On Mon, Jun 3, 2024 at 9:30 AM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote:
>
> Eric Dumazet <edumazet@xxxxxxxxxx> writes:
>
> > On Wed, May 29, 2024 at 1:21 PM Petr Machata <petrm@xxxxxxxxxx> wrote:
> >>
> >> When calculating hashes for the purpose of multipath forwarding, both IPv4
> >> and IPv6 code currently fall back on flow_hash_from_keys(). That uses a
> >> randomly-generated seed. That's a fine choice by default, but unfortunately
> >> some deployments may need a tighter control over the seed used.
> >>
> >> In this patch, make the seed configurable by adding a new sysctl key,
> >> net.ipv4.fib_multipath_hash_seed to control the seed. This seed is used
> >> specifically for multipath forwarding and not for the other concerns that
> >> flow_hash_from_keys() is used for, such as queue selection. Expose the knob
> >> as sysctl because other such settings, such as headers to hash, are also
> >> handled that way. Like those, the multipath hash seed is a per-netns
> >> variable.
> >>
> >> Despite being placed in the net.ipv4 namespace, the multipath seed sysctl
> >> is used for both IPv4 and IPv6, similarly to e.g. a number of TCP
> >> variables.
> >>
> > ...
> >
> >> + rtnl_lock();
> >> + old = rcu_replace_pointer_rtnl(net->ipv4.sysctl_fib_multipath_hash_seed,
> >> + mphs);
> >> + rtnl_unlock();
> >> +
> >
> > In case you keep RCU for the next version, please do not use rtnl_lock() here.
> >
> > A simple xchg() will work just fine.
> >
> > old = xchg((__force struct struct sysctl_fib_multipath_hash_seed
> > **)&net->ipv4.sysctl_fib_multipath_hash_seed,
> > mphs);
>
> We added a macro to do this kind of thing without triggering any of the
> RCU type linter warnings, in:
>
> 76c8eaafe4f0 ("rcu: Create an unrcu_pointer() to remove __rcu from a pointer")
>
> So as an alternative to open-coding the cast, something like this could
> work - I guess it's mostly a matter of taste:
>
> old = unrcu_pointer(xchg(&net->ipv4.sysctl_fib_multipath_hash_seed, RCU_INITIALIZER(mphs)));
Good to know, thanks.
Not sure why __kernel qualifier has been put there.
