freebsd-nq/share/man/man9/critical_enter.9

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.Dd March 22, 2001
.Dt CRITICAL_ENTER 9
.Os
.Sh NAME
.Nm cpu_critical_enter ,
.Nm cpu_critical_exit ,
.Nm critical_enter ,
.Nm critical_exit
.Nd enter and exit a critical region
.Sh SYNOPSIS
.In sys/param.h
.In sys/proc.h
.In sys/systm.h
.In machine/critical.h
.Ft void
.Fn cpu_critical_enter "void"
.Ft void
.Fn cpu_critical_exit "void"
.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 cpu_critical_enter
and
.Fn cpu_critical_exit
functions provide the machine dependent disabling of preemption, normally
by disabling interrupts on the local CPU.
.Pp
The
.Fn critical_enter
and
.Fn critical_exit
functions provide a machine independent wrapper around the machine dependent
API.
This wrapper currently saves state regarding nested critical sections.
Nearly all code should use these versions of the API.
.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.
.Sh EXAMPLES
This example demonstrates the use of
.Fn critical_enter
and
.Fn critical_exit
to guarantee atomic access to the DMA controller.
.Bd -literal -offset indent
int
isa_dmastatus(int chan)
{
        u_long  cnt = 0;
        int     ffport, waport;
        u_long  low1, high1, low2, high2;

	...
        critical_enter();
        outb(ffport, 0);
        low1 = inb(waport);
        high1 = inb(waport);
        outb(ffport, 0);
        low2 = inb(waport);
        high2 = inb(waport);
        critical_exit();
	...
}
.Ed
.Sh HISTORY
These functions were introduced in
.Fx 5.0 .