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Date:   Fri, 19 Jun 2020 01:53:38 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Cong Wang <xiyou.wangcong@...il.com>,
        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 Fri, 19 Jun 2020 at 01:46, Andrew Lunn <andrew@...n.ch> wrote:
>
> > > Hi Vladimir
> > >
> > > So you are suggesting this?
> > >
> > > > > +       ret = netdev_upper_dev_link(master, slave_dev, NULL);
> > >
> > >   Andrew
> >
> > Yes, basically this:
> >
> > diff --git a/net/dsa/slave.c b/net/dsa/slave.c
> > index 4c7f086a047b..6aff8cfc9cf1 100644
> > --- a/net/dsa/slave.c
> > +++ b/net/dsa/slave.c
> > @@ -1807,6 +1807,13 @@ int dsa_slave_create(struct dsa_port *port)
> >                            ret, slave_dev->name);
> >                 goto out_phy;
> >         }
> > +       rtnl_lock();
> > +       ret = netdev_upper_dev_link(master, slave_dev, NULL);
> > +       rtnl_unlock();
> > +       if (ret) {
> > +               unregister_netdevice(slave_dev);
> > +               goto out_phy;
> > +       }
> >
> >         return 0;
> >
> > @@ -1826,12 +1833,14 @@ int dsa_slave_create(struct dsa_port *port)
> >
> >  void dsa_slave_destroy(struct net_device *slave_dev)
> >  {
> > +       struct net_device *master = dsa_slave_to_master(slave_dev);
> >         struct dsa_port *dp = dsa_slave_to_port(slave_dev);
> >         struct dsa_slave_priv *p = netdev_priv(slave_dev);
> >
> >         netif_carrier_off(slave_dev);
> >         rtnl_lock();
> >         phylink_disconnect_phy(dp->pl);
> > +       netdev_upper_dev_unlink(master, slave_dev);
> >         rtnl_unlock();
> >
> >         dsa_slave_notify(slave_dev, DSA_PORT_UNREGISTER);
> >
> > Do you see a problem with it?
>
> I was initially not sure you could do this. But it looks like you can
> have N : M relationships between uppers and lowers. I suppose it does
> make sense. You can have multiple VLAN uppers to one base device. You
> can have multiple lowers to one bond device, etc.
>
> I wonder what 'side effects' there are for declaring this linkage. It
> is not something i've looked into before, since we never used it. So i
> don't see a problem with this, other than i don't know what problems
> we might run into :-)
>
>   Andrew
>

It was surprising to me as well, since I was used to the bridge model
(a port can have only one bridge master). But it looks like, that is
the difference between netdev_upper_dev_link and
netdev_master_upper_dev_link. This uses the former, and the bridge
layer uses the latter.

So I guess it is ok.

Thanks,
-Vladimir

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