.\"	$NetBSD: ucas.9,v 1.7 2021/10/04 20:25:20 andvar Exp $
.\"
.\" Copyright (c) 2019 Jason R. Thorpe.
.\" Copyright (c) 2011 YAMAMOTO Takashi,
.\" All rights reserved.
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.\" ------------------------------------------------------------
.Dd March 31, 2019
.Dt UCAS 9
.Os
.\" ------------------------------------------------------------
.Sh NAME
.Nm ucas
.Nd atomic memory operations on user-space address
.\" ------------------------------------------------------------
.Sh SYNOPSIS
.In sys/systm.h
.\" - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
.Ft int
.Fn ucas_ptr \
"volatile void *uptr" "void *old" "void *new" "void *retp"
.\" - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
.Ft int
.Fn ucas_int \
"volatile int *uptr" "int old" "int new" "int *retp"
.\" ------------------------------------------------------------
.Sh DESCRIPTION
These functions provide compare-and-swap (CAS) functionality on
user-space address.
.Pp
Except that they can be safely used for the kernel to access user-space
address, they are semantically equivalents of
.Xr atomic_cas 3 .
.Bl -tag -width uptr
.It Fa uptr
The pointer to the variable.
This should be a user-space pointer.
.It Fa old
The value to compare with the variable.
.It Fa new
The value to store to the variable.
.It Fa retp
The pointer to the memory to store the old value of the variable.
.El
.\" ------------------------------------------------------------
.Sh IMPLEMENTATION NOTES
The
.Nm ucas
functions are implemented in machine-independent code, but rely on
machine-dependent code to implement optimized primitives, if possible.
.Pp
The basic
.Nm ucas
primitives have the following signatures and are considered private to
the implementation and are not to be called by consumers of the
.Nm ucas
API:
.Bl -tag -width _ucas_32
.It Ft int Fn _ucas_32 \
"volatile uint32_t *uptr" "uint32_t old" "uint32_t new" "uint32_t *retp" ;
.It Ft int Fn _ucas_64 \
"volatile uint64_t *uptr" "uint64_t old" "uint64_t new" "uint64_t *retp" ;
.El
.Pp
If a platform is able to provide a CAS operation that meets the following
criteria, it should define
.Dv __HAVE_UCAS_FULL
in
.In machine/types.h
and provide complete machine-dependent implementations of
.Fn _ucas_32
.Po
and
.Fn _ucas_64 ,
if an
.Dv _LP64
platform
.Pc :
.Pp
.Bl -hyphen -compact
.It
Can be implemented using either native compare-and-swap operations or
load-locked / store-conditional code sequences.
.It
Can be used on uniprocessor and multiprocessor systems.
.It
Can operate across the kernel-userspace boundary.
.El
.Pp
If
.Dv __HAVE_UCAS_FULL
is not defined, then a generic implementation will be provided by
machine-dependent code.
This generic implementation is suitable for uniprocessor and multiprocessor
systems, but works on a
.Dq least-common denominator
principle.
In particular, kernel preemption is disabled during the critical section
.Po
which is comprised of
.Xr ufetch 9
and
.Xr ustore 9
operations
.Pc ,
and the multiprocessor implementation synchronizes with other CPUs using
interprocessor interrupts.
.Pp
If a particular platform wishes to use the generic implementation on
uniprocessors but an optimized implementation on multiprocessors, the
platform should define
.Dv __HAVE_UCAS_MP
in
.In machine/types.h
and provide
.Fn _ucas_32_mp
.Po
and
.Fn _ucas_64_mp ,
if an
.Dv _LP64
platform
.Pc .
.\" ------------------------------------------------------------
.Sh RETURN VALUES
On success, these functions return 0.
In that case, the caller can consult the value returned via
.Fa retp
to check the result of the CAS operation.
Otherwise, these functions return an appropriate
.Xr errno 9
error code, typically
.Dv EFAULT .
.\" ------------------------------------------------------------
.Sh SEE ALSO
.Xr atomic_cas 3 ,
.Xr intro 9
.\" ------------------------------------------------------------
.Sh BUGS
Conceptually, the
.Fa retp
argument of
.Fn ucas_ptr
would be of
.Dv void ** .
The current prototype is a compromise for usability.