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man page of critical_enter
critical_enter: enter and exit a critical region
critical_enter, critical_exit -- enter and exit a critical region
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.
These functions were introduced in FreeBSD 5.0.