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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 #include <linux/spinlock.h>
 23 
 24 struct kref {
 25         atomic_t refcount;
 26 };
 27 
 28 /**
 29  * kref_init - initialize object.
 30  * @kref: object in question.
 31  */
 32 static 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  */
 41 static 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  */
 68 static 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  */
 97 static 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  */
115 static 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 
134 static 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  */
167 static 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 

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