[PATCH v2] sched/fair: Fix integer underflow

[Pierre Gondois <pierre.gondois@arm.com>]

(struct sg_lb_stats).idle_cpus is of type 'unsigned int'.
(local->idle_cpus - busiest->idle_cpus) can underflow to UINT_MAX
for instance, and max_t(long, 0, UINT_MAX) will output UINT_MAX.

Use lsub_positive() instead of max_t().

Fixes: 16b0a7a1a0af ("sched/fair: Ensure tasks spreading in LLC during LB")
cc: stable@vger.kernel.org
Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
---
 kernel/sched/fair.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 9057584ec06d..6d9124499f52 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10775,8 +10775,8 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
 			 * idle CPUs.
 			 */
 			env->migration_type = migrate_task;
-			env->imbalance = max_t(long, 0,
-					       (local->idle_cpus - busiest->idle_cpus));
+			env->imbalance = local->idle_cpus;
+			lsub_positive(&env->imbalance, busiest->idle_cpus);
 		}
 
 #ifdef CONFIG_NUMA
-- 
2.25.1