sched: check for SD_SERIALIZE atomically in rebalance_domains()

Nothing really serious here, mainly just a matter of nit-picking :-/

From: Dmitry Adamushko <dmitry.adamushko@gmail.com>
For CONFIG_SCHED_DEBUG && CONFIG_SYSCT configs, sd->flags can be altered
while being manipulated in rebalance_domains(). Let's do an atomic check.
We rely here on the atomicity of read/write accesses for aligned words.

Signed-off-by: Dmitry Adamushko <dmitry.adamushko@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/sched.c

diff --git a/kernel/sched.c b/kernel/sched.c
index 8a1257b..90329f1 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3668,6 +3668,7 @@
 	/* Earliest time when we have to do rebalance again */
 	unsigned long next_balance = jiffies + 60*HZ;
 	int update_next_balance = 0;
+	int need_serialize;
 	cpumask_t tmp;
 
 	for_each_domain(cpu, sd) {
@@ -3685,8 +3686,9 @@
 		if (interval > HZ*NR_CPUS/10)
 			interval = HZ*NR_CPUS/10;
 
+		need_serialize = sd->flags & SD_SERIALIZE;
 
-		if (sd->flags & SD_SERIALIZE) {
+		if (need_serialize) {
 			if (!spin_trylock(&balancing))
 				goto out;
 		}
@@ -3702,7 +3704,7 @@
 			}
 			sd->last_balance = jiffies;
 		}
-		if (sd->flags & SD_SERIALIZE)
+		if (need_serialize)
 			spin_unlock(&balancing);
 out:
 		if (time_after(next_balance, sd->last_balance + interval)) {