[PATCH v3] nvme: core: shorten duration of multipath namespace rescan

[Christoph Hellwig]

> +	/*
> +	 * The controller queue is going to be frozen in
> +	 * nvme_update_ns_info_{generic,block}(). Every freeze implies waiting
> +	 * for an RCU grace period to pass. For multipath devices, we
> +	 * need to freeze the multipath queue, too. Start freezing the
> +	 * multipath queue now, lest we need to wait for two grace periods.
> +	 */
> +	if (nvme_ns_head_multipath(ns->head))
> +		blk_freeze_queue_start(ns->head->disk->queue);

I really don't like how we keep this state around the calls into
the command set specific nvme_update_ns_info_* calls, and how it
feels very open coded.

And while looking at it, I think we might even want the multipath
queue frozen over the actual update as well, as there is no point
in sending the I/O even to other paths while we are updating the
information, as they'll probably fail.

Here is what might make sense:

 1) move the code past the csi switch statement in nvme_update_ns_info
    into a new nvme_update_ns_info_common helper and all that
    from nvme_update_ns_info_block and nvme_update_ns_info_generic.
    Maybe having another helper just for the multipath updates might
    also be worthwhile.
 2) add new nvme_freeze_ns helpers that freeze the ns and nshead
    queues and use those in nvme_update_ns_info_block and
    nvme_update_ns_info_generic over the entire update
 3) update these new helpers to use the trick from this patch to
    only require a single grace period.