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=-4.1 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 51253C433DF for ; Tue, 4 Aug 2020 17:25:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3757421744 for ; Tue, 4 Aug 2020 17:25:26 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=efficios.com header.i=@efficios.com header.b="rzTDFuR7" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730054AbgHDRZZ (ORCPT ); Tue, 4 Aug 2020 13:25:25 -0400 Received: from mail.efficios.com ([167.114.26.124]:41690 "EHLO mail.efficios.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729986AbgHDRZV (ORCPT ); Tue, 4 Aug 2020 13:25:21 -0400 Received: from localhost (localhost [127.0.0.1]) by mail.efficios.com (Postfix) with ESMTP id 28EE02E0B39; Tue, 4 Aug 2020 13:25:20 -0400 (EDT) Received: from mail.efficios.com ([127.0.0.1]) by localhost (mail03.efficios.com [127.0.0.1]) (amavisd-new, port 10032) with ESMTP id ebHR5V_QCNWb; Tue, 4 Aug 2020 13:25:19 -0400 (EDT) Received: from localhost (localhost [127.0.0.1]) by mail.efficios.com (Postfix) with ESMTP id CAE1A2E0AC6; Tue, 4 Aug 2020 13:25:19 -0400 (EDT) DKIM-Filter: OpenDKIM Filter v2.10.3 mail.efficios.com CAE1A2E0AC6 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=efficios.com; s=default; t=1596561919; bh=LKgbLerUFgbYve9YdtBVomWfEyKussbk8GtMS0UFY4w=; h=Date:From:To:Message-ID:MIME-Version; b=rzTDFuR7kE9NHm3XEWdAut30byYgH5ZY0pRe7ozqd4YsIDVrkceWkHqNEWT1pjqhk bCm2d1d+gLtPTuv3rxx9TkZfXmAU1vF1eqBFROpYi96+8X5znR98wUdk3qYV026U29 NWLdeA9OsVGgbnoZ5g5DCE069qdUlgSUKNz7pPktBHEZ+0HJs4F8vBewR1tevUkV2E YTCdRSHfULJJ9S00rG9DMQ1xZUs6t6OPjD8AIQFz06imcZ3ZsUOCq/Lt8Cyic76XTr psJwMnb9kwp2bDHxtKyYhzthEHEBwH8JDq5MzkvXC+yENjR0Ow79zijFbEiNPTH1GT 4xE2lND6xgkzw== X-Virus-Scanned: amavisd-new at efficios.com Received: from mail.efficios.com ([127.0.0.1]) by localhost (mail03.efficios.com [127.0.0.1]) (amavisd-new, port 10026) with ESMTP id tokFWkSMtnbK; Tue, 4 Aug 2020 13:25:19 -0400 (EDT) Received: from mail03.efficios.com (mail03.efficios.com [167.114.26.124]) by mail.efficios.com (Postfix) with ESMTP id BB96F2E0A57; Tue, 4 Aug 2020 13:25:19 -0400 (EDT) Date: Tue, 4 Aug 2020 13:25:19 -0400 (EDT) From: Mathieu Desnoyers To: Peter Zijlstra Cc: linux-kernel , Will Deacon , paulmck , Nicholas Piggin , Andy Lutomirski , Thomas Gleixner , Linus Torvalds , Alan Stern , linux-mm Message-ID: <269299324.40115.1596561919633.JavaMail.zimbra@efficios.com> In-Reply-To: <20200804165118.GN2657@hirez.programming.kicks-ass.net> References: <20200728160010.3314-1-mathieu.desnoyers@efficios.com> <20200804143419.GL2657@hirez.programming.kicks-ass.net> <709073430.39864.1596552521779.JavaMail.zimbra@efficios.com> <20200804165118.GN2657@hirez.programming.kicks-ass.net> Subject: Re: [RFC PATCH 1/2] sched: Fix exit_mm vs membarrier MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-Originating-IP: [167.114.26.124] X-Mailer: Zimbra 8.8.15_GA_3955 (ZimbraWebClient - FF79 (Linux)/8.8.15_GA_3953) Thread-Topic: sched: Fix exit_mm vs membarrier Thread-Index: BO9QOzk5BkwuprAmMByJcjz3i1N+zw== Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org ----- On Aug 4, 2020, at 12:51 PM, Peter Zijlstra peterz@infradead.org wrote: > On Tue, Aug 04, 2020 at 10:48:41AM -0400, Mathieu Desnoyers wrote: >> Here is the scenario I have in mind: > >> Userspace variables: >> >> int x = 0, y = 0; >> >> CPU 0 CPU 1 >> Thread A Thread B >> (in thread group A) (in thread group B) >> >> x = 1 >> barrier() >> y = 1 >> exit() >> exit_mm() >> current->mm = NULL; >> r1 = load y >> membarrier() >> skips CPU 0 (no IPI) because its current mm is NULL >> r2 = load x >> BUG_ON(r1 == 1 && r2 == 0) >> > > Ah, yes of course. > > We really should have a bunch of these scenarios in membarrier.c. Good point. > > > > Now, the above cannot happen because we have an unconditional > atomic_dec_and_test() in do_exit() before exit_mm(), but I'm sure > relying on that is a wee bit dodgy. I am not against using this already existing barrier to provide the guarantee we need, but it would have to be documented in the code. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com