netdev - Re: [PATCH net-next] net-loopback: allow lo dev initial state to be controlled

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Date:   Mon, 16 Nov 2020 13:03:32 -0800
From:   Mahesh Bandewar (महेश बंडेवार) 
        <maheshb@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Ido Schimmel <idosch@...sch.org>, David Ahern <dsahern@...il.com>,
        Jian Yang <jianyang.kernel@...il.com>,
        David Miller <davem@...emloft.net>,
        linux-netdev <netdev@...r.kernel.org>,
        Jian Yang <jianyang@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next] net-loopback: allow lo dev initial state to be controlled

On Mon, Nov 16, 2020 at 12:34 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Mon, 16 Nov 2020 12:02:48 -0800 Mahesh Bandewar (महेश बंडेवार) wrote:
> > > > diff --git a/drivers/net/loopback.c b/drivers/net/loopback.c
> > > > index a1c77cc00416..76dc92ac65a2 100644
> > > > --- a/drivers/net/loopback.c
> > > > +++ b/drivers/net/loopback.c
> > > > @@ -219,6 +219,13 @@ static __net_init int loopback_net_init(struct net *net)
> > > >
> > > >       BUG_ON(dev->ifindex != LOOPBACK_IFINDEX);
> > > >       net->loopback_dev = dev;
> > > > +
> > > > +     if (sysctl_netdev_loopback_state) {
> > > > +             /* Bring loopback device UP */
> > > > +             rtnl_lock();
> > > > +             dev_open(dev, NULL);
> > > > +             rtnl_unlock();
> > > > +     }
> > >
> > > The only concern I have here is that it breaks notification ordering.
> > > Is there precedent for NETDEV_UP to be generated before all pernet ops
> > > ->init was called?
> > I'm not sure if any and didn't see any issues in our usage / tests.
> > I'm not even sure anyone is watching/monitoring for lo status as such.
>
> Ido, David, how does this sound to you?
>
> I can't think of any particular case where bringing the device up (and
> populating it's addresses) before per netns init is finished could be
> problematic. But if this is going to make kernel coding harder the
> minor convenience of the knob is probably not worth it.

+Eric Dumazet

I'm not sure why kernel coding should get harder, but happy to listen
to the opinions.