274 lines
8.3 KiB
C
274 lines
8.3 KiB
C
/*
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* Percpu refcounts:
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* (C) 2012 Google, Inc.
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* Author: Kent Overstreet <koverstreet@google.com>
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*
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* This implements a refcount with similar semantics to atomic_t - atomic_inc(),
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* atomic_dec_and_test() - but percpu.
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*
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* There's one important difference between percpu refs and normal atomic_t
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* refcounts; you have to keep track of your initial refcount, and then when you
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* start shutting down you call percpu_ref_kill() _before_ dropping the initial
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* refcount.
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*
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* The refcount will have a range of 0 to ((1U << 31) - 1), i.e. one bit less
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* than an atomic_t - this is because of the way shutdown works, see
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* percpu_ref_kill()/PERCPU_COUNT_BIAS.
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*
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* Before you call percpu_ref_kill(), percpu_ref_put() does not check for the
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* refcount hitting 0 - it can't, if it was in percpu mode. percpu_ref_kill()
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* puts the ref back in single atomic_t mode, collecting the per cpu refs and
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* issuing the appropriate barriers, and then marks the ref as shutting down so
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* that percpu_ref_put() will check for the ref hitting 0. After it returns,
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* it's safe to drop the initial ref.
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*
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* USAGE:
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*
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* See fs/aio.c for some example usage; it's used there for struct kioctx, which
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* is created when userspaces calls io_setup(), and destroyed when userspace
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* calls io_destroy() or the process exits.
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*
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* In the aio code, kill_ioctx() is called when we wish to destroy a kioctx; it
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* calls percpu_ref_kill(), then hlist_del_rcu() and synchronize_rcu() to remove
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* the kioctx from the proccess's list of kioctxs - after that, there can't be
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* any new users of the kioctx (from lookup_ioctx()) and it's then safe to drop
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* the initial ref with percpu_ref_put().
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*
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* Code that does a two stage shutdown like this often needs some kind of
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* explicit synchronization to ensure the initial refcount can only be dropped
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* once - percpu_ref_kill() does this for you, it returns true once and false if
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* someone else already called it. The aio code uses it this way, but it's not
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* necessary if the code has some other mechanism to synchronize teardown.
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* around.
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*/
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#ifndef _LINUX_PERCPU_REFCOUNT_H
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#define _LINUX_PERCPU_REFCOUNT_H
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#include <linux/atomic.h>
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#include <linux/kernel.h>
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#include <linux/percpu.h>
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#include <linux/rcupdate.h>
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#include <linux/gfp.h>
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struct percpu_ref;
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typedef void (percpu_ref_func_t)(struct percpu_ref *);
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/* flags set in the lower bits of percpu_ref->percpu_count_ptr */
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enum {
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__PERCPU_REF_ATOMIC = 1LU << 0, /* operating in atomic mode */
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__PERCPU_REF_DEAD = 1LU << 1, /* (being) killed */
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__PERCPU_REF_ATOMIC_DEAD = __PERCPU_REF_ATOMIC | __PERCPU_REF_DEAD,
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__PERCPU_REF_FLAG_BITS = 2,
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};
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/* @flags for percpu_ref_init() */
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enum {
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/*
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* Start w/ ref == 1 in atomic mode. Can be switched to percpu
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* operation using percpu_ref_switch_to_percpu(). If initialized
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* with this flag, the ref will stay in atomic mode until
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* percpu_ref_switch_to_percpu() is invoked on it.
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*/
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PERCPU_REF_INIT_ATOMIC = 1 << 0,
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/*
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* Start dead w/ ref == 0 in atomic mode. Must be revived with
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* percpu_ref_reinit() before used. Implies INIT_ATOMIC.
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*/
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PERCPU_REF_INIT_DEAD = 1 << 1,
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};
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struct percpu_ref {
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atomic_long_t count;
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/*
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* The low bit of the pointer indicates whether the ref is in percpu
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* mode; if set, then get/put will manipulate the atomic_t.
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*/
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unsigned long percpu_count_ptr;
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percpu_ref_func_t *release;
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percpu_ref_func_t *confirm_switch;
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bool force_atomic:1;
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struct rcu_head rcu;
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};
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int __must_check percpu_ref_init(struct percpu_ref *ref,
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percpu_ref_func_t *release, unsigned int flags,
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gfp_t gfp);
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void percpu_ref_exit(struct percpu_ref *ref);
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void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
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percpu_ref_func_t *confirm_switch);
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void percpu_ref_switch_to_percpu(struct percpu_ref *ref);
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void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
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percpu_ref_func_t *confirm_kill);
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void percpu_ref_reinit(struct percpu_ref *ref);
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/**
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* percpu_ref_kill - drop the initial ref
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* @ref: percpu_ref to kill
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*
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* Must be used to drop the initial ref on a percpu refcount; must be called
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* precisely once before shutdown.
