diff --git a/lib/libthr/thread/thr_rtld.c b/lib/libthr/thread/thr_rtld.c
index 6f5347de215..d17a4465dbf 100644
--- a/lib/libthr/thread/thr_rtld.c
+++ b/lib/libthr/thread/thr_rtld.c
@@ -140,6 +140,7 @@ _thr_rtld_wlock_acquire(void *lock)
 	THR_CRITICAL_ENTER(curthread);
 	while (_thr_rwlock_wrlock(&l->lock, NULL) != 0)
 		;
+	l->lock.rw_wowner = TID(curthread);
 	RESTORE_ERRNO();
 }
 
diff --git a/lib/libthr/thread/thr_umtx.h b/lib/libthr/thread/thr_umtx.h
index 5dae304528d..9103f8d1d55 100644
--- a/lib/libthr/thread/thr_umtx.h
+++ b/lib/libthr/thread/thr_umtx.h
@@ -37,7 +37,7 @@
 #else
 #define DEFAULT_UMUTEX	{0,0,{0,0},0,0,{0,0}}
 #endif
-#define DEFAULT_URWLOCK {0,0,0,0,{0,0,0,0}}
+#define DEFAULT_URWLOCK {0,0,0,0,0,{0,0,0}}
 
 int _umtx_op_err(void *, int op, u_long, void *, void *) __hidden;
 int __thr_umutex_lock(struct umutex *mtx, uint32_t id) __hidden;
diff --git a/sys/sys/_umtx.h b/sys/sys/_umtx.h
index d94f86b0f56..3903f36db91 100644
--- a/sys/sys/_umtx.h
+++ b/sys/sys/_umtx.h
@@ -56,7 +56,8 @@ struct urwlock {
 	__uint32_t		rw_flags;
 	__uint32_t		rw_blocked_readers;
 	__uint32_t		rw_blocked_writers;
-	__uint32_t		rw_spare[4];
+	__uint32_t		rw_wowner;
+	__uint32_t		rw_spare[3];
 };
 
 struct _usem {