[PATCH] sched_clock: prevent scd->clock from moving backwards

[Dave Kleikamp <shaggy@linux.vnet.ibm.com>]

On Thu, 2008-10-09 at 12:54 -0500, Dave Kleikamp wrote:

> I'll fix the patch and retest it before sending it again.

I'm definitely running with the new patch now.  Here goes:
 -------------------------------------------------
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>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>

diff --git a/kernel/sched_clock.c b/kernel/sched_clock.c
index e8ab096..8178724 100644
--- a/kernel/sched_clock.c
+++ b/kernel/sched_clock.c
@@ -118,13 +118,13 @@ static u64 __update_sched_clock(struct sched_clock_data *scd, u64 now)
 
 	/*
 	 * 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);

-- 
David Kleikamp
IBM Linux Technology Center