In the "Overlapping multiple requests" cases, the 3rd reqs (the break
point B) doesn't wait for the 2nd, and once resumed the I/O will just
continue.  This is because the 2nd is already waiting for the 1st, and
in wait_serialising_requests() there is:

    /* If the request is already (indirectly) waiting for us, or
     * will wait for us as soon as it wakes up, then just go on
     * (instead of producing a deadlock in the former case). */
    if (!req->waiting_for) {
        /* actually break */
        ...
    }

Consequently, the following "sleep 100; resume A" command races with the
completion of that request, and sometimes results in an unexpected
order of output:

> @@ -56,9 +56,9 @@
>  wrote XXX/XXX bytes at offset XXX
>  XXX bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
>  blkdebug: Resuming request 'B'
> +blkdebug: Resuming request 'A'
>  wrote XXX/XXX bytes at offset XXX
>  XXX bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
> -blkdebug: Resuming request 'A'
>  wrote XXX/XXX bytes at offset XXX
>  XXX bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
>  wrote XXX/XXX bytes at offset XXX

Filter out the "Resuming request" lines to make the output
deterministic.
Reported-by: NPatchew <no-reply@patchew.org>
Signed-off-by: NFam Zheng <famz@redhat.com>
Message-id: 20171113150026.4743-1-famz@redhat.com
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NMax Reitz <mreitz@redhat.com>