You could think of the kernel as a server that answers requests; these requests can come either from a process running on a CPU or an external device issuing an interrupt request. We make this analogy to underscore that parts of the kernel are not run serially, but in an interleaved way. Thus, they can give rise to race conditions, which must be controlled through proper synchronization techniques. A general introduction to these topics can be found in the section "An Overview of Unix Kernels" in Chapter 1.
We start this chapter by reviewing when, and to what extent, kernel requests are executed in an interleaved fashion. We then introduce the basic synchronization primitives implemented by the kernel and describe how they are applied in the most common cases. Finally, we illustrate a few practical examples.