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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 D918AC282DD for ; Thu, 9 Jan 2020 20:10:28 +0000 (UTC) Received: from lists.ozlabs.org (lists.ozlabs.org [203.11.71.2]) (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 5AA3820656 for ; Thu, 9 Jan 2020 20:10:28 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 5AA3820656 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.crashing.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Received: from lists.ozlabs.org (lists.ozlabs.org [IPv6:2401:3900:2:1::3]) by lists.ozlabs.org (Postfix) with ESMTP id 47ty0K6TvvzDqZB for ; Fri, 10 Jan 2020 07:10:25 +1100 (AEDT) Authentication-Results: lists.ozlabs.org; spf=permerror (SPF Permanent Error: Unknown mechanism found: ip:192.40.192.88/32) smtp.mailfrom=kernel.crashing.org (client-ip=63.228.1.57; helo=gate.crashing.org; envelope-from=segher@kernel.crashing.org; receiver=) Authentication-Results: lists.ozlabs.org; dmarc=none (p=none dis=none) header.from=kernel.crashing.org Received: from gate.crashing.org (gate.crashing.org [63.228.1.57]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 47txxn2skmzDqZw for ; Fri, 10 Jan 2020 07:08:13 +1100 (AEDT) Received: from gate.crashing.org (localhost.localdomain [127.0.0.1]) by gate.crashing.org (8.14.1/8.14.1) with ESMTP id 009K7ZKP004645; Thu, 9 Jan 2020 14:07:35 -0600 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 009K7XMf004644; Thu, 9 Jan 2020 14:07:33 -0600 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Thu, 9 Jan 2020 14:07:33 -0600 From: Segher Boessenkool To: Christophe Leroy Subject: Re: Surprising code generated for vdso_read_begin() Message-ID: <20200109200733.GS3191@gate.crashing.org> References: Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.4.2.3i X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: arnd@arndb.de, x86@kernel.org, linux-kernel@vger.kernel.org, linux-mips@vger.kernel.org, Paul Mackerras , luto@kernel.org, tglx@linutronix.de, vincenzo.frascino@arm.com, linuxppc-dev@lists.ozlabs.org, linux-arm-kernel@lists.infradead.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Thu, Jan 09, 2020 at 05:52:34PM +0000, Christophe Leroy wrote: > Wondering why we get something so complicated/redundant for > vdso_read_begin() > > static __always_inline u32 vdso_read_begin(const struct vdso_data *vd) > { > u32 seq; > > while ((seq = READ_ONCE(vd->seq)) & 1) > cpu_relax(); > > smp_rmb(); > return seq; > } > > > 6e0: 81 05 00 f0 lwz r8,240(r5) > 6e4: 71 09 00 01 andi. r9,r8,1 > 6e8: 41 82 00 10 beq 6f8 <__c_kernel_clock_gettime+0x158> > 6ec: 81 05 00 f0 lwz r8,240(r5) > 6f0: 71 0a 00 01 andi. r10,r8,1 > 6f4: 40 82 ff f8 bne 6ec <__c_kernel_clock_gettime+0x14c> > 6f8: > > r5 being vd pointer > > Why the first triplet, not only the second triplet ? Something wrong > with using READ_ONCE() for that ? It looks like the compiler did loop peeling. What GCC version is this? Please try current trunk (to become GCC 10), or at least GCC 9? Segher