fix race condition

src/parallel.cc

 	bool running;
 	std::function<void(void)> func;
 	std::condition_variable cond;
-	std::mutex mut;
 
 	std::thread thread;
 	Worker *next_worker;
 static int waiting = 0;
 
 static void workerFunc(Worker *w) {
-	std::unique_lock<std::mutex> lock(w->mut);
 	while (true) {
+		std::unique_lock<std::mutex> lock(workers_mut);
 		w->cond.wait(lock, [w] { return (bool)w->func || !w->running; });
 		if (!w->running)
 			break;
 
+		lock.unlock();
 		w->func();
+		lock.lock();
 
 		w->func = nullptr;
-		std::unique_lock<std::mutex> workers_lock(workers_mut);
 		w->next_worker = head;
 		head = w;
-		workers_lock.unlock();
-		workers_cond.notify_one();
+
 		threads_running -= 1;
+		lock.unlock();
+		workers_cond.notify_one();
 	}
 }
 
 	Worker *w = head;
 	while (w) {
 		w->running = false;
+		lock.unlock();
 		w->cond.notify_one();
-		w = w->next_worker;
-	}
-
-	w = head;
-	while (w) {
 		w->thread.join();
+		lock.lock();
 		w = w->next_worker;
 	}
 
 	head = head->next_worker;
 	threads_running += 1;
 	waiting -= 1;
-	lock.unlock();
 
 	// Launch!
 	w->func = std::move(func);
+	lock.unlock();
 	w->cond.notify_one();
 }