1.\" Copyright (c) 2001 John H. Baldwin <jhb@FreeBSD.org> 2.\" All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 13.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 14.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 15.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 16.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 17.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 18.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 19.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 20.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 21.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 22.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 23.\" SUCH DAMAGE. 24.\" 25.\" $FreeBSD$ 26.\" 27.Dd March 22, 2001 28.Dt CRITICAL_ENTER 9 29.Os 30.Sh NAME 31.Nm critical_enter , 32.Nm critical_exit 33.Nd enter and exit a critical region 34.Sh SYNOPSIS 35.Fd #include <sys/types.h> 36.Fd #include <sys/systm.h> 37.Ft critical_t 38.Fn critical_enter "void" 39.Ft void 40.Fn critical_exit "critical_t savecrit" 41.Sh DESCRIPTION 42These functions are used to prevent preemption in a critcal region of code. 43All that is guaranteed is that the current CPU will not be preempted by an 44external interrupt. 45The current CPU may still trigger faults and exceptions during a critical 46section. 47The 48.Fn critical_enter 49function returns an opaque value of type 50.Vt critical_t . 51This value must be passed to the 52.Fn critical_exit 53function when leaving the critical region. 54.Pp 55Note that these functions are not required to provide any inter-CPU 56synchronization, data protection, or memory ordering guarantees and thus 57should 58.Em not 59be used to protect shared data structures. 60.Pp 61These functions should be used with care as an infinite loop within a 62critical region will deadlock the CPU. 63.Sh RETURN VALUES 64The 65.Fn critical_enter 66function returns an opaque value that must be passed to a corresponding call to 67.Fn critical_exit . 68.Sh EXAMPLES 69This example demonstrates the use of 70.Fn critical_enter 71and 72.Fn critical_exit 73to guarantee atomic access to the DMA controller. 74.Bd -literal -offset indent 75int 76isa_dmastatus(int chan) 77{ 78 u_long cnt = 0; 79 int ffport, waport; 80 u_long low1, high1, low2, high2; 81 critical_t savecrit; 82 83 ... 84 savecrit = critical_enter(); 85 outb(ffport, 0); 86 low1 = inb(waport); 87 high1 = inb(waport); 88 outb(ffport, 0); 89 low2 = inb(waport); 90 high2 = inb(waport); 91 critical_exit(savecrit); 92 ... 93} 94.Ed 95.Sh HISTORY 96These functions were introduced in 97.Fx 5.0 . 98