dispatch_reentrant

gistfile1.m

// checks if already in current queue, prevents deadlock
void dispatch_sync_reentrant(dispatch_queue_t queue, dispatch_block_t block) {
	if (dispatch_get_current_queue() == queue) {
		block();
	}else {
		dispatch_sync(queue, block);
	}
}

// problem:

void not_really_reentrant() {
	dispatch_queue_t queueA = ...;
	dispatch_queue_t queueB = ...;
	dispatch_sync_reentrant(queueA, ^{
		dispatch_sync_reentrant(queueB, ^{
			dispatch_sync_reentrant(queueA, ^{
				// never reached
			});
		});
	});
}