sched/fair: Avoid integer overflow

sa->runnable_avg_sum is of type u32 but after shifting it by NICE_0_SHIFT
bits it is promoted to u64.  This of course makes no sense, since the
result will never be more then 32-bit long.  Casting sa->runnable_avg_sum
to u64 before it is shifted, fixes this problem.

Reviewed-by: Ben Segall <[email protected]>
Signed-off-by: Michal Nazarewicz <[email protected]>
Signed-off-by: Peter Zijlstra <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

kernel/sched/fair.c

@@ -2178,7 +2178,7 @@ static inline void __update_tg_runnable_avg(struct sched_avg *sa,
 	long contrib;
 
 	/* The fraction of a cpu used by this cfs_rq */
-	contrib = div_u64(sa->runnable_avg_sum << NICE_0_SHIFT,
+	contrib = div_u64((u64)sa->runnable_avg_sum << NICE_0_SHIFT,
 			  sa->runnable_avg_period + 1);
 	contrib -= cfs_rq->tg_runnable_contrib;