Branch data Line data Source code
1 : : // SPDX-License-Identifier: GPL-2.0-only
2 : : #define pr_fmt(fmt) "%s: " fmt "\n", __func__
3 : :
4 : : #include <linux/kernel.h>
5 : : #include <linux/sched.h>
6 : : #include <linux/wait.h>
7 : : #include <linux/percpu-refcount.h>
8 : :
9 : : /*
10 : : * Initially, a percpu refcount is just a set of percpu counters. Initially, we
11 : : * don't try to detect the ref hitting 0 - which means that get/put can just
12 : : * increment or decrement the local counter. Note that the counter on a
13 : : * particular cpu can (and will) wrap - this is fine, when we go to shutdown the
14 : : * percpu counters will all sum to the correct value
15 : : *
16 : : * (More precisely: because modular arithmetic is commutative the sum of all the
17 : : * percpu_count vars will be equal to what it would have been if all the gets
18 : : * and puts were done to a single integer, even if some of the percpu integers
19 : : * overflow or underflow).
20 : : *
21 : : * The real trick to implementing percpu refcounts is shutdown. We can't detect
22 : : * the ref hitting 0 on every put - this would require global synchronization
23 : : * and defeat the whole purpose of using percpu refs.
24 : : *
25 : : * What we do is require the user to keep track of the initial refcount; we know
26 : : * the ref can't hit 0 before the user drops the initial ref, so as long as we
27 : : * convert to non percpu mode before the initial ref is dropped everything
28 : : * works.
29 : : *
30 : : * Converting to non percpu mode is done with some RCUish stuff in
31 : : * percpu_ref_kill. Additionally, we need a bias value so that the
32 : : * atomic_long_t can't hit 0 before we've added up all the percpu refs.
33 : : */
34 : :
35 : : #define PERCPU_COUNT_BIAS (1LU << (BITS_PER_LONG - 1))
36 : :
37 : : static DEFINE_SPINLOCK(percpu_ref_switch_lock);
38 : : static DECLARE_WAIT_QUEUE_HEAD(percpu_ref_switch_waitq);
39 : :
40 : 1248 : static unsigned long __percpu *percpu_count_ptr(struct percpu_ref *ref)
41 : : {
42 : 1248 : return (unsigned long __percpu *)
43 : 1248 : (ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC_DEAD);
44 : : }
45 : :
46 : : /**
47 : : * percpu_ref_init - initialize a percpu refcount
48 : : * @ref: percpu_ref to initialize
49 : : * @release: function which will be called when refcount hits 0
50 : : * @flags: PERCPU_REF_INIT_* flags
51 : : * @gfp: allocation mask to use
52 : : *
53 : : * Initializes @ref. If @flags is zero, @ref starts in percpu mode with a
54 : : * refcount of 1; analagous to atomic_long_set(ref, 1). See the
55 : : * definitions of PERCPU_REF_INIT_* flags for flag behaviors.
56 : : *
57 : : * Note that @release must not sleep - it may potentially be called from RCU
58 : : * callback context by percpu_ref_kill().
