Merge tag 'for-linus-20160523' of git://git.infradead.org/linux-mtd
[linux-2.6-block.git] / include / linux / jump_label.h
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 <jbaron@redhat.com>
8  * Copyright (C) 2011-2012 Red Hat, Inc., Peter Zijlstra
9  *
10  * DEPRECATED API:
11  *
12  * The use of 'struct static_key' directly, is now DEPRECATED. In addition
13  * static_key_{true,false}() is also DEPRECATED. IE DO NOT use the following:
14  *
15  * struct static_key false = STATIC_KEY_INIT_FALSE;
16  * struct static_key true = STATIC_KEY_INIT_TRUE;
17  * static_key_true()
18  * static_key_false()
19  *
20  * The updated API replacements are:
21  *
22  * DEFINE_STATIC_KEY_TRUE(key);
23  * DEFINE_STATIC_KEY_FALSE(key);
24  * static_branch_likely()
25  * static_branch_unlikely()
26  *
27  * Jump labels provide an interface to generate dynamic branches using
28  * self-modifying code. Assuming toolchain and architecture support, if we
29  * define a "key" that is initially false via "DEFINE_STATIC_KEY_FALSE(key)",
30  * an "if (static_branch_unlikely(&key))" statement is an unconditional branch
31  * (which defaults to false - and the true block is placed out of line).
32  * Similarly, we can define an initially true key via
33  * "DEFINE_STATIC_KEY_TRUE(key)", and use it in the same
34  * "if (static_branch_unlikely(&key))", in which case we will generate an
35  * unconditional branch to the out-of-line true branch. Keys that are
36  * initially true or false can be using in both static_branch_unlikely()
37  * and static_branch_likely() statements.
38  *
39  * At runtime we can change the branch target by setting the key
40  * to true via a call to static_branch_enable(), or false using
41  * static_branch_disable(). If the direction of the branch is switched by
42  * these calls then we run-time modify the branch target via a
43  * no-op -> jump or jump -> no-op conversion. For example, for an
44  * initially false key that is used in an "if (static_branch_unlikely(&key))"
45  * statement, setting the key to true requires us to patch in a jump
46  * to the out-of-line of true branch.
47  *
48  * In addition to static_branch_{enable,disable}, we can also reference count
49  * the key or branch direction via static_branch_{inc,dec}. Thus,
50  * static_branch_inc() can be thought of as a 'make more true' and
51  * static_branch_dec() as a 'make more false'.
52  *
53  * Since this relies on modifying code, the branch modifying functions
54  * must be considered absolute slow paths (machine wide synchronization etc.).
55  * OTOH, since the affected branches are unconditional, their runtime overhead
56  * will be absolutely minimal, esp. in the default (off) case where the total
57  * effect is a single NOP of appropriate size. The on case will patch in a jump
58  * to the out-of-line block.
59  *
60  * When the control is directly exposed to userspace, it is prudent to delay the
61  * decrement to avoid high frequency code modifications which can (and do)
62  * cause significant performance degradation. Struct static_key_deferred and
63  * static_key_slow_dec_deferred() provide for this.
64  *
65  * Lacking toolchain and or architecture support, static keys fall back to a
66  * simple conditional branch.
