diff --git a/arch/um/kernel/sigio.c b/arch/um/kernel/sigio.c
index 2b0ab43..1c831f9 100644
--- a/arch/um/kernel/sigio.c
+++ b/arch/um/kernel/sigio.c
@@ -42,6 +42,7 @@
 	return(0);
 }
 
+/* These are called from os-Linux/sigio.c to protect its pollfds arrays. */
 static DEFINE_SPINLOCK(sigio_spinlock);
 
 void sigio_lock(void)