linux-kernel - Re: [PATCH] Fix bug in mm/thrash.c function grab_swap

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Date:	Thu, 10 May 2007 20:56:20 +0530
From:	"Satyam Sharma" <satyam.sharma@...il.com>
To:	mikukkon@....fi
Cc:	akpm@...gle.com, ashwin.chaugule@...unite.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Fix bug in mm/thrash.c function grab_swap_token()

On 5/10/07, Mika Kukkonen <mikukkon@...u.homelinux.net> wrote:
> Following bug was uncovered by compiling with '-W' flag:
>
>   CC      mm/thrash.o
> mm/thrash.c: In function 'grab_swap_token':
> mm/thrash.c:52: warning: comparison of unsigned expression < 0 is always false
>
> Field token_priority is unsigned, so decrementing first and the checking
> is not a good plan. Attached patch (compile tested) reverses the order.

Heh, yeah. Actually, decrementing first and then checking < 0 to clamp
to zero is _never_ a good plan. If the original author thought he was
optimizing speed that way, he was clearly mistaken.

> Btw, I'm not sure if likely() makes any sense in this new situation.

Right, if(likely(...)) without an else wouldn't make much sense ...

> Signed-off-by: Mika Kukkonen <mikukkon@....fi>
>
> diff --git a/mm/thrash.c b/mm/thrash.c
> index 9ef9071..c4c5205 100644
> --- a/mm/thrash.c
> +++ b/mm/thrash.c
> @@ -48,9 +48,8 @@ void grab_swap_token(void)
>                 if (current_interval < current->mm->last_interval)
>                         current->mm->token_priority++;
>                 else {
> -                       current->mm->token_priority--;
> -                       if (unlikely(current->mm->token_priority < 0))
> -                               current->mm->token_priority = 0;
> +                       if (likely(current->mm->token_priority > 0))
> +                               current->mm->token_priority--;
>                 }
>                 /* Check if we deserve the token */
>                 if (current->mm->token_priority >
>
-
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