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1#define pr_fmt(fmt) "%s: " fmt "\n", __func__
2
3#include <linux/kernel.h>
4#include <linux/sched.h>
5#include <linux/wait.h>
6#include <linux/percpu-refcount.h>
7
8/*
9 * Initially, a percpu refcount is just a set of percpu counters. Initially, we
10 * don't try to detect the ref hitting 0 - which means that get/put can just
11 * increment or decrement the local counter. Note that the counter on a
12 * particular cpu can (and will) wrap - this is fine, when we go to shutdown the
13 * percpu counters will all sum to the correct value
14 *
15 * (More precisely: because modular arithmetic is commutative the sum of all the
16 * percpu_count vars will be equal to what it would have been if all the gets
17 * and puts were done to a single integer, even if some of the percpu integers
18 * overflow or underflow).
19 *
20 * The real trick to implementing percpu refcounts is shutdown. We can't detect
21 * the ref hitting 0 on every put - this would require global synchronization
22 * and defeat the whole purpose of using percpu refs.
23 *
24 * What we do is require the user to keep track of the initial refcount; we know
25 * the ref can't hit 0 before the user drops the initial ref, so as long as we
26 * convert to non percpu mode before the initial ref is dropped everything
27 * works.
28 *
29 * Converting to non percpu mode is done with some RCUish stuff in
30 * percpu_ref_kill. Additionally, we need a bias value so that the
31 * atomic_long_t can't hit 0 before we've added up all the percpu refs.
32 */
33
34#define PERCPU_COUNT_BIAS (1LU << (BITS_PER_LONG - 1))
35
36static DEFINE_SPINLOCK(percpu_ref_switch_lock);
37static DECLARE_WAIT_QUEUE_HEAD(percpu_ref_switch_waitq);
38
39static unsigned long __percpu *percpu_count_ptr(struct percpu_ref *ref)
40{
41 return (unsigned long __percpu *)
42 (ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC_DEAD);
43}
44
45/**
46 * percpu_ref_init - initialize a percpu refcount
47 * @ref: percpu_ref to initialize
48 * @release: function which will be called when refcount hits 0
49 * @flags: PERCPU_REF_INIT_* flags
50 * @gfp: allocation mask to use
51 *
52 * Initializes @ref. If @flags is zero, @ref starts in percpu mode with a
53 * refcount of 1; analagous to atomic_long_set(ref, 1). See the
54 * definitions of PERCPU_REF_INIT_* flags for flag behaviors.
55 *
56 * Note that @release must not sleep - it may potentially be called from RCU
57 * callback context by percpu_ref_kill().
58 */
59int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release,
60 unsigned int flags, gfp_t gfp)
61{
62 size_t align = max_t(size_t, 1 << __PERCPU_REF_FLAG_BITS,
63 __alignof__(unsigned long));
64 unsigned long start_count = 0;
65
66 ref->percpu_count_ptr = (unsigned long)
67 __alloc_percpu_gfp(sizeof(unsigned long), align, gfp);
68 if (!ref->percpu_count_ptr)
69 return -ENOMEM;
70
71 ref->force_atomic = flags & PERCPU_REF_INIT_ATOMIC;
72
73 if (flags & (PERCPU_REF_INIT_ATOMIC | PERCPU_REF_INIT_DEAD))
74 ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;
75 else
76 start_count += PERCPU_COUNT_BIAS;
77
78 if (flags & PERCPU_REF_INIT_DEAD)
79 ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
80 else
81 start_count++;
82
83 atomic_long_set(&ref->count, start_count);
84
85 ref->release = release;
86 ref->confirm_switch = NULL;
87 return 0;
88}
89EXPORT_SYMBOL_GPL(percpu_ref_init);
90
91/**
92 * percpu_ref_exit - undo percpu_ref_init()
93 * @ref: percpu_ref to exit
94 *
95 * This function exits @ref. The caller is responsible for ensuring that
96 * @ref is no longer in active use. The usual places to invoke this
97 * function from are the @ref->release() callback or in init failure path
98 * where percpu_ref_init() succeeded but other parts of the initialization
99 * of the embedding object failed.
