diff --git a/sys/sys/mutex.h b/sys/sys/mutex.h
--- a/sys/sys/mutex.h
+++ b/sys/sys/mutex.h
@@ -37,6 +37,7 @@
 #include <sys/queue.h>
 #include <sys/_lock.h>
 #include <sys/_mutex.h>
+#include <sys/lock.h>
 
 #ifdef _KERNEL
 #include <sys/pcpu.h>