notifier: Add atomic_notifier_call_chain_is_empty()

Add atomic_notifier_call_chain_is_empty() that returns true if given
atomic call chain is empty.

The first user of this new notifier API function will be the kernel
power-off core code that will support power-off call chains. The core
code will need to check whether there is a power-off handler registered
at all in order to decide whether to halt machine or power it off.

Reviewed-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

diff --git a/include/linux/notifier.h b/include/linux/notifier.h
index 87069b8..95e2440 100644 (file)
--- a/include/linux/notifier.h
+++ b/include/linux/notifier.h
@@ -173,6 +173,8 @@ extern int blocking_notifier_call_chain_robust(struct blocking_notifier_head *nh
 extern int raw_notifier_call_chain_robust(struct raw_notifier_head *nh,
                unsigned long val_up, unsigned long val_down, void *v);
 
+extern bool atomic_notifier_call_chain_is_empty(struct atomic_notifier_head *nh);
+
 #define NOTIFY_DONE            0x0000          /* Don't care */
 #define NOTIFY_OK              0x0001          /* Suits me */
 #define NOTIFY_STOP_MASK       0x8000          /* Don't call further */

diff --git a/kernel/notifier.c b/kernel/notifier.c
index ba005eb..137b902 100644 (file)
--- a/kernel/notifier.c
+++ b/kernel/notifier.c
@@ -204,6 +204,19 @@ int atomic_notifier_call_chain(struct atomic_notifier_head *nh,
 EXPORT_SYMBOL_GPL(atomic_notifier_call_chain);
 NOKPROBE_SYMBOL(atomic_notifier_call_chain);
 
+/**
+ *     atomic_notifier_call_chain_is_empty - Check whether notifier chain is empty
+ *     @nh: Pointer to head of the atomic notifier chain
+ *
+ *     Checks whether notifier chain is empty.
+ *
+ *     Returns true is notifier chain is empty, false otherwise.
+ */
+bool atomic_notifier_call_chain_is_empty(struct atomic_notifier_head *nh)
+{
+       return !rcu_access_pointer(nh->head);
+}
+
 /*
  *     Blocking notifier chain routines.  All access to the chain is
  *     synchronized by an rwsem.