Re: [PATCH 3/3] pagewalk: use lockdep_assert_held for locking validation

[Matthew Wilcox]

On Thu, Aug 08, 2019 at 06:42:40PM +0300, Christoph Hellwig wrote:
> Use lockdep to check for held locks instead of using home grown
> asserts.
> 
> @@ -319,7 +319,7 @@ int walk_page_range(struct mm_struct *mm, unsigned long start,
>  	if (!walk.mm)
>  		return -EINVAL;
>  
> -	VM_BUG_ON_MM(!rwsem_is_locked(&walk.mm->mmap_sem), walk.mm);
> +	lockdep_assert_held(&walk.mm->mmap_sem);

It occurs to me that this is exactly the pattern that lockdep_pin_lock()
was designed for.  I'm pretty sure things will go badly if any callee
unlocks then relocks the lock.