100 */
101void percpu_ref_exit(struct percpu_ref *ref)
102{
103 unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
104
105 if (percpu_count) {
106 /* non-NULL confirm_switch indicates switching in progress */
107 WARN_ON_ONCE(ref->confirm_switch);
108 free_percpu(percpu_count);
109 ref->percpu_count_ptr = __PERCPU_REF_ATOMIC_DEAD;
110 }
111}
112EXPORT_SYMBOL_GPL(percpu_ref_exit);
113
114static void percpu_ref_call_confirm_rcu(struct rcu_head *rcu)
115{
116 struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);
117
118 ref->confirm_switch(ref);
119 ref->confirm_switch = NULL;
120 wake_up_all(&percpu_ref_switch_waitq);
121
122 /* drop ref from percpu_ref_switch_to_atomic() */
123 percpu_ref_put(ref);
124}
125
126static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu)
127{
128 struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);
129 unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
130 unsigned long count = 0;
131 int cpu;
132
133 for_each_possible_cpu(cpu)
134 count += *per_cpu_ptr(percpu_count, cpu);
135
136 pr_debug("global %ld percpu %ld",
137 atomic_long_read(&ref->count), (long)count);
138
139 /*
140 * It's crucial that we sum the percpu counters _before_ adding the sum
141 * to &ref->count; since gets could be happening on one cpu while puts
142 * happen on another, adding a single cpu's count could cause
143 * @ref->count to hit 0 before we've got a consistent value - but the
144 * sum of all the counts will be consistent and correct.
145 *
146 * Subtracting the bias value then has to happen _after_ adding count to
147 * &ref->count; we need the bias value to prevent &ref->count from
148 * reaching 0 before we add the percpu counts. But doing it at the same
149 * time is equivalent and saves us atomic operations:
150 */
151 atomic_long_add((long)count - PERCPU_COUNT_BIAS, &ref->count);
152
153 WARN_ONCE(atomic_long_read(&ref->count) <= 0,
154 "percpu ref (%pf) <= 0 (%ld) after switching to atomic",
155 ref->release, atomic_long_read(&ref->count));
156
157 /* @ref is viewed as dead on all CPUs, send out switch confirmation */
158 percpu_ref_call_confirm_rcu(rcu);
159}
160
161static void percpu_ref_noop_confirm_switch(struct percpu_ref *ref)
162{
163}
164
165static void __percpu_ref_switch_to_atomic(struct percpu_ref *ref,
166 percpu_ref_func_t *confirm_switch)
167{
168 if (ref->percpu_count_ptr & __PERCPU_REF_ATOMIC) {
169 if (confirm_switch)
170 confirm_switch(ref);
171 return;
172 }
173
174 /* switching from percpu to atomic */
175 ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;
176
177 /*
178 * Non-NULL ->confirm_switch is used to indicate that switching is
179 * in progress. Use noop one if unspecified.
180 */
181 ref->confirm_switch = confirm_switch ?: percpu_ref_noop_confirm_switch;
182
183 percpu_ref_get(ref); /* put after confirmation */
184 call_rcu_sched(&ref->rcu, percpu_ref_switch_to_atomic_rcu);
185}
186
187static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)
188{
189 unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
190 int cpu;
191
192 BUG_ON(!percpu_count);
193
194 if (!(ref->percpu_count_ptr & __PERCPU_REF_ATOMIC))
195 return;
196
197 atomic_long_add(PERCPU_COUNT_BIAS, &ref->count);
198
199 /*
200 * Restore per-cpu operation. smp_store_release() is paired with
201 * smp_read_barrier_depends() in __ref_is_percpu() and guarantees
202 * that the zeroing is visible to all percpu accesses which can see
203 * the following __PERCPU_REF_ATOMIC clearing.
