Most Linux device drivers are kernel modules, so I suppose most of the stories I have would be of the form "I needed to write a device driver for X" :-). Usersp

Most Linux device drivers are kernel modules, so I suppose most of the stories I have would be of the form "I needed to write a device driver for X" :-). Userspace drivers, while certainly in existence, are not very popular in Linux land. Other interesting stories would include: - Instrumenting certain filesystem operations (all modules share the same memory space; it's possible for one module to take a sneak peak at another's internal structures). This was back before dtrace & friends were a (useful) thing. - Real-time processing of instrumentation samples. Doing it in the kernel allowed us to avoid costly back-and-forth between user and kernel memory -- but we only did relatively simple processing, such as scaling, channel muxing/demuxing and the like. If you find yourself thinking about doing kernelside programming because carrying samples to and from userspace is too expensive, you should probably review your design first.