lib/percpu-refcount.c - linux - Git at Google

 // SPDX-License-Identifier: GPL-2.0-only
 #define pr_fmt(fmt) "%s: " fmt, __func__

 #include <linux/kernel.h>
 #include <linux/sched.h>
 #include <linux/wait.h>
 #include <linux/slab.h>
 #include <linux/mm.h>
 #include <linux/percpu-refcount.h>

 /*
  * Initially, a percpu refcount is just a set of percpu counters. Initially, we
  * don't try to detect the ref hitting 0 - which means that get/put can just
  * increment or decrement the local counter. Note that the counter on a
  * particular cpu can (and will) wrap - this is fine, when we go to shutdown the
  * percpu counters will all sum to the correct value
  *
  * (More precisely: because modular arithmetic is commutative the sum of all the
  * percpu_count vars will be equal to what it would have been if all the gets
  * and puts were done to a single integer, even if some of the percpu integers
  * overflow or underflow).
  *
  * The real trick to implementing percpu refcounts is shutdown. We can't detect
  * the ref hitting 0 on every put - this would require global synchronization
  * and defeat the whole purpose of using percpu refs.
  *
  * What we do is require the user to keep track of the initial refcount; we know
  * the ref can't hit 0 before the user drops the initial ref, so as long as we
  * convert to non percpu mode before the initial ref is dropped everything
  * works.
  *
  * Converting to non percpu mode is done with some RCUish stuff in
  * percpu_ref_kill. Additionally, we need a bias value so that the
  * atomic_long_t can't hit 0 before we've added up all the percpu refs.
  */

 #define PERCPU_COUNT_BIAS	(1LU << (BITS_PER_LONG - 1))

 static DEFINE_SPINLOCK(percpu_ref_switch_lock);
 static DECLARE_WAIT_QUEUE_HEAD(percpu_ref_switch_waitq);

 static unsigned long __percpu *percpu_count_ptr(struct percpu_ref *ref)
 {
 	return (unsigned long __percpu *)
 		(ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC_DEAD);
 }

 /**
  * percpu_ref_init - initialize a percpu refcount
  * @ref: percpu_ref to initialize
  * @release: function which will be called when refcount hits 0
  * @flags: PERCPU_REF_INIT_* flags
  * @gfp: allocation mask to use
  *
  * Initializes @ref.  @ref starts out in percpu mode with a refcount of 1 unless
  * @flags contains PERCPU_REF_INIT_ATOMIC or PERCPU_REF_INIT_DEAD.  These flags
  * change the start state to atomic with the latter setting the initial refcount
  * to 0.  See the definitions of PERCPU_REF_INIT_* flags for flag behaviors.
  *
  * Note that @release must not sleep - it may potentially be called from RCU
  * callback context by percpu_ref_kill().
  */
 int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release,
 		    unsigned int flags, gfp_t gfp)
 {
 	size_t align = max_t(size_t, 1 << __PERCPU_REF_FLAG_BITS,
 			     __alignof__(unsigned long));
 	unsigned long start_count = 0;
 	struct percpu_ref_data *data;

 	ref->percpu_count_ptr = (unsigned long)
 		__alloc_percpu_gfp(sizeof(unsigned long), align, gfp);
 	if (!ref->percpu_count_ptr)
 		return -ENOMEM;

 	data = kzalloc(sizeof(*ref->data), gfp);
 	if (!data) {
 		free_percpu((void __percpu *)ref->percpu_count_ptr);
 		ref->percpu_count_ptr = 0;
 		return -ENOMEM;
 	}

 	data->force_atomic = flags & PERCPU_REF_INIT_ATOMIC;
 	data->allow_reinit = flags & PERCPU_REF_ALLOW_REINIT;

 	if (flags & (PERCPU_REF_INIT_ATOMIC | PERCPU_REF_INIT_DEAD)) {
 		ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;
 		data->allow_reinit = true;
 	} else {
 		start_count += PERCPU_COUNT_BIAS;
 	}

 	if (flags & PERCPU_REF_INIT_DEAD)
 		ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
 	else
 		start_count++;

 	atomic_long_set(&data->count, start_count);

 	data->release = release;
 	data->confirm_switch = NULL;
 	data->ref = ref;
 	ref->data = data;
 	return 0;
 }
 EXPORT_SYMBOL_GPL(percpu_ref_init);

 static void __percpu_ref_exit(struct percpu_ref *ref)
 {
 	unsigned long __percpu *percpu_count = percpu_count_ptr(ref);

 	if (percpu_count) {
 		/* non-NULL confirm_switch indicates switching in progress */
 		WARN_ON_ONCE(ref->data && ref->data->confirm_switch);
 		free_percpu(percpu_count);
 		ref->percpu_count_ptr = __PERCPU_REF_ATOMIC_DEAD;
 	}
 }