204 */
205 for_each_possible_cpu(cpu)
206 *per_cpu_ptr(percpu_count, cpu) = 0;
207
208 smp_store_release(&ref->percpu_count_ptr,
209 ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC);
210}
211
212static void __percpu_ref_switch_mode(struct percpu_ref *ref,
213 percpu_ref_func_t *confirm_switch)
214{
215 lockdep_assert_held(&percpu_ref_switch_lock);
216
217 /*
218 * If the previous ATOMIC switching hasn't finished yet, wait for
219 * its completion. If the caller ensures that ATOMIC switching
220 * isn't in progress, this function can be called from any context.
221 */
222 wait_event_lock_irq(percpu_ref_switch_waitq, !ref->confirm_switch,
223 percpu_ref_switch_lock);
224
225 if (ref->force_atomic || (ref->percpu_count_ptr & __PERCPU_REF_DEAD))
226 __percpu_ref_switch_to_atomic(ref, confirm_switch);
227 else
228 __percpu_ref_switch_to_percpu(ref);
229}
230
231/**
232 * percpu_ref_switch_to_atomic - switch a percpu_ref to atomic mode
233 * @ref: percpu_ref to switch to atomic mode
234 * @confirm_switch: optional confirmation callback
235 *
236 * There's no reason to use this function for the usual reference counting.
237 * Use percpu_ref_kill[_and_confirm]().
238 *
239 * Schedule switching of @ref to atomic mode. All its percpu counts will
240 * be collected to the main atomic counter. On completion, when all CPUs
241 * are guaraneed to be in atomic mode, @confirm_switch, which may not
242 * block, is invoked. This function may be invoked concurrently with all
243 * the get/put operations and can safely be mixed with kill and reinit
244 * operations. Note that @ref will stay in atomic mode across kill/reinit
245 * cycles until percpu_ref_switch_to_percpu() is called.
246 *
247 * This function may block if @ref is in the process of switching to atomic
248 * mode. If the caller ensures that @ref is not in the process of
249 * switching to atomic mode, this function can be called from any context.
250 */
251void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
252 percpu_ref_func_t *confirm_switch)
253{
254 unsigned long flags;
255
256 spin_lock_irqsave(&percpu_ref_switch_lock, flags);
257
258 ref->force_atomic = true;
259 __percpu_ref_switch_mode(ref, confirm_switch);
260
261 spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
262}
263
264/**
265 * percpu_ref_switch_to_percpu - switch a percpu_ref to percpu mode
266 * @ref: percpu_ref to switch to percpu mode
267 *
268 * There's no reason to use this function for the usual reference counting.
269 * To re-use an expired ref, use percpu_ref_reinit().
270 *
271 * Switch @ref to percpu mode. This function may be invoked concurrently
272 * with all the get/put operations and can safely be mixed with kill and
273 * reinit operations. This function reverses the sticky atomic state set
274 * by PERCPU_REF_INIT_ATOMIC or percpu_ref_switch_to_atomic(). If @ref is
275 * dying or dead, the actual switching takes place on the following
276 * percpu_ref_reinit().
277 *
278 * This function may block if @ref is in the process of switching to atomic
279 * mode. If the caller ensures that @ref is not in the process of
280 * switching to atomic mode, this function can be called from any context.
281 */
282void percpu_ref_switch_to_percpu(struct percpu_ref *ref)
283{
284 unsigned long flags;
285
286 spin_lock_irqsave(&percpu_ref_switch_lock, flags);
287
288 ref->force_atomic = false;
289 __percpu_ref_switch_mode(ref, NULL);
290
291 spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
292}
293
294/**
295 * percpu_ref_kill_and_confirm - drop the initial ref and schedule confirmation
296 * @ref: percpu_ref to kill
297 * @confirm_kill: optional confirmation callback
298 *
299 * Equivalent to percpu_ref_kill() but also schedules kill confirmation if
300 * @confirm_kill is not NULL. @confirm_kill, which may not block, will be
301 * called after @ref is seen as dead from all CPUs at which point all
302 * further invocations of percpu_ref_tryget_live() will fail. See
303 * percpu_ref_tryget_live() for details.
