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=-5.8 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 57AE0C43331 for ; Mon, 30 Mar 2020 23:13:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 28D1020771 for ; Mon, 30 Mar 2020 23:13:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1585609998; bh=PpxCxvKLaRJ3LSmf1WVhJNuLiZcY8a+gLLy8BYzmxKs=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=CNDZljp8AJVb9f0L6e4iHLsxZOYe0008bG4TiHjq3NfM4z6YV7GgY8Px1vD/OHOwG FwTVxDE2Et0zMab8x2+nTZOeIDsa6O2UWYOPFI1xewGF9N5g4bCNm6UH/hHHTqb9f/ +He4YGTxiX0DozMHrIj5FByLMuY+2eWtKsZ3deF4= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729273AbgC3XNR (ORCPT ); Mon, 30 Mar 2020 19:13:17 -0400 Received: from mail.kernel.org ([198.145.29.99]:33052 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728880AbgC3XNQ (ORCPT ); Mon, 30 Mar 2020 19:13:16 -0400 Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (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 0B0B720733; Mon, 30 Mar 2020 23:13:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1585609996; bh=PpxCxvKLaRJ3LSmf1WVhJNuLiZcY8a+gLLy8BYzmxKs=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=bL+lH8/woQHRfbfj6X0OQm3jLCC/PyAVz624J3d3ldSRIkJT1jDwr7HQW+n0LG4Il CyGPUF3pinRoPEQQoJC46Cjh8gikh9/dc4Jec+VcryFNQe9hEtROWG/kt4gf4h1Djk e5Bz7vm38oee3hPXpXnLakPax4v+rbh77n/XESIQ= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id D45883523140; Mon, 30 Mar 2020 16:13:15 -0700 (PDT) Date: Mon, 30 Mar 2020 16:13:15 -0700 From: "Paul E. McKenney" To: Will Deacon Cc: Jann Horn , Peter Zijlstra , kernel list , Eric Dumazet , Kees Cook , Maddie Stone , Marco Elver , Thomas Gleixner , kernel-team , Kernel Hardening , Oleg Nesterov Subject: Re: [RFC PATCH 03/21] list: Annotate lockless list primitives with data_race() Message-ID: <20200330231315.GZ19865@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200324153643.15527-1-will@kernel.org> <20200324153643.15527-4-will@kernel.org> <20200324165128.GS20696@hirez.programming.kicks-ass.net> <20200324213200.GA21176@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200324213200.GA21176@willie-the-truck> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Mar 24, 2020 at 09:32:01PM +0000, Will Deacon wrote: > [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. The list_empty_careful() function (suitably annotated) returns false if the list is non-empty, including when it is in the process of becoming either empty or non-empty. It would be fine for the lockless use cases I have come across. Thanx, Paul