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Subject: Re: [PATCH] mm/page_counter: fix various data races
To:     Qian Cai <cai@lca.pw>, Dmitry Vyukov <dvyukov@google.com>
Cc:     Michal Hocko <mhocko@kernel.org>, akpm@linux-foundation.org,
        hannes@cmpxchg.org, elver@google.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20200129105224.4016-1-cai@lca.pw>
 <20200129120302.GJ24244@dhcp22.suse.cz>
From:   Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Message-ID: <59f892d0-5fc4-ae32-ce65-5a688d9180c8@I-love.SAKURA.ne.jp>
Date:   Wed, 29 Jan 2020 21:13:37 +0900
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On 2020/01/29 21:03, Michal Hocko wrote:
>> Fixes: 3e32cb2e0a12 ("mm: memcontrol: lockless page counters")
>> Signed-off-by: Qian Cai <cai@lca.pw>
> 
> Acked-by: Michal Hocko <mhocko@suse.com>

Please include

Reported-by: syzbot+f36cfe60b1006a94f9dc@syzkaller.appspotmail.com

for https://syzkaller.appspot.com/bug?id=744097b8b91cecd8b035a6f746bb12e4efc7669f .

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.

> 
>> ---
>>  mm/page_counter.c | 8 ++++----
>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/page_counter.c b/mm/page_counter.c
>> index de31470655f6..a17841150906 100644
>> --- a/mm/page_counter.c
>> +++ b/mm/page_counter.c
>> @@ -82,8 +82,8 @@ void page_counter_charge(struct page_counter *counter, unsigned long nr_pages)
>>  		 * This is indeed racy, but we can live with some
>>  		 * inaccuracy in the watermark.
>>  		 */
>> -		if (new > c->watermark)
>> -			c->watermark = new;
>> +		if (new > READ_ONCE(c->watermark))
>> +			WRITE_ONCE(c->watermark, new);
>>  	}
>>  }
>>  
>> @@ -135,8 +135,8 @@ bool page_counter_try_charge(struct page_counter *counter,
>>  		 * Just like with failcnt, we can live with some
>>  		 * inaccuracy in the watermark.
>>  		 */
>> -		if (new > c->watermark)
>> -			c->watermark = new;
>> +		if (new > READ_ONCE(c->watermark))
>> +			WRITE_ONCE(c->watermark, new);
>>  	}
>>  	return true;
>>  
>> -- 
>> 2.21.0 (Apple Git-122.2)
>