Unless you have left out critical code, you do not seem to be ensuring thread safety. You need a QWaitCondition and a QMutex. Check out the mandelbrot example.
Here is the relevant code for a safe construction of a worker thread.
Qt Code: 
  1. class JobThread : public QThread
  2. {
  3. public:
  4. 	JobThread(QObject *parent)
  5. 		: QThread(parent)
  6. 	 {
  7. 		abort = false;
  8. 		start(LowPriority);
  9. 	 }
  10.  
  11. 	 ~JobThread()
  12. 	 {
  13. 		mutex.lock();
  14. 		abort = true;
  15. 		condition.wakeOne();
  16. 		mutex.unlock();
  17. 		wait();
  18. 	 }
  19.  
  20. 	 void addJob(Job job)
  21. 	 {
  22. 		QMutexLocker locker(&mutex);
  23. 		this->jobs << job;
  24. 		condition.wakeOne();
  25. 	 }
  26.  
  27. 	 void run()
  28. 	 {
  29. 		 QMutexLocker lock(&mutex);
  30. 		 while(!abort) {
  31. 			 if (!jobs.isEmpty())
  32. 			 {
  33. 				 condition.wait(&mutex);
  34. 				 continue;
  35. 			 }
  36. 			 Job job = jobs.takeFirst();
  37. 			 lock.unlock();
  38.  
  39. 			 while (!abort) {
  40. 				 // do heavy computing
  41. 				 process(job);
  42. 				 emit progress(...);
  43. 			 }
  44. 			 lock.relock();
  45. 		 }
  46. 	 }
  47. private:
  48. 	mutable bool abort;
  49. 	QMutex mutex;
  50. 	QWaitCondition condition;
  51. 	QList<Job> jobs;
  52. };
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I would also consider using QtConcurrent.