59 : : */
60 : 1196 : int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release,
61 : : unsigned int flags, gfp_t gfp)
62 : : {
63 : 1196 : size_t align = max_t(size_t, 1 << __PERCPU_REF_FLAG_BITS,
64 : : __alignof__(unsigned long));
65 : 1196 : unsigned long start_count = 0;
66 : :
67 : 2392 : ref->percpu_count_ptr = (unsigned long)
68 : 1196 : __alloc_percpu_gfp(sizeof(unsigned long), align, gfp);
69 [ + - ]: 1196 : if (!ref->percpu_count_ptr)
70 : : return -ENOMEM;
71 : :
72 : 1196 : ref->force_atomic = flags & PERCPU_REF_INIT_ATOMIC;
73 : 1196 : ref->allow_reinit = flags & PERCPU_REF_ALLOW_REINIT;
74 : :
75 [ + + ]: 1196 : if (flags & (PERCPU_REF_INIT_ATOMIC | PERCPU_REF_INIT_DEAD)) {
76 : 156 : ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;
77 : 156 : ref->allow_reinit = true;
78 : : } else {
79 : : start_count += PERCPU_COUNT_BIAS;
80 : : }
81 : :
82 [ - + ]: 1196 : if (flags & PERCPU_REF_INIT_DEAD)
83 : 0 : ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
84 : : else
85 : 1196 : start_count++;
86 : :
87 : 1196 : atomic_long_set(&ref->count, start_count);
88 : :
89 : 1196 : ref->release = release;
90 : 1196 : ref->confirm_switch = NULL;
91 : 1196 : return 0;
92 : : }
93 : : EXPORT_SYMBOL_GPL(percpu_ref_init);
94 : :
95 : : /**
96 : : * percpu_ref_exit - undo percpu_ref_init()
97 : : * @ref: percpu_ref to exit
98 : : *
99 : : * This function exits @ref. The caller is responsible for ensuring that
100 : : * @ref is no longer in active use. The usual places to invoke this
101 : : * function from are the @ref->release() callback or in init failure path
102 : : * where percpu_ref_init() succeeded but other parts of the initialization
103 : : * of the embedding object failed.
104 : : */
105 : 728 : void percpu_ref_exit(struct percpu_ref *ref)
106 : : {
107 : 728 : unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
108 : :
109 [ + + ]: 728 : if (percpu_count) {
110 : : /* non-NULL confirm_switch indicates switching in progress */
111 [ - + ]: 364 : WARN_ON_ONCE(ref->confirm_switch);
112 : 364 : free_percpu(percpu_count);
113 : 364 : ref->percpu_count_ptr = __PERCPU_REF_ATOMIC_DEAD;
114 : : }
115 : 728 : }
116 : : EXPORT_SYMBOL_GPL(percpu_ref_exit);
117 : :
118 : 364 : static void percpu_ref_call_confirm_rcu(struct rcu_head *rcu)
119 : : {
120 : 364 : struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);
121 : :
122 : 364 : ref->confirm_switch(ref);
123 : 364 : ref->confirm_switch = NULL;
124 : 364 : wake_up_all(&percpu_ref_switch_waitq);
125 : :
126 [ + - ]: 364 : if (!ref->allow_reinit)
127 : 364 : percpu_ref_exit(ref);
128 : :
129 : : /* drop ref from percpu_ref_switch_to_atomic() */
130 : 364 : percpu_ref_put(ref);
131 : 364 : }
132 : :
133 : 364 : static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu)
134 : : {
135 : 364 : struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);
136 : 364 : unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
137 : 364 : unsigned long count = 0;
138 : 364 : int cpu;
139 : :
140 [ + + ]: 728 : for_each_possible_cpu(cpu)
141 : 364 : count += *per_cpu_ptr(percpu_count, cpu);
142 : :
143 : 364 : pr_debug("global %ld percpu %ld",
144 : : atomic_long_read(&ref->count), (long)count);
145 : :
146 : : /*
147 : : * It's crucial that we sum the percpu counters _before_ adding the sum
148 : : * to &ref->count; since gets could be happening on one cpu while puts
149 : : * happen on another, adding a single cpu's count could cause
150 : : * @ref->count to hit 0 before we've got a consistent value - but the
151 : : * sum of all the counts will be consistent and correct.