304 *
305 * This function normally doesn't block and can be called from any context
306 * but it may block if @confirm_kill is specified and @ref is in the
307 * process of switching to atomic mode by percpu_ref_switch_to_atomic().
308 */
309void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
310 percpu_ref_func_t *confirm_kill)
311{
312 unsigned long flags;
313
314 spin_lock_irqsave(&percpu_ref_switch_lock, flags);
315
316 WARN_ONCE(ref->percpu_count_ptr & __PERCPU_REF_DEAD,
317 "%s called more than once on %pf!", __func__, ref->release);
318
319 ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
320 __percpu_ref_switch_mode(ref, confirm_kill);
321 percpu_ref_put(ref);
322
323 spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
324}
325EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);
326
327/**
328 * percpu_ref_reinit - re-initialize a percpu refcount
329 * @ref: perpcu_ref to re-initialize
330 *
331 * Re-initialize @ref so that it's in the same state as when it finished
332 * percpu_ref_init() ignoring %PERCPU_REF_INIT_DEAD. @ref must have been
333 * initialized successfully and reached 0 but not exited.
334 *
335 * Note that percpu_ref_tryget[_live]() are safe to perform on @ref while
336 * this function is in progress.
337 */
338void percpu_ref_reinit(struct percpu_ref *ref)
339{
340 unsigned long flags;
341
342 spin_lock_irqsave(&percpu_ref_switch_lock, flags);
343
344 WARN_ON_ONCE(!percpu_ref_is_zero(ref));
345
346 ref->percpu_count_ptr &= ~__PERCPU_REF_DEAD;
347 percpu_ref_get(ref);
348 __percpu_ref_switch_mode(ref, NULL);
349
350 spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
351}
352EXPORT_SYMBOL_GPL(percpu_ref_reinit);
1#define pr_fmt(fmt) "%s: " fmt "\n", __func__
2
3#include <linux/kernel.h>
4#include <linux/sched.h>
5#include <linux/wait.h>
6#include <linux/percpu-refcount.h>
7
8/*
9 * Initially, a percpu refcount is just a set of percpu counters. Initially, we
10 * don't try to detect the ref hitting 0 - which means that get/put can just
11 * increment or decrement the local counter. Note that the counter on a
12 * particular cpu can (and will) wrap - this is fine, when we go to shutdown the
13 * percpu counters will all sum to the correct value
14 *
15 * (More precisely: because modular arithmetic is commutative the sum of all the
16 * percpu_count vars will be equal to what it would have been if all the gets
17 * and puts were done to a single integer, even if some of the percpu integers
18 * overflow or underflow).
19 *
20 * The real trick to implementing percpu refcounts is shutdown. We can't detect
21 * the ref hitting 0 on every put - this would require global synchronization
22 * and defeat the whole purpose of using percpu refs.
23 *
24 * What we do is require the user to keep track of the initial refcount; we know
25 * the ref can't hit 0 before the user drops the initial ref, so as long as we
26 * convert to non percpu mode before the initial ref is dropped everything
27 * works.
28 *
29 * Converting to non percpu mode is done with some RCUish stuff in
30 * percpu_ref_kill. Additionally, we need a bias value so that the
31 * atomic_long_t can't hit 0 before we've added up all the percpu refs.
