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>

1 files changed, 31 insertions, 0 deletions

diff --git a/net/mac80211/iface.c b/net/mac80211/iface.c
index 8dc2c2188d92..00562a8b99cf 100644
--- a/net/mac80211/iface.c
+++ b/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);
         }
 }