linux-kernel - Re: [PATCH] mm/page_counter: fix various data races

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Date:   Wed, 29 Jan 2020 22:47:45 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Marco Elver <elver@...gle.com>
Cc:     Qian Cai <cai@....pw>, Dmitry Vyukov <dvyukov@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/page_counter: fix various data races

On 2020/01/29 21:21, Marco Elver wrote:
>> By the way, can READ_ONCE()/WRITE_ONCE() really solve this warning?
>> The link above says read/write on the same location ( mm/page_counter.c:129 ).
>> I don't know how READ_ONCE()/WRITE_ONCE() can solve the race.
> 
> It avoids the *data* race, with *_ONCE telling the compiler to not
> optimize the accesses in concurrency-unfriendly ways.  Since *_ONCE is
> used, it conveys clear intent that the code here is meant to be
> concurrent, and KCSAN stops complaining (and assumes that the *logic*
> is correct).

I see. Unlike c->failcnt++ which involves read-modify-write, *_ONCE() can be used for
simple read (like c->watermark) or simple write (like c->watermark = new) case.