67  *
68  * Additional babbling in: Documentation/static-keys.txt
69  */
70
71 #if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
72 # define HAVE_JUMP_LABEL
73 #endif
74
75 #ifndef __ASSEMBLY__
76
77 #include <linux/types.h>
78 #include <linux/compiler.h>
79 #include <linux/bug.h>
80
81 extern bool static_key_initialized;
82
83 #define STATIC_KEY_CHECK_USE() WARN(!static_key_initialized,                  \
84                                     "%s used before call to jump_label_init", \
85                                     __func__)
86
87 #ifdef HAVE_JUMP_LABEL
88
89 struct static_key {
90         atomic_t enabled;
91 /* Set lsb bit to 1 if branch is default true, 0 ot */
92         struct jump_entry *entries;
93 #ifdef CONFIG_MODULES
94         struct static_key_mod *next;
95 #endif
96 };
97
98 #else
99 struct static_key {
100         atomic_t enabled;
101 };
102 #endif  /* HAVE_JUMP_LABEL */
103 #endif /* __ASSEMBLY__ */
104
105 #ifdef HAVE_JUMP_LABEL
106 #include <asm/jump_label.h>
107 #endif
108
109 #ifndef __ASSEMBLY__
110
111 enum jump_label_type {
112         JUMP_LABEL_NOP = 0,
113         JUMP_LABEL_JMP,
114 };
115
116 struct module;
117
118 #include <linux/atomic.h>
119
120 static inline int static_key_count(struct static_key *key)
121 {
122         return atomic_read(&key->enabled);
123 }
124
125 #ifdef HAVE_JUMP_LABEL
126
127 #define JUMP_TYPE_FALSE 0UL
128 #define JUMP_TYPE_TRUE  1UL
129 #define JUMP_TYPE_MASK  1UL
130
131 static __always_inline bool static_key_false(struct static_key *key)
132 {
133         return arch_static_branch(key, false);
134 }
135
136 static __always_inline bool static_key_true(struct static_key *key)
137 {
138         return !arch_static_branch(key, true);
139 }
140
141 extern struct jump_entry __start___jump_table[];
142 extern struct jump_entry __stop___jump_table[];
143
144 extern void jump_label_init(void);
145 extern void jump_label_lock(void);
146 extern void jump_label_unlock(void);
147 extern void arch_jump_label_transform(struct jump_entry *entry,
148                                       enum jump_label_type type);
149 extern void arch_jump_label_transform_static(struct jump_entry *entry,
150                                              enum jump_label_type type);
151 extern int jump_label_text_reserved(void *start, void *end);
152 extern void static_key_slow_inc(struct static_key *key);
153 extern void static_key_slow_dec(struct static_key *key);
154 extern void jump_label_apply_nops(struct module *mod);
155
156 #define STATIC_KEY_INIT_TRUE                                    \
157         { .enabled = ATOMIC_INIT(1),                            \
158           .entries = (void *)JUMP_TYPE_TRUE }
159 #define STATIC_KEY_INIT_FALSE                                   \
160         { .enabled = ATOMIC_INIT(0),                            \
161           .entries = (void *)JUMP_TYPE_FALSE }
162
163 #else  /* !HAVE_JUMP_LABEL */
164
165 static __always_inline void jump_label_init(void)
166 {
167         static_key_initialized = true;
168 }
169
170 static __always_inline bool static_key_false(struct static_key *key)
171 {
172         if (unlikely(static_key_count(key) > 0))
173                 return true;
174         return false;
175 }
176
177 static __always_inline bool static_key_true(struct static_key *key)
178 {
179         if (likely(static_key_count(key) > 0))
180                 return true;
181         return false;
182 }
183
184 static inline void static_key_slow_inc(struct static_key *key)
185 {
186         STATIC_KEY_CHECK_USE();
187         atomic_inc(&key->enabled);
188 }
189
190 static inline void static_key_slow_dec(struct static_key *key)
191 {
192         STATIC_KEY_CHECK_USE();
193         atomic_dec(&key->enabled);
194 }
195
196 static inline int jump_label_text_reserved(void *start, void *end)
197 {
198         return 0;
199 }
200
201 static inline void jump_label_lock(void) {}
202 static inline void jump_label_unlock(void) {}
203
204 static inline int jump_label_apply_nops(struct module *mod)
205 {
206         return 0;
207 }
208
209 #define STATIC_KEY_INIT_TRUE    { .enabled = ATOMIC_INIT(1) }
210 #define STATIC_KEY_INIT_FALSE   { .enabled = ATOMIC_INIT(0) }
211
212 #endif  /* HAVE_JUMP_LABEL */
213
214 #define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
215 #define jump_label_enabled static_key_enabled
216
217 static inline void static_key_enable(struct static_key *key)
218 {
219         int count = static_key_count(key);
220
221         WARN_ON_ONCE(count < 0 || count > 1);
222
223         if (!count)
224                 static_key_slow_inc(key);
225 }
226
227 static inline void static_key_disable(struct static_key *key)
228 {
229         int count = static_key_count(key);
230
231         WARN_ON_ONCE(count < 0 || count > 1);
232
233         if (count)
234                 static_key_slow_dec(key);
235 }
236
237 /* -------------------------------------------------------------------------- */
238
239 /*
240  * Two type wrappers around static_key, such that we can use compile time
241  * type differentiation to emit the right code.
242  *
243  * All the below code is macros in order to play type games.
