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_DISABLE = 0,
  90         JUMP_LABEL_ENABLE,
  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_LABEL_TYPE_FALSE_BRANCH    0UL
 105 #define JUMP_LABEL_TYPE_TRUE_BRANCH     1UL
 106 #define JUMP_LABEL_TYPE_MASK            1UL
 107
 108 static
 109 inline struct jump_entry *jump_label_get_entries(struct static_key *key)
 110 {
 111         return (struct jump_entry *)((unsigned long)key->entries
 112                                                 & ~JUMP_LABEL_TYPE_MASK);
 113 }
 114
 115 static inline bool jump_label_get_branch_default(struct static_key *key)
 116 {
 117         if (((unsigned long)key->entries & JUMP_LABEL_TYPE_MASK) ==
 118             JUMP_LABEL_TYPE_TRUE_BRANCH)
 119                 return true;
 120         return false;
 121 }
 122
 123 static __always_inline bool static_key_false(struct static_key *key)
 124 {
 125         return arch_static_branch(key);
 126 }
 127
 128 static __always_inline bool static_key_true(struct static_key *key)
 129 {
 130         return !static_key_false(key);
 131 }
 132
 133 extern struct jump_entry __start___jump_table[];
 134 extern struct jump_entry __stop___jump_table[];
 135
 136 extern void jump_label_init(void);
 137 extern void jump_label_lock(void);
 138 extern void jump_label_unlock(void);
 139 extern void arch_jump_label_transform(struct jump_entry *entry,
 140                                       enum jump_label_type type);
 141 extern void arch_jump_label_transform_static(struct jump_entry *entry,
 142                                              enum jump_label_type type);
 143 extern int jump_label_text_reserved(void *start, void *end);
 144 extern void static_key_slow_inc(struct static_key *key);
 145 extern void static_key_slow_dec(struct static_key *key);
 146 extern void jump_label_apply_nops(struct module *mod);
 147
 148 #define STATIC_KEY_INIT_TRUE ((struct static_key)               \
 149         { .enabled = ATOMIC_INIT(1),                            \
 150           .entries = (void *)JUMP_LABEL_TYPE_TRUE_BRANCH })
 151 #define STATIC_KEY_INIT_FALSE ((struct static_key)              \
 152         { .enabled = ATOMIC_INIT(0),                            \
 153           .entries = (void *)JUMP_LABEL_TYPE_FALSE_BRANCH })
 154
 155 #else  /* !HAVE_JUMP_LABEL */
 156
 157 static __always_inline void jump_label_init(void)
 158 {
 159         static_key_initialized = true;
 160 }
 161
 162 static __always_inline bool static_key_false(struct static_key *key)
 163 {
 164         if (unlikely(static_key_count(key) > 0))
 165                 return true;
 166         return false;
 167 }
 168
 169 static __always_inline bool static_key_true(struct static_key *key)
 170 {
 171         if (likely(static_key_count(key) > 0))
 172                 return true;
 173         return false;
 174 }
 175
 176 static inline void static_key_slow_inc(struct static_key *key)
 177 {
 178         STATIC_KEY_CHECK_USE();
 179         atomic_inc(&key->enabled);
 180 }
 181
 182 static inline void static_key_slow_dec(struct static_key *key)
 183 {
 184         STATIC_KEY_CHECK_USE();
 185         atomic_dec(&key->enabled);
 186 }
 187
 188 static inline int jump_label_text_reserved(void *start, void *end)
 189 {
 190         return 0;
 191 }
 192
 193 static inline void jump_label_lock(void) {}
 194 static inline void jump_label_unlock(void) {}
 195
 196 static inline int jump_label_apply_nops(struct module *mod)
 197 {
 198         return 0;
 199 }
 200
 201 #define STATIC_KEY_INIT_TRUE ((struct static_key) \
 202                 { .enabled = ATOMIC_INIT(1) })
 203 #define STATIC_KEY_INIT_FALSE ((struct static_key) \
 204                 { .enabled = ATOMIC_INIT(0) })
 205
 206 #endif  /* HAVE_JUMP_LABEL */
 207
 208 #define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
 209 #define jump_label_enabled static_key_enabled
 210
 211 static inline bool static_key_enabled(struct static_key *key)
 212 {
 213         return static_key_count(key) > 0;
 214 }
 215
 216 #endif  /* _LINUX_JUMP_LABEL_H */
 217
 218 #endif /* __ASSEMBLY__ */