From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932769Ab3LDQpd (ORCPT ); Wed, 4 Dec 2013 11:45:33 -0500 Received: from terminus.zytor.com ([198.137.202.10]:59157 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932508Ab3LDQpa (ORCPT ); Wed, 4 Dec 2013 11:45:30 -0500 Message-ID: <529F5C02.2060706@zytor.com> Date: Wed, 04 Dec 2013 08:44:50 -0800 From: "H. Peter Anvin" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.1.0 MIME-Version: 1.0 To: Ingo Molnar CC: Peter Zijlstra , Jesse Brandeburg , Linux Kernel Mailing List Subject: Re: Regression due to 0c44c2d0f459 x86: Use asm goto to implement better modify_and_test() functions References: <529E8719.4070202@zytor.com> <20131204085953.GC31778@gmail.com> In-Reply-To: <20131204085953.GC31778@gmail.com> X-Enigmail-Version: 1.6 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/04/2013 12:59 AM, Ingo Molnar wrote: > > * H. Peter Anvin wrote: > >> Hi guys, >> >> 0c44c2d0f459 x86: Use asm goto to implement better modify_and_test() >> functions >> >> causes a regression, because it incorrectly changed the constraints of >> bitops. >> >> Specifically, the GEN_BINARY_RMWcc() hardcodes a constraint as "er", but >> it needs to be "Ir" for the bitops themselves. "I" is correct (as >> opposed to "J" even on 64 bits, because we only generate the 64-bit >> version when we have a register operand. >> >> Unfortunately there isn't a way we can get gcc+gas to generate a version >> with an offset pointer. > > Does the regression manifest itself in any actual breakage - if yes, > how does it look like? (People experiencing similar symptoms will be > helped by seeing a fix matching their problems.) > It was discovered because it caused a build failure in a not-yet-submitted driver patch. This happens when someone uses test_and_set_bit() or another similar operation on a fixed bit index above 255; the assembler throws an error at that point and the build fails. *HOWEVER*, for bit indicies in the range 32-255, the current code will instead silently miscompile, as the CPU will truncate the argument to 5 bits. I don't know if there are any such instances in the current kernel, but it is entirely possible there is, with unknown but potentially disastrous results. -hpa