Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751536AbeACC6O (ORCPT + 1 other); Tue, 2 Jan 2018 21:58:14 -0500 Received: from ipmail06.adl2.internode.on.net ([150.101.137.129]:7899 "EHLO ipmail06.adl2.internode.on.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751148AbeACC6M (ORCPT ); Tue, 2 Jan 2018 21:58:12 -0500 Date: Wed, 3 Jan 2018 13:58:08 +1100 From: Dave Chinner To: Byungchul Park Cc: Matthew Wilcox , Theodore Ts'o , Byungchul Park , Thomas Gleixner , Peter Zijlstra , Ingo Molnar , Linus Torvalds , Amir Goldstein , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org, oleg@redhat.com, kernel-team@lge.com, daniel@ffwll.ch Subject: Re: About the try to remove cross-release feature entirely by Ingo Message-ID: <20180103025808.GD30682@dastard> References: <20171229014736.GA10341@X58A-UD3R> <20171229035146.GA11757@thunk.org> <20171229072851.GA12235@X58A-UD3R> <20171230061624.GA27959@bombadil.infradead.org> <20171230154041.GB3366@thunk.org> <20171230204417.GF27959@bombadil.infradead.org> <20171230224028.GC3366@thunk.org> <20171230230057.GB12995@thunk.org> <20180101101855.GA23567@bombadil.infradead.org> <06e73e69-9c78-07bc-3d06-a51accb2645b@lge.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <06e73e69-9c78-07bc-3d06-a51accb2645b@lge.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Return-Path: On Wed, Jan 03, 2018 at 11:28:44AM +0900, Byungchul Park wrote: > On 1/1/2018 7:18 PM, Matthew Wilcox wrote: > >On Sat, Dec 30, 2017 at 06:00:57PM -0500, Theodore Ts'o wrote: > >>Also, what to do with TCP connections which are created in userspace > >>(with some authentication exchanges happening in userspace), and then > >>passed into kernel space for use in kernel space, is an interesting > >>question. > > > >Yes! I'd love to have a lockdep expert weigh in here. I believe it's > >legitimate to change a lock's class after it's been used, essentially > >destroying it and reinitialising it. If not, it should be because it's > >a reasonable design for an object to need different lock classes for > >different phases of its existance. > > I also think it should be done ultimately. And I think it's very much > hard since it requires to change the dependency graph of lockdep but > anyway possible. It's up to lockdep maintainer's will though.. We used to do this in XFS to work around the fact that the memory reclaim context "locks" were too stupid to understand that an object referenced and locked above memory allocation could not be accessed below in memory reclaim because memory reclaim only accesses /unreferenced objects/. We played whack-a-mole with lockdep for years to get most of the false positives sorted out. Hence for a long time we had to re-initialise the lock context for the XFS inode iolock in ->evict_inode() so we could lock it for reclaim processing. Eventually we ended up completely reworking the inode reclaim locking in XFS primarily to get rid of all the nasty lockdep hacks we had strewn throughout the code. It was ~2012 we got rid of the last inode re-init code, IIRC. Yeah: commit 4f59af758f9092bc7b266ca919ce6067170e5172 Author: Christoph Hellwig Date: Wed Jul 4 11:13:33 2012 -0400 xfs: remove iolock lock classes Now that we never take the iolock during inode reclaim we don't need to play games with lock classes. Signed-off-by: Christoph Hellwig Reviewed-by: Rich Johnston Signed-off-by: Ben Myers We still have problems with lockdep false positives w.r.t. memory allocation contexts, mainly with code that can be called from both above and below memory allocation contexts. We've finally got __GFP_NOLOCKDEP to be able to annotate memory allocation points within such code paths, but that doesn't help with locks.... Byungchul, lockdep has a long, long history of having sharp edges and being very unfriendly to developers. We've all been scarred by lockdep at one time or another and so there's a fair bit of resistance to repeating past mistakes and allowing lockdep to inflict more scars on us.... Cheers, Dave. -- Dave Chinner david@fromorbit.com