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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT 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 D1046C4321D for ; Wed, 22 Aug 2018 07:50:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 989FE21486 for ; Wed, 22 Aug 2018 07:50:34 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 989FE21486 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=lge.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728353AbeHVLOP (ORCPT ); Wed, 22 Aug 2018 07:14:15 -0400 Received: from lgeamrelo11.lge.com ([156.147.23.51]:35317 "EHLO lgeamrelo11.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728116AbeHVLOP (ORCPT ); Wed, 22 Aug 2018 07:14:15 -0400 Received: from unknown (HELO lgeamrelo01.lge.com) (156.147.1.125) by 156.147.23.51 with ESMTP; 22 Aug 2018 16:50:30 +0900 X-Original-SENDERIP: 156.147.1.125 X-Original-MAILFROM: byungchul.park@lge.com Received: from unknown (HELO X58A-UD3R) (10.177.222.33) by 156.147.1.125 with ESMTP; 22 Aug 2018 16:50:30 +0900 X-Original-SENDERIP: 10.177.222.33 X-Original-MAILFROM: byungchul.park@lge.com Date: Wed, 22 Aug 2018 16:50:10 +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: <20180822075010.GA29722@X58A-UD3R> References: <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> <20180822024535.GA2414@X58A-UD3R> <1534910543.25523.50.camel@sipsolutions.net> <20180822054731.GB2414@X58A-UD3R> <1534921643.25523.56.camel@sipsolutions.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1534921643.25523.56.camel@sipsolutions.net> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Aug 22, 2018 at 09:07:23AM +0200, Johannes Berg wrote: > On Wed, 2018-08-22 at 14:47 +0900, Byungchul Park wrote: > > On Wed, Aug 22, 2018 at 06:02:23AM +0200, Johannes Berg wrote: > > > On Wed, 2018-08-22 at 11:45 +0900, Byungchul Park wrote: > > > > > > > That should've been adjusted as well when Ingo reverted Cross-release. > > > > > > I can't really say. > > > > What do you mean? > > I haven't followed any of this, so I just don't know. > > > > > 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. > > > > > > The commit doesn't do much more than this though. > > > > That also has named of lockdep_map for wq/work in a better way. > > What do you mean? Ah.. Not important thing. I just mentioned I changed lock names a bit when initializing lockdep_map instances which was suggested by Ingo. But no problem even if you revert the whole thing. I just informed it. ;) > > > > What's lacking is only lockdep annotations for wait_for_completion(). > > > > > > No, I disagree. Like I said before, we need the lockdep annotations on > > > > You seem to be confused. I was talking about wait_for_completion() in > > both flush_workqueue() and flush_work(). Without > > the wait_for_completion()s, nothing matters wrt what you are concerning. > > Yes and no. > > You're basically saying if we don't get to do a wait_for_completion(), > then we don't need any lockdep annotation. I'm saying this isn't true. Strictly no. But I'm just talking about the case in wq flush code. > Consider the following case: > > work_function() > { > mutex_lock(&mutex); > mutex_unlock(&mutex); > } > > other_function() > { > queue_work(&my_wq, &work); > > if (common_case) { > schedule_and_wait_for_something_that_takes_a_long_time() > } > > mutex_lock(&mutex); > flush_workqueue(&my_wq); > mutex_unlock(&mutex); > } > > > Clearly this code is broken, right? > > However, you'll almost never get lockdep to indicate that, because of > the "if (common_case)". Sorry I don't catch you. Why is that problem with the example? Please a deadlock example. > My argument basically is that the lockdep annotations in the workqueue > code should be entirely independent of the actual need to call > wait_for_completion(). No. Lockdep annotations always do with either wait_for_something or self event loop within a single context e.g. fs -> memory reclaim -> fs -> .. > Therefore, the commit should be reverted regardless of any cross-release No. That is necessary only when the wait_for_completion() cannot be tracked in checking dependencies automatically by cross-release. It might be the key to understand you, could you explain it more why you think lockdep annotations are independent of the actual need to call wait_for_completion()(or wait_for_something_else) hopefully with a deadlock example? > work (that I neither know and thus don't understand right now), since it > makes workqueue code rely on lockdep for the completion, whereas we Using wait_for_completion(), right? > really want to have annotations here even when we didn't actually need > to wait_for_completion(). Please an example of deadlock even w/o wait_for_completion(). > > johannes Byungchul