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Subject: Re: [PATCH] mm/page_counter: fix various data races
To:     Marco Elver <elver@google.com>
Cc:     Qian Cai <cai@lca.pw>, Dmitry Vyukov <dvyukov@google.com>,
        Michal Hocko <mhocko@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
References: <20200129105224.4016-1-cai@lca.pw>
 <20200129120302.GJ24244@dhcp22.suse.cz>
 <59f892d0-5fc4-ae32-ce65-5a688d9180c8@I-love.SAKURA.ne.jp>
 <CANpmjNOdFsU9gg7FSv7Pue0L2eAQ+5UHHaz9bgZ83r94prA4vQ@mail.gmail.com>
From:   Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Message-ID: <9575c1cc-1669-9492-d657-ad4ba6494e88@i-love.sakura.ne.jp>
Date:   Wed, 29 Jan 2020 22:47:45 +0900
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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.