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=-2.3 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,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 5AC55C282DD for ; Thu, 9 Jan 2020 20:08:19 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (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 2DC142072E for ; Thu, 9 Jan 2020 20:08:19 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="pmHqhnxW" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 2DC142072E Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.crashing.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+infradead-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=bombadil.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=wLA/cp55zrhYNF6rExxJ9+n0imFSSBtXyTPeu1Cd74o=; b=pmHqhnxW5U+vQb oN7WGUEGoMLqGMns5NlYy4gpjRp9fRjc/Ys32t2TPiN/fxjgQZbD3d2WIZFObPufSBfSgZqSxGdd4 fp/IL+fU+tkh/ph+ZTmFD/niefwxE8ZXbz/JoTEM6D+Igy2FBtWMlzXyuqNKQlg8afffqedyS5ctS DNKQvNGMj7BXU++Kg9HM4/O6DagY+a0VFLP2cxJcM3vSa/D8LlyqctmLPuE4/qGoWN1p7ZiN6YUXA azXAK0ZWa4pyvHs3prWYCTZIfxHTkjPPGr+4EUtB7/eoD0bH+L5lhJ3+3vdnfL6BzvAcfCGjUpJEf DlG2gD1MJC8NE2kMfrGA==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1ipe6E-0008HJ-Rj; Thu, 09 Jan 2020 20:08:10 +0000 Received: from gate.crashing.org ([63.228.1.57]) by bombadil.infradead.org with esmtps (Exim 4.92.3 #3 (Red Hat Linux)) id 1ipe6C-0008FX-FT for linux-arm-kernel@lists.infradead.org; Thu, 09 Jan 2020 20:08:09 +0000 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-Disposition: inline In-Reply-To: User-Agent: Mutt/1.4.2.3i X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200109_120808_664022_6CDB70C5 X-CRM114-Status: UNSURE ( 6.88 ) X-CRM114-Notice: Please train this message. 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: arnd@arndb.de, Benjamin Herrenschmidt , x86@kernel.org, linux-kernel@vger.kernel.org, linux-mips@vger.kernel.org, Paul Mackerras , luto@kernel.org, Michael Ellerman , tglx@linutronix.de, vincenzo.frascino@arm.com, linuxppc-dev@lists.ozlabs.org, linux-arm-kernel@lists.infradead.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org 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 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel