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=-3.8 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,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 450AAC433EC for ; Wed, 15 Jul 2020 08:14:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 27B14206E9 for ; Wed, 15 Jul 2020 08:14:38 +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="WDM6G+e3" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729690AbgGOIMp (ORCPT ); Wed, 15 Jul 2020 04:12:45 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49646 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729569AbgGOIMo (ORCPT ); Wed, 15 Jul 2020 04:12:44 -0400 Received: from merlin.infradead.org (merlin.infradead.org [IPv6:2001:8b0:10b:1231::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 63BA1C061755 for ; Wed, 15 Jul 2020 01:12:44 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.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; bh=uIbsP/4TYly3s1yM2udDpP8+Cyz0hKhcZJeKlcvBJuk=; b=WDM6G+e3sdPiUF/AmrUJIJV3nB Em9bl3v37rjrGJg6zrkIMhypVtr/gPqFDB6lk22NQPCqkmDUfVR6BgEJfc356XMSC+8DbuS6CdqCF N+hElOSoEl15nGYKOwUS4zvDmIqh4C2GhmPn2lu/7lGAfQJEO0aiHG2MqoiwwSDYacHj/Gh6Y4Eer smqFoQg6EuzilNj07sEc0hMMM4TXl4me9K07bqcBCzUAxcBKzlh5mSGDs7BuzBfdBYUTNHHwbZSIb cD8dej86nVQZntgoOYwE1aJXbSw5VvMp9fXGdzk/Gu1as0TSpGUcOL30/gRZQCGSxWHZRnBfikAEs S6d9pUYQ==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1jvcWg-0000g6-IQ; Wed, 15 Jul 2020 08:12:26 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id D120D300446; Wed, 15 Jul 2020 10:12:22 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id B73D423648704; Wed, 15 Jul 2020 10:12:22 +0200 (CEST) Date: Wed, 15 Jul 2020 10:12:22 +0200 From: Peter Zijlstra To: "Ahmed S. Darwish" Cc: Leo Yan , Will Deacon , Mark Rutland , Ingo Molnar , Arnaldo Carvalho de Melo , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Catalin Marinas , Thomas Gleixner , Paul Cercueil , "Ben Dooks (Codethink)" , Adrian Hunter , Kan Liang , jogness@linutronix.de, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 1/6] sched_clock: Expose struct clock_read_data Message-ID: <20200715081222.GB10769@hirez.programming.kicks-ass.net> References: <20200715020512.20991-1-leo.yan@linaro.org> <20200715020512.20991-2-leo.yan@linaro.org> <20200715055650.GB225020@debian-buster-darwi.lab.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200715055650.GB225020@debian-buster-darwi.lab.linutronix.de> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 15, 2020 at 07:56:50AM +0200, Ahmed S. Darwish wrote: > On Wed, Jul 15, 2020 at 10:05:07AM +0800, Leo Yan wrote: > > From: Peter Zijlstra > > > ... > > > > Provide struct clock_read_data and two (seqcount) helpers so that > > architectures (arm64 in specific) can expose the numbers to userspace. > > > ... > > > > +struct clock_read_data *sched_clock_read_begin(unsigned int *seq) > > +{ > > + *seq = raw_read_seqcount(&cd.seq); > > + return cd.read_data + (*seq & 1); > > +} > > + > ... > > Hmm, this seqcount_t is actually a latch seqcount. I know the original > code also used raw_read_seqcount(), but while at it, let's use the > proper read API for seqcount_t latchers: raw_read_seqcount_latch(). > > raw_read_seqcount_latch() has no read memory barrier though, and a > suspicious claim that READ_ONCE() pairs with an smp_wmb() (??). But if > its implementation is wrong, let's fix it there instead. It's supposed to be a dependent load, so READ_ONCE() is sufficient. Except, of course, the C standard has other ideas, so a compiler is allowed to wreck that, but they mostly don't :-)