Mutexes

• When the semaphore's ability to count is not needed, a simplified version of the semaphore, called a mutex, is sometimes used.
• A mutex is a variable that can be in one of two states: unlocked or locked. Two procedures are used with mutexes.
  • When a thread (or process) needs access to a critical region, it calls $mutex\_lock$.
  • If the mutex is current unlocked (meaning that the critical region is available), the call succeeds and the calling thread is free to enter the critical region.

  Figure 4: Some of the Pthreads calls relating to the mutexes.

  

  Figure 5: Using threads to solve the producer-consumer problem.

  
• On the other hand, if the mutex is already locked, the calling thread is blocked until the thread in the critical region is finished and calls $mutex\_unlock$.
• If multiple threads are blocked on the mutex, one of them is chosen at random and allowed to acquire the lock.
• With threads, there is no clock that stops threads that have run too long. Consequently, a thread that tries to acquire a lock by busy waiting will loop forever and never acquire the lock because it never allows any other thread to run and release the lock.
• That is where the difference between $enter\_region$ and $mutex\_lock$ comes in. When the later fails to acquire a lock, it calls $thread\_yield$ to give up the CPU to another thread.
• Consequently there is no busy waiting. When the thread runs the next time, it tests the lock again.