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
percpu_count_ptr(struct percpu_ref * ref)40 static unsigned long __percpu *percpu_count_ptr(struct percpu_ref *ref)
41 {
42 return (unsigned long __percpu *)
43 (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 */
percpu_ref_init(struct percpu_ref * ref,percpu_ref_func_t * release,unsigned int flags,gfp_t gfp)60 int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release,
61 unsigned int flags, gfp_t gfp)
62 {
63 size_t align = max_t(size_t, 1 << __PERCPU_REF_FLAG_BITS,
64 __alignof__(unsigned long));
65 unsigned long start_count = 0;
66
67 ref->percpu_count_ptr = (unsigned long)
68 __alloc_percpu_gfp(sizeof(unsigned long), align, gfp);
69 if (!ref->percpu_count_ptr)
70 return -ENOMEM;
71
72 ref->force_atomic = flags & PERCPU_REF_INIT_ATOMIC;
73 ref->allow_reinit = flags & PERCPU_REF_ALLOW_REINIT;
74
75 if (flags & (PERCPU_REF_INIT_ATOMIC | PERCPU_REF_INIT_DEAD)) {
76 ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;
77 ref->allow_reinit = true;
78 } else {
79 start_count += PERCPU_COUNT_BIAS;
80 }
81
82 if (flags & PERCPU_REF_INIT_DEAD)
83 ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
84 else
85 start_count++;
86
87 atomic_long_set(&ref->count, start_count);
88
89 ref->release = release;
90 ref->confirm_switch = NULL;
91 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 */
percpu_ref_exit(struct percpu_ref * ref)105 void percpu_ref_exit(struct percpu_ref *ref)
106 {
107 unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
108
109 if (percpu_count) {
110 /* non-NULL confirm_switch indicates switching in progress */
111 WARN_ON_ONCE(ref->confirm_switch);
112 free_percpu(percpu_count);
113 ref->percpu_count_ptr = __PERCPU_REF_ATOMIC_DEAD;
114 }
115 }
116 EXPORT_SYMBOL_GPL(percpu_ref_exit);
117
percpu_ref_call_confirm_rcu(struct rcu_head * rcu)118 static void percpu_ref_call_confirm_rcu(struct rcu_head *rcu)
119 {
120 struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);
121
122 ref->confirm_switch(ref);
123 ref->confirm_switch = NULL;
124 wake_up_all(&percpu_ref_switch_waitq);
125
126 if (!ref->allow_reinit)
127 percpu_ref_exit(ref);
128
129 /* drop ref from percpu_ref_switch_to_atomic() */
130 percpu_ref_put(ref);
131 }
132
percpu_ref_switch_to_atomic_rcu(struct rcu_head * rcu)133 static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu)
134 {
135 struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);
136 unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
137 unsigned long count = 0;
138 int cpu;
139
140 for_each_possible_cpu(cpu)
141 count += *per_cpu_ptr(percpu_count, cpu);
142
143 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 atomic_long_add((long)count - PERCPU_COUNT_BIAS, &ref->count);
159
160 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 percpu_ref_call_confirm_rcu(rcu);
166 }
167
percpu_ref_noop_confirm_switch(struct percpu_ref * ref)168 static void percpu_ref_noop_confirm_switch(struct percpu_ref *ref)
169 {
170 }
171
__percpu_ref_switch_to_atomic(struct percpu_ref * ref,percpu_ref_func_t * confirm_switch)172 static void __percpu_ref_switch_to_atomic(struct percpu_ref *ref,
173 percpu_ref_func_t *confirm_switch)
174 {
175 if (ref->percpu_count_ptr & __PERCPU_REF_ATOMIC) {
176 if (confirm_switch)
177 confirm_switch(ref);
178 return;
179 }
180
181 /* switching from percpu to atomic */
182 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 ref->confirm_switch = confirm_switch ?: percpu_ref_noop_confirm_switch;
189
190 percpu_ref_get(ref); /* put after confirmation */
191 call_rcu(&ref->rcu, percpu_ref_switch_to_atomic_rcu);
192 }
193
__percpu_ref_switch_to_percpu(struct percpu_ref * ref)194 static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)
195 {
