block/rnbd-clt: Avoid flush_workqueue(system_long_wq) usage

Flushing system-wide workqueues is dangerous and will be forbidden.

Replace system_long_wq with local rnbd_clt_wq.

Link: https://lkml.kernel.org/r/49925af7-78a8-a3dd-bce6-cfc02e1a9236@I-love.SAKURA.ne.jp

Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Jack Wang <jinpu.wang@ionos.com>
Reviewed-by: Santosh Kumar Pradhan <santosh.pradhan@ionos.com>
Link: https://lore.kernel.org/r/20220413123420.66470-1-jinpu.wang@ionos.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/rnbd/rnbd-clt.c

@@ -25,6 +25,7 @@ static int rnbd_client_major;
 static DEFINE_IDA(index_ida);
 static DEFINE_MUTEX(sess_lock);
 static LIST_HEAD(sess_list);
+static struct workqueue_struct *rnbd_clt_wq;
 
 /*
  * Maximum number of partitions an instance can have.
@@ -1759,12 +1760,12 @@ static void rnbd_destroy_sessions(void)
 			 * procedure takes minutes.
 			 */
 			INIT_WORK(&dev->unmap_on_rmmod_work, unmap_device_work);
-			queue_work(system_long_wq, &dev->unmap_on_rmmod_work);
+			queue_work(rnbd_clt_wq, &dev->unmap_on_rmmod_work);
 		}
 		rnbd_clt_put_sess(sess);
 	}
 	/* Wait for all scheduled unmap works */
-	flush_workqueue(system_long_wq);
+	flush_workqueue(rnbd_clt_wq);
 	WARN_ON(!list_empty(&sess_list));
 }
 
@@ -1789,6 +1790,14 @@ static int __init rnbd_client_init(void)
 		pr_err("Failed to load module, creating sysfs device files failed, err: %d\n",
 		       err);
 		unregister_blkdev(rnbd_client_major, "rnbd");
+		return err;
+	}
+	rnbd_clt_wq = alloc_workqueue("rnbd_clt_wq", 0, 0);
+	if (!rnbd_clt_wq) {
+		pr_err("Failed to load module, alloc_workqueue failed.\n");
+		rnbd_clt_destroy_sysfs_files();
+		unregister_blkdev(rnbd_client_major, "rnbd");
+		err = -ENOMEM;
 	}
 
 	return err;
@@ -1799,6 +1808,7 @@ static void __exit rnbd_client_exit(void)
 	rnbd_destroy_sessions();
 	unregister_blkdev(rnbd_client_major, "rnbd");
 	ida_destroy(&index_ida);
+	destroy_workqueue(rnbd_clt_wq);
 }
 
 module_init(rnbd_client_init);