Solution Issues

• Preemptive kernel
  • A process may be preempted when in kernel mode
• Non-preemptive kernel
  • A process may not be preempted when in kernel mode
• Preemptive kernels are difficult
  • Especially for SMP machines.
• Threads on multi-processors face independent OS.
• Preemptive kernels are essential
  • In embedded systems with RT guarantees
• Windows 2000, XP are non-preemptive
• Linux kernel became preemptive since Linux 2.6

Two Process Critical Section Solution

• Processes are P0 and P1.
• Processes share a common variable turn(=0 or 1).
• If turn= i, Pi is permitted to enter the critical section.

while (1) { While (turn != i); Critical section turn = 1-i; Remainder section }