projects/linux·blob v3.13 · refs

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1/*
2 * Percpu refcounts:
3 * (C) 2012 Google, Inc.
4 * Author: Kent Overstreet <[email protected]>
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 (this is a
61 * hack because we need to keep the pointer around for
62 * percpu_ref_kill_rcu())
63 */
64 unsigned __percpu *pcpu_count;
65 percpu_ref_func_t *release;
66 percpu_ref_func_t *confirm_kill;
67 struct rcu_head rcu;
68};
69
70int __must_check percpu_ref_init(struct percpu_ref *ref,
71 percpu_ref_func_t *release);
72void percpu_ref_cancel_init(struct percpu_ref *ref);
73void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
74 percpu_ref_func_t *confirm_kill);
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_STATUS_BITS 2
92#define PCPU_STATUS_MASK ((1 << PCPU_STATUS_BITS) - 1)
93#define PCPU_REF_PTR 0
94#define PCPU_REF_DEAD 1
95
96#define REF_STATUS(count) (((unsigned long) count) & PCPU_STATUS_MASK)
97
98/**
99 * percpu_ref_get - increment a percpu refcount
100 * @ref: percpu_ref to get
101 *
102 * Analagous to atomic_inc().
103 */
104static inline void percpu_ref_get(struct percpu_ref *ref)
105{
106 unsigned __percpu *pcpu_count;
107
108 rcu_read_lock_sched();
109
110 pcpu_count = ACCESS_ONCE(ref->pcpu_count);
111
112 if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR))
113 __this_cpu_inc(*pcpu_count);
114 else
115 atomic_inc(&ref->count);
116
117 rcu_read_unlock_sched();
118}
119
120/**
121 * percpu_ref_tryget - try to increment a percpu refcount
122 * @ref: percpu_ref to try-get
123 *
124 * Increment a percpu refcount unless it has already been killed. Returns
125 * %true on success; %false on failure.
126 *
127 * Completion of percpu_ref_kill() in itself doesn't guarantee that tryget
128 * will fail. For such guarantee, percpu_ref_kill_and_confirm() should be
129 * used. After the confirm_kill callback is invoked, it's guaranteed that
130 * no new reference will be given out by percpu_ref_tryget().
131 */
132static inline bool percpu_ref_tryget(struct percpu_ref *ref)
133{
134 unsigned __percpu *pcpu_count;
135 int ret = false;
136
137 rcu_read_lock_sched();
138
139 pcpu_count = ACCESS_ONCE(ref->pcpu_count);
140
141 if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR)) {
142 __this_cpu_inc(*pcpu_count);
143 ret = true;
144 }
145
146 rcu_read_unlock_sched();
147
148 return ret;
149}
150
151/**
152 * percpu_ref_put - decrement a percpu refcount
153 * @ref: percpu_ref to put
154 *
155 * Decrement the refcount, and if 0, call the release function (which was passed
156 * to percpu_ref_init())
157 */
158static inline void percpu_ref_put(struct percpu_ref *ref)
159{
160 unsigned __percpu *pcpu_count;
161
162 rcu_read_lock_sched();
163
164 pcpu_count = ACCESS_ONCE(ref->pcpu_count);
165
166 if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR))
167 __this_cpu_dec(*pcpu_count);
168 else if (unlikely(atomic_dec_and_test(&ref->count)))
169 ref->release(ref);
170
171 rcu_read_unlock_sched();
172}
173
174#endif