1 // SPDX-License-Identifier: GPL-2.0-only
2 #define pr_fmt(fmt) "%s: " fmt, __func__
3
4 #include <linux/kernel.h>
5 #include <linux/sched.h>
6 #include <linux/wait.h>
7 #include <linux/slab.h>
8 #include <linux/mm.h>
9 #include <linux/percpu-refcount.h>
10
11 /*
12 * Initially, a percpu refcount is just a set of percpu counters. Initially, we
13 * don't try to detect the ref hitting 0 - which means that get/put can just
14 * increment or decrement the local counter. Note that the counter on a
15 * particular cpu can (and will) wrap - this is fine, when we go to shutdown the
16 * percpu counters will all sum to the correct value
17 *
18 * (More precisely: because modular arithmetic is commutative the sum of all the
19 * percpu_count vars will be equal to what it would have been if all the gets
20 * and puts were done to a single integer, even if some of the percpu integers
21 * overflow or underflow).
22 *
23 * The real trick to implementing percpu refcounts is shutdown. We can't detect
24 * the ref hitting 0 on every put - this would require global synchronization
25 * and defeat the whole purpose of using percpu refs.
26 *
27 * What we do is require the user to keep track of the initial refcount; we know
28 * the ref can't hit 0 before the user drops the initial ref, so as long as we
29 * convert to non percpu mode before the initial ref is dropped everything
30 * works.
31 *
32 * Converting to non percpu mode is done with some RCUish stuff in
33 * percpu_ref_kill. Additionally, we need a bias value so that the
34 * atomic_long_t can't hit 0 before we've added up all the percpu refs.
35 */
36
37 #define PERCPU_COUNT_BIAS (1LU << (BITS_PER_LONG - 1))
38
39 static DEFINE_SPINLOCK(percpu_ref_switch_lock);
40 static DECLARE_WAIT_QUEUE_HEAD(percpu_ref_switch_waitq);
41
42 static unsigned long __percpu *percpu_count_ptr(struct percpu_ref *ref)
43 {
44 return (unsigned long __percpu *)
45 (ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC_DEAD);
46 }
47
48 /**
49 * percpu_ref_init - initialize a percpu refcount
50 * @ref: percpu_ref to initialize
51 * @release: function which will be called when refcount hits 0
52 * @flags: PERCPU_REF_INIT_* flags
53 * @gfp: allocation mask to use
54 *
55 * Initializes @ref. @ref starts out in percpu mode with a refcount of 1 unless
56 * @flags contains PERCPU_REF_INIT_ATOMIC or PERCPU_REF_INIT_DEAD. These flags
57 * change the start state to atomic with the latter setting the initial refcount
58 * to 0. See the definitions of PERCPU_REF_INIT_* flags for flag behaviors.
59 *
60 * Note that @release must not sleep - it may potentially be called from RCU
61 * callback context by percpu_ref_kill().
62 */
63 int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release,
64 unsigned int flags, gfp_t gfp)
65 {
66 size_t align = max_t(size_t, 1 << __PERCPU_REF_FLAG_BITS,
67 __alignof__(unsigned long));
68 unsigned long start_count = 0;
69 struct percpu_ref_data *data;
70
71 ref->percpu_count_ptr = (unsigned long)
72 __alloc_percpu_gfp(sizeof(unsigned long), align, gfp);
73 if (!ref->percpu_count_ptr)
74 return -ENOMEM;
75
76 data = kzalloc(sizeof(*ref->data), gfp);
77 if (!data) {
78 free_percpu((void __percpu *)ref->percpu_count_ptr);
79 ref->percpu_count_ptr = 0;
80 return -ENOMEM;
81 }
82
83 data->force_atomic = flags & PERCPU_REF_INIT_ATOMIC;
84 data->allow_reinit = flags & PERCPU_REF_ALLOW_REINIT;
85
86 if (flags & (PERCPU_REF_INIT_ATOMIC | PERCPU_REF_INIT_DEAD)) {
87 ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;
88 data->allow_reinit = true;
89 } else {
90 start_count += PERCPU_COUNT_BIAS;
91 }
92
93 if (flags & PERCPU_REF_INIT_DEAD)
94 ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
95 else
96 start_count++;
97
98 atomic_long_set(&data->count, start_count);
99
100 data->release = release;
101 data->confirm_switch = NULL;
102 data->ref = ref;
103 ref->data = data;
104 return 0;
105 }
106 EXPORT_SYMBOL_GPL(percpu_ref_init);
107
108 static void __percpu_ref_exit(struct percpu_ref *ref)
109 {
110 unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
111
112 if (percpu_count) {
113 /* non-NULL confirm_switch indicates switching in progress */
114 WARN_ON_ONCE(ref->data && ref->data->confirm_switch);
115 free_percpu(percpu_count);
116 ref->percpu_count_ptr = __PERCPU_REF_ATOMIC_DEAD;
117 }
118 }
119
120 /**
121 * percpu_ref_exit - undo percpu_ref_init()
122 * @ref: percpu_ref to exit
123 *
124 * This function exits @ref. The caller is responsible for ensuring that
125 * @ref is no longer in active use. The usual places to invoke this
126 * function from are the @ref->release() callback or in init failure path
127 * where percpu_ref_init() succeeded but other parts of the initialization
128 * of the embedding object failed.
