Tests performed with per device sysctl/sysfs entries disabled:

$ insmod /lib/modules/dummy.ko numdummies=8000
$ time rmmod dummy

Without the patch:    With the patch:
real    0m 3.65s      real    0m 0.27s
user    0m 0.00s      user    0m 0.00s
sys     0m 3.42s      sys     0m 0.24s

Signed-off-by: Octavian Purdila <opurdila@ixiacom.com>
---
 net/core/dev.c          |   28 ++++++++++++++++++++++++++--
 net/ipv4/fib_frontend.c |   13 ++++++++-----
 2 files changed, 34 insertions(+), 7 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 4b24d79..b0a14f0 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4720,7 +4720,8 @@ static void net_set_todo(struct net_device *dev)
 
 static void rollback_registered_many(struct list_head *head)
 {
-	struct net_device *dev;
+	struct net_device *dev, *aux, *fdev;
+	LIST_HEAD(rt_flush_list);
 
 	BUG_ON(dev_boot_phase);
 	ASSERT_RTNL();
@@ -4778,8 +4779,28 @@ static void rollback_registered_many(struct list_head *head)
 
 	synchronize_net();
 
-	list_for_each_entry(dev, head, unreg_list)
+	/* flush route cache by resending one NETDEV_UNREGISTER per namespace */
+	list_for_each_entry_safe(dev, aux, head, unreg_list) {
+		int needs_flush = 1;
+		list_for_each_entry(fdev, &rt_flush_list, unreg_list) {
+			if (dev_net(dev) == dev_net(fdev)) {
+				needs_flush = 0;
+				dev_put(dev);
+				break;
+			}
+		}
+		if (needs_flush) {
+			list_del(&dev->unreg_list);
+			list_add(&dev->unreg_list, &rt_flush_list);
+		}
+	}
+
+	list_for_each_entry_safe(dev, aux, &rt_flush_list, unreg_list) {
+		list_del_init(&dev->unreg_list);
+		call_netdevice_notifiers(NETDEV_UNREGISTER, dev);
+		list_add(&dev->unreg_list, head);
 		dev_put(dev);
+	}
 }
 
 static void rollback_registered(struct net_device *dev)
@@ -5374,6 +5395,9 @@ EXPORT_SYMBOL(unregister_netdevice_queue);
  *	unregister_netdevice_many - unregister many devices
  *	@head: list of devices
  *
+ *	WARNING: This function modifies the list. It may change the ...
[ message continues ]

Are you sure you want to overload NETDEV_UNREGISTER ?

Maybe it would be cleaner to add a new value, NETDEV_UNREGISTER_PERNET or something 
for the final loop...
--

[ message continues ]

The  list passed to unregister_netdevice_many(), as the "head" parameter, may 
be altered, e.g. order may change between the elements.

That is because we temporarily move the items from the list to the 
rt_flush_list for the flush. When we add the items back they may not be added in 
the same place.

Perhaps the confusion comes from the fact that I did not specified which list? 

Would it be better if I would add a dev_is_batch_unregister()  instead?


Hmm, I think that will allow us to get rid of the ugly test: never flush the 
cache for NETDEV_UNREGISTER, only flush it for NETDEV_UNREGISTER_PERNET. 

We just need to make sure to add NETDEV_UNREGISTER_PERNET in other places 
where NETDEV_UNREGISTER is called.

I'll try this in the next patch. Thanks for reviewing.

--

[ message continues ]

Ah, I got it now, confusion is that comment makes more sense for
rollback_registered_many() because when reading unregister_netdevice_many()
it is clear it doesnt change the list...

void unregister_netdevice_many(struct list_head *head)
{
        struct net_device *dev;

        if (!list_empty(head)) {
                rollback_registered_many(head);
                list_for_each_entry(dev, head, unreg_list)
                        net_set_todo(dev);
        }
}


--

[ message continues ]