macvlan: remove one synchronize_rcu() call

When one macvlan device is dismantled, we can avoid one
synchronize_rcu() call done after deletion from hash list, since caller
will perform a synchronize_net() call after its ndo_stop() call.

Add a new netdev->dismantle field to signal this dismantle intent.

Reduces RTNL hold time.
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
CC: Patrick McHardy <kaber@trash.net>
CC: Ben Greear <greearb@candelatech.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/macvlan.c

@@ -70,16 +70,17 @@ static void macvlan_hash_add(struct macvlan_dev *vlan)
 	hlist_add_head_rcu(&vlan->hlist, &port->vlan_hash[addr[5]]);
 }
 
-static void macvlan_hash_del(struct macvlan_dev *vlan)
+static void macvlan_hash_del(struct macvlan_dev *vlan, bool sync)
 {
 	hlist_del_rcu(&vlan->hlist);
-	synchronize_rcu();
+	if (sync)
+		synchronize_rcu();
 }
 
 static void macvlan_hash_change_addr(struct macvlan_dev *vlan,
 					const unsigned char *addr)
 {
-	macvlan_hash_del(vlan);
+	macvlan_hash_del(vlan, true);
 	/* Now that we are unhashed it is safe to change the device
 	 * address without confusing packet delivery.
 	 */
@@ -345,7 +346,7 @@ static int macvlan_stop(struct net_device *dev)
 	dev_uc_del(lowerdev, dev->dev_addr);
 
 hash_del:
-	macvlan_hash_del(vlan);
+	macvlan_hash_del(vlan, !dev->dismantle);
 	return 0;
 }
 

include/linux/netdevice.h

@@ -1293,7 +1293,9 @@ struct net_device {
 	       NETREG_UNREGISTERED,	/* completed unregister todo */
 	       NETREG_RELEASED,		/* called free_netdev */
 	       NETREG_DUMMY,		/* dummy device for NAPI poll */
-	} reg_state:16;
+	} reg_state:8;
+
+	bool dismantle; /* device is going do be freed */
 
 	enum {
 		RTNL_LINK_INITIALIZED,

net/core/dev.c

@@ -5126,7 +5126,7 @@ static void rollback_registered_many(struct list_head *head)
 			list_del(&dev->unreg_list);
 			continue;
 		}
-
+		dev->dismantle = true;
 		BUG_ON(dev->reg_state != NETREG_REGISTERED);
 	}