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=-14.3 required=3.0 tests=DKIMWL_WL_MED,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_IN_DEF_DKIM_WL autolearn=unavailable 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 5EEF3C35240 for ; Wed, 29 Jan 2020 12:22:04 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 23A9820732 for ; Wed, 29 Jan 2020 12:22:04 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="phRBcjau" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 23A9820732 Authentication-Results: mail.kernel.org; dmarc=fail (p=reject dis=none) header.from=google.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id A0AB56B0005; Wed, 29 Jan 2020 07:22:03 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 994B96B0006; Wed, 29 Jan 2020 07:22:03 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 85C3E6B0007; Wed, 29 Jan 2020 07:22:03 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0215.hostedemail.com [216.40.44.215]) by kanga.kvack.org (Postfix) with ESMTP id 6BB2D6B0005 for ; Wed, 29 Jan 2020 07:22:03 -0500 (EST) Received: from smtpin06.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with ESMTP id 2176B181AEF0B for ; Wed, 29 Jan 2020 12:22:03 +0000 (UTC) X-FDA: 76430583726.06.owner82_4c10f4883dd50 X-HE-Tag: owner82_4c10f4883dd50 X-Filterd-Recvd-Size: 5249 Received: from mail-ot1-f67.google.com (mail-ot1-f67.google.com [209.85.210.67]) by imf39.hostedemail.com (Postfix) with ESMTP for ; Wed, 29 Jan 2020 12:22:02 +0000 (UTC) Received: by mail-ot1-f67.google.com with SMTP id a15so15316575otf.1 for ; Wed, 29 Jan 2020 04:22:02 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=Kgn4GQBjvZyBAIu6NGCbaVyen3lY97LcGgNiimjGj/Q=; b=phRBcjaur7ITBOkqk5XTPrxyTXZOs2jwKLwhceVHmZrIGkzjAA+YGugG9jRKZhA6Cd kwEAM87ZSvXBKr6baOvtgQJhb7e1hO3Ppm4SJKTJtE5DN8nbSv4k5WpiHW8QaeTl651J l93XHr9fMHLJRqRfTqvcZwfSIHqGyxXnDdQi98uWUA2p9doiLia3Zl+YWF6GXmhSOFop 3I1JBPp59ih27dLmEn88Kby+o8RQgUgyL9AA1er0WLmmm2s0M4BA/1cRWtTkwSNGTaxL dnREovtzSOuKw8t/KR5ZmGQECMciE2XkcIwEEXh4R6TZ+JngFiNBjw5bAJwlK31KUSLE we3A== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=Kgn4GQBjvZyBAIu6NGCbaVyen3lY97LcGgNiimjGj/Q=; b=uFbGCXEmpd4ysFYGGoaSCvhnSgN0X66eGKudqMFbXSTzjlUTDdCa8GH7X8ju5bBVcU LbHV4mQygSLE4/QFHokfCR76iU8vOMXMocLccL4oePbLSbW1W835gEm4W8LPVpwrM8+l YH49Z3a5a0rAcizErNafVjS90DDtCuZcuaWLIv9woAb9nCfUCLbGnfFf+j5vkucv22T4 7LnDcg4pxQd0d2B4UPCA1soaFJSyAlkUqMWvw03Q1lE1Za7TQwlZf76BtMdwqtTLSMUP OB3iTVU9Z52KW8DSP2zx1qc+OcK8vL0jcOTJ6eAcgTmudFFeg+CCKbmqbpd+yBRJq5nD euXA== X-Gm-Message-State: APjAAAVOP6QJ0YGvlF+S7TL53eg0/cmlNkuSrnck3C0w8xaNQLPOeL/P LUyxg9p4Vcia/GsMoW+g92LzBYLFYvZiLF7Y1Ilt9g== X-Google-Smtp-Source: APXvYqzTfv+9BBZ36pv1LA1tPywV6VN5tvF9EncWG85nhPRdV9EPmBfwUGfUbafjTLYZXGT/tr30soJg+A6RqFedaxk= X-Received: by 2002:a05:6830:1d7b:: with SMTP id l27mr18932727oti.251.1580300521393; Wed, 29 Jan 2020 04:22:01 -0800 (PST) MIME-Version: 1.0 References: <20200129105224.4016-1-cai@lca.pw> <20200129120302.GJ24244@dhcp22.suse.cz> <59f892d0-5fc4-ae32-ce65-5a688d9180c8@I-love.SAKURA.ne.jp> In-Reply-To: <59f892d0-5fc4-ae32-ce65-5a688d9180c8@I-love.SAKURA.ne.jp> From: Marco Elver Date: Wed, 29 Jan 2020 13:21:49 +0100 Message-ID: Subject: Re: [PATCH] mm/page_counter: fix various data races To: Tetsuo Handa Cc: Qian Cai , Dmitry Vyukov , Michal Hocko , Andrew Morton , Johannes Weiner , Linux Memory Management List , LKML Content-Type: text/plain; charset="UTF-8" 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 Wed, 29 Jan 2020 at 13:13, Tetsuo Handa wrote: > > On 2020/01/29 21:03, Michal Hocko wrote: > >> Fixes: 3e32cb2e0a12 ("mm: memcontrol: lockless page counters") > >> Signed-off-by: Qian Cai > > > > Acked-by: Michal Hocko > > 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. 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). The race itself is still there, but as per comment in the file, apparently fine and not a logic bug. > > > >> --- > >> 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) > > >