sched: Fix dynamic power-balancing crash

This crash:
    
[ 1774.088275] divide error: 0000 [#1] SMP
[ 1774.100355] CPU 13
[ 1774.102498] Modules linked in:
[ 1774.105631] Pid: 30881, comm: hackbench Not tainted 2.6.31-rc8-tip-01308-g484d664-dirty #1629 X8DTN
[ 1774.114807] RIP: 0010:[<ffffffff81041c38>]  [<ffffffff81041c38>]
sched_balance_self+0x19b/0x2d4
    
Triggers because update_group_power() modifies the sd tree and does
temporary calculations there - not considering that other CPUs
could observe intermediate values, such as the zero initial value.
    
Calculate it in a temporary variable instead. (we need no memory
barrier as these are all statistical values anyway)
    
Got the same oops with the backport to -rt
Signed-off-by: Dinakar Guniguntala <dino@in.ibm.com>
Cc: John Stultz <johnstul@us.ibm.com>
Cc: Darren Hart <dvhltc@us.ibm.com>
Cc: John Kacur <jkacur@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/sched.c

@@ -3864,19 +3864,22 @@ static void update_group_power(struct sched_domain *sd, int cpu)
 {
 	struct sched_domain *child = sd->child;
 	struct sched_group *group, *sdg = sd->groups;
+	unsigned long power;
 
 	if (!child) {
 		update_cpu_power(sd, cpu);
 		return;
 	}
 
-	sdg->cpu_power = 0;
+	power = 0;
 
 	group = child->groups;
 	do {
-		sdg->cpu_power += group->cpu_power;
+		power += group->cpu_power;
 		group = group->next;
 	} while (group != child->groups);
+
+	sdg->cpu_power = power;
 }
 
 /**