129 */
130 void percpu_ref_exit(struct percpu_ref *ref)
131 {
132 struct percpu_ref_data *data = ref->data;
133 unsigned long flags;
134
135 __percpu_ref_exit(ref);
136
137 if (!data)
138 return;
139
140 spin_lock_irqsave(&percpu_ref_switch_lock, flags);
141 ref->percpu_count_ptr |= atomic_long_read(&ref->data->count) <<
142 __PERCPU_REF_FLAG_BITS;
143 ref->data = NULL;
144 spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
145
146 kfree(data);
147 }
148 EXPORT_SYMBOL_GPL(percpu_ref_exit);
149
150 static void percpu_ref_call_confirm_rcu(struct rcu_head *rcu)
151 {
152 struct percpu_ref_data *data = container_of(rcu,
153 struct percpu_ref_data, rcu);
154 struct percpu_ref *ref = data->ref;
155
156 data->confirm_switch(ref);
157 data->confirm_switch = NULL;
158 wake_up_all(&percpu_ref_switch_waitq);
159
160 if (!data->allow_reinit)
161 __percpu_ref_exit(ref);
162
163 /* drop ref from percpu_ref_switch_to_atomic() */
164 percpu_ref_put(ref);
165 }
166
167 static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu)
168 {
169 struct percpu_ref_data *data = container_of(rcu,
170 struct percpu_ref_data, rcu);
171 struct percpu_ref *ref = data->ref;
172 unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
173 static atomic_t underflows;
174 unsigned long count = 0;
175 int cpu;
176
177 for_each_possible_cpu(cpu)
178 count += *per_cpu_ptr(percpu_count, cpu);
179
180 pr_debug("global %lu percpu %lu\n",
181 atomic_long_read(&data->count), count);
182
183 /*
184 * It's crucial that we sum the percpu counters _before_ adding the sum
185 * to &ref->count; since gets could be happening on one cpu while puts
186 * happen on another, adding a single cpu's count could cause
187 * @ref->count to hit 0 before we've got a consistent value - but the
188 * sum of all the counts will be consistent and correct.
189 *
190 * Subtracting the bias value then has to happen _after_ adding count to
191 * &ref->count; we need the bias value to prevent &ref->count from
192 * reaching 0 before we add the percpu counts. But doing it at the same
193 * time is equivalent and saves us atomic operations:
194 */
195 atomic_long_add((long)count - PERCPU_COUNT_BIAS, &data->count);
196
197 if (WARN_ONCE(atomic_long_read(&data->count) <= 0,
198 "percpu ref (%ps) <= 0 (%ld) after switching to atomic",
199 data->release, atomic_long_read(&data->count)) &&
200 atomic_inc_return(&underflows) < 4) {
201 pr_err("%s(): percpu_ref underflow", __func__);
202 mem_dump_obj(data);
203 }
204
205 /* @ref is viewed as dead on all CPUs, send out switch confirmation */
206 percpu_ref_call_confirm_rcu(rcu);
207 }
208
209 static void percpu_ref_noop_confirm_switch(struct percpu_ref *ref)
210 {
211 }
212
213 static void __percpu_ref_switch_to_atomic(struct percpu_ref *ref,
214 percpu_ref_func_t *confirm_switch)
215 {
216 if (ref->percpu_count_ptr & __PERCPU_REF_ATOMIC) {
217 if (confirm_switch)
218 confirm_switch(ref);
219 return;
220 }
221
222 /* switching from percpu to atomic */
223 ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;
224
225 /*
226 * Non-NULL ->confirm_switch is used to indicate that switching is
227 * in progress. Use noop one if unspecified.
