net: qrtr: Make qrtr_port_lookup() use RCU

The important part of qrtr_port_lookup() wrt synchronization is that the
function returns a reference counted struct qrtr_sock, or fail.

As such we need only to ensure that an decrement of the object's
refcount happens inbetween the finding of the object in the idr and
qrtr_port_lookup()'s own increment of the object.

By using RCU and putting a synchronization point after we remove the
mapping from the idr, but before it can be released we achieve this -
with the benefit of not having to hold the mutex in qrtr_port_lookup().
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/qrtr/qrtr.c

@@ -646,11 +646,11 @@ static struct qrtr_sock *qrtr_port_lookup(int port)
 	if (port == QRTR_PORT_CTRL)
 		port = 0;
 
-	mutex_lock(&qrtr_port_lock);
+	rcu_read_lock();
 	ipc = idr_find(&qrtr_ports, port);
 	if (ipc)
 		sock_hold(&ipc->sk);
-	mutex_unlock(&qrtr_port_lock);
+	rcu_read_unlock();
 
 	return ipc;
 }
@@ -692,6 +692,10 @@ static void qrtr_port_remove(struct qrtr_sock *ipc)
 	mutex_lock(&qrtr_port_lock);
 	idr_remove(&qrtr_ports, port);
 	mutex_unlock(&qrtr_port_lock);
+
+	/* Ensure that if qrtr_port_lookup() did enter the RCU read section we
+	 * wait for it to up increment the refcount */
+	synchronize_rcu();
 }
 
 /* Assign port number to socket.