Re: [PATCH v2] net: set default_ethtool_ops in register_netdevice

[David Miller <davem@davemloft.net>]

From: Stanislaw Gruszka <sgruszka@redhat.com>
Date: Thu, 10 Jan 2013 09:36:42 +0100

> On Wed, Jan 09, 2013 at 11:57:38PM -0800, David Miller wrote:
>> From: Stanislaw Gruszka <sgruszka@redhat.com>
>> Date: Tue, 8 Jan 2013 16:38:51 +0100
>> 
>> > Since:
>> > 
>> > commit 2c60db037034d27f8c636403355d52872da92f81
>> > Author: Eric Dumazet <edumazet@google.com>
>> > Date:   Sun Sep 16 09:17:26 2012 +0000
>> > 
>> >     net: provide a default dev->ethtool_ops
>> > 
>> > wireless core does not correctly assign ethtool_ops. In order to fix
>> > the problem, move assignement of default_ethtool_ops to
>> > register_netdevice(). This is safe because both register_netdevice()
>> > and dev_ethtool() are protected by RTNL lock.
>> > 
>> > Patch is besed on hint of Michał Mirosław.
>> > 
>> > Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
>> > Cc: stable@vger.kernel.org # 3.7+
>> > ---
>> > v1 -> v2: change order of default_ethtool_ops initialization to avoid
>> > the problem. Change the subject accordingly.
>> 
>> I don't understand this.  Why is the assignment of default_ethtool_ops
>> at netdev allocation time not working?  Is wireless really not using
>> alloc_netdev*()?
> 
> It does. This is done on individual cfg80211 drivers , i.e. on mac80211
> or full mac drivers. After alloc_netdev*() call, some cfg80211 drivers
> provide they own ethtool_ops, but some do not. For them, wireless core
> provide generic cfg80211_ethtool_ops, which is assigned in
> NETDEV_REGISTER notify call: 
> 
> 	if (!dev->ethtool_ops)
> 		dev->ethtool_ops = &cfg80211_ethtool_ops;
> 
> But after Eric's commit, dev->ethtool_ops is no longer NULL (on cfg80211
> drivers without custom ethtool_ops), but points to &default_ethtool_ops.

The whole idea is to remove these kinds of NULL tests against
dev->ethtool_ops, thus creating the invariant that given any netdev
one must never be able to see a non-NULL value there.