vlan: remove one synchronize_net() call

At VLAN dismantle phase, unregister_vlan_dev() makes one
synchronize_net() call after vlan_group_set_device(grp, vlan_id, NULL).

This call can be safely removed because we are calling
unregister_netdevice_queue() to queue device for deletion, and this
process needs at least one rcu grace period to complete.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Ben Greear <greearb@candelatech.com>
Cc: Patrick McHardy <kaber@trash.net>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Jesse Gross <jesse@nicira.com>
Cc: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Acked-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/if_vlan.h

@@ -86,7 +86,6 @@ struct vlan_group {
 					    * the vlan is attached to.
 					    */
 	unsigned int		nr_vlans;
-	int			killall;
 	struct hlist_node	hlist;	/* linked list */
 	struct net_device **vlan_devices_arrays[VLAN_GROUP_ARRAY_SPLIT_PARTS];
 	struct rcu_head		rcu;

net/8021q/vlan.c

@@ -120,9 +120,10 @@ void unregister_vlan_dev(struct net_device *dev, struct list_head *head)
 	grp->nr_vlans--;
 
 	vlan_group_set_device(grp, vlan_id, NULL);
-	if (!grp->killall)
-		synchronize_net();
-
+	/* Because unregister_netdevice_queue() makes sure at least one rcu
+	 * grace period is respected before device freeing,
+	 * we dont need to call synchronize_net() here.
+	 */
 	unregister_netdevice_queue(dev, head);
 
 	/* If the group is now empty, kill off the group. */
@@ -478,9 +479,6 @@ static int vlan_device_event(struct notifier_block *unused, unsigned long event,
 		if (dev->reg_state != NETREG_UNREGISTERING)
 			break;
 
-		/* Delete all VLANs for this dev. */
-		grp->killall = 1;
-
 		for (i = 0; i < VLAN_N_VID; i++) {
 			vlandev = vlan_group_get_device(grp, i);
 			if (!vlandev)