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=-4.0 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,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 CBEE7C433DF for ; Wed, 15 Jul 2020 08:14:09 +0000 (UTC) Received: from merlin.infradead.org (merlin.infradead.org [205.233.59.134]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 9C6872067D for ; Wed, 15 Jul 2020 08:14:09 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="wdfTFWKj" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9C6872067D Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=merlin.20170209; h=Sender:Content-Transfer-Encoding: Content-Type:Cc:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References:Message-ID: Subject:To:From:Date:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=uE8cH4XU3GCRllKBY2Lncy7/WgclwM4meWISaL5zdMQ=; b=wdfTFWKjyBV6T6ulFgi+fYAW0 4DzAyYcWumOYSMDwdNavVdMM9UAaBigZQ6ETEs1Wdefuu+08d+6B/+KWiAnm15St2HHq2l9J4RYQ7 QXm/yNIh/0yUuE1qBQzwZjtoZZVqwNC0kwM1zcjpjdDRVp0bnMT9cQcqYmA9s/mHEozoCvk8YsfXm w6KwP9TiAPGNxPIhsS+sh6MwjIbxchLaM9OfalOzi28a85XluDA2A3FjP5S8m3yIjdRHTq47cEo82 H4rRq1oEaVUiz5wrXcQ5sA8UWP3RNyIPMTW+xRwpPM1O5tkfSZBwYaDLzw9R2i0CTifX9ngpX4DGy 68mxxhl0g==; Received: from localhost ([::1] helo=merlin.infradead.org) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1jvcWj-0000gV-Tb; Wed, 15 Jul 2020 08:12:29 +0000 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" 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-Disposition: inline In-Reply-To: <20200715055650.GB225020@debian-buster-darwi.lab.linutronix.de> X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Mark Rutland , Catalin Marinas , Alexander Shishkin , Jiri Olsa , Adrian Hunter , Arnaldo Carvalho de Melo , linux-kernel@vger.kernel.org, Paul Cercueil , Ingo Molnar , Leo Yan , Namhyung Kim , Thomas Gleixner , jogness@linutronix.de, Will Deacon , "Ben Dooks \(Codethink\)" , linux-arm-kernel@lists.infradead.org, Kan Liang Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.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 :-) _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel