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  * bit 0 => 1 if key is initially true
  94  *          0 if initially false
  95  * bit 1 => 1 if points to struct static_key_mod
  96  *          0 if points to struct jump_entry
  97  */
  98         union {
  99                 unsigned long type;
 100                 struct jump_entry *entries;
 101                 struct static_key_mod *next;
 102         };
 103 };
 104
 105 #else
 106 struct static_key {
 107         atomic_t enabled;
 108 };
 109 #endif  /* HAVE_JUMP_LABEL */
 110 #endif /* __ASSEMBLY__ */
 111
 112 #ifdef HAVE_JUMP_LABEL
 113 #include <asm/jump_label.h>
 114 #endif
 115
 116 #ifndef __ASSEMBLY__
 117
 118 enum jump_label_type {
 119         JUMP_LABEL_NOP = 0,
 120         JUMP_LABEL_JMP,
 121 };
 122
 123 struct module;
 124
 125 #ifdef HAVE_JUMP_LABEL
 126
 127 #define JUMP_TYPE_FALSE         0UL
 128 #define JUMP_TYPE_TRUE          1UL
 129 #define JUMP_TYPE_LINKED        2UL
 130 #define JUMP_TYPE_MASK          3UL
 131
 132 static __always_inline bool static_key_false(struct static_key *key)
 133 {
 134         return arch_static_branch(key, false);
 135 }
 136
 137 static __always_inline bool static_key_true(struct static_key *key)
 138 {
 139         return !arch_static_branch(key, true);
 140 }
 141
 142 extern struct jump_entry __start___jump_table[];
 143 extern struct jump_entry __stop___jump_table[];
 144
 145 extern void jump_label_init(void);
 146 extern void jump_label_lock(void);
 147 extern void jump_label_unlock(void);
 148 extern void arch_jump_label_transform(struct jump_entry *entry,
 149                                       enum jump_label_type type);
 150 extern void arch_jump_label_transform_static(struct jump_entry *entry,
 151                                              enum jump_label_type type);
 152 extern int jump_label_text_reserved(void *start, void *end);
 153 extern void static_key_slow_inc(struct static_key *key);
 154 extern void static_key_slow_dec(struct static_key *key);
 155 extern void jump_label_apply_nops(struct module *mod);
 156 extern int static_key_count(struct static_key *key);
 157 extern void static_key_enable(struct static_key *key);
 158 extern void static_key_disable(struct static_key *key);
 159
 160 /*
 161  * We should be using ATOMIC_INIT() for initializing .enabled, but
 162  * the inclusion of atomic.h is problematic for inclusion of jump_label.h
 163  * in 'low-level' headers. Thus, we are initializing .enabled with a
 164  * raw value, but have added a BUILD_BUG_ON() to catch any issues in
 165  * jump_label_init() see: kernel/jump_label.c.
