diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 6c724fde5fd8..fea67727f807 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -5413,6 +5413,9 @@ raid5_store_group_thread_cnt(struct mddev *mddev, const char *page, size_t len)
 	old_groups = conf->worker_groups;
 	old_group_cnt = conf->worker_cnt_per_group;
 
+	if (old_groups)
+		flush_workqueue(raid5_wq);
+
 	conf->worker_groups = NULL;
 	err = alloc_thread_groups(conf, new);
 	if (err) {