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=-7.6 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,USER_AGENT_SANE_1 autolearn=unavailable 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 58CDBC433E0 for ; Thu, 2 Jul 2020 17:59:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3B80B20870 for ; Thu, 2 Jul 2020 17:59:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1593712794; bh=vG1vBm6vstM6zzH+k1vdCUTH1v0SmoEIDUIITtr1D/0=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=ORev4BRQdgZ0fD1MfcNsLYMhQ3IzYPkLWzo6Onb3oRWwFMUzNq+P6LCk8Z4U7V6nz ys37Et7V8MSGYOmx/VuarvaD/06TTbsn3RV59kjsJ1kIbY6VkIDyUsEUd7MS8irYOy SUeiWCphpofOj997epxjI8x83CDYLK8HibBVeJyM= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727952AbgGBR7t (ORCPT ); Thu, 2 Jul 2020 13:59:49 -0400 Received: from mail.kernel.org ([198.145.29.99]:52286 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726349AbgGBR7t (ORCPT ); Thu, 2 Jul 2020 13:59:49 -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 69C4F20737; Thu, 2 Jul 2020 17:59:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1593712788; bh=vG1vBm6vstM6zzH+k1vdCUTH1v0SmoEIDUIITtr1D/0=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=YSECcQsGgN0Kb258rQalKoY0GD0gMkQKjSrIAw4B93WGNrTMgWQ/6hr+oWDw+SXRG xKlKIQ4BaG/mRIGsMf9Tu1kJIwAlxOy0d4WAWr1fTaubryYquozE9wywm8Z/2siJrf mogkDnM5b8tk2Qmi8jhn2YzWtbDFbssSH7swTEv0= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 52653352334B; Thu, 2 Jul 2020 10:59:48 -0700 (PDT) Date: Thu, 2 Jul 2020 10:59:48 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: Marco Elver , Nick Desaulniers , Sami Tolvanen , Masahiro Yamada , Will Deacon , Greg Kroah-Hartman , Kees Cook , clang-built-linux , Kernel Hardening , linux-arch , Linux ARM , Linux Kbuild mailing list , LKML , linux-pci@vger.kernel.org, "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" Subject: Re: [PATCH 00/22] add support for Clang LTO Message-ID: <20200702175948.GV9247@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200625085745.GD117543@hirez.programming.kicks-ass.net> <20200630191931.GA884155@elver.google.com> <20200630201243.GD4817@hirez.programming.kicks-ass.net> <20200630203016.GI9247@paulmck-ThinkPad-P72> <20200701114027.GO4800@hirez.programming.kicks-ass.net> <20200701140654.GL9247@paulmck-ThinkPad-P72> <20200701150512.GH4817@hirez.programming.kicks-ass.net> <20200701160338.GN9247@paulmck-ThinkPad-P72> <20200702082040.GB4781@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200702082040.GB4781@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-pci-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-pci@vger.kernel.org On Thu, Jul 02, 2020 at 10:20:40AM +0200, Peter Zijlstra wrote: > On Wed, Jul 01, 2020 at 09:03:38AM -0700, Paul E. McKenney wrote: > > > But it looks like we are going to have to tell the compiler. > > What does the current proposal look like? I can certainly annotate the > seqcount latch users, but who knows what other code is out there.... For pointers, yes, within the Linux kernel it is hopeless, thus the thought of a -fall-dependent-ptr or some such that makes the compiler pretend that each and every pointer is marked with the _Dependent_ptr qualifier. New non-Linux-kernel code might want to use his qualifier explicitly, perhaps something like the following: _Dependent_ptr struct foo *p; // Or maybe after the "*"? rcu_read_lock(); p = rcu_dereference(gp); // And so on... If a function is to take a dependent pointer as a function argument, then the corresponding parameter need the _Dependent_ptr marking. Ditto for return values. The proposal did not cover integers due to concerns about the number of optimization passes that would need to be reviewed to make that work. Nevertheless, using a marked integer would be safer than using an unmarked one, and if the review can be carried out, why not? Maybe something like this: _Dependent_ptr int idx; rcu_read_lock(); idx = READ_ONCE(gidx); d = rcuarray[idx]; rcu_read_unlock(); do_something_with(d); So use of this qualifier is quite reasonable. The prototype for GCC is here: https://github.com/AKG001/gcc/ Thanx, Paul