Re: [PATCH net] net: dsa: Keep a reference count on ethernet_dev

On 01/24/2017 09:39 AM, David Miller wrote:

From: Florian Fainelli <f.fainelli@gmail.com>
Date: Sat, 21 Jan 2017 09:40:54 -0800

quoted

of_find_net_device_by_node() just returns a reference to a net_device but does
not increment its reference count, which means that the master network device
can just vanish under our feet.

Fixes: 83c0afaec7b7 ("net: dsa: Add new binding implementation")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

This is fine, except now this netdev is completely locked into place with
no way to dynamically unload it.

If someone tries to modunload the driver for this ethernet device,
their screen will fill up with warning messages indicating that the
reference taken here in the DSA code is not going away.

You need to implement a netdev notifier that tears down this DSA
instance during an unregister event and releases the ethernet_dev.
Similar to how we handle protocol addresses bound to a netdev, etc.

I have been thinking about this a bit more, and this is what is going on:

- upon master network device unregister we can look up the
dsa_switch_tree in dev->dsa_ptr and call dsa_dst_unapply() that is
actually exactly what we want since it detaches the dsa_switch_tree from
the master network device

- upon master network device register, we can look up whether this
master netdev is the one of interest and re-attach the dangling switch
tree, but here we have two cases:

	- if we have an OF enabled system, doing a reverse look up of
net_device to device to device_node, and then looking it up in the
Device Tree is possible and reasonably simple, this works

	- if we have a platform data enabled system, doing a reverse lookup is
possible, but won't necessarily yield the expected results, because
platform data will have a device reference to the original master
netdev, and this one could be totally different the second time we probe
the master network device due to to unregister/register

Thoughts?
-- 
Florian