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.
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
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.