 /**
  * percpu_ref_exit - undo percpu_ref_init()
  * @ref: percpu_ref to exit
  *
  * This function exits @ref.  The caller is responsible for ensuring that
  * @ref is no longer in active use.  The usual places to invoke this
  * function from are the @ref->release() callback or in init failure path
  * where percpu_ref_init() succeeded but other parts of the initialization
  * of the embedding object failed.
  */
 void percpu_ref_exit(struct percpu_ref *ref)
 {
 	struct percpu_ref_data *data = ref->data;
 	unsigned long flags;

 	__percpu_ref_exit(ref);

 	if (!data)
 		return;

 	spin_lock_irqsave(&percpu_ref_switch_lock, flags);
 	ref->percpu_count_ptr |= atomic_long_read(&ref->data->count) <<
 		__PERCPU_REF_FLAG_BITS;
 	ref->data = NULL;
 	spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);

 	kfree(data);
 }
 EXPORT_SYMBOL_GPL(percpu_ref_exit);

 static void percpu_ref_call_confirm_rcu(struct rcu_head *rcu)
 {
 	struct percpu_ref_data *data = container_of(rcu,
 			struct percpu_ref_data, rcu);
 	struct percpu_ref *ref = data->ref;

 	data->confirm_switch(ref);
 	data->confirm_switch = NULL;
 	wake_up_all(&percpu_ref_switch_waitq);

 	if (!data->allow_reinit)
 		__percpu_ref_exit(ref);

 	/* drop ref from percpu_ref_switch_to_atomic() */
 	percpu_ref_put(ref);
 }

 static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu)
 {
 	struct percpu_ref_data *data = container_of(rcu,
 			struct percpu_ref_data, rcu);
 	struct percpu_ref *ref = data->ref;
 	unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
 	static atomic_t underflows;
 	unsigned long count = 0;
 	int cpu;

 	for_each_possible_cpu(cpu)
 		count += *per_cpu_ptr(percpu_count, cpu);

 	pr_debug("global %lu percpu %lu\n",
 		 atomic_long_read(&data->count), count);

 	/*
 	 * It's crucial that we sum the percpu counters _before_ adding the sum
 	 * to &ref->count; since gets could be happening on one cpu while puts
 	 * happen on another, adding a single cpu's count could cause
 	 * @ref->count to hit 0 before we've got a consistent value - but the
 	 * sum of all the counts will be consistent and correct.
 	 *
 	 * Subtracting the bias value then has to happen _after_ adding count to
 	 * &ref->count; we need the bias value to prevent &ref->count from
 	 * reaching 0 before we add the percpu counts. But doing it at the same
 	 * time is equivalent and saves us atomic operations:
 	 */
 	atomic_long_add((long)count - PERCPU_COUNT_BIAS, &data->count);

 	if (WARN_ONCE(atomic_long_read(&data->count) <= 0,
 		      "percpu ref (%ps) <= 0 (%ld) after switching to atomic",
 		      data->release, atomic_long_read(&data->count)) &&
 	    atomic_inc_return(&underflows) < 4) {
 		pr_err("%s(): percpu_ref underflow", __func__);
 		mem_dump_obj(data);
 	}

 	/* @ref is viewed as dead on all CPUs, send out switch confirmation */
 	percpu_ref_call_confirm_rcu(rcu);
 }

 static void percpu_ref_noop_confirm_switch(struct percpu_ref *ref)
 {
 }

 static void __percpu_ref_switch_to_atomic(struct percpu_ref *ref,
 					  percpu_ref_func_t *confirm_switch)
 {
 	if (ref->percpu_count_ptr & __PERCPU_REF_ATOMIC) {
 		if (confirm_switch)
 			confirm_switch(ref);
 		return;
 	}

 	/* switching from percpu to atomic */
 	ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;

 	/*
 	 * Non-NULL ->confirm_switch is used to indicate that switching is
 	 * in progress.  Use noop one if unspecified.
 	 */
 	ref->data->confirm_switch = confirm_switch ?:
 		percpu_ref_noop_confirm_switch;

 	percpu_ref_get(ref);	/* put after confirmation */
 	call_rcu_hurry(&ref->data->rcu,
 		       percpu_ref_switch_to_atomic_rcu);
 }

 static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)
 {
 	unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
 	int cpu;

