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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. #include <linux/spinlock.h>
  23.  
  24. struct kref {
  25.         atomic_t refcount;
  26. };
  27.  
  28. /**
  29.  * kref_init - initialize object.
  30.  * @kref: object in question.
  31.  */
  32. static inline void kref_init(struct kref *kref)
  33. {
  34.         atomic_set(&kref->refcount, 1);
  35. }
  36.  
  37. /**
  38.  * kref_get - increment refcount for object.
  39.  * @kref: object.
  40.  */
  41. static inline void kref_get(struct kref *kref)
  42. {
  43.         /* If refcount was 0 before incrementing then we have a race
  44.          * condition when this kref is freeing by some other thread right now.
  45.          * In this case one should use kref_get_unless_zero()
  46.          */
  47.         WARN_ON_ONCE(atomic_inc_return(&kref->refcount) < 2);
  48. }
  49.  
  50. /**
  51.  * kref_sub - subtract a number of refcounts for object.
  52.  * @kref: object.
  53.  * @count: Number of recounts to subtract.
  54.  * @release: pointer to the function that will clean up the object when the
  55.  *           last reference to the object is released.
  56.  *           This pointer is required, and it is not acceptable to pass kfree
  57.  *           in as this function.  If the caller does pass kfree to this
  58.  *           function, you will be publicly mocked mercilessly by the kref
  59.  *           maintainer, and anyone else who happens to notice it.  You have
  60.  *           been warned.
  61.  *
  62.  * Subtract @count from the refcount, and if 0, call release().
  63.  * Return 1 if the object was removed, otherwise return 0.  Beware, if this
  64.  * function returns 0, you still can not count on the kref from remaining in
  65.  * memory.  Only use the return value if you want to see if the kref is now
  66.  * gone, not present.
  67.  */
  68. static inline int kref_sub(struct kref *kref, unsigned int count,
  69.              void (*release)(struct kref *kref))
  70. {
  71.         WARN_ON(release == NULL);
  72.  
  73.         if (atomic_sub_and_test((int) count, &kref->refcount)) {
  74.                 release(kref);
  75.                 return 1;
  76.         }
  77.         return 0;
  78. }
  79.  
  80. /**
  81.  * kref_put - decrement refcount for object.
  82.  * @kref: object.
  83.  * @release: pointer to the function that will clean up the object when the
  84.  *           last reference to the object is released.
  85.  *           This pointer is required, and it is not acceptable to pass kfree
  86.  *           in as this function.  If the caller does pass kfree to this
  87.  *           function, you will be publicly mocked mercilessly by the kref
  88.  *           maintainer, and anyone else who happens to notice it.  You have
  89.  *           been warned.
  90.  *
  91.  * Decrement the refcount, and if 0, call release().
  92.  * Return 1 if the object was removed, otherwise return 0.  Beware, if this
  93.  * function returns 0, you still can not count on the kref from remaining in
  94.  * memory.  Only use the return value if you want to see if the kref is now
  95.  * gone, not present.
  96.  */
  97. static inline int kref_put(struct kref *kref, void (*release)(struct kref *kref))
  98. {
  99.         return kref_sub(kref, 1, release);
  100. }
  101.  
  102. /**
  103.  * kref_put_spinlock_irqsave - decrement refcount for object.
  104.  * @kref: object.
  105.  * @release: pointer to the function that will clean up the object when the
  106.  *           last reference to the object is released.
  107.  *           This pointer is required, and it is not acceptable to pass kfree
  108.  *           in as this function.
  109.  * @lock: lock to take in release case
  110.  *
  111.  * Behaves identical to kref_put with one exception.  If the reference count
  112.  * drops to zero, the lock will be taken atomically wrt dropping the reference
  113.  * count.  The release function has to call spin_unlock() without _irqrestore.
  114.  */
  115. static inline int kref_put_spinlock_irqsave(struct kref *kref,
  116.                 void (*release)(struct kref *kref),
  117.                 spinlock_t *lock)
  118. {
  119.         unsigned long flags;
  120.  
  121.         WARN_ON(release == NULL);
  122.         if (atomic_add_unless(&kref->refcount, -1, 1))
  123.                 return 0;
  124.         spin_lock_irqsave(lock, flags);
  125.         if (atomic_dec_and_test(&kref->refcount)) {
  126.                 release(kref);
  127.                 local_irq_restore(flags);
  128.                 return 1;
  129.         }
  130.         spin_unlock_irqrestore(lock, flags);
  131.         return 0;
  132. }
  133.  
  134. static inline int kref_put_mutex(struct kref *kref,
  135.                                  void (*release)(struct kref *kref),
  136.                                  struct mutex *lock)
  137. {
  138.         WARN_ON(release == NULL);
  139.         if (unlikely(!atomic_add_unless(&kref->refcount, -1, 1))) {
  140.                 mutex_lock(lock);
  141.                 if (unlikely(!atomic_dec_and_test(&kref->refcount))) {
  142.                         mutex_unlock(lock);
  143.                         return 0;
  144.                 }
  145.                 release(kref);
  146.                 return 1;
  147.         }
  148.         return 0;
  149. }
  150.  
  151. /**
  152.  * kref_get_unless_zero - Increment refcount for object unless it is zero.
  153.  * @kref: object.
  154.  *
  155.  * Return non-zero if the increment succeeded. Otherwise return 0.
  156.  *
  157.  * This function is intended to simplify locking around refcounting for
  158.  * objects that can be looked up from a lookup structure, and which are
  159.  * removed from that lookup structure in the object destructor.
  160.  * Operations on such objects require at least a read lock around
  161.  * lookup + kref_get, and a write lock around kref_put + remove from lookup
  162.  * structure. Furthermore, RCU implementations become extremely tricky.
  163.  * With a lookup followed by a kref_get_unless_zero *with return value check*
  164.  * locking in the kref_put path can be deferred to the actual removal from
  165.  * the lookup structure and RCU lookups become trivial.
  166.  */
  167. static inline int __must_check kref_get_unless_zero(struct kref *kref)
  168. {
  169.         return atomic_add_unless(&kref->refcount, 1, 0);
  170. }
  171. #endif /* _KREF_H_ */
  172.