[RFC PATCH v3 06/15] jump_label: Add forceful jump label type

[Valentin Schneider <vschneid@xxxxxxxxxx>]

Later commits will cause objtool to warn about non __ro_after_init static
keys being used in .noinstr sections in order to safely defer instruction
patching IPIs targeted at NOHZ_FULL CPUs.

Two such keys currently exist: mds_idle_clear and __sched_clock_stable,
which can both be modified at runtime.

As discussed at LPC 2024 during the realtime micro-conference, modifying
these specific keys incurs additional interference (SMT hotplug) or can
even be downright incompatible with NOHZ_FULL (unstable TSC).

Suppressing the IPI associated with modifying such keys is thus a minor
concern wrt NOHZ_FULL interference, so add a jump type that will be
leveraged by both the kernel (to know not to defer the IPI) and objtool (to
know not to generate the aforementioned warning).

Signed-off-by: Valentin Schneider <vschneid@xxxxxxxxxx>
---
 include/linux/jump_label.h | 26 +++++++++++++++++++-------
 1 file changed, 19 insertions(+), 7 deletions(-)

diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h
index f5a2727ca4a9a..93e729545b941 100644
--- a/include/linux/jump_label.h
+++ b/include/linux/jump_label.h
@@ -200,7 +200,8 @@ struct module;
 #define JUMP_TYPE_FALSE		0UL
 #define JUMP_TYPE_TRUE		1UL
 #define JUMP_TYPE_LINKED	2UL
-#define JUMP_TYPE_MASK		3UL
+#define JUMP_TYPE_FORCEFUL      4UL
+#define JUMP_TYPE_MASK		7UL
 
 static __always_inline bool static_key_false(struct static_key *key)
 {
@@ -244,12 +245,15 @@ extern enum jump_label_type jump_label_init_type(struct jump_entry *entry);
  * raw value, but have added a BUILD_BUG_ON() to catch any issues in
  * jump_label_init() see: kernel/jump_label.c.
  */
-#define STATIC_KEY_INIT_TRUE					\
-	{ .enabled = { 1 },					\
-	  { .type = JUMP_TYPE_TRUE } }
-#define STATIC_KEY_INIT_FALSE					\
-	{ .enabled = { 0 },					\
-	  { .type = JUMP_TYPE_FALSE } }
+#define __STATIC_KEY_INIT(_true, force)					\
+	{ .enabled = { _true },						\
+	  { .type = (_true ? JUMP_TYPE_TRUE : JUMP_TYPE_FALSE) |	\
+		    (force ? JUMP_TYPE_FORCEFUL : 0UL)} }
+
+#define STATIC_KEY_INIT_TRUE  __STATIC_KEY_INIT(true, false)
+#define STATIC_KEY_INIT_FALSE __STATIC_KEY_INIT(false, false)
+#define STATIC_KEY_INIT_TRUE_FORCE  __STATIC_KEY_INIT(true, true)
+#define STATIC_KEY_INIT_FALSE_FORCE __STATIC_KEY_INIT(false, true)
 
 #else  /* !CONFIG_JUMP_LABEL */
 
@@ -369,6 +373,8 @@ struct static_key_false {
 
 #define STATIC_KEY_TRUE_INIT  (struct static_key_true) { .key = STATIC_KEY_INIT_TRUE,  }
 #define STATIC_KEY_FALSE_INIT (struct static_key_false){ .key = STATIC_KEY_INIT_FALSE, }
+#define STATIC_KEY_TRUE_FORCE_INIT  (struct static_key_true) { .key = STATIC_KEY_INIT_TRUE_FORCE,  }
+#define STATIC_KEY_FALSE_FORCE_INIT (struct static_key_false){ .key = STATIC_KEY_INIT_FALSE_FORCE, }
 
 #define DEFINE_STATIC_KEY_TRUE(name)	\
 	struct static_key_true name = STATIC_KEY_TRUE_INIT
@@ -376,6 +382,9 @@ struct static_key_false {
 #define DEFINE_STATIC_KEY_TRUE_RO(name)	\
 	struct static_key_true name __ro_after_init = STATIC_KEY_TRUE_INIT
 
+#define DEFINE_STATIC_KEY_TRUE_FORCE(name)	\
+	struct static_key_true name = STATIC_KEY_TRUE_FORCE_INIT
+
 #define DECLARE_STATIC_KEY_TRUE(name)	\
 	extern struct static_key_true name
 
@@ -385,6 +394,9 @@ struct static_key_false {
 #define DEFINE_STATIC_KEY_FALSE_RO(name)	\
 	struct static_key_false name __ro_after_init = STATIC_KEY_FALSE_INIT
 
+#define DEFINE_STATIC_KEY_FALSE_FORCE(name)	\
+	struct static_key_false name = STATIC_KEY_FALSE_FORCE_INIT
+
 #define DECLARE_STATIC_KEY_FALSE(name)	\
 	extern struct static_key_false name
 
-- 
2.43.0