linux-kernel - Re: [PATCH 1/2] mm: memcg: switch to css_tryget() in get_mem_cgroup_from

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Message-ID: <20191122082002.GP23213@dhcp22.suse.cz>
Date:   Fri, 22 Nov 2019 09:20:02 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     Roman Gushchin <guro@...com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH 1/2] mm: memcg: switch to css_tryget() in
 get_mem_cgroup_from_mm()

On Thu 21-11-19 16:28:47, Michal Koutny wrote:
> On Wed, Nov 13, 2019 at 05:08:29PM +0000, Roman Gushchin <guro@...com> wrote:
> > The thing here is that css_tryget_online() cannot pin the online state,
> > so even if returned true, the cgroup can be offline at the return from
> > the function.
> Adding this for the record.
> 
> The actual offlining happens only from css_killed_ref_fn, which is
> percpu_ref_kill_and_confirm confirmation callback. That is only called
> after RCU grace period. So, css_tryget_online will pin the online state
> _inside_ rcu_read_{,un}lock section.

Thanks for the confirmation. Maybe we should start with a trivial patch
and document this first. We can go and cleanup bogus users on top.
-- 
Michal Hocko
SUSE Labs