2 * kref.h - library routines for handling generic reference counted objects
4 * Copyright (C) 2004 Greg Kroah-Hartman <greg@kroah.com>
5 * Copyright (C) 2004 IBM Corp.
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
11 * This file is released under the GPLv2.
18 #include <linux/bug.h>
19 #include <linux/atomic.h>
20 #include <linux/kernel.h>
21 #include <linux/mutex.h>
27 #define KREF_INIT(n) { .refcount = ATOMIC_INIT(n), }
30 * kref_init - initialize object.
31 * @kref: object in question.
33 static inline void kref_init(struct kref *kref)
35 atomic_set(&kref->refcount, 1);
38 static inline int kref_read(const struct kref *kref)
40 return atomic_read(&kref->refcount);
44 * kref_get - increment refcount for object.
47 static inline void kref_get(struct kref *kref)
49 /* If refcount was 0 before incrementing then we have a race
50 * condition when this kref is freeing by some other thread right now.
51 * In this case one should use kref_get_unless_zero()
53 WARN_ON_ONCE(atomic_inc_return(&kref->refcount) < 2);
57 * kref_put - decrement refcount for object.
59 * @release: pointer to the function that will clean up the object when the
60 * last reference to the object is released.
61 * This pointer is required, and it is not acceptable to pass kfree
62 * in as this function. If the caller does pass kfree to this
63 * function, you will be publicly mocked mercilessly by the kref
64 * maintainer, and anyone else who happens to notice it. You have
67 * Decrement the refcount, and if 0, call release().
68 * Return 1 if the object was removed, otherwise return 0. Beware, if this
69 * function returns 0, you still can not count on the kref from remaining in
70 * memory. Only use the return value if you want to see if the kref is now
73 static inline int kref_put(struct kref *kref, void (*release)(struct kref *kref))
75 WARN_ON(release == NULL);
77 if (atomic_dec_and_test(&kref->refcount)) {
84 static inline int kref_put_mutex(struct kref *kref,
85 void (*release)(struct kref *kref),
88 WARN_ON(release == NULL);
89 if (unlikely(!atomic_add_unless(&kref->refcount, -1, 1))) {
91 if (unlikely(!atomic_dec_and_test(&kref->refcount))) {
102 * kref_get_unless_zero - Increment refcount for object unless it is zero.
105 * Return non-zero if the increment succeeded. Otherwise return 0.
107 * This function is intended to simplify locking around refcounting for
108 * objects that can be looked up from a lookup structure, and which are
109 * removed from that lookup structure in the object destructor.
110 * Operations on such objects require at least a read lock around
111 * lookup + kref_get, and a write lock around kref_put + remove from lookup
112 * structure. Furthermore, RCU implementations become extremely tricky.
113 * With a lookup followed by a kref_get_unless_zero *with return value check*
114 * locking in the kref_put path can be deferred to the actual removal from
115 * the lookup structure and RCU lookups become trivial.
117 static inline int __must_check kref_get_unless_zero(struct kref *kref)
119 return atomic_add_unless(&kref->refcount, 1, 0);
121 #endif /* _KREF_H_ */