USB: appledisplay: Remove deprecated create_singlethread_workqueue

The workqueue "wq" is involved in controlling the brightness of an Apple Cinema Display over USB.

It has a single work item(&pdata->work) per appledisplay and hence doesn't require ordering. Also, it is not being used on a memory reclaim path.

Hence, the singlethreaded workqueue has been replaced with the use of system_wq.

System workqueues have been able to handle high level of concurrency for a long time now and hence it's not required to have a singlethreaded workqueue just to gain concurrency. Unlike a dedicated per-cpu workqueue created with create_singlethread_workqueue(), system_wq allows multiple work items to overlap executions even on the same CPU; however, a per-cpu workqueue doesn't have any CPU locality or global ordering guarantee unless the target CPU is explicitly specified and thus the increase of local concurrency shouldn't make any difference.

The work item is self-requeueing and needs to wait for the in-flight work item to finish before proceeding with destruction. Hence, it has been sync cancelled in appledisplay_disconnect(). This also ensures that there are no pending tasks while disconnecting the driver.

---
Changes in v2:

	-Removed an unncessary blank line that got inserted by mistake
	 in v1.

 drivers/usb/misc/appledisplay.c | 13 ++-----------
 1 file changed, 2 insertions(+), 11 deletions(-)

index a0a3827..188c8d2 100644
@@ -85,7 +85,6 @@ struct appledisplay {
 };
 
 static atomic_t count_displays = ATOMIC_INIT(0);
-static struct workqueue_struct *wq;
 
 static void appledisplay_complete(struct urb *urb)
 {

@@ -122,7 +121,7 @@ static void appledisplay_complete(struct urb *urb)
 	case ACD_BTN_BRIGHT_UP:
 	case ACD_BTN_BRIGHT_DOWN:
 		pdata->button_pressed = 1;
-		queue_delayed_work(wq, &pdata->work, 0);
+		schedule_delayed_work(&pdata->work, 0);
 		break;
 	case ACD_BTN_NONE:
 	default:

@@ -344,7 +343,7 @@ static void appledisplay_disconnect(struct usb_interface *iface)
 
 	if (pdata) {
 		usb_kill_urb(pdata->urb);
-		cancel_delayed_work(&pdata->work);
+		cancel_delayed_work_sync(&pdata->work);
 		backlight_device_unregister(pdata->bd);
 		usb_free_coherent(pdata->udev, ACD_URB_BUFFER_LEN,
 			pdata->urbdata, pdata->urb->transfer_dma);

@@ -365,19 +364,11 @@ static struct usb_driver appledisplay_driver = {
 
 static int __init appledisplay_init(void)
 {
-	wq = create_singlethread_workqueue("appledisplay");
-	if (!wq) {
-		printk(KERN_ERR "appledisplay: Could not create work queue\n");
-		return -ENOMEM;
-	}
-
 	return usb_register(&appledisplay_driver);
 }
 
 static void __exit appledisplay_exit(void)
 {
-	flush_workqueue(wq);
-	destroy_workqueue(wq);
 	usb_deregister(&appledisplay_driver);
 }
 

--
2.1.4

Thanks.

tejun