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From:   Qian Cai <cai@lca.pw>
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Subject: Re: [PATCH] mm/util: fix a data race in __vm_enough_memory()
Date:   Thu, 30 Jan 2020 06:50:27 -0500
Message-Id: <1135BD67-4CCB-4700-8150-44E7E323D385@lca.pw>
References: <20200130042011.GI6615@bombadil.infradead.org>
Cc:     akpm@linux-foundation.org, dennis@kernel.org, tj@kernel.org,
        cl@linux.com, elver@google.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
In-Reply-To: <20200130042011.GI6615@bombadil.infradead.org>
To:     Matthew Wilcox <willy@infradead.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On Jan 29, 2020, at 11:20 PM, Matthew Wilcox <willy@infradead.org> wrote:
>=20
> I'm really not a fan of exposing the internals of a percpu_counter outside=

> the percpu_counter.h file.  Why shouldn't this be fixed by putting the
> READ_ONCE() inside percpu_counter_read()?

It is because not all places suffer from a data race. For example, in __wb_u=
pdate_bandwidth(), it was protected by a lock. I was a bit worry about blind=
ly adding READ_ONCE() inside percpu_counter_read() might has unexpected side=
-effect. For example, it is unnecessary to have READ_ONCE() for a volatile v=
ariable. So, I thought just to keep the change minimal with a trade off by e=
xposing a bit internal details as you mentioned.

However, I had also copied the percpu maintainers to see if they have any pr=
eferences?=