1.\" Copyright (c) 2001,2002 John H. Baldwin <jhb@FreeBSD.org> 2.\" 3.\" Redistribution and use in source and binary forms, with or without 4.\" modification, are permitted provided that the following conditions 5.\" are met: 6.\" 1. Redistributions of source code must retain the above copyright 7.\" notice, this list of conditions and the following disclaimer. 8.\" 2. Redistributions in binary form must reproduce the above copyright 9.\" notice, this list of conditions and the following disclaimer in the 10.\" documentation and/or other materials provided with the distribution. 11.\" 12.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 13.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 14.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 15.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 16.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 17.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 18.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 19.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 20.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 21.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 22.\" SUCH DAMAGE. 23.\" 24.\" $FreeBSD$ 25.\" 26.Dd October 5, 2005 27.Dt CRITICAL_ENTER 9 28.Os 29.Sh NAME 30.Nm critical_enter , 31.Nm critical_exit 32.Nd enter and exit a critical region 33.Sh SYNOPSIS 34.In sys/param.h 35.In sys/systm.h 36.Ft void 37.Fn critical_enter "void" 38.Ft void 39.Fn critical_exit "void" 40.Sh DESCRIPTION 41These functions are used to prevent preemption in a critical region of code. 42All that is guaranteed is that the thread currently executing on a CPU will 43not be preempted. 44Specifically, a thread in a critical region will not migrate to another 45CPU while it is in a critical region. 46The current CPU may still trigger faults and exceptions during a critical 47section; however, these faults are usually fatal. 48.Pp 49The 50.Fn critical_enter 51and 52.Fn critical_exit 53functions manage a per-thread counter to handle nested critical sections. 54If a thread is made runnable that would normally preempt the current thread 55while the current thread is in a critical section, 56then the preemption will be deferred until the current thread exits the 57outermost critical section. 58.Pp 59Note that these functions are not required to provide any inter-CPU 60synchronization, data protection, or memory ordering guarantees and thus 61should 62.Em not 63be used to protect shared data structures. 64.Pp 65These functions should be used with care as an infinite loop within a 66critical region will deadlock the CPU. 67Also, they should not be interlocked with operations on mutexes, sx locks, 68semaphores, or other synchronization primitives. 69One exception to this is that spin mutexes include a critical section, 70so in certain cases critical sections may be interlocked with spin mutexes. 71.Sh HISTORY 72These functions were introduced in 73.Fx 5.0 . 74