Merge tag 'for-linus-20160523' of git://git.infradead.org/linux-mtd
[linux-2.6-block.git] / include / linux / percpu-refcount.h
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1/*
2 * Percpu refcounts:
3 * (C) 2012 Google, Inc.
4 * Author: Kent Overstreet <koverstreet@google.com>
5 *
6 * This implements a refcount with similar semantics to atomic_t - atomic_inc(),
7 * atomic_dec_and_test() - but percpu.
8 *
9 * There's one important difference between percpu refs and normal atomic_t
10 * refcounts; you have to keep track of your initial refcount, and then when you
11 * start shutting down you call percpu_ref_kill() _before_ dropping the initial
12 * refcount.
13 *
14 * The refcount will have a range of 0 to ((1U << 31) - 1), i.e. one bit less
15 * than an atomic_t - this is because of the way shutdown works, see
eecc16ba 16 * percpu_ref_kill()/PERCPU_COUNT_BIAS.
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17 *
18 * Before you call percpu_ref_kill(), percpu_ref_put() does not check for the
19 * refcount hitting 0 - it can't, if it was in percpu mode. percpu_ref_kill()
20 * puts the ref back in single atomic_t mode, collecting the per cpu refs and
21 * issuing the appropriate barriers, and then marks the ref as shutting down so
22 * that percpu_ref_put() will check for the ref hitting 0. After it returns,
23 * it's safe to drop the initial ref.
24 *
25 * USAGE:
26 *
27 * See fs/aio.c for some example usage; it's used there for struct kioctx, which
28 * is created when userspaces calls io_setup(), and destroyed when userspace
29 * calls io_destroy() or the process exits.
30 *
31 * In the aio code, kill_ioctx() is called when we wish to destroy a kioctx; it
d7cdb968 32 * calls percpu_ref_kill(), then hlist_del_rcu() and synchronize_rcu() to remove
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33 * the kioctx from the proccess's list of kioctxs - after that, there can't be
34 * any new users of the kioctx (from lookup_ioctx()) and it's then safe to drop
35 * the initial ref with percpu_ref_put().
36 *
37 * Code that does a two stage shutdown like this often needs some kind of
38 * explicit synchronization to ensure the initial refcount can only be dropped
39 * once - percpu_ref_kill() does this for you, it returns true once and false if
40 * someone else already called it. The aio code uses it this way, but it's not
41 * necessary if the code has some other mechanism to synchronize teardown.
42 * around.
43 */
44
45#ifndef _LINUX_PERCPU_REFCOUNT_H
46#define _LINUX_PERCPU_REFCOUNT_H
47
48#include <linux/atomic.h>
49#include <linux/kernel.h>
50#include <linux/percpu.h>
51#include <linux/rcupdate.h>
a34375ef 52#include <linux/gfp.h>
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53
54struct percpu_ref;
ac899061 55typedef void (percpu_ref_func_t)(struct percpu_ref *);
215e262f 56
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57/* flags set in the lower bits of percpu_ref->percpu_count_ptr */
58enum {
59 __PERCPU_REF_ATOMIC = 1LU << 0, /* operating in atomic mode */
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60 __PERCPU_REF_DEAD = 1LU << 1, /* (being) killed */
61 __PERCPU_REF_ATOMIC_DEAD = __PERCPU_REF_ATOMIC | __PERCPU_REF_DEAD,
62
63 __PERCPU_REF_FLAG_BITS = 2,
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64};
65
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66/* @flags for percpu_ref_init() */
67enum {
68 /*
69 * Start w/ ref == 1 in atomic mode. Can be switched to percpu
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70 * operation using percpu_ref_switch_to_percpu(). If initialized
71 * with this flag, the ref will stay in atomic mode until
72 * percpu_ref_switch_to_percpu() is invoked on it.
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73 */
74 PERCPU_REF_INIT_ATOMIC = 1 << 0,
75
76 /*
77 * Start dead w/ ref == 0 in atomic mode. Must be revived with
78 * percpu_ref_reinit() before used. Implies INIT_ATOMIC.
79 */
80 PERCPU_REF_INIT_DEAD = 1 << 1,
81};
82
215e262f 83struct percpu_ref {
e625305b 84 atomic_long_t count;
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85 /*
86 * The low bit of the pointer indicates whether the ref is in percpu
9a1049da 87 * mode; if set, then get/put will manipulate the atomic_t.
