10.12.2021 21:14, Rafael J. Wysocki пишет:

On Fri, Nov 26, 2021 at 7:01 PM Dmitry Osipenko [off-list ref] wrote:

quoted

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

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
---
 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);

The check is not reliable (racy) without locking, so I wouldn't export
anything like this to modules.

At least IMO it should be added along with a user.

I'll remove the export since it's indeed not obvious how other users may
want to use this function.