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.
Reported-by: Maxim Mikityanskiy <[email protected]> Signed-off-by: Paul E. McKenney <[email protected]> --- include/linux/rcupdate.h | 12 +++++++----- 1 file changed, 7 insertions(+), 5 deletions(-) diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index bfa765132de858..5e95acc33989b6 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 -- 2.40.1
