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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 {
96 : : atomic_long_t count;
97 : : /*
98 : : * The low bit of the pointer indicates whether the ref is in percpu
99 : : * mode; if set, then get/put will manipulate the atomic_t.
100 : : */
101 : : unsigned long percpu_count_ptr;
102 : : percpu_ref_func_t *release;
103 : : percpu_ref_func_t *confirm_switch;
104 : : bool force_atomic:1;
105 : : bool allow_reinit:1;
106 : : struct rcu_head rcu;
107 : : };
108 : :
109 : : int __must_check percpu_ref_init(struct percpu_ref *ref,
110 : : percpu_ref_func_t *release, unsigned int flags,
111 : : gfp_t gfp);
112 : : void percpu_ref_exit(struct percpu_ref *ref);
113 : : void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
114 : : percpu_ref_func_t *confirm_switch);
115 : : void percpu_ref_switch_to_atomic_sync(struct percpu_ref *ref);
116 : : void percpu_ref_switch_to_percpu(struct percpu_ref *ref);
117 : : void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
118 : : percpu_ref_func_t *confirm_kill);
119 : : void percpu_ref_resurrect(struct percpu_ref *ref);
120 : : void percpu_ref_reinit(struct percpu_ref *ref);
121 : :
122 : : /**
123 : : * percpu_ref_kill - drop the initial ref
124 : : * @ref: percpu_ref to kill
125 : : *
126 : : * Must be used to drop the initial ref on a percpu refcount; must be called
127 : : * precisely once before shutdown.
128 : : *
129 : : * Switches @ref into atomic mode before gathering up the percpu counters
130 : : * and dropping the initial ref.
131 : : *
132 : : * There are no implied RCU grace periods between kill and release.
133 : : */
134 : 588 : static inline void percpu_ref_kill(struct percpu_ref *ref)
135 : : {
136 : 840 : percpu_ref_kill_and_confirm(ref, NULL);
137 : 0 : }
138 : :
139 : : /*
140 : : * Internal helper. Don't use outside percpu-refcount proper. The
141 : : * function doesn't return the pointer and let the caller test it for NULL
142 : : * because doing so forces the compiler to generate two conditional
143 : : * branches as it can't assume that @ref->percpu_count is not NULL.
144 : : */
145 : 261591 : static inline bool __ref_is_percpu(struct percpu_ref *ref,
146 : : unsigned long __percpu **percpu_countp)
147 : : {
148 : 261591 : unsigned long percpu_ptr;
149 : :
150 : : /*
151 : : * The value of @ref->percpu_count_ptr is tested for
152 : : * !__PERCPU_REF_ATOMIC, which may be set asynchronously, and then
153 : : * used as a pointer. If the compiler generates a separate fetch
154 : : * when using it as a pointer, __PERCPU_REF_ATOMIC may be set in
155 : : * between contaminating the pointer value, meaning that
156 : : * READ_ONCE() is required when fetching it.
157 : : *
158 : : * The smp_read_barrier_depends() implied by READ_ONCE() pairs
159 : : * with smp_store_release() in __percpu_ref_switch_to_percpu().
160 : : */
161 [ - + ]: 259323 : percpu_ptr = READ_ONCE(ref->percpu_count_ptr);
162 : :
163 : : /*
164 : : * Theoretically, the following could test just ATOMIC; however,
165 : : * then we'd have to mask off DEAD separately as DEAD may be
166 : : * visible without ATOMIC if we race with percpu_ref_kill(). DEAD
167 : : * implies ATOMIC anyway. Test them together.
168 : : */
169 [ + + + + : 261591 : if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC_DEAD))
+ + + + +
+ + + ]
170 : : return false;
171 : :
172 : 247416 : *percpu_countp = (unsigned long __percpu *)percpu_ptr;
173 : 247416 : return true;
174 : : }
175 : :
176 : : /**
177 : : * percpu_ref_get_many - increment a percpu refcount
178 : : * @ref: percpu_ref to get
179 : : * @nr: number of references to get
180 : : *
181 : : * Analogous to atomic_long_add().
182 : : *
183 : : * This function is safe to call as long as @ref is between init and exit.
184 : : */
185 : 69366 : static inline void percpu_ref_get_many(struct percpu_ref *ref, unsigned long nr)
186 : : {
187 : 69366 : unsigned long __percpu *percpu_count;
188 : :
189 : 69366 : rcu_read_lock();
190 : :
191 [ + + ]: 69366 : if (__ref_is_percpu(ref, &percpu_count))
192 [ + - + - : 65166 : this_cpu_add(*percpu_count, nr);
+ - - - ]
193 : : else
194 : 4200 : atomic_long_add(nr, &ref->count);
195 : :
196 : 69366 : rcu_read_unlock();
197 : 69366 : }
198 : :
199 : : /**
200 : : * percpu_ref_get - increment a percpu refcount
201 : : * @ref: percpu_ref to get
202 : : *
203 : : * Analagous to atomic_long_inc().
204 : : *
205 : : * This function is safe to call as long as @ref is between init and exit.
206 : : */
207 : 69366 : static inline void percpu_ref_get(struct percpu_ref *ref)
208 : : {
209 : 69366 : percpu_ref_get_many(ref, 1);
210 : 3549 : }
211 : :
212 : : /**
213 : : * percpu_ref_tryget_many - try to increment a percpu refcount
214 : : * @ref: percpu_ref to try-get
215 : : * @nr: number of references to get
216 : : *
217 : : * Increment a percpu refcount by @nr unless its count already reached zero.
218 : : * Returns %true on success; %false on failure.
219 : : *
220 : : * This function is safe to call as long as @ref is between init and exit.
