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

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Message-ID: <87pmj2q0mf.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date:   Tue, 21 Jun 2022 09:14:00 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Qian Cai <quic_qiancai@...cinc.com>,
        Miaohe Lin <linmiaohe@...wei.com>
Cc:     Muchun Song <songmuchun@...edance.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

Qian Cai <quic_qiancai@...cinc.com> writes:

> 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()..."

Thanks for pointing me to this great article.  So although not required
by KCSAN strictly, WRITE_ONCE() is still good for documentation, etc.
Just like we have done for swap_info_struct->highest_bit, etc.

Best Regards,
Huang, Ying