mm/page_reporting: replace rcu_access_pointer() with rcu_dereference_protected()

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().

Link: https://lkml.kernel.org/r/20221228175942.149491-1-sj@kernel.org
Fixes: 36e66c554b5c ("mm: introduce Reported pages")
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/page_reporting.c b/mm/page_reporting.c
index 79a8554f024c27afb71916817073e5fc34d977d1..c65813a9dc78e7c91972845c3b37b477e8258ee0 100644 (file)
--- 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();