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    Subject[ 25/68] nohz: Fix stale jiffies update in tick_nohz_restart()
    3.2-stable review patch.  If anyone has any objections, please let me know.


    From: Neal Cardwell <>

    commit 6f103929f8979d2638e58d7f7fda0beefcb8ee7e upstream.

    Fix tick_nohz_restart() to not use a stale ktime_t "now" value when
    calling tick_do_update_jiffies64(now).

    If we reach this point in the loop it means that we crossed a tick
    boundary since we grabbed the "now" timestamp, so at this point "now"
    refers to a time in the old jiffy, so using the old value for "now" is
    incorrect, and is likely to give us a stale jiffies value.

    In particular, the first time through the loop the
    tick_do_update_jiffies64(now) call is always a no-op, since the
    caller, tick_nohz_restart_sched_tick(), will have already called
    tick_do_update_jiffies64(now) with that "now" value.

    Note that tick_nohz_stop_sched_tick() already uses the correct
    approach: when we notice we cross a jiffy boundary, grab a new
    timestamp with ktime_get(), and *then* update jiffies.

    Signed-off-by: Neal Cardwell <>
    Cc: Ben Segall <>
    Cc: Ingo Molnar <>
    Signed-off-by: Thomas Gleixner <>
    Signed-off-by: Greg Kroah-Hartman <>

    kernel/time/tick-sched.c | 4 ++--
    1 file changed, 2 insertions(+), 2 deletions(-)

    --- a/kernel/time/tick-sched.c
    +++ b/kernel/time/tick-sched.c
    @@ -508,9 +508,9 @@ static void tick_nohz_restart(struct tic
    hrtimer_get_expires(&ts->sched_timer), 0))
    - /* Update jiffies and reread time */
    - tick_do_update_jiffies64(now);
    + /* Reread time and update jiffies */
    now = ktime_get();
    + tick_do_update_jiffies64(now);

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      Last update: 2012-04-19 23:31    [W:0.022 / U:10.448 seconds]
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