152 : : *
153 : : * Subtracting the bias value then has to happen _after_ adding count to
154 : : * &ref->count; we need the bias value to prevent &ref->count from
155 : : * reaching 0 before we add the percpu counts. But doing it at the same
156 : : * time is equivalent and saves us atomic operations:
157 : : */
158 : 364 : atomic_long_add((long)count - PERCPU_COUNT_BIAS, &ref->count);
159 : :
160 [ - + - - ]: 364 : WARN_ONCE(atomic_long_read(&ref->count) <= 0,
161 : : "percpu ref (%ps) <= 0 (%ld) after switching to atomic",
162 : : ref->release, atomic_long_read(&ref->count));
163 : :
164 : : /* @ref is viewed as dead on all CPUs, send out switch confirmation */
165 : 364 : percpu_ref_call_confirm_rcu(rcu);
166 : 364 : }
167 : :
168 : 364 : static void percpu_ref_noop_confirm_switch(struct percpu_ref *ref)
169 : : {
170 : 364 : }
171 : :
172 : 676 : static void __percpu_ref_switch_to_atomic(struct percpu_ref *ref,
173 : : percpu_ref_func_t *confirm_switch)
174 : : {
175 [ + + ]: 676 : if (ref->percpu_count_ptr & __PERCPU_REF_ATOMIC) {
176 [ - + ]: 312 : if (confirm_switch)
177 : 0 : confirm_switch(ref);
178 : 312 : return;
179 : : }
180 : :
181 : : /* switching from percpu to atomic */
182 : 364 : ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;
183 : :
184 : : /*
185 : : * Non-NULL ->confirm_switch is used to indicate that switching is
186 : : * in progress. Use noop one if unspecified.
187 : : */
188 [ + - ]: 364 : ref->confirm_switch = confirm_switch ?: percpu_ref_noop_confirm_switch;
189 : :
190 : 364 : percpu_ref_get(ref); /* put after confirmation */
191 : 364 : call_rcu(&ref->rcu, percpu_ref_switch_to_atomic_rcu);
192 : : }
193 : :
194 : 156 : static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)
195 : : {
196 : 156 : unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
197 : 156 : int cpu;
198 : :
199 [ - + ]: 156 : BUG_ON(!percpu_count);
200 : :
201 [ + - ]: 156 : if (!(ref->percpu_count_ptr & __PERCPU_REF_ATOMIC))
202 : : return;
203 : :
204 [ - + + - ]: 156 : if (WARN_ON_ONCE(!ref->allow_reinit))
205 : : return;
206 : :
207 : 156 : atomic_long_add(PERCPU_COUNT_BIAS, &ref->count);
208 : :
209 : : /*
210 : : * Restore per-cpu operation. smp_store_release() is paired
211 : : * with READ_ONCE() in __ref_is_percpu() and guarantees that the
212 : : * zeroing is visible to all percpu accesses which can see the
213 : : * following __PERCPU_REF_ATOMIC clearing.
214 : : */
215 [ + + ]: 468 : for_each_possible_cpu(cpu)
216 : 156 : *per_cpu_ptr(percpu_count, cpu) = 0;
217 : :
218 : 156 : smp_store_release(&ref->percpu_count_ptr,
219 : : ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC);
220 : : }
221 : :
222 : 832 : static void __percpu_ref_switch_mode(struct percpu_ref *ref,
223 : : percpu_ref_func_t *confirm_switch)
224 : : {
225 : 832 : lockdep_assert_held(&percpu_ref_switch_lock);
226 : :
227 : : /*
228 : : * If the previous ATOMIC switching hasn't finished yet, wait for
229 : : * its completion. If the caller ensures that ATOMIC switching
230 : : * isn't in progress, this function can be called from any context.
231 : : */
232 [ - + - - ]: 832 : wait_event_lock_irq(percpu_ref_switch_waitq, !ref->confirm_switch,
233 : : percpu_ref_switch_lock);
234 : :
235 [ + + + + ]: 832 : if (ref->force_atomic || (ref->percpu_count_ptr & __PERCPU_REF_DEAD))
236 : 676 : __percpu_ref_switch_to_atomic(ref, confirm_switch);
237 : : else
238 : 156 : __percpu_ref_switch_to_percpu(ref);
239 : 832 : }
240 : :
241 : : /**
242 : : * percpu_ref_switch_to_atomic - switch a percpu_ref to atomic mode
243 : : * @ref: percpu_ref to switch to atomic mode
244 : : * @confirm_switch: optional confirmation callback
245 : : *
246 : : * There's no reason to use this function for the usual reference counting.
