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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Tue, 10 May 2022 20:14:00 +0200
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Subject: Re: [PATCH v8 01/27] notifier: Add atomic_notifier_call_chain_is_empty()
To:     Dmitry Osipenko <dmitry.osipenko@collabora.com>
Cc:     Thierry Reding <thierry.reding@gmail.com>,
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On Tue, May 10, 2022 at 1:33 AM Dmitry Osipenko
<dmitry.osipenko@collabora.com> wrote:
>
> Add atomic_notifier_call_chain_is_empty() that returns true if given
> atomic call chain is empty.

It would be good to mention a use case for it.

> Reviewed-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
> ---
>  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..aaf5b56452a6 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);
>
> +/**
> + *     atomicnotifier_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 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.
> --
> 2.35.1
>