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

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Date:   Wed, 4 Sep 2019 17:42:15 +0200
From:   Christoph Hellwig <hch@....de>
To:     Keith Busch <kbusch@...nel.org>
Cc:     Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...com>,
        Hannes Reinecke <hare@...e.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        Keith Busch <keith.busch@...el.com>,
        Logan Gunthorpe <logang@...tatee.com>
Subject: Re: [PATCH] nvme-core: Fix subsystem instance mismatches

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.