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 static inline void percpu_ref_kill(struct percpu_ref *ref)
135 {
136 percpu_ref_kill_and_confirm(ref, NULL);
137 }
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 static inline bool __ref_is_percpu(struct percpu_ref *ref,
146 unsigned long __percpu **percpu_countp)
147 {
148 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 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 if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC_DEAD))
170 return false;
171
172 *percpu_countp = (unsigned long __percpu *)percpu_ptr;
173 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 static inline void percpu_ref_get_many(struct percpu_ref *ref, unsigned long nr)
186 {
187 unsigned long __percpu *percpu_count;
188
189 rcu_read_lock_sched();
190
191 if (__ref_is_percpu(ref, &percpu_count))
192 this_cpu_add(*percpu_count, nr);
193 else
194 atomic_long_add(nr, &ref->count);
195
196 rcu_read_unlock_sched();
197 }
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 static inline void percpu_ref_get(struct percpu_ref *ref)
208 {
209 percpu_ref_get_many(ref, 1);
210 }
211
212 /**
213 * percpu_ref_tryget - try to increment a percpu refcount
214 * @ref: percpu_ref to try-get
215 *
216 * Increment a percpu refcount unless its count already reached zero.
217 * Returns %true on success; %false on failure.
218 *
219 * This function is safe to call as long as @ref is between init and exit.
220 */
221 static inline bool percpu_ref_tryget(struct percpu_ref *ref)
222 {
223 unsigned long __percpu *percpu_count;
224 bool ret;
225
226 rcu_read_lock_sched();
227
228 if (__ref_is_percpu(ref, &percpu_count)) {
229 this_cpu_inc(*percpu_count);
230 ret = true;
231 } else {
232 ret = atomic_long_inc_not_zero(&ref->count);
233 }
234
235 rcu_read_unlock_sched();
236
237 return ret;
238 }
239
240 /**
241 * percpu_ref_tryget_live - try to increment a live percpu refcount
242 * @ref: percpu_ref to try-get
243 *
244 * Increment a percpu refcount unless it has already been killed. Returns
245 * %true on success; %false on failure.
246 *
247 * Completion of percpu_ref_kill() in itself doesn't guarantee that this
248 * function will fail. For such guarantee, percpu_ref_kill_and_confirm()
249 * should be used. After the confirm_kill callback is invoked, it's
250 * guaranteed that no new reference will be given out by
251 * percpu_ref_tryget_live().
252 *
253 * This function is safe to call as long as @ref is between init and exit.
254 */
255 static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
256 {
257 unsigned long __percpu *percpu_count;
258 bool ret = false;
259
260 rcu_read_lock_sched();
261
262 if (__ref_is_percpu(ref, &percpu_count)) {
263 this_cpu_inc(*percpu_count);
264 ret = true;
265 } else if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) {
266 ret = atomic_long_inc_not_zero(&ref->count);
267 }
268
269 rcu_read_unlock_sched();
270
271 return ret;
272 }
273
274 /**
275 * percpu_ref_put_many - decrement a percpu refcount
276 * @ref: percpu_ref to put
277 * @nr: number of references to put
278 *
279 * Decrement the refcount, and if 0, call the release function (which was passed
280 * to percpu_ref_init())
281 *
282 * This function is safe to call as long as @ref is between init and exit.
283 */
284 static inline void percpu_ref_put_many(struct percpu_ref *ref, unsigned long nr)
285 {
286 unsigned long __percpu *percpu_count;
287
288 rcu_read_lock_sched();
289
290 if (__ref_is_percpu(ref, &percpu_count))
291 this_cpu_sub(*percpu_count, nr);
292 else if (unlikely(atomic_long_sub_and_test(nr, &ref->count)))
293 ref->release(ref);
294
295 rcu_read_unlock_sched();
296 }
297
298 /**
299 * percpu_ref_put - decrement a percpu refcount
300 * @ref: percpu_ref to put
301 *
302 * Decrement the refcount, and if 0, call the release function (which was passed
303 * to percpu_ref_init())
304 *
305 * This function is safe to call as long as @ref is between init and exit.
306 */
307 static inline void percpu_ref_put(struct percpu_ref *ref)
308 {
309 percpu_ref_put_many(ref, 1);
310 }
311
312 /**
313 * percpu_ref_is_dying - test whether a percpu refcount is dying or dead
314 * @ref: percpu_ref to test
315 *
316 * Returns %true if @ref is dying or dead.
317 *
318 * This function is safe to call as long as @ref is between init and exit
319 * and the caller is responsible for synchronizing against state changes.
320 */
321 static inline bool percpu_ref_is_dying(struct percpu_ref *ref)
322 {
323 return ref->percpu_count_ptr & __PERCPU_REF_DEAD;
324 }
325
326 /**
327 * percpu_ref_is_zero - test whether a percpu refcount reached zero
328 * @ref: percpu_ref to test
329 *
330 * Returns %true if @ref reached zero.
331 *
332 * This function is safe to call as long as @ref is between init and exit.
333 */
334 static inline bool percpu_ref_is_zero(struct percpu_ref *ref)
335 {
336 unsigned long __percpu *percpu_count;
337
338 if (__ref_is_percpu(ref, &percpu_count))
339 return false;
340 return !atomic_long_read(&ref->count);
341 }
342
343 #endif