linux-kernel - [PATCH v3 tip/core/rcu 10/14] mac80211: Apply ACCESS

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Date:	Fri, 11 Oct 2013 16:17:27 -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>,
	"John W. Linville" <linville@...driver.com>,
	Johannes Berg <johannes@...solutions.net>,
	"David S. Miller" <davem@...emloft.net>,
	linux-wireless@...r.kernel.org, netdev@...r.kernel.org
Subject: [PATCH v3 tip/core/rcu 10/14] mac80211: 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 uses in
sta_info_hash_del() are 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 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: "John W. Linville" <linville@...driver.com>
Cc: Johannes Berg <johannes@...solutions.net>
Cc: "David S. Miller" <davem@...emloft.net>
Cc: linux-wireless@...r.kernel.org
Cc: netdev@...r.kernel.org
---
 net/mac80211/sta_info.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/net/mac80211/sta_info.c b/net/mac80211/sta_info.c
index aeb967a0aeed..494f03c0831f 100644
--- a/net/mac80211/sta_info.c
+++ b/net/mac80211/sta_info.c
@@ -74,8 +74,8 @@ static int sta_info_hash_del(struct ieee80211_local *local,
 	if (!s)
 		return -ENOENT;
 	if (s == sta) {
-		rcu_assign_pointer(local->sta_hash[STA_HASH(sta->sta.addr)],
-				   s->hnext);
+		/* Both --rcu and visible, so ACCESS_ONCE() is OK. */
+		ACCESS_ONCE(local->sta_hash[STA_HASH(sta->sta.addr)]) = s->hnext;
 		return 0;
 	}
 
@@ -84,7 +84,8 @@ static int sta_info_hash_del(struct ieee80211_local *local,
 		s = rcu_dereference_protected(s->hnext,
 					lockdep_is_held(&local->sta_mtx));
 	if (rcu_access_pointer(s->hnext)) {
-		rcu_assign_pointer(s->hnext, sta->hnext);
+		/* Both --rcu and visible, so ACCESS_ONCE() is OK. */
+		ACCESS_ONCE(s->hnext) = sta->hnext;
 		return 0;
 	}
 
-- 
1.8.1.5

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