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=-5.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 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 50ACEC10DCE for ; Tue, 24 Mar 2020 21:32:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1EBE62070A for ; Tue, 24 Mar 2020 21:32:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1585085528; bh=WAEG2qvqlmwF1kcQfLLblGPBwt/+mnbg8XT6Bvt/JzM=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=S5/RPBR41Ztq+fe1mrYQHg2K8pFIZj0SjFx+Y3PparPM4VXVv7bX3SdizgbDHUPAa +hPpdpwVezEGFtDXvP++HRFdlROMeedQ1EIuzNBvTVDEAveI+A/NNgPQYT61yi1jMh 57Tfumpu+jjlOWu1V3tFdYznEA76alRqmXrZ5U1s= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728184AbgCXVcH (ORCPT ); Tue, 24 Mar 2020 17:32:07 -0400 Received: from mail.kernel.org ([198.145.29.99]:56188 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727023AbgCXVcG (ORCPT ); Tue, 24 Mar 2020 17:32:06 -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 4B51B206F6; Tue, 24 Mar 2020 21:32:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1585085526; bh=WAEG2qvqlmwF1kcQfLLblGPBwt/+mnbg8XT6Bvt/JzM=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=XX6bno7D7g8zvIKr88jks2+DO+aByMHZAU1xp4RtU01CeTnn/b6I7lqzXtSZsziOh AfEt7apQ8lT6qPLpDCTIiZ61oloIGP/8cmXQjODGz9mSHQTeVTMfQ660utCXUNa0OO /aUabDZwpOhhyegZa1iTUpYJBgg8u7z7F/lxGClY= Date: Tue, 24 Mar 2020 21:32:01 +0000 From: Will Deacon To: Jann Horn Cc: Peter Zijlstra , kernel list , Eric Dumazet , Kees Cook , Maddie Stone , Marco Elver , "Paul E . McKenney" , Thomas Gleixner , kernel-team , Kernel Hardening , Oleg Nesterov Subject: Re: [RFC PATCH 03/21] list: Annotate lockless list primitives with data_race() Message-ID: <20200324213200.GA21176@willie-the-truck> References: <20200324153643.15527-1-will@kernel.org> <20200324153643.15527-4-will@kernel.org> <20200324165128.GS20696@hirez.programming.kicks-ass.net> 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 [mutt crashed while I was sending this; apologies if you receive it twice] On Tue, Mar 24, 2020 at 05:56:15PM +0100, Jann Horn wrote: > On Tue, Mar 24, 2020 at 5:51 PM Peter Zijlstra wrote: > > On Tue, Mar 24, 2020 at 03:36:25PM +0000, Will Deacon wrote: > > > diff --git a/include/linux/list.h b/include/linux/list.h > > > index 4fed5a0f9b77..4d9f5f9ed1a8 100644 > > > --- a/include/linux/list.h > > > +++ b/include/linux/list.h > > > @@ -279,7 +279,7 @@ static inline int list_is_last(const struct list_head *list, > > > */ > > > static inline int list_empty(const struct list_head *head) > > > { > > > - return READ_ONCE(head->next) == head; > > > + return data_race(READ_ONCE(head->next) == head); > > > } > > > > list_empty() isn't lockless safe, that's what we have > > list_empty_careful() for. > > That thing looks like it could also use some READ_ONCE() sprinkled in... Crikey, how did I miss that? I need to spend some time understanding the ordering there. So it sounds like the KCSAN splats relating to list_empty() and loosely referred to by 1c97be677f72 ("list: Use WRITE_ONCE() when adding to lists and hlists") are indicative of real bugs and we should actually restore list_empty() to its former glory prior to 1658d35ead5d ("list: Use READ_ONCE() when testing for empty lists"). Alternatively, assuming list_empty_careful() does what it says on the tin, we could just make that the default. Will