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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
23struct kref {
24 atomic_t refcount;
25};
26
27/**
28 * kref_init - initialize object.
29 * @kref: object in question.
30 */
31static 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 */
40static 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 */
64static 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 */
93static inline int kref_put(struct kref *kref, void (*release)(struct kref *kref))
94{
95 return kref_sub(kref, 1, release);
96}
97
98static 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 */
131static 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_ */