1 | /* SPDX-License-Identifier: GPL-2.0 */ |
2 | /* |
3 | * Percpu refcounts: |
4 | * (C) 2012 Google, Inc. |
5 | * Author: Kent Overstreet <koverstreet@google.com> |
6 | * |
7 | * This implements a refcount with similar semantics to atomic_t - atomic_inc(), |
8 | * atomic_dec_and_test() - but percpu. |
9 | * |
10 | * There's one important difference between percpu refs and normal atomic_t |
11 | * refcounts; you have to keep track of your initial refcount, and then when you |
12 | * start shutting down you call percpu_ref_kill() _before_ dropping the initial |
13 | * refcount. |
14 | * |
15 | * The refcount will have a range of 0 to ((1U << 31) - 1), i.e. one bit less |
16 | * than an atomic_t - this is because of the way shutdown works, see |
17 | * percpu_ref_kill()/PERCPU_COUNT_BIAS. |
18 | * |
19 | * Before you call percpu_ref_kill(), percpu_ref_put() does not check for the |
20 | * refcount hitting 0 - it can't, if it was in percpu mode. percpu_ref_kill() |
21 | * puts the ref back in single atomic_t mode, collecting the per cpu refs and |
22 | * issuing the appropriate barriers, and then marks the ref as shutting down so |
23 | * that percpu_ref_put() will check for the ref hitting 0. After it returns, |
24 | * it's safe to drop the initial ref. |
25 | * |
26 | * USAGE: |
27 | * |
28 | * See fs/aio.c for some example usage; it's used there for struct kioctx, which |
29 | * is created when userspaces calls io_setup(), and destroyed when userspace |
30 | * calls io_destroy() or the process exits. |
31 | * |
32 | * In the aio code, kill_ioctx() is called when we wish to destroy a kioctx; it |
33 | * removes the kioctx from the proccess's table of kioctxs and kills percpu_ref. |
34 | * After that, there can't be any new users of the kioctx (from lookup_ioctx()) |
35 | * and it's then safe to drop the initial ref with percpu_ref_put(). |
36 | * |
37 | * Note that the free path, free_ioctx(), needs to go through explicit call_rcu() |
38 | * to synchronize with RCU protected lookup_ioctx(). percpu_ref operations don't |
39 | * imply RCU grace periods of any kind and if a user wants to combine percpu_ref |
40 | * with RCU protection, it must be done explicitly. |
41 | * |
42 | * Code that does a two stage shutdown like this often needs some kind of |
43 | * explicit synchronization to ensure the initial refcount can only be dropped |
44 | * once - percpu_ref_kill() does this for you, it returns true once and false if |
45 | * someone else already called it. The aio code uses it this way, but it's not |
46 | * necessary if the code has some other mechanism to synchronize teardown. |
47 | * around. |
48 | */ |
49 | |
50 | #ifndef _LINUX_PERCPU_REFCOUNT_H |
51 | #define _LINUX_PERCPU_REFCOUNT_H |
52 | |
53 | #include <linux/atomic.h> |
54 | #include <linux/percpu.h> |
55 | #include <linux/rcupdate.h> |
56 | #include <linux/types.h> |
57 | #include <linux/gfp.h> |
58 | |
59 | struct percpu_ref; |
60 | typedef void (percpu_ref_func_t)(struct percpu_ref *); |
61 | |
62 | /* flags set in the lower bits of percpu_ref->percpu_count_ptr */ |
63 | enum { |
64 | __PERCPU_REF_ATOMIC = 1LU << 0, /* operating in atomic mode */ |
65 | __PERCPU_REF_DEAD = 1LU << 1, /* (being) killed */ |
66 | __PERCPU_REF_ATOMIC_DEAD = __PERCPU_REF_ATOMIC | __PERCPU_REF_DEAD, |
67 | |
68 | __PERCPU_REF_FLAG_BITS = 2, |
69 | }; |
70 | |
71 | /* @flags for percpu_ref_init() */ |
72 | enum { |
73 | /* |
74 | * Start w/ ref == 1 in atomic mode. Can be switched to percpu |
