Re: [RFC][PATCH 6/7] memcg: don't need to get a reference to the parent

Michal Hocko
2013-04-04T15:34:25Z

On Wed 03-04-13 17:13:21, Li Zefan wrote:
> The cgroup core guarantees it's always safe to access the parent.
>
> Signed-off-by: Li Zefan <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>

I would just prefer if you put a comment into place where we used to
take a reference to parent so that it is more obvious in the code.
See bellow.

Acked-by: Michal Hocko <mhocko-AlSwsSmVLrQ@public.gmane.org>

> ---
> mm/memcontrol.c | 14 +-------------
> 1 file changed, 1 insertion(+), 13 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index ad576e8..45129cd 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6124,12 +6124,8 @@ static void mem_cgroup_get(struct mem_cgroup *memcg)
>
> static void __mem_cgroup_put(struct mem_cgroup *memcg, int count)
> {
> - if (atomic_sub_and_test(count, &memcg->refcnt)) {
> - struct mem_cgroup *parent = parent_mem_cgroup(memcg);
> + if (atomic_sub_and_test(count, &memcg->refcnt))
> call_rcu(&memcg->rcu_freeing, free_rcu);
> - if (parent)
> - mem_cgroup_put(parent);
> - }
> }
>
> static void mem_cgroup_put(struct mem_cgroup *memcg)
> @@ -6229,14 +6225,6 @@ mem_cgroup_css_online(struct cgroup *cont)
> res_counter_init(&memcg->res, &parent->res);
> res_counter_init(&memcg->memsw, &parent->memsw);
> res_counter_init(&memcg->kmem, &parent->kmem);
> -
> - /*
> - * We increment refcnt of the parent to ensure that we can
> - * safely access it on res_counter_charge/uncharge.
> - * This refcnt will be decremented when freeing this
> - * mem_cgroup(see mem_cgroup_put).
> - */
> - mem_cgroup_get(parent);

Maybe a little note would be nice
/*
* No need to take reference to parent because cgroup
* core guarantees its existence
*/
> } else {
> res_counter_init(&memcg->res, NULL);
> res_counter_init(&memcg->memsw, NULL);
> --
> 1.8.0.2
>
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--
Michal Hocko
SUSE Labs