Subject: Re: [PATCH net] bridge: notify user space of fdb port change

From: Jon Maxwell <[email-masked]>
Date: Wed, 14 May 2014 17:07:44 -0400 (EDT)

----- Original Message -----
> From: "Toshiaki Makita" <[email-masked]>
> To: "Jon Maxwell" <[email-masked]>, [email-masked]
> Cc: [email-masked], [email-masked], [email-masked], [email-masked],
> [email-masked], [email-masked], [email-masked]
> Sent: Wednesday, May 14, 2014 10:34:38 AM
> Subject: Re: [PATCH net] bridge: notify user space of fdb port change
> 
> (2014/05/13 16:55), Jon Maxwell wrote:
> > From: Jon Maxwell <[email-masked]>
> > 
> > There has been a number incidents recently where customers running KVM have
> > reported that VM hosts on different Hypervisors are unreachable. Based on
> > pcap traces we found that the bridge was broadcasting the ARP request out
> > onto the network. However some NICs have an inbuilt switch which on
> > occasions
> > were broadcasting the VMs ARP request back through the physical NIC on the
> > Hypervisor. This resulted in the bridge changing ports and incorrectly
> > learning
> > that the VMs mac address was external. As a result the ARP reply was
> > directed
> > back onto the external network and VM never updated it's ARP cache. This
> > patch
> > will notify the bridge command to identify such port toggling.
> > 
> > Signed-off-by: Jon Maxwell <[email-masked]>
> > ---
> >  net/bridge/br_fdb.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/net/bridge/br_fdb.c b/net/bridge/br_fdb.c
> > index 9203d5a..37742e2 100644
> > --- a/net/bridge/br_fdb.c
> > +++ b/net/bridge/br_fdb.c
> > @@ -507,6 +507,8 @@ void br_fdb_update(struct net_bridge *br, struct
> > net_bridge_port *source,
> >  					source->dev->name);
> >  		} else {
> >  			/* fastpath: update of existing entry */
> > +			if (source->port_no != fdb->dst->port_no)
> 
> It seems that we don't need to fetch port_no and it is enough to compare
> source and fdb->dst.

It may save a few instructions but I have not tested it.

> 
> > +				fdb_notify(br, fdb, RTM_NEWNEIGH);
> >  			fdb->dst = source;
> >  			fdb->updated = jiffies;
> >  			if (unlikely(added_by_user))
> > 
> 
> This notifies fdb entry before updating existing entry. Is this on purpose?
> I think we should notify the updated fdb entry.
> Similar code fdb_add_entry() does after updating it.

It was not on purpose but for this particular case there will be burst of notifies 
so it probably does not matter. However I agree it should be after the update.
I will do that along with adding the unlikely() conditional and resubmit.

> 
> Also, isn't it better to move update of dst into "if" block?

I would prefer to leave this portion as alone and only use the if 
statement for the notify.

> 
> 	if (source != fdb->dst) {
> 		fdb->dst = source;
> 		modified = true;
> 	}
> 	...
> 	if (modified) ...
> 
> Thanks,
> Toshiaki Makita
>