Line data Source code
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 0 : 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 0 : 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 0 : if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC_DEAD))
183 : return false;
184 :
185 0 : *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 0 : 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 0 : if (__ref_is_percpu(ref, &percpu_count))
205 0 : this_cpu_add(*percpu_count, nr);
206 : else
207 0 : atomic_long_add(nr, &ref->data->count);
208 :
209 : rcu_read_unlock();
210 0 : }
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 0 : percpu_ref_get_many(ref, 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 0 : 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 0 : if (__ref_is_percpu(ref, &percpu_count)) {
244 0 : this_cpu_add(*percpu_count, nr);
245 0 : ret = true;
246 : } else {
247 0 : ret = atomic_long_add_unless(&ref->data->count, nr, 0);
248 : }
249 :
250 : rcu_read_unlock();
251 :
252 0 : 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 0 : return percpu_ref_tryget_many(ref, 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 0 : static inline bool percpu_ref_tryget_live_rcu(struct percpu_ref *ref)
276 : {
277 : unsigned long __percpu *percpu_count;
278 0 : bool ret = false;
279 :
280 0 : WARN_ON_ONCE(!rcu_read_lock_held());
281 :
282 0 : if (likely(__ref_is_percpu(ref, &percpu_count))) {
283 0 : this_cpu_inc(*percpu_count);
284 0 : ret = true;
285 0 : } else if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) {
286 0 : ret = atomic_long_inc_not_zero(&ref->data->count);
287 : }
288 0 : 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 0 : bool ret = false;
309 :
310 : rcu_read_lock();
311 0 : 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 0 : 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 0 : if (__ref_is_percpu(ref, &percpu_count))
333 0 : this_cpu_sub(*percpu_count, nr);
334 0 : else if (unlikely(atomic_long_sub_and_test(nr, &ref->data->count)))
335 0 : ref->data->release(ref);
336 :
337 : rcu_read_unlock();
338 0 : }
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 0 : percpu_ref_put_many(ref, 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 0 : return ref->percpu_count_ptr & __PERCPU_REF_DEAD;
366 : }
367 :
368 : #endif
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