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

[SeongJae Park]

On Tue, 27 Dec 2022 19:55:09 +0000 Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:

> 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.

Good point, agreed.  Will do so in the next version.


Thanks,
SJ