linux-kernel - Re: [PATCH v2 tip/core/rcu 07/13] ipv6/ip6_tunnel: Apply rcu_access

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Message-ID: <1381356624.4971.26.camel@edumazet-glaptop.roam.corp.google.com>
Date:	Wed, 09 Oct 2013 15:10:24 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, darren@...art.com, fweisbec@...il.com,
	sbw@....edu, "David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>, netdev@...r.kernel.org
Subject: Re: [PATCH v2 tip/core/rcu 07/13] ipv6/ip6_tunnel: Apply
 rcu_access_pointer() to avoid sparse false positive

On Wed, 2013-10-09 at 14:57 -0700, Paul E. McKenney wrote:

> Hmmm...  I could use RCU_INIT_POINTER().  Something like the following?
> 
> 	RCU_INIT_POINTER(ACCESS_ONCE(*tp), t->next);
> 
> The ACCESS_ONCE() to prevent the compiler from doing anything stupid.
> Presumably the value of t->next cannot change, so a normal load suffices.
> 
> Or did you have something else in mind?

Well, *tp and t->next are both of the same type, with __rcu attribute.

struct ip6_tnl __rcu **tp;

So I meant :

ACCESS_ONCE(*tp) = t->next;

If really we can have a really stupid compiler.


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