From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Message-Id: <20120419210306.357709074@linuxfoundation.org> Date: Thu, 19 Apr 2012 14:03:41 -0700 From: Greg KH To: linux-kernel@vger.kernel.org, stable@vger.kernel.org Cc: torvalds@linux-foundation.org, akpm@linux-foundation.org, alan@lxorguk.ukuu.org.uk, Neal Cardwell , Ben Segall , Ingo Molnar , Thomas Gleixner Subject: [ 38/75] nohz: Fix stale jiffies update in tick_nohz_restart() In-Reply-To: <20120419210322.GA6478@kroah.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: 3.3-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 Link: http://lkml.kernel.org/r/1332875377-23014-1-git-send-email-ncardwell@google.com 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 @@ -538,9 +538,9 @@ static void tick_nohz_restart(struct tic hrtimer_get_expires(&ts->sched_timer), 0)) break; } - /* Update jiffies and reread time */ - tick_do_update_jiffies64(now); + /* Reread time and update jiffies */ now = ktime_get(); + tick_do_update_jiffies64(now); } }