Condition variable

Definition

A condition variable is a synchronisation object that lets a thread efficiently wait for a change to shared state that is protected by a lock.

APIs

A condition variable has three methods.

• CV::wait(Lock *lock*)

  • This call atomically releases the lock and suspends execution of the calling thread, placing the calling thread on the condition variable’s waiting list.
  • Later, when the calling thread is awakened, the lock is re-acquired before returning from the wait call.

• CV::signal()

  • This call takes one thread off the condition variable’s wait list and marks it as eligible to run (i.e., it puts the thread on the scheduler’s ready list).
  • If no threads are on the waiting list, signal has no effect.

• CV::broadcast()

  • This call takes all threads off the condition variable’s waiting list and marks them as eligible to run.
  • If no threads are on the waiting list, broadcast has no effect.

Properties

Memoryless

• The conditional variable itself has no internal state other than a queue of waiting threads.
  • Condition variables do not need their own state because they are always used inside shared objects that have their own state.
• If no threads are currently on the condition variable’s waiting list, a signal or broadcast has no effect.
• The condition variable has no memory of earlier calls to signal or broadcast.

Atomic Lock Release

• A thread always calls wait while holding a lock. The call to wait atomically releases the lock and puts the thread on the condition variable’s waiting list.
  • Atomicity ensures that there is no separation between checking the shared object’s state, deciding to wait, adding the waiting thread to the condition variable’s queue and releasing the lock, so that some other thread can access the shared object.

Non-immediate Return

• When a waiting thread is re-enabled, it is moved to the scheduler’s ready queue with no special priority, and the scheduler may run it at some later time.
• Furthermore, when the thread finally does run, it must re-acquire the lock, which means that other threads may have acquired and released the lock in the meantime, between the signal / broadcast occurs and when the waiter re-acquires the lock.
• Even if the desired predicate were true when signal / broadcast was called, it may no longer be true when wait returns.