168 lines
5.2 KiB
Groff
168 lines
5.2 KiB
Groff
.\" $NetBSD: ucas.9,v 1.4 2019/04/06 07:56:49 wiz Exp $
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.\"
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.\" Copyright (c) 2019 Jason R. Thorpe.
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.\" Copyright (c) 2011 YAMAMOTO Takashi,
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.\" All rights reserved.
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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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.\" ------------------------------------------------------------
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.Dd March 31, 2019
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.Dt UCAS 9
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.Os
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.\" ------------------------------------------------------------
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.Sh NAME
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.Nm ucas
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.Nd atomic memory operations on user-space address
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.\" ------------------------------------------------------------
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.Sh SYNOPSIS
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.In sys/systm.h
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.\" - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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.Ft int
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.Fn ucas_ptr \
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"volatile void *uptr" "void *old" "void *new" "void *retp"
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.\" - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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.Ft int
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.Fn ucas_int \
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"volatile int *uptr" "int old" "int new" "int *retp"
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.\" ------------------------------------------------------------
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.Sh DESCRIPTION
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These functions provide compare-and-swap (CAS) functionality on
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user-space address.
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.Pp
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Except that they can be safely used for the kernel to access user-space
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address, they are semantically equivalents of
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.Xr atomic_cas 3 .
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.Bl -tag -width uptr
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.It Fa uptr
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The pointer to the variable.
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This should be a user-space pointer.
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.It Fa old
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The value to compare with the variable.
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.It Fa new
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The value to store to the variable.
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.It Fa retp
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The pointer to the memory to store the old value of the variable.
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.El
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.\" ------------------------------------------------------------
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.Sh IMPLEMENTATION NOTES
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The
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.Nm ucas
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functions are implemented in machine-independent code, but rely on
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machine-dependent code to implement optimized primitives, if possible.
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.Pp
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The basic
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.Nm ucas
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primitives have the following signatures and are considered private to
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the implementation and are not to be called by consumers of the
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.Nm ucas
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API:
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.Bl -tag -width _ucas_32
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.It Ft int Fn _ucas_32 \
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"volatile uint32_t *uptr" "uint32_t old" "uint32_t new" "uint32_t *retp" ;
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.It Ft int Fn _ucas_64 \
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"volatile uint64_t *uptr" "uint64_t old" "uint64_t new" "uint64_t *retp" ;
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.El
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.Pp
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If a platform is able to provide a CAS operation that meets the following
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criteria, it should define
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.Dv __HAVE_UCAS_FULL
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in
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.In machine/types.h
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and provide complete machine-dependent implementations of
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.Fn _ucas_32
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.Po
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and
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.Fn _ucas_64 ,
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if an
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.Dv _LP64
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platform
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.Pc :
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.Pp
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.Bl -hyphen -compact
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.It
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Can be implemented using either native compare-and-swap operations or
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load-locked / store-conditional code sequences.
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.It
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Can be used on uniprocessor and multiprocessor systems.
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.It
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Can operate across the kernel-userpsace boundary.
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.El
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.Pp
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If
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.Dv __HAVE_UCAS_FULL
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is not defined, than a generic implementation will be provided by
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machine-dependent code.
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This generic implementation is suitable for uniprocessor and multiprocessor
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systems, but works on a
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.Dq least-common denominator
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principle.
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In particular, kernel preemption is disabled during the critical section
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.Po
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which is comprised of
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.Xr ufetch 9
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and
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.Xr ustore 9
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operations
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.Pc ,
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and the multiprocessor implementation synchronizes with other CPUs using
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interprocessor interrupts.
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.Pp
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If a particular platform wishes to use the generic implementation on
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uniprocessors but an optimized implementation on multiprocessors, the
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the platform should define
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.Dv __HAVE_UCAS_MP
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in
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.In machine/types.h
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and provide
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.Fn _ucas_32_mp
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.Po
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and
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.Fn _ucas_64_mp ,
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if an
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.Dv _LP64
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platform
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.Pc .
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.\" ------------------------------------------------------------
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.Sh RETURN VALUES
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On success, these functions return 0.
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In that case, the caller can consult the value returned via
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.Fa retp
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to check the result of the CAS operation.
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Otherwise, these functions return an appropriate
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.Xr errno 9
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error code, typically
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.Dv EFAULT .
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.\" ------------------------------------------------------------
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.Sh SEE ALSO
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.Xr atomic_cas 3 ,
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.Xr intro 9
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.\" ------------------------------------------------------------
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.Sh BUGS
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Conceptually, the
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.Fa retp
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argument of
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.Fn ucas_ptr
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would be of
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.Dv void ** .
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The current prototype is a compromise for usability.
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