sched/topology: Check variable group before dereferencing it

The 'group' variable in sched_domain_debug_one() is not checked
when firstly used in cpumask_test_cpu(cpu, sched_group_span(group)),
but it might be NULL (it is checked later in the following while loop)
and may cause NULL pointer dereference.

We need to check it before using to avoid NULL dereference.

Signed-off-by: Yi Wang <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Reviewed-by: Jiang Biao <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

kernel/sched/topology.c

@@ -47,7 +47,7 @@ static int sched_domain_debug_one(struct sched_domain *sd, int cpu, int level,
 	if (!cpumask_test_cpu(cpu, sched_domain_span(sd))) {
 		printk(KERN_ERR "ERROR: domain->span does not contain CPU%d\n", cpu);
 	}
-	if (!cpumask_test_cpu(cpu, sched_group_span(group))) {
+	if (group && !cpumask_test_cpu(cpu, sched_group_span(group))) {
 		printk(KERN_ERR "ERROR: domain->groups does not contain CPU%d\n", cpu);
 	}