From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from lgeamrelo11.lge.com ([156.147.23.51]:50340 "EHLO lgeamrelo11.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726469AbeHVGIl (ORCPT ); Wed, 22 Aug 2018 02:08:41 -0400 Date: Wed, 22 Aug 2018 11:45:35 +0900 From: Byungchul Park To: Johannes Berg Cc: Tejun Heo , Lai Jiangshan , linux-kernel@vger.kernel.org, linux-wireless@vger.kernel.org, kernel-team@lge.com Subject: Re: [PATCH 1/2] workqueue: skip lockdep wq dependency in cancel_work_sync() Message-ID: <20180822024535.GA2414@X58A-UD3R> (sfid-20180822_044619_145516_B0253157) References: <20180821120317.4115-1-johannes@sipsolutions.net> <20180821120317.4115-2-johannes@sipsolutions.net> <20180821160814.GP3978217@devbig004.ftw2.facebook.com> <1534871894.25523.34.camel@sipsolutions.net> <20180821172711.GR3978217@devbig004.ftw2.facebook.com> <1534872621.25523.39.camel@sipsolutions.net> <20180821175550.GS3978217@devbig004.ftw2.facebook.com> <1534879241.25523.44.camel@sipsolutions.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <1534879241.25523.44.camel@sipsolutions.net> Sender: linux-wireless-owner@vger.kernel.org List-ID: On Tue, Aug 21, 2018 at 09:20:41PM +0200, Johannes Berg wrote: > On Tue, 2018-08-21 at 10:55 -0700, Tejun Heo wrote: > > > > I'm not really sure what you think we might be missing? Am I missing > > > some case where cancel_work_sync() can possibly deadlock? Apart from the > > > issue I addressed in the second patch, obviously. > > > > Ah, that was me being slow. I thought you were skipping the work's > > lockdep_map. I can almost swear we had that before (the part you're > > adding on the second patch). Right, fd1a5b04dfb8 ("workqueue: Remove > > now redundant lock acquisitions wrt. workqueue flushes") removed it > > because it gets propagated through wait_for_completion(). Did we miss > > some cases with that change? > > Hmm. > > It doesn't seem to be working. > > No, ok, actually it probably *does*, but the point is similar to my > issue # 3 before - we don't do any of this unless the work is actually > running, but we really want the lockdep annotation *regardless* of that, > so that we catch the error unconditionally. > > So perhaps that commit just needs to be reverted entirely - I'd only > looked at a small subset of it, but the flush_workqueue() case has the > same problem - we only get to the completion when there's something to > flush, not when the workqueue happens to actually be empty. But again, > for lockdep we want to catch *potential* problems, not only *actual* > ones. > > The remaining part of the patch I'm not sure I fully understand (removal > of lockdep_init_map_crosslock()), but I suppose if we revert the other > bits we need to revert this as well. > > So please drop this patch, but revert Byungchul Park's commit > fd1a5b04dfb8 again, I don't think the lockdep annotations there are > really redundant as I just explained. That should've been adjusted as well when Ingo reverted Cross-release. It would be much easier to add each pair, acquire/release, before wait_for_completion() in both flush_workqueue() and flush_work() than reverting the whole commit. What's lacking is only lockdep annotations for wait_for_completion(). Byungchul > > johannes