linux-kernel - Re: [PATCH 4/8] cgroup: deactivate CSS's and mark cgroup dead before invoking ->pre

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Date:	Wed, 31 Oct 2012 17:42:25 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	Tejun Heo <tj@...nel.org>
CC:	<lizefan@...wei.com>, <hannes@...xchg.org>, <mhocko@...e.cz>,
	<bsingharora@...il.com>, <kamezawa.hiroyu@...fujitsu.com>,
	<cgroups@...r.kernel.org>, <containers@...ts.linux-foundation.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/8] cgroup: deactivate CSS's and mark cgroup dead before
 invoking ->pre_destroy()

On 10/31/2012 08:22 AM, Tejun Heo wrote:
> Because ->pre_destroy() could fail and can't be called under
> cgroup_mutex, cgroup destruction did something very ugly.
> 
>   1. Grab cgroup_mutex and verify it can be destroyed; fail otherwise.
> 
>   2. Release cgroup_mutex and call ->pre_destroy().
> 
>   3. Re-grab cgroup_mutex and verify it can still be destroyed; fail
>      otherwise.
> 
>   4. Continue destroying.
> 
> In addition to being ugly, it has been always broken in various ways.
> For example, memcg ->pre_destroy() expects the cgroup to be inactive
> after it's done but tasks can be attached and detached between #2 and
> #3 and the conditions that memcg verified in ->pre_destroy() might no
> longer hold by the time control reaches #3.
> 
> Now that ->pre_destroy() is no longer allowed to fail.  We can switch
> to the following.
> 
>   1. Grab cgroup_mutex and fail if it can't be destroyed; fail
>      otherwise.
> 
fail, or fail otherwise? Seems quite negative =)

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