cregit-Linux how code gets into the kernel

Release 4.12 include/linux/percpu-refcount.h

Directory: include/linux
/*
 * 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
 * calls percpu_ref_kill(), then hlist_del_rcu() and synchronize_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>
#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.
         */
	
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.
         */
	
PERCPU_REF_INIT_DEAD	= 1 << 1,
};


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_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.
 *
 * 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) { percpu_ref_kill_and_confirm(ref, NULL); }

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/* * 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. */ percpu_ptr = READ_ONCE(ref->percpu_count_ptr); /* paired with smp_store_release() in __percpu_ref_switch_to_percpu() */ smp_read_barrier_depends(); /* * 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; }

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/** * 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(); }

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/** * 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); }

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/** * 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; }

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/** * 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; }

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/** * 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(); }

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/** * 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); }

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/** * 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; }

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/** * 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); }

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#endif

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Directory: include/linux
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