Line data Source code
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 0 : return (unsigned long __percpu *)
45 0 : (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 0 : int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release,
64 : unsigned int flags, gfp_t gfp)
65 : {
66 0 : size_t align = max_t(size_t, 1 << __PERCPU_REF_FLAG_BITS,
67 : __alignof__(unsigned long));
68 0 : unsigned long start_count = 0;
69 : struct percpu_ref_data *data;
70 :
71 0 : ref->percpu_count_ptr = (unsigned long)
72 0 : __alloc_percpu_gfp(sizeof(unsigned long), align, gfp);
73 0 : if (!ref->percpu_count_ptr)
74 : return -ENOMEM;
75 :
76 0 : data = kzalloc(sizeof(*ref->data), gfp);
77 0 : if (!data) {
78 0 : free_percpu((void __percpu *)ref->percpu_count_ptr);
79 0 : ref->percpu_count_ptr = 0;
80 0 : return -ENOMEM;
81 : }
82 :
83 0 : data->force_atomic = flags & PERCPU_REF_INIT_ATOMIC;
84 0 : data->allow_reinit = flags & PERCPU_REF_ALLOW_REINIT;
85 :
86 0 : if (flags & (PERCPU_REF_INIT_ATOMIC | PERCPU_REF_INIT_DEAD)) {
87 0 : ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;
88 0 : data->allow_reinit = true;
89 : } else {
90 : start_count += PERCPU_COUNT_BIAS;
91 : }
92 :
93 0 : if (flags & PERCPU_REF_INIT_DEAD)
94 0 : ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
95 : else
96 0 : start_count++;
97 :
98 0 : atomic_long_set(&data->count, start_count);
99 :
100 0 : data->release = release;
101 0 : data->confirm_switch = NULL;
102 0 : data->ref = ref;
103 0 : ref->data = data;
104 0 : return 0;
105 : }
106 : EXPORT_SYMBOL_GPL(percpu_ref_init);
107 :
108 0 : static void __percpu_ref_exit(struct percpu_ref *ref)
109 : {
110 0 : unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
111 :
112 0 : if (percpu_count) {
113 : /* non-NULL confirm_switch indicates switching in progress */
114 0 : WARN_ON_ONCE(ref->data && ref->data->confirm_switch);
115 0 : free_percpu(percpu_count);
116 0 : ref->percpu_count_ptr = __PERCPU_REF_ATOMIC_DEAD;
117 : }
118 0 : }
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 0 : void percpu_ref_exit(struct percpu_ref *ref)
131 : {
132 0 : struct percpu_ref_data *data = ref->data;
133 : unsigned long flags;
134 :
135 0 : __percpu_ref_exit(ref);
136 :
137 0 : if (!data)
138 : return;
139 :
140 0 : spin_lock_irqsave(&percpu_ref_switch_lock, flags);
141 0 : ref->percpu_count_ptr |= atomic_long_read(&ref->data->count) <<
142 : __PERCPU_REF_FLAG_BITS;
143 0 : ref->data = NULL;
144 0 : spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
145 :
146 0 : kfree(data);
147 : }
148 : EXPORT_SYMBOL_GPL(percpu_ref_exit);
149 :
150 0 : static void percpu_ref_call_confirm_rcu(struct rcu_head *rcu)
151 : {
152 0 : struct percpu_ref_data *data = container_of(rcu,
153 : struct percpu_ref_data, rcu);
154 0 : struct percpu_ref *ref = data->ref;
155 :
156 0 : data->confirm_switch(ref);
157 0 : data->confirm_switch = NULL;
158 0 : wake_up_all(&percpu_ref_switch_waitq);
159 :
160 0 : if (!data->allow_reinit)
161 0 : __percpu_ref_exit(ref);
162 :
163 : /* drop ref from percpu_ref_switch_to_atomic() */
164 0 : percpu_ref_put(ref);
165 0 : }
166 :
167 0 : static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu)
168 : {
169 0 : struct percpu_ref_data *data = container_of(rcu,
170 : struct percpu_ref_data, rcu);
171 0 : struct percpu_ref *ref = data->ref;
172 0 : unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
173 : static atomic_t underflows;
174 0 : unsigned long count = 0;
175 : int cpu;
176 :
177 0 : for_each_possible_cpu(cpu)
178 0 : 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 0 : atomic_long_add((long)count - PERCPU_COUNT_BIAS, &data->count);
196 :
197 0 : if (WARN_ONCE(atomic_long_read(&data->count) <= 0,
198 : "percpu ref (%ps) <= 0 (%ld) after switching to atomic",
199 0 : data->release, atomic_long_read(&data->count)) &&
200 0 : atomic_inc_return(&underflows) < 4) {
201 0 : pr_err("%s(): percpu_ref underflow", __func__);
202 0 : mem_dump_obj(data);
203 : }
204 :
205 : /* @ref is viewed as dead on all CPUs, send out switch confirmation */
206 0 : percpu_ref_call_confirm_rcu(rcu);
207 0 : }
208 :
209 0 : static void percpu_ref_noop_confirm_switch(struct percpu_ref *ref)
210 : {
211 0 : }
212 :
213 0 : static void __percpu_ref_switch_to_atomic(struct percpu_ref *ref,
214 : percpu_ref_func_t *confirm_switch)
215 : {
216 0 : if (ref->percpu_count_ptr & __PERCPU_REF_ATOMIC) {
217 0 : if (confirm_switch)
218 0 : confirm_switch(ref);
219 : return;
220 : }
221 :
222 : /* switching from percpu to atomic */
