Understanding the Linux Kernel, Second Edition

Synchronization Primitives

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 a computation depends on how two or more interleaved kernel control paths are nested. A critical region is any section of code that must be completely executed by any kernel control path that enters 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. Table 5-1 lists the synchronization techniques used by the Linux kernel. The “Applies to” column indicates whether the synchronization technique applies to all CPUs in the system or to a single CPU. For instance, local interrupts disabling applies to just one CPU (other CPUs in the system are not affected); conversely, an atomic operation affects all CPUs in the system (atomic operations on several CPUs cannot interleave while accessing the same data structure).

Table 5-1. Various types of synchronization techniques used by the kernel

Technique	Description	Scope
Atomic operation	Atomic read-modify-write instruction to a counter	All CPUs
Memory barrier	Avoid instruction re-ordering	Local CPU
Spin lock	Lock with busy wait	All CPUs
Semaphore	Lock with blocking wait (sleep)	All CPUs
Local interrupt disabling	Forbid interrupt handling on a single CPU	Local CPU
Local softirq disabling ...