Implementing a FIFO mutex in pthreads

Question

I\'m trying to implement a binary tree supporting concurrent insertions (which could occur even between nodes), but without having to allocate a global lock or a separate mu

Answer 1

You could do something like this:

• define a "queued lock" that consists of a free/busy flag plus a linked-list of pthread condition variables. access to the queued_lock is protected by a mutex

• to lock the queued_lock:

  • seize the mutex
  • check the 'busy' flag
  • if not busy; set busy = true; release mutex; done
  • if busy; create a new condition @ end of queue & wait on it (releasing mutex)

• to unlock:

  • seize the mutex
  • if no other thread is queued, busy = false; release mutex; done
  • pthread_cond_signal the first waiting condition
  • do not clear the 'busy' flag - ownership is passing to the other thread
  • release mutex

• when waiting thread unblocked by pthread_cond_signal:

  • remove our condition var from head of queue
  • release mutex

Note that the mutex is locked only while the state of the queued_lock is being altered, not for the whole duration that the queued_lock is held.