The Solaris kernel is multithreaded. On a multiprocessor machine, multiple kernel threads can be running kernel code, and can do so concurrently. Kernel threads can also be preempted by other kernel threads at any time.
The multithreading of the kernel imposes some additional restrictions on device drivers. For more information on multithreading considerations, see Chapter 3, Multithreading. Device drivers must be coded to run as needed at the request of many different threads. For each thread, a driver must handle contention problems from overlapping I/O requests.