batman-adv: Remove atomic usage for tt.local_changes

The tt.local_changes atomic is either written with tt.changes_list_lock
or close to it (see batadv_tt_local_event()). Thus the performance gain
using an atomic was limited (or because of atomic_read() impact even
negative). Using atomic also comes with the need to be wary of potential
negative tt.local_changes value.

Simplify the tt.local_changes usage by removing the atomic property and
modifying it only with tt.changes_list_lock held.

Signed-off-by: Remi Pommarel <repk@triplefau.lt>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

net/batman-adv/soft-interface.c

@@ -783,13 +783,13 @@ static int batadv_softif_init_late(struct net_device *dev)
 	atomic_set(&bat_priv->mesh_state, BATADV_MESH_INACTIVE);
 	atomic_set(&bat_priv->bcast_seqno, 1);
 	atomic_set(&bat_priv->tt.vn, 0);
-	atomic_set(&bat_priv->tt.local_changes, 0);
 	atomic_set(&bat_priv->tt.ogm_append_cnt, 0);
 #ifdef CONFIG_BATMAN_ADV_BLA
 	atomic_set(&bat_priv->bla.num_requests, 0);
 #endif
 	atomic_set(&bat_priv->tp_num, 0);
 
+	WRITE_ONCE(bat_priv->tt.local_changes, 0);
 	bat_priv->tt.last_changeset = NULL;
 	bat_priv->tt.last_changeset_len = 0;
 	bat_priv->isolation_mark = 0;

net/batman-adv/translation-table.c

@@ -423,8 +423,8 @@ static void batadv_tt_local_event(struct batadv_priv *bat_priv,
 	struct batadv_tt_change_node *tt_change_node, *entry, *safe;
 	struct batadv_tt_common_entry *common = &tt_local_entry->common;
 	u8 flags = common->flags | event_flags;
-	bool event_removed = false;
 	bool del_op_requested, del_op_entry;
+	size_t changes;
 
 	tt_change_node = kmem_cache_alloc(batadv_tt_change_cache, GFP_ATOMIC);
 	if (!tt_change_node)
@@ -440,6 +440,7 @@ static void batadv_tt_local_event(struct batadv_priv *bat_priv,
 
 	/* check for ADD+DEL or DEL+ADD events */
 	spin_lock_bh(&bat_priv->tt.changes_list_lock);
+	changes = READ_ONCE(bat_priv->tt.local_changes);
 	list_for_each_entry_safe(entry, safe, &bat_priv->tt.changes_list,
 				 list) {
 		if (!batadv_compare_eth(entry->change.addr, common->addr))
@@ -468,21 +469,18 @@ static void batadv_tt_local_event(struct batadv_priv *bat_priv,
 del:
 		list_del(&entry->list);
 		kmem_cache_free(batadv_tt_change_cache, entry);
+		changes--;
 		kmem_cache_free(batadv_tt_change_cache, tt_change_node);
-		event_removed = true;
-		goto unlock;
+		goto update_changes;
 	}
 
 	/* track the change in the OGMinterval list */
 	list_add_tail(&tt_change_node->list, &bat_priv->tt.changes_list);
+	changes++;
 
-unlock:
+update_changes:
+	WRITE_ONCE(bat_priv->tt.local_changes, changes);
 	spin_unlock_bh(&bat_priv->tt.changes_list_lock);
-
-	if (event_removed)
-		atomic_dec(&bat_priv->tt.local_changes);
-	else
-		atomic_inc(&bat_priv->tt.local_changes);
 }
 
 /**
@@ -950,7 +948,7 @@ static void batadv_tt_tvlv_container_update(struct batadv_priv *bat_priv)
 	int tt_diff_entries_count = 0;
 	u16 tvlv_len;
 
-	tt_diff_entries_num = atomic_read(&bat_priv->tt.local_changes);
+	tt_diff_entries_num = READ_ONCE(bat_priv->tt.local_changes);
 	tt_diff_len = batadv_tt_len(tt_diff_entries_num);
 
 	/* if we have too many changes for one packet don't send any
@@ -970,7 +968,7 @@ static void batadv_tt_tvlv_container_update(struct batadv_priv *bat_priv)
 		goto container_register;
 
 	spin_lock_bh(&bat_priv->tt.changes_list_lock);
-	atomic_set(&bat_priv->tt.local_changes, 0);
+	WRITE_ONCE(bat_priv->tt.local_changes, 0);
 
 	list_for_each_entry_safe(entry, safe, &bat_priv->tt.changes_list,
 				 list) {
@@ -1380,7 +1378,7 @@ static void batadv_tt_changes_list_free(struct batadv_priv *bat_priv)
 		kmem_cache_free(batadv_tt_change_cache, entry);
 	}
 
-	atomic_set(&bat_priv->tt.local_changes, 0);
+	WRITE_ONCE(bat_priv->tt.local_changes, 0);
 	spin_unlock_bh(&bat_priv->tt.changes_list_lock);
 }
 
@@ -3634,7 +3632,7 @@ static void batadv_tt_local_commit_changes_nolock(struct batadv_priv *bat_priv)
 {
 	lockdep_assert_held(&bat_priv->tt.commit_lock);
 
-	if (atomic_read(&bat_priv->tt.local_changes) < 1) {
+	if (READ_ONCE(bat_priv->tt.local_changes) == 0) {
 		if (!batadv_atomic_dec_not_zero(&bat_priv->tt.ogm_append_cnt))
 			batadv_tt_tvlv_container_update(bat_priv);
 		return;

net/batman-adv/types.h

@@ -1022,7 +1022,7 @@ struct batadv_priv_tt {
 	atomic_t ogm_append_cnt;
 
 	/** @local_changes: changes registered in an originator interval */
-	atomic_t local_changes;
+	size_t local_changes;
 
 	/**
 	 * @changes_list: tracks tt local changes within an originator interval
@@ -1044,7 +1044,7 @@ struct batadv_priv_tt {
 	 */
 	struct list_head roam_list;
 
-	/** @changes_list_lock: lock protecting changes_list */
+	/** @changes_list_lock: lock protecting changes_list & local_changes */
 	spinlock_t changes_list_lock;
 
 	/** @req_list_lock: lock protecting req_list */