perf: Fix perf_event_{init,exit}_cpu stubs

  The original commit that introduced those stubs was already at fault,
but in the absence of a caller of perf_event_{init,exit}_cpu outside of
code that is compiled regardless of CONFIG_PERF_EVENTS, the build
failure cannot be observed. This was observed with the Android kernel to
produce a build failure similar to this:

    In file included from ./include/uapi/linux/posix_types.h:5,
                     from ./include/uapi/linux/types.h:14,
                     from ./include/linux/types.h:6,
                     from ./include/linux/limits.h:6,
                     from ./include/linux/kernel.h:7,
                     from ./include/linux/sched/mm.h:5,
                     from kernel/cpu.c:6:
    kernel/cpu.c: In function 'random_and_perf_prepare_fusion':
    ./include/linux/stddef.h:8:14: error: called object is not a function or function pointer
     #define NULL ((void *)0)
                  ^
    ./include/linux/perf_event.h:1607:29: note: in expansion of macro 'NULL'
     #define perf_event_init_cpu NULL
                                 ^~~~
    kernel/cpu.c:1686:2: note: in expansion of macro 'perf_event_init_cpu'
      perf_event_init_cpu(cpu);
      ^~~~~~~~~~~~~~~~~~~

Fixes: 00e16c3d68fc ("perf/core: Convert to hotplug state machine")
Reported-by: Anand Gore <anand.gore@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
---
 include/linux/perf_event.h | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)
  

On Thu, Nov 03, 2022 at 03:43:03PM -0700, Florian Fainelli wrote:
> The original commit that introduced those stubs was already at fault,
> but in the absence of a caller of perf_event_{init,exit}_cpu outside of
> code that is compiled regardless of CONFIG_PERF_EVENTS, the build
> failure cannot be observed. This was observed with the Android kernel to
> produce a build failure similar to this:
> 
>     In file included from ./include/uapi/linux/posix_types.h:5,
>                      from ./include/uapi/linux/types.h:14,
>                      from ./include/linux/types.h:6,
>                      from ./include/linux/limits.h:6,
>                      from ./include/linux/kernel.h:7,
>                      from ./include/linux/sched/mm.h:5,
>                      from kernel/cpu.c:6:
>     kernel/cpu.c: In function 'random_and_perf_prepare_fusion':
>     ./include/linux/stddef.h:8:14: error: called object is not a function or function pointer
>      #define NULL ((void *)0)
>                   ^
>     ./include/linux/perf_event.h:1607:29: note: in expansion of macro 'NULL'
>      #define perf_event_init_cpu NULL
>                                  ^~~~
>     kernel/cpu.c:1686:2: note: in expansion of macro 'perf_event_init_cpu'
>       perf_event_init_cpu(cpu);
>       ^~~~~~~~~~~~~~~~~~~

What is the actual problem reported here? Did you see all the other NULL
assignments in cpuhp_hp_states ?

> 
> Fixes: 00e16c3d68fc ("perf/core: Convert to hotplug state machine")
> Reported-by: Anand Gore <anand.gore@broadcom.com>
> Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
> ---
>  include/linux/perf_event.h | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 0031f7b4d9ab..592040e2398c 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -1678,8 +1678,15 @@ static struct device_attribute format_attr_##_name = __ATTR_RO(_name)
>  int perf_event_init_cpu(unsigned int cpu);
>  int perf_event_exit_cpu(unsigned int cpu);
>  #else
> -#define perf_event_init_cpu	NULL
> -#define perf_event_exit_cpu	NULL
> +static inline int perf_event_init_cpu(unsigned int cpu)
> +{
> +	return 0;
> +}
> +
> +static inline int perf_event_exit_cpu(unsigned int cpu)
> +{
> +	return 0;
> +}
>  #endif
>  
>  extern void __weak arch_perf_update_userpage(struct perf_event *event,
> -- 
> 2.25.1
>
  

