Discussion 11 handout
Group members (names & NetIDs)
Objectives
- Identify potential for and consequences of race conditions.
- Implement a concurrency abstraction using a mutex and condition variable.
Orientation
Download the dis11 project code and open it as a project in IntelliJ as you would an assignment.
Read BarrierDemo, then run it.
Is the requirement regarding subtask B being respected?
Unsynchronized attempt
Attempt to implement Barrier::await() without using synchronization (e.g. by “spinning”—checking for a condition in a loop to wait until it becomes true).
Ignore postAction for now.
Run BarrierDemo again.
What behavior do you see?
(different groups may see different behavior)
Synchronization
Where is synchronization needed?
Synchronization is not required everywhere, even when a class is shared between threads. It is important to recognize in which situations synchronization is required, so you can confidently avoid race conditions without unnecessary overhead.
Is synchronization required in the constructor? If so, which object should be synchronized on? If not, justify why not.
Is synchronization required in totalThreads()? If so, which object should be synchronized on? If not, justify why not.
Is synchronization required in awaitingThreads()? If so, which object should be synchronized on? If not, justify why not.
Await
Fix your implementation of await() so that it uses synchronization to avoid race conditions and to block without “spinning”.
Continue ignoring postAction for now.
Run BarrierDemo again.
Is the requirement regarding subtask B being respected?
postAction
Implement the requirement related to postAction.
Run BarrierDemo again.
Is the behavior what you would expect?
Reference the state diagram from lecture 20.
Which state is each thread in when postAction is executed?
Use the portion numbers from your most recent run.
Going further
What would happen if more than totalThreads called await() on a Barrier?