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    SubjectRe: [PATCH] mm: change memcg->oom_group access with atomic operations
    On Tue, Feb 21, 2023 at 9:47 AM Roman Gushchin <roman.gushchin@linux.dev> wrote:
    >
    > On Tue, Feb 21, 2023 at 01:51:29PM +0000, Matthew Wilcox wrote:
    > > On Mon, Feb 20, 2023 at 10:52:10PM -0800, Shakeel Butt wrote:
    > > > On Mon, Feb 20, 2023 at 9:17 PM 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?
    > > >
    > > > For this particular case, documenting such an access. Though I don't
    > > > think there are any architectures which may tear a one byte read/write
    > > > and merging/refetching is not an issue for this.
    > >
    > > Wouldn't a compiler be within its rights to implement a one byte store as:
    > >
    > > load-word
    > > modify-byte-in-word
    > > store-word
    > >
    > > and if this is a lockless store to a word which has an adjacent byte also
    > > being modified by another CPU, one of those CPUs can lose its store?
    > > And WRITE_ONCE would prevent the compiler from implementing the store
    > > in that way.
    >
    > Even then it's not an issue in this case, as we end up with either 0 or 1,
    > I don't see how we can screw things up here.
    >

    What do you mean by this is not an issue in this case? Yes, the
    oom_group usage will be ok but we can not say anything about the
    adjacent byte/fields.

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