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From:   Martin Zhao <findns94@gmail.com>
Date:   Wed, 22 Feb 2023 01:00:00 +0800
Message-ID: <CADfL_jBDNZiEWbnn+w9+FhSRPzVwP872XBbhYTZwny8Jzr4bDw@mail.gmail.com>
Subject: Re: [PATCH] mm: change memcg->oom_group access with atomic operations
To:     Roman Gushchin <roman.gushchin@linux.dev>
Cc:     Shakeel Butt <shakeelb@google.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, tangyeechou@gmail.com
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On Tue, Feb 21, 2023 at 1:17 PM Roman Gushchin <roman.gushchin@linux.dev> w=
rote:
>
> > On Feb 20, 2023, at 3:06 PM, Shakeel Butt <shakeelb@google.com> wrote:
> >
> > =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.
> >>>
> >>> Signed-off-by: Yue Zhao <findns94@gmail.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=E2=80=99s obviously not a big deal to put READ_ONCE()/WRITE_ONC=
E() here, but I struggle to imagine a scenario when it will make any differ=
ence. IMHO it=E2=80=99s easier to justify a proper atomic operation here, e=
ven if it=E2=80=99s most likely an overkill.
>
> My question is very simple: the commit log mentions =E2=80=9C=E2=80=A6 to=
 avoid concurrency problems=E2=80=9D, so I wonder what problems are these.

Thanks for your watching!
This topic is found in code review by coincidence, so no real issues
recorded for now. I checked other read/write callbacks about other knobs,
most of them use READ_ONCE/WRITE_ONCE on the user setting variable.
Actually I am curious as well why this interface doesn't use
READ_ONCE/WRITE_ONCE, is there any other synchronization mechanism I
didn't notice yet?

>
> Also there are other similar cgroup interfaces without READ_ONCE()/WRITE_=
ONCE().
>
> Thanks!