32 */
33
34#define PERCPU_COUNT_BIAS (1LU << (BITS_PER_LONG - 1))
35
36static DECLARE_WAIT_QUEUE_HEAD(percpu_ref_switch_waitq);
37
38static unsigned long __percpu *percpu_count_ptr(struct percpu_ref *ref)
39{
40 return (unsigned long __percpu *)
41 (ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC_DEAD);
42}
43
44/**
45 * percpu_ref_init - initialize a percpu refcount
46 * @ref: percpu_ref to initialize
47 * @release: function which will be called when refcount hits 0
48 * @flags: PERCPU_REF_INIT_* flags
49 * @gfp: allocation mask to use
50 *
51 * Initializes @ref. If @flags is zero, @ref starts in percpu mode with a
52 * refcount of 1; analagous to atomic_long_set(ref, 1). See the
53 * definitions of PERCPU_REF_INIT_* flags for flag behaviors.
54 *
55 * Note that @release must not sleep - it may potentially be called from RCU
56 * callback context by percpu_ref_kill().
57 */
58int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release,
59 unsigned int flags, gfp_t gfp)
60{
61 size_t align = max_t(size_t, 1 << __PERCPU_REF_FLAG_BITS,
62 __alignof__(unsigned long));
63 unsigned long start_count = 0;
64
65 ref->percpu_count_ptr = (unsigned long)
66 __alloc_percpu_gfp(sizeof(unsigned long), align, gfp);
67 if (!ref->percpu_count_ptr)
68 return -ENOMEM;
69
70 ref->force_atomic = flags & PERCPU_REF_INIT_ATOMIC;
71
72 if (flags & (PERCPU_REF_INIT_ATOMIC | PERCPU_REF_INIT_DEAD))
73 ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;
74 else
75 start_count += PERCPU_COUNT_BIAS;
76
77 if (flags & PERCPU_REF_INIT_DEAD)
78 ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
79 else
80 start_count++;
81
82 atomic_long_set(&ref->count, start_count);
83
84 ref->release = release;
85 return 0;
86}
87EXPORT_SYMBOL_GPL(percpu_ref_init);
88
89/**
90 * percpu_ref_exit - undo percpu_ref_init()
91 * @ref: percpu_ref to exit
92 *
93 * This function exits @ref. The caller is responsible for ensuring that
94 * @ref is no longer in active use. The usual places to invoke this
95 * function from are the @ref->release() callback or in init failure path
96 * where percpu_ref_init() succeeded but other parts of the initialization
97 * of the embedding object failed.
98 */
99void percpu_ref_exit(struct percpu_ref *ref)
100{
101 unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
102
103 if (percpu_count) {
104 free_percpu(percpu_count);
105 ref->percpu_count_ptr = __PERCPU_REF_ATOMIC_DEAD;
106 }
107}
108EXPORT_SYMBOL_GPL(percpu_ref_exit);
109
110static void percpu_ref_call_confirm_rcu(struct rcu_head *rcu)
111{
112 struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);
113
114 ref->confirm_switch(ref);
115 ref->confirm_switch = NULL;
116 wake_up_all(&percpu_ref_switch_waitq);
117
118 /* drop ref from percpu_ref_switch_to_atomic() */
119 percpu_ref_put(ref);
120}
121
122static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu)
123{
124 struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);
125 unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
126 unsigned long count = 0;
127 int cpu;
128
129 for_each_possible_cpu(cpu)
130 count += *per_cpu_ptr(percpu_count, cpu);
131
132 pr_debug("global %ld percpu %ld",
133 atomic_long_read(&ref->count), (long)count);
134
135 /*
136 * It's crucial that we sum the percpu counters _before_ adding the sum
137 * to &ref->count; since gets could be happening on one cpu while puts
138 * happen on another, adding a single cpu's count could cause
139 * @ref->count to hit 0 before we've got a consistent value - but the
140 * sum of all the counts will be consistent and correct.
