releases/2.6.27.5/sched_clock-prevent-scd-clock-from-moving-backwards.patch

From cebbert@redhat.com  Tue Nov  4 14:26:49 2008
From: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Date: Sun, 26 Oct 2008 18:20:14 -0400
Subject: sched_clock: prevent scd->clock from moving backwards
To: stable@kernel.org
Cc: Peter Zijlstra <peterz@infradead.org>, Molnar <mingo@elte.hu>, Ingo@hera.kernel.org
Message-ID: <20081026182014.3706d944@redhat.com>

From: Dave Kleikamp <shaggy@linux.vnet.ibm.com>

commit 5b7dba4ff834259a5623e03a565748704a8fe449 upstream

sched_clock: prevent scd->clock from moving backwards

When sched_clock_cpu() couples the clocks between two cpus, it may
increment scd->clock beyond the GTOD tick window that __update_sched_clock()
uses to clamp the clock.  A later call to __update_sched_clock() may move
the clock back to scd->tick_gtod + TICK_NSEC, violating the clock's
monotonic property.

This patch ensures that scd->clock will not be set backward.

Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Cc: Chuck Ebbert <cebbert@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
 kernel/sched_clock.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/kernel/sched_clock.c
+++ b/kernel/sched_clock.c
@@ -118,13 +118,13 @@ static u64 __update_sched_clock(struct s
 
 	/*
 	 * scd->clock = clamp(scd->tick_gtod + delta,
-	 * 		      max(scd->tick_gtod, scd->clock),
-	 * 		      scd->tick_gtod + TICK_NSEC);
+	 *		      max(scd->tick_gtod, scd->clock),
+	 *		      max(scd->clock, scd->tick_gtod + TICK_NSEC));
 	 */
 
 	clock = scd->tick_gtod + delta;
 	min_clock = wrap_max(scd->tick_gtod, scd->clock);
-	max_clock = scd->tick_gtod + TICK_NSEC;
+	max_clock = wrap_max(scd->clock, scd->tick_gtod + TICK_NSEC);
 
 	clock = wrap_max(clock, min_clock);
 	clock = wrap_min(clock, max_clock);