Question

An exam question:

A resource is shared between N threads such that access is on a first-come-first-served basis. Explain how you would implement a policy of first-come-first-served.

My answer would be along the lines of (Note: the course is Java-based):

Use a semaphore (binary) to control access to the shared resource. Then create a feeder class/thread to access the semaphore. The feeder thread uses a FIFO queue to implement first-come-first-served. When the semaphore is free it allows the thread a the head of the queue to access the shared resource.

I have just jotted out on paper the above approach and it seems to work.

The question is worth 8 marks in a 20 mark question so I doubt using new ReentrantLock(true) would suffice for full marks.

What do you guys think?

Answer 1

Your answer really doesn’t address the question. This is not just about locking but is also about queuing the threads waiting for the resource.

• How do you queue the threads waiting for the resource?
• If there are threads waiting for the resource, how do you wake them up so they consume the resource in order.
• If there are threads waiting for the resource, how do you make sure they run in order and avoid the race condition around a new thread grabbing it.

A possible solution would be something like the following code:

  lockResource(resource);
  try {
     ... consume the resource
  } finally {
     unlockResource(resource);
  }

The lockResource would do something like:

1. Acquire the lock on the resource.
2. Retrieve the lock object associated with the resource that contains a long accessNum and a long runningNum.
3. increment the accessNum value and store the results for the thread.
4. check the long runningNum value to see if == to my accessNum.
5. If it is == mine, unlock resource and continue running.
6. If more than me in the queue, wait() on resource.

When unlockResource(...) is called, the thread should:

1. Lock resource.
2. Set running++
3. Call notifyAll()
4. Unlock resource

If a thread is waiting and gets awoken:

1. Check to see if the accessNum == runningNum.
2. If true then unlock resource and continue running
3. If not then go back to wait()