Expand description
Safe Cell for uniprocessor（single cpu core）
UPSafeCell is used to wrap a static data structure which can access safely.
NOTICE: We should only use it in environment with uniprocessor（single cpu core）, and the kernel can not support task preempting in kernel mode （or trap in kernel mode）.
Structs
Wrap a static data structure inside it so that we are
able to access it without any unsafe.