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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 | |
eecc16ba | 16 | * percpu_ref_kill()/PERCPU_COUNT_BIAS. |
215e262f KO |
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 | |
d7cdb968 | 32 | * calls percpu_ref_kill(), then hlist_del_rcu() and synchronize_rcu() to remove |
215e262f KO |
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> | |
a34375ef | 52 | #include <linux/gfp.h> |
215e262f KO |
53 | |
54 | struct percpu_ref; | |
ac899061 | 55 | typedef void (percpu_ref_func_t)(struct percpu_ref *); |
215e262f | 56 | |
9e804d1f TH |
57 | /* flags set in the lower bits of percpu_ref->percpu_count_ptr */ |
58 | enum { | |
59 | __PERCPU_REF_ATOMIC = 1LU << 0, /* operating in atomic mode */ | |
27344a90 TH |
60 | __PERCPU_REF_DEAD = 1LU << 1, /* (being) killed */ |
61 | __PERCPU_REF_ATOMIC_DEAD = __PERCPU_REF_ATOMIC | __PERCPU_REF_DEAD, | |
62 | ||
63 | __PERCPU_REF_FLAG_BITS = 2, | |
9e804d1f TH |
64 | }; |
65 | ||
2aad2a86 TH |
66 | /* @flags for percpu_ref_init() */ |
67 | enum { | |
68 | /* | |
69 | * Start w/ ref == 1 in atomic mode. Can be switched to percpu | |
1cae13e7 TH |
70 | * operation using percpu_ref_switch_to_percpu(). If initialized |
71 | * with this flag, the ref will stay in atomic mode until | |
72 | * percpu_ref_switch_to_percpu() is invoked on it. | |
2aad2a86 TH |
73 | */ |
74 | PERCPU_REF_INIT_ATOMIC = 1 << 0, | |
75 | ||
76 | /* | |
77 | * Start dead w/ ref == 0 in atomic mode. Must be revived with | |
78 | * percpu_ref_reinit() before used. Implies INIT_ATOMIC. | |
79 | */ | |
80 | PERCPU_REF_INIT_DEAD = 1 << 1, | |
81 | }; | |
82 | ||
215e262f | 83 | struct percpu_ref { |
e625305b | 84 | atomic_long_t count; |
215e262f KO |
85 | /* |
86 | * The low bit of the pointer indicates whether the ref is in percpu | |
9a1049da | 87 | * mode; if set, then get/put will manipulate the atomic_t. |
215e262f | 88 | */ |
eecc16ba | 89 | unsigned long percpu_count_ptr; |
ac899061 | 90 | percpu_ref_func_t *release; |
9e804d1f | 91 | percpu_ref_func_t *confirm_switch; |
1cae13e7 | 92 | bool force_atomic:1; |
215e262f KO |
93 | struct rcu_head rcu; |
94 | }; | |
95 | ||
acac7883 | 96 | int __must_check percpu_ref_init(struct percpu_ref *ref, |
2aad2a86 TH |
97 | percpu_ref_func_t *release, unsigned int flags, |
98 | gfp_t gfp); | |
9a1049da | 99 | void percpu_ref_exit(struct percpu_ref *ref); |
490c79a6 TH |
100 | void percpu_ref_switch_to_atomic(struct percpu_ref *ref, |
101 | percpu_ref_func_t *confirm_switch); | |
f47ad457 | 102 | void percpu_ref_switch_to_percpu(struct percpu_ref *ref); |
dbece3a0 TH |
103 | void percpu_ref_kill_and_confirm(struct percpu_ref *ref, |
104 | percpu_ref_func_t *confirm_kill); | |
f47ad457 | 105 | void percpu_ref_reinit(struct percpu_ref *ref); |
dbece3a0 TH |
106 | |
107 | /** | |
108 | * percpu_ref_kill - drop the initial ref | |
109 | * @ref: percpu_ref to kill | |
110 | * | |
111 | * Must be used to drop the initial ref on a percpu refcount; must be called | |
112 | * precisely once before shutdown. | |
113 | * | |
114 | * Puts @ref in non percpu mode, then does a call_rcu() before gathering up the | |
115 | * percpu counters and dropping the initial ref. | |
