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Message-ID: <6599ad830804031156w79366866yed9f8c3b8acf71fb@mail.gmail.com>
Date: Thu, 3 Apr 2008 11:56:16 -0700
From: "Paul Menage" <menage@google.com>
To: balbir@linux.vnet.ibm.com
Subject: Re: [-mm] Add an owner to the mm_struct (v7)
Cc: "Dave Hansen" <dave@linux.vnet.ibm.com>,
       "Pavel Emelianov" <xemul@openvz.org>, "Hugh Dickins" <hugh@veritas.com>,
       "Sudhir Kumar" <skumar@linux.vnet.ibm.com>,
       "YAMAMOTO Takashi" <yamamoto@valinux.co.jp>, lizf@cn.fujitsu.com,
       linux-kernel@vger.kernel.org, taka@valinux.co.jp, linux-mm@kvack.org,
       "David Rientjes" <rientjes@google.com>,
       "Andrew Morton" <akpm@linux-foundation.org>,
       "KAMEZAWA Hiroyuki" <kamezawa.hiroyu@jp.fujitsu.com>
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On Thu, Apr 3, 2008 at 11:51 AM, Balbir Singh <balbir@linux.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 &&

Paul