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=-2.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_NEOMUTT 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 884ACC43143 for ; Tue, 2 Oct 2018 14:40:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2A0712082A for ; Tue, 2 Oct 2018 14:40:36 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 2A0712082A Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728234AbeJBVYR (ORCPT ); Tue, 2 Oct 2018 17:24:17 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:39652 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727562AbeJBVYR (ORCPT ); Tue, 2 Oct 2018 17:24:17 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id D22CC18A; Tue, 2 Oct 2018 07:40:33 -0700 (PDT) Received: from queper01-lin (queper01-lin.emea.arm.com [10.4.13.27]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id B44E23F5A0; Tue, 2 Oct 2018 07:40:29 -0700 (PDT) Date: Tue, 2 Oct 2018 15:40:28 +0100 From: Quentin Perret To: Peter Zijlstra Cc: rjw@rjwysocki.net, linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org, gregkh@linuxfoundation.org, mingo@redhat.com, dietmar.eggemann@arm.com, morten.rasmussen@arm.com, chris.redpath@arm.com, patrick.bellasi@arm.com, valentin.schneider@arm.com, vincent.guittot@linaro.org, thara.gopinath@linaro.org, viresh.kumar@linaro.org, tkjos@google.com, joel@joelfernandes.org, smuckle@google.com, adharmap@codeaurora.org, skannan@codeaurora.org, pkondeti@codeaurora.org, juri.lelli@redhat.com, edubezval@gmail.com, srinivas.pandruvada@linux.intel.com, currojerez@riseup.net, javi.merino@kernel.org Subject: Re: [PATCH v7 03/14] PM: Introduce an Energy Model management framework Message-ID: <20181002144025.wnanxibhdcnl23sf@queper01-lin> References: <20180912091309.7551-1-quentin.perret@arm.com> <20180912091309.7551-4-quentin.perret@arm.com> <20181002123031.GZ3439@hirez.programming.kicks-ass.net> <20181002125115.245r3ocusvyiexno@queper01-lin> <20181002134857.GE26858@hirez.programming.kicks-ass.net> <20181002140430.fpeiqzblbcaewg6n@queper01-lin> <20181002142924.GI26858@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181002142924.GI26858@hirez.programming.kicks-ass.net> User-Agent: NeoMutt/20171215 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tuesday 02 Oct 2018 at 16:29:24 (+0200), Peter Zijlstra wrote: > On Tue, Oct 02, 2018 at 03:05:23PM +0100, Quentin Perret wrote: > > On Tuesday 02 Oct 2018 at 15:48:57 (+0200), Peter Zijlstra wrote: > > > +/** > > > + * em_cpu_get() - Return the performance domain for a CPU > > > + * @cpu : CPU to find the performance domain for > > > + * > > > + * Return: the performance domain to which 'cpu' belongs, or NULL if it doesn't > > > + * exist. > > > + */ > > > +struct em_perf_domain *em_cpu_get(int cpu) > > > +{ > > > + return READ_ONCE(per_cpu(em_data, cpu)); > > > +} > > > +EXPORT_SYMBOL_GPL(em_cpu_get); > > > > > > But your read side doesn't take, not is required to take em_pd_mutex. > > > > > > At that point, the mutex_unlock() doesn't guarantee anything. > > > > > > A CPU observing the em_data store, doesn't need to observe the store > > > that filled the data structure it points to. > > > > Right but even if I add the smp_store_release(), I can still have a > > CPU observing em_data while another is in the process of updating it. > > So, if smp_store_release() doesn't guarantee that readers will see a > > complete update, do I actually get something interesting from it ? > > (That's not a rhetorical question, I'm actually wondering :-) > > I thought the update would fail if em_data was already set. > > That is, you can only set this thing up _once_ and then you'll have to > forever live with it. > > Or did I read that wrong? No no, that's correct. em_data is populated once and kept as-is forever. What I was trying to say is, when em_data is being populated for the first time, nothing prevents a reader from using em_cpu_get() concurrently. And in this case, it doesn't matter if you use smp_store_release() or not, the reader might see the table half-updated. So, basically, smp_store_release() doesn't guarantee that readers won't see a half-baked em_data. That's the point I'm trying to make at least :-)