linux-kernel - Re: check && lockdep_no_validate (Was: lockdep: Introduce wait-type checks)

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Date:	Sun, 12 Jan 2014 21:58:14 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: check && lockdep_no_validate (Was: lockdep: Introduce wait-type
 checks)

On Thu, Jan 09, 2014 at 06:54:48PM +0100, Oleg Nesterov wrote:
> But this connects to lockdep_no_validate. Not sure I understand what
> this class should actually do, but consider this code:
> 
> 	DEFINE_MUTEX(m1);
> 	DEFINE_MUTEX(m2);
> 	DEFINE_MUTEX(mx);
> 
> 	void lockdep_should_complain(void)
> 	{
> 		lockdep_set_novalidate_class(&mx);
> 
> 		// m1 -> mx -> m2
> 		mutex_lock(&m1);
> 		mutex_lock(&mx);
> 		mutex_lock(&m2);
> 		mutex_unlock(&m2);
> 		mutex_unlock(&mx);
> 		mutex_unlock(&m1);
> 
> 
> 		// m2 -> m1 ; should trigger the warning
> 		mutex_lock(&m2);
> 		mutex_lock(&m1);
> 		mutex_unlock(&m1);
> 		mutex_unlock(&m2);
> 	}
> 
> lockdep doesn't not detect the trivial possible deadlock.
> 
> The patch below seems to work but most probably it is not right, and
> I forgot everything (not too much) I knew about lockdep internals.
> 
> Oleg.
> 
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -1939,7 +1939,8 @@ check_prevs_add(struct task_struct *curr, struct held_lock *next)
>  		 * Only non-recursive-read entries get new dependencies
>  		 * added:
>  		 */
> -		if (hlock->read != 2) {
> +		if (hlock->read != 2 &&
> +		    hlock->instance->key != &__lockdep_no_validate__) {
>  			if (!check_prev_add(curr, hlock, next,
>  						distance, trylock_loop))
>  				return 0;
> 

Hmm, you are quite right indeed; although I would write it like:

  if (hlock->read != 2 && hlock->check == 2)

because the __lockdep_no_validate__ thing forces the ->check value to 1.
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