From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp1.linuxfoundation.org (smtp1.linux-foundation.org [172.17.192.35]) by mail.linuxfoundation.org (Postfix) with ESMTPS id 430C498C for ; Fri, 11 Aug 2017 23:10:53 +0000 (UTC) Received: from mail-oi0-f65.google.com (mail-oi0-f65.google.com [209.85.218.65]) by smtp1.linuxfoundation.org (Postfix) with ESMTPS id 2B3BC479 for ; Fri, 11 Aug 2017 23:10:53 +0000 (UTC) Received: by mail-oi0-f65.google.com with SMTP id e124so4627467oig.0 for ; Fri, 11 Aug 2017 16:10:53 -0700 (PDT) MIME-Version: 1.0 Sender: linus971@gmail.com In-Reply-To: <87zib6pm5s.fsf@notabene.neil.brown.name> References: <87efslsj7w.fsf@notabene.neil.brown.name> <878tiqr5eb.fsf@notabene.neil.brown.name> <87zib6pm5s.fsf@notabene.neil.brown.name> From: Linus Torvalds Date: Fri, 11 Aug 2017 16:10:51 -0700 Message-ID: To: NeilBrown Content-Type: text/plain; charset="UTF-8" Cc: "ksummit-discuss@lists.linuxfoundation.org" , Andy Lutomirski Subject: Re: [Ksummit-discuss] [MAINTAINER TOPIC] ABI feature gates? List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Fri, Aug 11, 2017 at 1:02 AM, NeilBrown wrote: > > What do you mean by "upgrade"? > Can I upgrade from 3.15 to 3.16-rc1? If not, why not? Yes.. Of course, bugs happen, and then they get fixed. But yes, even including things like -rc1 (or just "random untagged kernel of the day") you should (a) feel safe in always upgrading to any higher version (I *hope* you can always also downgrade to a lower kernel version, but obviously at some point user space may start depending on newer features that simply don't exist in older kernels). (b) also feel that if something breaks, it's a bug, and people will take it seriously and not dismiss it with some crazy "N+1 version" excuse. There are some cases where we may not be able to avoid breakage: the main two are "security issues" and "insanely old hardware". And even for security issues, we try really really hard to avoid breakage. And the key word in "insanely old hardware" is that "insanely" part. At some point it just gets too hard to test (and sometimes the hardware is too broken, like the original i386 non-working supervisor page fault workarounds). Now, it can get really interesting if somebody notices an ABI change so late that others have started to depend on that ABI change. At that point, it's a "damned if you do, damned if you don't". We've actually been able to handle even that occasionally (by just adjusting behavior automatically based on some pattern), but at some point it obviously is impossible to fix both cases. And then I say "if it took you three years to upgrade and notice a behavioral change that nobody else noticed, it's no longer _our_ fault". So there is _some_ onus on people actually testing and reporting these things, but I can't off-hand actually remember any case of this really being a major issue. So it's largely a theoretical thing. Linus