Spin locks

Before starting to talk about the condition variables and the true way that the sleep/notify technique should get implemented, let's go back a little bit and use busy-waiting together with mutexes to write a new solution for example 14.6. As a reminder, the example was about having first A and then B printed in the standard output.

The following is the proposed solution that uses a mutex equipped with the spin locking algorithm. The mutex acts as a memory barrier, so we won't have any memory visibility issues, and it effectively synchronizes the tasks P and Q over the Done shared flag:

Concurrent System {
  Shared State {
      Done : Boolean = False
      M : Mutex
  }
  Task P {
      1.1. print 'A'
      1.2. SpinLock(M)
      1.2. Done = True
      1.3. SpinUnlock(M)
  }
  Task Q {
      2.1  SpinLock(M)
      2.2. While Not Done {
      2.3.   SpinUnlock(M)
      2.4.   SpinLock(M)
      2.5. }
      2.6. SpinUnlock(M)
      2.4. print...