linux-kernel - Re: One comment on the __release

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Date:	Mon, 3 Oct 2011 19:24:58 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Wei Yang <weiyang.kernel@...il.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: One comment on the __release_region in kernel/resource.c

On Sun, 2 Oct 2011 21:57:07 +0800
Wei Yang <weiyang.kernel@...il.com> wrote:

> Dear experts,
> 
> I am viewing the source code of __release_region() in kernel/resource.c.
> And I have one comment for the performance issue.
> 
> For example, we have a resource tree like this.
> 10-89
>    20-79
>        30-49
>        55-59
>        60-64
>        65-69
>    80-89
> 100-279
> 
> If the caller wants to release a region of [50,59], the original code will
> execute four times in the for loop in the subtree of 20-79.
> 
> After changing the code below, it will execute two times instead.
> 
> By using the "git annotate", I see this code is committed by Linus as the
> initial version. So don't get more information about why this code is
> written
> in this way.
> 
> Maybe the case I thought will not happen in the real world?
> 
> Your comment is warmly welcome. :)
> 
> diff --git a/kernel/resource.c b/kernel/resource.c
> index 8461aea..81525b4 100644
> --- a/kernel/resource.c
> +++ b/kernel/resource.c
> @@ -931,7 +931,7 @@ void __release_region(struct resource *parent,
> resource_size_t start,
>        for (;;) {
>                struct resource *res = *p;
> 
> -               if (!res)
> +               if (!res || res->start > start)

Hmm ?
	res->start > end ?


Thanks,
-Kame

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