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=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no 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 ED626C4742C for ; Mon, 16 Nov 2020 09:30:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8AA182224F for ; Mon, 16 Nov 2020 09:30:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728375AbgKPI7Z (ORCPT ); Mon, 16 Nov 2020 03:59:25 -0500 Received: from lgeamrelo11.lge.com ([156.147.23.51]:59471 "EHLO lgeamrelo11.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726291AbgKPI7Y (ORCPT ); Mon, 16 Nov 2020 03:59:24 -0500 Received: from unknown (HELO lgeamrelo02.lge.com) (156.147.1.126) by 156.147.23.51 with ESMTP; 16 Nov 2020 17:59:23 +0900 X-Original-SENDERIP: 156.147.1.126 X-Original-MAILFROM: byungchul.park@lge.com Received: from unknown (HELO X58A-UD3R) (10.177.222.33) by 156.147.1.126 with ESMTP; 16 Nov 2020 17:59:23 +0900 X-Original-SENDERIP: 10.177.222.33 X-Original-MAILFROM: byungchul.park@lge.com Date: Mon, 16 Nov 2020 17:57:57 +0900 From: Byungchul Park To: Matthew Wilcox Cc: Steven Rostedt , Thomas Gleixner , Ingo Molnar , torvalds@linux-foundation.org, peterz@infradead.org, mingo@redhat.com, will@kernel.org, linux-kernel@vger.kernel.org, joel@joelfernandes.org, alexander.levin@microsoft.com, daniel.vetter@ffwll.ch, chris@chris-wilson.co.uk, duyuyang@gmail.com, johannes.berg@intel.com, tj@kernel.org, tytso@mit.edu, david@fromorbit.com, amir73il@gmail.com, bfields@fieldses.org, gregkh@linuxfoundation.org, kernel-team@lge.com Subject: Re: [RFC] Are you good with Lockdep? Message-ID: <20201116085757.GB26078@X58A-UD3R> References: <20201111050559.GA24438@X58A-UD3R> <20201111105441.GA78848@gmail.com> <20201111093609.1bd2b637@gandalf.local.home> <87d00jo55p.fsf@nanos.tec.linutronix.de> <20201112081030.GB14554@X58A-UD3R> <20201112092612.00a19239@gandalf.local.home> <20201112145251.GB17076@casper.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201112145251.GB17076@casper.infradead.org> User-Agent: Mutt/1.5.21 (2010-09-15) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Nov 12, 2020 at 02:52:51PM +0000, Matthew Wilcox wrote: > On Thu, Nov 12, 2020 at 09:26:12AM -0500, Steven Rostedt wrote: > > > FYI, roughly Lockdep is doing: > > > > > > 1. Dependency check > > > 2. Lock usage correctness check (including RCU) > > > 3. IRQ related usage correctness check with IRQFLAGS > > > > > > 2 and 3 should be there forever which is subtle and have gotten matured. > > > But 1 is not. I've been talking about 1. But again, it's not about > > > replacing it right away but having both for a while. I'm gonna try my > > > best to make it better. > > > > And I believe lockdep does handle 1. Perhaps show some tangible use case > > that you want to cover that you do not believe that lockdep can handle. If > > lockdep cannot handle it, it will show us where lockdep is lacking. If it > > can handle it, it will educate you on other ways that lockdep can be > > helpful in your development ;-) > > Something I believe lockdep is missing is a way to annotate "This lock > will be released by a softirq". If we had lockdep for lock_page(), this > would be a great case to show off. The filesystem locks the page, then > submits it to a device driver. On completion, the filesystem's bio > completion handler will be called in softirq context and unlock the page. > > So if the filesystem has another lock which is acquired by the completion > handler. we could get an ABBA deadlock that lockdep would be unable to see. > > There are other similar things; if you look at the remaining semaphore > users in the kernel, you'll see the general pattern is that they're > acquired in process context and then released in interrupt context. > If we had a way to transfer ownership of the semaphore to a generic > "interrupt context", they could become mutexes and lockdep could check > that nothing else will cause a deadlock. Yes. Those are exactly what Cross-release feature solves. Those problems can be achieved with Cross-release. But even with Cross-release, we still cannot solve the problem of (1) readlock handling (2) and false positives preventing further reporting. Thanks, Byungchul