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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 : :
53 : : struct percpu_ref;
54 : : typedef void (percpu_ref_func_t)(struct percpu_ref *);
55 : :
56 : : struct 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 : :
70 : : int __must_check percpu_ref_init(struct percpu_ref *ref,
71 : : percpu_ref_func_t *release);
72 : : void percpu_ref_cancel_init(struct percpu_ref *ref);
73 : : void 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 : : */
86 : : static inline void percpu_ref_kill(struct percpu_ref *ref)
87 : : {
88 : 0 : 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 : : */
104 : : static inline void percpu_ref_get(struct percpu_ref *ref)
105 : : {
106 : : unsigned __percpu *pcpu_count;
107 : :
108 : : rcu_read_lock_sched();
109 : :
110 : 0 : pcpu_count = ACCESS_ONCE(ref->pcpu_count);
111 : :
112 [ # # # # : 0 : if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR))
# # # # ]
113 : 0 : __this_cpu_inc(*pcpu_count);
114 : : else
115 : 0 : 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 : : */
132 : : static 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 : 0 : pcpu_count = ACCESS_ONCE(ref->pcpu_count);
140 : :
141 [ # # ]: 0 : if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR)) {
142 : 0 : __this_cpu_inc(*pcpu_count);
143 : : ret = true;
144 : : }
145 : :
146 : : rcu_read_unlock_sched();
147 : :
148 : 0 : 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 : : */
158 : : static inline void percpu_ref_put(struct percpu_ref *ref)
159 : : {
160 : : unsigned __percpu *pcpu_count;
161 : :
162 : : rcu_read_lock_sched();
163 : :
164 : 0 : pcpu_count = ACCESS_ONCE(ref->pcpu_count);
165 : :
166 [ # # # # : 0 : if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR))
# # # # #
# # # # #
# # ]
167 : 0 : __this_cpu_dec(*pcpu_count);
168 [ # # # # : 0 : else if (unlikely(atomic_dec_and_test(&ref->count)))
# # # # #
# # # # #
# # ]
169 : 0 : ref->release(ref);
170 : :
171 : : rcu_read_unlock_sched();
172 : : }
173 : :
174 : : #endif
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