static void replication_start(ReplicationState *rs, ReplicationMode mode,
                              Error **errp)
{
    BlockDriverState *bs = rs->opaque;
    BDRVReplicationState *s;
    BlockDriverState *top_bs;
    int64_t active_length, hidden_length, disk_length;
    AioContext *aio_context;
    Error *local_err = NULL;
    BlockJob *job;
    aio_context = bdrv_get_aio_context(bs);
    aio_context_acquire(aio_context);
    s = bs->opaque;
    if (s->stage != BLOCK_REPLICATION_NONE) {
        error_setg(errp, "Block replication is running or done");
        aio_context_release(aio_context);
        return;
    }
    if (s->mode != mode) {
        error_setg(errp, "The parameter mode's value is invalid, needs %d,"
                   " but got %d", s->mode, mode);
        aio_context_release(aio_context);
        return;
    }
    switch (s->mode) {
    case REPLICATION_MODE_PRIMARY:
        break;
    case REPLICATION_MODE_SECONDARY:
        s->active_disk = bs->file;
        if (!s->active_disk || !s->active_disk->bs ||
                                    !s->active_disk->bs->backing) {
            error_setg(errp, "Active disk doesn't have backing file");
            aio_context_release(aio_context);
            return;
        }
        s->hidden_disk = s->active_disk->bs->backing;
        if (!s->hidden_disk->bs || !s->hidden_disk->bs->backing) {
            error_setg(errp, "Hidden disk doesn't have backing file");
            aio_context_release(aio_context);
            return;
        }
        s->secondary_disk = s->hidden_disk->bs->backing;
        if (!s->secondary_disk->bs || !bdrv_has_blk(s->secondary_disk->bs)) {
            error_setg(errp, "The secondary disk doesn't have block backend");
            aio_context_release(aio_context);
            return;
        }
        /* verify the length */
        active_length = bdrv_getlength(s->active_disk->bs);
        hidden_length = bdrv_getlength(s->hidden_disk->bs);
        disk_length = bdrv_getlength(s->secondary_disk->bs);
        if (active_length < 0 || hidden_length < 0 || disk_length < 0 ||
            active_length != hidden_length || hidden_length != disk_length) {
            error_setg(errp, "Active disk, hidden disk, secondary disk's length"
                       " are not the same");
            aio_context_release(aio_context);
            return;
        }
        if (!s->active_disk->bs->drv->bdrv_make_empty ||
            !s->hidden_disk->bs->drv->bdrv_make_empty) {
            error_setg(errp,
                       "Active disk or hidden disk doesn't support make_empty");
            aio_context_release(aio_context);
            return;
        }
        /* reopen the backing file in r/w mode */
        reopen_backing_file(bs, true, &local_err);
        if (local_err) {
            error_propagate(errp, local_err);
            aio_context_release(aio_context);
            return;
        }
        /* start backup job now */
        error_setg(&s->blocker,
                   "Block device is in use by internal backup job");
        top_bs = bdrv_lookup_bs(s->top_id, s->top_id, NULL);
        if (!top_bs || !bdrv_is_root_node(top_bs) ||
            !check_top_bs(top_bs, bs)) {
            error_setg(errp, "No top_bs or it is invalid");
            reopen_backing_file(bs, false, NULL);
            aio_context_release(aio_context);
            return;
        }
        bdrv_op_block_all(top_bs, s->blocker);
        bdrv_op_unblock(top_bs, BLOCK_OP_TYPE_DATAPLANE, s->blocker);
        job = backup_job_create(NULL, s->secondary_disk->bs, s->hidden_disk->bs,
                                0, MIRROR_SYNC_MODE_NONE, NULL, false,
                                BLOCKDEV_ON_ERROR_REPORT,
                                BLOCKDEV_ON_ERROR_REPORT, BLOCK_JOB_INTERNAL,
                                backup_job_completed, bs, NULL, &local_err);
        if (local_err) {
            error_propagate(errp, local_err);
            backup_job_cleanup(bs);
            aio_context_release(aio_context);
            return;
        }
        block_job_start(job);
        break;
    default:
        aio_context_release(aio_context);
        abort();
    }
    s->stage = BLOCK_REPLICATION_RUNNING;
    if (s->mode == REPLICATION_MODE_SECONDARY) {
        secondary_do_checkpoint(s, errp);
    }
    s->error = 0;
    aio_context_release(aio_context);
}