[PATCH v3] mm/page_reporting: replace rcu_access_pointer() with rcu_dereference_protected()

[SeongJae Park]

Page reporting fetches pr_dev_info using rcu_access_pointer(), which is
for safely fetching a pointer that will not be dereferenced but could
concurrently updated.  The code indeed does not dereference pr_dev_info
after fetching it using rcu_access_pointer(), but it fetches the pointer
while concurrent updates to the pointer is avoided by holding the update
side lock, page_reporting_mutex.

In the case, rcu_dereference_protected() should be used instead because
it provides better readability and performance on some cases, as
rcu_dereference_protected() avoids use of READ_ONCE().  Replace the
rcu_access_pointer() calls with rcu_dereference_protected().

Fixes: 36e66c554b5c ("mm: introduce Reported pages")
Signed-off-by: SeongJae Park <sj@xxxxxxxxxx>
---
Changes from v2
(https://lore.kernel.org/linux-mm/20221228012902.1510901-1-sj@xxxxxxxxxx/)
- Fix typos (Miaohe Lin)
- Add Fixes: tag

Changes from v1
(https://lore.kernel.org/linux-mm/20221227192158.2553-1-sj@xxxxxxxxxx/)
- Explicitly set the protection condition (Matthew Wilcox)

 mm/page_reporting.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/page_reporting.c b/mm/page_reporting.c
index 79a8554f024c..c65813a9dc78 100644
--- a/mm/page_reporting.c
+++ b/mm/page_reporting.c
@@ -356,7 +356,8 @@ 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,
+				lockdep_is_held(&page_reporting_mutex))) {
 		err = -EBUSY;
 		goto err_out;
 	}
@@ -401,7 +402,8 @@ void page_reporting_unregister(struct page_reporting_dev_info *prdev)
 {
 	mutex_lock(&page_reporting_mutex);
 
-	if (rcu_access_pointer(pr_dev_info) == prdev) {
+	if (prdev == rcu_dereference_protected(pr_dev_info,
+				lockdep_is_held(&page_reporting_mutex))) {
 		/* Disable page reporting notification */
 		RCU_INIT_POINTER(pr_dev_info, NULL);
 		synchronize_rcu();
-- 
2.25.1