linux-kernel - Re: [PATCH] mm: change memcg->oom_group access with atomic operations

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Message-Id: <6563189C-7765-4FFA-A8F2-A5CC4860A1EF@linux.dev>
Date:   Mon, 20 Feb 2023 21:17:25 -0800
From:   Roman Gushchin <roman.gushchin@...ux.dev>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Yue Zhao <findns94@...il.com>, linux-mm@...ck.org,
        akpm@...ux-foundation.org, hannes@...xchg.org, mhocko@...nel.org,
        muchun.song@...ux.dev, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: change memcg->oom_group access with atomic operations

> On Feb 20, 2023, at 3:06 PM, Shakeel Butt <shakeelb@...gle.com> wrote:
> 
> On Mon, Feb 20, 2023 at 01:09:44PM -0800, Roman Gushchin wrote:
>>> On Mon, Feb 20, 2023 at 11:16:38PM +0800, Yue Zhao wrote:
>>> The knob for cgroup v2 memory controller: memory.oom.group
>>> will be read and written simultaneously by user space
>>> programs, thus we'd better change memcg->oom_group access
>>> with atomic operations to avoid concurrency problems.
>>> 
>>> Signed-off-by: Yue Zhao <findns94@...il.com>
>> 
>> Hi Yue!
>> 
>> I'm curious, have any seen any real issues which your patch is solving?
>> Can you, please, provide a bit more details.
>> 
> 
> IMHO such details are not needed. oom_group is being accessed
> concurrently and one of them can be a write access. At least
> READ_ONCE/WRITE_ONCE is needed here.

Needed for what?

I mean it’s obviously not a big deal to put READ_ONCE()/WRITE_ONCE() here, but I struggle to imagine a scenario when it will make any difference. IMHO it’s easier to justify a proper atomic operation here, even if it’s most likely an overkill.

My question is very simple: the commit log mentions “… to avoid concurrency problems”, so I wonder what problems are these.

Also there are other similar cgroup interfaces without READ_ONCE()/WRITE_ONCE().

Thanks!