247 : : * Use percpu_ref_kill[_and_confirm]().
248 : : *
249 : : * Schedule switching of @ref to atomic mode. All its percpu counts will
250 : : * be collected to the main atomic counter. On completion, when all CPUs
251 : : * are guaraneed to be in atomic mode, @confirm_switch, which may not
252 : : * block, is invoked. This function may be invoked concurrently with all
253 : : * the get/put operations and can safely be mixed with kill and reinit
254 : : * operations. Note that @ref will stay in atomic mode across kill/reinit
255 : : * cycles until percpu_ref_switch_to_percpu() is called.
256 : : *
257 : : * This function may block if @ref is in the process of switching to atomic
258 : : * mode. If the caller ensures that @ref is not in the process of
259 : : * switching to atomic mode, this function can be called from any context.
260 : : */
261 : 0 : void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
262 : : percpu_ref_func_t *confirm_switch)
263 : : {
264 : 0 : unsigned long flags;
265 : :
266 : 0 : spin_lock_irqsave(&percpu_ref_switch_lock, flags);
267 : :
268 : 0 : ref->force_atomic = true;
269 : 0 : __percpu_ref_switch_mode(ref, confirm_switch);
270 : :
271 : 0 : spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
272 : 0 : }
273 : : EXPORT_SYMBOL_GPL(percpu_ref_switch_to_atomic);
274 : :
275 : : /**
276 : : * percpu_ref_switch_to_atomic_sync - switch a percpu_ref to atomic mode
277 : : * @ref: percpu_ref to switch to atomic mode
278 : : *
279 : : * Schedule switching the ref to atomic mode, and wait for the
280 : : * switch to complete. Caller must ensure that no other thread
281 : : * will switch back to percpu mode.
282 : : */
283 : 0 : void percpu_ref_switch_to_atomic_sync(struct percpu_ref *ref)
284 : : {
285 : 0 : percpu_ref_switch_to_atomic(ref, NULL);
286 [ # # # # ]: 0 : wait_event(percpu_ref_switch_waitq, !ref->confirm_switch);
287 : 0 : }
288 : : EXPORT_SYMBOL_GPL(percpu_ref_switch_to_atomic_sync);
289 : :
290 : : /**
291 : : * percpu_ref_switch_to_percpu - switch a percpu_ref to percpu mode
292 : : * @ref: percpu_ref to switch to percpu mode
293 : : *
294 : : * There's no reason to use this function for the usual reference counting.
295 : : * To re-use an expired ref, use percpu_ref_reinit().
296 : : *
297 : : * Switch @ref to percpu mode. This function may be invoked concurrently
298 : : * with all the get/put operations and can safely be mixed with kill and
299 : : * reinit operations. This function reverses the sticky atomic state set
300 : : * by PERCPU_REF_INIT_ATOMIC or percpu_ref_switch_to_atomic(). If @ref is
301 : : * dying or dead, the actual switching takes place on the following
302 : : * percpu_ref_reinit().
303 : : *
304 : : * This function may block if @ref is in the process of switching to atomic
305 : : * mode. If the caller ensures that @ref is not in the process of
306 : : * switching to atomic mode, this function can be called from any context.
307 : : */
308 : 156 : void percpu_ref_switch_to_percpu(struct percpu_ref *ref)
309 : : {
310 : 156 : unsigned long flags;
311 : :
312 : 156 : spin_lock_irqsave(&percpu_ref_switch_lock, flags);
313 : :
314 : 156 : ref->force_atomic = false;
315 : 156 : __percpu_ref_switch_mode(ref, NULL);
316 : :
317 : 156 : spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
318 : 156 : }
319 : : EXPORT_SYMBOL_GPL(percpu_ref_switch_to_percpu);
320 : :
321 : : /**
322 : : * percpu_ref_kill_and_confirm - drop the initial ref and schedule confirmation
323 : : * @ref: percpu_ref to kill
324 : : * @confirm_kill: optional confirmation callback
325 : : *
326 : : * Equivalent to percpu_ref_kill() but also schedules kill confirmation if
327 : : * @confirm_kill is not NULL. @confirm_kill, which may not block, will be
328 : : * called after @ref is seen as dead from all CPUs at which point all
329 : : * further invocations of percpu_ref_tryget_live() will fail. See
330 : : * percpu_ref_tryget_live() for details.