75 | * operation using percpu_ref_switch_to_percpu(). If initialized |
76 | * with this flag, the ref will stay in atomic mode until |
77 | * percpu_ref_switch_to_percpu() is invoked on it. |
78 | * Implies ALLOW_REINIT. |
79 | */ |
80 | PERCPU_REF_INIT_ATOMIC = 1 << 0, |
81 | |
82 | /* |
83 | * Start dead w/ ref == 0 in atomic mode. Must be revived with |
84 | * percpu_ref_reinit() before used. Implies INIT_ATOMIC and |
85 | * ALLOW_REINIT. |
86 | */ |
87 | PERCPU_REF_INIT_DEAD = 1 << 1, |
88 | |
89 | /* |
90 | * Allow switching from atomic mode to percpu mode. |
91 | */ |
92 | PERCPU_REF_ALLOW_REINIT = 1 << 2, |
93 | }; |
94 | |
95 | struct percpu_ref_data { |
96 | atomic_long_t count; |
97 | percpu_ref_func_t *release; |
98 | percpu_ref_func_t *confirm_switch; |
99 | bool force_atomic:1; |
100 | bool allow_reinit:1; |
101 | struct rcu_head rcu; |
102 | struct percpu_ref *ref; |
103 | }; |
104 | |
105 | struct percpu_ref { |
106 | /* |
107 | * The low bit of the pointer indicates whether the ref is in percpu |
108 | * mode; if set, then get/put will manipulate the atomic_t. |
109 | */ |
110 | unsigned long percpu_count_ptr; |
111 | |
112 | /* |
113 | * 'percpu_ref' is often embedded into user structure, and only |
114 | * 'percpu_count_ptr' is required in fast path, move other fields |
115 | * into 'percpu_ref_data', so we can reduce memory footprint in |
116 | * fast path. |
117 | */ |
118 | struct percpu_ref_data *data; |
119 | }; |
120 | |
121 | int __must_check percpu_ref_init(struct percpu_ref *ref, |
122 | percpu_ref_func_t *release, unsigned int flags, |
123 | gfp_t gfp); |
124 | void percpu_ref_exit(struct percpu_ref *ref); |
125 | void percpu_ref_switch_to_atomic(struct percpu_ref *ref, |
126 | percpu_ref_func_t *confirm_switch); |
127 | void percpu_ref_switch_to_atomic_sync(struct percpu_ref *ref); |
128 | void percpu_ref_switch_to_percpu(struct percpu_ref *ref); |
129 | void percpu_ref_kill_and_confirm(struct percpu_ref *ref, |
130 | percpu_ref_func_t *confirm_kill); |
131 | void percpu_ref_resurrect(struct percpu_ref *ref); |
132 | void percpu_ref_reinit(struct percpu_ref *ref); |
133 | bool percpu_ref_is_zero(struct percpu_ref *ref); |
134 | |
135 | /** |
136 | * percpu_ref_kill - drop the initial ref |
137 | * @ref: percpu_ref to kill |
138 | * |
139 | * Must be used to drop the initial ref on a percpu refcount; must be called |
140 | * precisely once before shutdown. |
141 | * |
142 | * Switches @ref into atomic mode before gathering up the percpu counters |
143 | * and dropping the initial ref. |
144 | * |
145 | * There are no implied RCU grace periods between kill and release. |
146 | */ |
147 | static inline void percpu_ref_kill(struct percpu_ref *ref) |
148 | { |
149 | percpu_ref_kill_and_confirm(ref, NULL); |
150 | } |
151 | |
152 | /* |
153 | * Internal helper. Don't use outside percpu-refcount proper. The |
154 | * function doesn't return the pointer and let the caller test it for NULL |
155 | * because doing so forces the compiler to generate two conditional |
156 | * branches as it can't assume that @ref->percpu_count is not NULL. |
157 | */ |
158 | static inline bool __ref_is_percpu(struct percpu_ref *ref, |
159 | unsigned long __percpu **percpu_countp) |
160 | { |
161 | unsigned long percpu_ptr; |
162 | |
163 | /* |
164 | * The value of @ref->percpu_count_ptr is tested for |
165 | * !__PERCPU_REF_ATOMIC, which may be set asynchronously, and then |
166 | * used as a pointer. If the compiler generates a separate fetch |
