Re: [PATCH] mm/page_reporting: replace rcu_access_pointer() with rcu_dereference_protected()

[Matthew Wilcox]

On Tue, Dec 27, 2022 at 07:21:58PM +0000, SeongJae Park wrote:
> +++ b/mm/page_reporting.c
> @@ -356,7 +356,7 @@ int page_reporting_register(struct page_reporting_dev_info *prdev)
>  	mutex_lock(&page_reporting_mutex);
>  
>  	/* nothing to do if already in use */
> -	if (rcu_access_pointer(pr_dev_info)) {
> +	if (rcu_dereference_protected(pr_dev_info, true)) {

Pretty sure that passing a bare 'true' is an antipattern.
Instead, document _what_ lock protects us, ie:

	if (rcu_dereference_protected(pr_dev_info,
				      lockdep_is_held(&page_reporting_mutex))) {

Obviously, we took it just one line up, but if code moves around, it
may save us.