223 0 : 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 0 : ref->data->confirm_switch = confirm_switch ?:
230 : percpu_ref_noop_confirm_switch;
231 :
232 0 : percpu_ref_get(ref); /* put after confirmation */
233 0 : call_rcu_hurry(&ref->data->rcu,
234 : percpu_ref_switch_to_atomic_rcu);
235 : }
236 :
237 0 : static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)
238 : {
239 0 : unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
240 : int cpu;
241 :
242 0 : BUG_ON(!percpu_count);
243 :
244 0 : if (!(ref->percpu_count_ptr & __PERCPU_REF_ATOMIC))
245 : return;
246 :
247 0 : if (WARN_ON_ONCE(!ref->data->allow_reinit))
248 : return;
249 :
250 0 : 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 0 : for_each_possible_cpu(cpu)
259 0 : *per_cpu_ptr(percpu_count, cpu) = 0;
260 :
261 0 : smp_store_release(&ref->percpu_count_ptr,
262 : ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC);
263 : }
264 :
265 0 : static void __percpu_ref_switch_mode(struct percpu_ref *ref,
266 : percpu_ref_func_t *confirm_switch)
267 : {
268 0 : 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 0 : wait_event_lock_irq(percpu_ref_switch_waitq, !data->confirm_switch,
278 : percpu_ref_switch_lock);
279 :
280 0 : if (data->force_atomic || percpu_ref_is_dying(ref))
281 0 : __percpu_ref_switch_to_atomic(ref, confirm_switch);
282 : else
283 0 : __percpu_ref_switch_to_percpu(ref);
284 0 : }
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 0 : void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
307 : percpu_ref_func_t *confirm_switch)
308 : {
309 : unsigned long flags;
310 :
311 0 : spin_lock_irqsave(&percpu_ref_switch_lock, flags);
312 :
313 0 : ref->data->force_atomic = true;
314 0 : __percpu_ref_switch_mode(ref, confirm_switch);
315 :
316 0 : spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
317 0 : }
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 0 : void percpu_ref_switch_to_atomic_sync(struct percpu_ref *ref)
329 : {
330 0 : percpu_ref_switch_to_atomic(ref, NULL);
331 0 : wait_event(percpu_ref_switch_waitq, !ref->data->confirm_switch);
332 0 : }
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 0 : void percpu_ref_switch_to_percpu(struct percpu_ref *ref)
354 : {
355 : unsigned long flags;
356 :
357 0 : spin_lock_irqsave(&percpu_ref_switch_lock, flags);
358 :
359 0 : ref->data->force_atomic = false;
360 0 : __percpu_ref_switch_mode(ref, NULL);
361 :
362 0 : spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
363 0 : }
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 0 : void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
384 : percpu_ref_func_t *confirm_kill)
385 : {
386 : unsigned long flags;
387 :
388 0 : spin_lock_irqsave(&percpu_ref_switch_lock, flags);
389 :
390 0 : WARN_ONCE(percpu_ref_is_dying(ref),
391 : "%s called more than once on %ps!", __func__,
392 : ref->data->release);
393 :
394 0 : ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
395 0 : __percpu_ref_switch_mode(ref, confirm_kill);
396 0 : percpu_ref_put(ref);
397 :
398 0 : spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
399 0 : }
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 0 : bool percpu_ref_is_zero(struct percpu_ref *ref)
411 : {
412 : unsigned long __percpu *percpu_count;
413 : unsigned long count, flags;
414 :
415 0 : if (__ref_is_percpu(ref, &percpu_count))
416 : return false;
417 :
418 : /* protect us from being destroyed */
419 0 : spin_lock_irqsave(&percpu_ref_switch_lock, flags);
420 0 : if (ref->data)
421 0 : count = atomic_long_read(&ref->data->count);
422 : else
423 0 : count = ref->percpu_count_ptr >> __PERCPU_REF_FLAG_BITS;
424 0 : spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
425 :
426 0 : 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 0 : void percpu_ref_reinit(struct percpu_ref *ref)
442 : {
443 0 : WARN_ON_ONCE(!percpu_ref_is_zero(ref));
444 :
445 0 : percpu_ref_resurrect(ref);
446 0 : }
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 0 : void percpu_ref_resurrect(struct percpu_ref *ref)
464 : {
465 : unsigned long __percpu *percpu_count;
466 : unsigned long flags;
467 :
468 0 : spin_lock_irqsave(&percpu_ref_switch_lock, flags);
469 :
470 0 : WARN_ON_ONCE(!percpu_ref_is_dying(ref));
471 0 : WARN_ON_ONCE(__ref_is_percpu(ref, &percpu_count));
472 :
473 0 : ref->percpu_count_ptr &= ~__PERCPU_REF_DEAD;
474 0 : percpu_ref_get(ref);
475 0 : __percpu_ref_switch_mode(ref, NULL);
476 :
477 0 : spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
478 0 : }
479 : EXPORT_SYMBOL_GPL(percpu_ref_resurrect);
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