There are two types of kernel threads: those bound to an LWP and those not associated with an LWP. Threads not associated with LWPs are system threads, such as those created to handle hardware interrupts. For those threads bound to an LWP, there is one and only one kernel thread per LWP. On a multiprocessor system, several kernel threads can run simultaneously. Even on uniprocessors, running kernel threads can be preempted at any time to run other threads. Drivers are mainly concerned with kernel threads as most device driver routines run as kernel threads. Figure 4-1 illustrates the relationship between threads and lightweight processes.
A multithreaded kernel requires consideration of locking primitives and thread synchronization.