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215e262f KO |
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; | |
ac899061 | 54 | typedef void (percpu_ref_func_t)(struct percpu_ref *); |
215e262f KO |
55 | |
56 | struct percpu_ref { | |
57 | atomic_t count; | |
58 | /* | |
59 | * The low bit of the pointer indicates whether the ref is in percpu | |
9a1049da | 60 | * mode; if set, then get/put will manipulate the atomic_t. |
215e262f | 61 | */ |
7d742075 | 62 | unsigned long pcpu_count_ptr; |
ac899061 | 63 | percpu_ref_func_t *release; |
dbece3a0 | 64 | percpu_ref_func_t *confirm_kill; |
215e262f KO |
65 | struct rcu_head rcu; |
66 | }; | |
67 | ||
acac7883 TH |
68 | int __must_check percpu_ref_init(struct percpu_ref *ref, |
69 | percpu_ref_func_t *release); | |
9a1049da | 70 | void percpu_ref_exit(struct percpu_ref *ref); |
dbece3a0 TH |
71 | void percpu_ref_kill_and_confirm(struct percpu_ref *ref, |
72 | percpu_ref_func_t *confirm_kill); | |
73 | ||
74 | /** | |
75 | * percpu_ref_kill - drop the initial ref | |
76 | * @ref: percpu_ref to kill | |
77 | * | |
78 | * Must be used to drop the initial ref on a percpu refcount; must be called | |
79 | * precisely once before shutdown. | |
80 | * | |
81 | * Puts @ref in non percpu mode, then does a call_rcu() before gathering up the | |
82 | * percpu counters and dropping the initial ref. | |
83 | */ | |
84 | static inline void percpu_ref_kill(struct percpu_ref *ref) | |
85 | { | |
86 | return percpu_ref_kill_and_confirm(ref, NULL); | |
87 | } | |
215e262f | 88 | |
215e262f KO |
89 | #define PCPU_REF_DEAD 1 |
90 | ||
eae7975d TH |
91 | /* |
92 | * Internal helper. Don't use outside percpu-refcount proper. The | |
93 | * function doesn't return the pointer and let the caller test it for NULL | |
94 | * because doing so forces the compiler to generate two conditional | |
95 | * branches as it can't assume that @ref->pcpu_count is not NULL. | |
96 | */ | |
97 | static inline bool __pcpu_ref_alive(struct percpu_ref *ref, | |
98 | unsigned __percpu **pcpu_countp) | |
99 | { | |
7d742075 | 100 | unsigned long pcpu_ptr = ACCESS_ONCE(ref->pcpu_count_ptr); |
eae7975d TH |
101 | |
102 | if (unlikely(pcpu_ptr & PCPU_REF_DEAD)) | |
103 | return false; | |
104 | ||
105 | *pcpu_countp = (unsigned __percpu *)pcpu_ptr; | |
106 | return true; | |
107 | } | |
215e262f KO |
108 | |
109 | /** | |
110 | * percpu_ref_get - increment a percpu refcount | |
ac899061 | 111 | * @ref: percpu_ref to get |
215e262f KO |
112 | * |
113 | * Analagous to atomic_inc(). | |
114 | */ | |
115 | static inline void percpu_ref_get(struct percpu_ref *ref) | |
116 | { | |
117 | unsigned __percpu *pcpu_count; | |
118 | ||
a4244454 | 119 | rcu_read_lock_sched(); |
215e262f | 120 | |
eae7975d | 121 | if (__pcpu_ref_alive(ref, &pcpu_count)) |
0c36b390 | 122 | this_cpu_inc(*pcpu_count); |
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123 | else |
124 | atomic_inc(&ref->count); | |
125 | ||
a4244454 | 126 | rcu_read_unlock_sched(); |
215e262f KO |
127 | } |
128 | ||
4fb6e250 TH |
129 | /** |
130 | * percpu_ref_tryget - try to increment a percpu refcount | |
131 | * @ref: percpu_ref to try-get | |
132 | * | |
133 | * Increment a percpu refcount unless its count already reached zero. | |
134 | * Returns %true on success; %false on failure. | |
135 | * | |
136 | * The caller is responsible for ensuring that @ref stays accessible. | |
137 | */ | |
138 | static inline bool percpu_ref_tryget(struct percpu_ref *ref) | |
139 | { | |
140 | unsigned __percpu *pcpu_count; | |
141 | int ret = false; | |
142 | ||
143 | rcu_read_lock_sched(); | |
144 | ||
eae7975d | 145 | if (__pcpu_ref_alive(ref, &pcpu_count)) { |
315c5554 | 146 | this_cpu_inc(*pcpu_count); |
4fb6e250 TH |
147 | ret = true; |
148 | } else { | |
149 | ret = atomic_inc_not_zero(&ref->count); | |
150 | } | |
151 | ||
152 | rcu_read_unlock_sched(); | |
153 | ||
154 | return ret; | |
155 | } | |
156 | ||
dbece3a0 | 157 | /** |
2070d50e | 158 | * percpu_ref_tryget_live - try to increment a live percpu refcount |
dbece3a0 TH |
159 | * @ref: percpu_ref to try-get |
160 | * | |
161 | * Increment a percpu refcount unless it has already been killed. Returns | |
162 | * %true on success; %false on failure. | |
163 | * | |
164 | * Completion of percpu_ref_kill() in itself doesn't guarantee that tryget | |
165 | * will fail. For such guarantee, percpu_ref_kill_and_confirm() should be | |
166 | * used. After the confirm_kill callback is invoked, it's guaranteed that | |
167 | * no new reference will be given out by percpu_ref_tryget(). | |
4fb6e250 TH |
168 | * |
169 | * The caller is responsible for ensuring that @ref stays accessible. | |
dbece3a0 | 170 | */ |
2070d50e | 171 | static inline bool percpu_ref_tryget_live(struct percpu_ref *ref) |
dbece3a0 TH |
172 | { |
173 | unsigned __percpu *pcpu_count; | |
174 | int ret = false; | |
175 | ||
a4244454 | 176 | rcu_read_lock_sched(); |
dbece3a0 | 177 | |
eae7975d | 178 | if (__pcpu_ref_alive(ref, &pcpu_count)) { |
0c36b390 | 179 | this_cpu_inc(*pcpu_count); |
dbece3a0 TH |
180 | ret = true; |
181 | } | |
182 | ||
a4244454 | 183 | rcu_read_unlock_sched(); |
dbece3a0 TH |
184 | |
185 | return ret; | |
186 | } | |
187 | ||
215e262f KO |
188 | /** |
189 | * percpu_ref_put - decrement a percpu refcount | |
ac899061 | 190 | * @ref: percpu_ref to put |
215e262f KO |
191 | * |
192 | * Decrement the refcount, and if 0, call the release function (which was passed | |
193 | * to percpu_ref_init()) | |
194 | */ | |
195 | static inline void percpu_ref_put(struct percpu_ref *ref) | |
196 | { | |
197 | unsigned __percpu *pcpu_count; | |
198 | ||
a4244454 | 199 | rcu_read_lock_sched(); |
215e262f | 200 | |
eae7975d | 201 | if (__pcpu_ref_alive(ref, &pcpu_count)) |
0c36b390 | 202 | this_cpu_dec(*pcpu_count); |
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203 | else if (unlikely(atomic_dec_and_test(&ref->count))) |
204 | ref->release(ref); | |
205 | ||
a4244454 | 206 | rcu_read_unlock_sched(); |
215e262f KO |
207 | } |
208 | ||
209 | #endif |