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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Oct 09, 2013 at 02:42:29PM -0700, Eric Dumazet wrote:
> On Wed, 2013-10-09 at 14:29 -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
> > 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@intel.com>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > Cc: "David S. Miller" <davem@davemloft.net>
> > Cc: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
> > Cc: James Morris <jmorris@namei.org>
> > Cc: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
> > Cc: Patrick McHardy <kaber@trash.net>
> > Cc: netdev@vger.kernel.org
> > ---
> >  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.

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?

							Thanx, Paul