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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Chanho Min <chanho.min@lge.com>,
        Takashi Iwai <tiwai@suse.de>
Subject: [PATCH 3.18 45/54] ALSA: pcm: Call snd_pcm_unlink() conditionally at closing
Date:   Tue, 11 Dec 2018 16:41:33 +0100
Message-Id: <20181211151548.595323479@linuxfoundation.org>
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3.18-stable review patch.  If anyone has any objections, please let me know.

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

From: Takashi Iwai <tiwai@suse.de>

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@lge.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 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
@@ -2193,7 +2193,8 @@ 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)