netdev - Re: [PATCH net-next 1/4] net: mpls: Convert number of nexthops to u8

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Date:   Tue, 28 Mar 2017 09:25:54 -0600
From:   David Ahern <dsa@...ulusnetworks.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     netdev@...r.kernel.org, roopa@...ulusnetworks.com,
        rshearma@...cade.com
Subject: Re: [PATCH net-next 1/4] net: mpls: Convert number of nexthops to u8

On 3/27/17 4:54 PM, Eric W. Biederman wrote:
> It is absolutely a no-brainer to change rt_nhn to a u8.  And I very much
> appreciate all work to keep mpls_route into a single cache line.  As in
> practices that is one of the most important parts to performance.
> 
> Which leads to the functions mpls_ifup, mpls_ifdown, and
> mpls_select_multipath.
> 
> To make this all work correctly we need a couple of things.
> - A big fat comment on struct mpls_route and mpls_nh about how
>   and why these structures are modified and not replaced during
>   nexthop processing.  Including the fact that it all modifications
>   may only happen with rntl_lock held.
> 
> - The use of READ_ONCE and WRITE_ONCE on all rt->rt_nhn_alive accesses,
>   that happen after the route is installed (and is thus rcu reachable).
> 
> - The use of READ_ONCE and WRITE_ONCE on all nh->nh_flags accesses,
>   that happen after the route is installed (and is thus rcu reachable).

For both of these, mpls_select_multipath does need to use READ_ONCE to
read the nh_flags and rt_nhn_alive. In this case it is reading a value
that could change behind its back.

The READ_ONCE is not necessary for mpls_ifdown or mpls_ifup as these are
the functions that change the values. These 2 functions only need a
WRITE_ONCE for both struct members.