netdev - Re: neighbour netlink notifications delivered in wrong order

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Date:   Tue, 7 Jun 2022 09:29:45 -0700
From:   Francesco Ruggeri <fruggeri@...sta.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     Andy Roulin <aroulin@...dia.com>, netdev@...r.kernel.org
Subject: Re: neighbour netlink notifications delivered in wrong order

On Mon, Jun 6, 2022 at 8:19 PM Stephen Hemminger
<stephen@...workplumber.org> wrote:
>
> On Mon, 6 Jun 2022 19:07:04 -0700
> Andy Roulin <aroulin@...dia.com> wrote:
>
> > diff --git a/net/core/neighbour.c b/net/core/neighbour.c
> > index 54625287ee5b..a91dfcbfc01c 100644
> > --- a/net/core/neighbour.c
> > +++ b/net/core/neighbour.c
> > @@ -2531,23 +2531,19 @@ static int neigh_fill_info(struct sk_buff *skb,
> > struct neighbour *neigh,
> >       if (nla_put(skb, NDA_DST, neigh->tbl->key_len, neigh->primary_key))
> >               goto nla_put_failure;
> >
> > -     read_lock_bh(&neigh->lock);
> >       ndm->ndm_state   = neigh->nud_state;
>
> Accessing neighbor state outside of lock is not safe.
>
> But you should be able to use RCU here??

I think the patch removes the lock from neigh_fill_info but it then uses it
to protect all calls to neigh_fill_info, so the access should still be safe.
In case of __neigh_notify the lock also extends to protect rtnl_notify,
guaranteeing that the state cannot be changed while the notification
is in progress (I assume all state changes are protected by the same lock).
Andy, is that the idea?