From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933369Ab0JRXXo (ORCPT ); Mon, 18 Oct 2010 19:23:44 -0400 Received: from terminus.zytor.com ([198.137.202.10]:48052 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755064Ab0JRXXn (ORCPT ); Mon, 18 Oct 2010 19:23:43 -0400 Message-ID: <4CBCD6E3.2020208@zytor.com> Date: Mon, 18 Oct 2010 16:23:15 -0700 From: "H. Peter Anvin" User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.9) Gecko/20100921 Fedora/3.1.4-1.fc13 Thunderbird/3.1.4 MIME-Version: 1.0 To: David Rientjes CC: Thomas Gleixner , Frederic Weisbecker , LKML , Ingo Molnar Subject: Re: [PATCH 1/2] x86: Cleanup TIF value gaps in shift range References: <1287434158-24362-1-git-send-regression-fweisbec@gmail.com> <4CBCBBFC.1090500@zytor.com> <4CBCBF9D.1060409@zytor.com> In-Reply-To: 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 10/18/2010 03:00 PM, David Rientjes wrote: >> >> YES IT IS. In fact, it is completely and totally bananas bonkers. >> >> By not pushing for a proper maintainable ABI, you will have an >> indefinite forward compatibility problem, and when predictably it >> breaks, you'll complain. This is, however, backwards -- the right thing >> would have been to say "I need this, this isn't available, I should add >> a maintainable API and push it upstream", and perhaps add log parsing as >> a backwards-compatibility solution. > > The problem is deciding what should be an ABI and what shouldn't an ABI > when it isn't clear. It would be great if everything that is logged or > shown to userspace would be part of an ABI and would be available no > matter how much information is emitted to the log. That's not scalable, > so we have to decide what userspace may depend on and design ABIs that > provide that information in an extendable way that won't break or become > obsoleted in the foreseeable future. > > Since we can't do that for everything and we have no idea what users will > find to parse from the dmesg, I'm advocating that if a change is proposed, > like was in this case with ti->flags, and someone has a usecase where the > information isn't available in any other way, that they speak up and come > up with a maintainable solution so that we've identified the parties > involved and can change that log message if necessary. I only think that > should be done, though, when there is a compelling reason for the change. > > I think that was done in this case by suggesting an alternative (printing, > at minimum, "M" when a thread has TIF_MEMDIE set instead of the raw flag > bits), but I don't think the change itself was compelling enough that it > has to be done. That doesn't mean doing the change I suggested wasn't > still appropriate, but at least it was known as a prerequisite before > something like this should be merged. Note: I have already said we shouldn't change TIF_ flags. The thing I'm objecting to is that in very short order you have made multiple requests for API-type stability for things that are explicitly for human consumption, like dmesg and Sysrq information. Expecting *anything* in dmesg to remain stable in any way is aggravated insanity and completely unreasonable. -hpa