cls_cgroup: Fix rcu lockdep warning

Dave reported an rcu lockdep warning on 2.6.35.4 kernel

task->cgroups and task->cgroups->subsys[i] are protected by RCU.
So we avoid accessing invalid pointers here. This might happen,
for example, when you are deref-ing those pointers while someone
move @task from one cgroup to another.
Reported-by: Dave Jones <davej@redhat.com>
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/cls_cgroup.h

@@ -27,11 +27,17 @@ struct cgroup_cls_state
 #ifdef CONFIG_NET_CLS_CGROUP
 static inline u32 task_cls_classid(struct task_struct *p)
 {
+	int classid;
+
 	if (in_interrupt())
 		return 0;
 
-	return container_of(task_subsys_state(p, net_cls_subsys_id),
+	rcu_read_lock();
+	classid = container_of(task_subsys_state(p, net_cls_subsys_id),
 			       struct cgroup_cls_state, css)->classid;
+	rcu_read_unlock();
+
+	return classid;
 }
 #else
 extern int net_cls_subsys_id;