linux-kernel - [tip:timers/urgent] nohz: Fix stale jiffies update in tick_nohz

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Message-ID: <tip-6f103929f8979d2638e58d7f7fda0beefcb8ee7e@git.kernel.org>
Date:	Fri, 6 Apr 2012 04:34:43 -0700
From:	tip-bot for Neal Cardwell <ncardwell@...gle.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, bsegall@...gle.com,
	ncardwell@...gle.com, hpa@...or.com, mingo@...nel.org,
	tglx@...utronix.de, mingo@...e.hu
Subject: [tip:timers/urgent] nohz:
  Fix stale jiffies update in tick_nohz_restart()

Commit-ID:  6f103929f8979d2638e58d7f7fda0beefcb8ee7e
Gitweb:     http://git.kernel.org/tip/6f103929f8979d2638e58d7f7fda0beefcb8ee7e
Author:     Neal Cardwell <ncardwell@...gle.com>
AuthorDate: Tue, 27 Mar 2012 15:09:37 -0400
Committer:  Thomas Gleixner <tglx@...utronix.de>
CommitDate: Fri, 6 Apr 2012 13:24:17 +0200

nohz: Fix stale jiffies update in tick_nohz_restart()

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 <ncardwell@...gle.com>
Cc: Ben Segall <bsegall@...gle.com>
Cc: Ingo Molnar <mingo@...e.hu>
Cc: stable@...r.kernel.org
Link: http://lkml.kernel.org/r/1332875377-23014-1-git-send-email-ncardwell@google.com
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
 kernel/time/tick-sched.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 3526038..6a3a5b9 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -534,9 +534,9 @@ static void tick_nohz_restart(struct tick_sched *ts, ktime_t now)
 				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);
 	}
 }
 
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