228 */
229 ref->data->confirm_switch = confirm_switch ?:
230 percpu_ref_noop_confirm_switch;
231
232 percpu_ref_get(ref); /* put after confirmation */
233 call_rcu_hurry(&ref->data->rcu,
234 percpu_ref_switch_to_atomic_rcu);
235 }
236
237 static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)
238 {
239 unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
240 int cpu;
241
242 BUG_ON(!percpu_count);
243
244 if (!(ref->percpu_count_ptr & __PERCPU_REF_ATOMIC))
245 return;
246
247 if (WARN_ON_ONCE(!ref->data->allow_reinit))
248 return;
249
250 atomic_long_add(PERCPU_COUNT_BIAS, &ref->data->count);
251
252 /*
253 * Restore per-cpu operation. smp_store_release() is paired
254 * with READ_ONCE() in __ref_is_percpu() and guarantees that the
255 * zeroing is visible to all percpu accesses which can see the
256 * following __PERCPU_REF_ATOMIC clearing.
257 */
258 for_each_possible_cpu(cpu)
259 *per_cpu_ptr(percpu_count, cpu) = 0;
260
261 smp_store_release(&ref->percpu_count_ptr,
262 ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC);
263 }
264
265 static void __percpu_ref_switch_mode(struct percpu_ref *ref,
266 percpu_ref_func_t *confirm_switch)
267 {
268 struct percpu_ref_data *data = ref->data;
269
270 lockdep_assert_held(&percpu_ref_switch_lock);
271
272 /*
273 * If the previous ATOMIC switching hasn't finished yet, wait for
274 * its completion. If the caller ensures that ATOMIC switching
275 * isn't in progress, this function can be called from any context.
276 */
277 wait_event_lock_irq(percpu_ref_switch_waitq, !data->confirm_switch,
278 percpu_ref_switch_lock);
279
280 if (data->force_atomic || percpu_ref_is_dying(ref))
281 __percpu_ref_switch_to_atomic(ref, confirm_switch);
282 else
283 __percpu_ref_switch_to_percpu(ref);
284 }
285
286 /**
287 * percpu_ref_switch_to_atomic - switch a percpu_ref to atomic mode
288 * @ref: percpu_ref to switch to atomic mode
289 * @confirm_switch: optional confirmation callback
290 *
291 * There's no reason to use this function for the usual reference counting.
292 * Use percpu_ref_kill[_and_confirm]().
293 *
294 * Schedule switching of @ref to atomic mode. All its percpu counts will
295 * be collected to the main atomic counter. On completion, when all CPUs
296 * are guaraneed to be in atomic mode, @confirm_switch, which may not
297 * block, is invoked. This function may be invoked concurrently with all
298 * the get/put operations and can safely be mixed with kill and reinit
299 * operations. Note that @ref will stay in atomic mode across kill/reinit
300 * cycles until percpu_ref_switch_to_percpu() is called.
301 *
302 * This function may block if @ref is in the process of switching to atomic
303 * mode. If the caller ensures that @ref is not in the process of
304 * switching to atomic mode, this function can be called from any context.
305 */
306 void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
307 percpu_ref_func_t *confirm_switch)
308 {
309 unsigned long flags;
310
311 spin_lock_irqsave(&percpu_ref_switch_lock, flags);
312
313 ref->data->force_atomic = true;
314 __percpu_ref_switch_mode(ref, confirm_switch);
315
316 spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
317 }
318 EXPORT_SYMBOL_GPL(percpu_ref_switch_to_atomic);
319
320 /**
321 * percpu_ref_switch_to_atomic_sync - switch a percpu_ref to atomic mode
322 * @ref: percpu_ref to switch to atomic mode
323 *
324 * Schedule switching the ref to atomic mode, and wait for the
325 * switch to complete. Caller must ensure that no other thread
326 * will switch back to percpu mode.
327 */
328 void percpu_ref_switch_to_atomic_sync(struct percpu_ref *ref)
329 {
330 percpu_ref_switch_to_atomic(ref, NULL);
331 wait_event(percpu_ref_switch_waitq, !ref->data->confirm_switch);
332 }
333 EXPORT_SYMBOL_GPL(percpu_ref_switch_to_atomic_sync);
334
335 /**
336 * percpu_ref_switch_to_percpu - switch a percpu_ref to percpu mode
337 * @ref: percpu_ref to switch to percpu mode
338 *
339 * There's no reason to use this function for the usual reference counting.
340 * To re-use an expired ref, use percpu_ref_reinit().
341 *
342 * Switch @ref to percpu mode. This function may be invoked concurrently
343 * with all the get/put operations and can safely be mixed with kill and
344 * reinit operations. This function reverses the sticky atomic state set
345 * by PERCPU_REF_INIT_ATOMIC or percpu_ref_switch_to_atomic(). If @ref is
346 * dying or dead, the actual switching takes place on the following
347 * percpu_ref_reinit().
348 *
349 * This function may block if @ref is in the process of switching to atomic
350 * mode. If the caller ensures that @ref is not in the process of
351 * switching to atomic mode, this function can be called from any context.