141 *
142 * Subtracting the bias value then has to happen _after_ adding count to
143 * &ref->count; we need the bias value to prevent &ref->count from
144 * reaching 0 before we add the percpu counts. But doing it at the same
145 * time is equivalent and saves us atomic operations:
146 */
147 atomic_long_add((long)count - PERCPU_COUNT_BIAS, &ref->count);
148
149 WARN_ONCE(atomic_long_read(&ref->count) <= 0,
150 "percpu ref (%pf) <= 0 (%ld) after switching to atomic",
151 ref->release, atomic_long_read(&ref->count));
152
153 /* @ref is viewed as dead on all CPUs, send out switch confirmation */
154 percpu_ref_call_confirm_rcu(rcu);
155}
156
157static void percpu_ref_noop_confirm_switch(struct percpu_ref *ref)
158{
159}
160
161static void __percpu_ref_switch_to_atomic(struct percpu_ref *ref,
162 percpu_ref_func_t *confirm_switch)
163{
164 if (!(ref->percpu_count_ptr & __PERCPU_REF_ATOMIC)) {
165 /* switching from percpu to atomic */
166 ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;
167
168 /*
169 * Non-NULL ->confirm_switch is used to indicate that
170 * switching is in progress. Use noop one if unspecified.
171 */
172 WARN_ON_ONCE(ref->confirm_switch);
173 ref->confirm_switch =
174 confirm_switch ?: percpu_ref_noop_confirm_switch;
175
176 percpu_ref_get(ref); /* put after confirmation */
177 call_rcu_sched(&ref->rcu, percpu_ref_switch_to_atomic_rcu);
178 } else if (confirm_switch) {
179 /*
180 * Somebody already set ATOMIC. Switching may still be in
181 * progress. @confirm_switch must be invoked after the
182 * switching is complete and a full sched RCU grace period
183 * has passed. Wait synchronously for the previous
184 * switching and schedule @confirm_switch invocation.
185 */
186 wait_event(percpu_ref_switch_waitq, !ref->confirm_switch);
187 ref->confirm_switch = confirm_switch;
188
189 percpu_ref_get(ref); /* put after confirmation */
190 call_rcu_sched(&ref->rcu, percpu_ref_call_confirm_rcu);
191 }
192}
193
194/**
195 * percpu_ref_switch_to_atomic - switch a percpu_ref to atomic mode
196 * @ref: percpu_ref to switch to atomic mode
197 * @confirm_switch: optional confirmation callback
198 *
199 * There's no reason to use this function for the usual reference counting.
200 * Use percpu_ref_kill[_and_confirm]().
201 *
202 * Schedule switching of @ref to atomic mode. All its percpu counts will
203 * be collected to the main atomic counter. On completion, when all CPUs
204 * are guaraneed to be in atomic mode, @confirm_switch, which may not
205 * block, is invoked. This function may be invoked concurrently with all
206 * the get/put operations and can safely be mixed with kill and reinit
207 * operations. Note that @ref will stay in atomic mode across kill/reinit
208 * cycles until percpu_ref_switch_to_percpu() is called.
209 *
210 * This function normally doesn't block and can be called from any context
211 * but it may block if @confirm_kill is specified and @ref is already in
212 * the process of switching to atomic mode. In such cases, @confirm_switch
213 * will be invoked after the switching is complete.
214 *
215 * Due to the way percpu_ref is implemented, @confirm_switch will be called
216 * after at least one full sched RCU grace period has passed but this is an
217 * implementation detail and must not be depended upon.
218 */
219void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
220 percpu_ref_func_t *confirm_switch)
221{
222 ref->force_atomic = true;
223 __percpu_ref_switch_to_atomic(ref, confirm_switch);
224}
225
226static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)
227{
228 unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
229 int cpu;
230
231 BUG_ON(!percpu_count);
232
233 if (!(ref->percpu_count_ptr & __PERCPU_REF_ATOMIC))
234 return;
235
236 wait_event(percpu_ref_switch_waitq, !ref->confirm_switch);
237
238 atomic_long_add(PERCPU_COUNT_BIAS, &ref->count);
239
240 /*
241 * Restore per-cpu operation. smp_store_release() is paired with
242 * smp_read_barrier_depends() in __ref_is_percpu() and guarantees
243 * that the zeroing is visible to all percpu accesses which can see
244 * the following __PERCPU_REF_ATOMIC clearing.
