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