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

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Message-ID: <20251103093728.031042d0@mordecai>
Date: Mon, 3 Nov 2025 09:37:28 +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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Subject: Re: [PATCH v6 06/29] static_call: Add read-only-after-init static
 calls

On Fri, 31 Oct 2025 12:52:56 +0100
Valentin Schneider <vschneid@...hat.com> wrote:

> On 30/10/25 11:25, Petr Tesarik wrote:
> > 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.
> >  
> 
> I was about to write such a comment but I had another take; The _NOINSTR
> static key helpers are special and only relevant to IPI deferral; whereas
> the _RO helpers actually change the backing storage for the keys and as a
> bonus are used by the IPI deferral instrumentation.
> 
> IMO it's the same here for the static calls, it makes sense to mark the
> relevant ones as _RO regardless of IPI deferral.
> 
> I could however add a comment to ANNOTATE_NOINSTR_ALLOWED() itself,
> something like:
> 
> ```
> /*
>  * This is used to tell objtool that a given static key is safe to be used
>  * within .noinstr code, and it doesn't need to generate a warning about it.
>  *
>  * For more information, see tools/objtool/Documentation/objtool.txt,
>  * "non-RO static key usage in noinstr code"
>  */
> #define ANNOTATE_NOINSTR_ALLOWED(key) __ANNOTATE_NOINSTR_ALLOWED(key)
> ```

I agree, this makes more sense. Thank you!

Petr T