331 : : *
332 : : * This function normally doesn't block and can be called from any context
333 : : * but it may block if @confirm_kill is specified and @ref is in the
334 : : * process of switching to atomic mode by percpu_ref_switch_to_atomic().
335 : : *
336 : : * There are no implied RCU grace periods between kill and release.
337 : : */
338 : 520 : void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
339 : : percpu_ref_func_t *confirm_kill)
340 : : {
341 : 520 : unsigned long flags;
342 : :
343 : 520 : spin_lock_irqsave(&percpu_ref_switch_lock, flags);
344 : :
345 [ - + - - ]: 520 : WARN_ONCE(ref->percpu_count_ptr & __PERCPU_REF_DEAD,
346 : : "%s called more than once on %ps!", __func__, ref->release);
347 : :
348 : 520 : ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
349 : 520 : __percpu_ref_switch_mode(ref, confirm_kill);
350 : 520 : percpu_ref_put(ref);
351 : :
352 : 520 : spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
353 : 520 : }
354 : : EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);
355 : :
356 : : /**
357 : : * percpu_ref_reinit - re-initialize a percpu refcount
358 : : * @ref: perpcu_ref to re-initialize
359 : : *
360 : : * Re-initialize @ref so that it's in the same state as when it finished
361 : : * percpu_ref_init() ignoring %PERCPU_REF_INIT_DEAD. @ref must have been
362 : : * initialized successfully and reached 0 but not exited.
363 : : *
364 : : * Note that percpu_ref_tryget[_live]() are safe to perform on @ref while
365 : : * this function is in progress.
366 : : */
367 : 0 : void percpu_ref_reinit(struct percpu_ref *ref)
368 : : {
369 [ # # # # ]: 0 : WARN_ON_ONCE(!percpu_ref_is_zero(ref));
370 : :
371 : 0 : percpu_ref_resurrect(ref);
372 : 0 : }
373 : : EXPORT_SYMBOL_GPL(percpu_ref_reinit);
374 : :
375 : : /**
376 : : * percpu_ref_resurrect - modify a percpu refcount from dead to live
377 : : * @ref: perpcu_ref to resurrect
378 : : *
379 : : * Modify @ref so that it's in the same state as before percpu_ref_kill() was
380 : : * called. @ref must be dead but must not yet have exited.
381 : : *
382 : : * If @ref->release() frees @ref then the caller is responsible for
383 : : * guaranteeing that @ref->release() does not get called while this
384 : : * function is in progress.
385 : : *
386 : : * Note that percpu_ref_tryget[_live]() are safe to perform on @ref while
387 : : * this function is in progress.
388 : : */
389 : 156 : void percpu_ref_resurrect(struct percpu_ref *ref)
390 : : {
391 : 156 : unsigned long __percpu *percpu_count;
392 : 156 : unsigned long flags;
393 : :
394 : 156 : spin_lock_irqsave(&percpu_ref_switch_lock, flags);
395 : :
396 [ - + ]: 156 : WARN_ON_ONCE(!(ref->percpu_count_ptr & __PERCPU_REF_DEAD));
397 [ - + - + ]: 156 : WARN_ON_ONCE(__ref_is_percpu(ref, &percpu_count));
398 : :
399 : 156 : ref->percpu_count_ptr &= ~__PERCPU_REF_DEAD;
400 : 156 : percpu_ref_get(ref);
401 : 156 : __percpu_ref_switch_mode(ref, NULL);
402 : :
403 : 156 : spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
404 : 156 : }
405 : : EXPORT_SYMBOL_GPL(percpu_ref_resurrect);
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