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TOMOYO Linux Cross Reference
Linux/include/linux/jump_label.h

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  1 /* SPDX-License-Identifier: GPL-2.0 */
  2 #ifndef _LINUX_JUMP_LABEL_H
  3 #define _LINUX_JUMP_LABEL_H
  4 
  5 /*
  6  * Jump label support
  7  *
  8  * Copyright (C) 2009-2012 Jason Baron <jbaron@redhat.com>
  9  * Copyright (C) 2011-2012 Red Hat, Inc., Peter Zijlstra
 10  *
 11  * DEPRECATED API:
 12  *
 13  * The use of 'struct static_key' directly, is now DEPRECATED. In addition
 14  * static_key_{true,false}() is also DEPRECATED. IE DO NOT use the following:
 15  *
 16  * struct static_key false = STATIC_KEY_INIT_FALSE;
 17  * struct static_key true = STATIC_KEY_INIT_TRUE;
 18  * static_key_true()
 19  * static_key_false()
 20  *
 21  * The updated API replacements are:
 22  *
 23  * DEFINE_STATIC_KEY_TRUE(key);
 24  * DEFINE_STATIC_KEY_FALSE(key);
 25  * DEFINE_STATIC_KEY_ARRAY_TRUE(keys, count);
 26  * DEFINE_STATIC_KEY_ARRAY_FALSE(keys, count);
 27  * static_branch_likely()
 28  * static_branch_unlikely()
 29  *
 30  * Jump labels provide an interface to generate dynamic branches using
 31  * self-modifying code. Assuming toolchain and architecture support, if we
 32  * define a "key" that is initially false via "DEFINE_STATIC_KEY_FALSE(key)",
 33  * an "if (static_branch_unlikely(&key))" statement is an unconditional branch
 34  * (which defaults to false - and the true block is placed out of line).
 35  * Similarly, we can define an initially true key via
 36  * "DEFINE_STATIC_KEY_TRUE(key)", and use it in the same
 37  * "if (static_branch_unlikely(&key))", in which case we will generate an
 38  * unconditional branch to the out-of-line true branch. Keys that are
 39  * initially true or false can be using in both static_branch_unlikely()
 40  * and static_branch_likely() statements.
 41  *
 42  * At runtime we can change the branch target by setting the key
 43  * to true via a call to static_branch_enable(), or false using
 44  * static_branch_disable(). If the direction of the branch is switched by
 45  * these calls then we run-time modify the branch target via a
 46  * no-op -> jump or jump -> no-op conversion. For example, for an
 47  * initially false key that is used in an "if (static_branch_unlikely(&key))"
 48  * statement, setting the key to true requires us to patch in a jump
 49  * to the out-of-line of true branch.
 50  *
 51  * In addition to static_branch_{enable,disable}, we can also reference count
 52  * the key or branch direction via static_branch_{inc,dec}. Thus,
 53  * static_branch_inc() can be thought of as a 'make more true' and
 54  * static_branch_dec() as a 'make more false'.
 55  *
 56  * Since this relies on modifying code, the branch modifying functions
 57  * must be considered absolute slow paths (machine wide synchronization etc.).
 58  * OTOH, since the affected branches are unconditional, their runtime overhead
 59  * will be absolutely minimal, esp. in the default (off) case where the total
 60  * effect is a single NOP of appropriate size. The on case will patch in a jump
 61  * to the out-of-line block.
 62  *
 63  * When the control is directly exposed to userspace, it is prudent to delay the
 64  * decrement to avoid high frequency code modifications which can (and do)
 65  * cause significant performance degradation. Struct static_key_deferred and
 66  * static_key_slow_dec_deferred() provide for this.
 67  *
 68  * Lacking toolchain and or architecture support, static keys fall back to a
 69  * simple conditional branch.
 70  *
 71  * Additional babbling in: Documentation/static-keys.txt
 72  */
 73 
 74 #if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
 75 # define HAVE_JUMP_LABEL
 76 #endif
 77 
 78 #ifndef __ASSEMBLY__
 79 
 80 #include <linux/types.h>
 81 #include <linux/compiler.h>
 82 
 83 extern bool static_key_initialized;
 84 
 85 #define STATIC_KEY_CHECK_USE(key) WARN(!static_key_initialized,               \
 86                                     "%s(): static key '%pS' used before call to jump_label_init()", \
 87                                     __func__, (key))
 88 
 89 #ifdef HAVE_JUMP_LABEL
 90 
 91 struct static_key {
 92         atomic_t enabled;
 93 /*
 94  * Note:
 95  *   To make anonymous unions work with old compilers, the static
 96  *   initialization of them requires brackets. This creates a dependency
 97  *   on the order of the struct with the initializers. If any fields
 98  *   are added, STATIC_KEY_INIT_TRUE and STATIC_KEY_INIT_FALSE may need
 99  *   to be modified.
