Re: [PATCH] workqueue: not allow recursion run_workqueue

[Peter Zijlstra <peterz@infradead.org>]

On Wed, 2009-01-21 at 17:42 +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.

The patch looks good, but I'm utterly failing to comprehend this
changelog. What exactly can go wrong (other than the obvious too deep
nest and the fact that lockdep will complain)?

> Signed-off-by: Lai Jiangshan <laijs@cn.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();
> -	}
>  	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;
> +	WARN_ON(cwq->thread == current);
>  
> -		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;
>  }
> 
> 
> --
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