block/io: take bs->reqs_lock in bdrv_mark_request_serialising

block/io: take bs->reqs_lock in bdrv_mark_request_serialising

bdrv_mark_request_serialising is writing the overlap_offset and
overlap_bytes fields of BdrvTrackedRequest.  Take bs->reqs_lock
for the whole duration of it, and not just when waiting for
serialising requests, so that tracked_request_overlaps does not
look at a half-updated request.

The new code does not unlock/relock around retries.  This is unnecessary
because a retry is always preceded by a CoQueue wait, which already
releases and reacquires bs->reqs_lock.

Reported-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1578495356-46219-4-git-send-email-pbonzini@redhat.com
Message-Id: <1578495356-46219-4-git-send-email-pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>