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1// SPDX-License-Identifier: GPL-2.0-only2#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, we13 * don't try to detect the ref hitting 0 - which means that get/put can just14 * increment or decrement the local counter. Note that the counter on a15 * particular cpu can (and will) wrap - this is fine, when we go to shutdown the16 * percpu counters will all sum to the correct value17 *18 * (More precisely: because modular arithmetic is commutative the sum of all the19 * percpu_count vars will be equal to what it would have been if all the gets20 * and puts were done to a single integer, even if some of the percpu integers21 * overflow or underflow).22 *23 * The real trick to implementing percpu refcounts is shutdown. We can't detect24 * the ref hitting 0 on every put - this would require global synchronization25 * 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 know28 * the ref can't hit 0 before the user drops the initial ref, so as long as we29 * convert to non percpu mode before the initial ref is dropped everything30 * works.31 *32 * Converting to non percpu mode is done with some RCUish stuff in33 * percpu_ref_kill. Additionally, we need a bias value so that the34 * 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 39static DEFINE_SPINLOCK(percpu_ref_switch_lock);40static DECLARE_WAIT_QUEUE_HEAD(percpu_ref_switch_waitq);41 42static 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 refcount50 * @ref: percpu_ref to initialize51 * @release: function which will be called when refcount hits 052 * @flags: PERCPU_REF_INIT_* flags53 * @gfp: allocation mask to use54 *55 * Initializes @ref. @ref starts out in percpu mode with a refcount of 1 unless56 * @flags contains PERCPU_REF_INIT_ATOMIC or PERCPU_REF_INIT_DEAD. These flags57 * change the start state to atomic with the latter setting the initial refcount58 * 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 RCU61 * callback context by percpu_ref_kill().62 */63int 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 else96 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}106EXPORT_SYMBOL_GPL(percpu_ref_init);107 108static 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 exit123 *124 * This function exits @ref. The caller is responsible for ensuring that125 * @ref is no longer in active use. The usual places to invoke this126 * function from are the @ref->release() callback or in init failure path127 * where percpu_ref_init() succeeded but other parts of the initialization128 * of the embedding object failed.129 */130void 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}148EXPORT_SYMBOL_GPL(percpu_ref_exit);149 150static HWJS_SUSPENDS 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 167static HWJS_SUSPENDS 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 sum185 * to &ref->count; since gets could be happening on one cpu while puts186 * happen on another, adding a single cpu's count could cause187 * @ref->count to hit 0 before we've got a consistent value - but the188 * sum of all the counts will be consistent and correct.189 *190 * Subtracting the bias value then has to happen _after_ adding count to191 * &ref->count; we need the bias value to prevent &ref->count from192 * reaching 0 before we add the percpu counts. But doing it at the same193 * 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 209static void percpu_ref_noop_confirm_switch(struct percpu_ref *ref)210{211}212 213static HWJS_SUSPENDS 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 is227 * 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 237static 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 paired254 * with READ_ONCE() in __ref_is_percpu() and guarantees that the255 * zeroing is visible to all percpu accesses which can see the256 * 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 265static HWJS_SUSPENDS 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 for274 * its completion. If the caller ensures that ATOMIC switching275 * 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 else283 __percpu_ref_switch_to_percpu(ref);284}285 286/**287 * percpu_ref_switch_to_atomic - switch a percpu_ref to atomic mode288 * @ref: percpu_ref to switch to atomic mode289 * @confirm_switch: optional confirmation callback290 *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 will295 * be collected to the main atomic counter. On completion, when all CPUs296 * are guaraneed to be in atomic