As we saw in Chapter 1, Concurrency – An Introduction, a process is a container for threads. A process has executable code and global data; all threads share these things with other threads of the same process. As the following diagram shows, the binary executable code is read-only. It can be freely shared by threads as there is nothing mutable there.
The global data is mutable though and, as shown in the diagram, this is the source of concurrency bugs! Most of the techniques and patterns we will study in this book are ways to avoid such bugs.
Threads of the same process run concurrently. How is this achieved, given there is just one set of registers? Well, here's the answer: the thread context. This context helps a thread keep its runtime information independent of another thread. The thread context holds the register set and the stack.
The following diagram shows the various pieces:
When the scheduler preempts a running thread, it backs up the thread context—that...