Index: head/sys/sys/proc.h
===================================================================
--- head/sys/sys/proc.h
+++ head/sys/sys/proc.h
@@ -381,7 +381,10 @@
 } while (0)
 
 #define	TD_LOCKS_INC(td)	((td)->td_locks++)
-#define	TD_LOCKS_DEC(td)	((td)->td_locks--)
+#define	TD_LOCKS_DEC(td) do {						\
+	KASSERT((td)->td_locks > 0, ("thread %p owns no locks", (td)));	\
+	(td)->td_locks--;						\
+} while (0)
 #else
 #define	THREAD_LOCKPTR_ASSERT(td, lock)