linux-kernel - Re: [PATCH] [media] zc3xx: Remove deprecated create_singlethread

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Date:	Sat, 2 Jul 2016 09:42:10 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Bhaktipriya Shridhar <bhaktipriya96@...il.com>
Cc:	Hans de Goede <hdegoede@...hat.com>,
	Mauro Carvalho Chehab <mchehab@....samsung.com>,
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [media] zc3xx: Remove deprecated
 create_singlethread_workqueue

On Sat, Jul 02, 2016 at 04:19:28PM +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@...il.com>
> ---
>  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..ebdfed4d6 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);
> +	schedule_work(&sd->work);
> +	mutex_lock(&gspca_dev->usb_lock);

Shouldn't this be flush?

Thanks.

-- 
tejun