ipc,msg: introduce lockless functions to obtain the ipc object

Add msq_obtain_object() and msq_obtain_object_check(), which will allow
us to get the ipc object without acquiring the lock.  Just as with
semaphores, these functions are basically wrappers around
ipc_obtain_object*().

Signed-off-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

ipc/msg.c

@@ -166,6 +166,27 @@ static inline struct msg_queue *msg_lock_check(struct ipc_namespace *ns,
 	return container_of(ipcp, struct msg_queue, q_perm);
 }
 
+static inline struct msg_queue *msq_obtain_object(struct ipc_namespace *ns, int id)
+{
+	struct kern_ipc_perm *ipcp = ipc_obtain_object(&msg_ids(ns), id);
+
+	if (IS_ERR(ipcp))
+		return ERR_CAST(ipcp);
+
+	return container_of(ipcp, struct msg_queue, q_perm);
+}
+
+static inline struct msg_queue *msq_obtain_object_check(struct ipc_namespace *ns,
+							int id)
+{
+	struct kern_ipc_perm *ipcp = ipc_obtain_object_check(&msg_ids(ns), id);
+
+	if (IS_ERR(ipcp))
+		return ERR_CAST(ipcp);
+
+	return container_of(ipcp, struct msg_queue, q_perm);
+}
+
 static inline void msg_rmid(struct ipc_namespace *ns, struct msg_queue *s)
 {
 	ipc_rmid(&msg_ids(ns), &s->q_perm);