#ifndef _LINUX_JUMP_LABEL_H
  #define _LINUX_JUMP_LABEL_H
  
  /*
   * Jump label support
   *
   * Copyright (C) 2009-2012 Jason Baron <jbaron@redhat.com>
   * Copyright (C) 2011-2012 Peter Zijlstra <pzijlstr@redhat.com>
   *
   * Jump labels provide an interface to generate dynamic branches using
   * self-modifying code. Assuming toolchain and architecture support the result
   * of a "if (static_key_false(&key))" statement is a unconditional branch (which
   * defaults to false - and the true block is placed out of line).
   *
   * However at runtime we can change the branch target using
   * static_key_slow_{inc,dec}(). These function as a 'reference' count on the key
   * object and for as long as there are references all branches referring to
   * that particular key will point to the (out of line) true block.
   *
   * Since this relies on modifying code the static_key_slow_{inc,dec}() functions
   * must be considered absolute slow paths (machine wide synchronization etc.).
   * OTOH, since the affected branches are unconditional their runtime overhead
   * will be absolutely minimal, esp. in the default (off) case where the total
   * effect is a single NOP of appropriate size. The on case will patch in a jump
   * to the out-of-line block.
   *
   * When the control is directly exposed to userspace it is prudent to delay the
   * decrement to avoid high frequency code modifications which can (and do)
   * cause significant performance degradation. Struct static_key_deferred and
   * static_key_slow_dec_deferred() provide for this.
   *
   * Lacking toolchain and or architecture support, it falls back to a simple
   * conditional branch.
   *
   * struct static_key my_key = STATIC_KEY_INIT_TRUE;
   *
   *   if (static_key_true(&my_key)) {
   *   }
   *
   * will result in the true case being in-line and starts the key with a single
   * reference. Mixing static_key_true() and static_key_false() on the same key is not
   * allowed.
   *
   * Not initializing the key (static data is initialized to 0s anyway) is the
   * same as using STATIC_KEY_INIT_FALSE.
   *
  */
  
  #include <linux/types.h>
  #include <linux/compiler.h>
  #include <linux/bug.h>
  
  extern bool static_key_initialized;
  
  #define STATIC_KEY_CHECK_USE() WARN(!static_key_initialized,		      \
  				    "%s used before call to jump_label_init", \
  				    __func__)
  
  #if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
  
  struct static_key {
  	atomic_t enabled;
  /* Set lsb bit to 1 if branch is default true, 0 ot */
  	struct jump_entry *entries;
  #ifdef CONFIG_MODULES
  	struct static_key_mod *next;
  #endif
  };
  
  # include <asm/jump_label.h>
  # define HAVE_JUMP_LABEL
  #endif	/* CC_HAVE_ASM_GOTO && CONFIG_JUMP_LABEL */
  
  enum jump_label_type {
  	JUMP_LABEL_DISABLE = 0,
  	JUMP_LABEL_ENABLE,
  };
  
  struct module;
  
  #include <linux/atomic.h>
  #ifdef HAVE_JUMP_LABEL
  
  #define JUMP_LABEL_TYPE_FALSE_BRANCH	0UL
  #define JUMP_LABEL_TYPE_TRUE_BRANCH	1UL
  #define JUMP_LABEL_TYPE_MASK		1UL
  
  static
  inline struct jump_entry *jump_label_get_entries(struct static_key *key)
  {
  	return (struct jump_entry *)((unsigned long)key->entries
  						& ~JUMP_LABEL_TYPE_MASK);
  }
  
  static inline bool jump_label_get_branch_default(struct static_key *key)
  {
  	if (((unsigned long)key->entries & JUMP_LABEL_TYPE_MASK) ==
  	    JUMP_LABEL_TYPE_TRUE_BRANCH)
  		return true;
  	return false;
  }
  
  static __always_inline bool static_key_false(struct static_key *key)
  {
  	return arch_static_branch(key);
  }
  
  static __always_inline bool static_key_true(struct static_key *key)
  {
  	return !static_key_false(key);
  }
  
  extern struct jump_entry __start___jump_table[];
  extern struct jump_entry __stop___jump_table[];
  
  extern void jump_label_init(void);
  extern void jump_label_lock(void);
  extern void jump_label_unlock(void);
  extern void arch_jump_label_transform(struct jump_entry *entry,
  				      enum jump_label_type type);
  extern void arch_jump_label_transform_static(struct jump_entry *entry,
  					     enum jump_label_type type);
  extern int jump_label_text_reserved(void *start, void *end);
  extern void static_key_slow_inc(struct static_key *key);
  extern void static_key_slow_dec(struct static_key *key);
  extern void jump_label_apply_nops(struct module *mod);
  
  #define STATIC_KEY_INIT_TRUE ((struct static_key)		\
  	{ .enabled = ATOMIC_INIT(1),				\
  	  .entries = (void *)JUMP_LABEL_TYPE_TRUE_BRANCH })
  #define STATIC_KEY_INIT_FALSE ((struct static_key)		\
  	{ .enabled = ATOMIC_INIT(0),				\
  	  .entries = (void *)JUMP_LABEL_TYPE_FALSE_BRANCH })
  
  #else  /* !HAVE_JUMP_LABEL */
  
  struct static_key {
  	atomic_t enabled;
  };
  
  static __always_inline void jump_label_init(void)
  {
  	static_key_initialized = true;
  }
  
  static __always_inline bool static_key_false(struct static_key *key)
  {
  	if (unlikely(atomic_read(&key->enabled) > 0))
  		return true;
  	return false;
  }
  
  static __always_inline bool static_key_true(struct static_key *key)
  {
  	if (likely(atomic_read(&key->enabled) > 0))
  		return true;
  	return false;
  }
  
  static inline void static_key_slow_inc(struct static_key *key)
  {
  	STATIC_KEY_CHECK_USE();
  	atomic_inc(&key->enabled);
  }
  
  static inline void static_key_slow_dec(struct static_key *key)
  {
  	STATIC_KEY_CHECK_USE();
  	atomic_dec(&key->enabled);
  }
  
  static inline int jump_label_text_reserved(void *start, void *end)
  {
  	return 0;
  }
  
  static inline void jump_label_lock(void) {}
  static inline void jump_label_unlock(void) {}
  
  static inline int jump_label_apply_nops(struct module *mod)
  {
  	return 0;
  }
  
  #define STATIC_KEY_INIT_TRUE ((struct static_key) \
  		{ .enabled = ATOMIC_INIT(1) })
  #define STATIC_KEY_INIT_FALSE ((struct static_key) \
  		{ .enabled = ATOMIC_INIT(0) })
  
  #endif	/* HAVE_JUMP_LABEL */
  
  #define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
  #define jump_label_enabled static_key_enabled
  
  static inline bool static_key_enabled(struct static_key *key)
  {
  	return (atomic_read(&key->enabled) > 0);
  }
  
  #endif	/* _LINUX_JUMP_LABEL_H */