cgroups: add lock for child->cgroups in cgroup_post_fork()

When cgroup_post_fork() is called, child is seen by find_task_by_vpid(),
so child->cgroups maybe be changed, It'll incorrect.

child->cgroups<old>'s refcnt is decreased
child->cgroups<new>'s refcnt is increased
but child->cg_list is added to child->cgroups<old>'s list.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Reviewed-by: Paul Menage <menage@google.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: Balbir Singh <balbir@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/cgroup.c

@@ -2792,8 +2792,10 @@ void cgroup_post_fork(struct task_struct *child)
 {
 	if (use_task_css_set_links) {
 		write_lock(&css_set_lock);
+		task_lock(child);
 		if (list_empty(&child->cg_list))
 			list_add(&child->cg_list, &child->cgroups->tasks);
+		task_unlock(child);
 		write_unlock(&css_set_lock);
 	}
 }