7d74207512
percpu_ref->pcpu_count is a percpu pointer with a status flag in its lowest bit. As such, it always goes through arithmetic operations which is very cumbersome to do on a pointer. It has to be first casted to unsigned long and then back. Let's just make the field unsigned long so that we can skip the first casts. While at it, rename it to pcpu_counter_ptr to clarify that it's a pointer value. Signed-off-by: Tejun Heo <tj@kernel.org> Cc: Kent Overstreet <kmo@daterainc.com> Cc: Christoph Lameter <cl@linux-foundation.org>
212 lines
6.2 KiB
C
212 lines
6.2 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 (this is a
|
|
* hack because we need to keep the pointer around for
|
|
* percpu_ref_kill_rcu())
|
|
*/
|
|
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_cancel_init(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);
|
|
|
|
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();
|
|
}
|
|
|
|
#endif
|