221 : : */
222 : 5208 : static inline bool percpu_ref_tryget_many(struct percpu_ref *ref,
223 : : unsigned long nr)
224 : : {
225 : 5208 : unsigned long __percpu *percpu_count;
226 : 5208 : bool ret;
227 : :
228 : 5208 : rcu_read_lock();
229 : :
230 [ + + ]: 5208 : if (__ref_is_percpu(ref, &percpu_count)) {
231 [ + - + - : 5166 : this_cpu_add(*percpu_count, nr);
+ - - - ]
232 : : ret = true;
233 : : } else {
234 : 42 : ret = atomic_long_add_unless(&ref->count, nr, 0);
235 : : }
236 : :
237 : 5208 : rcu_read_unlock();
238 : :
239 : 5208 : return ret;
240 : : }
241 : :
242 : : /**
243 : : * percpu_ref_tryget - try to increment a percpu refcount
244 : : * @ref: percpu_ref to try-get
245 : : *
246 : : * Increment a percpu refcount unless its count already reached zero.
247 : : * Returns %true on success; %false on failure.
248 : : *
249 : : * This function is safe to call as long as @ref is between init and exit.
250 : : */
251 : 5208 : static inline bool percpu_ref_tryget(struct percpu_ref *ref)
252 : : {
253 : 5208 : return percpu_ref_tryget_many(ref, 1);
254 : : }
255 : :
256 : : /**
257 : : * percpu_ref_tryget_live - try to increment a live percpu refcount
258 : : * @ref: percpu_ref to try-get
259 : : *
260 : : * Increment a percpu refcount unless it has already been killed. Returns
261 : : * %true on success; %false on failure.
262 : : *
263 : : * Completion of percpu_ref_kill() in itself doesn't guarantee that this
264 : : * function will fail. For such guarantee, percpu_ref_kill_and_confirm()
265 : : * should be used. After the confirm_kill callback is invoked, it's
266 : : * guaranteed that no new reference will be given out by
267 : : * percpu_ref_tryget_live().
268 : : *
269 : : * This function is safe to call as long as @ref is between init and exit.
270 : : */
271 : 56190 : static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
272 : : {
273 : 56190 : unsigned long __percpu *percpu_count;
274 : 56190 : bool ret = false;
275 : :
276 : 56190 : rcu_read_lock();
277 : :
278 [ + + ]: 56190 : if (__ref_is_percpu(ref, &percpu_count)) {
279 : 54489 : this_cpu_inc(*percpu_count);
280 : 54489 : ret = true;
281 [ + + ]: 1701 : } else if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) {
282 : 1680 : ret = atomic_long_inc_not_zero(&ref->count);
283 : : }
284 : :
285 : 56190 : rcu_read_unlock();
286 : :
287 : 56190 : return ret;
288 : : }
289 : :
290 : : /**
291 : : * percpu_ref_put_many - decrement a percpu refcount
292 : : * @ref: percpu_ref to put
293 : : * @nr: number of references to put
294 : : *
295 : : * Decrement the refcount, and if 0, call the release function (which was passed
296 : : * to percpu_ref_init())
297 : : *
298 : : * This function is safe to call as long as @ref is between init and exit.
299 : : */
300 : 130281 : static inline void percpu_ref_put_many(struct percpu_ref *ref, unsigned long nr)
301 : : {
302 : 130281 : unsigned long __percpu *percpu_count;
303 : :
304 : 130281 : rcu_read_lock();
305 : :
306 [ + + ]: 130281 : if (__ref_is_percpu(ref, &percpu_count))
307 [ + - + - : 122595 : this_cpu_sub(*percpu_count, nr);
- + + - ]
308 [ + + ]: 7686 : else if (unlikely(atomic_long_sub_and_test(nr, &ref->count)))
309 : 840 : ref->release(ref);
310 : :
311 : 130281 : rcu_read_unlock();
312 : 130281 : }
313 : :
314 : : /**
315 : : * percpu_ref_put - decrement a percpu refcount
316 : : * @ref: percpu_ref to put
317 : : *
318 : : * Decrement the refcount, and if 0, call the release function (which was passed
319 : : * to percpu_ref_init())
320 : : *
321 : : * This function is safe to call as long as @ref is between init and exit.
322 : : */
323 : 130281 : static inline void percpu_ref_put(struct percpu_ref *ref)
324 : : {
325 : 130281 : percpu_ref_put_many(ref, 1);
326 : 7371 : }
327 : :
328 : : /**
329 : : * percpu_ref_is_dying - test whether a percpu refcount is dying or dead
330 : : * @ref: percpu_ref to test
331 : : *
332 : : * Returns %true if @ref is dying or dead.
333 : : *
334 : : * This function is safe to call as long as @ref is between init and exit
335 : : * and the caller is responsible for synchronizing against state changes.
336 : : */
337 : 770 : static inline bool percpu_ref_is_dying(struct percpu_ref *ref)
338 : : {
339 [ - + - + : 770 : return ref->percpu_count_ptr & __PERCPU_REF_DEAD;
+ - - - ]
340 : : }
341 : :
342 : : /**
343 : : * percpu_ref_is_zero - test whether a percpu refcount reached zero
344 : : * @ref: percpu_ref to test
345 : : *
346 : : * Returns %true if @ref reached zero.
347 : : *
348 : : * This function is safe to call as long as @ref is between init and exit.
349 : : */
350 : 294 : static inline bool percpu_ref_is_zero(struct percpu_ref *ref)
351 : : {
352 : 294 : unsigned long __percpu *percpu_count;
353 : :
354 [ - - - - : 294 : if (__ref_is_percpu(ref, &percpu_count))
+ - + - ]
355 : : return false;
356 [ + + + + ]: 294 : return !atomic_long_read(&ref->count);
357 : : }
358 : :
359 : : #endif
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