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=-6.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, SPF_HELO_NONE,SPF_PASS autolearn=ham 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 3A83AC2D0CE for ; Fri, 24 Jan 2020 05:43:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 087AA2071E for ; Fri, 24 Jan 2020 05:43:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1579844580; bh=UtXlQmAwVpIV4UVkULwSxcBxj3f1rwjkrvqd+IMtrm8=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=MUUrKVZx/frNDVrnz/SM61p1pvEWgrT6+YUcY2Y5d2eUs39M0WVD+newmT8NAxuNL liKaUWGHzHYYnbpIxSq4pPoVZiLxgty1iUaM27srCdBlO9BNKDjzJNA3mvoRiVSwrR 7wpEy13Dg5FTNI7YCwyYqPpZico4vXvZzjpuwC7o= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730418AbgAXFm7 (ORCPT ); Fri, 24 Jan 2020 00:42:59 -0500 Received: from mail.kernel.org ([198.145.29.99]:45980 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730222AbgAXFm7 (ORCPT ); Fri, 24 Jan 2020 00:42:59 -0500 Received: from sol.localdomain (c-107-3-166-239.hsd1.ca.comcast.net [107.3.166.239]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 3CEDE2070A; Fri, 24 Jan 2020 05:42:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1579844578; bh=UtXlQmAwVpIV4UVkULwSxcBxj3f1rwjkrvqd+IMtrm8=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=gHZoFfUfHkbxCFtJeTBNinZ/zDprtq6eg9/MVNYgeL82NiDWUtWTiqMDe4DOtiM70 xdTFSRWqRr9C87t/SulBsXb/TeI1wl6PGQYgFnLLS8shbSH78TyKf1/Z5uZqqWjwVT uWVf1KoxKRFRDgOcVd4Yu4ZLUxDsSDoMNw9deF+8= Date: Thu, 23 Jan 2020 21:42:56 -0800 From: Eric Biggers To: Gao Xiang Cc: linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, Alexander Viro , Daniel Rosenberg , Gabriel Krisman Bertazi Subject: Re: [PATCH] ext4: fix race conditions in ->d_compare() and ->d_hash() Message-ID: <20200124054256.GC832@sol.localdomain> References: <20200124041234.159740-1-ebiggers@kernel.org> <20200124050423.GA31271@hsiangkao-HP-ZHAN-66-Pro-G1> <20200124051601.GB832@sol.localdomain> <20200124053415.GC31271@hsiangkao-HP-ZHAN-66-Pro-G1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200124053415.GC31271@hsiangkao-HP-ZHAN-66-Pro-G1> Sender: linux-ext4-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org On Fri, Jan 24, 2020 at 01:34:23PM +0800, Gao Xiang wrote: > On Thu, Jan 23, 2020 at 09:16:01PM -0800, Eric Biggers wrote: > > [] > > > So we need READ_ONCE() to ensure that a consistent value is used. > > By the way, my understanding is all pointer could be accessed > atomicly guaranteed by compiler. In my opinion, we generally > use READ_ONCE() on pointers for other uses (such as, avoid > accessing a variable twice due to compiler optimization and > it will break some logic potentially or need some data > dependency barrier...) > > Thanks, > Gao Xiang But that *is* why we need READ_ONCE() here. Without it, there's no guarantee that the compiler doesn't load the variable twice. Please read: https://github.com/google/ktsan/wiki/READ_ONCE-and-WRITE_ONCE - Eric