include/linux/kref.h

   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_ */