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$FreeBSD: head/share/man/man9/critical_enter.9 94479 2002-04-12 05:06:38Z davidc $
.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 n sys/param.h n sys/proc.h n sys/systm.h n 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.
p 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.
p 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.
p 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.
p 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. d -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 .