mac80211: fix debugfs lockup

When debugfs_create_dir fails, sta_info_debugfs_add_work will not
terminate because it will find the same station again and again.
This is possible whenever debugfs fails for whatever reason; one
reason is a race condition in mac80211, unfortunately we cannot
do much about it, so just document it, it just means some station
may be missing from debugfs.
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: Robin Holt <holt@sgi.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/debugfs_sta.c

@@ -249,11 +249,22 @@ void ieee80211_sta_debugfs_add(struct sta_info *sta)
 	DECLARE_MAC_BUF(mbuf);
 	u8 *mac;
 
+	sta->debugfs.add_has_run = true;
+
 	if (!stations_dir)
 		return;
 
 	mac = print_mac(mbuf, sta->sta.addr);
 
+	/*
+	 * This might fail due to a race condition:
+	 * When mac80211 unlinks a station, the debugfs entries
+	 * remain, but it is already possible to link a new
+	 * station with the same address which triggers adding
+	 * it to debugfs; therefore, if the old station isn't
+	 * destroyed quickly enough the old station's debugfs
+	 * dir might still be around.
+	 */
 	sta->debugfs.dir = debugfs_create_dir(mac, stations_dir);
 	if (!sta->debugfs.dir)
 		return;

net/mac80211/sta_info.c

@@ -635,7 +635,12 @@ static void sta_info_debugfs_add_work(struct work_struct *work)
 
 		spin_lock_irqsave(&local->sta_lock, flags);
 		list_for_each_entry(tmp, &local->sta_list, list) {
-			if (!tmp->debugfs.dir) {
+			/*
+			 * debugfs.add_has_run will be set by
+			 * ieee80211_sta_debugfs_add regardless
+			 * of what else it does.
+			 */
+			if (!tmp->debugfs.add_has_run) {
 				sta = tmp;
 				__sta_info_pin(sta);
 				break;

net/mac80211/sta_info.h

@@ -300,6 +300,7 @@ struct sta_info {
 		struct dentry *inactive_ms;
 		struct dentry *last_seq_ctrl;
 		struct dentry *agg_status;
+		bool add_has_run;
 	} debugfs;
 #endif