Re: [PATCH v1 5/9] memcg: replace stats_flush_lock with an atomic

[Shakeel Butt]

On Tue, Mar 28, 2023 at 11:53 AM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
>
[...]
> > > +     if (atomic_xchg(&stats_flush_ongoing, 1))
> >
> > Have you profiled this? I wonder if we should replace the above with
> >
> >         if (atomic_read(&stats_flush_ongoing) || atomic_xchg(&stats_flush_ongoing, 1))
>
> I profiled the entire series with perf and I haven't noticed a notable
> difference between before and after the patch series -- but maybe some
> specific access patterns cause a regression, not sure.
>
> Does an atomic_cmpxchg() satisfy the same purpose? it's easier to read
> / more concise I guess.
>
> Something like
>
>     if (atomic_cmpxchg(&stats_flush_ongoing, 0, 1))
>
> WDYT?
>

No, I don't think cmpxchg will be any different from xchg(). On x86,
the cmpxchg will always write to stats_flush_ongoing and depending on
the comparison result, it will either be 0 or 1 here.

If you see the implementation of queued_spin_trylock(), it does the
same as well.