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Linux/lib/percpu-refcount.c

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  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/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 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.  @ref starts out in percpu mode with a refcount of 1 unless
 54  * @flags contains PERCPU_REF_INIT_ATOMIC or PERCPU_REF_INIT_DEAD.  These flags
 55  * change the start state to atomic with the latter setting the initial refcount
 56  * to 0.  See the definitions of PERCPU_REF_INIT_* flags for flag behaviors.
 57  *
 58  * Note that @release must not sleep - it may potentially be called from RCU
 59  * callback context by percpu_ref_kill().
 60  */
 61 int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release,
 62                     unsigned int flags, gfp_t gfp)
 63 {
 64         size_t align = max_t(size_t, 1 << __PERCPU_REF_FLAG_BITS,
 65                              __alignof__(unsigned long));
 66         unsigned long start_count = 0;
 67 
 68         ref->percpu_count_ptr = (unsigned long)
 69                 __alloc_percpu_gfp(sizeof(unsigned long), align, gfp);
 70         if (!ref->percpu_count_ptr)
 71                 return -ENOMEM;
 72 
 73         ref->force_atomic = flags & PERCPU_REF_INIT_ATOMIC;
 74         ref->allow_reinit = flags & PERCPU_REF_ALLOW_REINIT;
 75 
 76         if (flags & (PERCPU_REF_INIT_ATOMIC | PERCPU_REF_INIT_DEAD)) {
 77                 ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;
 78                 ref->allow_reinit = true;
 79         } else {
 80                 start_count += PERCPU_COUNT_BIAS;
 81         }
 82 
 83         if (flags & PERCPU_REF_INIT_DEAD)
 84                 ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
 85         else
 86                 start_count++;
 87 
 88         atomic_long_set(&ref->count, start_count);
 89 
 90         ref->release = release;
 91         ref->confirm_switch = NULL;
 92         return 0;
 93 }
 94 EXPORT_SYMBOL_GPL(percpu_ref_init);
 95 
 96 /**
 97  * percpu_ref_exit - undo percpu_ref_init()
 98  * @ref: percpu_ref to exit
 99  *
100  * This function exits @ref.  The caller is responsible for ensuring that
101  * @ref is no longer in active use.  The usual places to invoke this
102  * function from are the @ref->release() callback or in init failure path
103  * where percpu_ref_init() succeeded but other parts of the initialization
104  * of the embedding object failed.
105  */
106 void percpu_ref_exit(struct percpu_ref *ref)
107 {
108         unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
109 
110         if (percpu_count) {
111                 /* non-NULL confirm_switch indicates switching in progress */
112                 WARN_ON_ONCE(ref->confirm_switch);
113                 free_percpu(percpu_count);
114                 ref->percpu_count_ptr = __PERCPU_REF_ATOMIC_DEAD;
115         }
116 }
117 EXPORT_SYMBOL_GPL(percpu_ref_exit);
118 
119 static void percpu_ref_call_confirm_rcu(struct rcu_head *rcu)
120 {
121         struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);
122 
123         ref->confirm_switch(ref);
124         ref->confirm_switch = NULL;
125         wake_up_all(&percpu_ref_switch_waitq);
126 
127         if (!ref->allow_reinit)
128                 percpu_ref_exit(ref);
129 
130         /* drop ref from percpu_ref_switch_to_atomic() */
131         percpu_ref_put(ref);
132 }
133 
134 static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu)
135 {
136         struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);
137         unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
138         unsigned long count = 0;
139         int cpu;
140 
141         for_each_possible_cpu(cpu)
142                 count += *per_cpu_ptr(percpu_count, cpu);
143 
144         pr_debug("global %lu percpu %lu\n",
145                  atomic_long_read(&ref->count), count);
146 
147         /*
148          * It's crucial that we sum the percpu counters _before_ adding the sum
149          * to &ref->count; since gets could be happening on one cpu while puts
150          * happen on another, adding a single cpu's count could cause
151          * @ref->count to hit 0 before we've got a consistent value - but the
152          * sum of all the counts will be consistent and correct.
153          *
154          * Subtracting the bias value then has to happen _after_ adding count to
155          * &ref->count; we need the bias value to prevent &ref->count from
156          * reaching 0 before we add the percpu counts. But doing it at the same
157          * time is equivalent and saves us atomic operations:
158          */
159         atomic_long_add((long)count - PERCPU_COUNT_BIAS, &ref->count);
160 
161         WARN_ONCE(atomic_long_read(&ref->count) <= 0,
162                   "percpu ref (%ps) <= 0 (%ld) after switching to atomic",
163                   ref->release, atomic_long_read(&ref->count));
164 
165         /* @ref is viewed as dead on all CPUs, send out switch confirmation */
166         percpu_ref_call_confirm_rcu(rcu);
167 }
168 
169 static void percpu_ref_noop_confirm_switch(struct percpu_ref *ref)
170 {
171 }
172 
173 static void __percpu_ref_switch_to_atomic(struct percpu_ref *ref,
174                                           percpu_ref_func_t *confirm_switch)
175 {
176         if (ref->percpu_count_ptr & __PERCPU_REF_ATOMIC) {
177                 if (confirm_switch)
178                         confirm_switch(ref);
179                 return;
180         }
181 
182         /* switching from percpu to atomic */
183         ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;
184 
185         /*
186          * Non-NULL ->confirm_switch is used to indicate that switching is
187          * in progress.  Use noop one if unspecified.
