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

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Message-ID: <xhsmhqzujp9t3.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Fri, 31 Oct 2025 12:52:56 +0100
From: Valentin Schneider <vschneid@...hat.com>
To: Petr Tesarik <ptesarik@...e.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, rcu@...r.kernel.org,
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 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 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)
```

> Just my two cents,
> Petr T