netdev - Re: [PATCH net 2/3] net: nexthop: release IPv6 per-cpu dsts when replacing a nexthop group

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Date:   Sun, 21 Nov 2021 19:35:17 +0200
From:   Ido Schimmel <idosch@...sch.org>
To:     Nikolay Aleksandrov <razor@...ckwall.org>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        dsahern@...il.com, Nikolay Aleksandrov <nikolay@...dia.com>
Subject: Re: [PATCH net 2/3] net: nexthop: release IPv6 per-cpu dsts when
 replacing a nexthop group

On Sun, Nov 21, 2021 at 07:17:41PM +0200, Ido Schimmel wrote:
> On Sun, Nov 21, 2021 at 05:24:52PM +0200, Nikolay Aleksandrov wrote:
> > From: Nikolay Aleksandrov <nikolay@...dia.com>
> Can we avoid two synchronize_net() per resilient group by removing the
> one added here and instead do:
> 
> diff --git a/net/ipv4/nexthop.c b/net/ipv4/nexthop.c
> index a69a9e76f99f..a47ce43ab1ff 100644
> --- a/net/ipv4/nexthop.c
> +++ b/net/ipv4/nexthop.c
> @@ -2002,9 +2002,10 @@ static int replace_nexthop_grp(struct net *net, struct nexthop *old,
>  
>         rcu_assign_pointer(old->nh_grp, newg);
>  
> +       /* Make sure concurrent readers are not using 'oldg' anymore. */
> +       synchronize_net();
> +
>         if (newg->resilient) {
> -               /* Make sure concurrent readers are not using 'oldg' anymore. */
> -               synchronize_net();
>                 rcu_assign_pointer(oldg->res_table, tmp_table);
>                 rcu_assign_pointer(oldg->spare->res_table, tmp_table);
>         }

Discussed this with Nik. It is possible and would be a good cleanup for
net-next. For net it is best to leave synchronize_net() where it is so
that the patch will be easier to backport. Resilient nexthop groups were
only added in 5.13 whereas nexthop objects were added in 5.3