352 */
353 void percpu_ref_switch_to_percpu(struct percpu_ref *ref)
354 {
355 unsigned long flags;
356
357 spin_lock_irqsave(&percpu_ref_switch_lock, flags);
358
359 ref->data->force_atomic = false;
360 __percpu_ref_switch_mode(ref, NULL);
361
362 spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
363 }
364 EXPORT_SYMBOL_GPL(percpu_ref_switch_to_percpu);
365
366 /**
367 * percpu_ref_kill_and_confirm - drop the initial ref and schedule confirmation
368 * @ref: percpu_ref to kill
369 * @confirm_kill: optional confirmation callback
370 *
371 * Equivalent to percpu_ref_kill() but also schedules kill confirmation if
372 * @confirm_kill is not NULL. @confirm_kill, which may not block, will be
373 * called after @ref is seen as dead from all CPUs at which point all
374 * further invocations of percpu_ref_tryget_live() will fail. See
375 * percpu_ref_tryget_live() for details.
376 *
377 * This function normally doesn't block and can be called from any context
378 * but it may block if @confirm_kill is specified and @ref is in the
379 * process of switching to atomic mode by percpu_ref_switch_to_atomic().
380 *
381 * There are no implied RCU grace periods between kill and release.
382 */
383 void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
384 percpu_ref_func_t *confirm_kill)
385 {
386 unsigned long flags;
387
388 spin_lock_irqsave(&percpu_ref_switch_lock, flags);
389
390 WARN_ONCE(percpu_ref_is_dying(ref),
391 "%s called more than once on %ps!", __func__,
392 ref->data->release);
393
394 ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
395 __percpu_ref_switch_mode(ref, confirm_kill);
396 percpu_ref_put(ref);
397
398 spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
399 }
400 EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);
401
402 /**
403 * percpu_ref_is_zero - test whether a percpu refcount reached zero
404 * @ref: percpu_ref to test
405 *
406 * Returns %true if @ref reached zero.
407 *
408 * This function is safe to call as long as @ref is between init and exit.
409 */
410 bool percpu_ref_is_zero(struct percpu_ref *ref)
411 {
412 unsigned long __percpu *percpu_count;
413 unsigned long count, flags;
414
415 if (__ref_is_percpu(ref, &percpu_count))
416 return false;
417
418 /* protect us from being destroyed */
419 spin_lock_irqsave(&percpu_ref_switch_lock, flags);
420 if (ref->data)
421 count = atomic_long_read(&ref->data->count);
422 else
423 count = ref->percpu_count_ptr >> __PERCPU_REF_FLAG_BITS;
424 spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
425
426 return count == 0;
427 }
428 EXPORT_SYMBOL_GPL(percpu_ref_is_zero);
429
430 /**
431 * percpu_ref_reinit - re-initialize a percpu refcount
432 * @ref: perpcu_ref to re-initialize
433 *
434 * Re-initialize @ref so that it's in the same state as when it finished
435 * percpu_ref_init() ignoring %PERCPU_REF_INIT_DEAD. @ref must have been
436 * initialized successfully and reached 0 but not exited.
437 *
438 * Note that percpu_ref_tryget[_live]() are safe to perform on @ref while
439 * this function is in progress.
440 */
441 void percpu_ref_reinit(struct percpu_ref *ref)
442 {
443 WARN_ON_ONCE(!percpu_ref_is_zero(ref));
444
445 percpu_ref_resurrect(ref);
446 }
447 EXPORT_SYMBOL_GPL(percpu_ref_reinit);
448
449 /**
450 * percpu_ref_resurrect - modify a percpu refcount from dead to live
451 * @ref: perpcu_ref to resurrect
452 *
453 * Modify @ref so that it's in the same state as before percpu_ref_kill() was
454 * called. @ref must be dead but must not yet have exited.
455 *
456 * If @ref->release() frees @ref then the caller is responsible for
457 * guaranteeing that @ref->release() does not get called while this
458 * function is in progress.
459 *
460 * Note that percpu_ref_tryget[_live]() are safe to perform on @ref while
461 * this function is in progress.
462 */
463 void percpu_ref_resurrect(struct percpu_ref *ref)
464 {
465 unsigned long __percpu *percpu_count;
466 unsigned long flags;
467
468 spin_lock_irqsave(&percpu_ref_switch_lock, flags);
469
470 WARN_ON_ONCE(!percpu_ref_is_dying(ref));
471 WARN_ON_ONCE(__ref_is_percpu(ref, &percpu_count));
472
473 ref->percpu_count_ptr &= ~__PERCPU_REF_DEAD;
474 percpu_ref_get(ref);
475 __percpu_ref_switch_mode(ref, NULL);
476
477 spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
478 }
479 EXPORT_SYMBOL_GPL(percpu_ref_resurrect);
480
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