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

From: David Miller <davem-fT/PcQaiUtIeIZ0/mPfg9Q@public.gmane.org>
Date: Thu, 10 Jan 2013 02:07:29 -0800 (PST)

From: Stanislaw Gruszka <redacted>
Date: Thu, 10 Jan 2013 09:36:42 +0100

quoted

On Wed, Jan 09, 2013 at 11:57:38PM -0800, David Miller wrote:

quoted

From: Stanislaw Gruszka <redacted>
Date: Tue, 8 Jan 2013 16:38:51 +0100

quoted

Since:

commit 2c60db037034d27f8c636403355d52872da92f81
Author: Eric Dumazet [off-list ref]
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 <redacted>
Cc: stable-u79uwXL29TY76Z2rM5mHXA@public.gmane.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.

Of course I meant that we should never see a NULL value in
dev->ethtool_ops.

I would suggest fixing this by making the wireless core express it's
intentions, that it wants to use a different default ethtool ops, to
the netdev allocation layer.

You have two way to do this:

1) Add default_ethtool_ops argument to alloc_netdev*().

2) Add a "netdev_set_default_ethtool_ops(netdev, ops)"

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