Re: [PATCH v3] rtc: Avoid flush_scheduled_work() usage

[Alexandre Belloni <alexandre.belloni@bootlin.com>]

Hello,

On 20/05/2022 23:33:47+0900, Tetsuo Handa wrote:
> Flushing system-wide workqueues is dangerous and will be forbidden.
> Replace system_wq with local rtc_wq.
> 
> While we are at it, remove unused rtc_dev_exit().
> 
> Link: https://lkml.kernel.org/r/49925af7-78a8-a3dd-bce6-cfc02e1a9236@I-love.SAKURA.ne.jp
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> ---
> Changes in v3:
>   Fix build error.
>   Forgot to replace flush_scheduled_work().
> 
> Changes in v2:
>   Remove unused rtc_dev_exit().
> 
> Since rtc_dev_init() is built into vmlinux, there is no point with
> recovery.
> 
> This patch blindly converts schedule_work() into queue_work() within
> drivers/rtc/dev.c, based on an assumption that none of work items
> outside of drivers/rtc/dev.c needs to be handled by rtc_wq.
> Did I convert correctly?
> 

Yes and no, this could be a bit more clever and create the workqueue
only for the devices that actually need it. I worked on something after
seeing your first email a while ago but I needed more time to test it.

>  drivers/rtc/dev.c      | 18 ++++++++----------
>  drivers/rtc/rtc-core.h |  5 -----
>  2 files changed, 8 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/rtc/dev.c b/drivers/rtc/dev.c
> index 69325aeede1a..a7346f03a5b2 100644
> --- a/drivers/rtc/dev.c
> +++ b/drivers/rtc/dev.c
> @@ -17,6 +17,7 @@
>  #include "rtc-core.h"
>  
>  static dev_t rtc_devt;
> +static struct workqueue_struct *rtc_wq;
>  
>  #define RTC_DEV_MAX 16 /* 16 RTCs should be enough for everyone... */
>  
> @@ -62,7 +63,7 @@ static void rtc_uie_task(struct work_struct *work)
>  		rtc->uie_timer_active = 1;
>  		rtc->uie_task_active = 0;
>  		add_timer(&rtc->uie_timer);
> -	} else if (schedule_work(&rtc->uie_task) == 0) {
> +	} else if (queue_work(rtc_wq, &rtc->uie_task) == 0) {
>  		rtc->uie_task_active = 0;
>  	}
>  	spin_unlock_irq(&rtc->irq_lock);
> @@ -78,7 +79,7 @@ static void rtc_uie_timer(struct timer_list *t)
>  	spin_lock_irqsave(&rtc->irq_lock, flags);
>  	rtc->uie_timer_active = 0;
>  	rtc->uie_task_active = 1;
> -	if ((schedule_work(&rtc->uie_task) == 0))
> +	if (queue_work(rtc_wq, &rtc->uie_task) == 0)
>  		rtc->uie_task_active = 0;
>  	spin_unlock_irqrestore(&rtc->irq_lock, flags);
>  }
> @@ -96,7 +97,7 @@ static int clear_uie(struct rtc_device *rtc)
>  		}
>  		if (rtc->uie_task_active) {
>  			spin_unlock_irq(&rtc->irq_lock);
> -			flush_scheduled_work();
> +			flush_workqueue(rtc_wq);
>  			spin_lock_irq(&rtc->irq_lock);
>  		}
>  		rtc->uie_irq_active = 0;
> @@ -119,7 +120,7 @@ static int set_uie(struct rtc_device *rtc)
>  		rtc->stop_uie_polling = 0;
>  		rtc->oldsecs = tm.tm_sec;
>  		rtc->uie_task_active = 1;
> -		if (schedule_work(&rtc->uie_task) == 0)
> +		if (queue_work(rtc_wq, &rtc->uie_task) == 0)
>  			rtc->uie_task_active = 0;
>  	}
>  	rtc->irq_data = 0;
> @@ -562,13 +563,10 @@ void __init rtc_dev_init(void)
>  {
>  	int err;
>  
> +	rtc_wq = alloc_workqueue("rtc_wq", 0, 0);
> +	BUG_ON(!rtc_wq);
> +
>  	err = alloc_chrdev_region(&rtc_devt, 0, RTC_DEV_MAX, "rtc");
>  	if (err < 0)
>  		pr_err("failed to allocate char dev region\n");
>  }
> -
> -void __exit rtc_dev_exit(void)
> -{
> -	if (rtc_devt)
> -		unregister_chrdev_region(rtc_devt, RTC_DEV_MAX);

This is very unrelated and should be in a different patch.

> -}
> diff --git a/drivers/rtc/rtc-core.h b/drivers/rtc/rtc-core.h
> index 0abf98983e13..4b10a1b8f370 100644
> --- a/drivers/rtc/rtc-core.h
> +++ b/drivers/rtc/rtc-core.h
> @@ -2,7 +2,6 @@
>  #ifdef CONFIG_RTC_INTF_DEV
>  
>  extern void __init rtc_dev_init(void);
> -extern void __exit rtc_dev_exit(void);
>  extern void rtc_dev_prepare(struct rtc_device *rtc);
>  
>  #else
> @@ -11,10 +10,6 @@ static inline void rtc_dev_init(void)
>  {
>  }
>  
> -static inline void rtc_dev_exit(void)
> -{
> -}
> -
>  static inline void rtc_dev_prepare(struct rtc_device *rtc)
>  {
>  }
> -- 
> 2.34.1
> 
> 

-- 
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com