116 | */ | |
117 | static inline void percpu_ref_kill(struct percpu_ref *ref) | |
118 | { | |
119 | return percpu_ref_kill_and_confirm(ref, NULL); | |
120 | } | |
215e262f | 121 | |
eae7975d TH |
122 | /* |
123 | * Internal helper. Don't use outside percpu-refcount proper. The | |
124 | * function doesn't return the pointer and let the caller test it for NULL | |
125 | * because doing so forces the compiler to generate two conditional | |
eecc16ba | 126 | * branches as it can't assume that @ref->percpu_count is not NULL. |
eae7975d | 127 | */ |
9e804d1f TH |
128 | static inline bool __ref_is_percpu(struct percpu_ref *ref, |
129 | unsigned long __percpu **percpu_countp) | |
eae7975d | 130 | { |
eecc16ba | 131 | unsigned long percpu_ptr = ACCESS_ONCE(ref->percpu_count_ptr); |
eae7975d | 132 | |
2d722782 TH |
133 | /* paired with smp_store_release() in percpu_ref_reinit() */ |
134 | smp_read_barrier_depends(); | |
135 | ||
4aab3b5b TH |
136 | /* |
137 | * Theoretically, the following could test just ATOMIC; however, | |
138 | * then we'd have to mask off DEAD separately as DEAD may be | |
139 | * visible without ATOMIC if we race with percpu_ref_kill(). DEAD | |
140 | * implies ATOMIC anyway. Test them together. | |
141 | */ | |
142 | if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC_DEAD)) | |
eae7975d TH |
143 | return false; |
144 | ||
eecc16ba | 145 | *percpu_countp = (unsigned long __percpu *)percpu_ptr; |
eae7975d TH |
146 | return true; |
147 | } | |
215e262f KO |
148 | |
149 | /** | |
150 | * percpu_ref_get - increment a percpu refcount | |
ac899061 | 151 | * @ref: percpu_ref to get |
215e262f | 152 | * |
6251f997 TH |
153 | * Analagous to atomic_long_inc(). |
154 | * | |
155 | * This function is safe to call as long as @ref is between init and exit. | |
156 | */ | |
215e262f KO |
157 | static inline void percpu_ref_get(struct percpu_ref *ref) |
158 | { | |
eecc16ba | 159 | unsigned long __percpu *percpu_count; |
215e262f | 160 | |
a4244454 | 161 | rcu_read_lock_sched(); |
215e262f | 162 | |
9e804d1f | 163 | if (__ref_is_percpu(ref, &percpu_count)) |
eecc16ba | 164 | this_cpu_inc(*percpu_count); |
215e262f | 165 | else |
e625305b | 166 | atomic_long_inc(&ref->count); |
215e262f | 167 | |
a4244454 | 168 | rcu_read_unlock_sched(); |
215e262f KO |
169 | } |
170 | ||
4fb6e250 TH |
171 | /** |
172 | * percpu_ref_tryget - try to increment a percpu refcount | |
173 | * @ref: percpu_ref to try-get | |
174 | * | |
175 | * Increment a percpu refcount unless its count already reached zero. | |
176 | * Returns %true on success; %false on failure. | |
177 | * | |
6251f997 | 178 | * This function is safe to call as long as @ref is between init and exit. |
4fb6e250 TH |
179 | */ |
180 | static inline bool percpu_ref_tryget(struct percpu_ref *ref) | |
181 | { | |
eecc16ba | 182 | unsigned long __percpu *percpu_count; |
6251f997 | 183 | int ret; |
4fb6e250 TH |
184 | |
185 | rcu_read_lock_sched(); | |
186 | ||
9e804d1f | 187 | if (__ref_is_percpu(ref, &percpu_count)) { |
eecc16ba | 188 | this_cpu_inc(*percpu_count); |
4fb6e250 TH |
189 | ret = true; |
190 | } else { | |
e625305b | 191 | ret = atomic_long_inc_not_zero(&ref->count); |
4fb6e250 TH |
192 | } |
193 | ||
