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" <paul...@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...@intel.com>
> > Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> > Cc: "David S. Miller" <da...@davemloft.net>
> > Cc: Alexey Kuznetsov <kuz...@ms2.inr.ac.ru>
> > Cc: James Morris <jmor...@namei.org>
> > Cc: Hideaki YOSHIFUJI <yoshf...@linux-ipv6.org>
> > Cc: Patrick McHardy <ka...@trash.net>
> > Cc: net...@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
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