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

Dmitry Osipenko
2021-12-10T18:19:29Z

10.12.2021 21:14, Rafael J. Wysocki пишет:
> 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.
>

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