Re: [PATCH] memcg: don't do cleanup manually if mem_cgroup_css_online() fails

Glauber Costa
2013-04-02T13:36:59Z

On 04/02/2013 05:32 PM, Michal Hocko wrote:
> On Tue 02-04-13 16:22:23, Glauber Costa wrote:
>> On 04/02/2013 04:16 PM, Michal Hocko wrote:
>>> 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.
>>>
>>
>> static void mem_cgroup_css_free(struct cgroup *cont)
>> {
>> struct mem_cgroup *memcg = mem_cgroup_from_cont(cont);
>>
>> kmem_cgroup_destroy(memcg);
>>
>> mem_cgroup_put(memcg);
>> }
>>
>> kernel/cgroup.c:
>> err_free_all:
>> for_each_subsys(root, ss) {
>> if (cgrp->subsys[ss->subsys_id])
>> ss->css_free(cgrp);
>> }
>
> But we do not get to that path after online_css fails because that one
> jumps to err_destroy. So this is not it. Maybe css_free gets called from
> cgroup_diput but I got lost in the indirection.
>

Yes, it is called from diput:

call_rcu(&cgrp->rcu_head, cgroup_free_rcu);