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,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 734FFC433E2 for ; Wed, 15 Jul 2020 09:23:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5155020760 for ; Wed, 15 Jul 2020 09:23:53 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="iRcqbhoo"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="XvgIcEpx" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730645AbgGOJXw (ORCPT ); Wed, 15 Jul 2020 05:23:52 -0400 Received: from Galois.linutronix.de ([193.142.43.55]:49896 "EHLO galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729672AbgGOJXt (ORCPT ); Wed, 15 Jul 2020 05:23:49 -0400 Date: Wed, 15 Jul 2020 11:23:45 +0200 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1594805027; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=nVewUEkE+vEgqvpSZaR+rHv9rvICaRBJ6Hc/Y4Gfv04=; b=iRcqbhooQLbNQYs0t6EgUxItIJxhtP3b8s8woTQvVERuHgCybkfAetFUIzEPA2qoDta3XB hFGfqNFUsu3hRGjuhAphrutMLv6knCYWsKZM43Mb3AYZHtaVSpWCf9rN/cvu+IehElTs6w w7jkCecMreibSzUAgIGJ2NVV3RoeZlRT8WDBS9NXru7YSKwOWLV7KNvaQtm77bvWFv6rIw Tmf674X1SQV4U6ysvZST8Y6X5/9VyE6jietFR2lRK9R/Fnv4Xy0+oqlSKZzC+OAamfCMpL S6AUQbGmozxbFDdI/qPyin9SrTMah7JNfk2r2V4MAlBnzEXS+gnSSHhpJVaY6A== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1594805027; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=nVewUEkE+vEgqvpSZaR+rHv9rvICaRBJ6Hc/Y4Gfv04=; b=XvgIcEpx3pM+hW2dYCP5H7sBCeEboYeXT2fgmDCQ3cwOqyeGO6r/T+NNo/0VBxbVQMqjLC G1W0Dhzx1t3IK1CQ== From: "Ahmed S. Darwish" To: peterz@infradead.org 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: <20200715092345.GA231464@debian-buster-darwi.lab.linutronix.de> References: <20200715020512.20991-1-leo.yan@linaro.org> <20200715020512.20991-2-leo.yan@linaro.org> <20200715055650.GB225020@debian-buster-darwi.lab.linutronix.de> <20200715081222.GB10769@hirez.programming.kicks-ass.net> <20200715081443.GB43129@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200715081443.GB43129@hirez.programming.kicks-ass.net> 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 10:14:43AM +0200, peterz@infradead.org wrote: > On Wed, Jul 15, 2020 at 10:12:22AM +0200, Peter Zijlstra wrote: > > 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 :-) > > Also see: > > https://lkml.kernel.org/r/20200625085745.GD117543@hirez.programming.kicks-ass.net Oh, spot on. Can we then please replace the raw_read_seqcount(), in the original patch which started this discussion, with raw_read_seqcount_latch()? I see that you already did something similar for timekeeping.c: 7fc26327b756 ("seqlock: Introduce raw_read_seqcount_latch()"). Confession time: I have an internal patch series which introduces seqcount_latch_t. Having a standardized set of accessors for the seqcount latch read and write paths will make everything much more streamlined :-) Thanks, -- Ahmed S. Darwish Linutronix GmbH