sched: Mark parent and real_parent as __rcu

The parent and real_parent pointers should be considered __rcu,
since they should be held under either tasklist_lock or
rcu_read_lock.

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Link: http://lkml.kernel.org/r/20111214223925.GA27578@www.outflux.net
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/sched.h

@@ -1330,8 +1330,8 @@ struct task_struct {
 	 * older sibling, respectively.  (p->father can be replaced with 
 	 * p->real_parent->pid)
 	 */
-	struct task_struct *real_parent; /* real parent process */
-	struct task_struct *parent; /* recipient of SIGCHLD, wait4() reports */
+	struct task_struct __rcu *real_parent; /* real parent process */
+	struct task_struct __rcu *parent; /* recipient of SIGCHLD, wait4() reports */
 	/*
 	 * children/sibling forms the list of my natural children
 	 */