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=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 A45FAC76195 for ; Thu, 18 Jul 2019 11:04:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 775D6217F4 for ; Thu, 18 Jul 2019 11:04:53 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="jKN4G9K8" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2390062AbfGRLEw (ORCPT ); Thu, 18 Jul 2019 07:04:52 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:59400 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726495AbfGRLEv (ORCPT ); Thu, 18 Jul 2019 07:04:51 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=Mu6JgufiifeVCl5ahdzNFF6iFcPalKmCtlVpF9MQKr8=; b=jKN4G9K8EwPrqjC/8/JSP/m5S TcLMiwu4DvM/TqjWUr4BtZbfSEy0MjVH/ZTsf1dj+GBuHNoi9r6GoymAmvrVuL4RWPpnn7H+WVyay rCK4Y1pfOJH1jfPbRAY4kAS2Q3bVbOeA/eHxqhE47maCBr2kbFAvIZwB/7gsA4ZtGrELeouczRWsa 4J+6bKQ87LVxMn5Qfiwyia/jOs7bfCVOnDI3K4S0xiw8eTJ4N51Xld0jg8vq1EHsoETd5yIwGTpIr Lrfr/gT8b1WS7TqOkefy9ygY7HhrKYB/QGeUrbU6VX6ZuI6auNysqLKCLN0rCBA5kOLCGlLhKWaDt 1sz9FC9lQ==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.92 #3 (Red Hat Linux)) id 1ho4DQ-0001yJ-8K; Thu, 18 Jul 2019 11:04:48 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 1687B20197A71; Thu, 18 Jul 2019 13:04:46 +0200 (CEST) Date: Thu, 18 Jul 2019 13:04:46 +0200 From: Peter Zijlstra To: Jan Stancek Cc: Will Deacon , Waiman Long , linux-kernel@vger.kernel.org, dbueso@suse.de, mingo@redhat.com, jade alglave , paulmck@linux.vnet.ibm.com Subject: Re: [PATCH v2] locking/rwsem: add acquire barrier to read_slowpath exit when queue is empty Message-ID: <20190718110446.GC3419@hirez.programming.kicks-ass.net> References: <20190716185807.GJ3402@hirez.programming.kicks-ass.net> <20190717131335.b2ry43t2ov7ba4t4@willie-the-truck> <21ff5905-198b-6ea5-6c2a-9fb10cb48ea7@redhat.com> <20190717192200.GA17687@dustball.usersys.redhat.com> <20190718092640.52oliw3sid7gxyh6@willie-the-truck> <79224323.853324.1563447052432.JavaMail.zimbra@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <79224323.853324.1563447052432.JavaMail.zimbra@redhat.com> 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 On Thu, Jul 18, 2019 at 06:50:52AM -0400, Jan Stancek wrote: > > In writing this, I also noticed that we don't have any explicit ordering > > at the end of the reader slowpath when we wait on the queue but get woken > > immediately: > > > > if (!waiter.task) > > break; > > > > Am I missing something? > > I'm assuming this isn't problem, because set_current_state() on line above > is using smp_store_mb(). X = 0; X = 1; rwsem_down_read() rwsem_up_write(); for (;;) { set_current_state(TASK_UNINTERRUPTIBLE); rwsem_mark_wake() atomic_long_add(adjustment, &sem->count); smp_store_release(&waiter->task, NULL); if (!waiter.task) break; ... } r = X; can I think result in r==0 just fine, because there's nothing ordering the load of waiter.task with the store of X. It is exceedingly unlikely, but not impossible afaict.