netdev - [PATCH v3 tip/core/rcu 09/14] ipv6/sit: Apply ACCESS

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Message-Id: <1381533451-29018-9-git-send-email-paulmck@linux.vnet.ibm.com>
Date:	Fri, 11 Oct 2013 16:17:26 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	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, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"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: [PATCH v3 tip/core/rcu 09/14] ipv6/sit: Apply ACCESS_ONCE() to avoid sparse false positive

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
ipip6_tunnel_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 ACCESS_ONCE() as suggested by Eric Dumazet and 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/sit.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/sit.c b/net/ipv6/sit.c
index 7ee5cb96db34..9b976a4b463d 100644
--- a/net/ipv6/sit.c
+++ b/net/ipv6/sit.c
@@ -157,7 +157,8 @@ static void ipip6_tunnel_unlink(struct sit_net *sitn, struct ip_tunnel *t)
 	     (iter = rtnl_dereference(*tp)) != NULL;
 	     tp = &iter->next) {
 		if (t == iter) {
-			rcu_assign_pointer(*tp, t->next);
+			/* Both --rcu and visible, so ACCESS_ONCE() is OK. */
+			ACCESS_ONCE(*tp) = t->next;
 			break;
 		}
 	}
-- 
1.8.1.5

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