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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 28E2EC33CA4 for ; Sat, 11 Jan 2020 11:07:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0974920880 for ; Sat, 11 Jan 2020 11:07:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729445AbgAKLHz (ORCPT ); Sat, 11 Jan 2020 06:07:55 -0500 Received: from Galois.linutronix.de ([193.142.43.55]:32947 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729302AbgAKLHy (ORCPT ); Sat, 11 Jan 2020 06:07:54 -0500 Received: from p5b06da22.dip0.t-ipconnect.de ([91.6.218.34] helo=nanos.tec.linutronix.de) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1iqEcB-0000Ef-0n; Sat, 11 Jan 2020 12:07:35 +0100 Received: by nanos.tec.linutronix.de (Postfix, from userid 1000) id 45B3D100C52; Sat, 11 Jan 2020 12:07:34 +0100 (CET) From: Thomas Gleixner To: Christophe Leroy , Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , arnd@arndb.de, vincenzo.frascino@arm.com, luto@kernel.org Cc: linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-arm-kernel@lists.infradead.org, linux-mips@vger.kernel.org, x86@kernel.org Subject: Re: [RFC PATCH v2 07/10] lib: vdso: don't use READ_ONCE() in __c_kernel_time() In-Reply-To: References: <87lfqfrp7d.fsf@nanos.tec.linutronix.de> Date: Sat, 11 Jan 2020 12:07:34 +0100 Message-ID: <878smes13d.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Christophe Leroy writes: > > With READ_ONCE() the 64 bits are being read: > > Without the READ_ONCE() only 32 bits are read. That's the most optimal. > > Without READ_ONCE() but with a barrier() after the read, we should get > the same result but GCC (GCC 8.1) does less good: > > Assuming both part of the 64 bits data will fall into a single > cacheline, the second read is in the noise. They definitely are in the same cacheline. > So agreed to drop this change. We could be smart about this and force the compiler to issue a 32bit read for 32bit builds. See below. Not sure whether it's worth it, but OTOH it will take quite a while until the 32bit time interfaces die completely. Thanks, tglx 8<------------ --- a/include/vdso/datapage.h +++ b/include/vdso/datapage.h @@ -21,6 +21,18 @@ #define CS_RAW 1 #define CS_BASES (CS_RAW + 1) +#ifdef __LITTLE_ENDIAN +struct sec_hl { + u32 sec_l; + u32 sec_h; +}; +#else +struct sec_hl { + u32 sec_h; + u32 sec_l; +}; +#endif + /** * struct vdso_timestamp - basetime per clock_id * @sec: seconds @@ -35,7 +47,10 @@ * vdso_data.cs[x].shift. */ struct vdso_timestamp { - u64 sec; + union { + u64 sec; + struct sec_hl sec_hl; + }; u64 nsec; }; --- a/lib/vdso/gettimeofday.c +++ b/lib/vdso/gettimeofday.c @@ -165,8 +165,13 @@ static __maybe_unused int static __maybe_unused __kernel_old_time_t __cvdso_time(__kernel_old_time_t *time) { const struct vdso_data *vd = __arch_get_vdso_data(); - __kernel_old_time_t t = READ_ONCE(vd[CS_HRES_COARSE].basetime[CLOCK_REALTIME].sec); + __kernel_old_time_t t; +#if BITS_PER_LONG == 32 + t = READ_ONCE(vd[CS_HRES_COARSE].basetime[CLOCK_REALTIME].sec_hl.sec_l); +#else + t = READ_ONCE(vd[CS_HRES_COARSE].basetime[CLOCK_REALTIME].sec); +#endif if (time) *time = t;