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         /* 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_ */