Re: [PATCH RFC 2/4] backlight: use spin-lock to protect device list

[Jani Nikula <jani.nikula@linux.intel.com>]

On Wed, 10 Sep 2014, David Herrmann <dh.herrmann@gmail.com> wrote:
> There is really no reason to use a mutex to protect a simple list. Convert
> the list-lock to a simple spinlock instead.
>
> The spin-locks prepare for a backlight_find() helper, which should
> preferably be usable from atomic context. A mutex would prevent that, so
> use an irq-save spinlock instead.
>
> Signed-off-by: David Herrmann <dh.herrmann@gmail.com>

Reviewed-by: Jani Nikula <jani.nikula@intel.com>

> ---
>  drivers/video/backlight/backlight.c | 16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/video/backlight/backlight.c b/drivers/video/backlight/backlight.c
> index 726c6c6..33b64be 100644
> --- a/drivers/video/backlight/backlight.c
> +++ b/drivers/video/backlight/backlight.c
> @@ -16,13 +16,14 @@
>  #include <linux/err.h>
>  #include <linux/fb.h>
>  #include <linux/slab.h>
> +#include <linux/spinlock.h>
>  
>  #ifdef CONFIG_PMAC_BACKLIGHT
>  #include <asm/backlight.h>
>  #endif
>  
>  static LIST_HEAD(backlight_dev_list);
> -static DEFINE_MUTEX(backlight_dev_list_mutex);
> +static DEFINE_SPINLOCK(backlight_dev_list_lock);
>  static BLOCKING_NOTIFIER_HEAD(backlight_notifier);
>  
>  static const char *const backlight_types[] = {
> @@ -369,9 +370,9 @@ struct backlight_device *backlight_device_register(const char *name,
>  	mutex_unlock(&pmac_backlight_mutex);
>  #endif
>  
> -	mutex_lock(&backlight_dev_list_mutex);
> +	spin_lock_irq(&backlight_dev_list_lock);
>  	list_add(&new_bd->entry, &backlight_dev_list);
> -	mutex_unlock(&backlight_dev_list_mutex);
> +	spin_unlock_irq(&backlight_dev_list_lock);
>  
>  	blocking_notifier_call_chain(&backlight_notifier,
>  				     BACKLIGHT_REGISTERED, new_bd);
> @@ -384,15 +385,16 @@ bool backlight_device_registered(enum backlight_type type)
>  {
>  	bool found = false;
>  	struct backlight_device *bd;
> +	unsigned long flags;
>  
> -	mutex_lock(&backlight_dev_list_mutex);
> +	spin_lock_irqsave(&backlight_dev_list_lock, flags);
>  	list_for_each_entry(bd, &backlight_dev_list, entry) {
>  		if (bd->props.type == type) {
>  			found = true;
>  			break;
>  		}
>  	}
> -	mutex_unlock(&backlight_dev_list_mutex);
> +	spin_unlock_irqrestore(&backlight_dev_list_lock, flags);
>  
>  	return found;
>  }
> @@ -409,9 +411,9 @@ void backlight_device_unregister(struct backlight_device *bd)
>  	if (!bd)
>  		return;
>  
> -	mutex_lock(&backlight_dev_list_mutex);
> +	spin_lock_irq(&backlight_dev_list_lock);
>  	list_del(&bd->entry);
> -	mutex_unlock(&backlight_dev_list_mutex);
> +	spin_unlock_irq(&backlight_dev_list_lock);
>  
>  #ifdef CONFIG_PMAC_BACKLIGHT
>  	mutex_lock(&pmac_backlight_mutex);
> -- 
> 2.1.0
>
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-- 
Jani Nikula, Intel Open Source Technology Center