I am reading Java Concurrency in Practice, in “16.1.3 The Java Memory Model in 500 words or less”, it says:
The Java Memory Model is specified in terms of actions, which include reads and writes to variables, locks and unlocks of monitors, and starting and joining with threads. The JMM defines a partial ordering called happens-before on all actions within the program. To guarantee that the thread executing action B can see the results of action A (whether or not A and B occur in different threads), there must be a happens-before relationship between A and B. In the absence of a happens-before ordering between two operations, the JVM is free to reorder them as it pleases.
Even though actions are only partially ordered, synchronization actions—lock acquisition and release, and reads and writes of volatile variables—are totally ordered. This makes it sensible to describe happens-before in terms of “subsequent” lock acquisitions and reads of volatile variables.
About “partial ordering”, I have found this and this, but I don’t quite understand “Even though actions are only partially ordered, synchronization actions—lock acquisition and release, and reads and writes of volatile variables—are totally ordered.”. What does “synchronization actions are totally ordered” mean?