linux-kernel - Re: [PATCH v6 06/29] static_call: Add read-only-after-init static calls

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Message-ID: <20251030112251.5afcf9ed@mordecai>
Date: Thu, 30 Oct 2025 11:25:03 +0100
From: Petr Tesarik <ptesarik@...e.com>
To: Valentin Schneider <vschneid@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, rcu@...r.kernel.org,
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 linux-arch@...r.kernel.org, linux-trace-kernel@...r.kernel.org, Josh
 Poimboeuf <jpoimboe@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, Ingo
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Subject: Re: [PATCH v6 06/29] static_call: Add read-only-after-init static
 calls

On Fri, 10 Oct 2025 17:38:16 +0200
Valentin Schneider <vschneid@...hat.com> wrote:

> From: Josh Poimboeuf <jpoimboe@...nel.org>
> 
> Deferring a code patching IPI is unsafe if the patched code is in a
> noinstr region.  In that case the text poke code must trigger an
> immediate IPI to all CPUs, which can rudely interrupt an isolated NO_HZ
> CPU running in userspace.
> 
> If a noinstr static call only needs to be patched during boot, its key
> can be made ro-after-init to ensure it will never be patched at runtime.
> 
> Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
> ---
>  include/linux/static_call.h | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/include/linux/static_call.h b/include/linux/static_call.h
> index 78a77a4ae0ea8..ea6ca57e2a829 100644
> --- a/include/linux/static_call.h
> +++ b/include/linux/static_call.h
> @@ -192,6 +192,14 @@ extern long __static_call_return0(void);
>  	};								\
>  	ARCH_DEFINE_STATIC_CALL_TRAMP(name, _func)
>  
> +#define DEFINE_STATIC_CALL_RO(name, _func)				\
> +	DECLARE_STATIC_CALL(name, _func);				\
> +	struct static_call_key __ro_after_init STATIC_CALL_KEY(name) = {\
> +		.func = _func,						\
> +		.type = 1,						\
> +	};								\
> +	ARCH_DEFINE_STATIC_CALL_TRAMP(name, _func)
> +
>  #define DEFINE_STATIC_CALL_NULL(name, _func)				\
>  	DECLARE_STATIC_CALL(name, _func);				\
>  	struct static_call_key STATIC_CALL_KEY(name) = {		\
> @@ -200,6 +208,14 @@ extern long __static_call_return0(void);
>  	};								\
>  	ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name)
>  
> +#define DEFINE_STATIC_CALL_NULL_RO(name, _func)				\
> +	DECLARE_STATIC_CALL(name, _func);				\
> +	struct static_call_key __ro_after_init STATIC_CALL_KEY(name) = {\
> +		.func = NULL,						\
> +		.type = 1,						\
> +	};								\
> +	ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name)
> +

I think it would be a good idea to add a comment describing when these
macros are supposed to be used, similar to the explanation you wrote for
the _NOINSTR variants. Just to provide a clue for people adding a new
static key in the future, because the commit message may become a bit
hard to find if there are a few cleanup patches on top.

Just my two cents,
Petr T