From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757899AbZAVJhn (ORCPT ); Thu, 22 Jan 2009 04:37:43 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755551AbZAVJhQ (ORCPT ); Thu, 22 Jan 2009 04:37:16 -0500 Received: from mx2.mail.elte.hu ([157.181.151.9]:49396 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755247AbZAVJhO (ORCPT ); Thu, 22 Jan 2009 04:37:14 -0500 Date: Thu, 22 Jan 2009 10:36:49 +0100 From: Ingo Molnar To: Frederic Weisbecker Cc: Lai Jiangshan , Oleg Nesterov , Andrew Morton , Peter Zijlstra , Eric Dumazet , Linux Kernel Mailing List Subject: Re: [PATCH 2/3] workqueue: not allow recursion run_workqueue Message-ID: <20090122093649.GD24758@elte.hu> References: <497838F0.7020408@cn.fujitsu.com> <20090122093046.GC5891@nowhere> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090122093046.GC5891@nowhere> User-Agent: Mutt/1.5.18 (2008-05-17) X-ELTE-VirusStatus: clean X-ELTE-SpamScore: -1.5 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-1.5 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.3 -1.5 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Frederic Weisbecker wrote: > On Thu, Jan 22, 2009 at 05:14:24PM +0800, Lai Jiangshan wrote: > > 1) lockdep will complain when recursion run_workqueue > > 2) works is not run orderly when recursion run_workqueue > > > > 3) BUG! > > We use recursion run_workqueue to hidden deadlock when > > keventd trying to flush its own queue. > > > > It's bug. When flush_workqueue()(nested in a work callback)returns, > > the workqueue is not really flushed, the sequence statement of > > this work callback will do some thing bad. > > > > So we should not allow workqueue trying to flush its own queue. > > > > Signed-off-by: Lai Jiangshan > > --- > > diff --git a/kernel/workqueue.c b/kernel/workqueue.c > > index 2f44583..1129cde 100644 > > --- a/kernel/workqueue.c > > +++ b/kernel/workqueue.c > > @@ -48,8 +48,6 @@ struct cpu_workqueue_struct { > > > > struct workqueue_struct *wq; > > struct task_struct *thread; > > - > > - int run_depth; /* Detect run_workqueue() recursion depth */ > > } ____cacheline_aligned; > > > > /* > > @@ -262,13 +260,6 @@ EXPORT_SYMBOL_GPL(queue_delayed_work_on); > > static void run_workqueue(struct cpu_workqueue_struct *cwq) > > { > > spin_lock_irq(&cwq->lock); > > - cwq->run_depth++; > > - if (cwq->run_depth > 3) { > > - /* morton gets to eat his hat */ > > - printk("%s: recursion depth exceeded: %d\n", > > - __func__, cwq->run_depth); > > - dump_stack(); > > - } > > while (!list_empty(&cwq->worklist)) { > > struct work_struct *work = list_entry(cwq->worklist.next, > > struct work_struct, entry); > > @@ -311,7 +302,6 @@ static void run_workqueue(struct cpu_workqueue_struct *cwq) > > spin_lock_irq(&cwq->lock); > > cwq->current_work = NULL; > > } > > - cwq->run_depth--; > > spin_unlock_irq(&cwq->lock); > > } > > > > @@ -368,29 +358,20 @@ static void insert_wq_barrier(struct cpu_workqueue_struct *cwq, > > > > static int flush_cpu_workqueue(struct cpu_workqueue_struct *cwq) > > { > > - int active; > > + int active = 0; > > + struct wq_barrier barr; > > > > - if (cwq->thread == current) { > > - /* > > - * Probably keventd trying to flush its own queue. So simply run > > - * it by hand rather than deadlocking. > > - */ > > - run_workqueue(cwq); > > - active = 1; > > - } else { > > - struct wq_barrier barr; > > + BUG_ON(cwq->thread == current); > > Hi Lai, > > BUG_ON seems perhaps a bit too much for such case. The system > will run in an endless loop because of a mistake that will not have > necessarily a fatal end. > WARN_ON should be enough (plus the warn that lockdep will raise > too in this case). WARN_ONCE() is the best method usually - we want a one-time and expressive warning, not just a stack dump. (i.e. not WARN_ON and not WARN_ON_ONCE) Plus some thinking needs to be put into exiting from that function in a way that the system will still be usable enough to report the bug. Ingo