Re: [PATCH v2 tip/core/rcu 05/13] decnet: Apply rcu_access_pointer() to avoid sparse false positive

[Josh Triplett <josh@joshtriplett.org>]

On Wed, Oct 09, 2013 at 02:29:38PM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> 
> 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
> dn_insert_route() is legitimate: It is assigning a pointer to an element

Nit: "uses ... are", not "use ... is". :)

> 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@intel.com>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: Thomas Graf <tgraf@suug.ch>
> Cc: Gao feng <gaofeng@cn.fujitsu.com>
> Cc: Stephen Hemminger <shemminger@vyatta.com>
> Cc: linux-decnet-user@lists.sourceforge.net
> Cc: netdev@vger.kernel.org

With or without the above typo fix:

Reviewed-by: Josh Triplett <josh@joshtriplett.org>

> ---
>  net/decnet/dn_route.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/net/decnet/dn_route.c b/net/decnet/dn_route.c
> index fe32388ea24f..3b1357bcfc92 100644
> --- a/net/decnet/dn_route.c
> +++ b/net/decnet/dn_route.c
> @@ -345,7 +345,7 @@ static int dn_insert_route(struct dn_route *rt, unsigned int hash, struct dn_rou
>  			/* Put it first */
>  			*rthp = rth->dst.dn_next;
>  			rcu_assign_pointer(rth->dst.dn_next,
> -					   dn_rt_hash_table[hash].chain);
> +					   rcu_access_pointer(dn_rt_hash_table[hash].chain));
>  			rcu_assign_pointer(dn_rt_hash_table[hash].chain, rth);
>  
>  			dst_use(&rth->dst, now);
> @@ -358,7 +358,8 @@ static int dn_insert_route(struct dn_route *rt, unsigned int hash, struct dn_rou
>  		rthp = &rth->dst.dn_next;
>  	}
>  
> -	rcu_assign_pointer(rt->dst.dn_next, dn_rt_hash_table[hash].chain);
> +	rcu_assign_pointer(rt->dst.dn_next,
> +			   rcu_access_pointer(dn_rt_hash_table[hash].chain));
>  	rcu_assign_pointer(dn_rt_hash_table[hash].chain, rt);
>  
>  	dst_use(&rt->dst, now);
> -- 
> 1.8.1.5
>