linux-stable/include/linux/percpu-refcount.h
Roman Gushchin 09ed79d6d7 percpu_ref: introduce PERCPU_REF_ALLOW_REINIT flag
In most cases percpu reference counters are not switched to the
percpu mode after they reach the atomic mode. Some obvious exceptions
are reference counters which are initialized into the atomic
mode (using PERCPU_REF_INIT_ATOMIC and PERCPU_REF_INIT_DEAD flags),
and there are few other exceptions.

But in most cases there is no way back, and once the reference counter
is switched to the atomic mode, there is no reason to wait for
percpu_ref_exit() to release the percpu memory. Of course, the size
of a single counter is not so big, but because it can pin the whole
percpu block in memory, the memory footprint can be noticeable
(e.g. on my 32 CPUs machine a percpu block is 8Mb large).

To make releasing of the percpu memory as early as possible, let's
introduce the PERCPU_REF_ALLOW_REINIT flag with the following semantics:
it has to be set in order to switch a percpu reference counter to the
percpu mode after the initialization. PERCPU_REF_INIT_ATOMIC and
PERCPU_REF_INIT_DEAD flags will implicitly assume PERCPU_REF_ALLOW_REINIT.

This patch doesn't introduce any functional change to avoid any
regressions. It will be done later in the patchset after adjusting
all call sites, which are reviving percpu counters.

Signed-off-by: Roman Gushchin <guro@fb.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Dennis Zhou <dennis@kernel.org>
2019-05-09 10:49:47 -07:00

