NAME

       critical_enter, critical_exit — enter and exit a critical region

SYNOPSIS

       #include <sys/param.h>
       #include <sys/systm.h>

       void
       critical_enter(void);

       void
       critical_exit(void);

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.

       The  critical_enter() and 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.

       Note that these functions are not required to provide any inter-CPU synchronization, data protection,  or
       memory ordering guarantees and thus should not be used to protect shared data structures.

       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.

HISTORY

       These functions were introduced in FreeBSD 5.0.

Debian                                           October 5, 2005                               CRITICAL_ENTER(9)