167 | * when using it as a pointer, __PERCPU_REF_ATOMIC may be set in |
168 | * between contaminating the pointer value, meaning that |
169 | * READ_ONCE() is required when fetching it. |
170 | * |
171 | * The dependency ordering from the READ_ONCE() pairs |
172 | * with smp_store_release() in __percpu_ref_switch_to_percpu(). |
173 | */ |
174 | percpu_ptr = READ_ONCE(ref->percpu_count_ptr); |
175 | |
176 | /* |
177 | * Theoretically, the following could test just ATOMIC; however, |
178 | * then we'd have to mask off DEAD separately as DEAD may be |
179 | * visible without ATOMIC if we race with percpu_ref_kill(). DEAD |
180 | * implies ATOMIC anyway. Test them together. |
181 | */ |
182 | if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC_DEAD)) |
183 | return false; |
184 | |
185 | *percpu_countp = (unsigned long __percpu *)percpu_ptr; |
186 | return true; |
187 | } |
188 | |
189 | /** |
190 | * percpu_ref_get_many - increment a percpu refcount |
191 | * @ref: percpu_ref to get |
192 | * @nr: number of references to get |
193 | * |
194 | * Analogous to atomic_long_add(). |
195 | * |
196 | * This function is safe to call as long as @ref is between init and exit. |
197 | */ |
198 | static inline void percpu_ref_get_many(struct percpu_ref *ref, unsigned long nr) |
199 | { |
200 | unsigned long __percpu *percpu_count; |
201 | |
202 | rcu_read_lock(); |
203 | |
204 | if (__ref_is_percpu(ref, percpu_countp: &percpu_count)) |
205 | this_cpu_add(*percpu_count, nr); |
206 | else |
207 | atomic_long_add(i: nr, v: &ref->data->count); |
208 | |
209 | rcu_read_unlock(); |
210 | } |
211 | |
212 | /** |
213 | * percpu_ref_get - increment a percpu refcount |
214 | * @ref: percpu_ref to get |
215 | * |
216 | * Analogous to atomic_long_inc(). |
217 | * |
218 | * This function is safe to call as long as @ref is between init and exit. |
219 | */ |
220 | static inline void percpu_ref_get(struct percpu_ref *ref) |
221 | { |
222 | percpu_ref_get_many(ref, nr: 1); |
223 | } |
224 | |
225 | /** |
226 | * percpu_ref_tryget_many - try to increment a percpu refcount |
227 | * @ref: percpu_ref to try-get |
228 | * @nr: number of references to get |
229 | * |
230 | * Increment a percpu refcount by @nr unless its count already reached zero. |
231 | * Returns %true on success; %false on failure. |
232 | * |
233 | * This function is safe to call as long as @ref is between init and exit. |
234 | */ |
235 | static inline bool percpu_ref_tryget_many(struct percpu_ref *ref, |
236 | unsigned long nr) |
237 | { |
238 | unsigned long __percpu *percpu_count; |
239 | bool ret; |
240 | |
241 | rcu_read_lock(); |
242 | |
243 | if (__ref_is_percpu(ref, percpu_countp: &percpu_count)) { |
244 | this_cpu_add(*percpu_count, nr); |
245 | ret = true; |
246 | } else { |
247 | ret = atomic_long_add_unless(v: &ref->data->count, a: nr, u: 0); |
248 | } |
249 | |
250 | rcu_read_unlock(); |
251 | |
252 | return ret; |
253 | } |
254 | |
255 | /** |
256 | * percpu_ref_tryget - try to increment a percpu refcount |
257 | * @ref: percpu_ref to try-get |
258 | * |
259 | * Increment a percpu refcount unless its count already reached zero. |
260 | * Returns %true on success; %false on failure. |
261 | * |
262 | * This function is safe to call as long as @ref is between init and exit. |
263 | */ |
264 | static inline bool percpu_ref_tryget(struct percpu_ref *ref) |
265 | { |
266 | return percpu_ref_tryget_many(ref, nr: 1); |
267 | } |
268 | |
269 | /** |
270 | * percpu_ref_tryget_live_rcu - same as percpu_ref_tryget_live() but the |
