[PATCH v3 tip/core/rcu 10/14] mac80211: Apply ACCESS_ONCE() to avoid sparse false positive

[Paul E. McKenney]

From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>

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@xxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: "John W. Linville" <linville@xxxxxxxxxxxxx>
Cc: Johannes Berg <johannes@xxxxxxxxxxxxxxxx>
Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: linux-wireless@xxxxxxxxxxxxxxx
Cc: netdev@xxxxxxxxxxxxxxx
---
 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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