Re: [PATCH] perf: Fix perf_event_{init,exit}_cpu stubs

[Florian Fainelli <f.fainelli@gmail.com>]

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.
-- 
Florian