commit ecbadc21689bfeb4613fa27fca6c8f04203f6399
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..d52ba6736ec 100644
--- a/sys/kern/subr_workqueue.c
+++ b/sys/kern/subr_workqueue.c
@@ -57,6 +57,8 @@ struct workqueue {
 	void (*wq_func)(struct work *, void *);
 	void *wq_arg;
 	int wq_flags;
+	/* Internal flags */
+#define WQ_DRAINING	0x1000	/* Draining. No new work is allowed. */
 
 	char wq_name[MAXCOMLEN];
 	pri_t wq_prio;
@@ -271,6 +273,30 @@ 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;
+
+	wq->wq_flags |= WQ_DRAINING;
+
+	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);
+	}
+}
+
 void
 workqueue_destroy(struct workqueue *wq)
 {
@@ -298,6 +324,8 @@ workqueue_enqueue(struct workqueue *wq, struct work *wk0, struct cpu_info *ci)
 	q = workqueue_queue_lookup(wq, ci);
 
 	mutex_enter(&q->q_mutex);
+	KASSERTMSG((wq->wq_flags & WQ_DRAINING) == 0,
+	    "adding work to a draining workqueue");
 	SIMPLEQ_INSERT_TAIL(&q->q_queue, wk, wk_entry);
 	cv_signal(&q->q_cv);
 	mutex_exit(&q->q_mutex);
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 *);