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 21:06:21 +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 Tue, Jun 04, 2019 at 02:17:28PM -0600, David Ahern wrote:
> On 6/3/19 11:29 PM, Martin Lau wrote:
> > 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.
> 
> Not 'will be', 'there is' a solution now. Someone just needs to do the
> conversions and devise the tests for the impacted users.
I don't think everyone will convert to the new nexthop solution
immediately.

How about ensuring the existing usage stays solid first?
>> 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.