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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,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 8FCD2C43387 for ; Thu, 17 Jan 2019 19:43:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5D64320855 for ; Thu, 17 Jan 2019 19:43:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728937AbfAQTnz (ORCPT ); Thu, 17 Jan 2019 14:43:55 -0500 Received: from iolanthe.rowland.org ([192.131.102.54]:55744 "HELO iolanthe.rowland.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1728535AbfAQTnz (ORCPT ); Thu, 17 Jan 2019 14:43:55 -0500 Received: (qmail 2181 invoked by uid 2102); 17 Jan 2019 14:43:54 -0500 Received: from localhost (sendmail-bs@127.0.0.1) by localhost with SMTP; 17 Jan 2019 14:43:54 -0500 Date: Thu, 17 Jan 2019 14:43:54 -0500 (EST) From: Alan Stern X-X-Sender: stern@iolanthe.rowland.org To: Andrea Parri cc: LKMM Maintainers -- Akira Yokosawa , Boqun Feng , Daniel Lustig , David Howells , Jade Alglave , Luc Maranget , Nicholas Piggin , "Paul E. McKenney" , Peter Zijlstra , Will Deacon , Dmitry Vyukov , Nick Desaulniers , Subject: Re: Plain accesses and data races in the Linux Kernel Memory Model In-Reply-To: <20190116213658.GA3984@andrea> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 16 Jan 2019, Andrea Parri wrote: > Can the compiler (maybe, it does?) transform, at the C or at the "asm" > level, LB1's P0 in LB2's P0 (LB1 and LB2 are reported below)? > > C LB1 > > { > int *x = &a; > } > > P0(int **x, int *y) > { > int *r0; > > r0 = rcu_dereference(*x); > *r0 = 0; > smp_wmb(); > WRITE_ONCE(*y, 1); > } > > P1(int **x, int *y, int *b) > { > int r0; > > r0 = READ_ONCE(*y); > rcu_assign_pointer(*x, b); > } > > exists (0:r0=b /\ 1:r0=1) > > > C LB2 > > { > int *x = &a; > } > > P0(int **x, int *y) > { > int *r0; > > r0 = rcu_dereference(*x); > if (*r0) > *r0 = 0; > smp_wmb(); > WRITE_ONCE(*y, 1); > } > > P1(int **x, int *y, int *b) > { > int r0; > > r0 = READ_ONCE(*y); > rcu_assign_pointer(*x, b); > } > > exists (0:r0=b /\ 1:r0=1) > > LB1 and LB2 are data-race free, according to the patch; LB1's "exists" > clause is not satisfiable, while LB2's "exists" clause is satisfiable. Umm. Transforming *r0 = 0; to if (*r0 != 0) *r0 = 0; wouldn't work on Alpha if r0 was assigned from a plain read with no memory barrier between. But when r0 is assigned from an rcu_dereference call, or if there's no indirection (as in "if (a != 0) a = 0;"), the compiler is indeed allowed to perform this transformation. This means my definition of preserved writes was wrong; a write we thought had to be preserved could instead be transformed into a read. This objection throws a serious monkey wrench into my approach. For one thing, it implies that (as in the example) we can't expect smp_wmb() always to order plain writes. For another, it means we have to assume a lot more writes need not be preserved. I don't know. This may doom the effort to formalize dependencies to plain accesses. Or at least, those other than address dependencies from marked reads. Alan