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  1. /*
  2.  * kref.h - library routines for handling generic reference counted objects
  3.  *
  4.  * Copyright (C) 2004 Greg Kroah-Hartman <greg@kroah.com>
  5.  * Copyright (C) 2004 IBM Corp.
  6.  *
  7.  * based on kobject.h which was:
  8.  * Copyright (C) 2002-2003 Patrick Mochel <mochel@osdl.org>
  9.  * Copyright (C) 2002-2003 Open Source Development Labs
  10.  *
  11.  * This file is released under the GPLv2.
  12.  *
  13.  */
  14.  
  15. #ifndef _KREF_H_
  16. #define _KREF_H_
  17.  
  18. #include <linux/bug.h>
  19. #include <linux/atomic.h>
  20. #include <linux/kernel.h>
  21. #include <linux/mutex.h>
  22.  
  23. struct kref {
  24.         atomic_t refcount;
  25. };
  26.  
  27. /**
  28.  * kref_init - initialize object.
  29.  * @kref: object in question.
  30.  */
  31. static inline void kref_init(struct kref *kref)
  32. {
  33.         atomic_set(&kref->refcount, 1);
  34. }
  35.  
  36. /**
  37.  * kref_get - increment refcount for object.
  38.  * @kref: object.
  39.  */
  40. static inline void kref_get(struct kref *kref)
  41. {
  42.         /* If refcount was 0 before incrementing then we have a race
  43.          * condition when this kref is freeing by some other thread right now.
  44.          * In this case one should use kref_get_unless_zero()
  45.          */
  46.         WARN_ON_ONCE(atomic_inc_return(&kref->refcount) < 2);
  47. }
  48.  
  49. /**
  50.  * kref_sub - subtract a number of refcounts for object.
  51.  * @kref: object.
  52.  * @count: Number of recounts to subtract.
  53.  * @release: pointer to the function that will clean up the object when the
  54.  *           last reference to the object is released.
  55.  *           This pointer is required, and it is not acceptable to pass kfree
  56.  *           in as this function.  If the caller does pass kfree to this
  57.  *           function, you will be publicly mocked mercilessly by the kref
  58.  *           maintainer, and anyone else who happens to notice it.  You have
  59.  *           been warned.
  60.  *
  61.  * Subtract @count from the refcount, and if 0, call release().
  62.  * Return 1 if the object was removed, otherwise return 0.  Beware, if this
  63.  * function returns 0, you still can not count on the kref from remaining in
  64.  * memory.  Only use the return value if you want to see if the kref is now
  65.  * gone, not present.
  66.  */
  67. static inline int kref_sub(struct kref *kref, unsigned int count,
  68.              void (*release)(struct kref *kref))
  69. {
  70.         WARN_ON(release == NULL);
  71.  
  72.         if (atomic_sub_and_test((int) count, &kref->refcount)) {
  73.                 release(kref);
  74.                 return 1;
  75.         }
  76.         return 0;
  77. }
  78.  
  79. /**
  80.  * kref_put - decrement refcount for object.
  81.  * @kref: object.
  82.  * @release: pointer to the function that will clean up the object when the
  83.  *           last reference to the object is released.
  84.  *           This pointer is required, and it is not acceptable to pass kfree
  85.  *           in as this function.  If the caller does pass kfree to this
  86.  *           function, you will be publicly mocked mercilessly by the kref
  87.  *           maintainer, and anyone else who happens to notice it.  You have
  88.  *           been warned.
  89.  *
  90.  * Decrement the refcount, and if 0, call release().
  91.  * Return 1 if the object was removed, otherwise return 0.  Beware, if this
  92.  * function returns 0, you still can not count on the kref from remaining in
  93.  * memory.  Only use the return value if you want to see if the kref is now
  94.  * gone, not present.
  95.  */
  96. static inline int kref_put(struct kref *kref, void (*release)(struct kref *kref))
  97. {
  98.         return kref_sub(kref, 1, release);
  99. }
  100.  
  101. static inline int kref_put_mutex(struct kref *kref,
  102.                                  void (*release)(struct kref *kref),
  103.                                  struct mutex *lock)
  104. {
  105.         WARN_ON(release == NULL);
  106.         if (unlikely(!atomic_add_unless(&kref->refcount, -1, 1))) {
  107.                 mutex_lock(lock);
  108.                 if (unlikely(!atomic_dec_and_test(&kref->refcount))) {
  109.                         mutex_unlock(lock);
  110.                         return 0;
  111.                 }
  112.                 release(kref);
  113.                 return 1;
  114.         }
  115.         return 0;
  116. }
  117.  
  118. /**
  119.  * kref_get_unless_zero - Increment refcount for object unless it is zero.
  120.  * @kref: object.
  121.  *
  122.  * Return non-zero if the increment succeeded. Otherwise return 0.
  123.  *
  124.  * This function is intended to simplify locking around refcounting for
  125.  * objects that can be looked up from a lookup structure, and which are
  126.  * removed from that lookup structure in the object destructor.
  127.  * Operations on such objects require at least a read lock around
  128.  * lookup + kref_get, and a write lock around kref_put + remove from lookup
  129.  * structure. Furthermore, RCU implementations become extremely tricky.
  130.  * With a lookup followed by a kref_get_unless_zero *with return value check*
  131.  * locking in the kref_put path can be deferred to the actual removal from
  132.  * the lookup structure and RCU lookups become trivial.
  133.  */
  134. static inline int __must_check kref_get_unless_zero(struct kref *kref)
  135. {
  136.         return atomic_add_unless(&kref->refcount, 1, 0);
  137. }
  138. #endif /* _KREF_H_ */
  139.