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Date:   Wed, 4 Sep 2019 09:54:45 -0600
From:   Keith Busch <kbusch@kernel.org>
To:     Christoph Hellwig <hch@lst.de>
Cc:     Jens Axboe <axboe@fb.com>, Hannes Reinecke <hare@suse.com>,
        Sagi Grimberg <sagi@grimberg.me>,
        "Martin K . Petersen" <martin.petersen@oracle.com>,
        linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org,
        Keith Busch <keith.busch@intel.com>,
        Logan Gunthorpe <logang@deltatee.com>
Subject: Re: [PATCH] nvme-core: Fix subsystem instance mismatches
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On Wed, Sep 04, 2019 at 05:42:15PM +0200, Christoph Hellwig wrote:
> On Wed, Sep 04, 2019 at 08:44:27AM -0600, Keith Busch wrote:
> > Let me step through an example:
> > 
> >   Ctrl A gets instance 0.
> > 
> >   Its subsystem gets the same instance, and takes ref count on it:
> >   all namespaces in this subsystem will use '0'.
> > 
> >   Ctrl B gets instance 1, and it's in the same subsystem as Ctrl A so
> >   no new subsytem is allocated.
> > 
> >   Ctrl A is disconnected, dropping its ref on instance 0, but the
> >   subsystem still has its refcount, making it unavailable.
> > 
> >   Ctrl A is reconnected, and allocates instance 2 because 0 is still in
> >   use.
> > 
> > Now all the namespaces in this subsystem are prefixed with nvme0, but no
> > controller exists with the same prefix. We still have inevitable naming
> > mismatch, right?
> 
> I think th major confusion was that we can use the same handle for
> and unrelated subsystem vs controller, and that would avoid it.
>
> I don't see how we can avoid the controller is entirely different
> from namespace problem ever.

Can we just ensure there is never a matching controller then? This
patch will accomplish that and simpler than wrapping the instance in a
refcount'ed object:

http://lists.infradead.org/pipermail/linux-nvme/2019-May/024142.html