 	BUG_ON(!percpu_count);

 	if (!(ref->percpu_count_ptr & __PERCPU_REF_ATOMIC))
 		return;

 	if (WARN_ON_ONCE(!ref->data->allow_reinit))
 		return;

 	atomic_long_add(PERCPU_COUNT_BIAS, &ref->data->count);

 	/*
 	 * Restore per-cpu operation.  smp_store_release() is paired
 	 * with READ_ONCE() in __ref_is_percpu() and guarantees that the
 	 * zeroing is visible to all percpu accesses which can see the
 	 * following __PERCPU_REF_ATOMIC clearing.
 	 */
 	for_each_possible_cpu(cpu)
 		*per_cpu_ptr(percpu_count, cpu) = 0;

 	smp_store_release(&ref->percpu_count_ptr,
 			  ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC);
 }

 static void __percpu_ref_switch_mode(struct percpu_ref *ref,
 				     percpu_ref_func_t *confirm_switch)
 {
 	struct percpu_ref_data *data = ref->data;

 	lockdep_assert_held(&percpu_ref_switch_lock);

 	/*
 	 * If the previous ATOMIC switching hasn't finished yet, wait for
 	 * its completion.  If the caller ensures that ATOMIC switching
 	 * isn't in progress, this function can be called from any context.
 	 */
 	wait_event_lock_irq(percpu_ref_switch_waitq, !data->confirm_switch,
 			    percpu_ref_switch_lock);

 	if (data->force_atomic || percpu_ref_is_dying(ref))
 		__percpu_ref_switch_to_atomic(ref, confirm_switch);
 	else
 		__percpu_ref_switch_to_percpu(ref);
 }

 /**
  * percpu_ref_switch_to_atomic - switch a percpu_ref to atomic mode
  * @ref: percpu_ref to switch to atomic mode
  * @confirm_switch: optional confirmation callback
  *
  * There's no reason to use this function for the usual reference counting.
  * Use percpu_ref_kill[_and_confirm]().
  *
  * Schedule switching of @ref to atomic mode.  All its percpu counts will
  * be collected to the main atomic counter.  On completion, when all CPUs
  * are guaraneed to be in atomic mode, @confirm_switch, which may not
  * block, is invoked.  This function may be invoked concurrently with all
  * the get/put operations and can safely be mixed with kill and reinit
  * operations.  Note that @ref will stay in atomic mode across kill/reinit
  * cycles until percpu_ref_switch_to_percpu() is called.
  *
  * This function may block if @ref is in the process of switching to atomic
  * mode.  If the caller ensures that @ref is not in the process of
  * switching to atomic mode, this function can be called from any context.
  */
 void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
 				 percpu_ref_func_t *confirm_switch)
 {
 	unsigned long flags;

 	spin_lock_irqsave(&percpu_ref_switch_lock, flags);

 	ref->data->force_atomic = true;
 	__percpu_ref_switch_mode(ref, confirm_switch);

 	spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
 }
 EXPORT_SYMBOL_GPL(percpu_ref_switch_to_atomic);

 /**
  * percpu_ref_switch_to_atomic_sync - switch a percpu_ref to atomic mode
  * @ref: percpu_ref to switch to atomic mode
  *
  * Schedule switching the ref to atomic mode, and wait for the
  * switch to complete.  Caller must ensure that no other thread
  * will switch back to percpu mode.
  */
 void percpu_ref_switch_to_atomic_sync(struct percpu_ref *ref)
 {
 	percpu_ref_switch_to_atomic(ref, NULL);
 	wait_event(percpu_ref_switch_waitq, !ref->data->confirm_switch);
 }
 EXPORT_SYMBOL_GPL(percpu_ref_switch_to_atomic_sync);

 /**
  * percpu_ref_switch_to_percpu - switch a percpu_ref to percpu mode
  * @ref: percpu_ref to switch to percpu mode
  *
  * There's no reason to use this function for the usual reference counting.
  * To re-use an expired ref, use percpu_ref_reinit().
  *
  * Switch @ref to percpu mode.  This function may be invoked concurrently
  * with all the get/put operations and can safely be mixed with kill and
  * reinit operations.  This function reverses the sticky atomic state set
  * by PERCPU_REF_INIT_ATOMIC or percpu_ref_switch_to_atomic().  If @ref is
  * dying or dead, the actual switching takes place on the following
  * percpu_ref_reinit().
  *
  * This function may block if @ref is in the process of switching to atomic
  * mode.  If the caller ensures that @ref is not in the process of
  * switching to atomic mode, this function can be called from any context.
  */
 void percpu_ref_switch_to_percpu(struct percpu_ref *ref)
 {
 	unsigned long flags;

 	spin_lock_irqsave(&percpu_ref_switch_lock, flags);

 	ref->data->force_atomic = false;
 	__percpu_ref_switch_mode(ref, NULL);

 	spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
 }
 EXPORT_SYMBOL_GPL(percpu_ref_switch_to_percpu);