100  *
101  * bit 0 => 1 if key is initially true
102  *          0 if initially false
103  * bit 1 => 1 if points to struct static_key_mod
104  *          0 if points to struct jump_entry
105  */
106         union {
107                 unsigned long type;
108                 struct jump_entry *entries;
109                 struct static_key_mod *next;
110         };
111 };
112 
113 #else
114 struct static_key {
115         atomic_t enabled;
116 };
117 #endif  /* HAVE_JUMP_LABEL */
118 #endif /* __ASSEMBLY__ */
119 
120 #ifdef HAVE_JUMP_LABEL
121 #include <asm/jump_label.h>
122 #endif
123 
124 #ifndef __ASSEMBLY__
125 
126 enum jump_label_type {
127         JUMP_LABEL_NOP = 0,
128         JUMP_LABEL_JMP,
129 };
130 
131 struct module;
132 
133 #ifdef HAVE_JUMP_LABEL
134 
135 #define JUMP_TYPE_FALSE         0UL
136 #define JUMP_TYPE_TRUE          1UL
137 #define JUMP_TYPE_LINKED        2UL
138 #define JUMP_TYPE_MASK          3UL
139 
140 static __always_inline bool static_key_false(struct static_key *key)
141 {
142         return arch_static_branch(key, false);
143 }
144 
145 static __always_inline bool static_key_true(struct static_key *key)
146 {
147         return !arch_static_branch(key, true);
148 }
149 
150 extern struct jump_entry __start___jump_table[];
151 extern struct jump_entry __stop___jump_table[];
152 
153 extern void jump_label_init(void);
154 extern void jump_label_invalidate_initmem(void);
155 extern void jump_label_lock(void);
156 extern void jump_label_unlock(void);
157 extern void arch_jump_label_transform(struct jump_entry *entry,
158                                       enum jump_label_type type);
159 extern void arch_jump_label_transform_static(struct jump_entry *entry,
160                                              enum jump_label_type type);
161 extern int jump_label_text_reserved(void *start, void *end);
162 extern void static_key_slow_inc(struct static_key *key);
163 extern void static_key_slow_dec(struct static_key *key);
164 extern void static_key_slow_inc_cpuslocked(struct static_key *key);
165 extern void static_key_slow_dec_cpuslocked(struct static_key *key);
166 extern void jump_label_apply_nops(struct module *mod);
167 extern int static_key_count(struct static_key *key);
168 extern void static_key_enable(struct static_key *key);
169 extern void static_key_disable(struct static_key *key);
170 extern void static_key_enable_cpuslocked(struct static_key *key);
171 extern void static_key_disable_cpuslocked(struct static_key *key);
172 
173 /*
174  * We should be using ATOMIC_INIT() for initializing .enabled, but
175  * the inclusion of atomic.h is problematic for inclusion of jump_label.h
176  * in 'low-level' headers. Thus, we are initializing .enabled with a
177  * raw value, but have added a BUILD_BUG_ON() to catch any issues in
178  * jump_label_init() see: kernel/jump_label.c.
