Re: [PATCH v4 02/25] notifier: Add blocking_notifier_call_chain_is_empty()

Rafael J. Wysocki
2021-12-10T18:14:17Z

On Fri, Nov 26, 2021 at 7:01 PM Dmitry Osipenko <digetx@gmail.com> wrote:
>
> 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.