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