Subject: [PATCH v3 02/25] notifier: Add blocking_notifier_call_chain_is_empty()

From: Dmitry Osipenko <[email-masked]>
Date: Mon, 8 Nov 2021 03:45:01 +0300

Add blocking_notifier_call_chain_is_empty() that returns true if call
chain is empty.

Signed-off-by: Dmitry Osipenko <[email-masked]>
---
 include/linux/notifier.h |  2 ++
 kernel/notifier.c        | 14 ++++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/include/linux/notifier.h b/include/linux/notifier.h
index 4b80a815b666..924c9d7c8e73 100644
--- a/include/linux/notifier.h
+++ b/include/linux/notifier.h
@@ -173,6 +173,8 @@ int blocking_notifier_call_chain_robust(struct blocking_notifier_head *nh,
 int raw_notifier_call_chain_robust(struct raw_notifier_head *nh,
 		unsigned long val_up, unsigned long val_down, void *v);
 
+bool blocking_notifier_call_chain_is_empty(struct blocking_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 b8251dc0bc0f..b20cb7b9b1f0 100644
--- a/kernel/notifier.c
+++ b/kernel/notifier.c
@@ -322,6 +322,20 @@ int blocking_notifier_call_chain(struct blocking_notifier_head *nh,
 }
 EXPORT_SYMBOL_GPL(blocking_notifier_call_chain);
 
+/**
+ *	blocking_notifier_call_chain_is_empty - Check whether notifier chain is empty
+ *	@nh: Pointer to head of the blocking notifier chain
+ *
+ *	Checks whether notifier chain is empty.
+ *
+ *	Returns true is notifier chain is empty, false otherwise.
+ */
+bool blocking_notifier_call_chain_is_empty(struct blocking_notifier_head *nh)
+{
+	return !rcu_access_pointer(nh->head);
+}
+EXPORT_SYMBOL_GPL(blocking_notifier_call_chain_is_empty);
+
 /*
  *	Raw notifier chain routines.  There is no protection;
  *	the caller must provide it.  Use at your own risk!
-- 
2.33.1