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/kernel.h>
55 : #include <linux/percpu.h>
56 : #include <linux/rcupdate.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 50 : static inline void percpu_ref_kill(struct percpu_ref *ref)
148 : {
149 50 : percpu_ref_kill_and_confirm(ref, NULL);
150 0 : }
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 22626 : static inline bool __ref_is_percpu(struct percpu_ref *ref,
159 : unsigned long __percpu **percpu_countp)
160 : {
161 22626 : 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 22626 : 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 22464 : if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC_DEAD))
183 : return false;
184 :
185 22362 : *percpu_countp = (unsigned long __percpu *)percpu_ptr;
186 22362 : 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 2226 : static inline void percpu_ref_get_many(struct percpu_ref *ref, unsigned long nr)
199 : {
200 2226 : unsigned long __percpu *percpu_count;
201 :
202 2226 : rcu_read_lock();
203 :
204 2226 : if (__ref_is_percpu(ref, &percpu_count))
205 2172 : this_cpu_add(*percpu_count, nr);
206 : else
207 54 : atomic_long_add(nr, &ref->data->count);
208 :
209 2226 : rcu_read_unlock();
210 2226 : }
211 :
212 : /**
213 : * percpu_ref_get - increment a percpu refcount
214 : * @ref: percpu_ref to get
215 : *
216 : * Analagous to atomic_long_inc().
217 : *
218 : * This function is safe to call as long as @ref is between init and exit.
219 : */
220 2226 : static inline void percpu_ref_get(struct percpu_ref *ref)
221 : {
222 2226 : percpu_ref_get_many(ref, 1);
223 290 : }
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 : */
235 360 : static inline bool percpu_ref_tryget_many(struct percpu_ref *ref,
236 : unsigned long nr)
237 : {
238 360 : unsigned long __percpu *percpu_count;
239 360 : bool ret;
240 :
241 360 : rcu_read_lock();
242 :
243 360 : if (__ref_is_percpu(ref, &percpu_count)) {
244 360 : this_cpu_add(*percpu_count, nr);
245 : ret = true;
246 : } else {
247 0 : ret = atomic_long_add_unless(&ref->data->count, nr, 0);
248 : }
249 :
250 360 : rcu_read_unlock();
251 :
252 360 : 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 : */
264 360 : static inline bool percpu_ref_tryget(struct percpu_ref *ref)
265 : {
266 360 : return percpu_ref_tryget_many(ref, 1);
267 : }
268 :
269 : /**
270 : * percpu_ref_tryget_live - try to increment a live percpu refcount
271 : * @ref: percpu_ref to try-get
272 : *
273 : * Increment a percpu refcount unless it has already been killed. Returns
274 : * %true on success; %false on failure.
275 : *
276 : * Completion of percpu_ref_kill() in itself doesn't guarantee that this
277 : * function will fail. For such guarantee, percpu_ref_kill_and_confirm()
278 : * should be used. After the confirm_kill callback is invoked, it's
279 : * guaranteed that no new reference will be given out by
280 : * percpu_ref_tryget_live().
281 : *
282 : * This function is safe to call as long as @ref is between init and exit.
283 : */
284 8739 : static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
285 : {
286 8739 : unsigned long __percpu *percpu_count;
287 8739 : bool ret = false;
288 :
289 8739 : rcu_read_lock();
290 :
291 8740 : if (__ref_is_percpu(ref, &percpu_count)) {
292 8739 : this_cpu_inc(*percpu_count);
293 8739 : ret = true;
294 1 : } else if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) {
295 2 : ret = atomic_long_inc_not_zero(&ref->data->count);
296 : }
297 :
298 8740 : rcu_read_unlock();
299 :
300 8739 : return ret;
301 : }
302 :
303 : /**
304 : * percpu_ref_put_many - decrement a percpu refcount
305 : * @ref: percpu_ref to put
306 : * @nr: number of references to put
307 : *
308 : * Decrement the refcount, and if 0, call the release function (which was passed
309 : * to percpu_ref_init())
310 : *
311 : * This function is safe to call as long as @ref is between init and exit.
312 : */
313 11300 : static inline void percpu_ref_put_many(struct percpu_ref *ref, unsigned long nr)
314 : {
315 11300 : unsigned long __percpu *percpu_count;
316 :
317 11300 : rcu_read_lock();
318 :
319 11300 : if (__ref_is_percpu(ref, &percpu_count))
320 11091 : this_cpu_sub(*percpu_count, nr);
321 418 : else if (unlikely(atomic_long_sub_and_test(nr, &ref->data->count)))
322 52 : ref->data->release(ref);
323 :
324 11300 : rcu_read_unlock();
325 11300 : }
326 :
327 : /**
328 : * percpu_ref_put - decrement a percpu refcount
329 : * @ref: percpu_ref to put
330 : *
331 : * Decrement the refcount, and if 0, call the release function (which was passed
332 : * to percpu_ref_init())
333 : *
334 : * This function is safe to call as long as @ref is between init and exit.
335 : */
336 11300 : static inline void percpu_ref_put(struct percpu_ref *ref)
337 : {
338 11300 : percpu_ref_put_many(ref, 1);
339 555 : }
340 :
341 : /**
342 : * percpu_ref_is_dying - test whether a percpu refcount is dying or dead
343 : * @ref: percpu_ref to test
344 : *
345 : * Returns %true if @ref is dying or dead.
346 : *
347 : * This function is safe to call as long as @ref is between init and exit
348 : * and the caller is responsible for synchronizing against state changes.
349 : */
350 1 : static inline bool percpu_ref_is_dying(struct percpu_ref *ref)
351 : {
352 1 : return ref->percpu_count_ptr & __PERCPU_REF_DEAD;
353 : }
354 :
355 : #endif
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