From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: [PATCH tip/core/rcu 04/14] decnet: Apply ACCESS_ONCE() to avoid sparse false positive Date: Fri, 15 Nov 2013 16:40:07 -0800 Message-ID: <1384562417-817-4-git-send-email-paulmck@linux.vnet.ibm.com> References: <20131116003946.GA316@linux.vnet.ibm.com> <1384562417-817-1-git-send-email-paulmck@linux.vnet.ibm.com> Cc: 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, "Paul E. McKenney" , "David S. Miller" , Thomas Graf , Gao feng , Stephen Hemminger , linux-decnet-user@lists.sourceforge.net, netdev@vger.kernel.org To: linux-kernel@vger.kernel.org Return-path: Received: from e32.co.us.ibm.com ([32.97.110.150]:41137 "EHLO e32.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754109Ab3KPAke (ORCPT ); Fri, 15 Nov 2013 19:40:34 -0500 Received: from /spool/local by e32.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Fri, 15 Nov 2013 17:40:34 -0700 In-Reply-To: <1384562417-817-1-git-send-email-paulmck@linux.vnet.ibm.com> Sender: netdev-owner@vger.kernel.org List-ID: 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 dn_insert_route() 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 Signed-off-by: Paul E. McKenney Cc: "David S. Miller" Cc: Thomas Graf Cc: Gao feng Cc: Stephen Hemminger Cc: linux-decnet-user@lists.sourceforge.net Cc: netdev@vger.kernel.org --- net/decnet/dn_route.c | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-) diff --git a/net/decnet/dn_route.c b/net/decnet/dn_route.c index fe32388ea24f..a6ef8b025035 100644 --- a/net/decnet/dn_route.c +++ b/net/decnet/dn_route.c @@ -344,8 +344,9 @@ static int dn_insert_route(struct dn_route *rt, unsigned int hash, struct dn_rou if (compare_keys(&rth->fld, &rt->fld)) { /* Put it first */ *rthp = rth->dst.dn_next; - rcu_assign_pointer(rth->dst.dn_next, - dn_rt_hash_table[hash].chain); + /* Both --rcu and visible, so ACCESS_ONCE() is OK. */ + ACCESS_ONCE(rth->dst.dn_next) = + dn_rt_hash_table[hash].chain; rcu_assign_pointer(dn_rt_hash_table[hash].chain, rth); dst_use(&rth->dst, now); @@ -358,7 +359,8 @@ static int dn_insert_route(struct dn_route *rt, unsigned int hash, struct dn_rou rthp = &rth->dst.dn_next; } - rcu_assign_pointer(rt->dst.dn_next, dn_rt_hash_table[hash].chain); + /* Both --rcu and visible, so ACCESS_ONCE() is OK. */ + ACCESS_ONCE(rt->dst.dn_next) = dn_rt_hash_table[hash].chain; rcu_assign_pointer(dn_rt_hash_table[hash].chain, rt); dst_use(&rt->dst, now); -- 1.8.1.5