bcache: use delayed kworker fo asynchronous devices registration

This patch changes the asynchronous registration kworker to a delayed
kworker. There is probability queue_work() queues the async registration
kworker to the same CPU (even though very little), then the process
which writing sysfs interface to reigster bcache device may won't return
immeidately. queue_delayed_work() in this patch will delay 10 jiffies
before insert the kworker to run queue, which makes sure the registering
process may always returns to user space in time.

Fixes: 9e23ccf8 ("bcache: asynchronous devices registration")
Signed-off-by: Coly Li <colyli@suse.de>
Cc: Hannes Reinecke <hare@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/super.c

@@ -19,6 +19,7 @@
 #include <linux/genhd.h>
 #include <linux/idr.h>
 #include <linux/kthread.h>
+#include <linux/workqueue.h>
 #include <linux/module.h>
 #include <linux/random.h>
 #include <linux/reboot.h>
@@ -2380,7 +2381,7 @@ static bool bch_is_open(struct block_device *bdev)
 }
 
 struct async_reg_args {
-	struct work_struct reg_work;
+	struct delayed_work reg_work;
 	char *path;
 	struct cache_sb *sb;
 	struct cache_sb_disk *sb_disk;
@@ -2391,7 +2392,7 @@ static void register_bdev_worker(struct work_struct *work)
 {
 	int fail = false;
 	struct async_reg_args *args =
-		container_of(work, struct async_reg_args, reg_work);
+		container_of(work, struct async_reg_args, reg_work.work);
 	struct cached_dev *dc;
 
 	dc = kzalloc(sizeof(*dc), GFP_KERNEL);
@@ -2421,7 +2422,7 @@ static void register_cache_worker(struct work_struct *work)
 {
 	int fail = false;
 	struct async_reg_args *args =
-		container_of(work, struct async_reg_args, reg_work);
+		container_of(work, struct async_reg_args, reg_work.work);
 	struct cache *ca;
 
 	ca = kzalloc(sizeof(*ca), GFP_KERNEL);
@@ -2449,11 +2450,12 @@ static void register_cache_worker(struct work_struct *work)
 static void register_device_aync(struct async_reg_args *args)
 {
 	if (SB_IS_BDEV(args->sb))
-		INIT_WORK(&args->reg_work, register_bdev_worker);
+		INIT_DELAYED_WORK(&args->reg_work, register_bdev_worker);
 	else
-		INIT_WORK(&args->reg_work, register_cache_worker);
+		INIT_DELAYED_WORK(&args->reg_work, register_cache_worker);
 
-	queue_work(system_wq, &args->reg_work);
+	/* 10 jiffies is enough for a delay */
+	queue_delayed_work(system_wq, &args->reg_work, 10);
 }
 
 static ssize_t register_bcache(struct kobject *k, struct kobj_attribute *attr,