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    Subject[PATCH 3.2 39/92] hrtimer: Prevent all reprogramming if hang detected
    3.2.60-rc1 review patch.  If anyone has any objections, please let me know.

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

    From: Stuart Hayes <stuart.w.hayes@gmail.com>

    commit 6c6c0d5a1c949d2e084706f9e5fb1fccc175b265 upstream.

    If the last hrtimer interrupt detected a hang it sets hang_detected=1
    and programs the clock event device with a delay to let the system
    make progress.

    If hang_detected == 1, we prevent reprogramming of the clock event
    device in hrtimer_reprogram() but not in hrtimer_force_reprogram().

    This can lead to the following situation:

    hrtimer_interrupt()
    hang_detected = 1;
    program ce device to Xms from now (hang delay)

    We have two timers pending:
    T1 expires 50ms from now
    T2 expires 5s from now

    Now T1 gets canceled, which causes hrtimer_force_reprogram() to be
    invoked, which in turn programs the clock event device to T2 (5
    seconds from now).

    Any hrtimer_start after that will not reprogram the hardware due to
    hang_detected still being set. So we effectivly block all timers until
    the T2 event fires and cleans up the hang situation.

    Add a check for hang_detected to hrtimer_force_reprogram() which
    prevents the reprogramming of the hang delay in the hardware
    timer. The subsequent hrtimer_interrupt will resolve all outstanding
    issues.

    [ tglx: Rewrote subject and changelog and fixed up the comment in
    hrtimer_force_reprogram() ]

    Signed-off-by: Stuart Hayes <stuart.w.hayes@gmail.com>
    Link: http://lkml.kernel.org/r/53602DC6.2060101@gmail.com
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    kernel/hrtimer.c | 17 +++++++++++++++++
    1 file changed, 17 insertions(+)

    --- a/kernel/hrtimer.c
    +++ b/kernel/hrtimer.c
    @@ -567,6 +567,23 @@ hrtimer_force_reprogram(struct hrtimer_c

    cpu_base->expires_next.tv64 = expires_next.tv64;

    + /*
    + * If a hang was detected in the last timer interrupt then we
    + * leave the hang delay active in the hardware. We want the
    + * system to make progress. That also prevents the following
    + * scenario:
    + * T1 expires 50ms from now
    + * T2 expires 5s from now
    + *
    + * T1 is removed, so this code is called and would reprogram
    + * the hardware to 5s from now. Any hrtimer_start after that
    + * will not reprogram the hardware due to hang_detected being
    + * set. So we'd effectivly block all timers until the T2 event
    + * fires.
    + */
    + if (cpu_base->hang_detected)
    + return;
    +
    if (cpu_base->expires_next.tv64 != KTIME_MAX)
    tick_program_event(cpu_base->expires_next, 1);
    }


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