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>

4 files changed, 38 insertions, 2 deletions

diff --git a/net/mac80211/ieee80211_i.h b/net/mac80211/ieee80211_i.h
index 927cbde8c19c..eaf3603862b7 100644
--- a/net/mac80211/ieee80211_i.h
+++ b/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
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);
         }
 }
diff --git a/net/mac80211/main.c b/net/mac80211/main.c
index 210dfe3cf6c3..a109c06e8e4e 100644
--- a/net/mac80211/main.c
+++ b/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);
diff --git a/net/mac80211/util.c b/net/mac80211/util.c
index fc30f2940e1e..73c7d7345abd 100644
--- a/net/mac80211/util.c
+++ b/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);