The preemptible kernel

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

2.5 adds a preemptible kernel option
Kernel code can be interrupted too
Benefit: reduced latency for high-priority processes

Implications:
Anything can happen between two instructions
Uniprocessor machines have new concurrency issues
Care required with per-CPU variables