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[PATCH v2] USB: appledisplay: Remove deprecated create_singlethread_workqueue

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  [PATCH v2] USB: appledisplay: Remove deprecated  create_singlethread_workqueue Bhaktipriya Shridhar <bhaktipriya96@gmail.com> - 2016-07-26 18:50 +0200
    Re: [PATCH v2] USB: appledisplay: Remove deprecated  create_singlethread_workqueue Tejun Heo <tj@kernel.org> - 2016-07-26 19:00 +0200

#1450682 — [PATCH v2] 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.

Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>
---
 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(-)

diff --git a/drivers/usb/misc/appledisplay.c b/drivers/usb/misc/appledisplay.c
index a0a3827..188c8d2 100644
--- a/drivers/usb/misc/appledisplay.c
+++ b/drivers/usb/misc/appledisplay.c
@@ -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

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#1450691

On Tue, Jul 26, 2016 at 10:19:18PM +0530, Bhaktipriya Shridhar wrote:
> 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.
> 
> Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>

Acked-by: Tejun Heo <tj@kernel.org>

Thanks.

-- 
tejun

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