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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no 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 A4C65C33CB1 for ; Fri, 17 Jan 2020 16:01:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7A8822083E for ; Fri, 17 Jan 2020 16:01:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729238AbgAQQBP (ORCPT ); Fri, 17 Jan 2020 11:01:15 -0500 Received: from zeniv.linux.org.uk ([195.92.253.2]:34606 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727043AbgAQQBO (ORCPT ); Fri, 17 Jan 2020 11:01:14 -0500 Received: from viro by ZenIV.linux.org.uk with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1isU3Z-00ALHS-1l; Fri, 17 Jan 2020 16:01:09 +0000 Date: Fri, 17 Jan 2020 16:01:09 +0000 From: Al Viro To: Trond Myklebust Cc: "dhowells@redhat.com" , "hch@lst.de" , "osandov@osandov.com" , "miklos@szeredi.hu" , "linux-fsdevel@vger.kernel.org" , "amir73il@gmail.com" , "lsf-pc@lists.linux-foundation.org" Subject: Re: [LSF/MM/BPF TOPIC] Allowing linkat() to replace the destination Message-ID: <20200117160109.GL8904@ZenIV.linux.org.uk> References: <364531.1579265357@warthog.procyon.org.uk> <448106.1579272445@warthog.procyon.org.uk> <18dad9903c4f5c63300048e9ed2a8706ad31bc73.camel@hammerspace.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <18dad9903c4f5c63300048e9ed2a8706ad31bc73.camel@hammerspace.com> Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Fri, Jan 17, 2020 at 02:56:05PM +0000, Trond Myklebust wrote: > It sounds to me like we rather need a meta-topic about "How do we get > simple things done in the Linux fs community?" > > It shouldn't take a ticket to Palm Springs to perform something simple > like adding a new flag to a syscall. Sure - adding a new flag is trivial. Coming up with sane semantics for it, OTOH, can be rather non-trivial and in this case it is, unfortunately. "something like link(2), only it tolerates the existing target and atomically replaces it" does _not_ specify the semantics. Try to sit down for a few minutes and come up with the cases when behaviour is undefined by the above; it won't take longer than that. We can do it by asking the proponent to come up with full description to be included into the proposal, then have at it on fsdevel/linux-abi (as well as security lists). Doable, but not a small amount of PITA for original poster and dealing with questions/objections/etc. is certain to grow a large thread with many branches (and lots of bikeshedding thrown in) _and_ would include tons of roundtrips, so the latency (especially early on, while the proposal is still raw) will be a factor. It's not the question of how to implement it; it's what should it _do_. And "we'll tweak the behaviour in corner cases later on" is good in a lot of situations, but not for userland ABI. I'd been guilty of such fuckups several times and they are not cheap to fix afterwards ;-/