From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752359AbaEBNLD (ORCPT ); Fri, 2 May 2014 09:11:03 -0400 Received: from cantor2.suse.de ([195.135.220.15]:53107 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752316AbaEBNLA (ORCPT ); Fri, 2 May 2014 09:11:00 -0400 Date: Fri, 2 May 2014 15:10:58 +0200 (CEST) From: Jiri Kosina X-X-Sender: jikos@twin.jikos.cz To: Josh Poimboeuf cc: Seth Jennings , Masami Hiramatsu , Steven Rostedt , Frederic Weisbecker , Ingo Molnar , Jiri Slaby , linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH 0/2] kpatch: dynamic kernel patching In-Reply-To: Message-ID: References: User-Agent: Alpine 2.00 (LRH 1167 2008-08-23) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 1 May 2014, Josh Poimboeuf wrote: > kpatch vs kGraft > ---------------- > > I think the biggest difference between kpatch and kGraft is how they > ensure that the patch is applied atomically and safely. > > kpatch checks the backtraces of all tasks in stop_machine() to ensure > that no instances of the old function are running when the new function > is applied. I think the biggest downside of this approach is that > stop_machine() has to idle all other CPUs during the patching process, > so it inserts a small amount of latency (a few ms on an idle system). > > Instead, kGraft uses per-task consistency: each task either sees the old > version or the new version of the function. This gives a consistent > view with respect to functions, but _not_ data, because the old and new > functions are allowed to run simultaneously and share data. This could > be dangerous if a patch changes how a function uses a data structure. > The new function could make a data change that the old function wasn't > expecting. Please correct me if I am wrong, but with kPatch, you are also unable to do a "flip and forget" switch between functions that expect different format of in-memory data without performing a non-trivial all-memory lookup to find structures in question and perfoming corresponding transformations. What we can do with kGraft si to perform the patching in two steps (1) redirect to a temporary band-aid function that can handle both semantics of the data (persumably in highly sub-optimal way) (2) patching in (1) succeeds completely (kGraft claims victory), start a new round of patching with redirect to the final function which expects only the new semantics This basically implies that both aproaches need "human inspection" in this respect anyway. -- Jiri Kosina SUSE Labs