LXR linux/include/linux/jump

   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 a 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, it falls 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
  49#include <linux/types.h>
  50#include <linux/compiler.h>
  51#include <linux/bug.h>
  52
  53extern bool static_key_initialized;
  54
  55#define STATIC_KEY_CHECK_USE() WARN(!static_key_initialized,                  \
  56                                    "%s used before call to jump_label_init", \
  57                                    __func__)
  58
  59#if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
  60
  61struct static_key {
  62        atomic_t enabled;
  63/* Set lsb bit to 1 if branch is default true, 0 ot */
  64        struct jump_entry *entries;
  65#ifdef CONFIG_MODULES
  66        struct static_key_mod *next;
  67#endif
  68};
  69
  70# include <asm/jump_label.h>
  71# define HAVE_JUMP_LABEL
  72#endif  /* CC_HAVE_ASM_GOTO && CONFIG_JUMP_LABEL */
  73
  74enum jump_label_type {
  75        JUMP_LABEL_DISABLE = 0,
  76        JUMP_LABEL_ENABLE,
  77};
  78
  79struct module;
  80
  81#include <linux/atomic.h>
  82#ifdef HAVE_JUMP_LABEL
  83
  84#define JUMP_LABEL_TYPE_FALSE_BRANCH    0UL
  85#define JUMP_LABEL_TYPE_TRUE_BRANCH     1UL
  86#define JUMP_LABEL_TYPE_MASK            1UL
  87
  88static
  89inline struct jump_entry *jump_label_get_entries(struct static_key *key)
  90{
  91        return (struct jump_entry *)((unsigned long)key->entries
  92                                                & ~JUMP_LABEL_TYPE_MASK);
  93}
  94
  95static inline bool jump_label_get_branch_default(struct static_key *key)
  96{
  97        if (((unsigned long)key->entries & JUMP_LABEL_TYPE_MASK) ==
  98            JUMP_LABEL_TYPE_TRUE_BRANCH)
  99                return true;
 100        return false;
 101}
 102
 103static __always_inline bool static_key_false(struct static_key *key)
 104{
 105        return arch_static_branch(key);
 106}
 107
 108static __always_inline bool static_key_true(struct static_key *key)
 109{
 110        return !static_key_false(key);
 111}
 112
 113extern struct jump_entry __start___jump_table[];
 114extern struct jump_entry __stop___jump_table[];
 115
 116extern void jump_label_init(void);
 117extern void jump_label_lock(void);
 118extern void jump_label_unlock(void);
 119extern void arch_jump_label_transform(struct jump_entry *entry,
 120                                      enum jump_label_type type);
 121extern void arch_jump_label_transform_static(struct jump_entry *entry,
 122                                             enum jump_label_type type);
 123extern int jump_label_text_reserved(void *start, void *end);
 124extern void static_key_slow_inc(struct static_key *key);
 125extern void static_key_slow_dec(struct static_key *key);
 126extern void jump_label_apply_nops(struct module *mod);
 127
 128#define STATIC_KEY_INIT_TRUE ((struct static_key)               \
 129        { .enabled = ATOMIC_INIT(1),                            \
 130          .entries = (void *)JUMP_LABEL_TYPE_TRUE_BRANCH })
 131#define STATIC_KEY_INIT_FALSE ((struct static_key)              \
 132        { .enabled = ATOMIC_INIT(0),                            \
 133          .entries = (void *)JUMP_LABEL_TYPE_FALSE_BRANCH })
 134
 135#else  /* !HAVE_JUMP_LABEL */
 136
 137struct static_key {
 138        atomic_t enabled;
 139};
 140
 141static __always_inline void jump_label_init(void)
 142{
 143        static_key_initialized = true;
 144}
 145
 146static __always_inline bool static_key_false(struct static_key *key)
 147{
 148        if (unlikely(atomic_read(&key->enabled) > 0))
 149                return true;
 150        return false;
 151}
 152
 153static __always_inline bool static_key_true(struct static_key *key)
 154{
 155        if (likely(atomic_read(&key->enabled) > 0))
 156                return true;
 157        return false;
 158}
 159
 160static inline void static_key_slow_inc(struct static_key *key)
 161{
 162        STATIC_KEY_CHECK_USE();
 163        atomic_inc(&key->enabled);
 164}
 165
 166static inline void static_key_slow_dec(struct static_key *key)
 167{
 168        STATIC_KEY_CHECK_USE();
 169        atomic_dec(&key->enabled);
 170}
 171
 172static inline int jump_label_text_reserved(void *start, void *end)
 173{
 174        return 0;
 175}
 176
 177static inline void jump_label_lock(void) {}
 178static inline void jump_label_unlock(void) {}
 179
 180static inline int jump_label_apply_nops(struct module *mod)
 181{
 182        return 0;
 183}
 184
 185#define STATIC_KEY_INIT_TRUE ((struct static_key) \
 186                { .enabled = ATOMIC_INIT(1) })
 187#define STATIC_KEY_INIT_FALSE ((struct static_key) \
 188                { .enabled = ATOMIC_INIT(0) })
 189
 190#endif  /* HAVE_JUMP_LABEL */
 191
 192#define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
 193#define jump_label_enabled static_key_enabled
 194
 195static inline bool static_key_enabled(struct static_key *key)
 196{
 197        return (atomic_read(&key->enabled) > 0);
 198}
 199
 200#endif  /* _LINUX_JUMP_LABEL_H */
 201