On 11/4/2022 6:06 AM, Peter Zijlstra wrote:
> On Thu, Nov 03, 2022 at 03:43:03PM -0700, Florian Fainelli wrote:
>> The original commit that introduced those stubs was already at fault,
>> but in the absence of a caller of perf_event_{init,exit}_cpu outside of
>> code that is compiled regardless of CONFIG_PERF_EVENTS, the build
>> failure cannot be observed. This was observed with the Android kernel to
>> produce a build failure similar to this:
>>
>>      In file included from ./include/uapi/linux/posix_types.h:5,
>>                       from ./include/uapi/linux/types.h:14,
>>                       from ./include/linux/types.h:6,
>>                       from ./include/linux/limits.h:6,
>>                       from ./include/linux/kernel.h:7,
>>                       from ./include/linux/sched/mm.h:5,
>>                       from kernel/cpu.c:6:
>>      kernel/cpu.c: In function 'random_and_perf_prepare_fusion':
>>      ./include/linux/stddef.h:8:14: error: called object is not a function or function pointer
>>       #define NULL ((void *)0)
>>                    ^
>>      ./include/linux/perf_event.h:1607:29: note: in expansion of macro 'NULL'
>>       #define perf_event_init_cpu NULL
>>                                   ^~~~
>>      kernel/cpu.c:1686:2: note: in expansion of macro 'perf_event_init_cpu'
>>        perf_event_init_cpu(cpu);
>>        ^~~~~~~~~~~~~~~~~~~
> 
> What is the actual problem reported here? Did you see all the other NULL
> assignments in cpuhp_hp_states ?

Anand reported to me that the following downstream commit in the Android 
common kernel repository:

https://android.googlesource.com/kernel/common/+/ca927bd22ad8bd26fd8dcebf3c7f2a093385d8ea

was resulting in the build failure above.

While this is an Android inflicted change and I still intend to get a 
fix there, upon closer look, the build failure could be experienced 
upstream as well with any code calling perf_event_{init,exit}_cpu() as a 
regular function call as opposed to being used as a function pointer.

This felt worthy of fixing in the upstream kernel, hence this patch.
  

On 11/4/22 10:38, Florian Fainelli wrote:
> 
> 
> On 11/4/2022 6:06 AM, Peter Zijlstra wrote:
>> On Thu, Nov 03, 2022 at 03:43:03PM -0700, Florian Fainelli wrote:
>>> The original commit that introduced those stubs was already at fault,
>>> but in the absence of a caller of perf_event_{init,exit}_cpu outside of
>>> code that is compiled regardless of CONFIG_PERF_EVENTS, the build
>>> failure cannot be observed. This was observed with the Android kernel to
>>> produce a build failure similar to this:
>>>
>>>      In file included from ./include/uapi/linux/posix_types.h:5,
>>>                       from ./include/uapi/linux/types.h:14,
>>>                       from ./include/linux/types.h:6,
>>>                       from ./include/linux/limits.h:6,
>>>                       from ./include/linux/kernel.h:7,
>>>                       from ./include/linux/sched/mm.h:5,
>>>                       from kernel/cpu.c:6:
>>>      kernel/cpu.c: In function 'random_and_perf_prepare_fusion':
>>>      ./include/linux/stddef.h:8:14: error: called object is not a 
>>> function or function pointer
>>>       #define NULL ((void *)0)
>>>                    ^
>>>      ./include/linux/perf_event.h:1607:29: note: in expansion of 
>>> macro 'NULL'
>>>       #define perf_event_init_cpu NULL
>>>                                   ^~~~
>>>      kernel/cpu.c:1686:2: note: in expansion of macro 
>>> 'perf_event_init_cpu'
>>>        perf_event_init_cpu(cpu);
>>>        ^~~~~~~~~~~~~~~~~~~
>>
>> What is the actual problem reported here? Did you see all the other NULL
>> assignments in cpuhp_hp_states ?
> 
> Anand reported to me that the following downstream commit in the Android 
> common kernel repository:
> 
> https://android.googlesource.com/kernel/common/+/ca927bd22ad8bd26fd8dcebf3c7f2a093385d8ea
> 
> was resulting in the build failure above.
> 
> While this is an Android inflicted change and I still intend to get a 
> fix there, upon closer look, the build failure could be experienced 
> upstream as well with any code calling perf_event_{init,exit}_cpu() as a 
> regular function call as opposed to being used as a function pointer.
> 
> This felt worthy of fixing in the upstream kernel, hence this patch.

No response meaning it's a don't care because upstream is not (yet?) 
impacted?