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

Michal Hocko
2013-04-02T14:16:46Z

On Tue 02-04-13 14:16:00, 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:

OK, I was missing something but "there is one reference without put"
still holds...

cgroup_create
css = ss->css_alloc(cgrp)
mem_cgroup_css_alloc
atomic_set(&memcg->refcnt, 1)
online_css(ss, cgrp)
mem_cgroup_css_online
memcg_init_kmem
mem_cgroup_get # refcnt = 2
memcg_update_all_caches
memcg_update_cache_size # fails with ENOMEM
goto err_destroy
err_destroy:
cgroup_destroy_locked(cgrp)
offline_css
mem_cgroup_css_offline

There is one mem_cgroup_put from mem_cgroup_css_free from cgroup_diput
but besides that I do not see any put after the patch is applied. So I
think you really need to drop only the mem_cgroup_put on parent part.
--
Michal Hocko
SUSE Labs