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[2a00:1028:83b8:1e7a:3010:3bd6:8521:caf1]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-6407b428119sm9012005a12.24.2025.11.03.00.37.34 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 03 Nov 2025 00:37:36 -0800 (PST) Date: Mon, 3 Nov 2025 09:37:28 +0100 From: Petr Tesarik To: Valentin Schneider Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, rcu@vger.kernel.org, x86@kernel.org, linux-arm-kernel@lists.infradead.org, loongarch@lists.linux.dev, linux-riscv@lists.infradead.org, linux-arch@vger.kernel.org, linux-trace-kernel@vger.kernel.org, Josh Poimboeuf , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H. Peter Anvin" , Andy Lutomirski , Peter Zijlstra , Arnaldo Carvalho de Melo , Paolo Bonzini , Arnd Bergmann , Frederic Weisbecker , "Paul E. McKenney" , Jason Baron , Steven Rostedt , Ard Biesheuvel , Sami Tolvanen , "David S. Miller" , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Boqun Feng , Uladzislau Rezki , Mathieu Desnoyers , Mel Gorman , Andrew Morton , Masahiro Yamada , Han Shen , Rik van Riel , Jann Horn , Dan Carpenter , Oleg Nesterov , Juri Lelli , Clark Williams , Yair Podemsky , Marcelo Tosatti , Daniel Wagner Subject: Re: [PATCH v6 06/29] static_call: Add read-only-after-init static calls Message-ID: <20251103093728.031042d0@mordecai> In-Reply-To: References: <20251010153839.151763-1-vschneid@redhat.com> <20251010153839.151763-7-vschneid@redhat.com> <20251030112251.5afcf9ed@mordecai> X-Mailer: Claws Mail 4.3.1 (GTK 3.24.50; x86_64-suse-linux-gnu) Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Fri, 31 Oct 2025 12:52:56 +0100 Valentin Schneider wrote: > On 30/10/25 11:25, Petr Tesarik wrote: > > On Fri, 10 Oct 2025 17:38:16 +0200 > > Valentin Schneider wrote: > > > >> From: Josh Poimboeuf > >> > >> 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 > >> --- > >> 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