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>

3 files changed, 18 insertions, 1 deletions

diff --git a/net/mac80211/debugfs_sta.c b/net/mac80211/debugfs_sta.c
index b9902e425f09..189d0bafa91a 100644
--- a/net/mac80211/debugfs_sta.c
+++ b/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;
diff --git a/net/mac80211/sta_info.c b/net/mac80211/sta_info.c
index 9b72d15bc8dc..7fef8ea1f5ec 100644
--- a/net/mac80211/sta_info.c
+++ b/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;
diff --git a/net/mac80211/sta_info.h b/net/mac80211/sta_info.h
index a6b51862a89d..168a39a298bd 100644
--- a/net/mac80211/sta_info.h
+++ b/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