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/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         WARN_ON(!atomic_read(&kref->refcount));
  43         atomic_inc(&kref->refcount);
  44 }
  45
  46 /**
  47  * kref_sub - subtract a number of refcounts for object.
  48  * @kref: object.
  49  * @count: Number of recounts to subtract.
  50  * @release: pointer to the function that will clean up the object when the
  51  *           last reference to the object is released.
  52  *           This pointer is required, and it is not acceptable to pass kfree
  53  *           in as this function.  If the caller does pass kfree to this
  54  *           function, you will be publicly mocked mercilessly by the kref
  55  *           maintainer, and anyone else who happens to notice it.  You have
  56  *           been warned.
  57  *
  58  * Subtract @count from the refcount, and if 0, call release().
  59  * Return 1 if the object was removed, otherwise return 0.  Beware, if this
  60  * function returns 0, you still can not count on the kref from remaining in
  61  * memory.  Only use the return value if you want to see if the kref is now
  62  * gone, not present.
  63  */
  64 static inline int kref_sub(struct kref *kref, unsigned int count,
  65              void (*release)(struct kref *kref))
  66 {
  67         WARN_ON(release == NULL);
  68
  69         if (atomic_sub_and_test((int) count, &kref->refcount)) {
  70                 release(kref);
  71                 return 1;
  72         }
  73         return 0;
  74 }
  75
  76 /**
  77  * kref_put - decrement refcount for object.
  78  * @kref: object.
  79  * @release: pointer to the function that will clean up the object when the
  80  *           last reference to the object is released.
  81  *           This pointer is required, and it is not acceptable to pass kfree
  82  *           in as this function.  If the caller does pass kfree to this
  83  *           function, you will be publicly mocked mercilessly by the kref
  84  *           maintainer, and anyone else who happens to notice it.  You have
  85  *           been warned.
  86  *
  87  * Decrement the refcount, and if 0, call release().
  88  * Return 1 if the object was removed, otherwise return 0.  Beware, if this
  89  * function returns 0, you still can not count on the kref from remaining in
  90  * memory.  Only use the return value if you want to see if the kref is now
  91  * gone, not present.
  92  */
  93 static inline int kref_put(struct kref *kref, void (*release)(struct kref *kref))
  94 {
  95         return kref_sub(kref, 1, release);
  96 }
  97
  98 static inline int kref_put_mutex(struct kref *kref,
  99                                  void (*release)(struct kref *kref),
 100                                  struct mutex *lock)
 101 {
 102         WARN_ON(release == NULL);
 103         if (unlikely(!atomic_add_unless(&kref->refcount, -1, 1))) {
 104                 mutex_lock(lock);
 105                 if (unlikely(!atomic_dec_and_test(&kref->refcount))) {
 106                         mutex_unlock(lock);
 107                         return 0;
 108                 }
 109                 release(kref);
 110                 return 1;
 111         }
 112         return 0;
 113 }
 114
 115 /**
 116  * kref_get_unless_zero - Increment refcount for object unless it is zero.
 117  * @kref: object.
 118  *
 119  * Return non-zero if the increment succeeded. Otherwise return 0.
 120  *
 121  * This function is intended to simplify locking around refcounting for
 122  * objects that can be looked up from a lookup structure, and which are
 123  * removed from that lookup structure in the object destructor.
 124  * Operations on such objects require at least a read lock around
 125  * lookup + kref_get, and a write lock around kref_put + remove from lookup
 126  * structure. Furthermore, RCU implementations become extremely tricky.
 127  * With a lookup followed by a kref_get_unless_zero *with return value check*
 128  * locking in the kref_put path can be deferred to the actual removal from
 129  * the lookup structure and RCU lookups become trivial.
 130  */
 131 static inline int __must_check kref_get_unless_zero(struct kref *kref)
 132 {
 133         return atomic_add_unless(&kref->refcount, 1, 0);
 134 }
 135 #endif /* _KREF_H_ */