On Tue, Jul 12 2016, Lars Ellenberg wrote:
....

Instead, I suggest to distinguish between recursive calls to
generic_make_request(), and pushing back the remainder part in
blk_queue_split(), by pointing current->bio_lists to a
	struct recursion_to_iteration_bio_lists {
		struct bio_list recursion;
		struct bio_list queue;
	}

By providing each q->make_request_fn() with an empty "recursion"
bio_list, then merging any recursively submitted bios to the
head of the "queue" list, we can make the recursion-to-iteration
logic in generic_make_request() process deepest level bios first,
and "sibling" bios of the same level in "natural" order.

Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Roland Kammerer <redacted>

Reviewed-by: NeilBrown <redacted>

Thanks again for doing this - I think this is a very significant
improvement and could allow other simplifications.

NeilBrown