Re: [PATCH] nvme-core: Fix subsystem instance mismatches

[Christoph Hellwig]

On Tue, Sep 03, 2019 at 10:46:20AM -0600, Keith Busch wrote:
> Could we possibly make /dev/nvmeX be a subsystem handle without causing
> trouble for anyone? This would essentially be the same thing as today
> for non-CMIC controllers with a device-per-controller and only affects
> the CMIC ones.

A per-subsyste character device doesn't make sense, as a lot of admin
command require a specific controller.

If this really is an isue for people we'll just need to refcount the
handle allocation.  That is:

 - nvme_init_ctrl allocates a new nvme_instance or so object, which
   does the ida_simple_get.
 - we allocate a new subsystem that reuses the handle and grabs
   a reference in nvme_init_subsystem, then if we find an existing
   subsystem we drop that reference again.
 - last free of a ctrl or subsystem also drops a reference, with
   the final free releasing the ida