pcm_release_substream(struct snd_pcm_substream *substream)