From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2D01CC2D0DB for ; Wed, 29 Jan 2020 13:48:05 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id ED64420716 for ; Wed, 29 Jan 2020 13:48:04 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org ED64420716 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=i-love.sakura.ne.jp Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 954756B0005; Wed, 29 Jan 2020 08:48:04 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 8DE6E6B0006; Wed, 29 Jan 2020 08:48:04 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 7A5076B0007; Wed, 29 Jan 2020 08:48:04 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0078.hostedemail.com [216.40.44.78]) by kanga.kvack.org (Postfix) with ESMTP id 5FE5F6B0005 for ; Wed, 29 Jan 2020 08:48:04 -0500 (EST) Received: from smtpin14.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id 06116180AD801 for ; Wed, 29 Jan 2020 13:48:04 +0000 (UTC) X-FDA: 76430800488.14.brick39_636d526a03c45 X-HE-Tag: brick39_636d526a03c45 X-Filterd-Recvd-Size: 2830 Received: from www262.sakura.ne.jp (www262.sakura.ne.jp [202.181.97.72]) by imf23.hostedemail.com (Postfix) with ESMTP for ; Wed, 29 Jan 2020 13:48:02 +0000 (UTC) Received: from fsav110.sakura.ne.jp (fsav110.sakura.ne.jp [27.133.134.237]) by www262.sakura.ne.jp (8.15.2/8.15.2) with ESMTP id 00TDlpWv007890; Wed, 29 Jan 2020 22:47:51 +0900 (JST) (envelope-from penguin-kernel@i-love.sakura.ne.jp) Received: from www262.sakura.ne.jp (202.181.97.72) by fsav110.sakura.ne.jp (F-Secure/fsigk_smtp/550/fsav110.sakura.ne.jp); Wed, 29 Jan 2020 22:47:51 +0900 (JST) X-Virus-Status: clean(F-Secure/fsigk_smtp/550/fsav110.sakura.ne.jp) Received: from [192.168.1.9] (softbank126040062084.bbtec.net [126.40.62.84]) (authenticated bits=0) by www262.sakura.ne.jp (8.15.2/8.15.2) with ESMTPSA id 00TDlp73007874 (version=TLSv1.2 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO); Wed, 29 Jan 2020 22:47:51 +0900 (JST) (envelope-from penguin-kernel@i-love.sakura.ne.jp) Subject: Re: [PATCH] mm/page_counter: fix various data races To: Marco Elver Cc: Qian Cai , Dmitry Vyukov , Michal Hocko , Andrew Morton , Johannes Weiner , Linux Memory Management List , LKML References: <20200129105224.4016-1-cai@lca.pw> <20200129120302.GJ24244@dhcp22.suse.cz> <59f892d0-5fc4-ae32-ce65-5a688d9180c8@I-love.SAKURA.ne.jp> From: Tetsuo Handa Message-ID: <9575c1cc-1669-9492-d657-ad4ba6494e88@i-love.sakura.ne.jp> Date: Wed, 29 Jan 2020 22:47:45 +0900 User-Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.4.2 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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.