Past exam of the computer science course of the University of Oxford / 2015 / Concurrent programming / 3 / b / ii

Give the two waiting predicates different semaphores. empty has one permit initially; full has zero. A writer takes the empty permit before touching the slot and transfers permission to a reader by releasing full. A reader does the reverse.

class SemaphoreMemory[T] extends Memory[T] {
  private val empty: Semaphore = new CountingSemaphore(1)
  private val full: Semaphore = {
    val s = new CountingSemaphore(1)
    s.down
    s
  }
  private var slot: Option[T] = None

  def write(msg: T): Unit = {
    empty.down
    slot = Some(msg)
    full.up
  }
  def read: T = {
    full.down
    val result = slot.get
    slot = None
    empty.up
    result
  }
}

For a single slot, a separate mutex around slot is unnecessary. There is a unique circulating permission: either an empty permit is available, a writer holds it, a full permit is available, or a reader holds it. During the transition the sum of available permits can be zero; it is never necessary to assert that it is always one. In particular, the next writer cannot start until the preceding reader has finished reading, cleared the slot and released empty.

The monitor operations inside each semaphore also establish the required happens-before relation: the write precedes full.up, and a successful subsequent full.down makes it visible to the reader. Likewise, the reader's clearing precedes the next writer's successful empty.down. Slot accesses therefore need no unsynchronized visibility assumption. The invariant isOnly writers wait on empty, and only readers wait on full. With the single-notification semaphore implementation in part (a), a state change wakes at most one waiter of the appropriate kind, avoiding the thundering herd of part (i). This is an efficiency improvement, not yet a first-come-first-served guarantee.

If one or more readers arrive before the first writer, they block on the initially unavailable full semaphore. Their waits release its monitor, so they do not prevent a writer from obtaining the independent empty permit. The first write releases full and permits exactly one reader to take that value. Remaining readers wait for later writes; they never see uninitialized storage.