commit d801022e99a262133faf67859eb521a5d6ef1d8c
Author: Ryota Ozaki <ozaki-r@iij.ad.jp>
Date:   Wed Dec 20 15:08:25 2017 +0900

    Add workqueue_drain that waits for all pending works to finish
    
    It is useful when you want to safely call workqueue_destroy that requires that
    there remains no work in the queue. Without workqueue_drain the caller had to
    satisfy the constract by itself somehow.
    
    workqueue_drain has a contract that the caller must ensure no new work to be
    enqueued to the workqueue during workqueue_drain.

diff --git a/sys/kern/subr_workqueue.c b/sys/kern/subr_workqueue.c
index 7e651d1ebda..8a0ad9d5267 100644
--- a/sys/kern/subr_workqueue.c
+++ b/sys/kern/subr_workqueue.c
@@ -271,6 +271,38 @@ workqueue_create(struct workqueue **wqp, const char *name,
 	return error;
 }
 
+/*
+ * Wait for all pending works to finish.  The caller must ensure that no new
+ * work will be enqueued before calling workqueue_drain.
+ */
+void
+workqueue_drain(struct workqueue *wq)
+{
+	struct workqueue_queue *q;
+	struct cpu_info *ci;
+	CPU_INFO_ITERATOR cii;
+
+	for (CPU_INFO_FOREACH(cii, ci)) {
+		q = workqueue_queue_lookup(wq, ci);
+		KASSERT(q->q_worker != NULL);
+
+		mutex_enter(&q->q_mutex);
+		while (!SIMPLEQ_EMPTY(&q->q_queue))
+			cv_wait(&q->q_cv, &q->q_mutex);
+		mutex_exit(&q->q_mutex);
+	}
+
+#ifdef DEBUG
+	/* Check if no new work wasn't enqueued during the above loop. */
+	for (CPU_INFO_FOREACH(cii, ci)) {
+		q = workqueue_queue_lookup(wq, ci);
+		mutex_enter(&q->q_mutex);
+		KDASSERT(SIMPLEQ_EMPTY(&q->q_queue));
+		mutex_exit(&q->q_mutex);
+	}
+#endif
+}
+
 void
 workqueue_destroy(struct workqueue *wq)
 {
diff --git a/sys/sys/workqueue.h b/sys/sys/workqueue.h
index 26d974bced2..10cae3429ce 100644
--- a/sys/sys/workqueue.h
+++ b/sys/sys/workqueue.h
@@ -51,6 +51,7 @@ struct workqueue;
 int workqueue_create(struct workqueue **, const char *,
     void (*)(struct work *, void *), void *, pri_t, int, int);
 void workqueue_destroy(struct workqueue *);
+void workqueue_drain(struct workqueue *);
 
 void workqueue_enqueue(struct workqueue *, struct work *, struct cpu_info *);