229 lines
6.6 KiB
C
229 lines
6.6 KiB
C
/*
|
|
* 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_func_t)(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.
|
|
*/
|
|
unsigned long pcpu_count_ptr;
|
|
percpu_ref_func_t *release;
|
|
percpu_ref_func_t *confirm_kill;
|
|
struct rcu_head rcu;
|
|
};
|
|
|
|
int __must_check percpu_ref_init(struct percpu_ref *ref,
|
|
percpu_ref_func_t *release);
|
|
void percpu_ref_reinit(struct percpu_ref *ref);
|
|
void percpu_ref_exit(struct percpu_ref *ref);
|
|
void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
|
|
percpu_ref_func_t *confirm_kill);
|
|
|
|
/**
|
|
* percpu_ref_kill - drop the initial ref
|
|
* @ref: percpu_ref to kill
|
|
*
|
|
* Must be used to drop the initial ref on a percpu refcount; must be called
|
|
* precisely once before shutdown.
|
|
*
|
|
* Puts @ref in non percpu mode, then does a call_rcu() before gathering up the
|
|
* percpu counters and dropping the initial ref.
|
|
*/
|
|
static inline void percpu_ref_kill(struct percpu_ref *ref)
|
|
{
|
|
return percpu_ref_kill_and_confirm(ref, NULL);
|
|
}
|
|
|
|
#define PCPU_REF_DEAD 1
|
|
|
|
/*
|
|
* Internal helper. Don't use outside percpu-refcount proper. The
|
|
* function doesn't return the pointer and let the caller test it for NULL
|
|
* because doing so forces the compiler to generate two conditional
|
|
* branches as it can't assume that @ref->pcpu_count is not NULL.
|
|
*/
|
|
static inline bool __pcpu_ref_alive(struct percpu_ref *ref,
|
|
unsigned __percpu **pcpu_countp)
|
|
{
|
|
unsigned long pcpu_ptr = ACCESS_ONCE(ref->pcpu_count_ptr);
|
|
|
|
/* paired with smp_store_release() in percpu_ref_reinit() */
|
|
smp_read_barrier_depends();
|
|
|
|
if (unlikely(pcpu_ptr & PCPU_REF_DEAD))
|
|
return false;
|
|
|
|
*pcpu_countp = (unsigned __percpu *)pcpu_ptr;
|
|
return true;
|
|
}
|
|
|
|
/**
|
|
* percpu_ref_get - increment a percpu refcount
|
|
* @ref: percpu_ref to get
|
|
*
|
|
* Analagous to atomic_inc().
|
|
*/
|
|
static inline void percpu_ref_get(struct percpu_ref *ref)
|
|
{
|
|
unsigned __percpu *pcpu_count;
|
|
|
|
rcu_read_lock_sched();
|
|
|
|
if (__pcpu_ref_alive(ref, &pcpu_count))
|
|
this_cpu_inc(*pcpu_count);
|
|
else
|
|
atomic_inc(&ref->count);
|
|
|
|
rcu_read_unlock_sched();
|
|
}
|
|
|
|
/**
|
|
* percpu_ref_tryget - try to increment a percpu refcount
|
|
* @ref: percpu_ref to try-get
|
|
*
|
|
* Increment a percpu refcount unless its count already reached zero.
|
|
* Returns %true on success; %false on failure.
|
|
*
|
|
* The caller is responsible for ensuring that @ref stays accessible.
|
|
*/
|
|
static inline bool percpu_ref_tryget(struct percpu_ref *ref)
|
|
{
|
|
unsigned __percpu *pcpu_count;
|
|
int ret = false;
|
|
|
|
rcu_read_lock_sched();
|
|
|
|
if (__pcpu_ref_alive(ref, &pcpu_count)) {
|
|
this_cpu_inc(*pcpu_count);
|
|
ret = true;
|
|
} else {
|
|
ret = atomic_inc_not_zero(&ref->count);
|
|
}
|
|
|
|
rcu_read_unlock_sched();
|
|
|
|
return ret;
|
|
}
|
|
|
|
/**
|
|
* percpu_ref_tryget_live - try to increment a live percpu refcount
|
|
* @ref: percpu_ref to try-get
|
|
*
|
|
* Increment a percpu refcount unless it has already been killed. Returns
|
|
* %true on success; %false on failure.
|
|
*
|
|
* Completion of percpu_ref_kill() in itself doesn't guarantee that tryget
|
|
* will fail. For such guarantee, percpu_ref_kill_and_confirm() should be
|
|
* used. After the confirm_kill callback is invoked, it's guaranteed that
|
|
* no new reference will be given out by percpu_ref_tryget().
|
|
*
|
|
* The caller is responsible for ensuring that @ref stays accessible.
|
|
*/
|
|
static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
|
|
{
|
|
unsigned __percpu *pcpu_count;
|
|
int ret = false;
|
|
|
|
rcu_read_lock_sched();
|
|
|
|
if (__pcpu_ref_alive(ref, &pcpu_count)) {
|
|
this_cpu_inc(*pcpu_count);
|
|
ret = true;
|
|
}
|
|
|
|
rcu_read_unlock_sched();
|
|
|
|
return ret;
|
|
}
|
|
|
|
/**
|
|
* percpu_ref_put - decrement a percpu refcount
|
|
* @ref: percpu_ref to put
|
|
*
|
|
* 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_sched();
|
|
|
|
if (__pcpu_ref_alive(ref, &pcpu_count))
|
|
this_cpu_dec(*pcpu_count);
|
|
else if (unlikely(atomic_dec_and_test(&ref->count)))
|
|
ref->release(ref);
|
|
|
|
rcu_read_unlock_sched();
|
|
}
|
|
|
|
/**
|
|
* percpu_ref_is_zero - test whether a percpu refcount reached zero
|
|
* @ref: percpu_ref to test
|
|
*
|
|
* Returns %true if @ref reached zero.
|
|
*/
|
|
static inline bool percpu_ref_is_zero(struct percpu_ref *ref)
|
|
{
|
|
unsigned __percpu *pcpu_count;
|
|
|
|
if (__pcpu_ref_alive(ref, &pcpu_count))
|
|
return false;
|
|
return !atomic_read(&ref->count);
|
|
}
|
|
|
|
#endif
|