From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755806AbZAVJj5 (ORCPT ); Thu, 22 Jan 2009 04:39:57 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1757821AbZAVJjn (ORCPT ); Thu, 22 Jan 2009 04:39:43 -0500 Received: from fg-out-1718.google.com ([72.14.220.154]:38377 "EHLO fg-out-1718.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757820AbZAVJjm (ORCPT ); Thu, 22 Jan 2009 04:39:42 -0500 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=date:from:to:cc:subject:message-id:references:mime-version :content-type:content-disposition:in-reply-to:user-agent; b=Vd5/XVgIqIuY4tFpTtP8RQJPP9Lw1VXHEGdioFUBrRfw4OXJcbZLbBk9jA/MxpV8aX SWkPuf/9/4Rl6JzdfQENjN/g6pL85O+/AqyUJrN2ijAIEVn/sKkxfJ5a/Wt4Ysj5cHyW gJ403KruaXLKYJrSMm1EnaY+9LHz1Ff/uQgB0= Date: Thu, 22 Jan 2009 10:39:38 +0100 From: Frederic Weisbecker To: Lai Jiangshan Cc: Oleg Nesterov , Ingo Molnar , Andrew Morton , Peter Zijlstra , Eric Dumazet , Linux Kernel Mailing List Subject: Re: [PATCH 2/3] workqueue: not allow recursion run_workqueue Message-ID: <20090122093937.GA6009@nowhere> 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) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jan 22, 2009 at 10:30:46AM +0100, 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). > > Thanks. > Frederic. And perhaps add a comment for the developers who will encounter such a warn, and then fall down in this call site while searching which warned. To easily find the reason of the WARN. cwq->thread == current is perhaps not verbose enough to help the developer finding the source of the problem. They could solve the issue and say "Doh!" more quickly if they find in a one shot sight: /* Never flush a workqueue from a work */ :-) > > > - active = 0; > > - spin_lock_irq(&cwq->lock); > > - if (!list_empty(&cwq->worklist) || cwq->current_work != NULL) { > > - insert_wq_barrier(cwq, &barr, &cwq->worklist); > > - active = 1; > > - } > > - spin_unlock_irq(&cwq->lock); > > - > > - if (active) > > - wait_for_completion(&barr.done); > > + spin_lock_irq(&cwq->lock); > > + if (!list_empty(&cwq->worklist) || cwq->current_work != NULL) { > > + insert_wq_barrier(cwq, &barr, &cwq->worklist); > > + active = 1; > > } > > + spin_unlock_irq(&cwq->lock); > > + > > + if (active) > > + wait_for_completion(&barr.done); > > > > return active; > > } > >