Re: 2.6.24-rc6: possible recursive locking detected

[Oleg Nesterov <oleg@tv-sign.ru>]

On 01/07, Oleg Nesterov wrote:
>
> Consider this "just for illustration" patch,
> 
> --- t/kernel/lockdep.c	2007-11-09 12:57:31.000000000 +0300
> +++ t/kernel/lockdep.c	2008-01-07 19:43:50.000000000 +0300
> @@ -1266,10 +1266,13 @@ check_deadlock(struct task_struct *curr,
>  	struct held_lock *prev;
>  	int i;
>  
> -	for (i = 0; i < curr->lockdep_depth; i++) {
> +	for (i = curr->lockdep_depth; --i >= 0; ) {
>  		prev = curr->held_locks + i;
>  		if (prev->class != next->class)
>  			continue;
> +
> +		if (prev->trylock == -1)
> +			return 2;
>  		/*
>  		 * Allow read-after-read recursion of the same
>  		 * lock class (i.e. read_lock(lock)+read_lock(lock)):
> -------------------------------------------------------------------------
> 
> Now,
> 
> 	// trylock == -1
> 	#define	spin_mark_nested(l)	\
> 		lock_acquire(&(l)->dep_map, 0, -1, 0, 2, _THIS_IP_)
> 	#define	spin_unmark_nested(l)	\
> 		lock_release(&(l)->dep_map, 1, _THIS_IP_)
> 
> and ep_poll_safewake() can do:
> 
> 	/* Do really wake up now */
> 	spin_mark_nested(&wq->lock);
> 	wake_up(wq);
> 	spin_unmark_nested(&wq->lock);

I tested the patch above with the following code,

	wait_queue_head_t w1, w2, w3;

	init_waitqueue_head(&w1);
	init_waitqueue_head(&w2);
	init_waitqueue_head(&w3);

	local_irq_disable();
	spin_lock(&w1.lock);

	spin_mark_nested(&w2.lock);
	spin_lock(&w2.lock);

	spin_mark_nested(&w3.lock);
	wake_up(&w3);
	spin_unmark_nested(&w3.lock);

	spin_unlock(&w2.lock);
	spin_unmark_nested(&w2.lock);

	spin_unlock(&w1.lock);
	local_irq_enable();

seems to work. What do you think?

Oleg.