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

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Message-ID: <YrE9jxTAvLjeBTiT@FVFYT0MHHV2J.usts.net>
Date:   Tue, 21 Jun 2022 11:39:59 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     "Huang, Ying" <ying.huang@...el.com>,
        Qian Cai <quic_qiancai@...cinc.com>
Cc:     Miaohe Lin <linmiaohe@...wei.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 Tue, Jun 21, 2022 at 09:14:00AM +0800, Huang, Ying wrote:
> 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).

All right, CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC help us avoid KCSAN
complaining.

> > 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.
> 

+1

> Best Regards,
> Huang, Ying
>