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,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS 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 835A7C433E2 for ; Sat, 18 Jul 2020 01:02:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6244D2065F for ; Sat, 18 Jul 2020 01:02:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1595034173; bh=l2J74681mstclINahuFMUN0d6t9i4PUhKyWNqKN96Is=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=tazCzv/IjPLuO6RaitYev/NUYD4V4XgjZ1s2avpwl8B9wLBO59vrvAt5N71lWEWk2 XEUg78u346IE0qDCNV5TAJI+ziEkIJOsGuaVTldt2BjfcUYdyBOjie9qgQzohZ5Mkq FpRR8TtTAFc5YjQ136dwk1HsXWyIEGhrKa4LagN8= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727040AbgGRBCu (ORCPT ); Fri, 17 Jul 2020 21:02:50 -0400 Received: from mail.kernel.org ([198.145.29.99]:46124 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726742AbgGRBCt (ORCPT ); Fri, 17 Jul 2020 21:02:49 -0400 Received: from sol.localdomain (c-107-3-166-239.hsd1.ca.comcast.net [107.3.166.239]) (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 034992065F; Sat, 18 Jul 2020 01:02:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1595034169; bh=l2J74681mstclINahuFMUN0d6t9i4PUhKyWNqKN96Is=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=mMfoP2V3McMONoqqj2jyFhxrobrkhAd+7hj/xgHx65YV07lnQp0KGMYMH/Uiejxm7 7JdwseK21OmaAETUP+eE0qH12wT+BeK4RT5mDdDGPBbwR3ZeE4OQUXLKpMonCzSypb YOrGqK633zW10x3hiDHbbPo6TSBz+XffO0MsTZ5M= Date: Fri, 17 Jul 2020 18:02:47 -0700 From: Eric Biggers To: Alan Stern Cc: Matthew Wilcox , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, "Paul E . McKenney" , linux-fsdevel@vger.kernel.org, Akira Yokosawa , Andrea Parri , Boqun Feng , Daniel Lustig , "Darrick J . Wong" , Dave Chinner , David Howells , Jade Alglave , Luc Maranget , Nicholas Piggin , Peter Zijlstra , Will Deacon Subject: Re: [PATCH] tools/memory-model: document the "one-time init" pattern Message-ID: <20200718010247.GC2183@sol.localdomain> References: <20200717044427.68747-1-ebiggers@kernel.org> <20200717174750.GQ12769@casper.infradead.org> <20200717175138.GB1156312@rowland.harvard.edu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200717175138.GB1156312@rowland.harvard.edu> Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Fri, Jul 17, 2020 at 01:51:38PM -0400, Alan Stern wrote: > On Fri, Jul 17, 2020 at 06:47:50PM +0100, Matthew Wilcox wrote: > > On Thu, Jul 16, 2020 at 09:44:27PM -0700, Eric Biggers wrote: > ... > > > + /* on success, pairs with smp_load_acquire() above and below */ > > > + if (cmpxchg_release(&foo, NULL, p) != NULL) { > > > > Why do we have cmpxchg_release() anyway? Under what circumstances is > > cmpxchg() useful _without_ having release semantics? > > To answer just the last question: cmpxchg() is useful for lock > acquisition, in which case it needs to have acquire semantics rather > than release semantics. > To clarify, there are 4 versions of cmpxchg: cmpxchg(): does ACQUIRE and RELEASE (on success) cmpxchg_acquire(): does ACQUIRE only (on success) cmpxchg_release(): does RELEASE only (on success) cmpxchg_relaxed(): no barriers The problem here is that here we need RELEASE on success and ACQUIRE on failure. But no version guarantees any barrier on failure. So as far as I can tell, the best we can do is use cmpxchg_release() (or cmpxchg() which would be stronger but unnecessary), followed by a separate ACQUIRE on failure. - Eric