locking/static_keys: Add a new static_key interface
There are various problems and short-comings with the current
static_key interface:
- static_key_{true,false}() read like a branch depending on the key
value, instead of the actual likely/unlikely branch depending on
init value.
- static_key_{true,false}() are, as stated above, tied to the
static_key init values STATIC_KEY_INIT_{TRUE,FALSE}.
- we're limited to the 2 (out of 4) possible options that compile to
a default NOP because that's what our arch_static_branch() assembly
emits.
So provide a new static_key interface:
DEFINE_STATIC_KEY_TRUE(name);
DEFINE_STATIC_KEY_FALSE(name);
Which define a key of different types with an initial true/false
value.
Then allow:
static_branch_likely()
static_branch_unlikely()
to take a key of either type and emit the right instruction for the
case.
This means adding a second arch_static_branch_jump() assembly helper
which emits a JMP per default.
In order to determine the right instruction for the right state,
encode the branch type in the LSB of jump_entry::key.
This is the final step in removing the naming confusion that has led to
a stream of avoidable bugs such as:
a833581e372a ("x86, perf: Fix static_key bug in load_mm_cr4()")
... but it also allows new static key combinations that will give us
performance enhancements in the subsequent patches.
Tested-by: Rabin Vincent <rabin@rab.in> # arm
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> # ppc
Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com> # s390
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h
index 65f0eba..e337a19 100644
--- a/include/linux/jump_label.h
+++ b/include/linux/jump_label.h
@@ -107,12 +107,12 @@
static __always_inline bool static_key_false(struct static_key *key)
{
- return arch_static_branch(key);
+ return arch_static_branch(key, false);
}
static __always_inline bool static_key_true(struct static_key *key)
{
- return !static_key_false(key);
+ return !arch_static_branch(key, true);
}
extern struct jump_entry __start___jump_table[];
@@ -130,12 +130,12 @@
extern void static_key_slow_dec(struct static_key *key);
extern void jump_label_apply_nops(struct module *mod);
-#define STATIC_KEY_INIT_TRUE ((struct static_key) \
+#define STATIC_KEY_INIT_TRUE \
{ .enabled = ATOMIC_INIT(1), \
- .entries = (void *)JUMP_TYPE_TRUE })
-#define STATIC_KEY_INIT_FALSE ((struct static_key) \
+ .entries = (void *)JUMP_TYPE_TRUE }
+#define STATIC_KEY_INIT_FALSE \
{ .enabled = ATOMIC_INIT(0), \
- .entries = (void *)JUMP_TYPE_FALSE })
+ .entries = (void *)JUMP_TYPE_FALSE }
#else /* !HAVE_JUMP_LABEL */
@@ -183,10 +183,8 @@
return 0;
}
-#define STATIC_KEY_INIT_TRUE ((struct static_key) \
- { .enabled = ATOMIC_INIT(1) })
-#define STATIC_KEY_INIT_FALSE ((struct static_key) \
- { .enabled = ATOMIC_INIT(0) })
+#define STATIC_KEY_INIT_TRUE { .enabled = ATOMIC_INIT(1) }
+#define STATIC_KEY_INIT_FALSE { .enabled = ATOMIC_INIT(0) }
#endif /* HAVE_JUMP_LABEL */
@@ -218,6 +216,137 @@
static_key_slow_dec(key);
}
+/* -------------------------------------------------------------------------- */
+
+/*
+ * Two type wrappers around static_key, such that we can use compile time
+ * type differentiation to emit the right code.
+ *
+ * All the below code is macros in order to play type games.
+ */
+
+struct static_key_true {
+ struct static_key key;
+};
+
+struct static_key_false {
+ struct static_key key;
+};
+
+#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 DEFINE_STATIC_KEY_TRUE(name) \
+ struct static_key_true name = STATIC_KEY_TRUE_INIT
+
+#define DEFINE_STATIC_KEY_FALSE(name) \
+ struct static_key_false name = STATIC_KEY_FALSE_INIT
+
+#ifdef HAVE_JUMP_LABEL
+
+/*
+ * Combine the right initial value (type) with the right branch order
+ * to generate the desired result.
+ *
+ *
+ * type\branch| likely (1) | unlikely (0)
+ * -----------+-----------------------+------------------
+ * | |
+ * true (1) | ... | ...
+ * | NOP | JMP L
+ * | <br-stmts> | 1: ...
+ * | L: ... |
+ * | |
+ * | | L: <br-stmts>
+ * | | jmp 1b
+ * | |
+ * -----------+-----------------------+------------------
+ * | |
+ * false (0) | ... | ...
+ * | JMP L | NOP
+ * | <br-stmts> | 1: ...
+ * | L: ... |
+ * | |
+ * | | L: <br-stmts>
+ * | | jmp 1b
+ * | |
+ * -----------+-----------------------+------------------
+ *
+ * The initial value is encoded in the LSB of static_key::entries,
+ * type: 0 = false, 1 = true.
+ *
+ * The branch type is encoded in the LSB of jump_entry::key,
+ * branch: 0 = unlikely, 1 = likely.
+ *
+ * This gives the following logic table:
+ *
+ * enabled type branch instuction
+ * -----------------------------+-----------
+ * 0 0 0 | NOP
+ * 0 0 1 | JMP
+ * 0 1 0 | NOP
+ * 0 1 1 | JMP
+ *
+ * 1 0 0 | JMP
+ * 1 0 1 | NOP
+ * 1 1 0 | JMP
+ * 1 1 1 | NOP
+ *
+ * Which gives the following functions:
+ *
+ * dynamic: instruction = enabled ^ branch
+ * static: instruction = type ^ branch
+ *
+ * See jump_label_type() / jump_label_init_type().
+ */
+
+extern bool ____wrong_branch_error(void);
+
+#define static_branch_likely(x) \
+({ \
+ bool branch; \
+ if (__builtin_types_compatible_p(typeof(*x), struct static_key_true)) \
+ branch = !arch_static_branch(&(x)->key, true); \
+ else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
+ branch = !arch_static_branch_jump(&(x)->key, true); \
+ else \
+ branch = ____wrong_branch_error(); \
+ branch; \
+})
+
+#define static_branch_unlikely(x) \
+({ \
+ bool branch; \
+ if (__builtin_types_compatible_p(typeof(*x), struct static_key_true)) \
+ branch = arch_static_branch_jump(&(x)->key, false); \
+ else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
+ branch = arch_static_branch(&(x)->key, false); \
+ else \
+ branch = ____wrong_branch_error(); \
+ branch; \
+})
+
+#else /* !HAVE_JUMP_LABEL */
+
+#define static_branch_likely(x) likely(static_key_enabled(&(x)->key))
+#define static_branch_unlikely(x) unlikely(static_key_enabled(&(x)->key))
+
+#endif /* HAVE_JUMP_LABEL */
+
+/*
+ * Advanced usage; refcount, branch is enabled when: count != 0
+ */
+
+#define static_branch_inc(x) static_key_slow_inc(&(x)->key)
+#define static_branch_dec(x) static_key_slow_dec(&(x)->key)
+
+/*
+ * Normal usage; boolean enable/disable.
+ */
+
+#define static_branch_enable(x) static_key_enable(&(x)->key)
+#define static_branch_disable(x) static_key_disable(&(x)->key)
+
#endif /* _LINUX_JUMP_LABEL_H */
#endif /* __ASSEMBLY__ */