188          */
189         ref->confirm_switch = confirm_switch ?: percpu_ref_noop_confirm_switch;
190 
191         percpu_ref_get(ref);    /* put after confirmation */
192         call_rcu(&ref->rcu, percpu_ref_switch_to_atomic_rcu);
193 }
194 
195 static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)
196 {
197         unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
198         int cpu;
199 
200         BUG_ON(!percpu_count);
201 
202         if (!(ref->percpu_count_ptr & __PERCPU_REF_ATOMIC))
203                 return;
204 
205         if (WARN_ON_ONCE(!ref->allow_reinit))
206                 return;
207 
208         atomic_long_add(PERCPU_COUNT_BIAS, &ref->count);
209 
210         /*
211          * Restore per-cpu operation.  smp_store_release() is paired
212          * with READ_ONCE() in __ref_is_percpu() and guarantees that the
213          * zeroing is visible to all percpu accesses which can see the
214          * following __PERCPU_REF_ATOMIC clearing.
215          */
216         for_each_possible_cpu(cpu)
217                 *per_cpu_ptr(percpu_count, cpu) = 0;
218 
219         smp_store_release(&ref->percpu_count_ptr,
220                           ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC);
221 }
222 
223 static void __percpu_ref_switch_mode(struct percpu_ref *ref,
224                                      percpu_ref_func_t *confirm_switch)
225 {
226         lockdep_assert_held(&percpu_ref_switch_lock);
227 
228         /*
229          * If the previous ATOMIC switching hasn't finished yet, wait for
230          * its completion.  If the caller ensures that ATOMIC switching
231          * isn't in progress, this function can be called from any context.
232          */
233         wait_event_lock_irq(percpu_ref_switch_waitq, !ref->confirm_switch,
234                             percpu_ref_switch_lock);
235 
236         if (ref->force_atomic || (ref->percpu_count_ptr & __PERCPU_REF_DEAD))
237                 __percpu_ref_switch_to_atomic(ref, confirm_switch);
238         else
239                 __percpu_ref_switch_to_percpu(ref);
240 }
241 
242 /**
243  * percpu_ref_switch_to_atomic - switch a percpu_ref to atomic mode
244  * @ref: percpu_ref to switch to atomic mode
245  * @confirm_switch: optional confirmation callback
246  *
247  * There's no reason to use this function for the usual reference counting.
248  * Use percpu_ref_kill[_and_confirm]().
249  *
250  * Schedule switching of @ref to atomic mode.  All its percpu counts will
251  * be collected to the main atomic counter.  On completion, when all CPUs
252  * are guaraneed to be in atomic mode, @confirm_switch, which may not
253  * block, is invoked.  This function may be invoked concurrently with all
254  * the get/put operations and can safely be mixed with kill and reinit
255  * operations.  Note that @ref will stay in atomic mode across kill/reinit
256  * cycles until percpu_ref_switch_to_percpu() is called.
257  *
258  * This function may block if @ref is in the process of switching to atomic
259  * mode.  If the caller ensures that @ref is not in the process of
260  * switching to atomic mode, this function can be called from any context.
261  */
262 void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
263                                  percpu_ref_func_t *confirm_switch)
264 {
265         unsigned long flags;
266 
267         spin_lock_irqsave(&percpu_ref_switch_lock, flags);
268 
269         ref->force_atomic = true;
270         __percpu_ref_switch_mode(ref, confirm_switch);
271 
272         spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
273 }
274 EXPORT_SYMBOL_GPL(percpu_ref_switch_to_atomic);
275 
276 /**
277  * percpu_ref_switch_to_atomic_sync - switch a percpu_ref to atomic mode
278  * @ref: percpu_ref to switch to atomic mode
279  *
280  * Schedule switching the ref to atomic mode, and wait for the
281  * switch to complete.  Caller must ensure that no other thread
282  * will switch back to percpu mode.
283  */
284 void percpu_ref_switch_to_atomic_sync(struct percpu_ref *ref)
285 {
286         percpu_ref_switch_to_atomic(ref, NULL);
287         wait_event(percpu_ref_switch_waitq, !ref->confirm_switch);
288 }
289 EXPORT_SYMBOL_GPL(percpu_ref_switch_to_atomic_sync);
290 
291 /**
292  * percpu_ref_switch_to_percpu - switch a percpu_ref to percpu mode
293  * @ref: percpu_ref to switch to percpu mode
294  *
295  * There's no reason to use this function for the usual reference counting.
