Re: Correct usage of dev_base_lock in 2020

Vladimir Oltean
2020-11-30T21:13:34Z

On Mon, Nov 30, 2020 at 10:00:16PM +0100, Eric Dumazet wrote:
> On Mon, Nov 30, 2020 at 9:50 PM Vladimir Oltean <olteanv@gmail.com> wrote:
> >
> > On Mon, Nov 30, 2020 at 09:43:01PM +0100, Eric Dumazet wrote:
> > > Understood, but really dev_base_lock can only be removed _after_ we
> > > convert all usages to something else (mutex based, and preferably not
> > > the global RTNL)
> >
> > Sure.
> > A large part of getting rid of dev_base_lock seems to be just:
> > - deleting the bogus usage from mlx4 infiniband and friends
> > - converting procfs, sysfs and friends to netdev_lists_mutex
> > - renaming whatever is left into something related to the RFC 2863
> > operstate.
> >
> > > Focusing on dev_base_lock seems a distraction really.
> >
> > Maybe.
> > But it's going to be awkward to explain in words what the locking rules
> > are, when the read side can take optionally the dev_base_lock, RCU, or
> > netdev_lists_lock, and the write side can take optionally the dev_base_lock,
> > RTNL, or netdev_lists_lock. Not to mention that anybody grepping for
> > dev_base_lock will see the current usage and not make a lot out of it.
> >
> > I'm not really sure how to order this rework to be honest.
>
> We can not have a mix of RCU /rwlock/mutex. It must be one, because of
> bonding/teaming.
>
> So all existing uses of rwlock / RCU need to be removed.
>
> This is probably not trivial.

Now, "it's going to look nasty" is one thing, whereas "it won't work" is
completely different. I think it would work though, so could you expand
on why you're saying we can't have the mix? dev_change_name(),
list_netdevice() and unlist_netdevice() just need to take one more layer
of locking. The new netdev_lists_mutex would serve as a temporary
alternative to the RTNL mutex. Then we could gradually replace more and
more of the RTNL mutex with netdev_lists_mutex. The bonding driver can
certainly use the netdev_lists_mutex. It guarantees protection against
the three functions mentioned above, and it is sleepable, and it is not
the RTNL mutex. So can procfs and sysfs. Am I missing something?

> Perhaps you could add a temporary ndo_get_sleepable_stats64() so that
> drivers can be converted one at a time.

Yeah, been there, Jakub doesn't like it.