netdev - Re: net: netdev_sync_lower

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Message-ID: <CAM_iQpVgo3Few=uxVtHw4if30EveXzJ+j7jwiB0RZhT_HfiNPg@mail.gmail.com>
Date:   Mon, 18 May 2020 12:14:28 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Michał Mirosław <mirq-linux@...e.qmqm.pl>
Cc:     Jarod Wilson <jarod@...hat.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: net: netdev_sync_lower_features()

On Mon, May 18, 2020 at 1:32 AM Michał Mirosław <mirq-linux@...e.qmqm.pl> wrote:
>
> Hi!
>
> I just saw commit dd912306ff008 ("net: fix a potential recursive
> NETDEV_FEAT_CHANGE") landing in Linux master. The problem with it (or
> rather: with the netdev_sync_lower_features() function) is that
> netdev_update_features() is allowed to change more than one feature
> at a time, including force-enabling other feature than one that is
> being disabled. I think that a better fix would be to trigger
> notification only after all features are updated (outside of the loop).

Sounds reasonable. But I don't think this problem is introduced by
my commit, my commit merely skips the notification for the
failure.

> When you consider net effect of the function, the loop's added value
> is only to print debug messages. Other than that it's equivalent to:
>
> lower->wanted_features &= ~upper_disables;
> netdev_update_features(lower);
>
> The problem of spurious notification can be fixed in
> __netdev_update_features() by saving dev->features at the start and
> only return -1 when it really changed.

Please send a patch.

Thanks.