linux-kernel - Re: [RFC][PATCH 1/7] memcg: use css_get in sock_update

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Date:	Wed, 3 Apr 2013 16:58:48 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	Li Zefan <lizefan@...wei.com>
CC:	<linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
	Michal Hocko <mhocko@...e.cz>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [RFC][PATCH 1/7] memcg: use css_get in sock_update_memcg()

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@...wei.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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