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1/*
2 * Percpu refcounts:
3 * (C) 2012 Google, Inc.
4 * Author: Kent Overstreet <koverstreet@google.com>
5 *
6 * This implements a refcount with similar semantics to atomic_t - atomic_inc(),
7 * atomic_dec_and_test() - but percpu.
8 *
9 * There's one important difference between percpu refs and normal atomic_t
10 * refcounts; you have to keep track of your initial refcount, and then when you
11 * start shutting down you call percpu_ref_kill() _before_ dropping the initial
12 * refcount.
13 *
14 * The refcount will have a range of 0 to ((1U << 31) - 1), i.e. one bit less
15 * than an atomic_t - this is because of the way shutdown works, see
16 * percpu_ref_kill()/PCPU_COUNT_BIAS.
17 *
18 * Before you call percpu_ref_kill(), percpu_ref_put() does not check for the
19 * refcount hitting 0 - it can't, if it was in percpu mode. percpu_ref_kill()
20 * puts the ref back in single atomic_t mode, collecting the per cpu refs and
21 * issuing the appropriate barriers, and then marks the ref as shutting down so
22 * that percpu_ref_put() will check for the ref hitting 0. After it returns,
23 * it's safe to drop the initial ref.
24 *
25 * USAGE:
26 *
27 * See fs/aio.c for some example usage; it's used there for struct kioctx, which
28 * is created when userspaces calls io_setup(), and destroyed when userspace
29 * calls io_destroy() or the process exits.
30 *
31 * In the aio code, kill_ioctx() is called when we wish to destroy a kioctx; it
32 * calls percpu_ref_kill(), then hlist_del_rcu() and sychronize_rcu() to remove
33 * the kioctx from the proccess's list of kioctxs - after that, there can't be
34 * any new users of the kioctx (from lookup_ioctx()) and it's then safe to drop
35 * the initial ref with percpu_ref_put().
36 *
37 * Code that does a two stage shutdown like this often needs some kind of
38 * explicit synchronization to ensure the initial refcount can only be dropped
39 * once - percpu_ref_kill() does this for you, it returns true once and false if
40 * someone else already called it. The aio code uses it this way, but it's not
41 * necessary if the code has some other mechanism to synchronize teardown.
42 * around.
43 */
44
45#ifndef _LINUX_PERCPU_REFCOUNT_H
46#define _LINUX_PERCPU_REFCOUNT_H
47
48#include <linux/atomic.h>
49#include <linux/kernel.h>
50#include <linux/percpu.h>
51#include <linux/rcupdate.h>
52
53struct percpu_ref;
54typedef void (percpu_ref_func_t)(struct percpu_ref *);
55
56struct percpu_ref {
57 atomic_t count;
58 /*
59 * The low bit of the pointer indicates whether the ref is in percpu
60 * mode; if set, then get/put will manipulate the atomic_t.
61 */
62 unsigned long pcpu_count_ptr;
63 percpu_ref_func_t *release;
64 percpu_ref_func_t *confirm_kill;
65 struct rcu_head rcu;
66};
67
68int __must_check percpu_ref_init(struct percpu_ref *ref,
69 percpu_ref_func_t *release);
70void percpu_ref_reinit(struct percpu_ref *ref);
71void percpu_ref_exit(struct percpu_ref *ref);
72void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
73 percpu_ref_func_t *confirm_kill);
74void __percpu_ref_kill_expedited(struct percpu_ref *ref);
75
76/**
77 * percpu_ref_kill - drop the initial ref
78 * @ref: percpu_ref to kill
79 *
80 * Must be used to drop the initial ref on a percpu refcount; must be called
81 * precisely once before shutdown.
82 *
83 * Puts @ref in non percpu mode, then does a call_rcu() before gathering up the
84 * percpu counters and dropping the initial ref.
85 */
86static inline void percpu_ref_kill(struct percpu_ref *ref)
87{
88 return percpu_ref_kill_and_confirm(ref, NULL);
89}
90
91#define PCPU_REF_DEAD 1
92
93/*
94 * Internal helper. Don't use outside percpu-refcount proper. The
95 * function doesn't return the pointer and let the caller test it for NULL
96 * because doing so forces the compiler to generate two conditional
97 * branches as it can't assume that @ref->pcpu_count is not NULL.