 /**
  * percpu_ref_kill_and_confirm - drop the initial ref and schedule confirmation
  * @ref: percpu_ref to kill
  * @confirm_kill: optional confirmation callback
  *
  * Equivalent to percpu_ref_kill() but also schedules kill confirmation if
  * @confirm_kill is not NULL.  @confirm_kill, which may not block, will be
  * called after @ref is seen as dead from all CPUs at which point all
  * further invocations of percpu_ref_tryget_live() will fail.  See
  * percpu_ref_tryget_live() for details.
  *
  * This function normally doesn't block and can be called from any context
  * but it may block if @confirm_kill is specified and @ref is in the
  * process of switching to atomic mode by percpu_ref_switch_to_atomic().
  *
  * There are no implied RCU grace periods between kill and release.
  */
 void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
 				 percpu_ref_func_t *confirm_kill)
 {
 	unsigned long flags;

 	spin_lock_irqsave(&percpu_ref_switch_lock, flags);

 	WARN_ONCE(percpu_ref_is_dying(ref),
 		  "%s called more than once on %ps!", __func__,
 		  ref->data->release);

 	ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
 	__percpu_ref_switch_mode(ref, confirm_kill);
 	percpu_ref_put(ref);

 	spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
 }
 EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);

 /**
  * percpu_ref_is_zero - test whether a percpu refcount reached zero
  * @ref: percpu_ref to test
  *
  * Returns %true if @ref reached zero.
  *
  * This function is safe to call as long as @ref is between init and exit.
  */
 bool percpu_ref_is_zero(struct percpu_ref *ref)
 {
 	unsigned long __percpu *percpu_count;
 	unsigned long count, flags;

 	if (__ref_is_percpu(ref, &percpu_count))
 		return false;

 	/* protect us from being destroyed */
 	spin_lock_irqsave(&percpu_ref_switch_lock, flags);
 	if (ref->data)
 		count = atomic_long_read(&ref->data->count);
 	else
 		count = ref->percpu_count_ptr >> __PERCPU_REF_FLAG_BITS;
 	spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);

 	return count == 0;
 }
 EXPORT_SYMBOL_GPL(percpu_ref_is_zero);

 /**
  * percpu_ref_reinit - re-initialize a percpu refcount
  * @ref: perpcu_ref to re-initialize
  *
  * Re-initialize @ref so that it's in the same state as when it finished
  * percpu_ref_init() ignoring %PERCPU_REF_INIT_DEAD.  @ref must have been
  * initialized successfully and reached 0 but not exited.
  *
  * Note that percpu_ref_tryget[_live]() are safe to perform on @ref while
  * this function is in progress.
  */
 void percpu_ref_reinit(struct percpu_ref *ref)
 {
 	WARN_ON_ONCE(!percpu_ref_is_zero(ref));

 	percpu_ref_resurrect(ref);
 }
 EXPORT_SYMBOL_GPL(percpu_ref_reinit);

 /**
  * percpu_ref_resurrect - modify a percpu refcount from dead to live
  * @ref: perpcu_ref to resurrect
  *
  * Modify @ref so that it's in the same state as before percpu_ref_kill() was
  * called. @ref must be dead but must not yet have exited.
  *
  * If @ref->release() frees @ref then the caller is responsible for
  * guaranteeing that @ref->release() does not get called while this
  * function is in progress.
  *
  * Note that percpu_ref_tryget[_live]() are safe to perform on @ref while
  * this function is in progress.
  */
 void percpu_ref_resurrect(struct percpu_ref *ref)
 {
 	unsigned long __percpu *percpu_count;
 	unsigned long flags;

 	spin_lock_irqsave(&percpu_ref_switch_lock, flags);

 	WARN_ON_ONCE(!percpu_ref_is_dying(ref));
 	WARN_ON_ONCE(__ref_is_percpu(ref, &percpu_count));

 	ref->percpu_count_ptr &= ~__PERCPU_REF_DEAD;
 	percpu_ref_get(ref);
 	__percpu_ref_switch_mode(ref, NULL);

 	spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
 }
 EXPORT_SYMBOL_GPL(percpu_ref_resurrect);
	// SPDX-License-Identifier: GPL-2.0-only
	#define pr_fmt(fmt) "%s: " fmt, __func__

