The exact semantics of learning to use the mutex lock and the spinlock are quite simple (with appropriate abstraction within the kernel API set, making it even easier for the typical driver developer or module author). The critical question in this situation is a conceptual one: what really is the difference between the two locks? More to the point, under which circumstances should you use which lock? You will learn the answers to these questions in this section.
Taking our previous driver read method's pseudocode (Figure 6.5) as a base example, let's say that three threads – tA, tB, and tC – are running in parallel (on an SMP system) through this code. We shall solve this concurrency issue, while avoiding any data races, by taking or acquiring a lock prior to the start of the critical section (time t2), and release the lock (unlock) just after the end of the critical section code...
