linux-kernel - Re: [PATCH 1/2] mm, memcg: Avoid stale protection values when cgroup is above protection

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Message-ID: <CALOAHbCL_JJgcy9r99Kn81-o_t-fs_nQ+n7aKMHO-02QMCufEw@mail.gmail.com>
Date:   Thu, 30 Apr 2020 09:31:23 +0800
From:   Yafang Shao <laoar.shao@...il.com>
To:     Chris Down <chris@...isdown.name>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>, Roman Gushchin <guro@...com>,
        Linux MM <linux-mm@...ck.org>,
        Cgroups <cgroups@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm, memcg: Avoid stale protection values when cgroup
 is above protection

On Thu, Apr 30, 2020 at 9:16 AM Chris Down <chris@...isdown.name> wrote:
>
> Hi Yafang,
>
> Yafang Shao writes:
> >Would you pls. add some comments above these newly added WRITE_ONCE() ?
> >E.g.
> >What does them mean to fix ?
> >Why do we must add WRITE_ONCE() and READ_ONCE here and there all over
> >the memcg protection ?
> >Otherwise, it may be harder to understand by the others.
>
> There is already discussion in the changelogs for previous store tear
> improvements. For example, b3a7822e5e75 ("mm, memcg: prevent
> mem_cgroup_protected store tearing").
>

I'm sorry that I missed the changelog in the other one.
So you'd better add these commit log or comment to this one again.

> WRITE_ONCE and READ_ONCE are standard compiler barriers, in this case, to avoid
> store tears from writes in another thread (effective protection caching is
> designed by its very nature to permit racing, but tearing is non-ideal).
>
> You can find out more about them in the "COMPILER BARRIER" section in
> Documentation/memory-barriers.txt. I'm not really seeing the value of adding an
> extra comment about this specific use of them, unless you have some more
> explicit concern.

My concern is why we add these barriers to memcg protection
specifically but don't add these barriers to the other memebers like
memcg->oom_group which has the same issue ?
What is the difference between these members and that members ?


-- 
Thanks
Yafang