1.\" Copyright (c) 2001,2002 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 October 5, 2005 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.In sys/param.h 36.In sys/systm.h 37.Ft void 38.Fn critical_enter "void" 39.Ft void 40.Fn critical_exit "void" 41.Sh DESCRIPTION 42These functions are used to prevent preemption in a critical region of code. 43All that is guaranteed is that the thread currently executing on a CPU will 44not be preempted. 45Specifically, a thread in a critical region will not migrate to another 46CPU while it is in a critical region. 47The current CPU may still trigger faults and exceptions during a critical 48section; however, these faults are usually fatal. 49.Pp 50The 51.Fn critical_enter 52and 53.Fn critical_exit 54functions manage a per-thread counter to handle nested critical sections. 55If a thread is made runnable that would normally preempt the current thread 56while the current thread is in a critical section, 57then the preemption will be deferred until the current thread exits the 58outermost critical section. 59.Pp 60Note that these functions are not required to provide any inter-CPU 61synchronization, data protection, or memory ordering guarantees and thus 62should 63.Em not 64be used to protect shared data structures. 65.Pp 66These functions should be used with care as an infinite loop within a 67critical region will deadlock the CPU. 68Also, they should not be interlocked with operations on mutexes, sx locks, 69semaphores, or other synchronization primitives. 70One exception to this is that spin mutexes include a critical section, 71so in certain cases critical sections may be interlocked with spin mutexes. 72.Sh HISTORY 73These functions were introduced in 74.Fx 5.0 . 75