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

[Dmitry Osipenko]

10.12.2021 21:14, Rafael J. Wysocki пишет:
> On Fri, Nov 26, 2021 at 7:01 PM Dmitry Osipenko  wrote:
>>
>> Add blocking_notifier_call_chain_is_empty() that returns true if call
>> chain is empty.
>>
>> Signed-off-by: Dmitry Osipenko 
>> ---
>>  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_DONE0x  /* 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.

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

[Rafael J. Wysocki]

On Fri, Nov 26, 2021 at 7:01 PM Dmitry Osipenko  wrote:
>
> Add blocking_notifier_call_chain_is_empty() that returns true if call
> chain is empty.
>
> Signed-off-by: Dmitry Osipenko 
> ---
>  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_DONE0x  /* 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.

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

[Dmitry Osipenko]

Add blocking_notifier_call_chain_is_empty() that returns true if call
chain is empty.

Signed-off-by: Dmitry Osipenko 
---
 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_DONE0x  /* 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);
+
 /*
  * Raw notifier chain routines.  There is no protection;
  * the caller must provide it.  Use at your own risk!
-- 
2.33.1