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

Message ID 20221227192158.2553-1-sj@kernel.org
State New
Headers
Series mm/page_reporting: replace rcu_access_pointer() with rcu_dereference_protected() |

Commit Message

SeongJae Park Dec. 27, 2022, 7:21 p.m. UTC 
  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>
---
 mm/page_reporting.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Comments

Matthew Wilcox Dec. 27, 2022, 7:55 p.m. UTC | #1 
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.
SeongJae Park Dec. 27, 2022, 7:56 p.m. UTC | #2 
On Tue, 27 Dec 2022 19:55:09 +0000 Matthew Wilcox <willy@infradead.org> 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
diff mbox series

Patch

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();