pidns: make create_pid_namespace() accept parent pidns

create_pid_namespace() creates everything, but caller has to assign parent
pidns by hand, which is unnatural.  At the moment of call new ->level has
to be taken from somewhere and parent pidns is already available.
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Acked-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Reviewed-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/pid_namespace.c

@@ -67,9 +67,10 @@ err_alloc:
 	return NULL;
 }
 
-static struct pid_namespace *create_pid_namespace(unsigned int level)
+static struct pid_namespace *create_pid_namespace(struct pid_namespace *parent_pid_ns)
 {
 	struct pid_namespace *ns;
+	unsigned int level = parent_pid_ns->level + 1;
 	int i;
 
 	ns = kmem_cache_zalloc(pid_ns_cachep, GFP_KERNEL);
@@ -86,6 +87,7 @@ static struct pid_namespace *create_pid_namespace(unsigned int level)
 
 	kref_init(&ns->kref);
 	ns->level = level;
+	ns->parent = get_pid_ns(parent_pid_ns);
 
 	set_bit(0, ns->pidmap[0].page);
 	atomic_set(&ns->pidmap[0].nr_free, BITS_PER_PAGE - 1);
@@ -125,9 +127,7 @@ struct pid_namespace *copy_pid_ns(unsigned long flags, struct pid_namespace *old
 	if (flags & CLONE_THREAD)
 		goto out_put;
 
-	new_ns = create_pid_namespace(old_ns->level + 1);
-	if (!IS_ERR(new_ns))
-		new_ns->parent = get_pid_ns(old_ns);
+	new_ns = create_pid_namespace(old_ns);
 
 out_put:
 	put_pid_ns(old_ns);