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Date:   Mon, 28 Jan 2019 16:53:15 -0500
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc:     Michal Hocko <mhocko@kernel.org>,
        Arkadiusz =?utf-8?Q?Mi=C5=9Bkiewicz?= <a.miskiewicz@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Tejun Heo <tj@kernel.org>, cgroups@vger.kernel.org,
        Aleksa Sarai <asarai@suse.de>, Jay Kamat <jgkamat@fb.com>,
        Roman Gushchin <guro@fb.com>, linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-mm <linux-mm@kvack.org>
Subject: Re: [PATCH v3] oom, oom_reaper: do not enqueue same task twice
Message-ID: <20190128215315.GA2011@cmpxchg.org>
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Hi Tetsuo,

On Sun, Jan 27, 2019 at 11:57:38PM +0900, Tetsuo Handa wrote:
> From 9c9e935fc038342c48461aabca666f1b544e32b1 Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Date: Sun, 27 Jan 2019 23:51:37 +0900
> Subject: [PATCH v3] oom, oom_reaper: do not enqueue same task twice
> 
> Arkadiusz reported that enabling memcg's group oom killing causes
> strange memcg statistics where there is no task in a memcg despite
> the number of tasks in that memcg is not 0. It turned out that there
> is a bug in wake_oom_reaper() which allows enqueuing same task twice
> which makes impossible to decrease the number of tasks in that memcg
> due to a refcount leak.
> 
> This bug existed since the OOM reaper became invokable from
> task_will_free_mem(current) path in out_of_memory() in Linux 4.7,
> but memcg's group oom killing made it easier to trigger this bug by
> calling wake_oom_reaper() on the same task from one out_of_memory()
> request.

This changelog seems a little terse compared to how tricky this is.

Can you please include an explanation here *how* this bug is possible?
I.e. the race condition that causes the function te be entered twice
and the existing re-entrance check in there to fail.

> Fix this bug using an approach used by commit 855b018325737f76
> ("oom, oom_reaper: disable oom_reaper for oom_kill_allocating_task").
> As a side effect of this patch, this patch also avoids enqueuing
> multiple threads sharing memory via task_will_free_mem(current) path.