ba71f034aa
the standard atomic compare-and-swap operations. Tell some caveats. Manual page links, *_ni.3 -> atomic_cas.3 are coming up after a successful 'build.sh distribution'.
130 lines
4.5 KiB
Groff
130 lines
4.5 KiB
Groff
.\" $NetBSD: atomic_cas.3,v 1.2 2010/02/12 22:23:17 dyoung Exp $
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.\"
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.\" Copyright (c) 2007, 2010 The NetBSD Foundation, Inc.
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.\" All rights reserved.
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.\"
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.\" This code is derived from software contributed to The NetBSD Foundation
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.\" by Jason R. Thorpe.
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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 NETBSD FOUNDATION, INC. AND CONTRIBUTORS
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.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
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.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
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.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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.\" POSSIBILITY OF SUCH DAMAGE.
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.\"
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.Dd February 11, 2010
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.Dt ATOMIC_CAS 3
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.Os
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.Sh NAME
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.Nm atomic_cas ,
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.Nm atomic_cas_32 ,
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.Nm atomic_cas_uint ,
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.Nm atomic_cas_ulong ,
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.Nm atomic_cas_ptr ,
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.Nm atomic_cas_64 ,
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.Nm atomic_cas_32_ni ,
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.Nm atomic_cas_uint_ni ,
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.Nm atomic_cas_ulong_ni ,
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.Nm atomic_cas_ptr_ni ,
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.Nm atomic_cas_64_ni
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.Nd atomic compare-and-swap operations
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.\" .Sh LIBRARY
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.\" .Lb libc
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.Sh SYNOPSIS
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.In sys/atomic.h
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.Ft uint32_t
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.Fn atomic_cas_32 "volatile uint32_t *ptr" "uint32_t old" "uint32_t new"
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.Ft unsigned int
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.Fn atomic_cas_uint "volatile unsigned int *ptr" "unsigned int old" \
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"unsigned int new"
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.Ft unsigned long
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.Fn atomic_cas_ulong "volatile unsigned long *ptr" "unsigned long old" \
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"unsigned long new"
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.Ft void *
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.Fn atomic_cas_ptr "volatile void *ptr" "void *old" "void *new"
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.Ft uint64_t
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.Fn atomic_cas_64 "volatile uint64_t *ptr" "uint64_t old" "uint64_t new"
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.Ft uint32_t
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.Fn atomic_cas_32_ni "volatile uint32_t *ptr" "uint32_t old" "uint32_t new"
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.Ft unsigned int
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.Fn atomic_cas_uint_ni "volatile unsigned int *ptr" "unsigned int old" \
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"unsigned int new"
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.Ft unsigned long
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.Fn atomic_cas_ulong_ni "volatile unsigned long *ptr" "unsigned long old" \
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"unsigned long new"
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.Ft void *
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.Fn atomic_cas_ptr_ni "volatile void *ptr" "void *old" "void *new"
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.Ft uint64_t
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.Fn atomic_cas_64_ni "volatile uint64_t *ptr" "uint64_t old" "uint64_t new"
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.Sh DESCRIPTION
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The
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.Nm atomic_cas
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family of functions perform a compare-and-swap operation in an atomic fashion.
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The value of the variable referenced by
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.Fa ptr
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is compared against
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.Fa old .
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If the values are equal,
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.Fa new
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is stored in the variable referenced by
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.Fa ptr .
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.Pp
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The old value of the variable referenced by
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.Fa ptr
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is always returned regardless of whether or not the new value was stored.
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Applications can test for success of the operation by comparing the
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return value to the value passed as
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.Fa old ;
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if they are equal then the new value was stored.
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.Pp
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The non-interlocked variants,
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.Fn *_ni ,
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guarantee atomicity within the same CPU with respect to
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interrupts and preemption.
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For example, they are suitable for synchronizing compare-and-swap
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operations on a variable shared by a thread and an interrupt
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that are bound to the same CPU.
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The
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.Fn *_ni
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variants are not atomic with respect to different CPUs.
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.Fn *_ni
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variants should avoid the interprocessor synchronization overhead
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of the standard compare-and-swap operations.
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.Pp
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The 64-bit variants of these functions are available only on platforms
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that can support atomic 64-bit memory access.
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Applications can check for the availability of 64-bit atomic memory
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operations by testing if the pre-processor macro
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.Dv __HAVE_ATOMIC64_OPS
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is defined.
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.Sh SEE ALSO
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.Xr atomic_ops 3
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.Sh HISTORY
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The
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.Nm atomic_cas
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functions first appeared in
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.Nx 5.0 .
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.Sh NOTES
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On some architectures, a
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.Fn *_ni
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variant is merely an alias for the corresponding standard
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compare-and-swap operation.
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While the non-interlocked variant behaves correctly on those
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architectures, it does not avoid the interprocessor synchronization
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overhead.
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