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/workqueue.h>
  52
  53 #if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
  54
  55 struct static_key {
  56         atomic_t enabled;
  57 /* Set lsb bit to 1 if branch is default true, 0 ot */
  58         struct jump_entry *entries;
  59 #ifdef CONFIG_MODULES
  60         struct static_key_mod *next;
  61 #endif
  62 };
  63
  64 struct static_key_deferred {
  65         struct static_key key;
  66         unsigned long timeout;
  67         struct delayed_work work;
  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 #ifdef HAVE_JUMP_LABEL
  82
  83 #define JUMP_LABEL_TRUE_BRANCH 1UL
  84
  85 static
  86 inline struct jump_entry *jump_label_get_entries(struct static_key *key)
  87 {
  88         return (struct jump_entry *)((unsigned long)key->entries
  89                                                 & ~JUMP_LABEL_TRUE_BRANCH);
  90 }
  91
  92 static inline bool jump_label_get_branch_default(struct static_key *key)
  93 {
  94         if ((unsigned long)key->entries & JUMP_LABEL_TRUE_BRANCH)
  95                 return true;
  96         return false;
  97 }
  98
  99 static __always_inline bool static_key_false(struct static_key *key)
 100 {
 101         return arch_static_branch(key);
 102 }
 103
 104 static __always_inline bool static_key_true(struct static_key *key)
 105 {
 106         return !static_key_false(key);
 107 }
 108
 109 extern struct jump_entry __start___jump_table[];
 110 extern struct jump_entry __stop___jump_table[];
 111
 112 extern void jump_label_init(void);
 113 extern void jump_label_lock(void);
 114 extern void jump_label_unlock(void);
 115 extern void arch_jump_label_transform(struct jump_entry *entry,
 116                                       enum jump_label_type type);
 117 extern void arch_jump_label_transform_static(struct jump_entry *entry,
 118                                              enum jump_label_type type);
 119 extern int jump_label_text_reserved(void *start, void *end);
 120 extern void static_key_slow_inc(struct static_key *key);
 121 extern void static_key_slow_dec(struct static_key *key);
 122 extern void static_key_slow_dec_deferred(struct static_key_deferred *key);
 123 extern void jump_label_apply_nops(struct module *mod);
 124 extern void
 125 jump_label_rate_limit(struct static_key_deferred *key, unsigned long rl);
 126
 127 #define STATIC_KEY_INIT_TRUE ((struct static_key) \
 128         { .enabled = ATOMIC_INIT(1), .entries = (void *)1 })
 129 #define STATIC_KEY_INIT_FALSE ((struct static_key) \
 130         { .enabled = ATOMIC_INIT(0), .entries = (void *)0 })
 131
 132 #else  /* !HAVE_JUMP_LABEL */
 133
 134 #include <linux/atomic.h>
 135
 136 struct static_key {
 137         atomic_t enabled;
 138 };
 139
 140 static __always_inline void jump_label_init(void)
 141 {
 142 }
 143
 144 struct static_key_deferred {
 145         struct static_key  key;
 146 };
 147
 148 static __always_inline bool static_key_false(struct static_key *key)
 149 {
 150         if (unlikely(atomic_read(&key->enabled)) > 0)
 151                 return true;
 152         return false;
 153 }
 154
 155 static __always_inline bool static_key_true(struct static_key *key)
 156 {
 157         if (likely(atomic_read(&key->enabled)) > 0)
 158                 return true;
 159         return false;
 160 }
 161
 162 static inline void static_key_slow_inc(struct static_key *key)
 163 {
 164         atomic_inc(&key->enabled);
 165 }
 166
 167 static inline void static_key_slow_dec(struct static_key *key)
 168 {
 169         atomic_dec(&key->enabled);
 170 }
 171
 172 static inline void static_key_slow_dec_deferred(struct static_key_deferred *key)
 173 {
 174         static_key_slow_dec(&key->key);
 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 static inline void
 191 jump_label_rate_limit(struct static_key_deferred *key,
 192                 unsigned long rl)
 193 {
 194 }
 195
 196 #define STATIC_KEY_INIT_TRUE ((struct static_key) \
 197                 { .enabled = ATOMIC_INIT(1) })
 198 #define STATIC_KEY_INIT_FALSE ((struct static_key) \
 199                 { .enabled = ATOMIC_INIT(0) })
 200
 201 #endif  /* HAVE_JUMP_LABEL */
 202
 203 #define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
 204 #define jump_label_enabled static_key_enabled
 205
 206 static inline bool static_key_enabled(struct static_key *key)
 207 {
 208         return (atomic_read(&key->enabled) > 0);
 209 }
 210
 211 #endif  /* _LINUX_JUMP_LABEL_H */