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 #include <linux/types.h>
  49 #include <linux/compiler.h>
  50 #include <linux/bug.h>
  51
  52 extern 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
  60 struct 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
  72 struct static_key {
  73         atomic_t enabled;
  74 };
  75 #endif  /* CC_HAVE_ASM_GOTO && CONFIG_JUMP_LABEL */
  76
  77 enum jump_label_type {
  78         JUMP_LABEL_DISABLE = 0,
  79         JUMP_LABEL_ENABLE,
  80 };
  81
  82 struct module;
  83
  84 #include <linux/atomic.h>
  85
  86 static 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
  97 static
  98 inline 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
 104 static 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
 112 static __always_inline bool static_key_false(struct static_key *key)
 113 {
 114         return arch_static_branch(key);
 115 }
 116
 117 static __always_inline bool static_key_true(struct static_key *key)
 118 {
 119         return !static_key_false(key);
 120 }
 121
 122 extern struct jump_entry __start___jump_table[];
 123 extern struct jump_entry __stop___jump_table[];
 124
 125 extern void jump_label_init(void);
 126 extern void jump_label_lock(void);
 127 extern void jump_label_unlock(void);
 128 extern void arch_jump_label_transform(struct jump_entry *entry,
 129                                       enum jump_label_type type);
 130 extern void arch_jump_label_transform_static(struct jump_entry *entry,
 131                                              enum jump_label_type type);
 132 extern int jump_label_text_reserved(void *start, void *end);
 133 extern void static_key_slow_inc(struct static_key *key);
 134 extern void static_key_slow_dec(struct static_key *key);
 135 extern 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
 146 static __always_inline void jump_label_init(void)
 147 {
 148         static_key_initialized = true;
 149 }
 150
 151 static __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
 158 static __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
 165 static inline void static_key_slow_inc(struct static_key *key)
 166 {
 167         STATIC_KEY_CHECK_USE();
 168         atomic_inc(&key->enabled);
 169 }
 170
 171 static inline void static_key_slow_dec(struct static_key *key)
 172 {
 173         STATIC_KEY_CHECK_USE();
 174         atomic_dec(&key->enabled);
 175 }
 176
 177 static inline int jump_label_text_reserved(void *start, void *end)
 178 {
 179         return 0;
 180 }
 181
 182 static inline void jump_label_lock(void) {}
 183 static inline void jump_label_unlock(void) {}
 184
 185 static 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
 200 static 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 */