linux-kernel - Re: [PATCH v2 2/3] mm/swapfile: fix possible data races of inuse

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Message-ID: <YrDob8+9Xgig6mZw@qian>
Date:   Mon, 20 Jun 2022 17:36:47 -0400
From:   Qian Cai <quic_qiancai@...cinc.com>
To:     Muchun Song <songmuchun@...edance.com>
CC:     Miaohe Lin <linmiaohe@...wei.com>,
        "Huang, Ying" <ying.huang@...el.com>, <akpm@...ux-foundation.org>,
        <david@...hat.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/3] mm/swapfile: fix possible data races of
 inuse_pages

On Mon, Jun 20, 2022 at 10:20:07PM +0800, Muchun Song wrote:
> The lock does not protect the read sides. So the write side should be
> fixed by WRITTE_ONCE().

https://lwn.net/Articles/816854/

"Unmarked writes (aligned and up to word size) can be treated as if they had
used WRITE_ONCE() by building with
CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC=y (also selected by default).
Experience has shown that compilers are much less likely to destructively
optimize in-kernel writes than reads. Some developers might therefore
choose to use READ_ONCE() but omit the corresponding WRITE_ONCE(). Other
developers might prefer the documentation benefits and long-term peace of
mind accruing from explicit use of WRITE_ONCE()..."