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>
---

Changelog:

v10: - Corrected another doc-comment typo s/atomicnotifier/atomic_notifier/.

v9: - Extended commit message with a usage example.

    - Corrected doc-comment with s/blocking/atomic/.

 include/linux/notifier.h |  2 ++
 kernel/notifier.c        | 13 +++++++++++++
 2 files changed, 15 insertions(+)

diff --git a/include/linux/notifier.h b/include/linux/notifier.h
index 87069b8459af..95e2440037de 100644
--- 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 ba005ebf4730..137b902c985b 100644
--- 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.
-- 
2.35.3

On Thu, May 19, 2022 at 1:02 PM Dmitry Osipenko
<dmitry.osipenko@collabora.com> wrote:
>
> 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>
> ---
>
> Changelog:
>
> v10: - Corrected another doc-comment typo s/atomicnotifier/atomic_notifier/.
>
> v9: - Extended commit message with a usage example.
>
>     - Corrected doc-comment with s/blocking/atomic/.
>
>  include/linux/notifier.h |  2 ++
>  kernel/notifier.c        | 13 +++++++++++++
>  2 files changed, 15 insertions(+)
>
> diff --git a/include/linux/notifier.h b/include/linux/notifier.h
> index 87069b8459af..95e2440037de 100644
> --- 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 ba005ebf4730..137b902c985b 100644
> --- 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.
> --

Applied as 5.19 material, thanks!