linux.conf.au, January 2003
      PRINCIPLES: System Calls & Traps

We call these "user context"
Doing work for a process
Process accessed using the "current" global variable, eg
pid = current->pid;

We can switch to other tasks voluntarily, by calling "schedule()"
Sometimes called "sleeping".

We can be preempted by other tasks, too, unless we say not to.
Higher priority tasks take over the CPU.
Only if CONFIG_PREEMPT=y