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

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Message-ID: <Y/UEBc4qyLIqHGIA@P9FQF9L96D.corp.robot.car>
Date:   Tue, 21 Feb 2023 09:48:53 -0800
From:   Roman Gushchin <roman.gushchin@...ux.dev>
To:     Muchun Song <muchun.song@...ux.dev>
Cc:     Shakeel Butt <shakeelb@...gle.com>, Yue Zhao <findns94@...il.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: change memcg->oom_group access with atomic operations

On Tue, Feb 21, 2023 at 03:22:32PM +0800, Muchun Song wrote:
> 
> 
> > On Feb 21, 2023, at 13:17, Roman Gushchin <roman.gushchin@...ux.dev> wrote:
> > 
> >> On Feb 20, 2023, at 3:06 PM, Shakeel Butt <shakeelb@...gle.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@...il.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?
> > 
> > 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.
> 
> I think there is no difference in the assembly code between them in most
> cases. The only intention that I can think of is to avoid the potential
> complaint (data race) emitted by KCSAN.

+1

And it might be a totally good reason for this change, let's just make it clear,
instead of pretending to fix non-existing concurrency problems.

Thanks!