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TOMOYO Linux Cross Reference
Linux/include/linux/kref.h

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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 
 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_ */
139 

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