cpuset: hold callback_mutex in cpuset_post_clone()

Chaning cpuset->mems/cpuset->cpus should be protected under
callback_mutex.

cpuset_clone() doesn't follow this rule. It's ok because it's
called when creating and initializing a cgroup, but we'd better
hold the lock to avoid subtil break in the future.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Paul Menage <menage@google.com>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/cpuset.c

@@ -1840,8 +1840,10 @@ static void cpuset_post_clone(struct cgroup_subsys *ss,
 	cs = cgroup_cs(cgroup);
 	parent_cs = cgroup_cs(parent);
 
+	mutex_lock(&callback_mutex);
 	cs->mems_allowed = parent_cs->mems_allowed;
 	cpumask_copy(cs->cpus_allowed, parent_cs->cpus_allowed);
+	mutex_unlock(&callback_mutex);
 	return;
 }