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

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Message-Id: <20080402093157.e445acfb.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Wed, 2 Apr 2008 09:31:57 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Balbir Singh <balbir@...ux.vnet.ibm.com>
Cc:	Paul Menage <menage@...gle.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>
Subject: Re: [RFC][-mm] Add an owner to the mm_struct (v4)


On Tue, 01 Apr 2008 18:13:12 +0530
Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
> +	/*
> +	 * Search in the children
> +	 */
> +	list_for_each_entry(c, &p->children, sibling) {
> +		if (c->mm == mm)
> +			goto assign_new_owner;
> +	}
> +
This finds new owner when "current" is multi-threaded and
"current" called pthread_create(), right ?

> +	/*
> +	 * Search in the siblings
> +	 */
> +	list_for_each_entry(c, &p->parent->children, sibling) {
> +		if (c->mm == mm)
> +			goto assign_new_owner;
> +	}
> +
This finds new owner when "current" is multi-threaded and
"current" is just a child (means it doesn't call pthread_create()) ?


> +	/*
> +	 * Search through everything else. We should not get
> +	 * here often
> +	 */
> +	do_each_thread(g, c) {
> +		if (c->mm == mm)
> +			goto assign_new_owner;
> +	} while_each_thread(g, c);

Doing above in synchronized manner seems too heavy.
When this happen ? or Can this be done in lazy "on-demand" manner ?

+assign_new_owner:
+	rcu_read_unlock();
+	BUG_ON(c == p);
+	task_lock(c);
+	if (c->mm != mm) {
+		task_unlock(c);
+		goto retry;
+	}
+	cgroup_mm_owner_callbacks(mm->owner, c);
+	mm->owner = c;
+	task_unlock(c);
+}
Why rcu_read_unlock() before changing owner ? Is it safe ?

Thanks,
-Kame

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