296  * To re-use an expired ref, use percpu_ref_reinit().
297  *
298  * Switch @ref to percpu mode.  This function may be invoked concurrently
299  * with all the get/put operations and can safely be mixed with kill and
300  * reinit operations.  This function reverses the sticky atomic state set
301  * by PERCPU_REF_INIT_ATOMIC or percpu_ref_switch_to_atomic().  If @ref is
302  * dying or dead, the actual switching takes place on the following
303  * percpu_ref_reinit().
304  *
305  * This function may block if @ref is in the process of switching to atomic
306  * mode.  If the caller ensures that @ref is not in the process of
307  * switching to atomic mode, this function can be called from any context.
308  */
309 void percpu_ref_switch_to_percpu(struct percpu_ref *ref)
310 {
311         unsigned long flags;
312 
313         spin_lock_irqsave(&percpu_ref_switch_lock, flags);
314 
315         ref->force_atomic = false;
316         __percpu_ref_switch_mode(ref, NULL);
317 
318         spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
319 }
320 EXPORT_SYMBOL_GPL(percpu_ref_switch_to_percpu);
321 
322 /**
323  * percpu_ref_kill_and_confirm - drop the initial ref and schedule confirmation
324  * @ref: percpu_ref to kill
325  * @confirm_kill: optional confirmation callback
326  *
327  * Equivalent to percpu_ref_kill() but also schedules kill confirmation if
328  * @confirm_kill is not NULL.  @confirm_kill, which may not block, will be
329  * called after @ref is seen as dead from all CPUs at which point all
330  * further invocations of percpu_ref_tryget_live() will fail.  See
331  * percpu_ref_tryget_live() for details.
332  *
333  * This function normally doesn't block and can be called from any context
334  * but it may block if @confirm_kill is specified and @ref is in the
335  * process of switching to atomic mode by percpu_ref_switch_to_atomic().
336  *
337  * There are no implied RCU grace periods between kill and release.
338  */
339 void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
340                                  percpu_ref_func_t *confirm_kill)
341 {
342         unsigned long flags;
343 
344         spin_lock_irqsave(&percpu_ref_switch_lock, flags);
345 
346         WARN_ONCE(ref->percpu_count_ptr & __PERCPU_REF_DEAD,
347                   "%s called more than once on %ps!", __func__, ref->release);
348 
349         ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
350         __percpu_ref_switch_mode(ref, confirm_kill);
351         percpu_ref_put(ref);
352 
353         spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
354 }
355 EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);
356 
357 /**
358  * percpu_ref_reinit - re-initialize a percpu refcount
359  * @ref: perpcu_ref to re-initialize
360  *
361  * Re-initialize @ref so that it's in the same state as when it finished
362  * percpu_ref_init() ignoring %PERCPU_REF_INIT_DEAD.  @ref must have been
363  * initialized successfully and reached 0 but not exited.
364  *
365  * Note that percpu_ref_tryget[_live]() are safe to perform on @ref while
366  * this function is in progress.
367  */
368 void percpu_ref_reinit(struct percpu_ref *ref)
369 {
370         WARN_ON_ONCE(!percpu_ref_is_zero(ref));
371 
372         percpu_ref_resurrect(ref);
373 }
374 EXPORT_SYMBOL_GPL(percpu_ref_reinit);
375 
376 /**
377  * percpu_ref_resurrect - modify a percpu refcount from dead to live
378  * @ref: perpcu_ref to resurrect
379  *
380  * Modify @ref so that it's in the same state as before percpu_ref_kill() was
381  * called. @ref must be dead but must not yet have exited.
382  *
383  * If @ref->release() frees @ref then the caller is responsible for
384  * guaranteeing that @ref->release() does not get called while this
385  * function is in progress.
386  *
387  * Note that percpu_ref_tryget[_live]() are safe to perform on @ref while
388  * this function is in progress.
389  */
390 void percpu_ref_resurrect(struct percpu_ref *ref)
391 {
392         unsigned long __percpu *percpu_count;
393         unsigned long flags;
394 
395         spin_lock_irqsave(&percpu_ref_switch_lock, flags);
396 
397         WARN_ON_ONCE(!(ref->percpu_count_ptr & __PERCPU_REF_DEAD));
398         WARN_ON_ONCE(__ref_is_percpu(ref, &percpu_count));
399 
400         ref->percpu_count_ptr &= ~__PERCPU_REF_DEAD;
401         percpu_ref_get(ref);
402         __percpu_ref_switch_mode(ref, NULL);
403 
404         spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
405 }
406 EXPORT_SYMBOL_GPL(percpu_ref_resurrect);
407 

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