244  */
245
246 struct static_key_true {
247         struct static_key key;
248 };
249
250 struct static_key_false {
251         struct static_key key;
252 };
253
254 #define STATIC_KEY_TRUE_INIT  (struct static_key_true) { .key = STATIC_KEY_INIT_TRUE,  }
255 #define STATIC_KEY_FALSE_INIT (struct static_key_false){ .key = STATIC_KEY_INIT_FALSE, }
256
257 #define DEFINE_STATIC_KEY_TRUE(name)    \
258         struct static_key_true name = STATIC_KEY_TRUE_INIT
259
260 #define DEFINE_STATIC_KEY_FALSE(name)   \
261         struct static_key_false name = STATIC_KEY_FALSE_INIT
262
263 extern bool ____wrong_branch_error(void);
264
265 #define static_key_enabled(x)                                                   \
266 ({                                                                              \
267         if (!__builtin_types_compatible_p(typeof(*x), struct static_key) &&     \
268             !__builtin_types_compatible_p(typeof(*x), struct static_key_true) &&\
269             !__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
270                 ____wrong_branch_error();                                       \
271         static_key_count((struct static_key *)x) > 0;                           \
272 })
273
274 #ifdef HAVE_JUMP_LABEL
275
276 /*
277  * Combine the right initial value (type) with the right branch order
278  * to generate the desired result.
279  *
280  *
281  * type\branch| likely (1)            | unlikely (0)
282  * -----------+-----------------------+------------------
283  *            |                       |
284  *  true (1)  |    ...                |    ...
285  *            |    NOP                |    JMP L
286  *            |    <br-stmts>         | 1: ...
287  *            | L: ...                |
288  *            |                       |
289  *            |                       | L: <br-stmts>
290  *            |                       |    jmp 1b
291  *            |                       |
292  * -----------+-----------------------+------------------
293  *            |                       |
294  *  false (0) |    ...                |    ...
295  *            |    JMP L              |    NOP
296  *            |    <br-stmts>         | 1: ...
297  *            | L: ...                |
298  *            |                       |
299  *            |                       | L: <br-stmts>
300  *            |                       |    jmp 1b
301  *            |                       |
302  * -----------+-----------------------+------------------
303  *
304  * The initial value is encoded in the LSB of static_key::entries,
305  * type: 0 = false, 1 = true.
306  *
307  * The branch type is encoded in the LSB of jump_entry::key,
308  * branch: 0 = unlikely, 1 = likely.
309  *
310  * This gives the following logic table:
311  *
312  *      enabled type    branch    instuction
313  * -----------------------------+-----------
314  *      0       0       0       | NOP
315  *      0       0       1       | JMP
316  *      0       1       0       | NOP
317  *      0       1       1       | JMP
318  *
319  *      1       0       0       | JMP
320  *      1       0       1       | NOP
321  *      1       1       0       | JMP
322  *      1       1       1       | NOP
323  *
324  * Which gives the following functions:
325  *
326  *   dynamic: instruction = enabled ^ branch
327  *   static:  instruction = type ^ branch
328  *
329  * See jump_label_type() / jump_label_init_type().
330  */
331
332 #define static_branch_likely(x)                                                 \
333 ({                                                                              \
334         bool branch;                                                            \
335         if (__builtin_types_compatible_p(typeof(*x), struct static_key_true))   \
336                 branch = !arch_static_branch(&(x)->key, true);                  \
337         else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
338                 branch = !arch_static_branch_jump(&(x)->key, true);             \
339         else                                                                    \
340                 branch = ____wrong_branch_error();                              \
341         branch;                                                                 \
342 })
343
344 #define static_branch_unlikely(x)                                               \
345 ({                                                                              \
346         bool branch;                                                            \
347         if (__builtin_types_compatible_p(typeof(*x), struct static_key_true))   \
348                 branch = arch_static_branch_jump(&(x)->key, false);             \
349         else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
350                 branch = arch_static_branch(&(x)->key, false);                  \
351         else                                                                    \
352                 branch = ____wrong_branch_error();                              \
353         branch;                                                                 \
354 })
355
356 #else /* !HAVE_JUMP_LABEL */
357
358 #define static_branch_likely(x)         likely(static_key_enabled(&(x)->key))
359 #define static_branch_unlikely(x)       unlikely(static_key_enabled(&(x)->key))
360
361 #endif /* HAVE_JUMP_LABEL */
362
363 /*
364  * Advanced usage; refcount, branch is enabled when: count != 0
365  */
366
367 #define static_branch_inc(x)            static_key_slow_inc(&(x)->key)
368 #define static_branch_dec(x)            static_key_slow_dec(&(x)->key)
369
370 /*
371  * Normal usage; boolean enable/disable.
372  */
373
374 #define static_branch_enable(x)         static_key_enable(&(x)->key)
375 #define static_branch_disable(x)        static_key_disable(&(x)->key)
376
377 #endif  /* _LINUX_JUMP_LABEL_H */
378
379 #endif /* __ASSEMBLY__ */