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=-0.6 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 E51C0C352A4 for ; Mon, 10 Feb 2020 19:22:00 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 9D31420838 for ; Mon, 10 Feb 2020 19:22:00 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="ToQYS2ee" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9D31420838 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 46BF16B000C; Mon, 10 Feb 2020 14:22:00 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 41B746B000E; Mon, 10 Feb 2020 14:22:00 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 358456B0032; Mon, 10 Feb 2020 14:22:00 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0214.hostedemail.com [216.40.44.214]) by kanga.kvack.org (Postfix) with ESMTP id 1B1246B000C for ; Mon, 10 Feb 2020 14:22:00 -0500 (EST) Received: from smtpin06.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id C237540F1 for ; Mon, 10 Feb 2020 19:21:59 +0000 (UTC) X-FDA: 76475187558.06.flesh04_4b925412fe707 X-HE-Tag: flesh04_4b925412fe707 X-Filterd-Recvd-Size: 2633 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) by imf09.hostedemail.com (Postfix) with ESMTP for ; Mon, 10 Feb 2020 19:21:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=CiM52pTPVqz/skTBLeX1gYuocEamvKWE7EoYWXekqbE=; b=ToQYS2eeMN6pm+nzAjgrfjNuGk vx08Ib9g47C+wFMlqE6aEgHU+TDP9y769k0l5C5tTFhCrV4AuD33lgMv4AKJOW3I4/wpzQFGCmt7Q MlZTrZjdtonDDFjePt3EMml0AsAl+NF0OqM3TPqzKfKMbaIU0xlPMkax5sjaoJckafvIIv/T8b8LT KyhZZIX6Oni0VFLdoquOLWDQJcU401YytKrbPs0KKIC2KN/ke4KCvUx07wrbSyMt58ppqA5e7dOub R2zFcI2aTeD/OYDWzPgdAlffQICKo7+Yj9CIdVVFjaKjzZEnkIro//Y7ZztJm3jctCDlyAaaMJHN7 4bi6qvuA==; Received: from willy by bombadil.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1j1Ed1-0001zg-8H; Mon, 10 Feb 2020 19:21:55 +0000 Date: Mon, 10 Feb 2020 11:21:55 -0800 From: Matthew Wilcox To: Qian Cai Cc: akpm@linux-foundation.org, elver@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH -next] mm/filemap: fix a data race in filemap_fault() Message-ID: <20200210192155.GM8731@bombadil.infradead.org> References: <1581354029-20154-1-git-send-email-cai@lca.pw> <20200210172511.GL8731@bombadil.infradead.org> <1581362448.7365.38.camel@lca.pw> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1581362448.7365.38.camel@lca.pw> X-Bogosity: Ham, tests=bogofilter, spamicity=0.001034, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Feb 10, 2020 at 02:20:48PM -0500, Qian Cai wrote: > On Mon, 2020-02-10 at 09:25 -0800, Matthew Wilcox wrote: > > On Mon, Feb 10, 2020 at 12:00:29PM -0500, Qian Cai wrote: > > > @@ -2622,7 +2622,7 @@ void filemap_map_pages(struct vm_fault *vmf, > > > if (page->index >= max_idx) > > > goto unlock; > > > > > > - if (file->f_ra.mmap_miss > 0) > > > + if (data_race(file->f_ra.mmap_miss > 0)) > > > file->f_ra.mmap_miss--; > > > > How is this safe? Two threads can each see 1, and then both decrement the > > in-memory copy, causing it to end up at -1. > > Well, I meant to say it is safe from *data* races rather than all other races, > but it is a good catch for the underflow cases and makes some sense to fix them > together (so we don't need to touch the same lines over and over again). My point is that this is a legitimate warning from the sanitiser. The point of your patches should not be to remove all the warnings!