netdev - Re: [PATCH net-next 1/2] net: vxlan: when lower dev unregisters remove vxlan dev as well

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Message-ID: <20140113182232.355f9d20@nehalam.linuxnetplumber.net>
Date:	Mon, 13 Jan 2014 18:22:32 -0800
From:	Stephen Hemminger <stephen@...workplumber.org>
To:	Daniel Borkmann <dborkman@...hat.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [PATCH net-next 1/2] net: vxlan: when lower dev unregisters
 remove vxlan dev as well

On Mon, 13 Jan 2014 18:41:19 +0100
Daniel Borkmann <dborkman@...hat.com> wrote:

> We can create a vxlan device with an explicit underlying carrier.
> In that case, when the carrier link is being deleted from the
> system (e.g. due to module unload) we should also clean up all
> created vxlan devices on top of it since otherwise we're in an
> inconsistent state in vxlan device. In that case, the user needs
> to remove all such devices, while in case of other virtual devs
> that sit on top of physical ones, it is usually the case that
> these devices do unregister automatically as well and do not
> leave the burden on the user.
> 
> This work is not necessary when vxlan device was not created with
> a real underlying device, as connections can resume in that case
> when driver is plugged again. But at least for the other cases,
> we should go ahead and do the cleanup on removal.
> 
> We don't register the notifier during vxlan_newlink() here since
> I consider this event rather rare, and therefore we should not
> bloat vxlan's core structure unecessary. Also, we can simply make
> use of unregister_netdevice_many() to batch that. fdb is flushed
> upon ndo_stop().
> 
> E.g. `ip -d link show vxlan13` after carrier removal before
> this patch:
> 
> 5: vxlan13: <BROADCAST,MULTICAST> mtu 1450 qdisc noop state DOWN mode DEFAULT group default
>     link/ether 1e:47:da:6d:4d:99 brd ff:ff:ff:ff:ff:ff promiscuity 0
>     vxlan id 13 group 239.0.0.10 dev 2 port 32768 61000 ageing 300
>                                  ^^^^^
> Signed-off-by: Daniel Borkmann <dborkman@...hat.com>

Since vxlan is running over UDP socket. I wonder if this could be
done better by implementing something equivalent to SO_BINDTODEVICE.

What happens to a user land application which has a UDP socket
and has done SO_BINDTODEVICE and device is removed? Is there an asynchronous
error, can the application recover? Why can't vxlan use the same mechanism?

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