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

["Rafael J. Wysocki" <rafael@xxxxxxxxxx>]

On Fri, Nov 26, 2021 at 7:01 PM Dmitry Osipenko <digetx@xxxxxxxxx> wrote:
>
> Add blocking_notifier_call_chain_is_empty() that returns true if call
> chain is empty.
>
> Signed-off-by: Dmitry Osipenko <digetx@xxxxxxxxx>
> ---
>  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.