215e262f 88 */
eecc16ba 89 unsigned long percpu_count_ptr;
ac899061 90 percpu_ref_func_t *release;
9e804d1f 91 percpu_ref_func_t *confirm_switch;
1cae13e7 92 bool force_atomic:1;
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93 struct rcu_head rcu;
94};
95
acac7883 96int __must_check percpu_ref_init(struct percpu_ref *ref,
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97 percpu_ref_func_t *release, unsigned int flags,
98 gfp_t gfp);
9a1049da 99void percpu_ref_exit(struct percpu_ref *ref);
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100void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
101 percpu_ref_func_t *confirm_switch);
f47ad457 102void percpu_ref_switch_to_percpu(struct percpu_ref *ref);
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103void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
104 percpu_ref_func_t *confirm_kill);
f47ad457 105void percpu_ref_reinit(struct percpu_ref *ref);
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106
107/**
108 * percpu_ref_kill - drop the initial ref
109 * @ref: percpu_ref to kill
110 *
111 * Must be used to drop the initial ref on a percpu refcount; must be called
112 * precisely once before shutdown.
113 *
114 * Puts @ref in non percpu mode, then does a call_rcu() before gathering up the
115 * percpu counters and dropping the initial ref.
116 */
117static inline void percpu_ref_kill(struct percpu_ref *ref)
118{
4d414269 119 percpu_ref_kill_and_confirm(ref, NULL);
dbece3a0 120}
215e262f 121
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122/*
123 * Internal helper. Don't use outside percpu-refcount proper. The
124 * function doesn't return the pointer and let the caller test it for NULL
125 * because doing so forces the compiler to generate two conditional
eecc16ba 126 * branches as it can't assume that @ref->percpu_count is not NULL.
eae7975d 127 */
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128static inline bool __ref_is_percpu(struct percpu_ref *ref,
129 unsigned long __percpu **percpu_countp)
eae7975d 130{
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131 unsigned long percpu_ptr;
132
133 /*
134 * The value of @ref->percpu_count_ptr is tested for
135 * !__PERCPU_REF_ATOMIC, which may be set asynchronously, and then
136 * used as a pointer. If the compiler generates a separate fetch
137 * when using it as a pointer, __PERCPU_REF_ATOMIC may be set in
138 * between contaminating the pointer value, meaning that
139 * ACCESS_ONCE() is required when fetching it.
140 *
141 * Also, we need a data dependency barrier to be paired with
142 * smp_store_release() in __percpu_ref_switch_to_percpu().
143 *
144 * Use lockless deref which contains both.
145 */
146 percpu_ptr = lockless_dereference(ref->percpu_count_ptr);
2d722782 147
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148 /*
149 * Theoretically, the following could test just ATOMIC; however,
150 * then we'd have to mask off DEAD separately as DEAD may be
151 * visible without ATOMIC if we race with percpu_ref_kill(). DEAD
152 * implies ATOMIC anyway. Test them together.
153 */
154 if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC_DEAD))
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155 return false;
156
eecc16ba 157 *percpu_countp = (unsigned long __percpu *)percpu_ptr;
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158 return true;
159}
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160
161/**
e8ea14cc 162 * percpu_ref_get_many - increment a percpu refcount
ac899061 163 * @ref: percpu_ref to get
e8ea14cc 164 * @nr: number of references to get
215e262f 165 *
e8ea14cc 166 * Analogous to atomic_long_add().
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167 *
168 * This function is safe to call as long as @ref is between init and exit.
169 */
e8ea14cc 170static inline void percpu_ref_get_many(struct percpu_ref *ref, unsigned long nr)
215e262f 171{
eecc16ba 172 unsigned long __percpu *percpu_count;
215e262f 173
a4244454 174 rcu_read_lock_sched();
215e262f 175
9e804d1f 176 if (__ref_is_percpu(ref, &percpu_count))
e8ea14cc 177 this_cpu_add(*percpu_count, nr);
215e262f 178 else
e8ea14cc 179 atomic_long_add(nr, &ref->count);
215e262f 180
a4244454 181 rcu_read_unlock_sched();
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182}
183
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184/**
185 * percpu_ref_get - increment a percpu refcount
186 * @ref: percpu_ref to get
187 *
188 * Analagous to atomic_long_inc().
189 *
190 * This function is safe to call as long as @ref is between init and exit.
191 */
192static inline void percpu_ref_get(struct percpu_ref *ref)
193{
194 percpu_ref_get_many(ref, 1);
195}
196
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197/**
198 * percpu_ref_tryget - try to increment a percpu refcount
199 * @ref: percpu_ref to try-get
200 *
201 * Increment a percpu refcount unless its count already reached zero.
202 * Returns %true on success; %false on failure.
203 *
6251f997 204 * This function is safe to call as long as @ref is between init and exit.
