Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752745AbcLKCI7 (ORCPT ); Sat, 10 Dec 2016 21:08:59 -0500 Received: from mail-pg0-f68.google.com ([74.125.83.68]:33890 "EHLO mail-pg0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750756AbcLKCI5 (ORCPT ); Sat, 10 Dec 2016 21:08:57 -0500 Subject: Re: [PATCH v3 00/15] livepatch: hybrid consistency model To: Josh Poimboeuf References: <1481348777.28041.1.camel@gmail.com> <20161210171707.cpupmxyuhob4tc3i@treble> Cc: Jessica Yu , Jiri Kosina , Miroslav Benes , Petr Mladek , linux-s390@vger.kernel.org, Vojtech Pavlik , Peter Zijlstra , x86@kernel.org, Heiko Carstens , linux-kernel@vger.kernel.org, Andy Lutomirski , live-patching@vger.kernel.org, Jiri Slaby , linuxppc-dev@lists.ozlabs.org, Ingo Molnar , Chris J Arges From: Balbir Singh Message-ID: Date: Sun, 11 Dec 2016 13:08:33 +1100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.5.1 MIME-Version: 1.0 In-Reply-To: <20161210171707.cpupmxyuhob4tc3i@treble> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1793 Lines: 46 On 11/12/16 04:17, Josh Poimboeuf wrote: > On Sat, Dec 10, 2016 at 04:46:17PM +1100, Balbir Singh wrote: >> On Thu, 2016-12-08 at 12:08 -0600, Josh Poimboeuf wrote: >>> Dusting the cobwebs off the consistency model again. This is based on >>> linux-next/master. >>> >>> v1 was posted on 2015-02-09: >>> >>> https://lkml.kernel.org/r/cover.1423499826.git.jpoimboe@redhat.com >>> >>> v2 was posted on 2016-04-28: >>> >>> https://lkml.kernel.org/r/cover.1461875890.git.jpoimboe@redhat.com >>> >>> The biggest issue from v2 was finding a decent way to detect preemption >>> and page faults on the stack of a sleeping task. >> >> Could you please elaborate on this? Preemption of a sleeping task and >> faults as in the future (time) preemption and faults? > > The normal way for a task to go to sleep is to call schedule(). objtool > ensures the stack trace is reliable in that case, by making sure that > all functions save the frame pointer on the stack before calling out to > another function. > > But a task can also go to sleep in a few other ways. One way is by > preemption, where an interrupt handler interrupts the task and calls > preempt_schedule_irq(). It's preempted, not sleeping. It's on_rq but not on_cpu. Another way is by a page fault exception. In > both cases, there's no guarantee that the interrupted function saved the > frame pointer on the stack beforehand. So the stack trace might be > unreliable. Fortunately, interrupts and exceptions leave evidence > behind on the stack. So when walking the stack of a sleeping task, we > can detect when an IRQ or exception occurred, and consider such a stack > unreliable. > Thanks for the explanation. I presume a whole lot of this is arch specific code? I'll look at the patches as well Balbir