netdev - Re: [RFC PATCH net 3/4] ipv6: datagram: Update dst cache of a connected datagram sk during pmtu update

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Date:	Tue, 05 Apr 2016 19:56:54 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	xiyou.wangcong@...il.com
Cc:	kafai@...com, netdev@...r.kernel.org, edumazet@...gle.com,
	weiwan@...gle.com, kernel-team@...com
Subject: Re: [RFC PATCH net 3/4] ipv6: datagram: Update dst cache of a
 connected datagram sk during pmtu update

From: Cong Wang <xiyou.wangcong@...il.com>
Date: Mon, 4 Apr 2016 13:45:02 -0700

> On Sat, Apr 2, 2016 at 7:33 PM, Martin KaFai Lau <kafai@...com> wrote:
>> One thing to note is that this patch uses the addresses from the sk
>> instead of iph when updating sk->sk_dst_cache.  It is basically the
>> same logic that the __ip6_datagram_connect() is doing, so some
>> refactoring works in the first two patches.
>>
>> AFAIK, a UDP socket can become connected after sending out some
>> datagrams in un-connected state.  or It can be connected
>> multiple times to different destinations.  I did some quick
>> tests but I could be wrong.
>>
>> I am thinking if there could be a chance that the skb->data, which
>> has the original outgoing iph, is not related to the current
>> connected address.  If it is possible, we have to specifically
>> use the addresses in the sk instead of skb->data (i.e. iph) when
>> updating the sk->sk_dst_cache.
>>
>> If we need to use the sk addresses (and other info) to find out a
>> new dst for a connected udp socket, it is better not doing it while
>> the userland is connecting to somewhere else.
>>
>> If the above case is impossible, we can keep using the info from iph to
>> do the dst update for a connected-udp sk without taking the lock.
> 
> I see your point, but calling __ip6_datagram_connect() seems overkill
> here, we don't need to update so many things in the pmtu update context,
> at least IPv4 doesn't do that either. I don't think you have to do that.
> 
> So why just updating the dst cache (also some addr cache) here is not
> enough?

I think we are steadily getting closer to a version of this fix that
we have some agreement on, right?

Martin can you address Cong's feedback and spin another version of this
series?

Thanks.