194 | rcu_read_unlock_sched(); | |
195 | ||
196 | return ret; | |
197 | } | |
198 | ||
dbece3a0 | 199 | /** |
2070d50e | 200 | * percpu_ref_tryget_live - try to increment a live percpu refcount |
dbece3a0 TH |
201 | * @ref: percpu_ref to try-get |
202 | * | |
203 | * Increment a percpu refcount unless it has already been killed. Returns | |
204 | * %true on success; %false on failure. | |
205 | * | |
6251f997 TH |
206 | * Completion of percpu_ref_kill() in itself doesn't guarantee that this |
207 | * function will fail. For such guarantee, percpu_ref_kill_and_confirm() | |
208 | * should be used. After the confirm_kill callback is invoked, it's | |
209 | * guaranteed that no new reference will be given out by | |
210 | * percpu_ref_tryget_live(). | |
4fb6e250 | 211 | * |
6251f997 | 212 | * This function is safe to call as long as @ref is between init and exit. |
dbece3a0 | 213 | */ |
2070d50e | 214 | static inline bool percpu_ref_tryget_live(struct percpu_ref *ref) |
dbece3a0 | 215 | { |
eecc16ba | 216 | unsigned long __percpu *percpu_count; |
dbece3a0 TH |
217 | int ret = false; |
218 | ||
a4244454 | 219 | rcu_read_lock_sched(); |
dbece3a0 | 220 | |
9e804d1f | 221 | if (__ref_is_percpu(ref, &percpu_count)) { |
eecc16ba | 222 | this_cpu_inc(*percpu_count); |
dbece3a0 | 223 | ret = true; |
490c79a6 TH |
224 | } else if (!(ACCESS_ONCE(ref->percpu_count_ptr) & __PERCPU_REF_DEAD)) { |
225 | ret = atomic_long_inc_not_zero(&ref->count); | |
dbece3a0 TH |
226 | } |
227 | ||
a4244454 | 228 | rcu_read_unlock_sched(); |
dbece3a0 TH |
229 | |
230 | return ret; | |
231 | } | |
232 | ||
215e262f KO |
233 | /** |
234 | * percpu_ref_put - decrement a percpu refcount | |
ac899061 | 235 | * @ref: percpu_ref to put |
215e262f KO |
236 | * |
237 | * Decrement the refcount, and if 0, call the release function (which was passed | |
238 | * to percpu_ref_init()) | |
6251f997 TH |
239 | * |
240 | * This function is safe to call as long as @ref is between init and exit. | |
215e262f KO |
241 | */ |
242 | static inline void percpu_ref_put(struct percpu_ref *ref) | |
243 | { | |
eecc16ba | 244 | unsigned long __percpu *percpu_count; |
215e262f | 245 | |
a4244454 | 246 | rcu_read_lock_sched(); |
215e262f | 247 | |
9e804d1f | 248 | if (__ref_is_percpu(ref, &percpu_count)) |
eecc16ba | 249 | this_cpu_dec(*percpu_count); |
e625305b | 250 | else if (unlikely(atomic_long_dec_and_test(&ref->count))) |
215e262f KO |
251 | ref->release(ref); |
252 | ||
a4244454 | 253 | rcu_read_unlock_sched(); |
215e262f KO |
254 | } |
255 | ||
2d722782 TH |
256 | /** |
257 | * percpu_ref_is_zero - test whether a percpu refcount reached zero | |
258 | * @ref: percpu_ref to test | |
259 | * | |
260 | * Returns %true if @ref reached zero. | |
6251f997 TH |
261 | * |
262 | * This function is safe to call as long as @ref is between init and exit. | |
2d722782 TH |
263 | */ |
264 | static inline bool percpu_ref_is_zero(struct percpu_ref *ref) | |
265 | { | |
eecc16ba | 266 | unsigned long __percpu *percpu_count; |
2d722782 | 267 | |
9e804d1f | 268 | if (__ref_is_percpu(ref, &percpu_count)) |
2d722782 | 269 | return false; |
e625305b | 270 | return !atomic_long_read(&ref->count); |
2d722782 TH |
271 | } |
272 | ||
215e262f | 273 | #endif |