netdev - Re: [PATCH net] ipv6: fix suspecious RCU usage warning

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Date:   Tue, 9 Mar 2021 09:32:47 -0800
From:   Wei Wang <weiwan@...gle.com>
To:     Ido Schimmel <idosch@...sch.org>
Cc:     David Ahern <dsahern@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Ido Schimmel <idosch@...dia.com>,
        Petr Machata <petrm@...dia.com>,
        syzbot <syzkaller@...glegroups.com>,
        David Ahern <dsahern@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net] ipv6: fix suspecious RCU usage warning

On Tue, Mar 9, 2021 at 12:15 AM Ido Schimmel <idosch@...sch.org> wrote:
>
> On Mon, Mar 08, 2021 at 07:47:31PM -0700, David Ahern wrote:
> > [ cc Ido and Petr ]
> >
> > On 3/8/21 12:21 PM, Wei Wang wrote:
> > > diff --git a/include/net/nexthop.h b/include/net/nexthop.h
> > > index 7bc057aee40b..48956b144689 100644
> > > --- a/include/net/nexthop.h
> > > +++ b/include/net/nexthop.h
> > > @@ -410,31 +410,39 @@ static inline struct fib_nh *fib_info_nh(struct fib_info *fi, int nhsel)
> > >  int fib6_check_nexthop(struct nexthop *nh, struct fib6_config *cfg,
> > >                    struct netlink_ext_ack *extack);
> > >
> > > -static inline struct fib6_nh *nexthop_fib6_nh(struct nexthop *nh)
> > > +static inline struct fib6_nh *nexthop_fib6_nh(struct nexthop *nh,
> > > +                                         bool bh_disabled)
> >
> > Hi Wei: I would prefer not to have a second argument to nexthop_fib6_nh
> > for 1 code path, and a control path at that.
> >
> > >  {
> > >     struct nh_info *nhi;
> > >
> > >     if (nh->is_group) {
> > >             struct nh_group *nh_grp;
> > >
> > > -           nh_grp = rcu_dereference_rtnl(nh->nh_grp);
> > > +           if (bh_disabled)
> > > +                   nh_grp = rcu_dereference_bh_rtnl(nh->nh_grp);
> > > +           else
> > > +                   nh_grp = rcu_dereference_rtnl(nh->nh_grp);
> > >             nh = nexthop_mpath_select(nh_grp, 0);
> > >             if (!nh)
> > >                     return NULL;
> > >     }
> > >
> > > -   nhi = rcu_dereference_rtnl(nh->nh_info);
> > > +   if (bh_disabled)
> > > +           nhi = rcu_dereference_bh_rtnl(nh->nh_info);
> > > +   else
> > > +           nhi = rcu_dereference_rtnl(nh->nh_info);
> > >     if (nhi->family == AF_INET6)
> > >             return &nhi->fib6_nh;
> > >
> > >     return NULL;
> > >  }
> > >
> >
> > I am wary of duplicating code, but this helper is simple enough that it
> > should be ok with proper documentation.
> >
> > Ido/Petr: I think your resilient hashing patch set touches this helper.
> > How ugly does it get to have a second version?
>
> It actually doesn't touch this helper. Looks fine to me:


Thanks David and Ido.
To clarify, David, you suggest we add a separate function instead of
adding an extra parameter, right?

>
>
> diff --git a/include/net/nexthop.h b/include/net/nexthop.h
> index ba94868a21d5..6df9c12546fd 100644
> --- a/include/net/nexthop.h
> +++ b/include/net/nexthop.h
> @@ -496,6 +496,26 @@ static inline struct fib6_nh *nexthop_fib6_nh(struct nexthop *nh)
>         return NULL;
>  }
>
> +static inline struct fib6_nh *nexthop_fib6_nh_bh(struct nexthop *nh)
> +{
> +       struct nh_info *nhi;
> +
> +       if (nh->is_group) {
> +               struct nh_group *nh_grp;
> +
> +               nh_grp = rcu_dereference_bh(nh->nh_grp);
> +               nh = nexthop_mpath_select(nh_grp, 0);
> +               if (!nh)
> +                       return NULL;
> +       }
> +
> +       nhi = rcu_dereference_bh(nh->nh_info);
> +       if (nhi->family == AF_INET6)
> +               return &nhi->fib6_nh;
> +
> +       return NULL;
> +}
> +
>  static inline struct net_device *fib6_info_nh_dev(struct fib6_info *f6i)
>  {
>         struct fib6_nh *fib6_nh;
> diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
> index ef9d022e693f..679699e953f1 100644
> --- a/net/ipv6/ip6_fib.c
> +++ b/net/ipv6/ip6_fib.c
> @@ -2486,7 +2486,7 @@ static int ipv6_route_native_seq_show(struct seq_file *seq, void *v)
>         const struct net_device *dev;
>
>         if (rt->nh)
> -               fib6_nh = nexthop_fib6_nh(rt->nh);
> +               fib6_nh = nexthop_fib6_nh_bh(rt->nh);
>
>         seq_printf(seq, "%pi6 %02x ", &rt->fib6_dst.addr, rt->fib6_dst.plen);