[PATCH v2] zc3xx: Remove deprecated create_singlethread_workqueue

[PATCH v2] zc3xx: Remove deprecated create_singlethread_workqueue

[Bhaktipriya Shridhar]

The workqueue "work_thread" is involved in updating parameters for
transfers. It has a single work item(&sd->work) 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.

Work item has been flushed in sd_stop0() to ensure that there are no
pending tasks while disconnecting the driver.

Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>
---
 Changes in v2:
	-Added call to flush_work

 drivers/media/usb/gspca/zc3xx.c | 13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/drivers/media/usb/gspca/zc3xx.c b/drivers/media/usb/gspca/zc3xx.c
index c5d8ee6..5f7254d 100644
--- a/drivers/media/usb/gspca/zc3xx.c
+++ b/drivers/media/usb/gspca/zc3xx.c
@@ -53,7 +53,6 @@ struct sd {
 	struct v4l2_ctrl *jpegqual;

 	struct work_struct work;
-	struct workqueue_struct *work_thread;

 	u8 reg08;		/* webcam compression quality */

@@ -6826,8 +6825,7 @@ static int sd_start(struct gspca_dev *gspca_dev)
 		return gspca_dev->usb_err;

 	/* Start the transfer parameters update thread */
-	sd->work_thread = create_singlethread_workqueue(KBUILD_MODNAME);
-	queue_work(sd->work_thread, &sd->work);
+	schedule_work(&sd->work);

 	return 0;
 }
@@ -6838,12 +6836,9 @@ static void sd_stop0(struct gspca_dev *gspca_dev)
 {
 	struct sd *sd = (struct sd *) gspca_dev;

-	if (sd->work_thread != NULL) {
-		mutex_unlock(&gspca_dev->usb_lock);
-		destroy_workqueue(sd->work_thread);
-		mutex_lock(&gspca_dev->usb_lock);
-		sd->work_thread = NULL;
-	}
+	mutex_unlock(&gspca_dev->usb_lock);
+	flush_work(&sd->work);
+	mutex_lock(&gspca_dev->usb_lock);
 	if (!gspca_dev->present)
 		return;
 	send_unknown(gspca_dev, sd->sensor);
--
2.1.4

Re: [PATCH v2] zc3xx: Remove deprecated create_singlethread_workqueue

[Tejun Heo]

On Sat, Jul 09, 2016 at 01:22:29PM +0530, Bhaktipriya Shridhar wrote:
> The workqueue "work_thread" is involved in updating parameters for
> transfers. It has a single work item(&sd->work) 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.
> 
> Work item has been flushed in sd_stop0() to ensure 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