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#else
  73struct static_key {
  74        atomic_t enabled;
  75};
  76#endif  /* CC_HAVE_ASM_GOTO && CONFIG_JUMP_LABEL */
  77
  78enum jump_label_type {
  79        JUMP_LABEL_DISABLE = 0,
  80        JUMP_LABEL_ENABLE,
  81};
  82
  83struct module;
  84
  85#include <linux/atomic.h>
  86
  87static inline int static_key_count(struct static_key *key)
  88{
  89        return atomic_read(&key->enabled);
  90}
  91
  92#ifdef HAVE_JUMP_LABEL
  93
  94#define JUMP_LABEL_TYPE_FALSE_BRANCH    0UL
  95#define JUMP_LABEL_TYPE_TRUE_BRANCH     1UL
  96#define JUMP_LABEL_TYPE_MASK            1UL
  97
  98static
  99inline struct jump_entry *jump_label_get_entries(struct static_key *key)
 100{
 101        return (struct jump_entry *)((unsigned long)key->entries
 102                                                & ~JUMP_LABEL_TYPE_MASK);
 103}
 104
 105static inline bool jump_label_get_branch_default(struct static_key *key)
 106{
 107        if (((unsigned long)key->entries & JUMP_LABEL_TYPE_MASK) ==
 108            JUMP_LABEL_TYPE_TRUE_BRANCH)
 109                return true;
 110        return false;
 111}
 112
 113static __always_inline bool static_key_false(struct static_key *key)
 114{
 115        return arch_static_branch(key);
 116}
 117
 118static __always_inline bool static_key_true(struct static_key *key)
 119{
 120        return !static_key_false(key);
 121}
 122
 123extern struct jump_entry __start___jump_table[];
 124extern struct jump_entry __stop___jump_table[];
 125
 126extern void jump_label_init(void);
 127extern void jump_label_lock(void);
 128extern void jump_label_unlock(void);
 129extern void arch_jump_label_transform(struct jump_entry *entry,
 130                                      enum jump_label_type type);
 131extern void arch_jump_label_transform_static(struct jump_entry *entry,
 132                                             enum jump_label_type type);
 133extern int jump_label_text_reserved(void *start, void *end);
 134extern void static_key_slow_inc(struct static_key *key);
 135extern void static_key_slow_dec(struct static_key *key);
 136extern void jump_label_apply_nops(struct module *mod);
 137
 138#define STATIC_KEY_INIT_TRUE ((struct static_key)               \
 139        { .enabled = ATOMIC_INIT(1),                            \
 140          .entries = (void *)JUMP_LABEL_TYPE_TRUE_BRANCH })
 141#define STATIC_KEY_INIT_FALSE ((struct static_key)              \
 142        { .enabled = ATOMIC_INIT(0),                            \
 143          .entries = (void *)JUMP_LABEL_TYPE_FALSE_BRANCH })
 144
 145#else  /* !HAVE_JUMP_LABEL */
 146
 147static __always_inline void jump_label_init(void)
 148{
 149        static_key_initialized = true;
 150}
 151
 152static __always_inline bool static_key_false(struct static_key *key)
 153{
 154        if (unlikely(static_key_count(key) > 0))
 155                return true;
 156        return false;
 157}
 158
 159static __always_inline bool static_key_true(struct static_key *key)
 160{
 161        if (likely(static_key_count(key) > 0))
 162                return true;
 163        return false;
 164}
 165
 166static inline void static_key_slow_inc(struct static_key *key)
 167{
 168        STATIC_KEY_CHECK_USE();
 169        atomic_inc(&key->enabled);
 170}
 171
 172static inline void static_key_slow_dec(struct static_key *key)
 173{
 174        STATIC_KEY_CHECK_USE();
 175        atomic_dec(&key->enabled);
 176}
 177
 178static inline int jump_label_text_reserved(void *start, void *end)
 179{
 180        return 0;
 181}
 182
 183static inline void jump_label_lock(void) {}
 184static inline void jump_label_unlock(void) {}
 185
 186static inline int jump_label_apply_nops(struct module *mod)
 187{
 188        return 0;
 189}
 190
 191#define STATIC_KEY_INIT_TRUE ((struct static_key) \
 192                { .enabled = ATOMIC_INIT(1) })
 193#define STATIC_KEY_INIT_FALSE ((struct static_key) \
 194                { .enabled = ATOMIC_INIT(0) })
 195
 196#endif  /* HAVE_JUMP_LABEL */
 197
 198#define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
 199#define jump_label_enabled static_key_enabled
 200
 201static inline bool static_key_enabled(struct static_key *key)
 202{
 203        return static_key_count(key) > 0;
 204}
 205
 206#endif  /* _LINUX_JUMP_LABEL_H */
 207