linux/include/linux/jump_label.h
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   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/workqueue.h>
  52
  53#if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
  54
  55struct static_key {
  56        atomic_t enabled;
  57/* Set lsb bit to 1 if branch is default true, 0 ot */
  58        struct jump_entry *entries;
  59#ifdef CONFIG_MODULES
  60        struct static_key_mod *next;
  61#endif
  62};
  63
  64struct static_key_deferred {
  65        struct static_key key;
  66        unsigned long timeout;
  67        struct delayed_work work;
  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#ifdef HAVE_JUMP_LABEL
  82
  83#define JUMP_LABEL_TRUE_BRANCH 1UL
  84
  85static
  86inline struct jump_entry *jump_label_get_entries(struct static_key *key)
  87{
  88        return (struct jump_entry *)((unsigned long)key->entries
  89                                                & ~JUMP_LABEL_TRUE_BRANCH);
  90}
  91
  92static inline bool jump_label_get_branch_default(struct static_key *key)
  93{
  94        if ((unsigned long)key->entries & JUMP_LABEL_TRUE_BRANCH)
  95                return true;
  96        return false;
  97}
  98
  99static __always_inline bool static_key_false(struct static_key *key)
 100{
 101        return arch_static_branch(key);
 102}
 103
 104static __always_inline bool static_key_true(struct static_key *key)
 105{
 106        return !static_key_false(key);
 107}
 108
 109extern struct jump_entry __start___jump_table[];
 110extern struct jump_entry __stop___jump_table[];
 111
 112extern void jump_label_init(void);
 113extern void jump_label_lock(void);
 114extern void jump_label_unlock(void);
 115extern void arch_jump_label_transform(struct jump_entry *entry,
 116                                      enum jump_label_type type);
 117extern void arch_jump_label_transform_static(struct jump_entry *entry,
 118                                             enum jump_label_type type);
 119extern int jump_label_text_reserved(void *start, void *end);
 120extern void static_key_slow_inc(struct static_key *key);
 121extern void static_key_slow_dec(struct static_key *key);
 122extern void static_key_slow_dec_deferred(struct static_key_deferred *key);
 123extern void jump_label_apply_nops(struct module *mod);
 124extern void
 125jump_label_rate_limit(struct static_key_deferred *key, unsigned long rl);
 126
 127#define STATIC_KEY_INIT_TRUE ((struct static_key) \
 128        { .enabled = ATOMIC_INIT(1), .entries = (void *)1 })
 129#define STATIC_KEY_INIT_FALSE ((struct static_key) \
 130        { .enabled = ATOMIC_INIT(0), .entries = (void *)0 })
 131
 132#else  /* !HAVE_JUMP_LABEL */
 133
 134#include <linux/atomic.h>
 135
 136struct static_key {
 137        atomic_t enabled;
 138};
 139
 140static __always_inline void jump_label_init(void)
 141{
 142}
 143
 144struct static_key_deferred {
 145        struct static_key  key;
 146};
 147
 148static __always_inline bool static_key_false(struct static_key *key)
 149{
 150        if (unlikely(atomic_read(&key->enabled)) > 0)
 151                return true;
 152        return false;
 153}
 154
 155static __always_inline bool static_key_true(struct static_key *key)
 156{
 157        if (likely(atomic_read(&key->enabled)) > 0)
 158                return true;
 159        return false;
 160}
 161
 162static inline void static_key_slow_inc(struct static_key *key)
 163{
 164        atomic_inc(&key->enabled);
 165}
 166
 167static inline void static_key_slow_dec(struct static_key *key)
 168{
 169        atomic_dec(&key->enabled);
 170}
 171
 172static inline void static_key_slow_dec_deferred(struct static_key_deferred *key)
 173{
 174        static_key_slow_dec(&key->key);
 175}
 176
 177static inline int jump_label_text_reserved(void *start, void *end)
 178{
 179        return 0;
 180}
 181
 182static inline void jump_label_lock(void) {}
 183static inline void jump_label_unlock(void) {}
 184
 185static inline int jump_label_apply_nops(struct module *mod)
 186{
 187        return 0;
 188}
 189
 190static inline void
 191jump_label_rate_limit(struct static_key_deferred *key,
 192                unsigned long rl)
 193{
 194}
 195
 196#define STATIC_KEY_INIT_TRUE ((struct static_key) \
 197                { .enabled = ATOMIC_INIT(1) })
 198#define STATIC_KEY_INIT_FALSE ((struct static_key) \
 199                { .enabled = ATOMIC_INIT(0) })
 200
 201#endif  /* HAVE_JUMP_LABEL */
 202
 203#define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
 204#define jump_label_enabled static_key_enabled
 205
 206static inline bool static_key_enabled(struct static_key *key)
 207{
 208        return (atomic_read(&key->enabled) > 0);
 209}
 210
 211#endif  /* _LINUX_JUMP_LABEL_H */
 212
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