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From:   Roman Gushchin <roman.gushchin@linux.dev>
MIME-Version: 1.0
Subject: Re: [PATCH] mm: change memcg->oom_group access with atomic operations
Date:   Mon, 20 Feb 2023 21:17:25 -0800
Message-Id: <6563189C-7765-4FFA-A8F2-A5CC4860A1EF@linux.dev>
References: <20230220230624.lkobqeagycx7bi7p@google.com>
Cc:     Yue Zhao <findns94@gmail.com>, linux-mm@kvack.org,
        akpm@linux-foundation.org, hannes@cmpxchg.org, mhocko@kernel.org,
        muchun.song@linux.dev, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org
In-Reply-To: <20230220230624.lkobqeagycx7bi7p@google.com>
To:     Shakeel Butt <shakeelb@google.com>
Precedence: bulk

> On Feb 20, 2023, at 3:06 PM, Shakeel Butt <shakeelb@google.com> wrote:
>=20
> =EF=BB=BFOn 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.
>>>=20
>>> Signed-off-by: Yue Zhao <findns94@gmail.com>
>>=20
>> Hi Yue!
>>=20
>> I'm curious, have any seen any real issues which your patch is solving?
>> Can you, please, provide a bit more details.
>>=20
>=20
> 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=E2=80=99s 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=E2=80=99s easier to justify a proper atomic operation here, even i=
f it=E2=80=99s most likely an overkill.

My question is very simple: the commit log mentions =E2=80=9C=E2=80=A6 to av=
oid concurrency problems=E2=80=9D, so I wonder what problems are these.

Also there are other similar cgroup interfaces without READ_ONCE()/WRITE_ONC=
E().

Thanks!=