Index: head/sys/kern/kern_umtx.c
===================================================================
--- head/sys/kern/kern_umtx.c
+++ head/sys/kern/kern_umtx.c
@@ -2456,7 +2456,8 @@
 			break;
 		}
 
-		if (owner == UMUTEX_CONTESTED) {
+		if (rv == 0) {
+			MPASS(owner == UMUTEX_CONTESTED);
 			rv = suword32(&m->m_ceilings[0], ceiling);
 			rv1 = suword32(&m->m_owner, UMUTEX_CONTESTED);
 			error = (rv == 0 && rv1 == 0) ? 0: EFAULT;