The preemptible kernel

Linux kernel code has always been non-preemptible
Runs until it voluntarily releases the processor
Interrupts excepted

2.6 adds a preemptible kernel option
Kernel code can be interrupted too
Reduces latency for high-priority processes
No preemption when spinlocks are held

Implications:
Anything can happen between two instructions
Uniprocessor machines look like SMP
Care required with per-CPU variables