eefd941ba2
- Remove references to cpu_critical_*() as they no longer exist. - Explain that any preemptions that occur during a critical section are deferred until the current thread exits the section. - Remove a bogus example usage of a critical section. - Note that one can interlock critical sections with spin mutexes in certain situations. MFC after: 3 days
75 lines
3.0 KiB
Groff
75 lines
3.0 KiB
Groff
.\" Copyright (c) 2001,2002 John H. Baldwin <jhb@FreeBSD.org>
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd October 5, 2005
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.Dt CRITICAL_ENTER 9
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.Os
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.Sh NAME
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.Nm critical_enter ,
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.Nm critical_exit
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.Nd enter and exit a critical region
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.Sh SYNOPSIS
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.In sys/param.h
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.In sys/systm.h
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.Ft void
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.Fn critical_enter "void"
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.Ft void
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.Fn critical_exit "void"
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.Sh DESCRIPTION
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These functions are used to prevent preemption in a critical region of code.
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All that is guaranteed is that the thread currently executing on a CPU will
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not be preempted.
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Specifically, a thread in a critical region will not migrate to another
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CPU while it is in a critical region.
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The current CPU may still trigger faults and exceptions during a critical
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section; however, these faults are usually fatal.
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.Pp
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The
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.Fn critical_enter
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and
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.Fn critical_exit
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functions manage a per-thread counter to handle nested critical sections.
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If a thread is made runnable that would normally preempt the current thread
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while the current thread is in a critical section,
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then the preemption will be deferred until the current thread exits the
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outermost critical section.
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.Pp
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Note that these functions are not required to provide any inter-CPU
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synchronization, data protection, or memory ordering guarantees and thus
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should
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.Em not
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be used to protect shared data structures.
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.Pp
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These functions should be used with care as an infinite loop within a
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critical region will deadlock the CPU.
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Also, they should not be interlocked with operations on mutexes, sx locks,
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semaphores, or other synchronization primitives.
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One exception to this is that spin mutexes include a critical section,
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so in certain cases critical sections may be interlocked with spin mutexes.
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.Sh HISTORY
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These functions were introduced in
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.Fx 5.0 .
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