342 lines
10 KiB
C

/* SPDX-License-Identifier: GPL-2.0 */
/*
* 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()/PERCPU_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
* removes the kioctx from the proccess's table of kioctxs and kills percpu_ref.
* 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().
*
* Note that the free path, free_ioctx(), needs to go through explicit call_rcu()
* to synchronize with RCU protected lookup_ioctx(). percpu_ref operations don't
* imply RCU grace periods of any kind and if a user wants to combine percpu_ref
* with RCU protection, it must be done explicitly.
*
* 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 *);
/* flags set in the lower bits of percpu_ref->percpu_count_ptr */
enum {
__PERCPU_REF_ATOMIC = 1LU << 0, /* operating in atomic mode */
__PERCPU_REF_DEAD = 1LU << 1, /* (being) killed */
__PERCPU_REF_ATOMIC_DEAD = __PERCPU_REF_ATOMIC | __PERCPU_REF_DEAD,
__PERCPU_REF_FLAG_BITS = 2,
};
/* @flags for percpu_ref_init() */
enum {
/*
* Start w/ ref == 1 in atomic mode. Can be switched to percpu
* operation using percpu_ref_switch_to_percpu(). If initialized
* with this flag, the ref will stay in atomic mode until
* percpu_ref_switch_to_percpu() is invoked on it.
* Implies ALLOW_REINIT.
*/
PERCPU_REF_INIT_ATOMIC = 1 << 0,
/*
* Start dead w/ ref == 0 in atomic mode. Must be revived with
* percpu_ref_reinit() before used. Implies INIT_ATOMIC and
* ALLOW_REINIT.
*/
PERCPU_REF_INIT_DEAD = 1 << 1,
/*
* Allow switching from atomic mode to percpu mode.
*/
PERCPU_REF_ALLOW_REINIT = 1 << 2,
};
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 percpu_count_ptr;
percpu_ref_func_t *release;
percpu_ref_func_t *confirm_switch;
bool force_atomic:1;
struct rcu_head rcu;
};
int __must_check percpu_ref_init(struct percpu_ref *ref,
percpu_ref_func_t *release, unsigned int flags,
gfp_t gfp);
void percpu_ref_exit(struct percpu_ref *ref);
void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
percpu_ref_func_t *confirm_switch);
void percpu_ref_switch_to_atomic_sync(struct percpu_ref *ref);
void percpu_ref_switch_to_percpu(struct percpu_ref *ref);
void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
percpu_ref_func_t *confirm_kill);
void percpu_ref_resurrect(struct percpu_ref *ref);
void percpu_ref_reinit(struct percpu_ref *ref);
/**
* 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.
*
* Switches @ref into atomic mode before gathering up the percpu counters
* and dropping the initial ref.
*
* There are no implied RCU grace periods between kill and release.
*/
static inline void percpu_ref_kill(struct percpu_ref *ref)
{
percpu_ref_kill_and_confirm(ref, NULL);
}
/*
* 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->percpu_count is not NULL.
*/
static inline bool __ref_is_percpu(struct percpu_ref *ref,
unsigned long __percpu **percpu_countp)
{
unsigned long percpu_ptr;
/*
* The value of @ref->percpu_count_ptr is tested for
* !__PERCPU_REF_ATOMIC, which may be set asynchronously, and then
* used as a pointer. If the compiler generates a separate fetch
* when using it as a pointer, __PERCPU_REF_ATOMIC may be set in
* between contaminating the pointer value, meaning that
* READ_ONCE() is required when fetching it.
*
* The smp_read_barrier_depends() implied by READ_ONCE() pairs
* with smp_store_release() in __percpu_ref_switch_to_percpu().
*/
percpu_ptr = READ_ONCE(ref->percpu_count_ptr);
/*
* Theoretically, the following could test just ATOMIC; however,
* then we'd have to mask off DEAD separately as DEAD may be
* visible without ATOMIC if we race with percpu_ref_kill(). DEAD
* implies ATOMIC anyway. Test them together.
*/
if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC_DEAD))
return false;
*percpu_countp = (unsigned long __percpu *)percpu_ptr;
return true;
}
/**
* percpu_ref_get_many - increment a percpu refcount
* @ref: percpu_ref to get
* @nr: number of references to get
*
* Analogous to atomic_long_add().
*
* This function is safe to call as long as @ref is between init and exit.
*/
static inline void percpu_ref_get_many(struct percpu_ref *ref, unsigned long nr)
{
unsigned long __percpu *percpu_count;
rcu_read_lock_sched();
if (__ref_is_percpu(ref, &percpu_count))
this_cpu_add(*percpu_count, nr);
else
atomic_long_add(nr, &ref->count);
rcu_read_unlock_sched();
}
/**
* percpu_ref_get - increment a percpu refcount
* @ref: percpu_ref to get
*
* Analagous to atomic_long_inc().
*
* This function is safe to call as long as @ref is between init and exit.
*/
static inline void percpu_ref_get(struct percpu_ref *ref)
{
percpu_ref_get_many(ref, 1);
}
/**
* 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.
*
* This function is safe to call as long as @ref is between init and exit.
*/
static inline bool percpu_ref_tryget(struct percpu_ref *ref)
{
unsigned long __percpu *percpu_count;
bool ret;
rcu_read_lock_sched();
if (__ref_is_percpu(ref, &percpu_count)) {
this_cpu_inc(*percpu_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 this
* function 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_live().
*
* This function is safe to call as long as @ref is between init and exit.
*/
static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
{
unsigned long __percpu *percpu_count;
bool ret = false;
rcu_read_lock_sched();
if (__ref_is_percpu(ref, &percpu_count)) {
this_cpu_inc(*percpu_count);
ret = true;
} else if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) {
ret = atomic_long_inc_not_zero(&ref->count);
}
rcu_read_unlock_sched();
return ret;
}
/**
* percpu_ref_put_many - decrement a percpu refcount
* @ref: percpu_ref to put
* @nr: number of references to put
*
* Decrement the refcount, and if 0, call the release function (which was passed
* to percpu_ref_init())
*
* This function is safe to call as long as @ref is between init and exit.
*/
static inline void percpu_ref_put_many(struct percpu_ref *ref, unsigned long nr)
{
unsigned long __percpu *percpu_count;
rcu_read_lock_sched();
if (__ref_is_percpu(ref, &percpu_count))
this_cpu_sub(*percpu_count, nr);
else if (unlikely(atomic_long_sub_and_test(nr, &ref->count)))
ref->release(ref);
rcu_read_unlock_sched();
}
/**
* 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())
*
* This function is safe to call as long as @ref is between init and exit.
*/
static inline void percpu_ref_put(struct percpu_ref *ref)
{
percpu_ref_put_many(ref, 1);
}
/**
* percpu_ref_is_dying - test whether a percpu refcount is dying or dead
* @ref: percpu_ref to test
*
* Returns %true if @ref is dying or dead.
*
* This function is safe to call as long as @ref is between init and exit
* and the caller is responsible for synchronizing against state changes.
*/
static inline bool percpu_ref_is_dying(struct percpu_ref *ref)
{
return ref->percpu_count_ptr & __PERCPU_REF_DEAD;
}
/**
* 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.
*/
static inline bool percpu_ref_is_zero(struct percpu_ref *ref)
{
unsigned long __percpu *percpu_count;
if (__ref_is_percpu(ref, &percpu_count))
return false;
return !atomic_long_read(&ref->count);
}
#endif