CS 3733 Operating Systems

• Mutex lock:
  • Simplest synchronization mechanism.
  • Variable with two atomic operations: lock and unlock
  • Used to assure mutual exclusion among cooperating processes or cooperating threads.
  • Should not hold a lock while executing code of long or undetermined duration.

• Condition variables:
  • Condition variables allow for blocking and unblocking.
  • Operations on condition variables are cond_wait and cond_signal
  • Need to guarantee exclusive access so condition variables are always associated with a mutex lock.
  • Steps for using condition variables:
    • Acquire mutex lock.
    • Test predicate.
    • If the predicate is true, do some work and release mutex lock.
    • If the predicate is false, do a wait and go to test.
  • The following code does some work in a critical section when predicate is true. M and V are the mutex lock and the condition variable:

       
          lock(M) 
          while (!predicate) cond_wait(V,M);
          ...do work
          unlock(M)
    

  • The following pseudocode wakes up the next process waiting on the condition variable v:

          lock(M)
          cond_signal(V)
          unlock(M) 
    

    If the implementation (such as in the case of POSIX condition variables) only wakes up one waiting process (thread), a cond_broadcast operation is also provided.