On Fri, 31 Oct 2025 12:52:56 +0100 Valentin Schneider <[email protected]> wrote:
> 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) > ``` I agree, this makes more sense. Thank you! Petr T
