Index: sys/kern/kern_umtx.c
===================================================================
--- sys/kern/kern_umtx.c
+++ sys/kern/kern_umtx.c
@@ -616,6 +616,43 @@
 	wakeup(uq);
 }
 
+/*
+ * Wake up a maximum of n_wake threads that are waiting on an userland
+ * object identified by key. The remaining threads are removed from queue
+ * identified by key and added to the queue identified by key2 (requeued).
+ * The n_requeue specifies an upper limit on the number of threads that
+ * are requeued to the second queue.
+ */
+int
+umtxq_requeue(struct umtx_key *key, int n_wake, struct umtx_key *key2,
+    int n_requeue)
+{
+	struct umtxq_queue *uh, *uh2;
+	struct umtx_q *uq;
+	int ret;
+
+	ret = 0;
+	UMTXQ_LOCKED_ASSERT(umtxq_getchain(key));
+	UMTXQ_LOCKED_ASSERT(umtxq_getchain(key2));
+	uh = umtxq_queue_lookup(key, UMTX_SHARED_QUEUE);
+	uh2 = umtxq_queue_lookup(key2, UMTX_SHARED_QUEUE);
+	if (uh != NULL) {
+		while ((uq = TAILQ_FIRST(&uh->head)) != NULL) {
+			if (++ret <= n_wake) {
+				umtxq_remove(uq);
+				wakeup_one(uq);
+			} else {
+				umtxq_remove(uq);
+				uq->uq_key = *key2;
+				umtxq_insert(uq);
+				if (ret - n_wake == n_requeue)
+					break;
+			}
+		}
+	}
+	return (ret);
+}
+
 static inline int
 tstohz(const struct timespec *tsp)
 {
Index: sys/sys/umtxvar.h
===================================================================
--- sys/sys/umtxvar.h
+++ sys/sys/umtxvar.h
@@ -211,6 +211,7 @@
 struct umtxq_chain *umtxq_getchain(struct umtx_key *);
 void umtxq_insert_queue(struct umtx_q *, int);
 void umtxq_remove_queue(struct umtx_q *, int);
+int umtxq_requeue(struct umtx_key *, int, struct umtx_key *, int);
 int umtxq_signal_mask(struct umtx_key *, int, u_int);
 int umtxq_sleep(struct umtx_q *, const char *,
     struct umtx_abs_timeout *);