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 Peter Zijlstra <pzijlstr@redhat.com>
   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 an 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, jump labels fall 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 #if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
  49 # define HAVE_JUMP_LABEL
  50 #endif
  51
  52 #ifndef __ASSEMBLY__
  53
  54 #include <linux/types.h>
  55 #include <linux/compiler.h>
  56 #include <linux/bug.h>
  57
  58 extern bool static_key_initialized;
  59
  60 #define STATIC_KEY_CHECK_USE() WARN(!static_key_initialized,                  \
  61                                     "%s used before call to jump_label_init", \
  62                                     __func__)
  63
  64 #ifdef HAVE_JUMP_LABEL
  65
  66 struct static_key {
  67         atomic_t enabled;
  68 /* Set lsb bit to 1 if branch is default true, 0 ot */
  69         struct jump_entry *entries;
  70 #ifdef CONFIG_MODULES
  71         struct static_key_mod *next;
  72 #endif
  73 };
  74
  75 #else
  76 struct static_key {
  77         atomic_t enabled;
  78 };
  79 #endif  /* HAVE_JUMP_LABEL */
  80 #endif /* __ASSEMBLY__ */
  81
  82 #ifdef HAVE_JUMP_LABEL
  83 #include <asm/jump_label.h>
  84 #endif
  85
  86 #ifndef __ASSEMBLY__
  87
  88 enum jump_label_type {
  89         JUMP_LABEL_NOP = 0,
  90         JUMP_LABEL_JMP,
  91 };
  92
  93 struct module;
  94
  95 #include <linux/atomic.h>
  96
  97 static inline int static_key_count(struct static_key *key)
  98 {
  99         return atomic_read(&key->enabled);
 100 }
 101
 102 #ifdef HAVE_JUMP_LABEL
 103
 104 #define JUMP_TYPE_FALSE 0UL
 105 #define JUMP_TYPE_TRUE  1UL
 106 #define JUMP_TYPE_MASK  1UL
 107
 108 static __always_inline bool static_key_false(struct static_key *key)
 109 {
 110         return arch_static_branch(key, false);
 111 }
 112
 113 static __always_inline bool static_key_true(struct static_key *key)
 114 {
 115         return !arch_static_branch(key, true);
 116 }
 117
 118 extern struct jump_entry __start___jump_table[];
 119 extern struct jump_entry __stop___jump_table[];
 120
 121 extern void jump_label_init(void);
 122 extern void jump_label_lock(void);
 123 extern void jump_label_unlock(void);
 124 extern void arch_jump_label_transform(struct jump_entry *entry,
 125                                       enum jump_label_type type);
 126 extern void arch_jump_label_transform_static(struct jump_entry *entry,
 127                                              enum jump_label_type type);
 128 extern int jump_label_text_reserved(void *start, void *end);
 129 extern void static_key_slow_inc(struct static_key *key);
 130 extern void static_key_slow_dec(struct static_key *key);
 131 extern void jump_label_apply_nops(struct module *mod);
 132
 133 #define STATIC_KEY_INIT_TRUE                                    \
 134         { .enabled = ATOMIC_INIT(1),                            \
 135           .entries = (void *)JUMP_TYPE_TRUE }
 136 #define STATIC_KEY_INIT_FALSE                                   \
 137         { .enabled = ATOMIC_INIT(0),                            \
 138           .entries = (void *)JUMP_TYPE_FALSE }
 139
 140 #else  /* !HAVE_JUMP_LABEL */
 141
 142 static __always_inline void jump_label_init(void)
 143 {
 144         static_key_initialized = true;
 145 }
 146
 147 static __always_inline bool static_key_false(struct static_key *key)
 148 {
 149         if (unlikely(static_key_count(key) > 0))
 150                 return true;
 151         return false;
 152 }
 153
 154 static __always_inline bool static_key_true(struct static_key *key)
 155 {
 156         if (likely(static_key_count(key) > 0))
 157                 return true;
 158         return false;
 159 }
 160
 161 static inline void static_key_slow_inc(struct static_key *key)
 162 {
 163         STATIC_KEY_CHECK_USE();
 164         atomic_inc(&key->enabled);
 165 }
 166
 167 static inline void static_key_slow_dec(struct static_key *key)
 168 {
 169         STATIC_KEY_CHECK_USE();
 170         atomic_dec(&key->enabled);
 171 }
 172
 173 static inline int jump_label_text_reserved(void *start, void *end)
 174 {
 175         return 0;
 176 }
 177
 178 static inline void jump_label_lock(void) {}
 179 static inline void jump_label_unlock(void) {}
 180
 181 static inline int jump_label_apply_nops(struct module *mod)
 182 {
 183         return 0;
 184 }
 185
 186 #define STATIC_KEY_INIT_TRUE    { .enabled = ATOMIC_INIT(1) }
 187 #define STATIC_KEY_INIT_FALSE   { .enabled = ATOMIC_INIT(0) }
 188
 189 #endif  /* HAVE_JUMP_LABEL */
 190
 191 #define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
 192 #define jump_label_enabled static_key_enabled
 193
 194 static inline bool static_key_enabled(struct static_key *key)
 195 {
 196         return static_key_count(key) > 0;
 197 }
 198
 199 static inline void static_key_enable(struct static_key *key)
 200 {
 201         int count = static_key_count(key);
 202
 203         WARN_ON_ONCE(count < 0 || count > 1);
 204
 205         if (!count)
 206                 static_key_slow_inc(key);
 207 }
 208
 209 static inline void static_key_disable(struct static_key *key)
 210 {
 211         int count = static_key_count(key);
 212
 213         WARN_ON_ONCE(count < 0 || count > 1);
 214
 215         if (count)
 216                 static_key_slow_dec(key);
 217 }
 218
 219 /* -------------------------------------------------------------------------- */
 220
 221 /*
 222  * Two type wrappers around static_key, such that we can use compile time
 223  * type differentiation to emit the right code.
