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

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Date:	Tue, 10 Feb 2009 12:53:44 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	peterz@...radead.org, mingo@...e.hu, fweisbec@...il.com,
	oleg@...sign.ru, dada1@...mosbay.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] workqueue: not allow recursion run_workqueue

On Fri, 06 Feb 2009 09:46:29 +0800
Lai Jiangshan <laijs@...fujitsu.com> wrote:

> Hi, Ingo
> 
> This is new changelog, I didn't change the patch,
> except use WARN_ON instead BUG_ON.
> 
> Thanks, Lai
> 
> From: Lai Jiangshan <laijs@...fujitsu.com>
> 
> 1) lockdep will complain when recursion run_workqueue()
> 2) The recursive implement of run_workqueue() makes flush_workqueue()
>    and it's doc are inconsistent. It may hide deadlock and other bugs.
> 3) recursion run_workqueue() will poison cwq->current_work,
>    but flush_work() and __cancel_work_timer() ...etc. need
>    reliable cwq->current_work.
> 
> Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
> ---
> 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();
> -	}

I never did get to eat my hat.


Do we know of any cases where there was ever any recursion here?  Even
single-level recursion?

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