![Tejun Heo](/assets/img/avatar_default.png)
percpu_ref internally uses sched-RCU to implement the percpu -> atomic mode switching and the documentation suggested that this could be depended upon. This doesn't seem like a good idea. * percpu_ref uses sched-RCU which has different grace periods regular RCU. Users may combine percpu_ref with regular RCU usage and incorrectly believe that regular RCU grace periods are performed by percpu_ref. This can lead to, for example, use-after-free due to premature freeing. * percpu_ref has a grace period when switching from percpu to atomic mode. It doesn't have one between the last put and release. This distinction is subtle and can lead to surprising bugs. * percpu_ref allows starting in and switching to atomic mode manually for debugging and other purposes. This means that there may not be any grace periods from kill to release. This patch makes it clear that the grace periods are percpu_ref's internal implementation detail and can't be depended upon by the users. Signed-off-by: Tejun Heo <tj@kernel.org> Cc: Kent Overstreet <kent.overstreet@gmail.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Tejun Heo <tj@kernel.org>
335 lines
10 KiB
C
335 lines
10 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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/*
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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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* removes the kioctx from the proccess's table of kioctxs and kills percpu_ref.
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* After that, there can't be any new users of the kioctx (from lookup_ioctx())
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* and it's then safe to drop the initial ref with percpu_ref_put().
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*
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* Note that the free path, free_ioctx(), needs to go through explicit call_rcu()
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* to synchronize with RCU protected lookup_ioctx(). percpu_ref operations don't
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* imply RCU grace periods of any kind and if a user wants to combine percpu_ref
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* with RCU protection, it must be done explicitly.
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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_atomic_sync(struct percpu_ref *ref);
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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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* Switches @ref into atomic mode before gathering up the percpu counters
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* and dropping the initial ref.
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*
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* There are no implied RCU grace periods between kill and release.
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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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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;
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/*
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* The value of @ref->percpu_count_ptr is tested for
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* !__PERCPU_REF_ATOMIC, which may be set asynchronously, and then
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* used as a pointer. If the compiler generates a separate fetch
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* when using it as a pointer, __PERCPU_REF_ATOMIC may be set in
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* between contaminating the pointer value, meaning that
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* READ_ONCE() is required when fetching it.
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*
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* The smp_read_barrier_depends() implied by READ_ONCE() pairs
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* with smp_store_release() in __percpu_ref_switch_to_percpu().
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*/
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percpu_ptr = READ_ONCE(ref->percpu_count_ptr);
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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_many - increment a percpu refcount
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* @ref: percpu_ref to get
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* @nr: number of references to get
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*
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* Analogous to atomic_long_add().
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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_many(struct percpu_ref *ref, unsigned long nr)
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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_add(*percpu_count, nr);
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else
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atomic_long_add(nr, &ref->count);
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rcu_read_unlock_sched();
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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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percpu_ref_get_many(ref, 1);
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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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bool 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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bool 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 (!(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_many - decrement a percpu refcount
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* @ref: percpu_ref to put
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* @nr: number of references 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_many(struct percpu_ref *ref, unsigned long nr)
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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_sub(*percpu_count, nr);
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else if (unlikely(atomic_long_sub_and_test(nr, &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_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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percpu_ref_put_many(ref, 1);
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}
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/**
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* percpu_ref_is_dying - test whether a percpu refcount is dying or dead
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* @ref: percpu_ref to test
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*
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* Returns %true if @ref is dying or dead.
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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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* and the caller is responsible for synchronizing against state changes.
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*/
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static inline bool percpu_ref_is_dying(struct percpu_ref *ref)
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{
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return ref->percpu_count_ptr & __PERCPU_REF_DEAD;
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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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