Re: [PATCH v2 tip/core/rcu 07/13] ipv6/ip6_tunnel: Applyrcu_access_pointer() to avoid sparse false positive

[Eric Dumazet]

On Wed, 2013-10-09 at 14:29 -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> 
> The sparse checking for rcu_assign_pointer() was recently upgraded
> to reject non-__kernel address spaces.  This also rejects __rcu,
> which is almost always the right thing to do.  However, the use in
> ip6_tnl_unlink() is legitimate: It is assigning a pointer to an element
> from an RCU-protected list, and all elements of this list are already
> visible to caller.
> 
> This commit therefore silences this false positive by laundering the
> pointer using rcu_access_pointer() as suggested by Josh Triplett.
> 
> Reported-by: kbuild test robot <fengguang.wu@xxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
> Cc: Alexey Kuznetsov <kuznet@xxxxxxxxxxxxx>
> Cc: James Morris <jmorris@xxxxxxxxx>
> Cc: Hideaki YOSHIFUJI <yoshfuji@xxxxxxxxxxxxxx>
> Cc: Patrick McHardy <kaber@xxxxxxxxx>
> Cc: netdev@xxxxxxxxxxxxxxx
> ---
>  net/ipv6/ip6_tunnel.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/ipv6/ip6_tunnel.c b/net/ipv6/ip6_tunnel.c
> index 61355f7f4da5..ecc0166e1a9c 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);
> +			rcu_assign_pointer(*tp, rcu_access_pointer(t->next));
>  			break;
>  		}
>  	}

Then it seems a mere "*tp = t->next;" would be enough  ?

We do not really need a barrier.



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