mac80211: Add RTNL warning for workqueue

The workqueue provided by mac80211 should not be used for
scheduled tasks that acquire the RTNL lock. This could be done
when the driver uses the function ieee80211_iterate_active_interfaces()
within the scheduled work. Such behavior will end in locking
dependencies problems when an interface is being removed.

This patch will add a notification about the RTNL locking and
the mac80211 workqueue to prevent driver developers from
blindly using it.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

include/net/mac80211.h

@@ -738,8 +738,11 @@ enum ieee80211_hw_flags {
  * @conf: &struct ieee80211_conf, device configuration, don't use.
  *
  * @workqueue: single threaded workqueue available for driver use,
- *	allocated by mac80211 on registration and flushed on
- *	unregistration.
+ *	allocated by mac80211 on registration and flushed when an
+ *	interface is removed.
+ *	NOTICE: All work performed on this workqueue should NEVER
+ *	acquire the RTNL lock (i.e. Don't use the function
+ *	ieee80211_iterate_active_interfaces())
  *
  * @priv: pointer to private area that was allocated for driver use
  *	along with this structure.