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=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham 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 A1FA1C3279B for ; Tue, 10 Jul 2018 13:11:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4B44220883 for ; Tue, 10 Jul 2018 13:11:28 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 4B44220883 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933283AbeGJNLZ (ORCPT ); Tue, 10 Jul 2018 09:11:25 -0400 Received: from foss.arm.com ([217.140.101.70]:46188 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754201AbeGJNLY (ORCPT ); Tue, 10 Jul 2018 09:11:24 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 26A62ED1; Tue, 10 Jul 2018 06:11:24 -0700 (PDT) Received: from e103592.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.72.51.249]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 7E01F3F5A0; Tue, 10 Jul 2018 06:11:22 -0700 (PDT) Date: Tue, 10 Jul 2018 14:11:20 +0100 From: Dave Martin To: "Yandong.Zhao" Cc: zhaoxb@thundersoft.com, ard.biesheuvel@linaro.org, catalin.marinas@arm.com, will.deacon@arm.com, linux-kernel@vger.kernel.org, zhaoyd@thundersoft.com, linux-arm-kernel@lists.infradead.org, fanlc0801@thundersoft.com Subject: Re: [PATCH] arm64: neon: Fix function may_use_simd() return error status Message-ID: <20180710131119.GI9486@e103592.cambridge.arm.com> References: <1531189300-26216-1-git-send-email-yandong77520@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1531189300-26216-1-git-send-email-yandong77520@gmail.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 10, 2018 at 10:21:40AM +0800, Yandong.Zhao wrote: > From: Yandong Zhao > > Operations for contexts where we do not want to do any checks for > preemptions. Unless strictly necessary, always use this_cpu_read() > instead. Because of the kernel_neon_busy here we have to make sure > that it is the current cpu. I find this wording a bit confusing. Does the following make look OK to you? --8<-- It does not matter if the caller of may_use_simd() migrates to another cpu after the call, but it is still important that the kernel_neon_busy percpu instance that is read matches the cpu the task is running on at the time of the read. This means that raw_cpu_read() is not sufficient. kernel_neon_busy may appear true if the caller migrates during the execution of raw_cpu_read() and the next task to be scheduled in on the initial cpu calls kernel_neon_begin(). This patch replaces raw_cpu_read() with this_cpu_read() to protect against this race. -->8-- > > Signed-off-by: Yandong Zhao > --- > arch/arm64/include/asm/simd.h | 5 +++-- > 1 file changed, 3 insertions(+), 2 deletions(-) > > diff --git a/arch/arm64/include/asm/simd.h b/arch/arm64/include/asm/simd.h > index fa8b3fe..8b97f8b 100644 > --- a/arch/arm64/include/asm/simd.h > +++ b/arch/arm64/include/asm/simd.h > @@ -29,7 +29,8 @@ > static __must_check inline bool may_use_simd(void) > { > /* > - * The raw_cpu_read() is racy if called with preemption enabled. > + * The this_cpu_read() is racy if called with preemption enabled, > + * since the task my subsequently migrate to another CPU. "my" -> "may" (apologies if I was responsible for that typo) [...] Cheers ---Dave