linux-kernel - Re: [PATCH 5/5] drivers: char: Replace bit operation functions with IDA allocator.

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Date:   Fri, 7 Apr 2017 17:45:20 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Varsha Rao <rvarsha016@...il.com>
Cc:     Matthew Wilcox <mawilcox@...rosoft.com>,
        Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/5] drivers: char: Replace bit operation functions with
 IDA allocator.

On Fri, Apr 07, 2017 at 05:04:41PM +0530, Varsha Rao wrote:
> Replace bit operation functions with IDA allocator functions. As IDA
> allocation is simpler.

But why does this matter?

> 
> Signed-off-by: Varsha Rao <rvarsha016@...il.com>
> ---
>  drivers/char/misc.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/char/misc.c b/drivers/char/misc.c
> index c9cd1ea..5786281 100644
> --- a/drivers/char/misc.c
> +++ b/drivers/char/misc.c
> @@ -60,7 +60,7 @@ static DEFINE_MUTEX(misc_mtx);
>   * Assigned numbers, used for dynamic minors
>   */
>  #define DYNAMIC_MINORS 64 /* like dynamic majors */
> -static DECLARE_BITMAP(misc_minors, DYNAMIC_MINORS);
> +static DEFINE_IDA(misc_minors_ida);
>  
>  #ifdef CONFIG_PROC_FS
>  static void *misc_seq_start(struct seq_file *seq, loff_t *pos)
> @@ -193,14 +193,18 @@ int misc_register(struct miscdevice *misc)
>  	mutex_lock(&misc_mtx);
>  
>  	if (is_dynamic) {
> -		int i = find_first_zero_bit(misc_minors, DYNAMIC_MINORS);
> +		int i = ida_simple_get(&misc_minors_ida, 0,
> +				       DYNAMIC_MINORS, GFP_KERNEL);
>  
>  		if (i >= DYNAMIC_MINORS) {
>  			err = -EBUSY;
>  			goto out;
> -		}
> +		} else if (i < 0) {
> +			err = i;
> +			goto out;
> +		} else {
>  		misc->minor = DYNAMIC_MINORS - i - 1;
> -		set_bit(i, misc_minors);
> +		}

Your indentation is now incorrect :(

>  	} else {
>  		struct miscdevice *c;
>  
> @@ -222,7 +226,7 @@ int misc_register(struct miscdevice *misc)
>  			int i = DYNAMIC_MINORS - misc->minor - 1;
>  
>  			if (i < DYNAMIC_MINORS && i >= 0)
> -				clear_bit(i, misc_minors);
> +				ida_simple_remove(&misc_minors_ida, i);
>  			misc->minor = MISC_DYNAMIC_MINOR;
>  		}
>  		err = PTR_ERR(misc->this_device);
> @@ -258,7 +262,7 @@ void misc_deregister(struct miscdevice *misc)
>  	list_del(&misc->list);
>  	device_destroy(misc_class, MKDEV(MISC_MAJOR, misc->minor));
>  	if (i < DYNAMIC_MINORS && i >= 0)
> -		clear_bit(i, misc_minors);
> +		ida_simple_remove(&misc_minors_ida, i);

As much as I like the ida interface, I don't see why it is required to
use it here, you have not provided any justification for this patch at
all :(

thanks,

greg k-h