Question

The following code has concurrency issues. Here inDegVec is a global int *, and getGUID() and getTime() are innocuous accessors for POD variables.

void operator () ( GNode& src,
                      tbb::parallel_do_feeder<GNode>& feed_it )
        const 
{

            std::list<GNode> addNodes;             

            // some thread-safe code that populates addNodes

            while (!addNodes.empty()) {
                feed_it.add(addNodes.front());
                addNodes.pop_front();
            } 

            if (inDegVec[srcAVI->getUID()] == 0) {
                // srcAVI is still the minimum among its neighbors
                if (srcAVI->getTime() < colTime) {
                    //lwl.push (src);
                    //tbb::mutex::scoped_lock lock(histMutex);
                    assert(addNodes.empty());
                    feed_it.add(src);
                }  
           } 


};

If I wrap the while/if blocks in a mutex, the code works. But why is this necessary? Is parallel_do_feeder::add() not thread-safe? The example code I’ve seen (such as http://llpanorama.wordpress.com/2008/03/09/parallel_do-parallel-done/) use no synchronization, suggesting that it is… and I don’t see a discussion of parallel_do_feeder’s thread safety in the TBB manual.

Answer 1

Yes, parallel_do_feeder is thread safe.

Is the copy constructor of GNode thread safe? The instance of GNode passed to parallel_do_feeder::add() is copied into a task that will process it (possibly in another thread).