[RFC 1/2] netvsc: reference counting fix

[David Miller]

From: Stephen Hemminger <stephen at networkplumber.org>
Date: Sat, 13 Aug 2016 11:35:59 -0700

> This is how I think it should be fixed, but not tested yet.
> 
> Subjec: netvsc: use device not module reference counts
> 
> Fix how the cross-device reference counting is handled.  When VF is
> associated with the synthetic interface, the VF driver module should
> still be able to be unloaded.  The module unload code will callback
> with NETDEV_UNREGISTER event which breaks the connection safely.
> (Fixes 9f4b5ba5db4 hv_netvsc: Implement support for VF drivers on Hyper-V)
> 
> Signed-off-by: Stephen Hemminger <sthemmin at linuxonhyperv.com>

This might not work.

It is assumed that when a netdev unregister happens, it may be done so
at any point in time.

Therefore that NETDEV_UNREGISTER event must eliminate any and all
references to a given netdev.  And that works perfectly fine right now
with all existing subsystems that take references to netdevs.

You'll have to add something so that a NETDEV_UNREGISTER event tears
this VF down and thus releases it's reference to the synthetic device.