What makes concurrent programming hard?

There are a number of reasons why concurrent programming is hard, and, if you have written concurrent programs before, you have most likely already encountered the ones listed here:

• Sharing state between multiple threads in a safe manner is hard. Whenever we have data that can be read and written to at the same time, we need some way of protecting that data from data races. You will see many examples of this later on.
• Concurrent programs are usually more complicated to reason about because of the multiple parallel execution flows.
• Concurrency complicates debugging. Bugs that occur because of data races can be very hard to debug since they are dependent on how threads are scheduled. These kinds of bugs can be hard to reproduce and, in the worst-case scenario, they may even cease to exist when running the program using a debugger. Sometimes an innocent debug trace to the console can change the way a multithreaded program...