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*
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* Puts @ref in non percpu mode, then does a call_rcu() before gathering up the
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* percpu counters and dropping the initial ref.
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*/
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static inline void percpu_ref_kill(struct percpu_ref *ref)
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{
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return percpu_ref_kill_and_confirm(ref, NULL);
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}
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/*
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* Internal helper. Don't use outside percpu-refcount proper. The
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* function doesn't return the pointer and let the caller test it for NULL
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* because doing so forces the compiler to generate two conditional
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* branches as it can't assume that @ref->percpu_count is not NULL.
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*/
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static inline bool __ref_is_percpu(struct percpu_ref *ref,
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unsigned long __percpu **percpu_countp)
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{
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unsigned long percpu_ptr = ACCESS_ONCE(ref->percpu_count_ptr);
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/* paired with smp_store_release() in percpu_ref_reinit() */
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smp_read_barrier_depends();
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/*
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* Theoretically, the following could test just ATOMIC; however,
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* then we'd have to mask off DEAD separately as DEAD may be
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* visible without ATOMIC if we race with percpu_ref_kill(). DEAD
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* implies ATOMIC anyway. Test them together.
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*/
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if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC_DEAD))
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return false;
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*percpu_countp = (unsigned long __percpu *)percpu_ptr;
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return true;
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}
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/**
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* percpu_ref_get - increment a percpu refcount
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* @ref: percpu_ref to get
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*
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* Analagous to atomic_long_inc().
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*
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* This function is safe to call as long as @ref is between init and exit.
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*/
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static inline void percpu_ref_get(struct percpu_ref *ref)
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{
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unsigned long __percpu *percpu_count;
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rcu_read_lock_sched();
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if (__ref_is_percpu(ref, &percpu_count))
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this_cpu_inc(*percpu_count);
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else
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atomic_long_inc(&ref->count);
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rcu_read_unlock_sched();
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}
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/**
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* percpu_ref_tryget - try to increment a percpu refcount
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* @ref: percpu_ref to try-get
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*
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* Increment a percpu refcount unless its count already reached zero.
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* Returns %true on success; %false on failure.
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*
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* This function is safe to call as long as @ref is between init and exit.
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*/
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static inline bool percpu_ref_tryget(struct percpu_ref *ref)
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{
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unsigned long __percpu *percpu_count;
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int ret;
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rcu_read_lock_sched();
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if (__ref_is_percpu(ref, &percpu_count)) {
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this_cpu_inc(*percpu_count);
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ret = true;
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} else {
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ret = atomic_long_inc_not_zero(&ref->count);
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}
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rcu_read_unlock_sched();
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return ret;
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}
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/**
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* percpu_ref_tryget_live - try to increment a live percpu refcount
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* @ref: percpu_ref to try-get
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*
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* Increment a percpu refcount unless it has already been killed. Returns
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* %true on success; %false on failure.
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*
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* Completion of percpu_ref_kill() in itself doesn't guarantee that this
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* function will fail. For such guarantee, percpu_ref_kill_and_confirm()
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* should be used. After the confirm_kill callback is invoked, it's
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* guaranteed that no new reference will be given out by
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* percpu_ref_tryget_live().
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*
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* This function is safe to call as long as @ref is between init and exit.
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*/
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static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
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{
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unsigned long __percpu *percpu_count;
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int ret = false;
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rcu_read_lock_sched();
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if (__ref_is_percpu(ref, &percpu_count)) {
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this_cpu_inc(*percpu_count);
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ret = true;
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} else if (!(ACCESS_ONCE(ref->percpu_count_ptr) & __PERCPU_REF_DEAD)) {
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ret = atomic_long_inc_not_zero(&ref->count);
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}
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rcu_read_unlock_sched();
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return ret;
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}
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/**
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* percpu_ref_put - decrement a percpu refcount
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* @ref: percpu_ref to put
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*
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* Decrement the refcount, and if 0, call the release function (which was passed
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* to percpu_ref_init())
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*
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* This function is safe to call as long as @ref is between init and exit.
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*/
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static inline void percpu_ref_put(struct percpu_ref *ref)
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{
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unsigned long __percpu *percpu_count;
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rcu_read_lock_sched();
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if (__ref_is_percpu(ref, &percpu_count))
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this_cpu_dec(*percpu_count);
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else if (unlikely(atomic_long_dec_and_test(&ref->count)))
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ref->release(ref);
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rcu_read_unlock_sched();
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}
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/**
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* percpu_ref_is_zero - test whether a percpu refcount reached zero
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* @ref: percpu_ref to test
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*
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* Returns %true if @ref reached zero.
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*
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* This function is safe to call as long as @ref is between init and exit.
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*/
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static inline bool percpu_ref_is_zero(struct percpu_ref *ref)
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{
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unsigned long __percpu *percpu_count;
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if (__ref_is_percpu(ref, &percpu_count))
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return false;
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return !atomic_long_read(&ref->count);
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}
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#endif
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