linux-kernel - Re: [PATCH v9 2/9] preempt: Introduce __preempt_count_{sub, add}

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Message-ID: <20250228091509.8985B18-hca@linux.ibm.com>
Date: Fri, 28 Feb 2025 10:15:09 +0100
From: Heiko Carstens <hca@...ux.ibm.com>
To: Lyude Paul <lyude@...hat.com>
Cc: rust-for-linux@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Boqun Feng <boqun.feng@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
        Juergen Christ <jchrist@...ux.ibm.com>,
        Ilya Leoshkevich <iii@...ux.ibm.com>,
        "moderated list:ARM64 PORT (AARCH64 ARCHITECTURE)" <linux-arm-kernel@...ts.infradead.org>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:S390 ARCHITECTURE" <linux-s390@...r.kernel.org>,
        "open list:GENERIC INCLUDE/ASM HEADER FILES" <linux-arch@...r.kernel.org>
Subject: Re: [PATCH v9 2/9] preempt: Introduce __preempt_count_{sub,
 add}_return()

On Thu, Feb 27, 2025 at 05:10:13PM -0500, Lyude Paul wrote:
> From: Boqun Feng <boqun.feng@...il.com>
> 
> Signed-off-by: Boqun Feng <boqun.feng@...il.com>
> Signed-off-by: Lyude Paul <lyude@...hat.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?