Upgrading boost::shared_lock to exclusive lock

Could someone please explain the correct usage for boost::upgrade_lock. The following code results in a deadlock

//Global
typedef boost::shared_mutex Mutex;
typedef boost::shared_lock<Mutex> ReadLock;
typedef boost::upgrade_lock<Mutex> UpgradeLock; 
typedef boost::upgrade_to_unique_lock<Mutex> WriteLock;
Mutex sharedMutex;


//Multi threaded reader and writer
{
    ReadLock read(sharedMutex);

    for (int ii = 0; ii < vec.size(); ++ii) {
        Element e = vec[ii];

        if (e.needsUpdating()) {
            UpgradeLock upgrade(sharedMutex);

            WriteLock write(upgrade)

            //Do stuff
        }
    }
}

It doesn't deadlock if i unlock the read lock with read.unlock() before upgrading. But it seems that this shouldn't be necessary?