acac7883ee
Two small changes. * Unlike most init functions, percpu_ref_init() allocates memory and may fail. Let's mark it with __must_check in case the caller forgets. * percpu_ref_kill_rcu() is unnecessarily using ACCESS_ONCE() to dereference @ref->pcpu_count, which can be misleading. The pointer is guaranteed to be valid and visible and can't change underneath the function. Drop ACCESS_ONCE(). Signed-off-by: Tejun Heo <tj@kernel.org>
125 lines
3.9 KiB
C
125 lines
3.9 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()/PCPU_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 sychronize_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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struct percpu_ref;
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typedef void (percpu_ref_func_t)(struct percpu_ref *);
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struct percpu_ref {
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atomic_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 (this is a
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* hack because we need to keep the pointer around for
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* percpu_ref_kill_rcu())
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*/
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unsigned __percpu *pcpu_count;
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percpu_ref_func_t *release;
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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);
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void percpu_ref_kill(struct percpu_ref *ref);
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#define PCPU_STATUS_BITS 2
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#define PCPU_STATUS_MASK ((1 << PCPU_STATUS_BITS) - 1)
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#define PCPU_REF_PTR 0
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#define PCPU_REF_DEAD 1
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#define REF_STATUS(count) (((unsigned long) count) & PCPU_STATUS_MASK)
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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_inc().
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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 __percpu *pcpu_count;
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rcu_read_lock();
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pcpu_count = ACCESS_ONCE(ref->pcpu_count);
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if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR))
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__this_cpu_inc(*pcpu_count);
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else
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atomic_inc(&ref->count);
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rcu_read_unlock();
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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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static inline void percpu_ref_put(struct percpu_ref *ref)
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{
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unsigned __percpu *pcpu_count;
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rcu_read_lock();
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pcpu_count = ACCESS_ONCE(ref->pcpu_count);
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if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR))
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__this_cpu_dec(*pcpu_count);
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else if (unlikely(atomic_dec_and_test(&ref->count)))
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ref->release(ref);
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rcu_read_unlock();
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}
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#endif
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