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Message-ID: <ZIcarHx0qbfC2iyy@FVFF77S0Q05N>
Date: Mon, 12 Jun 2023 14:16:28 +0100
From: Mark Rutland <mark.rutland@....com>
To: Marc Zyngier <maz@...nel.org>,
Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, Reiji Watanabe <reijiw@...gle.com>,
Will Deacon <will@...nel.org>
Subject: Re: [PATCH] perf/core: Drop __weak attribute from
arch_perf_update_userpage() prototype
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@...rids:~/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@...gle.com>
> Signed-off-by: Marc Zyngier <maz@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Will Deacon <will@...nel.org>
> Tested-by: Reiji Watanabe <reijiw@...gle.com>
FWIW, regardless of the above:
Acked-by: Mark Rutland <mark.rutland@....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
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