From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757029AbZAVLGv (ORCPT ); Thu, 22 Jan 2009 06:06:51 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753983AbZAVLGm (ORCPT ); Thu, 22 Jan 2009 06:06:42 -0500 Received: from fg-out-1718.google.com ([72.14.220.153]:64329 "EHLO fg-out-1718.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753934AbZAVLGl (ORCPT ); Thu, 22 Jan 2009 06:06:41 -0500 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :cc:content-type:content-transfer-encoding; b=N46mrr+KMLyI5JB05glUkp9M0+M7h1paGMVWC4j+Z7aIr6vDrgZFIFKUYaKc0vfiMi +zIPA8ddzTY8Xn3X7uLbZ6CORsqBkFfYXtE3SsguYE1SHaEt088kbUuCEQS+prxyfUSq ZRKtO/AauW3MVVTOgkp4baxQmIqZkg4AEWfpM= MIME-Version: 1.0 In-Reply-To: <20090122093649.GD24758@elte.hu> References: <497838F0.7020408@cn.fujitsu.com> <20090122093046.GC5891@nowhere> <20090122093649.GD24758@elte.hu> Date: Thu, 22 Jan 2009 12:06:39 +0100 Message-ID: Subject: Re: [PATCH 2/3] workqueue: not allow recursion run_workqueue From: =?ISO-8859-1?Q?Fr=E9d=E9ric_Weisbecker?= To: Ingo Molnar Cc: Lai Jiangshan , Oleg Nesterov , Andrew Morton , Peter Zijlstra , Eric Dumazet , Linux Kernel Mailing List Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 2009/1/22 Ingo Molnar : > > * 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 > Ok. Oh but I haven't seen that Oleg said he prefered bug_on, because the system will deadlock instead....hmm... Or perhaps keeping the things like the old way, but with a WARN_ONCE: if (cwq->thread == current) { /* * Don't ever think to flush workqueue from a work */ WARN_ONCE(1); run_workqueue(cwq); active = 1; } And then, the workqueue will flush...so it will behave correctly but will warn on this bad developer idea of flushing from a work. Actually I don't understand when Lai says that it will actually not flush.