linux-kernel - Re: [PATCH v4 02/25] notifier: Add blocking_notifier_call_chain_is

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Message-ID: <9307a684-0b0b-0488-75be-7845d9b973a1@gmail.com>
Date:   Fri, 10 Dec 2021 21:19:29 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Russell King <linux@...linux.org.uk>,
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Subject: Re: [PATCH v4 02/25] notifier: Add
 blocking_notifier_call_chain_is_empty()

10.12.2021 21:14, Rafael J. Wysocki пишет:
> On Fri, Nov 26, 2021 at 7:01 PM Dmitry Osipenko <digetx@...il.com> wrote:
>>
>> Add blocking_notifier_call_chain_is_empty() that returns true if call
>> chain is empty.
>>
>> Signed-off-by: Dmitry Osipenko <digetx@...il.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.