From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH v2 tip/core/rcu 07/13] ipv6/ip6_tunnel: Apply rcu_access_pointer() to avoid sparse false positive Date: Wed, 9 Oct 2013 14:57:47 -0700 Message-ID: <20131009215747.GA5790@linux.vnet.ibm.com> References: <20131009212920.GA15413@linux.vnet.ibm.com> <1381354186-16285-1-git-send-email-paulmck@linux.vnet.ibm.com> <1381354186-16285-7-git-send-email-paulmck@linux.vnet.ibm.com> <1381354949.4971.20.camel@edumazet-glaptop.roam.corp.google.com> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, darren@dvhart.com, fweisbec@gmail.com, sbw@mit.edu, "David S. Miller" , Alexey Kuznetsov , James Morris , Hideaki YOSHIFUJI , Patrick McHardy , netdev@vger.kernel.org To: Eric Dumazet Return-path: Content-Disposition: inline In-Reply-To: <1381354949.4971.20.camel@edumazet-glaptop.roam.corp.google.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org 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" > > > > 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 > > Signed-off-by: Paul E. McKenney > > Cc: "David S. Miller" > > Cc: Alexey Kuznetsov > > Cc: James Morris > > Cc: Hideaki YOSHIFUJI > > Cc: Patrick McHardy > > 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