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

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Date:   Mon, 16 Nov 2020 12:34:47 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     "Mahesh Bandewar (महेश बंडेवार) " <maheshb@...gle.com>,
        Ido Schimmel <idosch@...sch.org>,
        David Ahern <dsahern@...il.com>
Cc:     Jian Yang <jianyang.kernel@...il.com>,
        David Miller <davem@...emloft.net>,
        linux-netdev <netdev@...r.kernel.org>,
        Jian Yang <jianyang@...gle.com>
Subject: Re: [PATCH net-next] net-loopback: allow lo dev initial state to be
 controlled

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.