271 | * caller is responsible for taking RCU. |
272 | * |
273 | * This function is safe to call as long as @ref is between init and exit. |
274 | */ |
275 | static inline bool percpu_ref_tryget_live_rcu(struct percpu_ref *ref) |
276 | { |
277 | unsigned long __percpu *percpu_count; |
278 | bool ret = false; |
279 | |
280 | WARN_ON_ONCE(!rcu_read_lock_held()); |
281 | |
282 | if (likely(__ref_is_percpu(ref, &percpu_count))) { |
283 | this_cpu_inc(*percpu_count); |
284 | ret = true; |
285 | } else if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) { |
286 | ret = atomic_long_inc_not_zero(v: &ref->data->count); |
287 | } |
288 | return ret; |
289 | } |
290 | |
291 | /** |
292 | * percpu_ref_tryget_live - try to increment a live percpu refcount |
293 | * @ref: percpu_ref to try-get |
294 | * |
295 | * Increment a percpu refcount unless it has already been killed. Returns |
296 | * %true on success; %false on failure. |
297 | * |
298 | * Completion of percpu_ref_kill() in itself doesn't guarantee that this |
299 | * function will fail. For such guarantee, percpu_ref_kill_and_confirm() |
300 | * should be used. After the confirm_kill callback is invoked, it's |
301 | * guaranteed that no new reference will be given out by |
302 | * percpu_ref_tryget_live(). |
303 | * |
304 | * This function is safe to call as long as @ref is between init and exit. |
305 | */ |
306 | static inline bool percpu_ref_tryget_live(struct percpu_ref *ref) |
307 | { |
308 | bool ret = false; |
309 | |
310 | rcu_read_lock(); |
311 | ret = percpu_ref_tryget_live_rcu(ref); |
312 | rcu_read_unlock(); |
313 | return ret; |
314 | } |
315 | |
316 | /** |
317 | * percpu_ref_put_many - decrement a percpu refcount |
318 | * @ref: percpu_ref to put |
319 | * @nr: number of references to put |
320 | * |
321 | * Decrement the refcount, and if 0, call the release function (which was passed |
322 | * to percpu_ref_init()) |
323 | * |
324 | * This function is safe to call as long as @ref is between init and exit. |
325 | */ |
326 | static inline void percpu_ref_put_many(struct percpu_ref *ref, unsigned long nr) |
327 | { |
328 | unsigned long __percpu *percpu_count; |
329 | |
330 | rcu_read_lock(); |
331 | |
332 | if (__ref_is_percpu(ref, percpu_countp: &percpu_count)) |
333 | this_cpu_sub(*percpu_count, nr); |
334 | else if (unlikely(atomic_long_sub_and_test(nr, &ref->data->count))) |
335 | ref->data->release(ref); |
336 | |
337 | rcu_read_unlock(); |
338 | } |
339 | |
340 | /** |
341 | * percpu_ref_put - decrement a percpu refcount |
342 | * @ref: percpu_ref to put |
343 | * |
344 | * Decrement the refcount, and if 0, call the release function (which was passed |
345 | * to percpu_ref_init()) |
346 | * |
347 | * This function is safe to call as long as @ref is between init and exit. |
348 | */ |
349 | static inline void percpu_ref_put(struct percpu_ref *ref) |
350 | { |
351 | percpu_ref_put_many(ref, nr: 1); |
352 | } |
353 | |
354 | /** |
355 | * percpu_ref_is_dying - test whether a percpu refcount is dying or dead |
356 | * @ref: percpu_ref to test |
357 | * |
358 | * Returns %true if @ref is dying or dead. |
359 | * |
360 | * This function is safe to call as long as @ref is between init and exit |
361 | * and the caller is responsible for synchronizing against state changes. |
362 | */ |
363 | static inline bool percpu_ref_is_dying(struct percpu_ref *ref) |
364 | { |
365 | return ref->percpu_count_ptr & __PERCPU_REF_DEAD; |
366 | } |
367 | |
368 | #endif |
369 | |