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Subject[PATCH tip/core/rcu 08/14] ipv6/sit: Apply ACCESS_ONCE() to avoid sparse false positive
Date
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
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@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/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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