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=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 BACFCC4321D for ; Tue, 21 Aug 2018 17:30:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 839B521837 for ; Tue, 21 Aug 2018 17:30:30 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 839B521837 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=sipsolutions.net 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 S1727307AbeHUUvc (ORCPT ); Tue, 21 Aug 2018 16:51:32 -0400 Received: from s3.sipsolutions.net ([144.76.43.62]:35674 "EHLO sipsolutions.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726626AbeHUUvb (ORCPT ); Tue, 21 Aug 2018 16:51:31 -0400 Received: by sipsolutions.net with esmtpsa (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.91) (envelope-from ) id 1fsAU5-0007Sl-FJ; Tue, 21 Aug 2018 19:30:25 +0200 Message-ID: <1534872621.25523.39.camel@sipsolutions.net> Subject: Re: [PATCH 1/2] workqueue: skip lockdep wq dependency in cancel_work_sync() From: Johannes Berg To: Tejun Heo Cc: Lai Jiangshan , linux-kernel@vger.kernel.org, linux-wireless@vger.kernel.org Date: Tue, 21 Aug 2018 19:30:21 +0200 In-Reply-To: <20180821172711.GR3978217@devbig004.ftw2.facebook.com> (sfid-20180821_192716_762024_DE7C68A2) 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> (sfid-20180821_192716_762024_DE7C68A2) Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.26.6 (3.26.6-1.fc27) Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, > On Tue, Aug 21, 2018 at 07:18:14PM +0200, Johannes Berg wrote: > > > But this can lead to a deadlock. I'd much rather err on the side of > > > discouraging complex lock dancing around ordered workqueues, no? > > > > What can lead to a deadlock? > > Oh not this particular case, but I was wondering whether we'd be > missing legitimate possible deadlock cases by skipping lockdep for all > cancel_work_sync()'s as they can actually flush. I don't see how? This is only relevant in ordered/single-threaded WQs, but even there it doesn't matter doesn't matter as explained? I'm actually seeing a false positive report from lockdep, because it *is* flushing, i.e. I'm running into the case of the work actually running, i.e. the "_sync" part of "cancel_work_sync()" is kicking in, but in that case a single-threaded WQ can't have anything executing *before* it, so we don't need to generate a lockdep dependency - and in fact don't *want* to create one to avoid the false positive. 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. johannes