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Message-ID: <6599ad830803280706j54376243if56ccca0281f685d@mail.gmail.com>
Date: Fri, 28 Mar 2008 07:06:52 -0700
From: "Paul Menage" <menage@google.com>
To: balbir@linux.vnet.ibm.com
Subject: Re: [-mm] Add an owner to the mm_struct (v2)
Cc: "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 Fri, Mar 28, 2008 at 5:54 AM, Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
>
>  Thinking more, I don't think it makes sense for us to overload task_lock() to do
>  the mm->owner handling (we don't want to mix lock domains). task_lock() is used
>  for several things
>
>  1. We don't want to make task_lock() rules more complicated by having it protect
>  an mm member to save space
>  2. We don't want more contention on task_lock()
>

This isn't to save space, it's to provide correctness. We *have* to
hold task_lock(new_owner) before setting mm->owner = new_owner,
otherwise we have no guarantee that new_owner is still a user of mm.

Paul