mac80211: optimise AP stop RCU handling

If there are VLANs, stopping an AP is inefficient as it
calls rcu_barrier() once for each interface (the VLANs
and the AP itself). Optimise this by moving rcu_barrier()
out of the station cleanups and calling it only once for
all interfaces combined.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/cfg.c

@@ -1010,8 +1010,13 @@ static int ieee80211_stop_ap(struct wiphy *wiphy, struct net_device *dev)
 		kfree_rcu(old_probe_resp, rcu_head);
 
 	list_for_each_entry(vlan, &sdata->u.ap.vlans, u.vlan.list)
-		sta_info_flush(vlan);
-	sta_info_flush(sdata);
+		sta_info_flush_defer(vlan);
+	sta_info_flush_defer(sdata);
+	rcu_barrier();
+	list_for_each_entry(vlan, &sdata->u.ap.vlans, u.vlan.list)
+		sta_info_flush_cleanup(vlan);
+	sta_info_flush_cleanup(sdata);
+
 	ieee80211_bss_info_change_notify(sdata, BSS_CHANGED_BEACON_ENABLED);
 
 	drv_stop_ap(sdata->local, sdata);

net/mac80211/iface.c

@@ -863,12 +863,15 @@ static void ieee80211_do_stop(struct ieee80211_sub_if_data *sdata,
 		cancel_work_sync(&sdata->work);
 		/*
 		 * When we get here, the interface is marked down.
-		 * sta_info_flush_cleanup() calls rcu_barrier to
-		 * wait for the station call_rcu() calls to complete,
-		 * here we require it to wait for the RX path in case
-		 * it is using the interface and enqueuing frames at
-		 * this very time on another CPU.
+		 *
+		 * sta_info_flush_cleanup() requires rcu_barrier()
+		 * first to wait for the station call_rcu() calls
+		 * to complete, here we need at least sychronize_rcu()
+		 * it to wait for the RX path in case it is using the
+		 * interface and enqueuing frames at this very time on
+		 * another CPU.
 		 */
+		rcu_barrier();
 		sta_info_flush_cleanup(sdata);
 
 		skb_queue_purge(&sdata->skb_queue);

net/mac80211/sta_info.c

@@ -920,8 +920,6 @@ int sta_info_flush_defer(struct ieee80211_sub_if_data *sdata)
 
 void sta_info_flush_cleanup(struct ieee80211_sub_if_data *sdata)
 {
-	rcu_barrier();
-
 	ieee80211_cleanup_sdata_stas(sdata);
 	cancel_work_sync(&sdata->cleanup_stations_wk);
 }

net/mac80211/sta_info.h

@@ -549,6 +549,19 @@ void sta_info_recalc_tim(struct sta_info *sta);
 void sta_info_init(struct ieee80211_local *local);
 void sta_info_stop(struct ieee80211_local *local);
 int sta_info_flush_defer(struct ieee80211_sub_if_data *sdata);
+
+/**
+ * sta_info_flush_cleanup - flush the sta_info cleanup queue
+ * @sdata: the interface
+ *
+ * Flushes the sta_info cleanup queue for a given interface;
+ * this is necessary before the interface is removed or, for
+ * AP/mesh interfaces, before it is deconfigured.
+ *
+ * Note an rcu_barrier() must precede the function, after all
+ * stations have been flushed/removed to ensure the call_rcu()
+ * calls that add stations to the cleanup queue have completed.
+ */
 void sta_info_flush_cleanup(struct ieee80211_sub_if_data *sdata);
 
 /**
@@ -562,6 +575,7 @@ static inline int sta_info_flush(struct ieee80211_sub_if_data *sdata)
 {
 	int ret = sta_info_flush_defer(sdata);
 
+	rcu_barrier();
 	sta_info_flush_cleanup(sdata);
 
 	return ret;