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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 51D65C43331 for ; Wed, 1 Apr 2020 08:40:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 27C83214DB for ; Wed, 1 Apr 2020 08:40:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1585730422; bh=yOZ6t/Gl6Iy7HVC0u60CLMokCXrPywPPg/GEC4Tb6K8=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=mZ3Zhz2q5Vzi9GPIDGOai6lTp+L8pP9LxJY4N15401rcuFJ0IRDaT2L8sIPC3mxoL p1yg0mdh+fPZQmSDtJeLrNhTLiqYIilQe3lbAV2X9d5MHqAqR2qiBkg2SbRKmrpgwb Y6p7VhS+cVCUaALpzMIyDoJ2A+0VyVv4GeBBtsso= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732132AbgDAIkV (ORCPT ); Wed, 1 Apr 2020 04:40:21 -0400 Received: from mail.kernel.org ([198.145.29.99]:49198 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728225AbgDAIkU (ORCPT ); Wed, 1 Apr 2020 04:40:20 -0400 Received: from willie-the-truck (236.31.169.217.in-addr.arpa [217.169.31.236]) (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 183F8208FE; Wed, 1 Apr 2020 08:40:17 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1585730419; bh=yOZ6t/Gl6Iy7HVC0u60CLMokCXrPywPPg/GEC4Tb6K8=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=rTTk8b3G6QIpfgO26cm3aA8BrUITT9Y4fB1u6SUaeS/clr7LIkwPxJuHpPqBgPZHn GlDqC9tLDR8JRJltMeLXcNqXe7H7MY606DxVN2a7EqGogHa7wo7NjDH7O+keIQZHN9 bDBGIsrFgjnLSafMyMWS3zZAisL5LPN7wrkdGmPs= Date: Wed, 1 Apr 2020 09:40:15 +0100 From: Will Deacon To: Marco Elver Cc: LKML , Eric Dumazet , Jann Horn , Kees Cook , Maddie Stone , "Paul E . McKenney" , Peter Zijlstra , Thomas Gleixner , kernel-team@android.com, kernel-hardening@lists.openwall.com Subject: Re: [RFC PATCH 03/21] list: Annotate lockless list primitives with data_race() Message-ID: <20200401084014.GC16446@willie-the-truck> References: <20200324153643.15527-1-will@kernel.org> <20200324153643.15527-4-will@kernel.org> <20200331131002.GA30975@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Apr 01, 2020 at 08:34:36AM +0200, Marco Elver wrote: > On Tue, 31 Mar 2020 at 15:10, Will Deacon wrote: > > On Tue, Mar 24, 2020 at 05:23:30PM +0100, Marco Elver wrote: > > > Then, my suggestion would be to mark the write with data_race() and > > > just leave this as a READ_ONCE(). Having a data_race() somewhere only > > > makes KCSAN stop reporting the race if the paired access is also > > > marked (be it with data_race() or _ONCE, etc.). > > > > The problem with taking that approach is that it ends up much of the > > list implementation annotated with either WRITE_ONCE() or data_race(), > > meaning that concurrent, racy list operations will no longer be reported > > by KCSAN. I think that's a pretty big deal and I'm strongly against > > annotating the internals of library code such as this because it means > > that buggy callers will largely go undetected. > > > > The situation we have here is that some calls, e.g. hlist_empty() are > > safe even in the presence of a racy write and I'd like to suppress KCSAN > > reports without annotating the writes at all. > > > > > Alternatively, if marking the write is impossible, you can surround > > > the access with kcsan_disable_current()/kcsan_enable_current(). Or, as > > > a last resort, just leaving as-is is fine too, because KCSAN's default > > > config (still) has KCSAN_ASSUME_PLAIN_WRITES_ATOMIC selected. > > > > Hmm, I suppose some bright spark will want to change the default at the some > > point though, no? ;) I'll look at using > > kcsan_disable_current()/kcsan_enable_current(), thanks. > > I think this will come up again (it did already come up in some other > patch I reviewed, and Paul also mentioned it), so it seems best to > change data_race() to match the intuitive semantics of just completely > ignoring the access marked with it. I.e. marking accesses racing with > accesses marked with data_race() is now optional: > https://lkml.kernel.org/r/20200331193233.15180-1-elver@google.com /me goes look. Thanks! > In which case, the original patch you had here works just fine. Ah yes, so now data_race(READ_ONCE(...)) does make sense as a combination. It's tempting to wrap that up as an accessor, but actually forcing people to spell it out might not be a bad thing after all. Will