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