 166  */
 167 #define STATIC_KEY_INIT_TRUE                                    \
 168         { .enabled = { 1 },                                     \
 169           .entries = (void *)JUMP_TYPE_TRUE }
 170 #define STATIC_KEY_INIT_FALSE                                   \
 171         { .enabled = { 0 },                                     \
 172           .entries = (void *)JUMP_TYPE_FALSE }
 173
 174 #else  /* !HAVE_JUMP_LABEL */
 175
 176 #include <linux/atomic.h>
 177 #include <linux/bug.h>
 178
 179 static inline int static_key_count(struct static_key *key)
 180 {
 181         return atomic_read(&key->enabled);
 182 }
 183
 184 static __always_inline void jump_label_init(void)
 185 {
 186         static_key_initialized = true;
 187 }
 188
 189 static __always_inline bool static_key_false(struct static_key *key)
 190 {
 191         if (unlikely(static_key_count(key) > 0))
 192                 return true;
 193         return false;
 194 }
 195
 196 static __always_inline bool static_key_true(struct static_key *key)
 197 {
 198         if (likely(static_key_count(key) > 0))
 199                 return true;
 200         return false;
 201 }
 202
 203 static inline void static_key_slow_inc(struct static_key *key)
 204 {
 205         STATIC_KEY_CHECK_USE();
 206         atomic_inc(&key->enabled);
 207 }
 208
 209 static inline void static_key_slow_dec(struct static_key *key)
 210 {
 211         STATIC_KEY_CHECK_USE();
 212         atomic_dec(&key->enabled);
 213 }
 214
 215 static inline int jump_label_text_reserved(void *start, void *end)
 216 {
 217         return 0;
 218 }
 219
 220 static inline void jump_label_lock(void) {}
 221 static inline void jump_label_unlock(void) {}
 222
 223 static inline int jump_label_apply_nops(struct module *mod)
 224 {
 225         return 0;
 226 }
 227
 228 static inline void static_key_enable(struct static_key *key)
 229 {
 230         int count = static_key_count(key);
 231
 232         WARN_ON_ONCE(count < 0 || count > 1);
 233
 234         if (!count)
 235                 static_key_slow_inc(key);
 236 }
 237
 238 static inline void static_key_disable(struct static_key *key)
 239 {
 240         int count = static_key_count(key);
 241
 242         WARN_ON_ONCE(count < 0 || count > 1);
 243
 244         if (count)
 245                 static_key_slow_dec(key);
 246 }
 247
 248 #define STATIC_KEY_INIT_TRUE    { .enabled = ATOMIC_INIT(1) }
 249 #define STATIC_KEY_INIT_FALSE   { .enabled = ATOMIC_INIT(0) }
 250
 251 #endif  /* HAVE_JUMP_LABEL */
 252
 253 #define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
 254 #define jump_label_enabled static_key_enabled
 255
 256 /* -------------------------------------------------------------------------- */
 257
 258 /*
 259  * Two type wrappers around static_key, such that we can use compile time
 260  * type differentiation to emit the right code.
 261  *
 262  * All the below code is macros in order to play type games.
 263  */
 264
 265 struct static_key_true {
 266         struct static_key key;
 267 };
 268
 269 struct static_key_false {
 270         struct static_key key;
 271 };
 272
 273 #define STATIC_KEY_TRUE_INIT  (struct static_key_true) { .key = STATIC_KEY_INIT_TRUE,  }
 274 #define STATIC_KEY_FALSE_INIT (struct static_key_false){ .key = STATIC_KEY_INIT_FALSE, }
 275
 276 #define DEFINE_STATIC_KEY_TRUE(name)    \
 277         struct static_key_true name = STATIC_KEY_TRUE_INIT
 278
 279 #define DECLARE_STATIC_KEY_TRUE(name)   \
 280         extern struct static_key_true name
 281
 282 #define DEFINE_STATIC_KEY_FALSE(name)   \
 283         struct static_key_false name = STATIC_KEY_FALSE_INIT
 284
 285 #define DECLARE_STATIC_KEY_FALSE(name)  \
 286         extern struct static_key_false name
 287
 288 #define DEFINE_STATIC_KEY_ARRAY_TRUE(name, count)               \
 289         struct static_key_true name[count] = {                  \
 290                 [0 ... (count) - 1] = STATIC_KEY_TRUE_INIT,     \
 291         }
 292
 293 #define DEFINE_STATIC_KEY_ARRAY_FALSE(name, count)              \
 294         struct static_key_false name[count] = {                 \
 295                 [0 ... (count) - 1] = STATIC_KEY_FALSE_INIT,    \
 296         }
 297
 298 extern bool ____wrong_branch_error(void);
 299
 300 #define static_key_enabled(x)                                                   \
 301 ({                                                                              \
 302         if (!__builtin_types_compatible_p(typeof(*x), struct static_key) &&     \
 303             !__builtin_types_compatible_p(typeof(*x), struct static_key_true) &&\
 304             !__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
 305                 ____wrong_branch_error();                                       \
 306         static_key_count((struct static_key *)x) > 0;                           \
 307 })
 308
 309 #ifdef HAVE_JUMP_LABEL
 310
 311 /*
 312  * Combine the right initial value (type) with the right branch order
 313  * to generate the desired result.
