mac80211: add interface list lock

Using only the RTNL has a number of problems, most notably that
ieee80211_iterate_active_interfaces() and other interface list
traversals cannot be done from the internal workqueue because it
needs to be flushed under the RTNL.

This patch introduces a new mutex that protects the interface list
against modifications. A more detailed explanation is part of the
code change.
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

include/net/mac80211.h

@@ -928,9 +928,8 @@ enum ieee80211_hw_flags {
  * @workqueue: single threaded workqueue available for driver use,
  *	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())
+ *	NOTICE: All work performed on this workqueue must not
+ *	acquire the RTNL lock.
  *
  * @priv: pointer to private area that was allocated for driver use
  *	along with this structure.

net/mac80211/ieee80211_i.h

@@ -643,7 +643,9 @@ struct ieee80211_local {
 	struct crypto_blkcipher *wep_rx_tfm;
 	u32 wep_iv;
 
+	/* see iface.c */
 	struct list_head interfaces;
+	struct mutex iflist_mtx;
 
 	/*
 	 * Key lock, protects sdata's key_list and sta_info's

net/mac80211/iface.c

@@ -21,6 +21,23 @@
 #include "mesh.h"
 #include "led.h"
 
+/**
+ * DOC: Interface list locking
+ *
+ * The interface list in each struct ieee80211_local is protected
+ * three-fold:
+ *
+ * (1) modifications may only be done under the RTNL
+ * (2) modifications and readers are protected against each other by
+ *     the iflist_mtx.
+ * (3) modifications are done in an RCU manner so atomic readers
+ *     can traverse the list in RCU-safe blocks.
+ *
+ * As a consequence, reads (traversals) of the list can be protected
+ * by either the RTNL, the iflist_mtx or RCU.
+ */
+
+
 static int ieee80211_change_mtu(struct net_device *dev, int new_mtu)
 {
 	int meshhdrlen;
@@ -800,7 +817,9 @@ int ieee80211_if_add(struct ieee80211_local *local, const char *name,
 					    params->mesh_id_len,
 					    params->mesh_id);
 
+	mutex_lock(&local->iflist_mtx);
 	list_add_tail_rcu(&sdata->list, &local->interfaces);
+	mutex_unlock(&local->iflist_mtx);
 
 	if (new_dev)
 		*new_dev = ndev;
@@ -816,7 +835,10 @@ void ieee80211_if_remove(struct ieee80211_sub_if_data *sdata)
 {
 	ASSERT_RTNL();
 
+	mutex_lock(&sdata->local->iflist_mtx);
 	list_del_rcu(&sdata->list);
+	mutex_unlock(&sdata->local->iflist_mtx);
+
 	synchronize_rcu();
 	unregister_netdevice(sdata->dev);
 }
@@ -832,7 +854,16 @@ void ieee80211_remove_interfaces(struct ieee80211_local *local)
 	ASSERT_RTNL();
 
 	list_for_each_entry_safe(sdata, tmp, &local->interfaces, list) {
+		/*
+		 * we cannot hold the iflist_mtx across unregister_netdevice,
+		 * but we only need to hold it for list modifications to lock
+		 * out readers since we're under the RTNL here as all other
+		 * writers.
+		 */
+		mutex_lock(&local->iflist_mtx);
 		list_del(&sdata->list);
+		mutex_unlock(&local->iflist_mtx);
+
 		unregister_netdevice(sdata->dev);
 	}
 }

net/mac80211/main.c

@@ -758,6 +758,7 @@ struct ieee80211_hw *ieee80211_alloc_hw(size_t priv_data_len,
 	local->hw.conf.radio_enabled = true;
 
 	INIT_LIST_HEAD(&local->interfaces);
+	mutex_init(&local->iflist_mtx);
 
 	spin_lock_init(&local->key_lock);
 
@@ -1008,6 +1009,8 @@ void ieee80211_free_hw(struct ieee80211_hw *hw)
 {
 	struct ieee80211_local *local = hw_to_local(hw);
 
+	mutex_destroy(&local->iflist_mtx);
+
 	wiphy_free(local->hw.wiphy);
 }
 EXPORT_SYMBOL(ieee80211_free_hw);

net/mac80211/util.c

@@ -468,7 +468,7 @@ void ieee80211_iterate_active_interfaces(
 	struct ieee80211_local *local = hw_to_local(hw);
 	struct ieee80211_sub_if_data *sdata;
 
-	rtnl_lock();
+	mutex_lock(&local->iflist_mtx);
 
 	list_for_each_entry(sdata, &local->interfaces, list) {
 		switch (sdata->vif.type) {
@@ -489,7 +489,7 @@ void ieee80211_iterate_active_interfaces(
 				 &sdata->vif);
 	}
 
-	rtnl_unlock();
+	mutex_unlock(&local->iflist_mtx);
 }
 EXPORT_SYMBOL_GPL(ieee80211_iterate_active_interfaces);