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Date:   Fri, 22 Nov 2019 09:20:02 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>
Cc:     Roman Gushchin <guro@fb.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Kernel Team <Kernel-team@fb.com>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>,
        Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH 1/2] mm: memcg: switch to css_tryget() in
 get_mem_cgroup_from_mm()
Message-ID: <20191122082002.GP23213@dhcp22.suse.cz>
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On Thu 21-11-19 16:28:47, Michal Koutny wrote:
> On Wed, Nov 13, 2019 at 05:08:29PM +0000, Roman Gushchin <guro@fb.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