diff --git a/sys/kern/kern_umtx.c b/sys/kern/kern_umtx.c
--- a/sys/kern/kern_umtx.c
+++ b/sys/kern/kern_umtx.c
@@ -2616,6 +2616,10 @@
 			}
 		} else if (owner == UMUTEX_RB_NOTRECOV) {
 			error = ENOTRECOVERABLE;
+		} else if (owner == UMUTEX_CONTESTED) {
+			/* Spurious failure, retry. */
+			umtxq_unbusy_unlocked(&uq->uq_key);
+			continue;
 		}
 
 		if (try != 0)
@@ -2834,6 +2838,14 @@
 		if (error != 0)
 			break;
 
+		/*
+		 * We may have gotten a spurious failure.
+		 */
+		if (owner == UMUTEX_CONTESTED) {
+			umtxq_unbusy_unlocked(&uq->uq_key);
+			continue;
+		}
+
 		/*
 		 * We set the contested bit, sleep. Otherwise the lock changed
 		 * and we need to retry or we lost a race to the thread