Re: [PATCH] perf/core: Drop __weak attribute from arch_perf_update_userpage() prototype

[Mark Rutland <mark.rutland@arm.com>]

On Sat, Jun 03, 2023 at 09:25:19AM +0100, Marc Zyngier wrote:
> Reiji reports that the arm64 implementation of arch_perf_update_userpage()
> is now ignored and replaced by the dummy stub in core code.
> This seems to happen since the PMUv3 driver was moved to driver/perf.

I guess we should have a Cc stable then?

The below implies this has always been on dodgy ground, and so it's probably
inaccurate to give this a Fixes tag pointing to the move.

> As it turns out, dropping the __weak attribute from the *prototype*
> of the function solves the problem. You're right, this doesn't seem
> to make much sense. And yet... It appears that both symbols get
> flagged as weak, and that the first one to appear in the link order
> wins:
> 
> $ nm drivers/perf/arm_pmuv3.o|grep arch_perf_update_userpage
> 0000000000001db0 W arch_perf_update_userpage

Ah, so having it on th *declaration* will apply to any *definition*. :/

That suggests this is a bad pattern generally, and we should probably remove
the other __weak instances in headers. Lukcily it seems there aren't that many:

[mark@lakrids:~/src/linux]% git grep __weak -- **/*.h | wc -l
50

IMO we'd should aim to remove __weak entirely; it causes a number of weird
things like this and it'd be much easier to manage with a small amount of
ifdeffery.

Peter, thoughts?

> Dropping the attribute from the prototype restores the expected
> behaviour, and arm64 is able to enjoy arch_perf_update_userpage()
> again.
> 
> Reported-by: Reiji Watanabe <reijiw@google.com>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Tested-by: Reiji Watanabe <reijiw@google.com>

FWIW, regardless of the above:

Acked-by: Mark Rutland <mark.rutland@arm.com>

> ---
>  include/linux/perf_event.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index d5628a7b5eaa..c8dcfdbda1f4 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -1845,9 +1845,9 @@ int perf_event_exit_cpu(unsigned int cpu);
>  #define perf_event_exit_cpu	NULL
>  #endif
>  
> -extern void __weak arch_perf_update_userpage(struct perf_event *event,
> -					     struct perf_event_mmap_page *userpg,
> -					     u64 now);
> +extern void arch_perf_update_userpage(struct perf_event *event,
> +				      struct perf_event_mmap_page *userpg,
> +				      u64 now);
>  
>  #ifdef CONFIG_MMU
>  extern __weak u64 arch_perf_get_page_size(struct mm_struct *mm, unsigned long addr);

Luckily, arch_perf_get_page_size() has no callers or definition since commit:

  8af26be062721e52 ("perf/core: Fix arch_perf_get_page_size()")

... so we can just delete that prototype.

Mark