Linux Ethernet Bridging - Nov 2013 - [Bridge] [PATCH tip/core/rcu 10/14] bridge/br_mdb: Apply ACCESS_ONCE() to avoid sparse false positive

From: "Paul E. McKenney" <paulmck at 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
__br_mdb_del() is legitimate: They are 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 these false positives by laundering
the pointers using ACCESS_ONCE() as suggested by Eric Dumazet and Josh
Triplett.

Reported-by: kbuild test robot <fengguang.wu at intel.com>
Signed-off-by: Paul E. McKenney <paulmck at linux.vnet.ibm.com>
Cc: Stephen Hemminger <stephen at networkplumber.org>
Cc: "David S. Miller" <davem at davemloft.net>
Cc: bridge at lists.linux-foundation.org
Cc: netdev at vger.kernel.org
---
 net/bridge/br_mdb.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/bridge/br_mdb.c b/net/bridge/br_mdb.c
index 85a09bb5ca51..de7197ba8695 100644
--- a/net/bridge/br_mdb.c
+++ b/net/bridge/br_mdb.c
@@ -447,7 +447,7 @@ static int __br_mdb_del(struct net_bridge *br, struct
br_mdb_entry *entry)
 		if (p->port->state == BR_STATE_DISABLED)
 			goto unlock;
 
-		rcu_assign_pointer(*pp, p->next);
+		ACCESS_ONCE(*pp) = p->next; /* OK: Both --rcu and visible. */
 		hlist_del_init(&p->mglist);
 		del_timer(&p->timer);
 		call_rcu_bh(&p->rcu, br_multicast_free_pg);
-- 
1.8.1.5