linux-kernel - [PATCH] mm/page_reporting: replace rcu_access_pointer() with rcu_dereference

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Message-Id: <20221227192158.2553-1-sj@kernel.org>
Date:   Tue, 27 Dec 2022 19:21:58 +0000
From:   SeongJae Park <sj@...nel.org>
To:     akpm@...ux-foundation.org
Cc:     alexander.h.duyck@...ux.intel.com, paulmck@...nel.org,
        linux-mm@...ck.org, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org, SeongJae Park <sj@...nel.org>
Subject: [PATCH] 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 fetcing it using rcu_access_pointer(), but it fetches the pointer
while concurrent updtes to the pointer is avoided by holding the update
side lock, page_reporting_mutex.

In the case, rcu_dereference_protected() is recommended 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().

Signed-off-by: SeongJae Park <sj@...nel.org>
---
 mm/page_reporting.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/page_reporting.c b/mm/page_reporting.c
index 79a8554f024c..079e0ebe1fb7 100644
--- a/mm/page_reporting.c
+++ 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)) {
 		err = -EBUSY;
 		goto err_out;
 	}
@@ -401,7 +401,7 @@ void page_reporting_unregister(struct page_reporting_dev_info *prdev)
 {
 	mutex_lock(&page_reporting_mutex);

-	if (rcu_access_pointer(pr_dev_info) == prdev) {
+	if (rcu_dereference_protected(pr_dev_info, true) == prdev) {
 		/* Disable page reporting notification */
 		RCU_INIT_POINTER(pr_dev_info, NULL);
 		synchronize_rcu();
-- 
2.25.1