sched: Add missing rcu_dereference() around ->real_parent usage

Wrap another ->real_parent dereference while under rcu_read_lock.
Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Glauber Costa <glommer@parallels.com>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Link: http://lkml.kernel.org/r/20111215164918.GA13003@www.outflux.net
[ tidied up the changelog ]
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched/core.c

@@ -4784,7 +4784,7 @@ void sched_show_task(struct task_struct *p)
 	free = stack_not_used(p);
 #endif
 	printk(KERN_CONT "%5lu %5d %6d 0x%08lx\n", free,
-		task_pid_nr(p), task_pid_nr(p->real_parent),
+		task_pid_nr(p), task_pid_nr(rcu_dereference(p->real_parent)),
 		(unsigned long)task_thread_info(p)->flags);
 
 	show_stack(p, NULL);