179  */
180 #define STATIC_KEY_INIT_TRUE                                    \
181         { .enabled = { 1 },                                     \
182           { .entries = (void *)JUMP_TYPE_TRUE } }
183 #define STATIC_KEY_INIT_FALSE                                   \
184         { .enabled = { 0 },                                     \
185           { .entries = (void *)JUMP_TYPE_FALSE } }
186 
187 #else  /* !HAVE_JUMP_LABEL */
188 
189 #include <linux/atomic.h>
190 #include <linux/bug.h>
191 
192 static inline int static_key_count(struct static_key *key)
193 {
194         return atomic_read(&key->enabled);
195 }
196 
197 static __always_inline void jump_label_init(void)
198 {
199         static_key_initialized = true;
200 }
201 
202 static inline void jump_label_invalidate_initmem(void) {}
203 
204 static __always_inline bool static_key_false(struct static_key *key)
205 {
206         if (unlikely(static_key_count(key) > 0))
207                 return true;
208         return false;
209 }
210 
211 static __always_inline bool static_key_true(struct static_key *key)
212 {
213         if (likely(static_key_count(key) > 0))
214                 return true;
215         return false;
216 }
217 
218 static inline void static_key_slow_inc(struct static_key *key)
219 {
220         STATIC_KEY_CHECK_USE(key);
221         atomic_inc(&key->enabled);
222 }
223 
224 static inline void static_key_slow_dec(struct static_key *key)
225 {
226         STATIC_KEY_CHECK_USE(key);
227         atomic_dec(&key->enabled);
228 }
229 
230 #define static_key_slow_inc_cpuslocked(key) static_key_slow_inc(key)
231 #define static_key_slow_dec_cpuslocked(key) static_key_slow_dec(key)
232 
233 static inline int jump_label_text_reserved(void *start, void *end)
234 {
235         return 0;
236 }
237 
238 static inline void jump_label_lock(void) {}
239 static inline void jump_label_unlock(void) {}
240 
241 static inline int jump_label_apply_nops(struct module *mod)
242 {
243         return 0;
244 }
245 
246 static inline void static_key_enable(struct static_key *key)
247 {
248         STATIC_KEY_CHECK_USE(key);
249 
250         if (atomic_read(&key->enabled) != 0) {
251                 WARN_ON_ONCE(atomic_read(&key->enabled) != 1);
252                 return;
253         }
254         atomic_set(&key->enabled, 1);
255 }
256 
257 static inline void static_key_disable(struct static_key *key)
258 {
259         STATIC_KEY_CHECK_USE(key);
260 
261         if (atomic_read(&key->enabled) != 1) {
262                 WARN_ON_ONCE(atomic_read(&key->enabled) != 0);
263                 return;
264         }
265         atomic_set(&key->enabled, 0);
266 }
267 
268 #define static_key_enable_cpuslocked(k)         static_key_enable((k))
269 #define static_key_disable_cpuslocked(k)        static_key_disable((k))
270 
271 #define STATIC_KEY_INIT_TRUE    { .enabled = ATOMIC_INIT(1) }
272 #define STATIC_KEY_INIT_FALSE   { .enabled = ATOMIC_INIT(0) }
273 
274 #endif  /* HAVE_JUMP_LABEL */
275 
276 #define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
277 #define jump_label_enabled static_key_enabled
278 
279 /* -------------------------------------------------------------------------- */
280 
281 /*
282  * Two type wrappers around static_key, such that we can use compile time
283  * type differentiation to emit the right code.
284  *
285  * All the below code is macros in order to play type games.
286  */
287 
288 struct static_key_true {
289         struct static_key key;
290 };
291 
292 struct static_key_false {
293         struct static_key key;
294 };
295 
296 #define STATIC_KEY_TRUE_INIT  (struct static_key_true) { .key = STATIC_KEY_INIT_TRUE,  }
297 #define STATIC_KEY_FALSE_INIT (struct static_key_false){ .key = STATIC_KEY_INIT_FALSE, }
298 
299 #define DEFINE_STATIC_KEY_TRUE(name)    \
300         struct static_key_true name = STATIC_KEY_TRUE_INIT
301 
302 #define DECLARE_STATIC_KEY_TRUE(name)   \
303         extern struct static_key_true name
304 
305 #define DEFINE_STATIC_KEY_FALSE(name)   \
306         struct static_key_false name = STATIC_KEY_FALSE_INIT
307 
308 #define DECLARE_STATIC_KEY_FALSE(name)  \
309         extern struct static_key_false name
310 
311 #define DEFINE_STATIC_KEY_ARRAY_TRUE(name, count)               \
312         struct static_key_true name[count] = {                  \
313                 [0 ... (count) - 1] = STATIC_KEY_TRUE_INIT,     \
314         }
315 
316 #define DEFINE_STATIC_KEY_ARRAY_FALSE(name, count)              \
317         struct static_key_false name[count] = {                 \
318                 [0 ... (count) - 1] = STATIC_KEY_FALSE_INIT,    \
319         }
320 
321 extern bool ____wrong_branch_error(void);
322 
323 #define static_key_enabled(x)                                                   \
324 ({                                                                              \
325         if (!__builtin_types_compatible_p(typeof(*x), struct static_key) &&     \
326             !__builtin_types_compatible_p(typeof(*x), struct static_key_true) &&\
327             !__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
328                 ____wrong_branch_error();                                       \
329         static_key_count((struct static_key *)x) > 0;                           \
330 })
331 
332 #ifdef HAVE_JUMP_LABEL
333 
334 /*
335  * Combine the right initial value (type) with the right branch order
336  * to generate the desired result.
