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    SubjectRe: [PATCH] mm: change memcg->oom_group access with atomic operations
    On Tue, Feb 21, 2023 at 03:22:32PM +0800, Muchun Song wrote:
    >
    >
    > > On Feb 21, 2023, at 13:17, Roman Gushchin <roman.gushchin@linux.dev> wrote:
    > >
    > >> On Feb 20, 2023, at 3:06 PM, Shakeel Butt <shakeelb@google.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@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’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.
    >
    > I think there is no difference in the assembly code between them in most
    > cases. The only intention that I can think of is to avoid the potential
    > complaint (data race) emitted by KCSAN.

    +1

    And it might be a totally good reason for this change, let's just make it clear,
    instead of pretending to fix non-existing concurrency problems.

    Thanks!

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