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Linus Torvalds1da177e2005-04-16 15:20:36 -07001/*
Robert P. J. Day932fb062010-03-13 07:58:13 -05002 * kref.h - library routines for handling generic reference counted objects
Linus Torvalds1da177e2005-04-16 15:20:36 -07003 *
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
Greg Kroah-Hartman6261dde2011-12-14 11:19:07 -080018#include <linux/bug.h>
19#include <linux/atomic.h>
James Bottomley67175b82012-01-17 21:14:05 +000020#include <linux/kernel.h>
Al Viro8ad5db82012-08-17 20:10:46 -040021#include <linux/mutex.h>
Linus Torvalds1da177e2005-04-16 15:20:36 -070022
23struct kref {
24 atomic_t refcount;
25};
26
Peter Zijlstra4af679c2011-12-13 10:36:20 +010027/**
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);
Peter Zijlstra4af679c2011-12-13 10:36:20 +010034}
Linus Torvalds1da177e2005-04-16 15:20:36 -070035
Peter Zijlstra4af679c2011-12-13 10:36:20 +010036/**
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);
Peter Zijlstra4af679c2011-12-13 10:36:20 +010044}
45
46/**
Peter Zijlstra4af679c2011-12-13 10:36:20 +010047 * 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
Greg Kroah-Hartman6261dde2011-12-14 11:19:07 -080053 * 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.
Peter Zijlstra4af679c2011-12-13 10:36:20 +010057 *
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);
Peter Zijlstra4af679c2011-12-13 10:36:20 +010068
69 if (atomic_sub_and_test((int) count, &kref->refcount)) {
70 release(kref);
71 return 1;
72 }
73 return 0;
74}
Peter Zijlstra47dbd7d2011-12-10 11:43:43 +010075
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
Greg Kroah-Hartman6261dde2011-12-14 11:19:07 -080082 * 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.
Peter Zijlstra47dbd7d2011-12-10 11:43:43 +010086 *
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}
Al Viro8ad5db82012-08-17 20:10:46 -040097
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}
Thomas Hellstrom4b20db32012-11-06 11:31:49 +0000114
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}
Linus Torvalds1da177e2005-04-16 15:20:36 -0700135#endif /* _KREF_H_ */