drivers/staging/lustre/lustre/include/lu_ref.h

   1 /*
   2  * Copyright (c) 2008, 2010, Oracle and/or its affiliates. All rights reserved.
   3  * Use is subject to license terms.
   4  *
   5  * Copyright (c) 2012, Intel Corporation.
   6  *
   7  *   Author: Nikita Danilov <nikita.danilov@sun.com>
   8  *
   9  *   This file is part of Lustre, http://www.lustre.org.
  10  *
  11  *   Lustre is free software; you can redistribute it and/or
  12  *   modify it under the terms of version 2 of the GNU General Public
  13  *   License as published by the Free Software Foundation.
  14  *
  15  *   Lustre is distributed in the hope that it will be useful,
  16  *   but WITHOUT ANY WARRANTY; without even the implied warranty of
  17  *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  18  *   GNU General Public License for more details.
  19  *
  20  *   You should have received a copy of the GNU General Public License
  21  *   along with Lustre; if not, write to the Free Software
  22  *   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  23  *
  24  */
  25
  26 #ifndef __LUSTRE_LU_REF_H
  27 #define __LUSTRE_LU_REF_H
  28
  29 #include <linux/list.h>
  30
  31 /** \defgroup lu_ref lu_ref
  32  *
  33  * An interface to track references between objects. Mostly for debugging.
  34  *
  35  * Suppose there is a reference counted data-structure struct foo. To track
  36  * who acquired references to instance of struct foo, add lu_ref field to it:
  37  *
  38  * \code
  39  *       struct foo {
  40  *               atomic_t      foo_refcount;
  41  *               struct lu_ref foo_reference;
  42  *               ...
  43  *       };
  44  * \endcode
  45  *
  46  * foo::foo_reference has to be initialized by calling
  47  * lu_ref_init(). Typically there will be functions or macros to increment and
  48  * decrement foo::foo_refcount, let's say they are foo_get(struct foo *foo)
  49  * and foo_put(struct foo *foo), respectively.
  50  *
  51  * Whenever foo_get() is called to acquire a reference on a foo, lu_ref_add()
  52  * has to be called to insert into foo::foo_reference a record, describing
  53  * acquired reference. Dually, lu_ref_del() removes matching record. Typical
  54  * usages are:
  55  *
  56  * \code
  57  *      struct bar *bar;
  58  *
  59  *      // bar owns a reference to foo.
  60  *      bar->bar_foo = foo_get(foo);
  61  *      lu_ref_add(&foo->foo_reference, "bar", bar);
  62  *
  63  *      ...
  64  *
  65  *      // reference from bar to foo is released.
  66  *      lu_ref_del(&foo->foo_reference, "bar", bar);
  67  *      foo_put(bar->bar_foo);
  68  *
  69  *
  70  *      // current thread acquired a temporary reference to foo.
  71  *      foo_get(foo);
  72  *      lu_ref_add(&foo->reference, __FUNCTION__, current);
  73  *
  74  *      ...
  75  *
  76  *      // temporary reference is released.
  77  *      lu_ref_del(&foo->reference, __FUNCTION__, current);
  78  *      foo_put(foo);
  79  * \endcode
  80  *
  81  * \e Et \e cetera. Often it makes sense to include lu_ref_add() and
  82  * lu_ref_del() calls into foo_get() and foo_put(). When an instance of struct
  83  * foo is destroyed, lu_ref_fini() has to be called that checks that no
  84  * pending references remain. lu_ref_print() can be used to dump a list of
  85  * pending references, while hunting down a leak.
  86  *
  87  * For objects to which a large number of references can be acquired,
  88  * lu_ref_del() can become cpu consuming, as it has to scan the list of
  89  * references. To work around this, remember result of lu_ref_add() (usually
  90  * in the same place where pointer to struct foo is stored), and use
  91  * lu_ref_del_at():
  92  *
  93  * \code
  94  *      // There is a large number of bar's for a single foo.
  95  *      bar->bar_foo     = foo_get(foo);
  96  *      bar->bar_foo_ref = lu_ref_add(&foo->foo_reference, "bar", bar);
  97  *
  98  *      ...
  99  *
 100  *      // reference from bar to foo is released.
 101  *      lu_ref_del_at(&foo->foo_reference, bar->bar_foo_ref, "bar", bar);
 102  *      foo_put(bar->bar_foo);
 103  * \endcode
 104  *
 105  * lu_ref interface degrades gracefully in case of memory shortages.
 106  *
 107  * @{
 108  */
 109
 110
 111 struct lu_ref  {};
 112
 113 static inline void lu_ref_init(struct lu_ref *ref)
 114 {
 115 }
 116
 117 static inline void lu_ref_fini(struct lu_ref *ref)
 118 {
 119 }
 120
 121 static inline struct lu_ref_link *lu_ref_add(struct lu_ref *ref,
 122                                              const char *scope,
 123                                              const void *source)
 124 {
 125         return NULL;
 126 }
 127
 128 static inline struct lu_ref_link *lu_ref_add_atomic(struct lu_ref *ref,
 129                                                     const char *scope,
 130                                                     const void *source)
 131 {
 132         return NULL;
 133 }
 134
 135 static inline void lu_ref_del(struct lu_ref *ref, const char *scope,
 136                               const void *source)
 137 {
 138 }
 139
 140 static inline void lu_ref_set_at(struct lu_ref *ref, struct lu_ref_link *link,
 141                                  const char *scope, const void *source0,
 142                                  const void *source1)
 143 {
 144 }
 145
 146 static inline void lu_ref_del_at(struct lu_ref *ref, struct lu_ref_link *link,
 147                                  const char *scope, const void *source)
 148 {
 149 }
 150
 151 static inline int lu_ref_global_init(void)
 152 {
 153         return 0;
 154 }
 155
 156 static inline void lu_ref_global_fini(void)
 157 {
 158 }
 159
 160 static inline void lu_ref_print(const struct lu_ref *ref)
 161 {
 162 }
 163
 164 static inline void lu_ref_print_all(void)
 165 {
 166 }
 167
 168 /** @} lu */
 169
 170 #endif /* __LUSTRE_LU_REF_H */