From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg0-x244.google.com (mail-pg0-x244.google.com [IPv6:2607:f8b0:400e:c05::244]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 3tbqCG6Nm3zDvk4 for ; Sun, 11 Dec 2016 13:08:58 +1100 (AEDT) Received: by mail-pg0-x244.google.com with SMTP id 3so6667870pgd.0 for ; Sat, 10 Dec 2016 18:08:58 -0800 (PST) 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 MIME-Version: 1.0 In-Reply-To: <20161210171707.cpupmxyuhob4tc3i@treble> Content-Type: text/plain; charset=utf-8 List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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