netdev - Re: [RFC PATCH net-next] ip6: Do not expire uncached routes for mtu invalidation

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Date:	Mon, 08 Sep 2014 10:20:26 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alex Gartrell <agartrell@...com>
Cc:	davem@...emloft.net, edumazet@...gle.com, netdev@...r.kernel.org,
	kernel-team@...com, ps@...com
Subject: Re: [RFC PATCH net-next] ip6: Do not expire uncached routes for mtu
 invalidation

On Mon, 2014-09-08 at 10:12 -0700, Alex Gartrell wrote:
> Thank you for taking a look, Eric.
> 
> I'll admit that I have a distinct lack of confidence that I've got the 
> right solution to the problem here, but I've made it about as far as I 
> can without getting your collective comments, so it's much appreciated.
..
> >>
> >>   static inline void rt6_set_from(struct rt6_info *rt, struct rt6_info *from)
> >> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> >> index f74b041..a509a06 100644
> >> --- a/net/ipv6/route.c
> >> +++ b/net/ipv6/route.c
> >> @@ -947,8 +947,19 @@ restart:
> >>   		nrt = rt6_alloc_cow(rt, &fl6->daddr, &fl6->saddr);
> >>   	else if (!(rt->dst.flags & DST_HOST))
> >>   		nrt = rt6_alloc_clone(rt, &fl6->daddr);
> >> -	else
> >> +	else {
> >> +		if (!(rt->rt6i_flags & RTF_EXPIRES) && rt->dst.expires &&
> >> +		    time_after(jiffies, rt->dst.expires)) {
> >> +			/* Uncached routes may have expires set if we
> >> +			 * intend to expire the MTU but not the dest
> >> +			 * itself.  In that case, we should reset the mtu
> >> +			 * before handing it back */
> >> +			dst_metric_set(&rt->dst, RTAX_MTU, 0);
> >> +			rt6_clean_expires(rt);
> >> +			rt->rt6i_flags &= ~RTF_MODIFIED;
> >
> > Many cpus can perform this at the same time on same route, this looks
> > racy.
> 
> Initially I was just going to agree with you here, but taking another 
> look at ip_vs_xmit at least, there doesn't appear to be any special 
> locking before invoking ->update_pmtu, which is playing with rt6i_flags 
> and dst.expires as well.  Is that racy as well or is there something 
> else I'm missing here?
> 
> There are other ways to skin this particular cat though, and I've got no 
> specific attachment to any of them.  The most logical thing to do IMO is 
> clone the route when it may be necessary to do so, but given the fact 
> that that was very deliberately undone in 7343ff3 "ipv6: Don't create 
> clones of host routes," I'm not sure that it's the right thing to do or 
> that it won't require major surgery.

Have you followed thread started yesterday ?

https://patchwork.ozlabs.org/patch/386739/

Reverting 7343ff3 "ipv6: Don't create clones of host routes" was
considered as a matter of fact, when I replied :

"This means we have to clone all routes."



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