+ /*
+ * Run the aio requests. As soon as one request can't be submitted
+ * successfully, fail all requests that are not yet submitted (we must
+ * return failure for all requests anyway)
+ *
+ * num_requests cannot be set to the right value immediately: If
+ * bdrv_aio_writev fails for some request, num_requests would be too high
+ * and therefore multiwrite_cb() would never recognize the multiwrite
+ * request as completed. We also cannot use the loop variable i to set it
+ * when the first request fails because the callback may already have been
+ * called for previously submitted requests. Thus, num_requests must be
+ * incremented for each request that is submitted.
+ *
+ * The problem that callbacks may be called early also means that we need
+ * to take care that num_requests doesn't become 0 before all requests are
+ * submitted - multiwrite_cb() would consider the multiwrite request
+ * completed. A dummy request that is "completed" by a manual call to
+ * multiwrite_cb() takes care of this.
+ */
+ mcb->num_requests = 1;
+