linux-kernel - Re: [PATCH] memcg: don't do cleanup manually if mem_cgroup_css

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Date:	Tue, 2 Apr 2013 14:16:00 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Li Zefan <lizefan@...wei.com>
Cc:	Glauber Costa <glommer@...allels.com>,
	Johannes Weiner <hannes@...xchg.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [PATCH] memcg: don't do cleanup manually if
 mem_cgroup_css_online() fails

On Tue 02-04-13 15:35:28, Li Zefan wrote:
[...]
> @@ -6247,16 +6247,7 @@ mem_cgroup_css_online(struct cgroup *cont)
>  
>  	error = memcg_init_kmem(memcg, &mem_cgroup_subsys);
>  	mutex_unlock(&memcg_create_mutex);
> -	if (error) {
> -		/*
> -		 * We call put now because our (and parent's) refcnts
> -		 * are already in place. mem_cgroup_put() will internally
> -		 * call __mem_cgroup_free, so return directly
> -		 */
> -		mem_cgroup_put(memcg);
> -		if (parent->use_hierarchy)
> -			mem_cgroup_put(parent);
> -	}
> +
>  	return error;
>  }

The mem_cgroup_put(parent) part is incorrect because mem_cgroup_put goes
up the hierarchy already but I do not think mem_cgroup_put(memcg) should
go away as well. Who is going to free the last reference then?

Maybe I am missing something but we have:
cgroup_create
  css = ss->css_alloc(cgrp)
    mem_cgroup_css_alloc
      atomic_set(&memcg->refcnt, 1)
  online_css(ss, cgrp)
    mem_cgroup_css_online
      error = memcg_init_kmem		# fails
  goto err_destroy
err_destroy:
  cgroup_destroy_locked(cgrp)
    offline_css
      mem_cgroup_css_offline

no mem_cgroup_put on the way.

-- 
Michal Hocko
SUSE Labs
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