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* [PATCH v2] mm/page_reporting: replace rcu_access_pointer() with rcu_dereference_protected()
@ 2022-12-28  1:29 SeongJae Park
  2022-12-28  1:45 ` Miaohe Lin
                   ` (2 more replies)
  0 siblings, 3 replies; 5+ messages in thread
From: SeongJae Park @ 2022-12-28  1:29 UTC (permalink / raw)
  To: akpm
  Cc: alexander.h.duyck, paulmck, linux-mm, rcu, linux-kernel, 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 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@kernel.org>
---
Changes from v1
(https://lore.kernel.org/linux-mm/20221227192158.2553-1-sj@kernel.org/)
- 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..5c557a3e1423 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_order))) {
 		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



^ permalink raw reply related	[flat|nested] 5+ messages in thread
end of thread, other threads:[~2022-12-28  6:48 UTC | newest]

Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed)
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2022-12-28  1:29 [PATCH v2] mm/page_reporting: replace rcu_access_pointer() with rcu_dereference_protected() SeongJae Park
2022-12-28  1:45 ` Miaohe Lin
2022-12-28  2:09   ` SeongJae Park
2022-12-28  4:55 ` kernel test robot
2022-12-28  6:47 ` kernel test robot

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