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=unavailable 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 869DDC10F11 for ; Wed, 24 Apr 2019 19:51:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 601F420674 for ; Wed, 24 Apr 2019 19:51:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732030AbfDXTvt (ORCPT ); Wed, 24 Apr 2019 15:51:49 -0400 Received: from Galois.linutronix.de ([146.0.238.70]:55610 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725970AbfDXTvs (ORCPT ); Wed, 24 Apr 2019 15:51:48 -0400 Received: from p5de0b374.dip0.t-ipconnect.de ([93.224.179.116] helo=nanos) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1hJNv9-0005Zr-Gt; Wed, 24 Apr 2019 21:51:07 +0200 Date: Wed, 24 Apr 2019 21:51:05 +0200 (CEST) From: Thomas Gleixner To: Peter Zijlstra cc: LKML , Josh Poimboeuf , x86@kernel.org, Andy Lutomirski , Steven Rostedt , Alexander Potapenko , Alexey Dobriyan , Andrew Morton , Pekka Enberg , linux-mm@kvack.org, David Rientjes , Christoph Lameter , Catalin Marinas , Dmitry Vyukov , Andrey Ryabinin , kasan-dev@googlegroups.com, Mike Rapoport , Akinobu Mita , iommu@lists.linux-foundation.org, Robin Murphy , Christoph Hellwig , Marek Szyprowski , Johannes Thumshirn , David Sterba , Chris Mason , Josef Bacik , linux-btrfs@vger.kernel.org, dm-devel@redhat.com, Mike Snitzer , Alasdair Kergon , intel-gfx@lists.freedesktop.org, Joonas Lahtinen , Maarten Lankhorst , dri-devel@lists.freedesktop.org, David Airlie , Jani Nikula , Daniel Vetter , Rodrigo Vivi , linux-arch@vger.kernel.org Subject: Re: [patch V2 18/29] lockdep: Move stack trace logic into check_prev_add() In-Reply-To: <20190424194505.GR11158@hirez.programming.kicks-ass.net> Message-ID: References: <20190418084119.056416939@linutronix.de> <20190418084254.729689921@linutronix.de> <20190424194505.GR11158@hirez.programming.kicks-ass.net> User-Agent: Alpine 2.21 (DEB 202 2017-01-01) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 24 Apr 2019, Peter Zijlstra wrote: > On Thu, Apr 18, 2019 at 10:41:37AM +0200, Thomas Gleixner wrote: > > There is only one caller of check_prev_add() which hands in a zeroed struct > > stack trace and a function pointer to save_stack(). Inside check_prev_add() > > the stack_trace struct is checked for being empty, which is always > > true. Based on that one code path stores a stack trace which is unused. The > > comment there does not make sense either. It's all leftovers from > > historical lockdep code (cross release). > > I was more or less expecting a revert of: > > ce07a9415f26 ("locking/lockdep: Make check_prev_add() able to handle external stack_trace") > > And then I read the comment that went with the "static struct > stack_trace trace" that got removed (in the above commit) and realized > that your patch will consume more stack entries. > > The problem is when the held lock stack in check_prevs_add() has multple > trylock entries on top, in that case we call check_prev_add() multiple > times, and this patch will then save the exact same stack-trace multiple > times, consuming static resources. > > Possibly we should copy what stackdepot does (but we cannot use it > directly because stackdepot uses locks; but possible we can share bits), > but that is a patch for another day I think. > > So while convoluted, perhaps we should retain this code for now. Uurg, what a mess.