 224  *
 225  * All the below code is macros in order to play type games.
 226  */
 227
 228 struct static_key_true {
 229         struct static_key key;
 230 };
 231
 232 struct static_key_false {
 233         struct static_key key;
 234 };
 235
 236 #define STATIC_KEY_TRUE_INIT  (struct static_key_true) { .key = STATIC_KEY_INIT_TRUE,  }
 237 #define STATIC_KEY_FALSE_INIT (struct static_key_false){ .key = STATIC_KEY_INIT_FALSE, }
 238
 239 #define DEFINE_STATIC_KEY_TRUE(name)    \
 240         struct static_key_true name = STATIC_KEY_TRUE_INIT
 241
 242 #define DEFINE_STATIC_KEY_FALSE(name)   \
 243         struct static_key_false name = STATIC_KEY_FALSE_INIT
 244
 245 #ifdef HAVE_JUMP_LABEL
 246
 247 /*
 248  * Combine the right initial value (type) with the right branch order
 249  * to generate the desired result.
 250  *
 251  *
 252  * type\branch| likely (1)            | unlikely (0)
 253  * -----------+-----------------------+------------------
 254  *            |                       |
 255  *  true (1)  |    ...                |    ...
 256  *            |    NOP                |    JMP L
 257  *            |    <br-stmts>         | 1: ...
 258  *            | L: ...                |
 259  *            |                       |
 260  *            |                       | L: <br-stmts>
 261  *            |                       |    jmp 1b
 262  *            |                       |
 263  * -----------+-----------------------+------------------
 264  *            |                       |
 265  *  false (0) |    ...                |    ...
 266  *            |    JMP L              |    NOP
 267  *            |    <br-stmts>         | 1: ...
 268  *            | L: ...                |
 269  *            |                       |
 270  *            |                       | L: <br-stmts>
 271  *            |                       |    jmp 1b
 272  *            |                       |
 273  * -----------+-----------------------+------------------
 274  *
 275  * The initial value is encoded in the LSB of static_key::entries,
 276  * type: 0 = false, 1 = true.
 277  *
 278  * The branch type is encoded in the LSB of jump_entry::key,
 279  * branch: 0 = unlikely, 1 = likely.
 280  *
 281  * This gives the following logic table:
 282  *
 283  *      enabled type    branch    instuction
 284  * -----------------------------+-----------
 285  *      0       0       0       | NOP
 286  *      0       0       1       | JMP
 287  *      0       1       0       | NOP
 288  *      0       1       1       | JMP
 289  *
 290  *      1       0       0       | JMP
 291  *      1       0       1       | NOP
 292  *      1       1       0       | JMP
 293  *      1       1       1       | NOP
 294  *
 295  * Which gives the following functions:
 296  *
 297  *   dynamic: instruction = enabled ^ branch
 298  *   static:  instruction = type ^ branch
 299  *
 300  * See jump_label_type() / jump_label_init_type().
 301  */
 302
 303 extern bool ____wrong_branch_error(void);
 304
 305 #define static_branch_likely(x)                                                 \
 306 ({                                                                              \
 307         bool branch;                                                            \
 308         if (__builtin_types_compatible_p(typeof(*x), struct static_key_true))   \
 309                 branch = !arch_static_branch(&(x)->key, true);                  \
 310         else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
 311                 branch = !arch_static_branch_jump(&(x)->key, true);             \
 312         else                                                                    \
 313                 branch = ____wrong_branch_error();                              \
 314         branch;                                                                 \
 315 })
 316
 317 #define static_branch_unlikely(x)                                               \
 318 ({                                                                              \
 319         bool branch;                                                            \
 320         if (__builtin_types_compatible_p(typeof(*x), struct static_key_true))   \
 321                 branch = arch_static_branch_jump(&(x)->key, false);             \
 322         else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
 323                 branch = arch_static_branch(&(x)->key, false);                  \
 324         else                                                                    \
 325                 branch = ____wrong_branch_error();                              \
 326         branch;                                                                 \
 327 })
 328
 329 #else /* !HAVE_JUMP_LABEL */
 330
 331 #define static_branch_likely(x)         likely(static_key_enabled(&(x)->key))
 332 #define static_branch_unlikely(x)       unlikely(static_key_enabled(&(x)->key))
 333
 334 #endif /* HAVE_JUMP_LABEL */
 335
 336 /*
 337  * Advanced usage; refcount, branch is enabled when: count != 0
 338  */
 339
 340 #define static_branch_inc(x)            static_key_slow_inc(&(x)->key)
 341 #define static_branch_dec(x)            static_key_slow_dec(&(x)->key)
 342
 343 /*
 344  * Normal usage; boolean enable/disable.
 345  */
 346
 347 #define static_branch_enable(x)         static_key_enable(&(x)->key)
 348 #define static_branch_disable(x)        static_key_disable(&(x)->key)
 349
 350 #endif  /* _LINUX_JUMP_LABEL_H */
 351
 352 #endif /* __ASSEMBLY__ */