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    Subject[PATCH 3.13.y-ckt 097/136] ALSA: hrtimer: Fix stall by hrtimer_cancel()
    Date
    3.13.11-ckt34 -stable review patch.  If anyone has any objections, please let me know.

    ---8<------------------------------------------------------------

    From: Takashi Iwai <tiwai@suse.de>

    commit 2ba1fe7a06d3624f9a7586d672b55f08f7c670f3 upstream.

    hrtimer_cancel() waits for the completion from the callback, thus it
    must not be called inside the callback itself. This was already a
    problem in the past with ALSA hrtimer driver, and the early commit
    [fcfdebe70759: ALSA: hrtimer - Fix lock-up] tried to address it.

    However, the previous fix is still insufficient: it may still cause a
    lockup when the ALSA timer instance reprograms itself in its callback.
    Then it invokes the start function even in snd_timer_interrupt() that
    is called in hrtimer callback itself, results in a CPU stall. This is
    no hypothetical problem but actually triggered by syzkaller fuzzer.

    This patch tries to fix the issue again. Now we call
    hrtimer_try_to_cancel() at both start and stop functions so that it
    won't fall into a deadlock, yet giving some chance to cancel the queue
    if the functions have been called outside the callback. The proper
    hrtimer_cancel() is called in anyway at closing, so this should be
    enough.

    Reported-and-tested-by: Dmitry Vyukov <dvyukov@google.com>
    Signed-off-by: Takashi Iwai <tiwai@suse.de>
    Signed-off-by: Kamal Mostafa <kamal@canonical.com>
    ---
    sound/core/hrtimer.c | 3 ++-
    1 file changed, 2 insertions(+), 1 deletion(-)

    diff --git a/sound/core/hrtimer.c b/sound/core/hrtimer.c
    index b8b31c4..14d483d 100644
    --- a/sound/core/hrtimer.c
    +++ b/sound/core/hrtimer.c
    @@ -90,7 +90,7 @@ static int snd_hrtimer_start(struct snd_timer *t)
    struct snd_hrtimer *stime = t->private_data;

    atomic_set(&stime->running, 0);
    - hrtimer_cancel(&stime->hrt);
    + hrtimer_try_to_cancel(&stime->hrt);
    hrtimer_start(&stime->hrt, ns_to_ktime(t->sticks * resolution),
    HRTIMER_MODE_REL);
    atomic_set(&stime->running, 1);
    @@ -101,6 +101,7 @@ static int snd_hrtimer_stop(struct snd_timer *t)
    {
    struct snd_hrtimer *stime = t->private_data;
    atomic_set(&stime->running, 0);
    + hrtimer_try_to_cancel(&stime->hrt);
    return 0;
    }

    --
    1.9.1
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