linux-kernel - Re: [-mm] Add an owner to the mm

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Message-ID: <47F52FC0.60305@linux.vnet.ibm.com>
Date:	Fri, 04 Apr 2008 00:58:00 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Paul Menage <menage@...gle.com>
CC:	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Pavel Emelianov <xemul@...nvz.org>,
	Hugh Dickins <hugh@...itas.com>,
	Sudhir Kumar <skumar@...ux.vnet.ibm.com>,
	YAMAMOTO Takashi <yamamoto@...inux.co.jp>, lizf@...fujitsu.com,
	linux-kernel@...r.kernel.org, taka@...inux.co.jp,
	linux-mm@...ck.org, David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [-mm] Add an owner to the mm_struct (v7)

Paul Menage wrote:
> On Thu, Apr 3, 2008 at 11:51 AM, Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
>>  >> +     * delay_group_leader() ensures that if the group leader is around
>>  >> +     * we need not select a new owner.
>>  >> +     */
>>  >> +    ret = (mm && (atomic_read(&mm->mm_users) > 1) && (mm->owner == p) &&
>>  >> +            !delay_group_leader(p));
>>  >> +    return ret;
>>  >> +}
>>  >
>>  > Ugh.  Could you please spell this out a bit more.  I find that stuff
>>  > above really hard to read.  Something like:
>>  >
>>  >       if (!mm)
>>  >               return 0;
>>  >       if (atomic_read(&mm->mm_users) <= 1)
>>  >               return 0;
>>  >       if (mm->owner != p)
>>  >               return 0;
>>  >       if (delay_group_leader(p))
>>  >               return 0;
>>  >       return 1;
>>  >
>>
>>  The problem with code above is 4 branch instructions and the code I have just 4
>>  AND operations.
> 
> They'll be completely equivalent to the compiler, due to the
> short-circuit evaluation of &&

Aahh.. Yes.. my bad.. I keep under-estimating compilers and their potential to
optimize

Form (2) seems more readable, I'll move to that



-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL
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