net: dsa: setup master before ports

It is said that as soon as a network interface is registered, all its
resources should have already been prepared, so that it is available for
sending and receiving traffic. One of the resources needed by a DSA
slave interface is the master.

dsa_tree_setup
-> dsa_tree_setup_ports
   -> dsa_port_setup
      -> dsa_slave_create
         -> register_netdevice
-> dsa_tree_setup_master
   -> dsa_master_setup
      -> sets up master->dsa_ptr, which enables reception

Therefore, there is a short period of time after register_netdevice()
during which the master isn't prepared to pass traffic to the DSA layer
(master->dsa_ptr is checked by eth_type_trans). Same thing during
unregistration, there is a time frame in which packets might be missed.

Note that this change opens us to another race: dsa_master_find_slave()
will get invoked potentially earlier than the slave creation, and later
than the slave deletion. Since dp->slave starts off as a NULL pointer,
the earlier calls aren't a problem, but the later calls are. To avoid
use-after-free, we should zeroize dp->slave before calling
dsa_slave_destroy().

In practice I cannot really test real life improvements brought by this
change, since in my systems, netdevice creation races with PHY autoneg
which takes a few seconds to complete, and that masks quite a few races.
Effects might be noticeable in a setup with fixed links all the way to
an external system.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/dsa2.c

@@ -561,6 +561,7 @@ static void dsa_port_teardown(struct dsa_port *dp)
 	struct devlink_port *dlp = &dp->devlink_port;
 	struct dsa_switch *ds = dp->ds;
 	struct dsa_mac_addr *a, *tmp;
+	struct net_device *slave;
 
 	if (!dp->setup)
 		return;
@@ -582,9 +583,11 @@ static void dsa_port_teardown(struct dsa_port *dp)
 		dsa_port_link_unregister_of(dp);
 		break;
 	case DSA_PORT_TYPE_USER:
-		if (dp->slave) {
-			dsa_slave_destroy(dp->slave);
+		slave = dp->slave;
+
+		if (slave) {
 			dp->slave = NULL;
+			dsa_slave_destroy(slave);
 		}
 		break;
 	}
@@ -1134,17 +1137,17 @@ static int dsa_tree_setup(struct dsa_switch_tree *dst)
 	if (err)
 		goto teardown_cpu_ports;
 
-	err = dsa_tree_setup_ports(dst);
+	err = dsa_tree_setup_master(dst);
 	if (err)
 		goto teardown_switches;
 
-	err = dsa_tree_setup_master(dst);
+	err = dsa_tree_setup_ports(dst);
 	if (err)
-		goto teardown_ports;
+		goto teardown_master;
 
 	err = dsa_tree_setup_lags(dst);
 	if (err)
-		goto teardown_master;
+		goto teardown_ports;
 
 	dst->setup = true;
 
@@ -1152,10 +1155,10 @@ static int dsa_tree_setup(struct dsa_switch_tree *dst)
 
 	return 0;
 
-teardown_master:
-	dsa_tree_teardown_master(dst);
 teardown_ports:
 	dsa_tree_teardown_ports(dst);
+teardown_master:
+	dsa_tree_teardown_master(dst);
 teardown_switches:
 	dsa_tree_teardown_switches(dst);
 teardown_cpu_ports:
@@ -1173,10 +1176,10 @@ static void dsa_tree_teardown(struct dsa_switch_tree *dst)
 
 	dsa_tree_teardown_lags(dst);
 
-	dsa_tree_teardown_master(dst);
-
 	dsa_tree_teardown_ports(dst);
 
+	dsa_tree_teardown_master(dst);
+
 	dsa_tree_teardown_switches(dst);
 
 	dsa_tree_teardown_cpu_ports(dst);