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 <[email protected]> Signed-off-by: Dmitry Osipenko <[email protected]> Signed-off-by: Rafael J. Wysocki <[email protected]>
torvalds · May 19, 2022 · 13dfd97 · 13dfd97
1 parent 42226c9
commit 13dfd97
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diff --git 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
@@ -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.