Subject: Re: [PATCH] net: linkwatch: ignore events for unregistered netdevs

From: Jakub Kicinski <[email-masked]>
Date: Mon, 25 Apr 2022 08:36:09 -0700

On Mon, 25 Apr 2022 08:31:23 -0700 Eric Dumazet wrote:
> > Jann described a case where someone does
> >
> >     CPU 0      CPU 1     CPU 2
> >
> >   dev_hold()
> >    ------  #unregister -------
> >              dev_hold()
> >                          dev_put()
> >
> > Our check for refcount == 0 goes over the CPUs one by one,
> > so if it sums up CPUs 0 and 1 at the "unregister" point above
> > and CPU2 after the CPU1 hold and CPU2 release it will "miss"
> > one refcount.
> >
> > That's a problem unless doing a dev_hold() on a netdev we only have
> > a reference on is illegal.  
> 
> What is 'illegal' is trying to keep using the device after #unregister.
> 
> We have barriers to prevent that.
> 
> Somehow a layer does not care about the barriers and pretends the
> device is still good to use.
> 
> It is of course perfectly fine to stack multiple dev_hold() from one
> path (if these do not leak, but this is a different issue)

So we'd need something like

WARN_ON(dev->reg_state != NETREG_REGISTERED && !rtnl_held())

in dev_hold()?