netdev - Re: Race condition in route lookup

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Date:   Tue, 15 Oct 2019 09:42:49 -0700
From:   Wei Wang <weiwan@...gle.com>
To:     David Ahern <dsahern@...il.com>
Cc:     Martin Lau <kafai@...com>, Ido Schimmel <idosch@...sch.org>,
        Jesse Hathaway <jesse@...ki-mvuki.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: Race condition in route lookup

On Tue, Oct 15, 2019 at 7:45 AM David Ahern <dsahern@...il.com> wrote:
>
> On 10/14/19 1:26 PM, Martin Lau wrote:
> >
> > AFAICT, even for the route that are affected by fib6_update_sernum_upto_root(),
> > I don't see the RTF_PCPU route is re-created.  v6 sk does
> > dst_check() => re-lookup the fib6 =>
> > found the same RTF_PCPU (but does not re-create it) =>
> > update the sk with new cookie in ip6_dst_store()
> >
Hmm... That is a good point. Why does v4 need to recreate the dst
cache even though the route itself is not changed?
Now that I think about it, I agree with Martin's previous comment: it
probably is because v4 code does not cache rt->rt_genid into the
socket and every user of the rt is sharing the same rt_genid stored in
the route itself.

>
> That's fine. The pcpu cache is per nexthop (fib6_nh) for a specific
> gateway/device.
>
> The invalidate forces another lookup for the intended destination after
> the change to the fib. If the lookup resolves to the same fib entry and
> nexthop, then re-using the same cached dst/rt6_info is ok.