* Jump label support
*
* Copyright (C) 2009-2012 Jason Baron <jbaron@redhat.com>
- * Copyright (C) 2011-2012 Peter Zijlstra <pzijlstr@redhat.com>
+ * Copyright (C) 2011-2012 Red Hat, Inc., Peter Zijlstra
+ *
+ * DEPRECATED API:
+ *
+ * The use of 'struct static_key' directly, is now DEPRECATED. In addition
+ * static_key_{true,false}() is also DEPRECATED. IE DO NOT use the following:
+ *
+ * struct static_key false = STATIC_KEY_INIT_FALSE;
+ * struct static_key true = STATIC_KEY_INIT_TRUE;
+ * static_key_true()
+ * static_key_false()
+ *
+ * The updated API replacements are:
+ *
+ * DEFINE_STATIC_KEY_TRUE(key);
+ * DEFINE_STATIC_KEY_FALSE(key);
+ * static_branch_likely()
+ * static_branch_unlikely()
*
* Jump labels provide an interface to generate dynamic branches using
- * self-modifying code. Assuming toolchain and architecture support, the result
- * of a "if (static_key_false(&key))" statement is an unconditional branch (which
- * defaults to false - and the true block is placed out of line).
+ * self-modifying code. Assuming toolchain and architecture support, if we
+ * define a "key" that is initially false via "DEFINE_STATIC_KEY_FALSE(key)",
+ * an "if (static_branch_unlikely(&key))" statement is an unconditional branch
+ * (which defaults to false - and the true block is placed out of line).
+ * Similarly, we can define an initially true key via
+ * "DEFINE_STATIC_KEY_TRUE(key)", and use it in the same
+ * "if (static_branch_unlikely(&key))", in which case we will generate an
+ * unconditional branch to the out-of-line true branch. Keys that are
+ * initially true or false can be using in both static_branch_unlikely()
+ * and static_branch_likely() statements.
+ *
+ * At runtime we can change the branch target by setting the key
+ * to true via a call to static_branch_enable(), or false using
+ * static_branch_disable(). If the direction of the branch is switched by
+ * these calls then we run-time modify the branch target via a
+ * no-op -> jump or jump -> no-op conversion. For example, for an
+ * initially false key that is used in an "if (static_branch_unlikely(&key))"
+ * statement, setting the key to true requires us to patch in a jump
+ * to the out-of-line of true branch.
*
- * However at runtime we can change the branch target using
- * static_key_slow_{inc,dec}(). These function as a 'reference' count on the key
- * object, and for as long as there are references all branches referring to
- * that particular key will point to the (out of line) true block.
+ * In addition to static_branch_{enable,disable}, we can also reference count
+ * the key or branch direction via static_branch_{inc,dec}. Thus,
+ * static_branch_inc() can be thought of as a 'make more true' and
+ * static_branch_dec() as a 'make more false'.
*
- * Since this relies on modifying code, the static_key_slow_{inc,dec}() functions
+ * Since this relies on modifying code, the branch modifying functions
* must be considered absolute slow paths (machine wide synchronization etc.).
* OTOH, since the affected branches are unconditional, their runtime overhead
* will be absolutely minimal, esp. in the default (off) case where the total
* cause significant performance degradation. Struct static_key_deferred and
* static_key_slow_dec_deferred() provide for this.
*
- * Lacking toolchain and or architecture support, jump labels fall back to a simple
- * conditional branch.
- *
- * struct static_key my_key = STATIC_KEY_INIT_TRUE;
- *
- * if (static_key_true(&my_key)) {
- * }
+ * Lacking toolchain and or architecture support, static keys fall back to a
+ * simple conditional branch.
*
- * will result in the true case being in-line and starts the key with a single
- * reference. Mixing static_key_true() and static_key_false() on the same key is not
- * allowed.
- *
- * Not initializing the key (static data is initialized to 0s anyway) is the
- * same as using STATIC_KEY_INIT_FALSE.
+ * Additional babbling in: Documentation/static-keys.txt
*/
#if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
#ifndef __ASSEMBLY__
enum jump_label_type {
- JUMP_LABEL_DISABLE = 0,
- JUMP_LABEL_ENABLE,
+ JUMP_LABEL_NOP = 0,
+ JUMP_LABEL_JMP,
};
struct module;
#ifdef HAVE_JUMP_LABEL
-#define JUMP_LABEL_TYPE_FALSE_BRANCH 0UL
-#define JUMP_LABEL_TYPE_TRUE_BRANCH 1UL
-#define JUMP_LABEL_TYPE_MASK 1UL
-
-static
-inline struct jump_entry *jump_label_get_entries(struct static_key *key)
-{
- return (struct jump_entry *)((unsigned long)key->entries
- & ~JUMP_LABEL_TYPE_MASK);
-}
-
-static inline bool jump_label_get_branch_default(struct static_key *key)
-{
- if (((unsigned long)key->entries & JUMP_LABEL_TYPE_MASK) ==
- JUMP_LABEL_TYPE_TRUE_BRANCH)
- return true;
- return false;
-}
+#define JUMP_TYPE_FALSE 0UL
+#define JUMP_TYPE_TRUE 1UL
+#define JUMP_TYPE_MASK 1UL
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[];
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_LABEL_TYPE_TRUE_BRANCH })
-#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_LABEL_TYPE_FALSE_BRANCH })
+ .entries = (void *)JUMP_TYPE_FALSE }
#else /* !HAVE_JUMP_LABEL */
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 */
#define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
#define jump_label_enabled static_key_enabled
-static inline bool static_key_enabled(struct static_key *key)
+static inline void static_key_enable(struct static_key *key)
+{
+ int count = static_key_count(key);
+
+ WARN_ON_ONCE(count < 0 || count > 1);
+
+ if (!count)
+ static_key_slow_inc(key);
+}
+
+static inline void static_key_disable(struct static_key *key)
{
- return static_key_count(key) > 0;
+ int count = static_key_count(key);
+
+ WARN_ON_ONCE(count < 0 || count > 1);
+
+ if (count)
+ 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
+
+extern bool ____wrong_branch_error(void);
+
+#define static_key_enabled(x) \
+({ \
+ if (!__builtin_types_compatible_p(typeof(*x), struct static_key) && \
+ !__builtin_types_compatible_p(typeof(*x), struct static_key_true) &&\
+ !__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
+ ____wrong_branch_error(); \
+ static_key_count((struct static_key *)x) > 0; \
+})
+
+#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().
+ */
+
+#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__ */
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