xskmap: Move non-standard list manipulation to helper

Add a helper in list.h for the non-standard way of clearing a list that is
used in xskmap. This makes it easier to reuse it in the other map types,
and also makes sure this usage is not forgotten in any list refactorings in
the future.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

include/linux/list.h

@@ -106,6 +106,20 @@ static inline void __list_del(struct list_head * prev, struct list_head * next)
 	WRITE_ONCE(prev->next, next);
 }
 
+/*
+ * Delete a list entry and clear the 'prev' pointer.
+ *
+ * This is a special-purpose list clearing method used in the networking code
+ * for lists allocated as per-cpu, where we don't want to incur the extra
+ * WRITE_ONCE() overhead of a regular list_del_init(). The code that uses this
+ * needs to check the node 'prev' pointer instead of calling list_empty().
+ */
+static inline void __list_del_clearprev(struct list_head *entry)
+{
+	__list_del(entry->prev, entry->next);
+	entry->prev = NULL;
+}
+
 /**
  * list_del - deletes entry from list.
  * @entry: the element to delete from the list.

kernel/bpf/xskmap.c

@@ -145,8 +145,7 @@ void __xsk_map_flush(struct bpf_map *map)
 
 	list_for_each_entry_safe(xs, tmp, flush_list, flush_node) {
 		xsk_flush(xs);
-		__list_del(xs->flush_node.prev, xs->flush_node.next);
-		xs->flush_node.prev = NULL;
+		__list_del_clearprev(&xs->flush_node);
 	}
 }