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

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Date:	Wed, 9 Oct 2013 15:56:18 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	mingo@...nel.org, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	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, Oct 09, 2013 at 03:51:17PM -0700, Eric Dumazet wrote:
> On Wed, 2013-10-09 at 15:36 -0700, Paul E. McKenney wrote:
> 
> > That would work, though it would probably give sparse complaints.
> 
> No sparse error here, as I said types are correct and SPARSE_RCU ready :
> 
> diff --git a/net/ipv6/ip6_tunnel.c b/net/ipv6/ip6_tunnel.c
> index 583b77e..28f8495 100644
> --- a/net/ipv6/ip6_tunnel.c
> +++ b/net/ipv6/ip6_tunnel.c
> @@ -245,7 +245,7 @@ ip6_tnl_unlink(struct ip6_tnl_net *ip6n, struct ip6_tnl *t)
>              (iter = rtnl_dereference(*tp)) != NULL;
>              tp = &iter->next) {
>                 if (t == iter) {
> -                       rcu_assign_pointer(*tp, t->next);
> +                       ACCESS_ONCE(*tp) = t->next;
>                         break;
>                 }
>         }

I'd be really hesitant to introduce that type of direct assignment to an
__rcu pointer without wrapping it in some appropriately named macro, or
at the very least adding a comment.

- Josh Triplett
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