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

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On Mon, Feb 20, 2023 at 9:17 PM Roman Gushchin <roman.gushchin@xxxxxxxxx> wrote:
> > On Feb 20, 2023, at 3:06 PM, Shakeel Butt <shakeelb@xxxxxxxxxx> 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:
> >>> 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@xxxxxxxxx>
> >>
> >> 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.

> 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()

Yeah and those are v1 interfaces e.g. oom_kill_disable, swappiness,
soft_limit. These definitely need [READ|WRITE]_ONCE primitive.

Yue, can you update your patch and convert all accesses to these
fields through [READ|WRITE]_ONCE ?

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