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);