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205 */
206static inline bool percpu_ref_tryget(struct percpu_ref *ref)
207{
eecc16ba 208 unsigned long __percpu *percpu_count;
6251f997 209 int ret;
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210
211 rcu_read_lock_sched();
212
9e804d1f 213 if (__ref_is_percpu(ref, &percpu_count)) {
eecc16ba 214 this_cpu_inc(*percpu_count);
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215 ret = true;
216 } else {
e625305b 217 ret = atomic_long_inc_not_zero(&ref->count);
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218 }
219
220 rcu_read_unlock_sched();
221
222 return ret;
223}
224
dbece3a0 225/**
2070d50e 226 * percpu_ref_tryget_live - try to increment a live percpu refcount
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227 * @ref: percpu_ref to try-get
228 *
229 * Increment a percpu refcount unless it has already been killed. Returns
230 * %true on success; %false on failure.
231 *
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232 * Completion of percpu_ref_kill() in itself doesn't guarantee that this
233 * function will fail. For such guarantee, percpu_ref_kill_and_confirm()
234 * should be used. After the confirm_kill callback is invoked, it's
235 * guaranteed that no new reference will be given out by
236 * percpu_ref_tryget_live().
4fb6e250 237 *
6251f997 238 * This function is safe to call as long as @ref is between init and exit.
dbece3a0 239 */
2070d50e 240static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
dbece3a0 241{
eecc16ba 242 unsigned long __percpu *percpu_count;
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243 int ret = false;
244
a4244454 245 rcu_read_lock_sched();
dbece3a0 246
9e804d1f 247 if (__ref_is_percpu(ref, &percpu_count)) {
eecc16ba 248 this_cpu_inc(*percpu_count);
dbece3a0 249 ret = true;
6810e4a3 250 } else if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) {
490c79a6 251 ret = atomic_long_inc_not_zero(&ref->count);
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252 }
253
a4244454 254 rcu_read_unlock_sched();
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255
256 return ret;
257}
258
215e262f 259/**
e8ea14cc 260 * percpu_ref_put_many - decrement a percpu refcount
ac899061 261 * @ref: percpu_ref to put
e8ea14cc 262 * @nr: number of references to put
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263 *
264 * Decrement the refcount, and if 0, call the release function (which was passed
265 * to percpu_ref_init())
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266 *
267 * This function is safe to call as long as @ref is between init and exit.
215e262f 268 */
e8ea14cc 269static inline void percpu_ref_put_many(struct percpu_ref *ref, unsigned long nr)
215e262f 270{
eecc16ba 271 unsigned long __percpu *percpu_count;
215e262f 272
a4244454 273 rcu_read_lock_sched();
215e262f 274
9e804d1f 275 if (__ref_is_percpu(ref, &percpu_count))
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276 this_cpu_sub(*percpu_count, nr);
277 else if (unlikely(atomic_long_sub_and_test(nr, &ref->count)))
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278 ref->release(ref);
279
a4244454 280 rcu_read_unlock_sched();
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281}
282
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283/**
284 * percpu_ref_put - decrement a percpu refcount
285 * @ref: percpu_ref to put
286 *
287 * Decrement the refcount, and if 0, call the release function (which was passed
288 * to percpu_ref_init())
289 *
290 * This function is safe to call as long as @ref is between init and exit.
291 */
292static inline void percpu_ref_put(struct percpu_ref *ref)
293{
294 percpu_ref_put_many(ref, 1);
295}
296
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297/**
298 * percpu_ref_is_dying - test whether a percpu refcount is dying or dead
299 * @ref: percpu_ref to test
300 *
301 * Returns %true if @ref is dying or dead.
302 *
303 * This function is safe to call as long as @ref is between init and exit
304 * and the caller is responsible for synchronizing against state changes.
305 */
306static inline bool percpu_ref_is_dying(struct percpu_ref *ref)
307{
308 return ref->percpu_count_ptr & __PERCPU_REF_DEAD;
309}
310
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311/**
312 * percpu_ref_is_zero - test whether a percpu refcount reached zero
313 * @ref: percpu_ref to test
314 *
315 * Returns %true if @ref reached zero.
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316 *
317 * This function is safe to call as long as @ref is between init and exit.
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318 */
319static inline bool percpu_ref_is_zero(struct percpu_ref *ref)
320{
eecc16ba 321 unsigned long __percpu *percpu_count;
2d722782 322
9e804d1f 323 if (__ref_is_percpu(ref, &percpu_count))
2d722782 324 return false;
e625305b 325 return !atomic_long_read(&ref->count);
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326}
327
215e262f 328#endif