Date:   Tue, 21 Feb 2023 18:18:03 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Roman Gushchin <roman.gushchin@linux.dev>
Cc:     Shakeel Butt <shakeelb@google.com>, Yue Zhao <findns94@gmail.com>,
        linux-mm@kvack.org, akpm@linux-foundation.org, hannes@cmpxchg.org,
        mhocko@kernel.org, muchun.song@linux.dev, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: change memcg->oom_group access with atomic operations
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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.