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

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Date: Mon, 3 Jun 2024 06:53:56 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Toke Høiland-Jørgensen <toke@...hat.com>
Cc: Eric Dumazet <edumazet@...gle.com>, Petr Machata <petrm@...dia.com>,
	"David S. Miller" <davem@...emloft.net>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	netdev@...r.kernel.org, Ido Schimmel <idosch@...dia.com>,
	David Ahern <dsahern@...nel.org>, Jonathan Corbet <corbet@....net>,
	linux-doc@...r.kernel.org, Simon Horman <horms@...nel.org>
Subject: Re: [PATCH net-next 2/4] net: ipv4: Add a sysctl to set multipath
 hash seed

On Mon, Jun 03, 2024 at 10:58:18AM +0200, Toke Høiland-Jørgensen wrote:
> Eric Dumazet <edumazet@...gle.com> writes:
> 
> > On Mon, Jun 3, 2024 at 9:30 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
> >>
> >> Eric Dumazet <edumazet@...gle.com> writes:
> >>
> >> > On Wed, May 29, 2024 at 1:21 PM Petr Machata <petrm@...dia.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)));
> >
> > Good to know, thanks.
> >
> > Not sure why __kernel qualifier has been put there.
> 
> Not sure either. Paul, care to enlighten us? :)

Because __kernel says "just plain kernel access".  Here are the options:

# define __kernel       __attribute__((address_space(0)))
# define __user         __attribute__((noderef, address_space(__user)))
# define __iomem        __attribute__((noderef, address_space(__iomem)))
# define __percpu       __attribute__((noderef, address_space(__percpu)))
# define __rcu          __attribute__((noderef, address_space(__rcu)))

So casting to __kernel removes the __rcu, thus avoiding the sparse
complaint.

							Thanx, Paul