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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); | |
2d722782 | 70 | void percpu_ref_reinit(struct percpu_ref *ref); |
9a1049da | 71 | void percpu_ref_exit(struct percpu_ref *ref); |
dbece3a0 TH |
72 | void percpu_ref_kill_and_confirm(struct percpu_ref *ref, |
73 | percpu_ref_func_t *confirm_kill); | |
74 | ||
75 | /** | |
76 | * percpu_ref_kill - drop the initial ref | |
77 | * @ref: percpu_ref to kill | |
78 | * | |
79 | * Must be used to drop the initial ref on a percpu refcount; must be called | |
80 | * precisely once before shutdown. | |
81 | * | |
82 | * Puts @ref in non percpu mode, then does a call_rcu() before gathering up the | |
83 | * percpu counters and dropping the initial ref. | |
84 | */ | |
85 | static inline void percpu_ref_kill(struct percpu_ref *ref) | |
86 | { | |
87 | return percpu_ref_kill_and_confirm(ref, NULL); | |
88 | } | |
215e262f | 89 | |
215e262f KO |
90 | #define PCPU_REF_DEAD 1 |
91 | ||
eae7975d TH |
92 | /* |
93 | * Internal helper. Don't use outside percpu-refcount proper. The | |
94 | * function doesn't return the pointer and let the caller test it for NULL | |
95 | * because doing so forces the compiler to generate two conditional | |
96 | * branches as it can't assume that @ref->pcpu_count is not NULL. | |
97 | */ | |
98 | static inline bool __pcpu_ref_alive(struct percpu_ref *ref, | |
99 | unsigned __percpu **pcpu_countp) | |
100 | { | |
7d742075 | 101 | unsigned long pcpu_ptr = ACCESS_ONCE(ref->pcpu_count_ptr); |
eae7975d | 102 | |
2d722782 TH |
103 | /* paired with smp_store_release() in percpu_ref_reinit() */ |
104 | smp_read_barrier_depends(); | |
105 | ||
eae7975d TH |
106 | if (unlikely(pcpu_ptr & PCPU_REF_DEAD)) |
107 | return false; | |
108 | ||
109 | *pcpu_countp = (unsigned __percpu *)pcpu_ptr; | |
110 | return true; | |
111 | } | |
215e262f KO |
112 | |
113 | /** | |
114 | * percpu_ref_get - increment a percpu refcount | |
ac899061 | 115 | * @ref: percpu_ref to get |
215e262f KO |
116 | * |
117 | * Analagous to atomic_inc(). | |
118 | */ | |
119 | static inline void percpu_ref_get(struct percpu_ref *ref) | |
120 | { | |
121 | unsigned __percpu *pcpu_count; | |
122 | ||
a4244454 | 123 | rcu_read_lock_sched(); |
215e262f | 124 | |
eae7975d | 125 | if (__pcpu_ref_alive(ref, &pcpu_count)) |
0c36b390 | 126 | this_cpu_inc(*pcpu_count); |
215e262f KO |
127 | else |
128 | atomic_inc(&ref->count); | |
129 | ||
a4244454 | 130 | rcu_read_unlock_sched(); |
215e262f KO |
131 | } |
132 | ||
4fb6e250 TH |
133 | /** |
134 | * percpu_ref_tryget - try to increment a percpu refcount | |
135 | * @ref: percpu_ref to try-get | |
136 | * | |
137 | * Increment a percpu refcount unless its count already reached zero. | |
138 | * Returns %true on success; %false on failure. | |
139 | * | |
140 | * The caller is responsible for ensuring that @ref stays accessible. | |
141 | */ | |
142 | static inline bool percpu_ref_tryget(struct percpu_ref *ref) | |
143 | { | |
144 | unsigned __percpu *pcpu_count; | |
145 | int ret = false; | |
146 | ||
147 | rcu_read_lock_sched(); | |
148 | ||
eae7975d | 149 | if (__pcpu_ref_alive(ref, &pcpu_count)) { |
315c5554 | 150 | this_cpu_inc(*pcpu_count); |
4fb6e250 TH |
151 | ret = true; |
152 | } else { | |
153 | ret = atomic_inc_not_zero(&ref->count); | |
154 | } | |
155 | ||
156 | rcu_read_unlock_sched(); | |
157 | ||
158 | return ret; | |
159 | } | |
160 | ||
dbece3a0 | 161 | /** |
2070d50e | 162 | * percpu_ref_tryget_live - try to increment a live percpu refcount |
dbece3a0 TH |
163 | * @ref: percpu_ref to try-get |
164 | * | |
165 | * Increment a percpu refcount unless it has already been killed. Returns | |
166 | * %true on success; %false on failure. | |
167 | * | |
168 | * Completion of percpu_ref_kill() in itself doesn't guarantee that tryget | |
169 | * will fail. For such guarantee, percpu_ref_kill_and_confirm() should be | |
170 | * used. After the confirm_kill callback is invoked, it's guaranteed that | |
171 | * no new reference will be given out by percpu_ref_tryget(). | |
4fb6e250 TH |
172 | * |
173 | * The caller is responsible for ensuring that @ref stays accessible. | |
dbece3a0 | 174 | */ |
2070d50e | 175 | static inline bool percpu_ref_tryget_live(struct percpu_ref *ref) |
dbece3a0 TH |
176 | { |
177 | unsigned __percpu *pcpu_count; | |
178 | int ret = false; | |
179 | ||
a4244454 | 180 | rcu_read_lock_sched(); |
dbece3a0 | 181 | |
eae7975d | 182 | if (__pcpu_ref_alive(ref, &pcpu_count)) { |
0c36b390 | 183 | this_cpu_inc(*pcpu_count); |
dbece3a0 TH |
184 | ret = true; |
185 | } | |
186 | ||
a4244454 | 187 | rcu_read_unlock_sched(); |
dbece3a0 TH |
188 | |
189 | return ret; | |
190 | } | |
191 | ||
215e262f KO |
192 | /** |
193 | * percpu_ref_put - decrement a percpu refcount | |
ac899061 | 194 | * @ref: percpu_ref to put |
215e262f KO |
195 | * |
196 | * Decrement the refcount, and if 0, call the release function (which was passed | |
197 | * to percpu_ref_init()) | |
198 | */ | |
199 | static inline void percpu_ref_put(struct percpu_ref *ref) | |
200 | { | |
201 | unsigned __percpu *pcpu_count; | |
202 | ||
a4244454 | 203 | rcu_read_lock_sched(); |
215e262f | 204 | |
eae7975d | 205 | if (__pcpu_ref_alive(ref, &pcpu_count)) |
0c36b390 | 206 | this_cpu_dec(*pcpu_count); |
215e262f KO |
207 | else if (unlikely(atomic_dec_and_test(&ref->count))) |
208 | ref->release(ref); | |
209 | ||
a4244454 | 210 | rcu_read_unlock_sched(); |
215e262f KO |
211 | } |
212 | ||
2d722782 TH |
213 | /** |
214 | * percpu_ref_is_zero - test whether a percpu refcount reached zero | |
215 | * @ref: percpu_ref to test | |
216 | * | |
217 | * Returns %true if @ref reached zero. | |
218 | */ | |
219 | static inline bool percpu_ref_is_zero(struct percpu_ref *ref) | |
220 | { | |
221 | unsigned __percpu *pcpu_count; | |
222 | ||
223 | if (__pcpu_ref_alive(ref, &pcpu_count)) | |
224 | return false; | |
225 | return !atomic_read(&ref->count); | |
226 | } | |
227 | ||
215e262f | 228 | #endif |