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

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Date:	Thu, 24 Mar 2016 18:51:20 -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 Thu, 2016-03-24 at 17:15 -0700, Cong Wang wrote:

> My understanding is that bh_lock_sock() prevents concurrent
> access to sock struct. Since this is in softirq context, multiple
> CPU's could call this function concurrently, the whole pmtu
> update needs to be done atomically.
> 
> UDP, on the other hand, doesn't do this logic, it just looks up
> for dst and save it in sk_dst_cache.

Two ICMP messages processed on two different cpus can still get two
different sockets pointing to the same dst.

I do not see how dst pmtu update could be protected by a lock on each
socket. It would require a dst lock , or some simple memory writes that
do not require special synchro.