linux-kernel - Re: [PATCH v4 33/40] sh: mach-x3proto: optimize ilsel

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Date: Fri, 21 Jun 2024 10:48:44 +0200
From: John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
To: Yury Norov <yury.norov@...il.com>, linux-kernel@...r.kernel.org, 
 Yoshinori Sato <ysato@...rs.sourceforge.jp>, Rich Felker <dalias@...c.org>,
 Geert Uytterhoeven <geert+renesas@...der.be>, linux-sh@...r.kernel.org
Cc: Alexey Klimov <alexey.klimov@...aro.org>, Bart Van Assche
 <bvanassche@....org>, Jan Kara <jack@...e.cz>, Linus Torvalds
 <torvalds@...ux-foundation.org>, Matthew Wilcox <willy@...radead.org>, 
 Mirsad Todorovac <mirsad.todorovac@....unizg.hr>, Rasmus Villemoes
 <linux@...musvillemoes.dk>, Sergey Shtylyov <s.shtylyov@....ru>
Subject: Re: [PATCH v4 33/40] sh: mach-x3proto: optimize ilsel_enable()

Hi Yury,

thanks for your patch!

On Thu, 2024-06-20 at 10:56 -0700, Yury Norov wrote:
> Simplify ilsel_enable() by using find_and_set_bit().
> 
> Geert also pointed the bug in the old implementation:
> 
> 	I don't think the old code worked as intended: the first time
> 	no free bit is found, bit would have been ILSEL_LEVELS, and
> 	test_and_set_bit() would have returned false, thus terminating
> 	the loop, and continuing with an out-of-range bit value? Hence
> 	to work correctly, bit ILSEL_LEVELS of ilsel_level_map should
> 	have been initialized to one?  Or am I missing something?
> 
> The new code does not have that issue.
> 
> CC: John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
> Signed-off-by: Yury Norov <yury.norov@...il.com>
> Reviewed-by: Geert Uytterhoeven <geert+renesas@...der.be>
> ---
>  arch/sh/boards/mach-x3proto/ilsel.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/sh/boards/mach-x3proto/ilsel.c b/arch/sh/boards/mach-x3proto/ilsel.c
> index f0d5eb41521a..35b585e154f0 100644
> --- a/arch/sh/boards/mach-x3proto/ilsel.c
> +++ b/arch/sh/boards/mach-x3proto/ilsel.c
> @@ -8,6 +8,7 @@
>   */
>  #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
>  
> +#include <linux/find_atomic.h>
>  #include <linux/init.h>
>  #include <linux/kernel.h>
>  #include <linux/module.h>
> @@ -99,8 +100,8 @@ int ilsel_enable(ilsel_source_t set)
>  	}
>  
>  	do {
> -		bit = find_first_zero_bit(&ilsel_level_map, ILSEL_LEVELS);
> -	} while (test_and_set_bit(bit, &ilsel_level_map));
> +		bit = find_and_set_bit(&ilsel_level_map, ILSEL_LEVELS);
> +	} while (bit >= ILSEL_LEVELS);
>  
>  	__ilsel_enable(set, bit);

I will need to take a closer look at the whole code in ilsel_enable() to understand what's
happening here. If Geert's explanation is correct, it sounds more like you're fixing a bug
and saying you're optimizing the function in the patch subject would sound more like an
euphemism.

Also, I think we should add a Fixes tag if possible in case your patch fixes an actual bug.

I will have a closer look over the weekend.

Adrian

-- 
 .''`.  John Paul Adrian Glaubitz
: :' :  Debian Developer
`. `'   Physicist
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