Re: [PATCH] lockdep: Check if nested lock is actually held

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Thu, 2012-09-13 at 11:39 +0200, Maarten Lankhorst wrote:
> It is considered good form to lock the lock you claim to be nested in.

Uhm yeah.. cute. You actually found a site where this triggered?

> Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
> ---
> diff --git a/kernel/lockdep.c b/kernel/lockdep.c
> index ea9ee45..7175447 100644
> --- a/kernel/lockdep.c
> +++ b/kernel/lockdep.c
> @@ -2998,6 +2998,43 @@ EXPORT_SYMBOL_GPL(lockdep_init_map);
>  
>  struct lock_class_key __lockdep_no_validate__;
>  
> +static int
> +print_lock_nested_lock_not_held(struct task_struct *curr,
> +				struct held_lock *lock,
> +				struct lockdep_map *nest,
> +				unsigned long ip)
> +{
> +	if (!debug_locks_off())
> +		return 0;
> +	if (debug_locks_silent)
> +		return 0;
> +
> +	printk("\n");
> +	printk("==================================\n");
> +	printk("[ BUG: Nested lock was not taken ]\n");
> +	print_kernel_ident();
> +	printk("----------------------------------\n");
> +
> +	printk("%s/%d is trying to lock:\n", curr->comm, task_pid_nr(curr));
> +	print_lock(lock);
> +
> +	printk("\nbut this task is not holding:\n");
> +	printk("%s\n", nest->name);
> +
> +	printk("\nstack backtrace:\n");
> +	dump_stack();
> +
> +	printk("\nother info that might help us debug this:\n");
> +	lockdep_print_held_locks(curr);
> +
> +	printk("\nstack backtrace:\n");
> +	dump_stack();
> +
> +	return 0;
> +}
> +
> +static int __lock_is_held(struct lockdep_map *lock);
> +
>  /*
>   * This gets called for every mutex_lock*()/spin_lock*() operation.
>   * We maintain the dependency maps and validate the locking attempt:
> @@ -3139,6 +3176,10 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
>  	}
>  	chain_key = iterate_chain_key(chain_key, id);
>  
> +	if (nest_lock && !__lock_is_held(nest_lock))
> +		return print_lock_nested_lock_not_held(curr, hlock,
> +						       nest_lock, ip);

At this time we've already set hlock->nest_lock, so I've shortened the
argument list here a little.

>  	if (!validate_chain(curr, lock, hlock, chain_head, chain_key))
>  		return 0;
>  
>