linux/lib/percpu-refcount.c

// SPDX-License-Identifier: GPL-2.0-only
#define pr_fmt(fmt)

#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

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)
{}

/**
 * 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)
{}
EXPORT_SYMBOL_GPL();

static void __percpu_ref_exit(struct percpu_ref *ref)
{}

/**
 * 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)
{}
EXPORT_SYMBOL_GPL();

static void percpu_ref_call_confirm_rcu(struct rcu_head *rcu)
{}

static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *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)
{}

static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)
{}

static void __percpu_ref_switch_mode(struct percpu_ref *ref,
				     percpu_ref_func_t *confirm_switch)
{}

/**
 * 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)
{}
EXPORT_SYMBOL_GPL();

/**
 * 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)
{}
EXPORT_SYMBOL_GPL();

/**
 * 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)
{}
EXPORT_SYMBOL_GPL();

/**
 * 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)
{}
EXPORT_SYMBOL_GPL();

/**
 * 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)
{}
EXPORT_SYMBOL_GPL();

/**
 * 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)
{}
EXPORT_SYMBOL_GPL();

/**
 * 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)
{}
EXPORT_SYMBOL_GPL();