From mboxrd@z Thu Jan  1 00:00:00 1970
From: Eric Dumazet <eric.dumazet@gmail.com>
Subject: Re: [RFC PATCH 0/2] Faster/parallel SYN handling to mitigate SYN
 floods
Date: Wed, 30 May 2012 06:45:37 +0200
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Cc: christoph.paasch@uclouvain.be, netdev@vger.kernel.org,
	"David S. Miller" <davem@davemloft.net>,
	Martin Topholm <mph@hoth.dk>, Florian Westphal <fw@strlen.de>,
	opurdila@ixiacom.com,
	Hans Schillstrom <hans.schillstrom@ericsson.com>,
	Andi Kleen <andi@firstfloor.org>
To: Jesper Dangaard Brouer <brouer@redhat.com>
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On Tue, 2012-05-29 at 22:17 +0200, Jesper Dangaard Brouer wrote:
> On Mon, 2012-05-28 at 18:14 +0200, Christoph Paasch wrote:
> 

> > Concerning (1):
> > I think, there are places where you may have troube because you don't
> > hold the lock.
> > E.g., in tcp_make_synack (called by tcp_v4_send_synack from your
> > tcp_v4_syn_conn_limit) there is:
> > 
> > if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
> > 	(req->window_clamp > tcp_full_space(sk) ||
> > 	 req->window_clamp == 0))
> > 	req->window_clamp = tcp_full_space(sk);
> > 
> > Thus, tcp_full_space(sk) may have different values between the check and
> > setting req->window_clamp.
> 
> This should be simply solved by using a local stack variable, for
> storing the result from tcp_full_space(sk).  Its likely that GCC already
> does this behind our back.
> 

Thats not the proper way to handle that situation.

A local stack variable makes no such guarantee. You need ACCESS_ONCE().

This is exactly the kind of things that RCU takes care of.