Update deliveries value atomically to avoid potential race condition

Author: seabaylea

tests/dispatch_read2.c

@@ -348,14 +348,15 @@ test_writes_reads_eagain(void) // rdar://problem/8333366
 
 	dispatch_queue_t q = dispatch_get_global_queue(0,0);
 	dispatch_group_t g = dispatch_group_create();
-	__block size_t siz_acc = 0, deliveries = 0;
+	volatile __block size_t deliveries = 0;
+	__block size_t siz_acc = 0;
 	__block void (^b)(dispatch_data_t, int);
 	b = Block_copy(^(dispatch_data_t data, int err) {
 		if (err) {
 			test_errno("dispatch_read", err, 0);
 			test_stop();
 		}
-		deliveries++;
+		OSAtomicIncrement32(&deliveries);
 		siz_acc += dispatch_data_get_size(data);
 		if (siz_acc < siz) {
 			dispatch_group_enter(g);