mac80211: make ieee80211_iterate_active_interfaces not need rtnl

Interface iteration in mac80211 can be done without holding any
locks because I converted it to RCU. Initially, I thought this
wouldn't be needed for ieee80211_iterate_active_interfaces but
it's turning out that multi-BSS AP support can be much simpler
in a driver if ieee80211_iterate_active_interfaces can be called
without holding locks. This converts it to use RCU, it adds a
requirement that the callback it invokes cannot sleep.
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/mac80211.h

@@ -1411,10 +1411,9 @@ void ieee80211_scan_completed(struct ieee80211_hw *hw);
  *
  * This function iterates over the interfaces associated with a given
  * hardware that are currently active and calls the callback for them.
- * Must be called under RTNL.
  *
  * @hw: the hardware struct of which the interfaces should be iterated over
- * @iterator: the iterator function to call
+ * @iterator: the iterator function to call, cannot sleep
  * @data: first argument of the iterator function
  */
 void ieee80211_iterate_active_interfaces(struct ieee80211_hw *hw,

net/mac80211/util.c

@@ -482,10 +482,9 @@ void ieee80211_iterate_active_interfaces(struct ieee80211_hw *hw,
 	struct ieee80211_local *local = hw_to_local(hw);
 	struct ieee80211_sub_if_data *sdata;
 
-	ASSERT_RTNL();
+	rcu_read_lock();
 
-	/* we hold the RTNL here so can safely walk the list */
-	list_for_each_entry(sdata, &local->interfaces, list) {
+	list_for_each_entry_rcu(sdata, &local->interfaces, list) {
 		switch (sdata->type) {
 		case IEEE80211_IF_TYPE_INVALID:
 		case IEEE80211_IF_TYPE_MNTR:
@@ -503,5 +502,7 @@ void ieee80211_iterate_active_interfaces(struct ieee80211_hw *hw,
 			iterator(data, sdata->dev->dev_addr,
 				 sdata->dev->ifindex);
 	}
+
+	rcu_read_unlock();
 }
 EXPORT_SYMBOL_GPL(ieee80211_iterate_active_interfaces);