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

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Message-ID: <Y/TMYa8DrocppXRu@casper.infradead.org>
Date:   Tue, 21 Feb 2023 13:51:29 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Roman Gushchin <roman.gushchin@...ux.dev>,
        Yue Zhao <findns94@...il.com>, linux-mm@...ck.org,
        akpm@...ux-foundation.org, hannes@...xchg.org, mhocko@...nel.org,
        muchun.song@...ux.dev, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: change memcg->oom_group access with atomic operations

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@...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?
> 
> 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.