netdev - Re: [RFT Patch net 1/2] ipv6: invalidate the socket cached route on pmtu events if possible

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Date:	Fri, 25 Mar 2016 11:11:17 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Cong Wang <xiyou.wangcong@...il.com>
Cc:	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	Wei Wang <weiwan@...gle.com>,
	Steffen Klassert <steffen.klassert@...unet.com>,
	Martin KaFai Lau <kafai@...com>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Julian Anastasov <ja@....bg>
Subject: Re: [RFT Patch net 1/2] ipv6: invalidate the socket cached route on
 pmtu events if possible

On Fri, 2016-03-25 at 10:17 -0700, Cong Wang wrote:

> 1) sock lock protects the whole update: the whole check, update, recheck,
> set logic, to make sure another CPU will not do the same to the same socket
> at the same time.
> 
> 2) the dst itself is safe, because it is always refcounted, and we use xchg()
> to change the pointer in sk_dst_cache.
> 
> Or am I still missing anything here?

As TCP always lock the socket before doing its heavy stuff,
it can use a variant of sk_dst_cache manipulations that do not use extra
atomic operations.

But UDP gets xchg() to safely exchange sk_dst_cache, because we do not
feel locking the socket is needed for UDP for typical uses (! cork)

If you hold the socket lock in ICMP handler, then it would be
inconsistent with udp sendmsg() where we do not hold the socket lock.

Since I believe udp sendmsg() is fine, I do believe you do not need to
lock the socket, and then care about socket being owned by the user.