netdev - Re: [PATCH v2 net-next 4/7] ipv6: Plumb support for nexthop object in a fib6

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Date:   Tue, 4 Jun 2019 05:29:32 +0000
From:   Martin Lau <kafai@...com>
To:     David Ahern <dsahern@...il.com>
CC:     Wei Wang <weiwan@...gle.com>, David Ahern <dsahern@...nel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "idosch@...lanox.com" <idosch@...lanox.com>,
        "saeedm@...lanox.com" <saeedm@...lanox.com>
Subject: Re: [PATCH v2 net-next 4/7] ipv6: Plumb support for nexthop object in
 a fib6_info

On Mon, Jun 03, 2019 at 07:36:06PM -0600, David Ahern wrote:
> On 6/3/19 6:58 PM, Martin Lau wrote:
> > I have concern on calling ip6_create_rt_rcu() in general which seems
> > to trace back to this commit
> > dec9b0e295f6 ("net/ipv6: Add rt6_info create function for ip6_pol_route_lookup")
> > 
> > This rt is not tracked in pcpu_rt, rt6_uncached_list or exception bucket.
> > In particular, how to react to NETDEV_UNREGISTER/DOWN like
> > the rt6_uncached_list_flush_dev() does and calls dev_put()?
> > 
> > The existing callers seem to do dst_release() immediately without
> > caching it, but still concerning.
> 
> those are the callers that don't care about the dst_entry, but are
> forced to deal with it. Removing the tie between fib lookups an
> dst_entry is again the right solution.
Great to know that there will be a solution.  It would be great
if there is patch (or repo) to show how that may look like on
those rt6_lookup() callers.

Before that,
although it seems fine now (__ip6_route_redirect() is
harder to confirm since rt is passed around but it
seems to be ok),
instead of risking for "unregister_netdevice: waiting for eth0 to become free"
in case some future patch is caching this rt,
why pcpu_rt cannot be used in all occasions? and also
avoid re-creating the same rt.