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Subject: Re: [PATCH v2 3/3] nvme: fire discovery log page change events to
 userspace
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Christoph Hellwig <hch@lst.de>
Cc:     linux-nvme@lists.infradead.org,
        Keith Busch <keith.busch@intel.com>,
        James Smart <james.smart@broadcom.com>,
        linux-kernel@vger.kernel.org
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 <20190826065639.GA11036@lst.de> <20190826075916.GA30396@kroah.com>
From:   Sagi Grimberg <sagi@grimberg.me>
Message-ID: <ac168168-fed2-2b57-493e-e88261ead73b@grimberg.me>
Date:   Thu, 29 Aug 2019 11:21:02 -0700
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>>>> You are correct that this information can be derived from sysfs, but the
>>>> main reason why we add these here, is because in udev rule we can't
>>>> just go ahead and start looking these up and parsing these..
>>>>
>>>> We could send the discovery aen with NVME_CTRL_NAME and have
>>>> then have systemd run something like:
>>>>
>>>> nvme connect-all -d nvme0 --sysfs
>>>>
>>>> and have nvme-cli retrieve all this stuff from sysfs?
>>>
>>> Actually that may be a problem.
>>>
>>> There could be a hypothetical case where after the event was fired
>>> and before it was handled, the discovery controller went away and
>>> came back again with a different controller instance, and the old
>>> instance is now a different discovery controller.
>>>
>>> This is why we need this information in the event. And we verify this
>>> information in sysfs in nvme-cli.
>>
>> Well, that must be a usual issue with uevents, right?  Don't we usually
>> have a increasing serial number for that or something?
> 
> Yes we do, userspace should use it to order events.  Does udev not
> handle that properly today?

The problem is not ordering of events, its really about the fact that
the chardev can be removed and reallocated for a different controller
(could be a completely different discovery controller) by the time
that userspace handles the event.