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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham 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 1DA25C43387 for ; Sun, 13 Jan 2019 13:05:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id DFD5E20836 for ; Sun, 13 Jan 2019 13:05:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726564AbfAMNFu (ORCPT ); Sun, 13 Jan 2019 08:05:50 -0500 Received: from verein.lst.de ([213.95.11.211]:39960 "EHLO newverein.lst.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726221AbfAMNFu (ORCPT ); Sun, 13 Jan 2019 08:05:50 -0500 Received: by newverein.lst.de (Postfix, from userid 107) id 29BC468DD2; Sun, 13 Jan 2019 14:05:48 +0100 (CET) Received: from blackhole (p5B0D8EE3.dip0.t-ipconnect.de [91.13.142.227]) by newverein.lst.de (Postfix) with ESMTPSA id A9EE067329; Sun, 13 Jan 2019 14:05:44 +0100 (CET) Date: Sun, 13 Jan 2019 14:05:38 +0100 From: Torsten Duwe To: Balbir Singh Cc: Segher Boessenkool , Joe Lawrence , Jiri Kosina , linux-kernel@vger.kernel.org, live-patching@vger.kernel.org, linuxppc-dev@lists.ozlabs.org Subject: Re: ppc64le reliable stack unwinder and scheduled tasks Message-ID: <20190113140512.44eac0e0@blackhole> In-Reply-To: <20190113123356.GA26056@350D> References: <7f468285-b149-37e2-e782-c9e538b997a9@redhat.com> <20190112010914.GB10427@350D> <20190112084541.GK14180@gate.crashing.org> <20190113123356.GA26056@350D> Organization: LST e.V. X-Mailer: Claws Mail 3.16.0 (GTK+ 2.24.32; x86_64-suse-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, 13 Jan 2019 23:33:56 +1100 Balbir Singh wrote: > On Sat, Jan 12, 2019 at 02:45:41AM -0600, Segher Boessenkool wrote: > > On Sat, Jan 12, 2019 at 12:09:14PM +1100, Balbir Singh wrote: > > > Could you please define interesting frame on top a bit more? > > > Usually the topmost return address is in LR > > > > There is no reliable way (other than DWARF unwind info) to find out > > where the value of LR at function entry currently lives (if > > anywhere). It may or may not be still available in LR, it may or > > may not be saved to the return stack slot. It can also live in > > some GPR, or in some other stack slot. > > > > (The same is true for all other registers). > > > > The only thing the ABI guarantees you is that you can find all > > stack frames via the back chain. If you want more you can use some > > heuristics and do some heroics (like GDB does), but this is not > > fully reliable. Using DWARF unwind info is, but that requires big > > tables. > > > > Thanks, so are you suggesting that a reliable stack is not possible on > ppc64le? Even with the restricted scope of the kernel? The LR value location is _always_ hard to determine for the topmost frame. This is not a problem for voluntarily sleeping tasks, because the topmost function will always be well known. It is a problem for tasks preempted by an interrupt, or those handling an exception, that's why these need to report "unreliable". Note that this is a very general problem, across _all_ RISC-like architectures. It should thus be handled as generically as possible. Torsten