98 */
99static inline bool __pcpu_ref_alive(struct percpu_ref *ref,
100 unsigned __percpu **pcpu_countp)
101{
102 unsigned long pcpu_ptr = ACCESS_ONCE(ref->pcpu_count_ptr);
103
104 /* paired with smp_store_release() in percpu_ref_reinit() */
105 smp_read_barrier_depends();
106
107 if (unlikely(pcpu_ptr & PCPU_REF_DEAD))
108 return false;
109
110 *pcpu_countp = (unsigned __percpu *)pcpu_ptr;
111 return true;
112}
113
114/**
115 * percpu_ref_get - increment a percpu refcount
116 * @ref: percpu_ref to get
117 *
118 * Analagous to atomic_inc().
119 */
120static inline void percpu_ref_get(struct percpu_ref *ref)
121{
122 unsigned __percpu *pcpu_count;
123
124 rcu_read_lock_sched();
125
126 if (__pcpu_ref_alive(ref, &pcpu_count))
127 this_cpu_inc(*pcpu_count);
128 else
129 atomic_inc(&ref->count);
130
131 rcu_read_unlock_sched();
132}
133
134/**
135 * percpu_ref_tryget - try to increment a percpu refcount
136 * @ref: percpu_ref to try-get
137 *
138 * Increment a percpu refcount unless its count already reached zero.
139 * Returns %true on success; %false on failure.
140 *
141 * The caller is responsible for ensuring that @ref stays accessible.
142 */
143static inline bool percpu_ref_tryget(struct percpu_ref *ref)
144{
145 unsigned __percpu *pcpu_count;
146 int ret = false;
147
148 rcu_read_lock_sched();
149
150 if (__pcpu_ref_alive(ref, &pcpu_count)) {
151 this_cpu_inc(*pcpu_count);
152 ret = true;
153 } else {
154 ret = atomic_inc_not_zero(&ref->count);
155 }
156
157 rcu_read_unlock_sched();
158
159 return ret;
160}
161
162/**
163 * percpu_ref_tryget_live - try to increment a live percpu refcount
164 * @ref: percpu_ref to try-get
165 *
166 * Increment a percpu refcount unless it has already been killed. Returns
167 * %true on success; %false on failure.
168 *
169 * Completion of percpu_ref_kill() in itself doesn't guarantee that tryget
170 * will fail. For such guarantee, percpu_ref_kill_and_confirm() should be
171 * used. After the confirm_kill callback is invoked, it's guaranteed that
172 * no new reference will be given out by percpu_ref_tryget().
173 *
174 * The caller is responsible for ensuring that @ref stays accessible.
175 */
176static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
177{
178 unsigned __percpu *pcpu_count;
179 int ret = false;
180
181 rcu_read_lock_sched();
182
183 if (__pcpu_ref_alive(ref, &pcpu_count)) {
184 this_cpu_inc(*pcpu_count);
185 ret = true;
186 }
187
188 rcu_read_unlock_sched();
189
190 return ret;
191}
192
193/**
194 * percpu_ref_put - decrement a percpu refcount
195 * @ref: percpu_ref to put
196 *
197 * Decrement the refcount, and if 0, call the release function (which was passed
198 * to percpu_ref_init())
199 */
200static inline void percpu_ref_put(struct percpu_ref *ref)
201{
202 unsigned __percpu *pcpu_count;
203
204 rcu_read_lock_sched();
205
206 if (__pcpu_ref_alive(ref, &pcpu_count))
207 this_cpu_dec(*pcpu_count);
208 else if (unlikely(atomic_dec_and_test(&ref->count)))
209 ref->release(ref);
210
211 rcu_read_unlock_sched();
212}
213
214/**
215 * percpu_ref_is_zero - test whether a percpu refcount reached zero
216 * @ref: percpu_ref to test
217 *
218 * Returns %true if @ref reached zero.
219 */
220static inline bool percpu_ref_is_zero(struct percpu_ref *ref)
221{
222 unsigned __percpu *pcpu_count;
223
224 if (__pcpu_ref_alive(ref, &pcpu_count))
225 return false;
226 return !atomic_read(&ref->count);
227}
228
229#endif