	#include <linux/kernel.h>
	#include <linux/sched.h>
	#include <linux/wait.h>
	#include <linux/slab.h>
	#include <linux/mm.h>
	#include <linux/percpu-refcount.h>

	/*
	* Initially, a percpu refcount is just a set of percpu counters. Initially, we
	* don't try to detect the ref hitting 0 - which means that get/put can just
	* increment or decrement the local counter. Note that the counter on a
	* particular cpu can (and will) wrap - this is fine, when we go to shutdown the
	* percpu counters will all sum to the correct value
	*
	* (More precisely: because modular arithmetic is commutative the sum of all the
	* percpu_count vars will be equal to what it would have been if all the gets
	* and puts were done to a single integer, even if some of the percpu integers
	* overflow or underflow).
	*
	* The real trick to implementing percpu refcounts is shutdown. We can't detect
	* the ref hitting 0 on every put - this would require global synchronization
	* and defeat the whole purpose of using percpu refs.
	*
	* What we do is require the user to keep track of the initial refcount; we know
	* the ref can't hit 0 before the user drops the initial ref, so as long as we
	* convert to non percpu mode before the initial ref is dropped everything
	* works.
	*
	* Converting to non percpu mode is done with some RCUish stuff in
	* percpu_ref_kill. Additionally, we need a bias value so that the
	* atomic_long_t can't hit 0 before we've added up all the percpu refs.
	*/

	#define PERCPU_COUNT_BIAS (1LU << (BITS_PER_LONG - 1))

	static DEFINE_SPINLOCK(percpu_ref_switch_lock);
	static DECLARE_WAIT_QUEUE_HEAD(percpu_ref_switch_waitq);

	static unsigned long __percpu percpu_count_ptr(struct percpu_ref ref)
	{
	return (unsigned long __percpu *)
	(ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC_DEAD);
	}

	/**
	* percpu_ref_init - initialize a percpu refcount
	* @ref: percpu_ref to initialize
	* @release: function which will be called when refcount hits 0
	* @flags: PERCPU_REF_INIT_* flags
	* @gfp: allocation mask to use
	*
	* Initializes @ref. @ref starts out in percpu mode with a refcount of 1 unless
	* @flags contains PERCPU_REF_INIT_ATOMIC or PERCPU_REF_INIT_DEAD. These flags
	* change the start state to atomic with the latter setting the initial refcount
	* to 0. See the definitions of PERCPU_REF_INIT_* flags for flag behaviors.
	*
	* Note that @release must not sleep - it may potentially be called from RCU
	* callback context by percpu_ref_kill().
	*/
	int percpu_ref_init(struct percpu_ref ref, percpu_ref_func_t release,
	unsigned int flags, gfp_t gfp)
	{
	size_t align = max_t(size_t, 1 << __PERCPU_REF_FLAG_BITS,
	__alignof__(unsigned long));
	unsigned long start_count = 0;
	struct percpu_ref_data *data;

	ref->percpu_count_ptr = (unsigned long)
	__alloc_percpu_gfp(sizeof(unsigned long), align, gfp);
	if (!ref->percpu_count_ptr)
	return -ENOMEM;

	data = kzalloc(sizeof(*ref->data), gfp);
	if (!data) {
	free_percpu((void __percpu *)ref->percpu_count_ptr);
	ref->percpu_count_ptr = 0;
	return -ENOMEM;
	}

	data->force_atomic = flags & PERCPU_REF_INIT_ATOMIC;
	data->allow_reinit = flags & PERCPU_REF_ALLOW_REINIT;

	if (flags & (PERCPU_REF_INIT_ATOMIC \| PERCPU_REF_INIT_DEAD)) {
	ref->percpu_count_ptr \|= __PERCPU_REF_ATOMIC;
	data->allow_reinit = true;
	} else {
	start_count += PERCPU_COUNT_BIAS;
	}

	if (flags & PERCPU_REF_INIT_DEAD)
	ref->percpu_count_ptr \|= __PERCPU_REF_DEAD;
	else
	start_count++;

	atomic_long_set(&data->count, start_count);

	data->release = release;
	data->confirm_switch = NULL;
	data->ref = ref;
	ref->data = data;
	return 0;
	}
	EXPORT_SYMBOL_GPL(percpu_ref_init);

	static void __percpu_ref_exit(struct percpu_ref *ref)
	{
	unsigned long __percpu *percpu_count = percpu_count_ptr(ref);

	if (percpu_count) {
	/* non-NULL confirm_switch indicates switching in progress */
	WARN_ON_ONCE(ref->data && ref->data->confirm_switch);
	free_percpu(percpu_count);
	ref->percpu_count_ptr = __PERCPU_REF_ATOMIC_DEAD;
	}
	}

