ipc,msg: shorten critical region in msgctl_down

Instead of holding the ipc lock for the entire function, use the
ipcctl_pre_down_nolock and only acquire the lock for specific commands:
RMID and SET.
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

@@ -410,11 +410,10 @@ static int msgctl_down(struct ipc_namespace *ns, int msqid, int cmd,
 	down_write(&msg_ids(ns).rw_mutex);
 	rcu_read_lock();
 
-	ipcp = ipcctl_pre_down(ns, &msg_ids(ns), msqid, cmd,
+	ipcp = ipcctl_pre_down_nolock(ns, &msg_ids(ns), msqid, cmd,
 				      &msqid64.msg_perm, msqid64.msg_qbytes);
 	if (IS_ERR(ipcp)) {
 		err = PTR_ERR(ipcp);
-		/* the ipc lock is not held upon failure */
 		goto out_unlock1;
 	}
 
@@ -422,10 +421,11 @@ static int msgctl_down(struct ipc_namespace *ns, int msqid, int cmd,
 
 	err = security_msg_queue_msgctl(msq, cmd);
 	if (err)
-		goto out_unlock0;
+		goto out_unlock1;
 
 	switch (cmd) {
 	case IPC_RMID:
+		ipc_lock_object(&msq->q_perm);
 		/* freeque unlocks the ipc object and rcu */
 		freeque(ns, ipcp);
 		goto out_up;
@@ -433,9 +433,10 @@ static int msgctl_down(struct ipc_namespace *ns, int msqid, int cmd,
 		if (msqid64.msg_qbytes > ns->msg_ctlmnb &&
 		    !capable(CAP_SYS_RESOURCE)) {
 			err = -EPERM;
-			goto out_unlock0;
+			goto out_unlock1;
 		}
 
+		ipc_lock_object(&msq->q_perm);
 		err = ipc_update_perm(&msqid64.msg_perm, ipcp);
 		if (err)
 			goto out_unlock0;
@@ -454,6 +455,7 @@ static int msgctl_down(struct ipc_namespace *ns, int msqid, int cmd,
 		break;
 	default:
 		err = -EINVAL;
+		goto out_unlock1;
 	}
 
 out_unlock0: