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 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#include <linux/types.h>
  49#include <linux/compiler.h>
  50#include <linux/bug.h>
  51
  52extern bool static_key_initialized;
  53
  54#define STATIC_KEY_CHECK_USE() WARN(!static_key_initialized,                  \
  55                                    "%s used before call to jump_label_init", \
  56                                    __func__)
  57
  58#if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
  59
  60struct static_key {
  61        atomic_t enabled;
  62/* Set lsb bit to 1 if branch is default true, 0 ot */
  63        struct jump_entry *entries;
  64#ifdef CONFIG_MODULES
  65        struct static_key_mod *next;
  66#endif
  67};
  68
  69# include <asm/jump_label.h>
  70# define HAVE_JUMP_LABEL
  71#else
  72struct static_key {
  73        atomic_t enabled;
  74};
  75#endif  /* CC_HAVE_ASM_GOTO && CONFIG_JUMP_LABEL */
  76
  77enum jump_label_type {
  78        JUMP_LABEL_DISABLE = 0,
  79        JUMP_LABEL_ENABLE,
  80};
  81
  82struct module;
  83
  84#include <linux/atomic.h>
  85
  86static inline int static_key_count(struct static_key *key)
  87{
  88        return atomic_read(&key->enabled);
  89}
  90
  91#ifdef HAVE_JUMP_LABEL
  92
  93#define JUMP_LABEL_TYPE_FALSE_BRANCH    0UL
  94#define JUMP_LABEL_TYPE_TRUE_BRANCH     1UL
  95#define JUMP_LABEL_TYPE_MASK            1UL
  96
  97static
  98inline struct jump_entry *jump_label_get_entries(struct static_key *key)
  99{
 100        return (struct jump_entry *)((unsigned long)key->entries
 101                                                & ~JUMP_LABEL_TYPE_MASK);
 102}
 103
 104static inline bool jump_label_get_branch_default(struct static_key *key)
 105{
 106        if (((unsigned long)key->entries & JUMP_LABEL_TYPE_MASK) ==
 107            JUMP_LABEL_TYPE_TRUE_BRANCH)
 108                return true;
 109        return false;
 110}
 111
 112static __always_inline bool static_key_false(struct static_key *key)
 113{
 114        return arch_static_branch(key);
 115}
 116
 117static __always_inline bool static_key_true(struct static_key *key)
 118{
 119        return !static_key_false(key);
 120}
 121
 122extern struct jump_entry __start___jump_table[];
 123extern struct jump_entry __stop___jump_table[];
 124
 125extern void jump_label_init(void);
 126extern void jump_label_lock(void);
 127extern void jump_label_unlock(void);
 128extern void arch_jump_label_transform(struct jump_entry *entry,
 129                                      enum jump_label_type type);
 130extern void arch_jump_label_transform_static(struct jump_entry *entry,
 131                                             enum jump_label_type type);
 132extern int jump_label_text_reserved(void *start, void *end);
 133extern void static_key_slow_inc(struct static_key *key);
 134extern void static_key_slow_dec(struct static_key *key);
 135extern void jump_label_apply_nops(struct module *mod);
 136
 137#define STATIC_KEY_INIT_TRUE ((struct static_key)               \
 138        { .enabled = ATOMIC_INIT(1),                            \
 139          .entries = (void *)JUMP_LABEL_TYPE_TRUE_BRANCH })
 140#define STATIC_KEY_INIT_FALSE ((struct static_key)              \
 141        { .enabled = ATOMIC_INIT(0),                            \
 142          .entries = (void *)JUMP_LABEL_TYPE_FALSE_BRANCH })
 143
 144#else  /* !HAVE_JUMP_LABEL */
 145
 146static __always_inline void jump_label_init(void)
 147{
 148        static_key_initialized = true;
 149}
 150
 151static __always_inline bool static_key_false(struct static_key *key)
 152{
 153        if (unlikely(static_key_count(key) > 0))
 154                return true;
 155        return false;
 156}
 157
 158static __always_inline bool static_key_true(struct static_key *key)
 159{
 160        if (likely(static_key_count(key) > 0))
 161                return true;
 162        return false;
 163}
 164
 165static inline void static_key_slow_inc(struct static_key *key)
 166{
 167        STATIC_KEY_CHECK_USE();
 168        atomic_inc(&key->enabled);
 169}
 170
 171static inline void static_key_slow_dec(struct static_key *key)
 172{
 173        STATIC_KEY_CHECK_USE();
 174        atomic_dec(&key->enabled);
 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
 190#define STATIC_KEY_INIT_TRUE ((struct static_key) \
 191                { .enabled = ATOMIC_INIT(1) })
 192#define STATIC_KEY_INIT_FALSE ((struct static_key) \
 193                { .enabled = ATOMIC_INIT(0) })
 194
 195#endif  /* HAVE_JUMP_LABEL */
 196
 197#define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
 198#define jump_label_enabled static_key_enabled
 199
 200static inline bool static_key_enabled(struct static_key *key)
 201{
 202        return static_key_count(key) > 0;
 203}
 204
 205#endif  /* _LINUX_JUMP_LABEL_H */
 206