	/**
	* percpu_ref_exit - undo percpu_ref_init()
	* @ref: percpu_ref to exit
	*
	* This function exits @ref. The caller is responsible for ensuring that
	* @ref is no longer in active use. The usual places to invoke this
	* function from are the @ref->release() callback or in init failure path
	* where percpu_ref_init() succeeded but other parts of the initialization
	* of the embedding object failed.
	*/
	void percpu_ref_exit(struct percpu_ref *ref)
	{
	struct percpu_ref_data *data = ref->data;
	unsigned long flags;

	__percpu_ref_exit(ref);

	if (!data)
	return;

	spin_lock_irqsave(&percpu_ref_switch_lock, flags);
	ref->percpu_count_ptr \|= atomic_long_read(&ref->data->count) <<
	__PERCPU_REF_FLAG_BITS;
	ref->data = NULL;
	spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);

	kfree(data);
	}
	EXPORT_SYMBOL_GPL(percpu_ref_exit);

	static void percpu_ref_call_confirm_rcu(struct rcu_head *rcu)
	{
	struct percpu_ref_data *data = container_of(rcu,
	struct percpu_ref_data, rcu);
	struct percpu_ref *ref = data->ref;

	data->confirm_switch(ref);
	data->confirm_switch = NULL;
	wake_up_all(&percpu_ref_switch_waitq);

	if (!data->allow_reinit)
	__percpu_ref_exit(ref);

	/* drop ref from percpu_ref_switch_to_atomic() */
	percpu_ref_put(ref);
	}

	static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu)
	{
	struct percpu_ref_data *data = container_of(rcu,
	struct percpu_ref_data, rcu);
	struct percpu_ref *ref = data->ref;
	unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
	static atomic_t underflows;
	unsigned long count = 0;
	int cpu;

	for_each_possible_cpu(cpu)
	count += *per_cpu_ptr(percpu_count, cpu);

	pr_debug("global %lu percpu %lu\n",
	atomic_long_read(&data->count), count);

	/*
	* It's crucial that we sum the percpu counters _before_ adding the sum
	* to &ref->count; since gets could be happening on one cpu while puts
	* happen on another, adding a single cpu's count could cause
	* @ref->count to hit 0 before we've got a consistent value - but the
	* sum of all the counts will be consistent and correct.
	*
	* Subtracting the bias value then has to happen _after_ adding count to
	* &ref->count; we need the bias value to prevent &ref->count from
	* reaching 0 before we add the percpu counts. But doing it at the same
	* time is equivalent and saves us atomic operations:
	*/
	atomic_long_add((long)count - PERCPU_COUNT_BIAS, &data->count);

	if (WARN_ONCE(atomic_long_read(&data->count) <= 0,
	"percpu ref (%ps) <= 0 (%ld) after switching to atomic",
	data->release, atomic_long_read(&data->count)) &&
	atomic_inc_return(&underflows) < 4) {
	pr_err("%s(): percpu_ref underflow", __func__);
	mem_dump_obj(data);
	}

	/* @ref is viewed as dead on all CPUs, send out switch confirmation */
	percpu_ref_call_confirm_rcu(rcu);
	}

	static void percpu_ref_noop_confirm_switch(struct percpu_ref *ref)
	{
	}

	static void __percpu_ref_switch_to_atomic(struct percpu_ref *ref,
	percpu_ref_func_t *confirm_switch)
	{
	if (ref->percpu_count_ptr & __PERCPU_REF_ATOMIC) {
	if (confirm_switch)
	confirm_switch(ref);
	return;
	}

	/* switching from percpu to atomic */
	ref->percpu_count_ptr \|= __PERCPU_REF_ATOMIC;

	/*
	* Non-NULL ->confirm_switch is used to indicate that switching is
	* in progress. Use noop one if unspecified.
	*/
	ref->data->confirm_switch = confirm_switch ?:
	percpu_ref_noop_confirm_switch;

	percpu_ref_get(ref); /* put after confirmation */
	call_rcu_hurry(&ref->data->rcu,
	percpu_ref_switch_to_atomic_rcu);
	}

	static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)
	{
	unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
	int cpu;

