Re: [PATCH net-next 2/4] net: ipv4: Add a sysctl to set multipath hash seed

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Eric Dumazet <edumazet@google.com> writes:

> On Wed, May 29, 2024 at 1:21 PM Petr Machata <petrm@nvidia.com> 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)));

-Toke