linux-kernel - Re: [-mm][PATCH 4/4] Add memrlimit controller accounting and control (v5)

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Date:	Wed, 21 May 2008 13:20:32 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Balbir Singh <balbir@...ux.vnet.ibm.com>
Cc:	linux-mm@...ck.org, Sudhir Kumar <skumar@...ux.vnet.ibm.com>,
	YAMAMOTO Takashi <yamamoto@...inux.co.jp>,
	Paul Menage <menage@...gle.com>, lizf@...fujitsu.com,
	linux-kernel@...r.kernel.org, Pavel Emelianov <xemul@...nvz.org>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [-mm][PATCH 4/4] Add memrlimit controller accounting and
	control (v5)

On Wed, May 21, 2008 at 09:00:12PM +0530, Balbir Singh wrote:

[..]
> +static void memrlimit_cgroup_move_task(struct cgroup_subsys *ss,
> +					struct cgroup *cgrp,
> +					struct cgroup *old_cgrp,
> +					struct task_struct *p)
> +{
> +	struct mm_struct *mm;
> +	struct memrlimit_cgroup *memrcg, *old_memrcg;
> +
> +	mm = get_task_mm(p);
> +	if (mm == NULL)
> +		return;
> +
> +	/*
> +	 * Hold mmap_sem, so that total_vm does not change underneath us
> +	 */
> +	down_read(&mm->mmap_sem);
> +
> +	rcu_read_lock();
> +	if (p != rcu_dereference(mm->owner))
> +		goto out;
> +

Hi Balbir,

How does rcu help here? We are not dereferencing mm->owner. So even if
task_struct it was pointing to goes away, should not be a problem.

OTOH, while updating the mm->owner in mmm_update_next_owner(), we
are not using rcu_assing_pointer() and synchronize_rcu()/call_rcu(). Is
this the right usage if mm->owner is rcu protected?

Thanks
Vivek
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