User Mode and Kernel Mode
See also: Kernel preemptionIn any given system design, some operations performed by the system may not be preemptible. This usually applies to kernel functions and service interrupts which, if not permitted to run to completion, would tend to produce race conditions resulting in deadlock. Barring the scheduler from preempting tasks while they are processing kernel functions simplifies the kernel design at the expense of system responsiveness. The distinction between user mode and kernel mode, which determines privilege level within the system, may also be used to distinguish whether a task is currently preemptible.
Most modern systems have preemptive kernels, designed to permit tasks to be preempted even when in kernel mode. Examples of such systems are Solaris 2.0/SunOS 5.0, Windows NT, the Linux kernel 2.6 and 3.x, AIX and some BSD systems (NetBSD, since version 5).
Read more about this topic: Preemption (computing)
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