337  *
338  *
339  * type\branch| likely (1)            | unlikely (0)
340  * -----------+-----------------------+------------------
341  *            |                       |
342  *  true (1)  |    ...                |    ...
343  *            |    NOP                |    JMP L
344  *            |    <br-stmts>         | 1: ...
345  *            | L: ...                |
346  *            |                       |
347  *            |                       | L: <br-stmts>
348  *            |                       |    jmp 1b
349  *            |                       |
350  * -----------+-----------------------+------------------
351  *            |                       |
352  *  false (0) |    ...                |    ...
353  *            |    JMP L              |    NOP
354  *            |    <br-stmts>         | 1: ...
355  *            | L: ...                |
356  *            |                       |
357  *            |                       | L: <br-stmts>
358  *            |                       |    jmp 1b
359  *            |                       |
360  * -----------+-----------------------+------------------
361  *
362  * The initial value is encoded in the LSB of static_key::entries,
363  * type: 0 = false, 1 = true.
364  *
365  * The branch type is encoded in the LSB of jump_entry::key,
366  * branch: 0 = unlikely, 1 = likely.
367  *
368  * This gives the following logic table:
369  *
370  *      enabled type    branch    instuction
371  * -----------------------------+-----------
372  *      0       0       0       | NOP
373  *      0       0       1       | JMP
374  *      0       1       0       | NOP
375  *      0       1       1       | JMP
376  *
377  *      1       0       0       | JMP
378  *      1       0       1       | NOP
379  *      1       1       0       | JMP
380  *      1       1       1       | NOP
381  *
382  * Which gives the following functions:
383  *
384  *   dynamic: instruction = enabled ^ branch
385  *   static:  instruction = type ^ branch
386  *
387  * See jump_label_type() / jump_label_init_type().
388  */
389 
390 #define static_branch_likely(x)                                                 \
391 ({                                                                              \
392         bool branch;                                                            \
393         if (__builtin_types_compatible_p(typeof(*x), struct static_key_true))   \
394                 branch = !arch_static_branch(&(x)->key, true);                  \
395         else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
396                 branch = !arch_static_branch_jump(&(x)->key, true);             \
397         else                                                                    \
398                 branch = ____wrong_branch_error();                              \
399         likely(branch);                                                         \
400 })
401 
402 #define static_branch_unlikely(x)                                               \
403 ({                                                                              \
404         bool branch;                                                            \
405         if (__builtin_types_compatible_p(typeof(*x), struct static_key_true))   \
406                 branch = arch_static_branch_jump(&(x)->key, false);             \
407         else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
408                 branch = arch_static_branch(&(x)->key, false);                  \
409         else                                                                    \
410                 branch = ____wrong_branch_error();                              \
411         unlikely(branch);                                                       \
412 })
413 
414 #else /* !HAVE_JUMP_LABEL */
415 
416 #define static_branch_likely(x)         likely(static_key_enabled(&(x)->key))
417 #define static_branch_unlikely(x)       unlikely(static_key_enabled(&(x)->key))
418 
419 #endif /* HAVE_JUMP_LABEL */
420 
421 /*
422  * Advanced usage; refcount, branch is enabled when: count != 0
423  */
424 
425 #define static_branch_inc(x)            static_key_slow_inc(&(x)->key)
426 #define static_branch_dec(x)            static_key_slow_dec(&(x)->key)
427 #define static_branch_inc_cpuslocked(x) static_key_slow_inc_cpuslocked(&(x)->key)
428 #define static_branch_dec_cpuslocked(x) static_key_slow_dec_cpuslocked(&(x)->key)
429 
430 /*
431  * Normal usage; boolean enable/disable.
432  */
433 
434 #define static_branch_enable(x)                 static_key_enable(&(x)->key)
435 #define static_branch_disable(x)                static_key_disable(&(x)->key)
436 #define static_branch_enable_cpuslocked(x)      static_key_enable_cpuslocked(&(x)->key)
437 #define static_branch_disable_cpuslocked(x)     static_key_disable_cpuslocked(&(x)->key)
438 
439 #endif /* __ASSEMBLY__ */
440 
441 #endif  /* _LINUX_JUMP_LABEL_H */
442 

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