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=-2.4 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,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 EB359C33C99 for ; Fri, 10 Jan 2020 06:45:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B539C20678 for ; Fri, 10 Jan 2020 06:45:50 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=c-s.fr header.i=@c-s.fr header.b="ueN7Vpxn" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731533AbgAJGpu (ORCPT ); Fri, 10 Jan 2020 01:45:50 -0500 Received: from pegase1.c-s.fr ([93.17.236.30]:39753 "EHLO pegase1.c-s.fr" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731530AbgAJGpu (ORCPT ); Fri, 10 Jan 2020 01:45:50 -0500 Received: from localhost (mailhub1-int [192.168.12.234]) by localhost (Postfix) with ESMTP id 47vD5S13wRz9v3tv; Fri, 10 Jan 2020 07:45:48 +0100 (CET) Authentication-Results: localhost; dkim=pass reason="1024-bit key; insecure key" header.d=c-s.fr header.i=@c-s.fr header.b=ueN7Vpxn; dkim-adsp=pass; dkim-atps=neutral X-Virus-Scanned: Debian amavisd-new at c-s.fr Received: from pegase1.c-s.fr ([192.168.12.234]) by localhost (pegase1.c-s.fr [192.168.12.234]) (amavisd-new, port 10024) with ESMTP id Av3l7EVHXBCa; Fri, 10 Jan 2020 07:45:48 +0100 (CET) Received: from messagerie.si.c-s.fr (messagerie.si.c-s.fr [192.168.25.192]) by pegase1.c-s.fr (Postfix) with ESMTP id 47vD5R6Z0Hz9v3tj; Fri, 10 Jan 2020 07:45:47 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=c-s.fr; s=mail; t=1578638747; bh=5TApiD5N/34vXaM+pQ7UUlq0/+wq9o7jjNc+pK8cRCg=; h=Subject:To:Cc:References:From:Date:In-Reply-To:From; b=ueN7VpxnzBcnWuZoF/iIk0abFzI/Og6E2TcMJ9xPs+8bq1JLbuwjMI10oN+xH1GW4 DzTR5wk+czh2b56nyJo+0UKVXRL7LBJYds2dWPvb77f70oXNepmKK0hmzElSjKYQPC 5TkwvFKiglrjZkvvPDDUdnzAbqHeggFMFpJlVGmE= Received: from localhost (localhost [127.0.0.1]) by messagerie.si.c-s.fr (Postfix) with ESMTP id C0BAF8B77D; Fri, 10 Jan 2020 07:45:48 +0100 (CET) X-Virus-Scanned: amavisd-new at c-s.fr Received: from messagerie.si.c-s.fr ([127.0.0.1]) by localhost (messagerie.si.c-s.fr [127.0.0.1]) (amavisd-new, port 10023) with ESMTP id 7nJRtiLkHgXr; Fri, 10 Jan 2020 07:45:48 +0100 (CET) Received: from [172.25.230.102] (po15451.idsi0.si.c-s.fr [172.25.230.102]) by messagerie.si.c-s.fr (Postfix) with ESMTP id 4C2CD8B755; Fri, 10 Jan 2020 07:45:45 +0100 (CET) Subject: Re: Surprising code generated for vdso_read_begin() To: Segher Boessenkool Cc: Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , arnd@arndb.de, tglx@linutronix.de, vincenzo.frascino@arm.com, luto@kernel.org, x86@kernel.org, linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-mips@vger.kernel.org References: <20200109200733.GS3191@gate.crashing.org> From: Christophe Leroy Message-ID: <77a8bf25-6615-6c0a-56d4-eae7aa8a8f09@c-s.fr> Date: Fri, 10 Jan 2020 07:45:44 +0100 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.9.1 MIME-Version: 1.0 In-Reply-To: <20200109200733.GS3191@gate.crashing.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: fr Content-Transfer-Encoding: 8bit Sender: linux-mips-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-mips@vger.kernel.org Le 09/01/2020 à 21:07, Segher Boessenkool a écrit : > 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? > It is with GCC 5.5 https://mirrors.edge.kernel.org/pub/tools/crosstool/ doesn't have more recent than 8.1 With 8.1, the problem doesn't show up. Thanks Christophe