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
  52 #if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
  53
  54 struct static_key {
  55         atomic_t enabled;
  56 /* Set lsb bit to 1 if branch is default true, 0 ot */
  57         struct jump_entry *entries;
  58 #ifdef CONFIG_MODULES
  59         struct static_key_mod *next;
  60 #endif
  61 };
  62
  63 # include <asm/jump_label.h>
  64 # define HAVE_JUMP_LABEL
  65 #endif  /* CC_HAVE_ASM_GOTO && CONFIG_JUMP_LABEL */
  66
  67 enum jump_label_type {
  68         JUMP_LABEL_DISABLE = 0,
  69         JUMP_LABEL_ENABLE,
  70 };
  71
  72 struct module;
  73
  74 #include <linux/atomic.h>
  75 #ifdef HAVE_JUMP_LABEL
  76
  77 #define JUMP_LABEL_TRUE_BRANCH 1UL
  78
  79 static
  80 inline struct jump_entry *jump_label_get_entries(struct static_key *key)
  81 {
  82         return (struct jump_entry *)((unsigned long)key->entries
  83                                                 & ~JUMP_LABEL_TRUE_BRANCH);
  84 }
  85
  86 static inline bool jump_label_get_branch_default(struct static_key *key)
  87 {
  88         if ((unsigned long)key->entries & JUMP_LABEL_TRUE_BRANCH)
  89                 return true;
  90         return false;
  91 }
  92
  93 static __always_inline bool static_key_false(struct static_key *key)
  94 {
  95         return arch_static_branch(key);
  96 }
  97
  98 static __always_inline bool static_key_true(struct static_key *key)
  99 {
 100         return !static_key_false(key);
 101 }
 102
 103 extern struct jump_entry __start___jump_table[];
 104 extern struct jump_entry __stop___jump_table[];
 105
 106 extern void jump_label_init(void);
 107 extern void jump_label_lock(void);
 108 extern void jump_label_unlock(void);
 109 extern void arch_jump_label_transform(struct jump_entry *entry,
 110                                       enum jump_label_type type);
 111 extern void arch_jump_label_transform_static(struct jump_entry *entry,
 112                                              enum jump_label_type type);
 113 extern int jump_label_text_reserved(void *start, void *end);
 114 extern void static_key_slow_inc(struct static_key *key);
 115 extern void static_key_slow_dec(struct static_key *key);
 116 extern void jump_label_apply_nops(struct module *mod);
 117
 118 #define STATIC_KEY_INIT_TRUE ((struct static_key) \
 119         { .enabled = ATOMIC_INIT(1), .entries = (void *)1 })
 120 #define STATIC_KEY_INIT_FALSE ((struct static_key) \
 121         { .enabled = ATOMIC_INIT(0), .entries = (void *)0 })
 122
 123 #else  /* !HAVE_JUMP_LABEL */
 124
 125 struct static_key {
 126         atomic_t enabled;
 127 };
 128
 129 static __always_inline void jump_label_init(void)
 130 {
 131 }
 132
 133 static __always_inline bool static_key_false(struct static_key *key)
 134 {
 135         if (unlikely(atomic_read(&key->enabled)) > 0)
 136                 return true;
 137         return false;
 138 }
 139
 140 static __always_inline bool static_key_true(struct static_key *key)
 141 {
 142         if (likely(atomic_read(&key->enabled)) > 0)
 143                 return true;
 144         return false;
 145 }
 146
 147 static inline void static_key_slow_inc(struct static_key *key)
 148 {
 149         atomic_inc(&key->enabled);
 150 }
 151
 152 static inline void static_key_slow_dec(struct static_key *key)
 153 {
 154         atomic_dec(&key->enabled);
 155 }
 156
 157 static inline int jump_label_text_reserved(void *start, void *end)
 158 {
 159         return 0;
 160 }
 161
 162 static inline void jump_label_lock(void) {}
 163 static inline void jump_label_unlock(void) {}
 164
 165 static inline int jump_label_apply_nops(struct module *mod)
 166 {
 167         return 0;
 168 }
 169
 170 #define STATIC_KEY_INIT_TRUE ((struct static_key) \
 171                 { .enabled = ATOMIC_INIT(1) })
 172 #define STATIC_KEY_INIT_FALSE ((struct static_key) \
 173                 { .enabled = ATOMIC_INIT(0) })
 174
 175 #endif  /* HAVE_JUMP_LABEL */
 176
 177 #define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
 178 #define jump_label_enabled static_key_enabled
 179
 180 static inline bool static_key_enabled(struct static_key *key)
 181 {
 182         return (atomic_read(&key->enabled) > 0);
 183 }
 184
 185 #endif  /* _LINUX_JUMP_LABEL_H */