	BUG_ON(!percpu_count);

	if (!(ref->percpu_count_ptr & __PERCPU_REF_ATOMIC))
	return;

	if (WARN_ON_ONCE(!ref->data->allow_reinit))
	return;

	atomic_long_add(PERCPU_COUNT_BIAS, &ref->data->count);

	/*
	* Restore per-cpu operation. smp_store_release() is paired
	* with READ_ONCE() in __ref_is_percpu() and guarantees that the
	* zeroing is visible to all percpu accesses which can see the
	* following __PERCPU_REF_ATOMIC clearing.
	*/
	for_each_possible_cpu(cpu)
	*per_cpu_ptr(percpu_count, cpu) = 0;

	smp_store_release(&ref->percpu_count_ptr,
	ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC);
	}

	static void __percpu_ref_switch_mode(struct percpu_ref *ref,
	percpu_ref_func_t *confirm_switch)
	{
	struct percpu_ref_data *data = ref->data;

	lockdep_assert_held(&percpu_ref_switch_lock);

	/*
	* If the previous ATOMIC switching hasn't finished yet, wait for
	* its completion. If the caller ensures that ATOMIC switching
	* isn't in progress, this function can be called from any context.
	*/
	wait_event_lock_irq(percpu_ref_switch_waitq, !data->confirm_switch,
	percpu_ref_switch_lock);

	if (data->force_atomic \|\| percpu_ref_is_dying(ref))
	__percpu_ref_switch_to_atomic(ref, confirm_switch);
	else
	__percpu_ref_switch_to_percpu(ref);
	}

	/**
	* percpu_ref_switch_to_atomic - switch a percpu_ref to atomic mode
	* @ref: percpu_ref to switch to atomic mode
	* @confirm_switch: optional confirmation callback
	*
	* There's no reason to use this function for the usual reference counting.
	* Use percpu_ref_kill[_and_confirm]().
	*
	* Schedule switching of @ref to atomic mode. All its percpu counts will
	* be collected to the main atomic counter. On completion, when all CPUs
	* are guaraneed to be in atomic mode, @confirm_switch, which may not
	* block, is invoked. This function may be invoked concurrently with all
	* the get/put operations and can safely be mixed with kill and reinit
	* operations. Note that @ref will stay in atomic mode across kill/reinit
	* cycles until percpu_ref_switch_to_percpu() is called.
	*
	* This function may block if @ref is in the process of switching to atomic
	* mode. If the caller ensures that @ref is not in the process of
	* switching to atomic mode, this function can be called from any context.
	*/
	void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
	percpu_ref_func_t *confirm_switch)
	{
	unsigned long flags;

	spin_lock_irqsave(&percpu_ref_switch_lock, flags);

	ref->data->force_atomic = true;
	__percpu_ref_switch_mode(ref, confirm_switch);

	spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
	}
	EXPORT_SYMBOL_GPL(percpu_ref_switch_to_atomic);

	/**
	* percpu_ref_switch_to_atomic_sync - switch a percpu_ref to atomic mode
	* @ref: percpu_ref to switch to atomic mode
	*
	* Schedule switching the ref to atomic mode, and wait for the
	* switch to complete. Caller must ensure that no other thread
	* will switch back to percpu mode.
	*/
	void percpu_ref_switch_to_atomic_sync(struct percpu_ref *ref)
	{
	percpu_ref_switch_to_atomic(ref, NULL);
	wait_event(percpu_ref_switch_waitq, !ref->data->confirm_switch);
	}
	EXPORT_SYMBOL_GPL(percpu_ref_switch_to_atomic_sync);

	/**
	* percpu_ref_switch_to_percpu - switch a percpu_ref to percpu mode
	* @ref: percpu_ref to switch to percpu mode
	*
	* There's no reason to use this function for the usual reference counting.
	* To re-use an expired ref, use percpu_ref_reinit().
	*
	* Switch @ref to percpu mode. This function may be invoked concurrently
	* with all the get/put operations and can safely be mixed with kill and
	* reinit operations. This function reverses the sticky atomic state set
	* by PERCPU_REF_INIT_ATOMIC or percpu_ref_switch_to_atomic(). If @ref is
	* dying or dead, the actual switching takes place on the following
	* percpu_ref_reinit().
	*
	* This function may block if @ref is in the process of switching to atomic
	* mode. If the caller ensures that @ref is not in the process of
	* switching to atomic mode, this function can be called from any context.
	*/
	void percpu_ref_switch_to_percpu(struct percpu_ref *ref)
	{
	unsigned long flags;

	spin_lock_irqsave(&percpu_ref_switch_lock, flags);

	ref->data->force_atomic = false;
	__percpu_ref_switch_mode(ref, NULL);

	spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
	}
	EXPORT_SYMBOL_GPL(percpu_ref_switch_to_percpu);

