netdev - Re: [PATCH v2 tip/core/rcu 05/13] decnet: Apply rcu_access

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Date:	Wed, 9 Oct 2013 15:28:42 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <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,
	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>,
	Thomas Graf <tgraf@...g.ch>, Gao feng <gaofeng@...fujitsu.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	linux-decnet-user@...ts.sourceforge.net, netdev@...r.kernel.org
Subject: Re: [PATCH v2 tip/core/rcu 05/13] decnet: Apply rcu_access_pointer()
 to avoid sparse false positive

On Wed, Oct 09, 2013 at 02:29:38PM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@...ux.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@...el.com>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: "David S. Miller" <davem@...emloft.net>
> Cc: Thomas Graf <tgraf@...g.ch>
> Cc: Gao feng <gaofeng@...fujitsu.com>
> Cc: Stephen Hemminger <shemminger@...tta.com>
> Cc: linux-decnet-user@...ts.sourceforge.net
> Cc: netdev@...r.kernel.org

With or without the above typo fix:

Reviewed-by: Josh Triplett <josh@...htriplett.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
> 
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