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membarrier: Add manual page

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Add a minimal membarrier man page that documents the available cmd
values and errors that can be returned.  We can add more information and
iterate on it in the tree.

Reviewed by:	kib
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46967
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Ed Maste 2024-10-06 13:47:34 -04:00
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.\" SPDX-License-Identifier: BSD-2-Clause
.\"
.\" Copyright (c) 2024 The FreeBSD Foundation
.\"
.\" This documentation was written by Ed Maste <emaste@FreeBSD.org>
.\" under sponsorship from the FreeBSD Foundation.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHORS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHORS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.Dd October 6, 2024
.Dt MEMBARRIER 2
.Os
.Sh NAME
.Nm membarrier
.Nd memory barrier
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/membarrier.h
.Ft int
.Fn membarrier "int cmd" "unsigned flags" "int cpu_id"
.Sh DESCRIPTION
The
.Nm
system call provides a memory barrier, ensuring ordering between memory
accesses performed by different threads or processes within the same address
space.
.Pp
The following values for
.Fa cmd
may be specified:
.Bl -tag -width Dv
.It Dv MEMBARRIER_CMD_QUERY
Query supported commands.
A bitmask of commands supported by the kernel is returned.
.It Dv MEMBARRIER_CMD_GLOBAL
Issue a memory barrier for all threads from all processes.
.It Dv MEMBARRIER_CMD_SHARED
This is an alias for
.Dv MEMBARRIER_CMD_GLOBAL .
.It Dv MEMBARRIER_CMD_GLOBAL_EXPEDITED
Execute a memory barrier on all running threads of all processes registered
with
.Dv MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED
.It Dv MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED
Register the process to receive
.Dv MEMBARRIER_CMD_GLOBAL_EXPEDITED
memory barriers.
.It Dv MEMBARRIER_CMD_PRIVATE_EXPEDITED
Execute a memory barrier on each running thread belonging to the same process
as the thread calling
.Nm .
.It Dv MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED
Register the process to receive
.Dv MEMBARRIER_CMD_PRIVATE_EXPEDITED
memory barriers.
.It Dv MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE
In addition to the guarantees provided by
.Dv MEMBARRIER_CMD_PRIVATE_EXPEDITED
it also executes machine-specific serialization instructions, which
ensures that all possible speculative and out-of-order activities on the target
CPU are fenced.
.It Dv MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE
Register the process to receive
.Dv MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE
memory barriers.
.El
.Pp
The following
.Fa cmd
values are defined for source compatibility but are not yet supported:
.Pp
.Bl -tag -width Dv -compact
.It Dv MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ
.It Dv MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ
.El
.Pp
The
.Fa flags
argument must be 0.
The
.Fa cpu_id
argument is ignored.
.Sh RETURN VALUES
If the
.Dv cmd
is MEMBARRIER_CMD_QUERY
a bitmask of supported commands is returned.
Otherwise, on success,
.Nm
returns 0.
On error, -1 is returned and
.Va errno
is set to indicate the error.
.Sh ERRORS
.Nm
may fail with the following errors:
.Bl -tag -width Er
.It Bq Er EINVAL
.Fa cmd
does not specify a valid command.
.It Bq Er EINVAL
.Fa flags
is not 0.
.It Bq Er EPERM
An attempt was made to use
.Dv MEMBARRIER_CMD_GLOBAL_EXPEDITED ,
.Dv MEMBARRIER_CMD_PRIVATE_EXPEDITED ,
or
.Dv MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE
by a process that did not previously register use with the corresponding
.Dv MEMBARRIER_CMD_REGISTER_*
.Fa cmd .
.El
.\" .Sh SEE ALSO
.Sh STANDARDS
The
.Nm
system call originated in Linux.
This implementation aims to be source-compatible with the Linux implementation.
Certain
.Fa cmd
and
.Fa flags
values are currently not supported by
.Fx .
.Sh HISTORY
The
.Nm
function was introduced in
.Fx 14.1 .