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.\" 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 October 5, 2005
.Dt CRITICAL_ENTER 9
.Os
.Sh NAME
.Nm critical_enter ,
.Nm critical_exit
.Nd enter and exit a critical region
.Sh SYNOPSIS
.In sys/param.h
.In sys/systm.h
.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 critical_enter
and
.Fn critical_exit
functions manage a per-thread counter to handle nested critical sections.
If a thread is made runnable that would normally preempt the current thread
while the current thread is in a critical section,
then the preemption will be deferred until the current thread exits the
outermost critical section.
.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, they should not be interlocked with operations on mutexes, sx locks,
semaphores, or other synchronization primitives.
One exception to this is that spin mutexes include a critical section,
so in certain cases critical sections may be interlocked with spin mutexes.
.Sh HISTORY
These functions were introduced in
.Fx 5.0 .
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