linux-kernel - Re: [PATCH 2/3] workqueue: not allow recursion run

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Date:	Thu, 22 Jan 2009 18:23:12 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Lai Jiangshan <laijs@...fujitsu.com>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Eric Dumazet <dada1@...mosbay.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] workqueue: not allow recursion run_workqueue

On 01/22, Frederic Weisbecker wrote:
>
> On Thu, Jan 22, 2009 at 05:14:24PM +0800, Lai Jiangshan wrote:
> >  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.

Confused. Why do you think the system will run in an endless loop?
cwq-thread will exit.

> WARN_ON should be enough (plus the warn that lockdep will raise
> too in this case).

and if cwq-thread proceeds after WARN_ON() it will be "lost" anyway
because it will sleep forever.

Not that I think BUG_ON() is much better, except it is more "loud".


As for the patch itself, I completely agree with Peter.

Oleg.

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