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    Subject[PATCH 4.4 76/91] ALSA: pcm: Call snd_pcm_unlink() conditionally at closing
    Date
    4.4-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
    @@ -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)

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