net: factorize sync-rcu call in unregister_netdevice_many

Add dev_close_many and dev_deactivate_many to factorize another
sync-rcu operation on the netdevice unregister path.

$ modprobe dummy numdummies=10000
$ ip link set dev dummy* up
$ time rmmod dummy

Without the patch           With the patch

real    0m 24.63s           real    0m 5.15s
user    0m 0.00s            user    0m 0.00s
sys     0m 6.05s            sys     0m 5.14s

Signed-off-by: Octavian Purdila <opurdila@ixiacom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_generic.c

@@ -810,20 +810,35 @@ static bool some_qdisc_is_busy(struct net_device *dev)
 	return false;
 }
 
-void dev_deactivate(struct net_device *dev)
+void dev_deactivate_many(struct list_head *head)
 {
-	netdev_for_each_tx_queue(dev, dev_deactivate_queue, &noop_qdisc);
-	if (dev_ingress_queue(dev))
-		dev_deactivate_queue(dev, dev_ingress_queue(dev), &noop_qdisc);
+	struct net_device *dev;
 
-	dev_watchdog_down(dev);
+	list_for_each_entry(dev, head, unreg_list) {
+		netdev_for_each_tx_queue(dev, dev_deactivate_queue,
+					 &noop_qdisc);
+		if (dev_ingress_queue(dev))
+			dev_deactivate_queue(dev, dev_ingress_queue(dev),
+					     &noop_qdisc);
+
+		dev_watchdog_down(dev);
+	}
 
 	/* Wait for outstanding qdisc-less dev_queue_xmit calls. */
 	synchronize_rcu();
 
 	/* Wait for outstanding qdisc_run calls. */
-	while (some_qdisc_is_busy(dev))
-		yield();
+	list_for_each_entry(dev, head, unreg_list)
+		while (some_qdisc_is_busy(dev))
+			yield();
+}
+
+void dev_deactivate(struct net_device *dev)
+{
+	LIST_HEAD(single);
+
+	list_add(&dev->unreg_list, &single);
+	dev_deactivate_many(&single);
 }
 
 static void dev_init_scheduler_queue(struct net_device *dev,