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    Subject[patch 52/58] clockevents: prevent endless loop in tick_handle_periodic()
    2.6.29-stable review patch.  If anyone has any objections, please let us know.

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

    From: john stultz <johnstul@us.ibm.com>

    commit 74a03b69d1b5ce00a568e142ca97e76b7f5239c6 upstream.

    tick_handle_periodic() can lock up hard when a one shot clock event
    device is used in combination with jiffies clocksource.

    Avoid an endless loop issue by requiring that a highres valid
    clocksource be installed before we call tick_periodic() in a loop when
    using ONESHOT mode. The result is we will only increment jiffies once
    per interrupt until a continuous hardware clocksource is available.

    Without this, we can run into a endless loop, where each cycle through
    the loop, jiffies is updated which increments time by tick_period or
    more (due to clock steering), which can cause the event programming to
    think the next event was before the newly incremented time and fail
    causing tick_periodic() to be called again and the whole process loops
    forever.

    [ Impact: prevent hard lock up ]

    Signed-off-by: John Stultz <johnstul@us.ibm.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Cc: stable@kernel.org
    Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

    --- a/kernel/time/tick-common.c
    +++ b/kernel/time/tick-common.c
    @@ -93,7 +93,17 @@ void tick_handle_periodic(struct clock_event_device *dev)
    for (;;) {
    if (!clockevents_program_event(dev, next, ktime_get()))
    return;
    - tick_periodic(cpu);
    + /*
    + * Have to be careful here. If we're in oneshot mode,
    + * before we call tick_periodic() in a loop, we need
    + * to be sure we're using a real hardware clocksource.
    + * Otherwise we could get trapped in an infinite
    + * loop, as the tick_periodic() increments jiffies,
    + * when then will increment time, posibly causing
    + * the loop to trigger again and again.
    + */
    + if (timekeeping_valid_for_hres())
    + tick_periodic(cpu);
    next = ktime_add(next, tick_period);
    }
    }



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