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Writing a Generalized Concurrent Queue


Now let's look at the first of the two key methods: Produce. The goal is to allow multiple producers, and to let them run as concurrently as possible:

  void Produce( const T& t ) {
    Node* tmp = new Node( new T(t) );
    while( producerLock.exchange(true) )
      { } 	// acquire exclusivity
    last->next = tmp;		 // publish to consumers
    last = tmp;		 		// swing last forward
    producerLock = false; 		// release exclusivity

First, we want to do as much work as possible outside the critical section of code that actually updates the queue. In this case, we can do all of the allocation and construction of the new node and its value concurrently with any number of other producers and consumers.

Second, we "commit" the change by getting exclusive access to the tail of the queue. The while loop keeps trying to set the producerLock to true until the old value was false because while the old value was true, it means someone else already has exclusivity. The way to read this while loop is, "until I get to be the one to change producerLock from false to true," which means that this thread has acquired exclusivity. Then we can update last->next and last itself, which are two separate writes and cannot be done as a single atomic operation on most processors without some sort of lock. Finally, we release exclusivity on the tail of the queue by setting producerLock to false.


Likewise, we want to support any number of threads calling Consume, and let them run as concurrently as possible. First, we get exclusivity, this time on the head end of the queue:

  bool Consume( T& result ) {
    while( consumerLock.exchange(true) ) 
      { }	 // acquire exclusivity

Next, we read the head node's next pointer. If it's not null, we need to take out the first value but we want to do as little work as possible here inside the exclusive critical section:

    Node* theFirst = first;
    Node* theNext = first-> next;
    if( theNext != nullptr ) { 	 // if queue is nonempty
      T* val = theNext->value;	 // take it out
      theNext->value = nullptr;  // of the Node
      first = theNext;		 	// swing first forward
      consumerLock = false; 	       	// release exclusivity

Now we're done touching the list, and other consumers can make progress while we do the remaining copying and cleanup work off to the side:

      result = *val;  	// now copy it back
      delete val;  		// clean up the value
      delete theFirst;  	// and the old dummy
      return true;	 	// and report success

Otherwise, if theNext was null, the list was empty and we can immediately release exclusion and return that status:

    consumerLock = false;   // release exclusivity
    return false; 	               // report queue was empty

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