[PATCH v2 10/61] kernel/fork: Use maple tree for dup_mmap() during forking

[Hillf Danton <hdanton@xxxxxxxx>]

On Tue, 17 Aug 2021 15:47:11 +0000 Liam R. Howlett wrote:
> 
>  static inline void mmget(struct mm_struct *mm)
>  {
> +	mt_set_in_rcu(&mm->mm_mt);
>  	atomic_inc(&mm->mm_users);
>  }
> 
>  static inline bool mmget_not_zero(struct mm_struct *mm)
>  {
> +	/*
> +	 * There is a race below during task tear down that can cause the maple
> +	 * tree to enter rcu mode with only a single user.  If this race
> +	 * happens, the result would be that the maple tree nodes would remain
> +	 * active for an extra RCU read cycle.
> +	 */
> +	mt_set_in_rcu(&mm->mm_mt);
>  	return atomic_inc_not_zero(&mm->mm_users);
>  }

Nit, leave the mm with zero refcount intact.

 	if (atomic_inc_not_zero(&mm->mm_users)) {
		mt_set_in_rcu(&mm->mm_mt);
		return true;
	}
	return false;