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jump_label.h
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1 #ifndef _LINUX_JUMP_LABEL_H
2 #define _LINUX_JUMP_LABEL_H
3 
4 /*
5  * Jump label support
6  *
7  * Copyright (C) 2009-2012 Jason Baron <[email protected]>
8  * Copyright (C) 2011-2012 Peter Zijlstra <[email protected]>
9  *
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 a unconditional branch (which
13  * defaults to false - and the true block is placed out of line).
14  *
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.
19  *
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.
26  *
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.
31  *
32  * Lacking toolchain and or architecture support, it falls back to a simple
33  * conditional branch.
34  *
35  * struct static_key my_key = STATIC_KEY_INIT_TRUE;
36  *
37  * if (static_key_true(&my_key)) {
38  * }
39  *
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
42  * allowed.
43  *
44  * Not initializing the key (static data is initialized to 0s anyway) is the
45  * same as using STATIC_KEY_INIT_FALSE.
46  *
47 */
48 
49 #include <linux/types.h>
50 #include <linux/compiler.h>
51 #include <linux/workqueue.h>
52 
53 #if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
54 
55 struct static_key {
57 /* Set lsb bit to 1 if branch is default true, 0 ot */
58  struct jump_entry *entries;
59 #ifdef CONFIG_MODULES
60  struct static_key_mod *next;
61 #endif
62 };
63 
64 struct static_key_deferred {
65  struct static_key key;
66  unsigned long timeout;
67  struct delayed_work work;
68 };
69 
70 # include <asm/jump_label.h>
71 # define HAVE_JUMP_LABEL
72 #endif /* CC_HAVE_ASM_GOTO && CONFIG_JUMP_LABEL */
73 
77 };
78 
79 struct module;
80 
81 #ifdef HAVE_JUMP_LABEL
82 
83 #define JUMP_LABEL_TRUE_BRANCH 1UL
84 
85 static
86 inline struct jump_entry *jump_label_get_entries(struct static_key *key)
87 {
88  return (struct jump_entry *)((unsigned long)key->entries
89  & ~JUMP_LABEL_TRUE_BRANCH);
90 }
91 
92 static inline bool jump_label_get_branch_default(struct static_key *key)
93 {
94  if ((unsigned long)key->entries & JUMP_LABEL_TRUE_BRANCH)
95  return true;
96  return false;
97 }
98 
99 static __always_inline bool static_key_false(struct static_key *key)
100 {
101  return arch_static_branch(key);
102 }
103 
104 static __always_inline bool static_key_true(struct static_key *key)
105 {
106  return !static_key_false(key);
107 }
108 
109 extern struct jump_entry __start___jump_table[];
110 extern struct jump_entry __stop___jump_table[];
111 
112 extern void jump_label_init(void);
113 extern void jump_label_lock(void);
114 extern void jump_label_unlock(void);
115 extern void arch_jump_label_transform(struct jump_entry *entry,
116  enum jump_label_type type);
117 extern void arch_jump_label_transform_static(struct jump_entry *entry,
118  enum jump_label_type type);
119 extern int jump_label_text_reserved(void *start, void *end);
120 extern void static_key_slow_inc(struct static_key *key);
121 extern void static_key_slow_dec(struct static_key *key);
122 extern void static_key_slow_dec_deferred(struct static_key_deferred *key);
123 extern void jump_label_apply_nops(struct module *mod);
124 extern void
125 jump_label_rate_limit(struct static_key_deferred *key, unsigned long rl);
126 
127 #define STATIC_KEY_INIT_TRUE ((struct static_key) \
128  { .enabled = ATOMIC_INIT(1), .entries = (void *)1 })
129 #define STATIC_KEY_INIT_FALSE ((struct static_key) \
130  { .enabled = ATOMIC_INIT(0), .entries = (void *)0 })
131 
132 #else /* !HAVE_JUMP_LABEL */
133 
134 #include <linux/atomic.h>
135 
136 struct static_key {
138 };
139 
140 static __always_inline void jump_label_init(void)
141 {
142 }
143 
145  struct static_key key;
146 };
147 
148 static __always_inline bool static_key_false(struct static_key *key)
149 {
150  if (unlikely(atomic_read(&key->enabled)) > 0)
151  return true;
152  return false;
153 }
154 
155 static __always_inline bool static_key_true(struct static_key *key)
156 {
157  if (likely(atomic_read(&key->enabled)) > 0)
158  return true;
159  return false;
160 }
161 
162 static inline void static_key_slow_inc(struct static_key *key)
163 {
164  atomic_inc(&key->enabled);
165 }
166 
167 static inline void static_key_slow_dec(struct static_key *key)
168 {
169  atomic_dec(&key->enabled);
170 }
171 
172 static inline void static_key_slow_dec_deferred(struct static_key_deferred *key)
173 {
174  static_key_slow_dec(&key->key);
175 }
176 
177 static inline int jump_label_text_reserved(void *start, void *end)
178 {
179  return 0;
180 }
181 
182 static inline void jump_label_lock(void) {}
183 static inline void jump_label_unlock(void) {}
184 
185 static inline int jump_label_apply_nops(struct module *mod)
186 {
187  return 0;
188 }
189 
190 static inline void
191 jump_label_rate_limit(struct static_key_deferred *key,
192  unsigned long rl)
193 {
194 }
195 
196 #define STATIC_KEY_INIT_TRUE ((struct static_key) \
197  { .enabled = ATOMIC_INIT(1) })
198 #define STATIC_KEY_INIT_FALSE ((struct static_key) \
199  { .enabled = ATOMIC_INIT(0) })
200 
201 #endif /* HAVE_JUMP_LABEL */
202 
203 #define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
204 #define jump_label_enabled static_key_enabled
205 
206 static inline bool static_key_enabled(struct static_key *key)
207 {
208  return (atomic_read(&key->enabled) > 0);
209 }
210 
211 #endif /* _LINUX_JUMP_LABEL_H */