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linux-next/include/linux/percpu-refcount.h
Tejun Heo e625305b39 percpu-refcount: make percpu_ref based on longs instead of ints
percpu_ref is currently based on ints and the number of refs it can
cover is (1 << 31).  This makes it impossible to use a percpu_ref to
count memory objects or pages on 64bit machines as it may overflow.
This forces those users to somehow aggregate the references before
contributing to the percpu_ref which is often cumbersome and sometimes
challenging to get the same level of performance as using the
percpu_ref directly.

While using ints for the percpu counters makes them pack tighter on
64bit machines, the possible gain from using ints instead of longs is
extremely small compared to the overall gain from per-cpu operation.
This patch makes percpu_ref based on longs so that it can be used to
directly count memory objects or pages.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Kent Overstreet <kmo@daterainc.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
2014-09-20 01:27:25 -04:00

230 lines
6.7 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>
#include <linux/gfp.h>
struct percpu_ref;
typedef void (percpu_ref_func_t)(struct percpu_ref *);
struct percpu_ref {
atomic_long_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, gfp_t gfp);
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 long __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 long __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 long __percpu *pcpu_count;
rcu_read_lock_sched();
if (__pcpu_ref_alive(ref, &pcpu_count))
this_cpu_inc(*pcpu_count);
else
atomic_long_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 long __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_long_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 long __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 long __percpu *pcpu_count;
rcu_read_lock_sched();
if (__pcpu_ref_alive(ref, &pcpu_count))
this_cpu_dec(*pcpu_count);
else if (unlikely(atomic_long_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 long __percpu *pcpu_count;
if (__pcpu_ref_alive(ref, &pcpu_count))
return false;
return !atomic_long_read(&ref->count);
}
#endif