245 */
246 for_each_possible_cpu(cpu)
247 *per_cpu_ptr(percpu_count, cpu) = 0;
248
249 smp_store_release(&ref->percpu_count_ptr,
250 ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC);
251}
252
253/**
254 * percpu_ref_switch_to_percpu - switch a percpu_ref to percpu mode
255 * @ref: percpu_ref to switch to percpu mode
256 *
257 * There's no reason to use this function for the usual reference counting.
258 * To re-use an expired ref, use percpu_ref_reinit().
259 *
260 * Switch @ref to percpu mode. This function may be invoked concurrently
261 * with all the get/put operations and can safely be mixed with kill and
262 * reinit operations. This function reverses the sticky atomic state set
263 * by PERCPU_REF_INIT_ATOMIC or percpu_ref_switch_to_atomic(). If @ref is
264 * dying or dead, the actual switching takes place on the following
265 * percpu_ref_reinit().
266 *
267 * This function normally doesn't block and can be called from any context
268 * but it may block if @ref is in the process of switching to atomic mode
269 * by percpu_ref_switch_atomic().
270 */
271void percpu_ref_switch_to_percpu(struct percpu_ref *ref)
272{
273 ref->force_atomic = false;
274
275 /* a dying or dead ref can't be switched to percpu mode w/o reinit */
276 if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD))
277 __percpu_ref_switch_to_percpu(ref);
278}
279
280/**
281 * percpu_ref_kill_and_confirm - drop the initial ref and schedule confirmation
282 * @ref: percpu_ref to kill
283 * @confirm_kill: optional confirmation callback
284 *
285 * Equivalent to percpu_ref_kill() but also schedules kill confirmation if
286 * @confirm_kill is not NULL. @confirm_kill, which may not block, will be
287 * called after @ref is seen as dead from all CPUs at which point all
288 * further invocations of percpu_ref_tryget_live() will fail. See
289 * percpu_ref_tryget_live() for details.
290 *
291 * This function normally doesn't block and can be called from any context
292 * but it may block if @confirm_kill is specified and @ref is in the
293 * process of switching to atomic mode by percpu_ref_switch_atomic().
294 *
295 * Due to the way percpu_ref is implemented, @confirm_switch will be called
296 * after at least one full sched RCU grace period has passed but this is an
297 * implementation detail and must not be depended upon.
298 */
299void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
300 percpu_ref_func_t *confirm_kill)
301{
302 WARN_ONCE(ref->percpu_count_ptr & __PERCPU_REF_DEAD,
303 "%s called more than once on %pf!", __func__, ref->release);
304
305 ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
306 __percpu_ref_switch_to_atomic(ref, confirm_kill);
307 percpu_ref_put(ref);
308}
309EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);
310
311/**
312 * percpu_ref_reinit - re-initialize a percpu refcount
313 * @ref: perpcu_ref to re-initialize
314 *
315 * Re-initialize @ref so that it's in the same state as when it finished
316 * percpu_ref_init() ignoring %PERCPU_REF_INIT_DEAD. @ref must have been
317 * initialized successfully and reached 0 but not exited.
318 *
319 * Note that percpu_ref_tryget[_live]() are safe to perform on @ref while
320 * this function is in progress.
321 */
322void percpu_ref_reinit(struct percpu_ref *ref)
323{
324 WARN_ON_ONCE(!percpu_ref_is_zero(ref));
325
326 ref->percpu_count_ptr &= ~__PERCPU_REF_DEAD;
327 percpu_ref_get(ref);
328 if (!ref->force_atomic)
329 __percpu_ref_switch_to_percpu(ref);
330}
331EXPORT_SYMBOL_GPL(percpu_ref_reinit);