diff --git a/include/haproxy/fd-t.h b/include/haproxy/fd-t.h
index 1bffb26c4..5e17b6fe0 100644
--- a/include/haproxy/fd-t.h
+++ b/include/haproxy/fd-t.h
@@ -116,6 +116,10 @@ struct fdlist {
 
 /* info about one given fd. Note: only align on cache lines when using threads;
  * 32-bit small archs can put everything in 32-bytes when threads are disabled.
+ *
+ * NOTE: DO NOT REORDER THIS STRUCT AT ALL! Some code parts rely on exact field
+ * ordering, for example fd_takeover() and fd_set_running() want running_mask
+ * immediately followed by thread_mask to perform a double-word-CAS on them.
  */
 struct fdtab {
 	unsigned long running_mask;          /* mask of thread IDs currently using the fd */