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Date:   Fri, 15 Nov 2019 09:45:10 -0800
From:   Tejun Heo <tj@kernel.org>
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Roman Gushchin <guro@fb.com>,
        Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.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>
Subject: Re: [PATCH 1/2] mm: memcg: switch to css_tryget() in
 get_mem_cgroup_from_mm()
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On Fri, Nov 15, 2019 at 06:40:31PM +0100, Michal Hocko wrote:
> I am sorry but I do not follow. How can css_tryget_online provide the
> same semantic when the css can go offline right after the tryget call
> returns so it is effectivelly undistinguishable from the case when the
> css was already online before the call was made. Or is my understanding
> of what Roman's said earlier in the thread?

It's *exactly* the same semantics as opening a file and deleting it
and keeping using it vs. deleting a file and then trying to open it.
You can't give out a new reference when the entity's visibility is
expected to be gone already and that's the exactly the condition that
tryget_online avoids.  I don't know how to better explain it if the
file analogy doesn't work.

Thanks.

-- 
tejun