 314  *
 315  *
 316  * type\branch| likely (1)            | unlikely (0)
 317  * -----------+-----------------------+------------------
 318  *            |                       |
 319  *  true (1)  |    ...                |    ...
 320  *            |    NOP                |    JMP L
 321  *            |    <br-stmts>         | 1: ...
 322  *            | L: ...                |
 323  *            |                       |
 324  *            |                       | L: <br-stmts>
 325  *            |                       |    jmp 1b
 326  *            |                       |
 327  * -----------+-----------------------+------------------
 328  *            |                       |
 329  *  false (0) |    ...                |    ...
 330  *            |    JMP L              |    NOP
 331  *            |    <br-stmts>         | 1: ...
 332  *            | L: ...                |
 333  *            |                       |
 334  *            |                       | L: <br-stmts>
 335  *            |                       |    jmp 1b
 336  *            |                       |
 337  * -----------+-----------------------+------------------
 338  *
 339  * The initial value is encoded in the LSB of static_key::entries,
 340  * type: 0 = false, 1 = true.
 341  *
 342  * The branch type is encoded in the LSB of jump_entry::key,
 343  * branch: 0 = unlikely, 1 = likely.
 344  *
 345  * This gives the following logic table:
 346  *
 347  *      enabled type    branch    instuction
 348  * -----------------------------+-----------
 349  *      0       0       0       | NOP
 350  *      0       0       1       | JMP
 351  *      0       1       0       | NOP
 352  *      0       1       1       | JMP
 353  *
 354  *      1       0       0       | JMP
 355  *      1       0       1       | NOP
 356  *      1       1       0       | JMP
 357  *      1       1       1       | NOP
 358  *
 359  * Which gives the following functions:
 360  *
 361  *   dynamic: instruction = enabled ^ branch
 362  *   static:  instruction = type ^ branch
 363  *
 364  * See jump_label_type() / jump_label_init_type().
 365  */
 366
 367 #define static_branch_likely(x)                                                 \
 368 ({                                                                              \
 369         bool branch;                                                            \
 370         if (__builtin_types_compatible_p(typeof(*x), struct static_key_true))   \
 371                 branch = !arch_static_branch(&(x)->key, true);                  \
 372         else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
 373                 branch = !arch_static_branch_jump(&(x)->key, true);             \
 374         else                                                                    \
 375                 branch = ____wrong_branch_error();                              \
 376         branch;                                                                 \
 377 })
 378
 379 #define static_branch_unlikely(x)                                               \
 380 ({                                                                              \
 381         bool branch;                                                            \
 382         if (__builtin_types_compatible_p(typeof(*x), struct static_key_true))   \
 383                 branch = arch_static_branch_jump(&(x)->key, false);             \
 384         else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
 385                 branch = arch_static_branch(&(x)->key, false);                  \
 386         else                                                                    \
 387                 branch = ____wrong_branch_error();                              \
 388         branch;                                                                 \
 389 })
 390
 391 #else /* !HAVE_JUMP_LABEL */
 392
 393 #define static_branch_likely(x)         likely(static_key_enabled(&(x)->key))
 394 #define static_branch_unlikely(x)       unlikely(static_key_enabled(&(x)->key))
 395
 396 #endif /* HAVE_JUMP_LABEL */
 397
 398 /*
 399  * Advanced usage; refcount, branch is enabled when: count != 0
 400  */
 401
 402 #define static_branch_inc(x)            static_key_slow_inc(&(x)->key)
 403 #define static_branch_dec(x)            static_key_slow_dec(&(x)->key)
 404
 405 /*
 406  * Normal usage; boolean enable/disable.
 407  */
 408
 409 #define static_branch_enable(x)         static_key_enable(&(x)->key)
 410 #define static_branch_disable(x)        static_key_disable(&(x)->key)
 411
 412 #endif /* __ASSEMBLY__ */
 413
 414 #endif  /* _LINUX_JUMP_LABEL_H */