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#include <linux/spinlock.h>
23
24struct kref {
25	atomic_t refcount;
26};
27
28/**
29 * kref_init - initialize object.
30 * @kref: object in question.
31 */
32static inline void kref_init(struct kref *kref)
33{
34	atomic_set(&kref->refcount, 1);
35}
36
37/**
38 * kref_get - increment refcount for object.
39 * @kref: object.
40 */
41static inline void kref_get(struct kref *kref)
42{
43	/* If refcount was 0 before incrementing then we have a race
44	 * condition when this kref is freeing by some other thread right now.
45	 * In this case one should use kref_get_unless_zero()
46	 */
47	WARN_ON_ONCE(atomic_inc_return(&kref->refcount) < 2);
48}
49
50/**
51 * kref_sub - subtract a number of refcounts for object.
52 * @kref: object.
53 * @count: Number of recounts to subtract.
54 * @release: pointer to the function that will clean up the object when the
55 *	     last reference to the object is released.
56 *	     This pointer is required, and it is not acceptable to pass kfree
57 *	     in as this function.  If the caller does pass kfree to this
58 *	     function, you will be publicly mocked mercilessly by the kref
59 *	     maintainer, and anyone else who happens to notice it.  You have
60 *	     been warned.
61 *
62 * Subtract @count from the refcount, and if 0, call release().
63 * Return 1 if the object was removed, otherwise return 0.  Beware, if this
64 * function returns 0, you still can not count on the kref from remaining in
65 * memory.  Only use the return value if you want to see if the kref is now
66 * gone, not present.
67 */
68static inline int kref_sub(struct kref *kref, unsigned int count,
69	     void (*release)(struct kref *kref))
70{
71	WARN_ON(release == NULL);
72
73	if (atomic_sub_and_test((int) count, &kref->refcount)) {
74		release(kref);
75		return 1;
76	}
77	return 0;
78}
79
80/**
81 * kref_put - decrement refcount for object.
82 * @kref: object.
83 * @release: pointer to the function that will clean up the object when the
84 *	     last reference to the object is released.
85 *	     This pointer is required, and it is not acceptable to pass kfree
86 *	     in as this function.  If the caller does pass kfree to this
87 *	     function, you will be publicly mocked mercilessly by the kref
88 *	     maintainer, and anyone else who happens to notice it.  You have
89 *	     been warned.
90 *
91 * Decrement the refcount, and if 0, call release().
92 * Return 1 if the object was removed, otherwise return 0.  Beware, if this
93 * function returns 0, you still can not count on the kref from remaining in
94 * memory.  Only use the return value if you want to see if the kref is now
95 * gone, not present.
96 */
97static inline int kref_put(struct kref *kref, void (*release)(struct kref *kref))
98{
99	return kref_sub(kref, 1, release);
100}
101
102/**
103 * kref_put_spinlock_irqsave - decrement refcount for object.
104 * @kref: object.
105 * @release: pointer to the function that will clean up the object when the
106 *	     last reference to the object is released.
107 *	     This pointer is required, and it is not acceptable to pass kfree
108 *	     in as this function.
109 * @lock: lock to take in release case
110 *
111 * Behaves identical to kref_put with one exception.  If the reference count
112 * drops to zero, the lock will be taken atomically wrt dropping the reference
113 * count.  The release function has to call spin_unlock() without _irqrestore.
114 */
115static inline int kref_put_spinlock_irqsave(struct kref *kref,
116		void (*release)(struct kref *kref),
117		spinlock_t *lock)
118{
119	unsigned long flags;
120
121	WARN_ON(release == NULL);
122	if (atomic_add_unless(&kref->refcount, -1, 1))
123		return 0;
124	spin_lock_irqsave(lock, flags);
125	if (atomic_dec_and_test(&kref->refcount)) {
126		release(kref);
127		local_irq_restore(flags);
128		return 1;
129	}
130	spin_unlock_irqrestore(lock, flags);
131	return 0;
132}
133
134static inline int kref_put_mutex(struct kref *kref,
135				 void (*release)(struct kref *kref),
136				 struct mutex *lock)
137{
138	WARN_ON(release == NULL);
139	if (unlikely(!atomic_add_unless(&kref->refcount, -1, 1))) {
140		mutex_lock(lock);
141		if (unlikely(!atomic_dec_and_test(&kref->refcount))) {
142			mutex_unlock(lock);
143			return 0;
144		}
145		release(kref);
146		return 1;
147	}
148	return 0;
149}
150
151/**
152 * kref_get_unless_zero - Increment refcount for object unless it is zero.
153 * @kref: object.
154 *
155 * Return non-zero if the increment succeeded. Otherwise return 0.
156 *
157 * This function is intended to simplify locking around refcounting for
158 * objects that can be looked up from a lookup structure, and which are
159 * removed from that lookup structure in the object destructor.
160 * Operations on such objects require at least a read lock around
161 * lookup + kref_get, and a write lock around kref_put + remove from lookup
162 * structure. Furthermore, RCU implementations become extremely tricky.
163 * With a lookup followed by a kref_get_unless_zero *with return value check*
164 * locking in the kref_put path can be deferred to the actual removal from
165 * the lookup structure and RCU lookups become trivial.
166 */
167static inline int __must_check kref_get_unless_zero(struct kref *kref)
168{
169	return atomic_add_unless(&kref->refcount, 1, 0);
170}
171#endif /* _KREF_H_ */
172