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=-8.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 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 C2E93C433DF for ; Wed, 20 May 2020 17:39:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8B4FA207D4 for ; Wed, 20 May 2020 17:39:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1589996399; bh=dERhy1cVIOn/6h748BVlmDC9dX7ACeMBsQGjJZhXqIM=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=jMlRP1kjfnVU6r2xIiK9frBlRWw9X5TxxPsHYXh0Kxsg45qIaZz63EUG0A22fuTXk C9UgG/Frr+Irpzuq8+c1HyM+jiTG3ztDtmsNbGKyGIq8T0GUelDWcWYl7vFeUekcMc gA+dSq2zw7ggATzYby1wV1XvB4askcqYbS/U1KLY= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726871AbgETRj6 (ORCPT ); Wed, 20 May 2020 13:39:58 -0400 Received: from mail.kernel.org ([198.145.29.99]:51542 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726727AbgETRj6 (ORCPT ); Wed, 20 May 2020 13:39:58 -0400 Received: from willie-the-truck (236.31.169.217.in-addr.arpa [217.169.31.236]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id E74BA2075F; Wed, 20 May 2020 17:39:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1589996398; bh=dERhy1cVIOn/6h748BVlmDC9dX7ACeMBsQGjJZhXqIM=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=zty+BQ7zbhCHFe1nFd2ZcRTOkVrTaNPEvw2wfo9naH4EQuluD/fVWs6kF7dscCMTe IoBGpQJ1ah1trxZ2Qu94YjFT71AfNYvCtoyKSv33xRVxeL3XClIit17cYyWvMKjNQz GE1yd2dj5/PiZawkS8GcN6vr19Rx5fZM6/He3zPE= Date: Wed, 20 May 2020 18:39:53 +0100 From: Will Deacon To: Catalin Marinas Cc: Anshuman Khandual , linux-arm-kernel@lists.infradead.org, mark.rutland@arm.com, Suzuki K Poulose , Mark Brown , linux-kernel@vger.kernel.org Subject: Re: [PATCH] arm64/cpufeature: Move BUG_ON() inside get_arm64_ftr_reg() Message-ID: <20200520173953.GA27629@willie-the-truck> References: <1589937774-20479-1-git-send-email-anshuman.khandual@arm.com> <20200520122012.GA25815@willie-the-truck> <20200520154711.GD18302@gaia> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200520154711.GD18302@gaia> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 20, 2020 at 04:47:11PM +0100, Catalin Marinas wrote: > On Wed, May 20, 2020 at 01:20:13PM +0100, Will Deacon wrote: > > On Wed, May 20, 2020 at 06:52:54AM +0530, Anshuman Khandual wrote: > > > There is no way to proceed when requested register could not be searched in > > > arm64_ftr_reg[]. Requesting for a non present register would be an error as > > > well. Hence lets just BUG_ON() when the search fails in get_arm64_ftr_reg() > > > rather than checking for return value and doing the same in some individual > > > callers. > > > > > > But there are some callers that dont BUG_ON() upon search failure. It adds > > > an argument 'failsafe' that provides required switch between callers based > > > on whether they could proceed or not. > > > > > > Cc: Catalin Marinas > > > Cc: Will Deacon > > > Cc: Suzuki K Poulose > > > Cc: Mark Brown > > > Cc: linux-arm-kernel@lists.infradead.org > > > Cc: linux-kernel@vger.kernel.org > > > > > > Signed-off-by: Anshuman Khandual > > > --- > > > Applies on next-20200518 that has recent cpufeature changes from Will. > > > > > > arch/arm64/kernel/cpufeature.c | 26 +++++++++++++------------- > > > 1 file changed, 13 insertions(+), 13 deletions(-) > > > > > > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c > > > index bc5048f152c1..62767cc540c3 100644 > > > --- a/arch/arm64/kernel/cpufeature.c > > > +++ b/arch/arm64/kernel/cpufeature.c > > > @@ -557,7 +557,7 @@ static int search_cmp_ftr_reg(const void *id, const void *regp) > > > * - NULL on failure. It is upto the caller to decide > > > * the impact of a failure. > > > */ > > > -static struct arm64_ftr_reg *get_arm64_ftr_reg(u32 sys_id) > > > +static struct arm64_ftr_reg *get_arm64_ftr_reg(u32 sys_id, bool failsafe) > > > > Generally, I'm not a big fan of boolean arguments because they are really > > opaque at the callsite. It also seems bogus to me that we don't trust the > > caller to pass a valid sys_id, but we trust it to get "failsafe" right, > > which seems to mean "I promise to check the result isn't NULL before > > dereferencing it." > > > > So I don't see how this patch improves anything. I'd actually be more > > inclined to stick a WARN() in get_arm64_ftr_reg() when it returns NULL and > > have the callers handle NULL by returning early, getting rid of all the > > BUG_ONs in here. Sure, the system might end up in a funny state, but we > > WARN()d about it and tried to keep going (and Linus has some strong opinions > > on this too). > > Such WARN can be triggered by the user via emulate_sys_reg(), so we > can't really have it in get_arm64_ftr_reg() without a 'failsafe' option. Ah yes, that would be bad. In which case, I don't think the existing code should change. Will