e89004612a
The former is intended for use in vmspace_exit(). The latter is to encourage use of explicit loads rather than relying on the volatile qualifier. This works better with kernel sanitizers, which can intercept atomic(9) calls, and makes tricky lockless code easier to read by not forcing the reader to remember which variables are declared volatile. Reviewed by: kib, mjg, mmel MFC after: 2 weeks Sponsored by: The FreeBSD Foundation Differential Revision: https://reviews.freebsd.org/D27056
196 lines
5.9 KiB
Groff
196 lines
5.9 KiB
Groff
.\"
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.\" Copyright (c) 2009 Hudson River Trading LLC
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.\" Written by: John H. Baldwin <jhb@FreeBSD.org>
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.\" All rights reserved.
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.\"
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.\" Copyright (c) 2019 The FreeBSD Foundation, Inc.
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.\"
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.\" Parts of this documentation was written by
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.\" Konstantin Belousov <kib@FreeBSD.org> under sponsorship
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.\" from the FreeBSD Foundation.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd November 2, 2020
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.Dt REFCOUNT 9
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.Os
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.Sh NAME
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.Nm refcount ,
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.Nm refcount_init ,
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.Nm refcount_acquire ,
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.Nm refcount_release
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.Nd manage a simple reference counter
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.Sh SYNOPSIS
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.In sys/param.h
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.In sys/refcount.h
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.Ft void
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.Fn refcount_init "volatile u_int *count" "u_int value"
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.Ft u_int
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.Fn refcount_load "volatile u_int *count"
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.Ft void
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.Fn refcount_acquire "volatile u_int *count"
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.Ft bool
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.Fn refcount_acquire_checked "volatile u_int *count"
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.Ft bool
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.Fn refcount_acquire_if_not_zero "volatile u_int *count"
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.Ft bool
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.Fn refcount_release "volatile u_int *count"
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.Ft bool
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.Fn refcount_release_if_last "volatile u_int *count"
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.Ft bool
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.Fn refcount_release_if_not_last "volatile u_int *count"
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.Sh DESCRIPTION
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The
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.Nm
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functions provide an API to manage a simple reference counter.
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The caller provides the storage for the counter in an unsigned integer.
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A pointer to this integer is passed via
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.Fa count .
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Usually the counter is used to manage the lifetime of an object and is
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stored as a member of the object.
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.Pp
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Currently all functions are implemented as static inline.
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.Pp
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The
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.Fn refcount_init
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function is used to set the initial value of the counter to
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.Fa value .
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It is normally used when creating a reference-counted object.
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.Pp
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The
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.Fn refcount_load
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function returns a snapshot of the counter value.
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This value may immediately become out-of-date in the absence of external
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synchronization.
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.Fn refcount_load
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should be used instead of relying on the properties of the
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.Vt volatile
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qualifier.
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.Pp
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The
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.Fn refcount_acquire
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function is used to acquire a new reference.
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The caller is responsible for ensuring that it holds a valid reference
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while obtaining a new reference.
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For example,
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if an object is stored on a list and the list holds a reference on the
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object, then holding a lock that protects the list provides sufficient
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protection for acquiring a new reference.
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.Pp
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The
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.Fn refcount_acquire_checked
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variant performs the same operation as
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.Fn refcount_acquire ,
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but additionally checks that the
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.Fa count
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value does not overflow as result of the operation.
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It returns
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.Dv true
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if the reference was sucessfully obtained, and
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.Dv false
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if it was not, due to the overflow.
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.Pp
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The
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.Fn refcount_acquire_if_not_zero
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function is yet another variant of
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.Fn refcount_acquire ,
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which only obtains the reference when some reference already exists.
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In other words,
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.Fa *count
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must be already greater than zero for the function to succeed, in which
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case the return value is
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.Dv true ,
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otherwise
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.Dv false
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is returned.
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.Pp
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The
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.Fn refcount_release
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function is used to release an existing reference.
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The function returns true if the reference being released was
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the last reference;
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otherwise, it returns false.
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.Pp
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The
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.Fn refcount_release_if_last
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and
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.Fn refcount_release_if_not_last
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functions are variants of
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.Fn refcount_release
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which only drop the reference when it is or is not the last reference,
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respectively.
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In other words,
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.Fn refcount_release_if_last
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returns
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.Dv true
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when
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.Fa *count
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is equal to one, in which case it is decremented to zero.
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Otherwise,
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.Fa *count
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is not modified and the function returns
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.Dv false .
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Similarly,
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.Fn refcount_release_if_not_last
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returns
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.Dv true
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when
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.Fa *count
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is greater than one, in which case
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.Fa *count
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is decremented.
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Otherwise, if
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.Fa *count
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is equal to one, the reference is not released and the function returns
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.Dv false .
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.Pp
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Note that these routines do not provide any inter-CPU synchronization or
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data protection for managing the counter.
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The caller is responsible for any additional synchronization needed by
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consumers of any containing objects.
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In addition,
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the caller is also responsible for managing the life cycle of any containing
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objects including explicitly releasing any resources when the last reference
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is released.
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.Pp
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The
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.Fn refcount_release
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unconditionally executes a release fence (see
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.Xr atomic 9 ) before releasing the reference, which
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synchronizes with an acquire fence executed right before
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returning the
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.Dv true
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value.
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This ensures that the destructor, supposedly executed by the caller after
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the last reference was dropped, sees all updates done during the lifetime
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of the object.
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.Sh RETURN VALUES
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The
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.Nm refcount_release
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function returns true when releasing the last reference and false when
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releasing any other reference.
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.Sh HISTORY
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These functions were introduced in
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.Fx 6.0 .
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