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     critical_enter, critical_exit — enter and exit a critical region


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




     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

     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.


     These functions were introduced in FreeBSD 5.0.