From: Paul E. McKenney Date: Fri, 8 May 2026 17:43:53 +0000 (-0700) Subject: rcu: Document rcu_access_pointer() feeding into cmpxchg() X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=e6cb527255c9f873851bd18d2bc375f6e6abb311;p=thirdparty%2Fkernel%2Flinux.git rcu: Document rcu_access_pointer() feeding into cmpxchg() This commit documents the rcu_access_pointer() use case for fetching the old value of an RCU-protected pointer within a lockless updater for use by an atomic cmpxchg() operation. Reviewed-by: Frederic Weisbecker Reported-by: Maxim Mikityanskiy Signed-off-by: Paul E. McKenney Signed-off-by: Uladzislau Rezki (Sony) --- diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index bfa765132de85..5e95acc33989b 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -592,11 +592,13 @@ context_unsafe( \ * lockdep checks for being in an RCU read-side critical section. This is * useful when the value of this pointer is accessed, but the pointer is * not dereferenced, for example, when testing an RCU-protected pointer - * against NULL. Although rcu_access_pointer() may also be used in cases - * where update-side locks prevent the value of the pointer from changing, - * you should instead use rcu_dereference_protected() for this use case. - * Within an RCU read-side critical section, there is little reason to - * use rcu_access_pointer(). + * against NULL. Within an RCU read-side critical section, there is little + * reason to use rcu_access_pointer(). Although rcu_access_pointer() may + * also be used in cases where update-side locks prevent the value of the + * pointer from changing, you should instead use rcu_dereference_protected() + * for this use case. It is also permissible to use rcu_access_pointer() + * within lockless updaters to obtain the old value for an atomic operation, + * for example, for cmpxchg(). * * It is usually best to test the rcu_access_pointer() return value * directly in order to avoid accidental dereferences being introduced