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

[Shakeel Butt]

+Paul & Marco

On Tue, Feb 21, 2023 at 5:51 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> 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@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: 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@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.
>
> 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.
>

Thanks Willy for pointing this out. If the compiler can really do this
then [READ|WRITE]_ONCE are required here. I always have big bad
compiler lwn article open in a tab. I couldn't map this transformation
to ones mentioned in that article. Do we have name of this one?

thanks,
Shakeel