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$FreeBSD: head/share/man/man9/critical_enter.9 89124 2002-01-09 11:43:48Z mpp $
.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/types.h n sys/systm.h .Ft critical_t .Fn cpu_critical_enter "void" .Ft void .Fn cpu_critical_exit "critical_t savecrit" .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. The .Fn cpu_critical_enter function returns an opaque value of type .Vt critical_t . This value must be passed to the .Fn cpu_critical_exit function when leaving the critical region.
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 as well as the .Vt critical_t value inside of a critical region in per-thread variables. 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, the machine dependent and machine independent functions should not be interlocked, and neither pair of functions should be interlocked with operations on mutexes, sx locks, semaphores, or other synchronization primitives. .Sh RETURN VALUES The .Fn cpu_critical_enter function returns an opaque value that must be passed to a corresponding call to .Fn cpu_critical_exit . .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 .