Re: [RFC][PATCH 1/7] memcg: use css_get in sock_update_memcg()

Glauber Costa
2013-04-03T12:58:48Z

On 04/03/2013 01:11 PM, Li Zefan wrote:
> Use css_get/css_put instead of mem_cgroup_get/put.
>
> Note, if at the same time someone is moving @current to a different
> cgroup and removing the old cgroup, css_tryget() may return false,
> and sock->sk_cgrp won't be initialized.
>
> Signed-off-by: Li Zefan <lizefan@huawei.com>
> ---
> mm/memcontrol.c | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 23d0f6e..43ca91d 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -536,15 +536,15 @@ void sock_update_memcg(struct sock *sk)
> */
> if (sk->sk_cgrp) {
> BUG_ON(mem_cgroup_is_root(sk->sk_cgrp->memcg));
> - mem_cgroup_get(sk->sk_cgrp->memcg);
> + css_get(&sk->sk_cgrp->memcg->css);
> return;
> }
>
> rcu_read_lock();
> memcg = mem_cgroup_from_task(current);
> cg_proto = sk->sk_prot->proto_cgroup(memcg);
> - if (!mem_cgroup_is_root(memcg) && memcg_proto_active(cg_proto)) {
> - mem_cgroup_get(memcg);
> + if (!mem_cgroup_is_root(memcg) &&
> + memcg_proto_active(cg_proto) && css_tryget(&memcg->css)) {
> sk->sk_cgrp = cg_proto;
> }

What happens if this tryget fails ? Won't we leak a reference here? We
will put regardless when the socket is released, and this may go
negative. No?

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