From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752321AbdLKW4Q (ORCPT ); Mon, 11 Dec 2017 17:56:16 -0500 Received: from bombadil.infradead.org ([65.50.211.133]:38333 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751274AbdLKW4M (ORCPT ); Mon, 11 Dec 2017 17:56:12 -0500 Date: Mon, 11 Dec 2017 14:56:09 -0800 From: Darren Hart To: Jiri Slaby Cc: Linus Torvalds , Ingo Molnar , Linux Kernel Mailing List , Peter Zijlstra , Thomas Gleixner Subject: Re: [PATCH 1/1] futex: futex_wake_op, fix sign_extend32 sign bits Message-ID: <20171211225609.GB27831@fury> References: <20171130143544.21009-1-jslaby@suse.cz> <10cccd1f-76a1-13be-1173-670c3fcdf921@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <10cccd1f-76a1-13be-1173-670c3fcdf921@suse.cz> User-Agent: Mutt/1.8.0 (2017-02-23) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Dec 11, 2017 at 08:37:11AM +0100, Jiri Slaby wrote: > On 12/10/2017, 09:50 PM, Linus Torvalds wrote: > > On Thu, Nov 30, 2017 at 6:35 AM, Jiri Slaby wrote: > >> sign_extend32 counts the sign bit parameter from 0, not from 1. So we > >> have to use "11" for 12th bit, not "12". > > > > This interface is crap. It really doesn't make much sense. I wonder > > how many people have gotten this wrong, but it's hard to tell. > > I tend to agree, because it really surprised me. So at that time I > searched for most (all?) uses of the interface, checked them and all of > them *seem* to be fine. > > > I'm applying this directly to my tree since I didn't see anybody else > > react to it, but the whole pattern worries me. > > > > Also, clearly nobody actually uses the odder corners of futex ops > > anyway. Maybe we should deprecate them entirely? > > > > Jiri, did you notice by testing, or what? > > I noticed it by coincidence while fixing the strace build test failures > -- e78c38f6bdd9 (futex: futex_wake_op, do not fail on invalid op). I > compiled a bit modified futex_atomic_op_inuser in userspace to test the > conversion and the added check and it did not work. > > And yes, somebody (tglx?) noted already that this interface is old and > perhaps unused. The only use I know of for FUTEX_WAKE_OP is glibc lll_futex_wake_unlock(). and that is limited to a single operation. At the very least, we need to add a futex_wake_op test to the kselftests, something that's been nagging me for a very long time. There are some 120 different combinations of op and cmp and condition value. Assuming this isn't urgent, I've added it to my projects list. -- Darren Hart VMware Open Source Technology Center