Re: [PATCH v9 2/9] preempt: Introduce __preempt_count_{sub, add}_return()

Heiko Carstens
2025-02-28T09:15:46Z

On Thu, Feb 27, 2025 at 05:10:13PM -0500, Lyude Paul wrote:
> From: Boqun Feng <boqun.feng@gmail.com>
>
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> Signed-off-by: Lyude Paul <lyude@redhat.com>
> ---
> arch/arm64/include/asm/preempt.h | 18 ++++++++++++++++++
> arch/s390/include/asm/preempt.h | 19 +++++++++++++++++++
> arch/x86/include/asm/preempt.h | 10 ++++++++++
> include/asm-generic/preempt.h | 14 ++++++++++++++
> 4 files changed, 61 insertions(+)
...
> diff --git a/arch/s390/include/asm/preempt.h b/arch/s390/include/asm/preempt.h
> index 6ccd033acfe52..67a6e265e9fff 100644
> --- a/arch/s390/include/asm/preempt.h
> +++ b/arch/s390/include/asm/preempt.h
> @@ -98,6 +98,25 @@ static __always_inline bool should_resched(int preempt_offset)
> return unlikely(READ_ONCE(get_lowcore()->preempt_count) == preempt_offset);
> }
>
> +static __always_inline int __preempt_count_add_return(int val)
> +{
> + /*
> + * With some obscure config options and CONFIG_PROFILE_ALL_BRANCHES
> + * enabled, gcc 12 fails to handle __builtin_constant_p().
> + */
> + if (!IS_ENABLED(CONFIG_PROFILE_ALL_BRANCHES)) {
> + if (__builtin_constant_p(val) && (val >= -128) && (val <= 127)) {
> + return val + __atomic_add_const(val, &get_lowcore()->preempt_count);
> + }
> + }
> + return val + __atomic_add(val, &get_lowcore()->preempt_count);
> +}

This should just be

static __always_inline int __preempt_count_add_return(int val)
{
return val + __atomic_add(val, &get_lowcore()->preempt_count);
}

since __atomic_add_const() won't return the original value.

Well.. at least it should not, but the way it is currently implemented it
indeed does sometimes depending on config options - there is room for
improvement. That's my fault - going to address that.

I couldn't find any cover letter for the whole patch series which describes
what this is about, and why it is needed.
It looks like some Rust enablement?