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>

1 files changed, 3 insertions, 3 deletions

diff --git a/kernel/sched_clock.c b/kernel/sched_clock.c
index e8ab096ddfe3..81787248b60f 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);