Understanding the Linux Kernel

11.2. Synchronization Techniques

Chapter 1 introduced the concepts of race condition and critical region for processes. The same definitions apply to kernel control paths. In this chapter, a race condition can occur when the outcome of some computation depends on how two or more interleaved kernel control paths are nested. A critical region is any section of code that should be completely executed by each kernel control path that begins it, before another kernel control path can enter it.

We now examine how kernel control paths can be interleaved while avoiding race conditions among shared data. We'll distinguish four broad types of synchronization techniques:

• Nonpreemptability of processes in Kernel Mode

• Atomic operations

• Interrupt disabling

• Locking

11.2.1. Nonpreemptability of Processes in Kernel Mode

As already pointed out, the Linux kernel is not preemptive, that is, a running process cannot be preempted (replaced by a higher-priority process) while it remains in Kernel Mode. In particular, the following assertions always hold in Linux:

• No process running in Kernel Mode may be replaced by another process, except when the former voluntarily relinquishes control of the CPU.^[1]

  ^[1] Of course, all context switches are performed in Kernel Mode. However, a context switch may occur only when the current process is going to return in User Mode.

• Interrupt or exception handling can interrupt a process running in Kernel Mode; however, when the interrupt handler terminates, the kernel control ...