196 unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
197 int cpu;
198
199 BUG_ON(!percpu_count);
200
201 if (!(ref->percpu_count_ptr & __PERCPU_REF_ATOMIC))
202 return;
203
204 if (WARN_ON_ONCE(!ref->allow_reinit))
205 return;
206
207 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 for_each_possible_cpu(cpu)
216 *per_cpu_ptr(percpu_count, cpu) = 0;
217
218 smp_store_release(&ref->percpu_count_ptr,
219 ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC);
220 }
221
__percpu_ref_switch_mode(struct percpu_ref * ref,percpu_ref_func_t * confirm_switch)222 static void __percpu_ref_switch_mode(struct percpu_ref *ref,
223 percpu_ref_func_t *confirm_switch)
224 {
225 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 wait_event_lock_irq(percpu_ref_switch_waitq, !ref->confirm_switch,
233 percpu_ref_switch_lock);
234
235 if (ref->force_atomic || (ref->percpu_count_ptr & __PERCPU_REF_DEAD))
236 __percpu_ref_switch_to_atomic(ref, confirm_switch);
237 else
238 __percpu_ref_switch_to_percpu(ref);
239 }
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 */
percpu_ref_switch_to_atomic(struct percpu_ref * ref,percpu_ref_func_t * confirm_switch)261 void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
262 percpu_ref_func_t *confirm_switch)
263 {
264 unsigned long flags;
265
266 spin_lock_irqsave(&percpu_ref_switch_lock, flags);
267
268 ref->force_atomic = true;
269 __percpu_ref_switch_mode(ref, confirm_switch);
270
271 spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
272 }
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 */
percpu_ref_switch_to_atomic_sync(struct percpu_ref * ref)283 void percpu_ref_switch_to_atomic_sync(struct percpu_ref *ref)
284 {
285 percpu_ref_switch_to_atomic(ref, NULL);
286 wait_event(percpu_ref_switch_waitq, !ref->confirm_switch);
287 }
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 */
percpu_ref_switch_to_percpu(struct percpu_ref * ref)308 void percpu_ref_switch_to_percpu(struct percpu_ref *ref)
309 {
310 unsigned long flags;
311
312 spin_lock_irqsave(&percpu_ref_switch_lock, flags);
313
314 ref->force_atomic = false;
315 __percpu_ref_switch_mode(ref, NULL);
316
317 spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
318 }
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 */
percpu_ref_kill_and_confirm(struct percpu_ref * ref,percpu_ref_func_t * confirm_kill)338 void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
339 percpu_ref_func_t *confirm_kill)
340 {
341 unsigned long flags;
342
343 spin_lock_irqsave(&percpu_ref_switch_lock, flags);
344
345 WARN_ONCE(ref->percpu_count_ptr & __PERCPU_REF_DEAD,
346 "%s called more than once on %ps!", __func__, ref->release);
347
348 ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
349 __percpu_ref_switch_mode(ref, confirm_kill);
350 percpu_ref_put(ref);
351
352 spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
353 }
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 */
percpu_ref_reinit(struct percpu_ref * ref)367 void percpu_ref_reinit(struct percpu_ref *ref)
368 {
369 WARN_ON_ONCE(!percpu_ref_is_zero(ref));
370
371 percpu_ref_resurrect(ref);
372 }
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 */
percpu_ref_resurrect(struct percpu_ref * ref)389 void percpu_ref_resurrect(struct percpu_ref *ref)
390 {
391 unsigned long __percpu *percpu_count;
392 unsigned long flags;
393
394 spin_lock_irqsave(&percpu_ref_switch_lock, flags);
395
396 WARN_ON_ONCE(!(ref->percpu_count_ptr & __PERCPU_REF_DEAD));
397 WARN_ON_ONCE(__ref_is_percpu(ref, &percpu_count));
398
399 ref->percpu_count_ptr &= ~__PERCPU_REF_DEAD;
400 percpu_ref_get(ref);
401 __percpu_ref_switch_mode(ref, NULL);
402
403 spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
404 }
405 EXPORT_SYMBOL_GPL(percpu_ref_resurrect);
406