	/**
	* percpu_ref_kill_and_confirm - drop the initial ref and schedule confirmation
	* @ref: percpu_ref to kill
	* @confirm_kill: optional confirmation callback
	*
	* Equivalent to percpu_ref_kill() but also schedules kill confirmation if
	* @confirm_kill is not NULL. @confirm_kill, which may not block, will be
	* called after @ref is seen as dead from all CPUs at which point all
	* further invocations of percpu_ref_tryget_live() will fail. See
	* percpu_ref_tryget_live() for details.
	*
	* This function normally doesn't block and can be called from any context
	* but it may block if @confirm_kill is specified and @ref is in the
	* process of switching to atomic mode by percpu_ref_switch_to_atomic().
	*
	* There are no implied RCU grace periods between kill and release.
	*/
	void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
	percpu_ref_func_t *confirm_kill)
	{
	unsigned long flags;

	spin_lock_irqsave(&percpu_ref_switch_lock, flags);

	WARN_ONCE(percpu_ref_is_dying(ref),
	"%s called more than once on %ps!", __func__,
	ref->data->release);

	ref->percpu_count_ptr \|= __PERCPU_REF_DEAD;
	__percpu_ref_switch_mode(ref, confirm_kill);
	percpu_ref_put(ref);

	spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
	}
	EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);

	/**
	* percpu_ref_is_zero - test whether a percpu refcount reached zero
	* @ref: percpu_ref to test
	*
	* Returns %true if @ref reached zero.
	*
	* This function is safe to call as long as @ref is between init and exit.
	*/
	bool percpu_ref_is_zero(struct percpu_ref *ref)
	{
	unsigned long __percpu *percpu_count;
	unsigned long count, flags;

	if (__ref_is_percpu(ref, &percpu_count))
	return false;

	/* protect us from being destroyed */
	spin_lock_irqsave(&percpu_ref_switch_lock, flags);
	if (ref->data)
	count = atomic_long_read(&ref->data->count);
	else
	count = ref->percpu_count_ptr >> __PERCPU_REF_FLAG_BITS;
	spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);

	return count == 0;
	}
	EXPORT_SYMBOL_GPL(percpu_ref_is_zero);

	/**
	* percpu_ref_reinit - re-initialize a percpu refcount
	* @ref: perpcu_ref to re-initialize
	*
	* Re-initialize @ref so that it's in the same state as when it finished
	* percpu_ref_init() ignoring %PERCPU_REF_INIT_DEAD. @ref must have been
	* initialized successfully and reached 0 but not exited.
	*
	* Note that percpu_ref_tryget[_live]() are safe to perform on @ref while
	* this function is in progress.
	*/
	void percpu_ref_reinit(struct percpu_ref *ref)
	{
	WARN_ON_ONCE(!percpu_ref_is_zero(ref));

	percpu_ref_resurrect(ref);
	}
	EXPORT_SYMBOL_GPL(percpu_ref_reinit);

	/**
	* percpu_ref_resurrect - modify a percpu refcount from dead to live
	* @ref: perpcu_ref to resurrect
	*
	* Modify @ref so that it's in the same state as before percpu_ref_kill() was
	* called. @ref must be dead but must not yet have exited.
	*
	* If @ref->release() frees @ref then the caller is responsible for
	* guaranteeing that @ref->release() does not get called while this
	* function is in progress.
	*
	* Note that percpu_ref_tryget[_live]() are safe to perform on @ref while
	* this function is in progress.
	*/
	void percpu_ref_resurrect(struct percpu_ref *ref)
	{
	unsigned long __percpu *percpu_count;
	unsigned long flags;

	spin_lock_irqsave(&percpu_ref_switch_lock, flags);

	WARN_ON_ONCE(!percpu_ref_is_dying(ref));
	WARN_ON_ONCE(__ref_is_percpu(ref, &percpu_count));

	ref->percpu_count_ptr &= ~__PERCPU_REF_DEAD;
	percpu_ref_get(ref);
	__percpu_ref_switch_mode(ref, NULL);

	spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
	}
	EXPORT_SYMBOL_GPL(percpu_ref_resurrect);