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 and static_key_false() is
  46  * equivalent with static_branch().
  47  *
  48 */
  49
  50 #include <linux/types.h>
  51 #include <linux/compiler.h>
  52 #include <linux/workqueue.h>
  53
  54 #if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
  55
  56 struct static_key {
  57         atomic_t enabled;
  58 /* Set lsb bit to 1 if branch is default true, 0 ot */
  59         struct jump_entry *entries;
  60 #ifdef CONFIG_MODULES
  61         struct static_key_mod *next;
  62 #endif
  63 };
  64
  65 struct static_key_deferred {
  66         struct static_key key;
  67         unsigned long timeout;
  68         struct delayed_work work;
  69 };
  70
  71 # include <asm/jump_label.h>
  72 # define HAVE_JUMP_LABEL
  73 #endif  /* CC_HAVE_ASM_GOTO && CONFIG_JUMP_LABEL */
  74
  75 enum jump_label_type {
  76         JUMP_LABEL_DISABLE = 0,
  77         JUMP_LABEL_ENABLE,
  78 };
  79
  80 struct module;
  81
  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 /* Deprecated. Please use 'static_key_false() instead. */
 111 static __always_inline bool static_branch(struct static_key *key)
 112 {
 113         return arch_static_branch(key);
 114 }
 115
 116 extern struct jump_entry __start___jump_table[];
 117 extern struct jump_entry __stop___jump_table[];
 118
 119 extern void jump_label_init(void);
 120 extern void jump_label_lock(void);
 121 extern void jump_label_unlock(void);
 122 extern void arch_jump_label_transform(struct jump_entry *entry,
 123                                       enum jump_label_type type);
 124 extern void arch_jump_label_transform_static(struct jump_entry *entry,
 125                                              enum jump_label_type type);
 126 extern int jump_label_text_reserved(void *start, void *end);
 127 extern void static_key_slow_inc(struct static_key *key);
 128 extern void static_key_slow_dec(struct static_key *key);
 129 extern void static_key_slow_dec_deferred(struct static_key_deferred *key);
 130 extern void jump_label_apply_nops(struct module *mod);
 131 extern void
 132 jump_label_rate_limit(struct static_key_deferred *key, unsigned long rl);
 133
 134 #define STATIC_KEY_INIT_TRUE ((struct static_key) \
 135         { .enabled = ATOMIC_INIT(1), .entries = (void *)1 })
 136 #define STATIC_KEY_INIT_FALSE ((struct static_key) \
 137         { .enabled = ATOMIC_INIT(0), .entries = (void *)0 })
 138
 139 #else  /* !HAVE_JUMP_LABEL */
 140
 141 #include <linux/atomic.h>
 142
 143 struct static_key {
 144         atomic_t enabled;
 145 };
 146
 147 static __always_inline void jump_label_init(void)
 148 {
 149 }
 150
 151 struct static_key_deferred {
 152         struct static_key  key;
 153 };
 154
 155 static __always_inline bool static_key_false(struct static_key *key)
 156 {
 157         if (unlikely(atomic_read(&key->enabled)) > 0)
 158                 return true;
 159         return false;
 160 }
 161
 162 static __always_inline bool static_key_true(struct static_key *key)
 163 {
 164         if (likely(atomic_read(&key->enabled)) > 0)
 165                 return true;
 166         return false;
 167 }
 168
 169 /* Deprecated. Please use 'static_key_false() instead. */
 170 static __always_inline bool static_branch(struct static_key *key)
 171 {
 172         if (unlikely(atomic_read(&key->enabled)) > 0)
 173                 return true;
 174         return false;
 175 }
 176
 177 static inline void static_key_slow_inc(struct static_key *key)
 178 {
 179         atomic_inc(&key->enabled);
 180 }
 181
 182 static inline void static_key_slow_dec(struct static_key *key)
 183 {
 184         atomic_dec(&key->enabled);
 185 }
 186
 187 static inline void static_key_slow_dec_deferred(struct static_key_deferred *key)
 188 {
 189         static_key_slow_dec(&key->key);
 190 }
 191
 192 static inline int jump_label_text_reserved(void *start, void *end)
 193 {
 194         return 0;
 195 }
 196
 197 static inline void jump_label_lock(void) {}
 198 static inline void jump_label_unlock(void) {}
 199
 200 static inline int jump_label_apply_nops(struct module *mod)
 201 {
 202         return 0;
 203 }
 204
 205 static inline void
 206 jump_label_rate_limit(struct static_key_deferred *key,
 207                 unsigned long rl)
 208 {
 209 }
 210
 211 #define STATIC_KEY_INIT_TRUE ((struct static_key) \
 212                 { .enabled = ATOMIC_INIT(1) })
 213 #define STATIC_KEY_INIT_FALSE ((struct static_key) \
 214                 { .enabled = ATOMIC_INIT(0) })
 215
 216 #endif  /* HAVE_JUMP_LABEL */
 217
 218 #define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
 219 #define jump_label_enabled static_key_enabled
 220
 221 static inline bool static_key_enabled(struct static_key *key)
 222 {
 223         return (atomic_read(&key->enabled) > 0);
 224 }
 225
 226 #endif  /* _LINUX_JUMP_LABEL_H */