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/*
* Percpu refcounts:
* (C) 2012 Google, Inc.
* Author: Kent Overstreet <koverstreet@google.com>
*
* This implements a refcount with similar semantics to atomic_t - atomic_inc(),
* atomic_dec_and_test() - but percpu.
*
* There's one important difference between percpu refs and normal atomic_t
* refcounts; you have to keep track of your initial refcount, and then when you
* start shutting down you call percpu_ref_kill() _before_ dropping the initial
* refcount.
*
* The refcount will have a range of 0 to ((1U << 31) - 1), i.e. one bit less
* than an atomic_t - this is because of the way shutdown works, see
* percpu_ref_kill()/PCPU_COUNT_BIAS.
*
* Before you call percpu_ref_kill(), percpu_ref_put() does not check for the
* refcount hitting 0 - it can't, if it was in percpu mode. percpu_ref_kill()
* puts the ref back in single atomic_t mode, collecting the per cpu refs and
* issuing the appropriate barriers, and then marks the ref as shutting down so
* that percpu_ref_put() will check for the ref hitting 0. After it returns,
* it's safe to drop the initial ref.
*
* USAGE:
*
* See fs/aio.c for some example usage; it's used there for struct kioctx, which
* is created when userspaces calls io_setup(), and destroyed when userspace
* calls io_destroy() or the process exits.
*
* In the aio code, kill_ioctx() is called when we wish to destroy a kioctx; it
* calls percpu_ref_kill(), then hlist_del_rcu() and sychronize_rcu() to remove
* the kioctx from the proccess's list of kioctxs - after that, there can't be
* any new users of the kioctx (from lookup_ioctx()) and it's then safe to drop
* the initial ref with percpu_ref_put().
*
* Code that does a two stage shutdown like this often needs some kind of
* explicit synchronization to ensure the initial refcount can only be dropped
* once - percpu_ref_kill() does this for you, it returns true once and false if
* someone else already called it. The aio code uses it this way, but it's not
* necessary if the code has some other mechanism to synchronize teardown.
* around.
*/
#ifndef _LINUX_PERCPU_REFCOUNT_H
#define _LINUX_PERCPU_REFCOUNT_H
#include <linux/atomic.h>
#include <linux/kernel.h>
#include <linux/percpu.h>
#include <linux/rcupdate.h>
struct percpu_ref;
typedef void (percpu_ref_release)(struct percpu_ref *);
struct percpu_ref {
atomic_t count;
/*
* The low bit of the pointer indicates whether the ref is in percpu
* mode; if set, then get/put will manipulate the atomic_t (this is a
* hack because we need to keep the pointer around for
* percpu_ref_kill_rcu())
*/
unsigned __percpu *pcpu_count;
percpu_ref_release *release;
struct rcu_head rcu;
};
int percpu_ref_init(struct percpu_ref *, percpu_ref_release *);
void percpu_ref_kill(struct percpu_ref *ref);
#define PCPU_STATUS_BITS 2
#define PCPU_STATUS_MASK ((1 << PCPU_STATUS_BITS) - 1)
#define PCPU_REF_PTR 0
#define PCPU_REF_DEAD 1
#define REF_STATUS(count) (((unsigned long) count) & PCPU_STATUS_MASK)
/**
* percpu_ref_get - increment a percpu refcount
*
* Analagous to atomic_inc().
*/
static inline void percpu_ref_get(struct percpu_ref *ref)
{
unsigned __percpu *pcpu_count;
rcu_read_lock();
pcpu_count = ACCESS_ONCE(ref->pcpu_count);
if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR))
__this_cpu_inc(*pcpu_count);
else
atomic_inc(&ref->count);
rcu_read_unlock();
}
/**
* percpu_ref_put - decrement a percpu refcount
*
* Decrement the refcount, and if 0, call the release function (which was passed
* to percpu_ref_init())
*/
static inline void percpu_ref_put(struct percpu_ref *ref)
{
unsigned __percpu *pcpu_count;
rcu_read_lock();
pcpu_count = ACCESS_ONCE(ref->pcpu_count);
if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR))
__this_cpu_dec(*pcpu_count);
else if (unlikely(atomic_dec_and_test(&ref->count)))
ref->release(ref);
rcu_read_unlock();
}
#endif
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