.\" Copyright (c) 2001,2002 John H. Baldwin <jhb@FreeBSD.org>
.\" All rights reserved.
.\"
.\" 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 AUTHOR 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 AUTHOR 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.
.\"
.\" $FreeBSD$
.\"
.Dd March 22, 2001
.Dt CRITICAL_ENTER 9
.Os
.Sh NAME
.Nm cpu_critical_enter ,
.Nm cpu_critical_exit ,
.Nm critical_enter ,
.Nm critical_exit
.Nd enter and exit a critical region
.Sh SYNOPSIS
.In sys/types.h
.In sys/systm.h
.Ft critical_t
.Fn cpu_critical_enter "void"
.Ft void
.Fn cpu_critical_exit "critical_t savecrit"
.Ft void
.Fn critical_enter "void"
.Ft void
.Fn critical_exit "void"
.Sh DESCRIPTION
These functions are used to prevent preemption in a critical region of code.
All that is guaranteed is that the thread currently executing on a CPU will
not be preempted.
Specifically, a thread in a critical region will not migrate to another
CPU while it is in a critical region.
The current CPU may still trigger faults and exceptions during a critical
section; however, these faults are usually fatal.
.Pp
The
.Fn cpu_critical_enter
and
.Fn cpu_critical_exit
functions provide the machine dependent disabling of preemption, normally
by disabling interrupts on the local CPU.
The
.Fn cpu_critical_enter
function returns an opaque value of type
.Vt critical_t .
This value must be passed to the
.Fn cpu_critical_exit
function when leaving the critical region.
.Pp
The
.Fn critical_enter
and
.Fn critical_exit
functions provide a machine independent wrapper around the machine dependent
API.
This wrapper currently saves state regarding nested critical sections as
well as the
.Vt critical_t
value inside of a critical region in per-thread variables.
Nearly all code should use these versions of the API.
.Pp
Note that these functions are not required to provide any inter-CPU
synchronization, data protection, or memory ordering guarantees and thus
should
.Em not
be used to protect shared data structures.
.Pp
These functions should be used with care as an infinite loop within a
critical region will deadlock the CPU.
Also, the machine dependent and machine independent functions should not
be interlocked, and neither pair of functions should be interlocked with
operations on mutexes, sx locks, semaphores, or other synchronization
primitives.
.Sh RETURN VALUES
The
.Fn cpu_critical_enter
function returns an opaque value that must be passed to a corresponding call to
.Fn cpu_critical_exit .
.Sh EXAMPLES
This example demonstrates the use of
.Fn critical_enter
and
.Fn critical_exit
to guarantee atomic access to the DMA controller.
.Bd -literal -offset indent
int
isa_dmastatus(int chan)
{
        u_long  cnt = 0;
        int     ffport, waport;
        u_long  low1, high1, low2, high2;

	...
        critical_enter();
        outb(ffport, 0);
        low1 = inb(waport);
        high1 = inb(waport);
        outb(ffport, 0);
        low2 = inb(waport);
        high2 = inb(waport);
        critical_exit();
	...
}
.Ed
.Sh HISTORY
These functions were introduced in
.Fx 5.0 .