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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.