include/linux/kref.h

   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/spinlock.h>
  19 #include <linux/refcount.h>
  20
  21 struct kref {
  22         refcount_t refcount;
  23 };
  24
  25 #define KREF_INIT(n)    { .refcount = REFCOUNT_INIT(n), }
  26
  27 /**
  28  * kref_init - initialize object.
  29  * @kref: object in question.
  30  */
  31 static inline void kref_init(struct kref *kref)
  32 {
  33         refcount_set(&kref->refcount, 1);
  34 }
  35
  36 static inline unsigned int kref_read(const struct kref *kref)
  37 {
  38         return refcount_read(&kref->refcount);
  39 }
  40
  41 /**
  42  * kref_get - increment refcount for object.
  43  * @kref: object.
  44  */
  45 static inline void kref_get(struct kref *kref)
  46 {
  47         refcount_inc(&kref->refcount);
  48 }
  49
  50 /**
  51  * kref_put - decrement refcount for object.
  52  * @kref: object.
  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  * Decrement 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_put(struct kref *kref, void (*release)(struct kref *kref))
  68 {
  69         if (refcount_dec_and_test(&kref->refcount)) {
  70                 release(kref);
  71                 return 1;
  72         }
  73         return 0;
  74 }
  75
  76 static inline int kref_put_mutex(struct kref *kref,
  77                                  void (*release)(struct kref *kref),
  78                                  struct mutex *lock)
  79 {
  80         if (refcount_dec_and_mutex_lock(&kref->refcount, lock)) {
  81                 release(kref);
  82                 return 1;
  83         }
  84         return 0;
  85 }
  86
  87 static inline int kref_put_lock(struct kref *kref,
  88                                 void (*release)(struct kref *kref),
  89                                 spinlock_t *lock)
  90 {
  91         if (refcount_dec_and_lock(&kref->refcount, lock)) {
  92                 release(kref);
  93                 return 1;
  94         }
  95         return 0;
  96 }
  97
  98 /**
  99  * kref_get_unless_zero - Increment refcount for object unless it is zero.
 100  * @kref: object.
 101  *
 102  * Return non-zero if the increment succeeded. Otherwise return 0.
 103  *
 104  * This function is intended to simplify locking around refcounting for
 105  * objects that can be looked up from a lookup structure, and which are
 106  * removed from that lookup structure in the object destructor.
 107  * Operations on such objects require at least a read lock around
 108  * lookup + kref_get, and a write lock around kref_put + remove from lookup
 109  * structure. Furthermore, RCU implementations become extremely tricky.
 110  * With a lookup followed by a kref_get_unless_zero *with return value check*
 111  * locking in the kref_put path can be deferred to the actual removal from
 112  * the lookup structure and RCU lookups become trivial.
 113  */
 114 static inline int __must_check kref_get_unless_zero(struct kref *kref)
 115 {
 116         return refcount_inc_not_zero(&kref->refcount);
 117 }
 118 #endif /* _KREF_H_ */