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 A4551C433E1 for ; Sat, 18 Jul 2020 00:59:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 799842065E for ; Sat, 18 Jul 2020 00:59:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1595033945; bh=vG8PG1WGBlJ0AL5gm2JsIz6j0+sz4k0vgDpRJOM4PMQ=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=lHpmiyrHJ4LLuDi1/40u+jOh8/efPul5FwQgkp3ptid8ayz61usjmGC1CFWqz5efN xu5+OzkP3qjA8hQS0h9RmCWxQjLUXTl+0gZ4lomdrJPe4c4prNDnuV5kD7+0ijPoAF RbPr1CL3P2FCwsgNmClBnNNKPFaHZy+ssKQZwUC4= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727916AbgGRA7B (ORCPT ); Fri, 17 Jul 2020 20:59:01 -0400 Received: from mail.kernel.org ([198.145.29.99]:42858 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726742AbgGRA67 (ORCPT ); Fri, 17 Jul 2020 20:58:59 -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 B814A2065E; Sat, 18 Jul 2020 00:58:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1595033939; bh=vG8PG1WGBlJ0AL5gm2JsIz6j0+sz4k0vgDpRJOM4PMQ=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=thlrGQyy/UZAZa70V3vjI5b/9CwLgKXaXVTnc9hc8tyntpaGvy7M9F3kizE34zxT8 UnoVA/HY5M5gQUq/nTl2Q/60WtpRPGHSjW1XM4QBu/VOSsfmOPu+qqfzPdTjVZkqHN gOXHcBzM1LF2zmDa9gKvbb5uByy3pFHg1H3Ce10c= Date: Fri, 17 Jul 2020 17:58:57 -0700 From: Eric Biggers To: "Darrick J. Wong" Cc: linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, "Paul E . McKenney" , linux-fsdevel@vger.kernel.org, Akira Yokosawa , Alan Stern , Andrea Parri , Boqun Feng , Daniel Lustig , 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: <20200718005857.GB2183@sol.localdomain> References: <20200717044427.68747-1-ebiggers@kernel.org> <20200717205340.GR7625@magnolia> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200717205340.GR7625@magnolia> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jul 17, 2020 at 01:53:40PM -0700, Darrick J. Wong wrote: > > +There are also cases in which the smp_load_acquire() can be replaced by > > +the more lightweight READ_ONCE(). (smp_store_release() is still > > +required.) Specifically, if all initialized memory is transitively > > +reachable from the pointer itself, then there is no control dependency > > I don't quite understand what "transitively reachable from the pointer > itself" means? Does that describe the situation where all the objects > reachable through the object that the global struct foo pointer points > at are /only/ reachable via that global pointer? > The intent is that "transitively reachable" means that all initialized memory can be reached by dereferencing the pointer in some way, e.g. p->a->b[5]->c. It could also be the case that allocating the object initializes some global or static data, which isn't reachable in that way. Access to that data would then be a control dependency, which a data dependency barrier wouldn't work for. It's possible I misunderstood something. (Note the next paragraph does say that using READ_ONCE() is discouraged, exactly for this reason -- it can be hard to tell whether it's correct.) Suggestions of what to write here are appreciated. - Eric