mode, @confirm_switch, which may not297 * block, is invoked. This function may be invoked concurrently with all298 * the get/put operations and can safely be mixed with kill and reinit299 * operations. Note that @ref will stay in atomic mode across kill/reinit300 * cycles until percpu_ref_switch_to_percpu() is called.301 *302 * This function may block if @ref is in the process of switching to atomic303 * mode. If the caller ensures that @ref is not in the process of304 * switching to atomic mode, this function can be called from any context.305 */306void 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}318EXPORT_SYMBOL_GPL(percpu_ref_switch_to_atomic);319 320/**321 * percpu_ref_switch_to_atomic_sync - switch a percpu_ref to atomic mode322 * @ref: percpu_ref to switch to atomic mode323 *324 * Schedule switching the ref to atomic mode, and wait for the325 * switch to complete. Caller must ensure that no other thread326 * will switch back to percpu mode.327 */328void 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}333EXPORT_SYMBOL_GPL(percpu_ref_switch_to_atomic_sync);334 335/**336 * percpu_ref_switch_to_percpu - switch a percpu_ref to percpu mode337 * @ref: percpu_ref to switch to percpu mode338 *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 concurrently343 * with all the get/put operations and can safely be mixed with kill and344 * reinit operations. This function reverses the sticky atomic state set345 * by PERCPU_REF_INIT_ATOMIC or percpu_ref_switch_to_atomic(). If @ref is346 * dying or dead, the actual switching takes place on the following347 * percpu_ref_reinit().348 *349 * This function may block if @ref is in the process of switching to atomic350 * mode. If the caller ensures that @ref is not in the process of351 * switching to atomic mode, this function can be called from any context.352 */353void 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}364EXPORT_SYMBOL_GPL(percpu_ref_switch_to_percpu);365 366/**367 * percpu_ref_kill_and_confirm - drop the initial ref and schedule confirmation368 * @ref: percpu_ref to kill369 * @confirm_kill: optional confirmation callback370 *371 * Equivalent to percpu_ref_kill() but also schedules kill confirmation if372 * @confirm_kill is not NULL. @confirm_kill, which may not block, will be373 * called after @ref is seen as dead from all CPUs at which point all374 * further invocations of percpu_ref_tryget_live() will fail. See375 * percpu_ref_tryget_live() for details.376 *377 * This function normally doesn't block and can be called from any context378 * but it may block if @confirm_kill is specified and @ref is in the379 * 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 */383void 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}400EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);401 402/**403 * percpu_ref_is_zero - test whether a percpu refcount reached zero404 * @ref: percpu_ref to test405 *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 */410bool 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 else423 count = ref->percpu_count_ptr >> __PERCPU_REF_FLAG_BITS;424 spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);425 426 return count == 0;427}428EXPORT_SYMBOL_GPL(percpu_ref_is_zero);429 430/**431 * percpu_ref_reinit - re-initialize a percpu refcount432 * @ref: perpcu_ref to re-initialize433 *434 * Re-initialize @ref so that it's in the same state as when it finished435 * percpu_ref_init() ignoring %PERCPU_REF_INIT_DEAD. @ref must have been436 * initialized successfully and reached 0 but not exited.437 *438 * Note that percpu_ref_tryget[_live]() are safe to perform on @ref while439 * this function is in progress.440 */441void percpu_ref_reinit(struct percpu_ref *ref)442{443 WARN_ON_ONCE(!percpu_ref_is_zero(ref));444 445 percpu_ref_resurrect(ref);446}447EXPORT_SYMBOL_GPL(percpu_ref_reinit);448 449/**450 * percpu_ref_resurrect - modify a percpu refcount from dead to live451 * @ref: perpcu_ref to resurrect452 *453 * Modify @ref so that it's in the same state as before percpu_ref_kill() was454 * called. @ref must be dead but must not yet have exited.455 *456 * If @ref->release() frees @ref then the caller is responsible for457 * guaranteeing that @ref->release() does not get called while this458 * function is in progress.459 *460 * Note that percpu_ref_tryget[_live]() are safe to perform on @ref while461 * this function is in progress.462 */463void 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}479EXPORT_SYMBOL_GPL(percpu_ref_resurrect);480