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

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Message-ID: <Y/UK2zRi+WNGrWEs@casper.infradead.org>
Date:   Tue, 21 Feb 2023 18:18:03 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Roman Gushchin <roman.gushchin@...ux.dev>
Cc:     Shakeel Butt <shakeelb@...gle.com>, 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 Tue, Feb 21, 2023 at 09:47:05AM -0800, Roman Gushchin wrote:
> > 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.
> 
> Even then it's not an issue in this case, as we end up with either 0 or 1,
> I don't see how we can screw things up here.

Thread 1:
	load word containing oom_group and oom_lock

Thread 2:
	store to oom_lock

Thread 1:
	store word containing oom_group and oom_lock

Thread 2's store has been lost.