netdev - Re: [PATCH v2 tip/core/rcu 07/13] ipv6/ip6_tunnel: Apply rcu_access

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Message-ID: <20131009215747.GA5790@linux.vnet.ibm.com>
Date:	Wed, 9 Oct 2013 14:57:47 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, 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>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>, netdev@...r.kernel.org
Subject: Re: [PATCH v2 tip/core/rcu 07/13] ipv6/ip6_tunnel: Apply
 rcu_access_pointer() to avoid sparse false positive

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@...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
> > 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@...el.com>
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > Cc: "David S. Miller" <davem@...emloft.net>
> > Cc: Alexey Kuznetsov <kuznet@....inr.ac.ru>
> > Cc: James Morris <jmorris@...ei.org>
> > Cc: Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>
> > Cc: Patrick McHardy <kaber@...sh.net>
> > Cc: netdev@...r.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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