1 #ifndef _LINUX_JUMP_LABEL_H
2 #define _LINUX_JUMP_LABEL_H
7 * Copyright (C) 2009-2012 Jason Baron <jbaron@redhat.com>
8 * Copyright (C) 2011-2012 Peter Zijlstra <pzijlstr@redhat.com>
10 * Jump labels provide an interface to generate dynamic branches using
11 * self-modifying code. Assuming toolchain and architecture support, the result
12 * of a "if (static_key_false(&key))" statement is an unconditional branch (which
13 * defaults to false - and the true block is placed out of line).
15 * However at runtime we can change the branch target using
16 * static_key_slow_{inc,dec}(). These function as a 'reference' count on the key
17 * object, and for as long as there are references all branches referring to
18 * that particular key will point to the (out of line) true block.
20 * Since this relies on modifying code, the static_key_slow_{inc,dec}() functions
21 * must be considered absolute slow paths (machine wide synchronization etc.).
22 * OTOH, since the affected branches are unconditional, their runtime overhead
23 * will be absolutely minimal, esp. in the default (off) case where the total
24 * effect is a single NOP of appropriate size. The on case will patch in a jump
25 * to the out-of-line block.
27 * When the control is directly exposed to userspace, it is prudent to delay the
28 * decrement to avoid high frequency code modifications which can (and do)
29 * cause significant performance degradation. Struct static_key_deferred and
30 * static_key_slow_dec_deferred() provide for this.
32 * Lacking toolchain and or architecture support, jump labels fall back to a simple
35 * struct static_key my_key = STATIC_KEY_INIT_TRUE;
37 * if (static_key_true(&my_key)) {
40 * will result in the true case being in-line and starts the key with a single
41 * reference. Mixing static_key_true() and static_key_false() on the same key is not
44 * Not initializing the key (static data is initialized to 0s anyway) is the
45 * same as using STATIC_KEY_INIT_FALSE.
48 #if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
49 # define HAVE_JUMP_LABEL
54 #include <linux/types.h>
55 #include <linux/compiler.h>
56 #include <linux/bug.h>
58 extern bool static_key_initialized;
60 #define STATIC_KEY_CHECK_USE() WARN(!static_key_initialized, \
61 "%s used before call to jump_label_init", \
64 #ifdef HAVE_JUMP_LABEL
68 /* Set lsb bit to 1 if branch is default true, 0 ot */
69 struct jump_entry *entries;
71 struct static_key_mod *next;
79 #endif /* HAVE_JUMP_LABEL */
80 #endif /* __ASSEMBLY__ */
82 #ifdef HAVE_JUMP_LABEL
83 #include <asm/jump_label.h>
88 enum jump_label_type {
95 #include <linux/atomic.h>
97 static inline int static_key_count(struct static_key *key)
99 return atomic_read(&key->enabled);
102 #ifdef HAVE_JUMP_LABEL
104 #define JUMP_TYPE_FALSE 0UL
105 #define JUMP_TYPE_TRUE 1UL
106 #define JUMP_TYPE_MASK 1UL
108 static __always_inline bool static_key_false(struct static_key *key)
110 return arch_static_branch(key);
113 static __always_inline bool static_key_true(struct static_key *key)
115 return !static_key_false(key);
118 extern struct jump_entry __start___jump_table[];
119 extern struct jump_entry __stop___jump_table[];
121 extern void jump_label_init(void);
122 extern void jump_label_lock(void);
123 extern void jump_label_unlock(void);
124 extern void arch_jump_label_transform(struct jump_entry *entry,
125 enum jump_label_type type);
126 extern void arch_jump_label_transform_static(struct jump_entry *entry,
127 enum jump_label_type type);
128 extern int jump_label_text_reserved(void *start, void *end);
129 extern void static_key_slow_inc(struct static_key *key);
130 extern void static_key_slow_dec(struct static_key *key);
131 extern void jump_label_apply_nops(struct module *mod);
133 #define STATIC_KEY_INIT_TRUE ((struct static_key) \
134 { .enabled = ATOMIC_INIT(1), \
135 .entries = (void *)JUMP_TYPE_TRUE })
136 #define STATIC_KEY_INIT_FALSE ((struct static_key) \
137 { .enabled = ATOMIC_INIT(0), \
138 .entries = (void *)JUMP_TYPE_FALSE })
140 #else /* !HAVE_JUMP_LABEL */
142 static __always_inline void jump_label_init(void)
144 static_key_initialized = true;
147 static __always_inline bool static_key_false(struct static_key *key)
149 if (unlikely(static_key_count(key) > 0))
154 static __always_inline bool static_key_true(struct static_key *key)
156 if (likely(static_key_count(key) > 0))
161 static inline void static_key_slow_inc(struct static_key *key)
163 STATIC_KEY_CHECK_USE();
164 atomic_inc(&key->enabled);
167 static inline void static_key_slow_dec(struct static_key *key)
169 STATIC_KEY_CHECK_USE();
170 atomic_dec(&key->enabled);
173 static inline int jump_label_text_reserved(void *start, void *end)
178 static inline void jump_label_lock(void) {}
179 static inline void jump_label_unlock(void) {}
181 static inline int jump_label_apply_nops(struct module *mod)
186 #define STATIC_KEY_INIT_TRUE ((struct static_key) \
187 { .enabled = ATOMIC_INIT(1) })
188 #define STATIC_KEY_INIT_FALSE ((struct static_key) \
189 { .enabled = ATOMIC_INIT(0) })
191 #endif /* HAVE_JUMP_LABEL */
193 #define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
194 #define jump_label_enabled static_key_enabled
196 static inline bool static_key_enabled(struct static_key *key)
198 return static_key_count(key) > 0;
201 #endif /* _LINUX_JUMP_LABEL_H */
203 #endif /* __ASSEMBLY__ */