Subject: Re: [PATCH net-next 3/3] ipv6: obsolete cached dst when removing them from fib tree

From: Paolo Abeni <[email-masked]>
Date: Tue, 17 Oct 2017 22:02:58 +0200

On Tue, 2017-10-17 at 11:58 -0700, Wei Wang wrote:
> On Tue, Oct 17, 2017 at 10:40 AM, Paolo Abeni <[email-masked]> wrote:
> > The commit 2b760fcf5cfb ("ipv6: hook up exception table to store
> > dst cache") partially reverted 1e2ea8ad37be ("ipv6: set
> > dst.obsolete when a cached route has expired").
> > 
> > This change brings back the dst obsoleting and push it a step
> > farther: cached dst are always obsoleted when removed from the
> > fib tree, and removal by time expiration is now performed
> > regardless of dst->__refcnt, to be consistent with what we
> > already do for RTF_GATEWAY dst.
> > 
> > Fixes: 2b760fcf5cfb ("ipv6: hook up exception table to store dst cache")
> > Signed-off-by: Paolo Abeni <[email-masked]>
> > ---
> >  net/ipv6/route.c | 13 +++++++++++--
> >  1 file changed, 11 insertions(+), 2 deletions(-)
> > 
> > diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> > index 8b25a31b6b03..fce740049e3e 100644
> > --- a/net/ipv6/route.c
> > +++ b/net/ipv6/route.c
> > @@ -1147,6 +1147,12 @@ static void rt6_remove_exception(struct rt6_exception_bucket *bucket,
> >         if (!bucket || !rt6_ex)
> >                 return;
> > 
> > +       /* sockets, flow cache, etc. can hold a refence to this dst, be sure
> > +        * they will drop it.
> > +        */
> > +       if (rt6_ex->rt6i)
> > +               rt6_ex->rt6i->dst.obsolete = DST_OBSOLETE_FORCE_CHK;
> > +
> 
> Hmm... I don't really think it is needed. rt6 is created with
> rt6->dst.obsolete set to DST_OBSOLETE_FORCE_CHK. And by the time the
> above function is called, it should still be that value.
> Furthermore, the later call rt6_release() calls dst_dev_put() which
> sets rt6->dst.obsolete to DST_OBSOLETE_DEAD to indicate this route has
> been removed from the tree.

You are right, this looks as not needed, if we keep the chunck below.

> >         net = dev_net(rt6_ex->rt6i->dst.dev);
> >         rt6_ex->rt6i->rt6i_node = NULL;
> >         hlist_del_rcu(&rt6_ex->hlist);
> > @@ -1575,8 +1581,11 @@ static void rt6_age_examine_exception(struct rt6_exception_bucket *bucket,
> >  {
> >         struct rt6_info *rt = rt6_ex->rt6i;
> > 
> > -       if (atomic_read(&rt->dst.__refcnt) == 1 &&
> > -           time_after_eq(now, rt->dst.lastuse + gc_args->timeout)) {
> > +       /* we are pruning and obsoleting the exception route even if others
> > +        * have still reference to it, so that on next dst_check() such
> > +        * reference can be dropped
> > +        */
> > +       if (time_after_eq(now, rt->dst.lastuse + gc_args->timeout)) {
> 
> Why do we want to change this behavior? Before my patch series, cached
> routes were only deleted from the tree in fib6_age() when
> rt->dst.__refcnt == 1, isn't it?

yes, but that really looks like a relic from ancient past more than
something really needed. We already remove from the dst from fib tree
regardless of the refcnt if the gateway validation fails - a few lines
below in the same function.

Waiting for __refcnt going down will let the kernel keep the exception
entry around for much longer - potentially forever, if e.g. we have a
reference in a socket dst cache and the application stops processing
packets. 

Meanwhile others sockets may grab more references to (and use) the same
aged-out dst.

The commit 1e2ea8ad37be ("ipv6: set dst.obsolete when a cached route
has expired") was the solution to the above issue prior to the recent
refactor.

Cheers,

Paolo