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Date:   Thu, 18 Jun 2020 23:33:44 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     netdev <netdev@...r.kernel.org>,
        syzbot+f3a0e80c34b3fc28ac5e@...kaller.appspotmail.com,
        Taehee Yoo <ap420073@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [Patch net] net: change addr_list_lock back to static key

On Thu, 18 Jun 2020 at 23:06, Cong Wang <xiyou.wangcong@...il.com> wrote:
>
> On Thu, Jun 18, 2020 at 12:56 PM Cong Wang <xiyou.wangcong@...il.com> wrote:
> >
> > On Thu, Jun 18, 2020 at 12:40 PM Vladimir Oltean <olteanv@...il.com> wrote:
> > >
> > > It's me with the stacked DSA devices again:
> >
> > It looks like DSA never uses netdev API to link master
> > device with slave devices? If so, their dev->lower_level
> > are always 1, therefore triggers this warning.
> >
> > I think it should call one of these netdev_upper_dev_link()
> > API's when creating a slave device.
> >
>
> I don't know whether DSA is too special to use the API, but
> something like this should work:
>
> diff --git a/net/dsa/slave.c b/net/dsa/slave.c
> index 4c7f086a047b..f7a2a281e7f0 100644
> --- a/net/dsa/slave.c
> +++ b/net/dsa/slave.c
> @@ -1807,6 +1807,11 @@ int dsa_slave_create(struct dsa_port *port)
>                            ret, slave_dev->name);
>                 goto out_phy;
>         }
> +       ret = netdev_upper_dev_link(slave_dev, master, NULL);
> +       if (ret) {
> +               unregister_netdevice(slave_dev);
> +               goto out_phy;
> +       }
>
>         return 0;
>
> @@ -1832,6 +1837,7 @@ void dsa_slave_destroy(struct net_device *slave_dev)
>         netif_carrier_off(slave_dev);
>         rtnl_lock();
>         phylink_disconnect_phy(dp->pl);
> +       netdev_upper_dev_unlink(slave_dev, dp->master);
>         rtnl_unlock();
>
>         dsa_slave_notify(slave_dev, DSA_PORT_UNREGISTER);

Thanks. This is a good approximation of what needed to be done:
- netdev_upper_dev_link needs to be under rtnl,
- "dp->master" should be "dsa_slave_to_master(slave_dev)" since it's
actually a union if you look at struct dsa_port).
- And, most importantly, I think the hierarchy should be reversed: a
(virtual) DSA switch port net device (slave) should be an upper of the
(real) DSA master (the host port). Think of it like this: a DSA switch
is a sort of port multiplier for a host port, based on a frame header.
But, it works!

Do you mind if I submit your modified patch to "net"? What would be an
adequate Fixes: tag?

Cheers,
-Vladimir

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