From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754716AbZLRPlh (ORCPT ); Fri, 18 Dec 2009 10:41:37 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754488AbZLRPlg (ORCPT ); Fri, 18 Dec 2009 10:41:36 -0500 Received: from bombadil.infradead.org ([18.85.46.34]:56299 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754069AbZLRPle (ORCPT ); Fri, 18 Dec 2009 10:41:34 -0500 Subject: Re: workqueue thing From: Peter Zijlstra To: Linus Torvalds Cc: Tejun Heo , awalls@radix.net, linux-kernel@vger.kernel.org, jeff@garzik.org, mingo@elte.hu, akpm@linux-foundation.org, jens.axboe@oracle.com, rusty@rustcorp.com.au, cl@linux-foundation.org, dhowells@redhat.com, arjan@linux.intel.com, avi@redhat.com, johannes@sipsolutions.net, andi@firstfloor.org In-Reply-To: References: <1261141088-2014-1-git-send-email-tj@kernel.org> <1261143924.20899.169.camel@laptop> Content-Type: text/plain; charset="UTF-8" Date: Fri, 18 Dec 2009 16:39:48 +0100 Message-ID: <1261150788.20899.299.camel@laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.28.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 2009-12-18 at 07:30 -0800, Linus Torvalds wrote: > > On Fri, 18 Dec 2009, Peter Zijlstra wrote: > > > > r1. The first design goal of cmwq is solving the issues the current > > > workqueue implementation has including hard to detect > > > deadlocks, > > > > lockdep is quite proficient at finding these these days. > > I don't think so. > > The reason it is not is that workqueues fundamentally do _different_ > things in the same context, adn lockdep has no clue what-so-ever. > > IOW, if you hold a lock, and then do 'flush_workqueue()', lockdep has no > idea that maybe one of the entries on a workqueue might need the lock that > you are holding. But I don't think lockdep sees the dependency that gets > created by the flush - because it's not a direct code execution > dependency. > > It's not a deadlock _directly_ due to lock ordering, but indirectly due to > waiting for unrelated code that needs locks. > > Now, maybe lockdep could be _taught_ to consider workqueues themselves to > be 'locks', and ordering those pseudo-locks wrt the real locks they take. > So if workqueue Q takes lock A, the fact that it is _taken_ in a workqueue > makes the ordering be Q->A. Then, if somebody does a "flush_workqueue" > while holding lock B, the flush implies a "lock ordering" of B->Q (where > "Q" is the set of all workqueues that could be flushed). That's exactly what it does.. 4e6045f134784f4b158b3c0f7a282b04bd816887 eb13ba873881abd5e15af784756a61af635e665e a67da70dc0955580665f5444f318b92e69a3c272