urcu/ref.h

   1 #ifndef _URCU_REF_H
   2 #define _URCU_REF_H
   3
   4 /*
   5  * Userspace RCU - Reference counting
   6  *
   7  * Copyright (C) 2009 Novell Inc.
   8  * Copyright (C) 2010 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   9  *
  10  * Author: Jan Blunck <jblunck@suse.de>
  11  *
  12  * This program is free software; you can redistribute it and/or modify it
  13  * under the terms of the GNU Lesser General Public License version 2.1 as
  14  * published by the Free  Software Foundation.
  15  */
  16
  17 #include <assert.h>
  18 #include <stdbool.h>
  19 #include <limits.h>
  20 #include <stdlib.h>
  21 #include <urcu/uatomic.h>
  22
  23 struct urcu_ref {
  24         long refcount; /* ATOMIC */
  25 };
  26
  27 static inline void urcu_ref_set(struct urcu_ref *ref, long val)
  28 {
  29         uatomic_set(&ref->refcount, val);
  30 }
  31
  32 static inline void urcu_ref_init(struct urcu_ref *ref)
  33 {
  34         urcu_ref_set(ref, 1);
  35 }
  36
  37 static inline void urcu_ref_get(struct urcu_ref *ref)
  38 {
  39         long old, _new, res;
  40
  41         old = uatomic_read(&ref->refcount);
  42         for (;;) {
  43                 if (old == LONG_MAX) {
  44                         abort();
  45                 }
  46                 _new = old + 1;
  47                 res = uatomic_cmpxchg(&ref->refcount, old, _new);
  48                 if (res == old) {
  49                         return;
  50                 }
  51                 old = res;
  52         }
  53 }
  54
  55 static inline void urcu_ref_put(struct urcu_ref *ref,
  56                                 void (*release)(struct urcu_ref *))
  57 {
  58         long res = uatomic_sub_return(&ref->refcount, 1);
  59         assert (res >= 0);
  60         if (res == 0)
  61                 release(ref);
  62 }
  63
  64 /*
  65  * urcu_ref_get_unless_zero
  66  *
  67  * Allows getting a reference atomically if the reference count is not
  68  * zero. Returns true if the reference is taken, false otherwise. This
  69  * needs to be used in conjunction with another synchronization
  70  * technique (e.g.  RCU or mutex) to ensure existence of the reference
  71  * count. False is also returned in case incrementing the refcount would
  72  * result in an overflow.
  73  */
  74 static inline bool urcu_ref_get_unless_zero(struct urcu_ref *ref)
  75 {
  76         long old, _new, res;
  77
  78         old = uatomic_read(&ref->refcount);
  79         for (;;) {
  80                 if (old == 0 || old == LONG_MAX)
  81                         return false;   /* Failure. */
  82                 _new = old + 1;
  83                 res = uatomic_cmpxchg(&ref->refcount, old, _new);
  84                 if (res == old) {
  85                         return true;    /* Success. */
  86                 }
  87                 old = res;
  88         }
  89 }
  90
  91 #endif /* _URCU_REF_H */