On 30/10/25 11:25, Petr Tesarik wrote: > On Fri, 10 Oct 2025 17:38:16 +0200 > Valentin Schneider <[email protected]> wrote: > >> From: Josh Poimboeuf <[email protected]> >> >> 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 <[email protected]> >> --- >> 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
