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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>
Cc:     Christoph Hellwig <hch@lst.de>, 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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 <20190830062036.GA15257@kroah.com>
From:   Sagi Grimberg <sagi@grimberg.me>
Message-ID: <73e3d2ca-33e0-3133-9dfb-62b07e5b09c4@grimberg.me>
Date:   Fri, 30 Aug 2019 11:14:39 -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.
> 
> So?  You will have gotten the remove and then new addition uevent in
> order showing you this.  So your userspace code knows that something
> went away and then came back properly so you should be kept in sync.

Still don't understand how this is ok...

I have /dev/nvme0 represents a network endpoint that I would discover
from, it is raising me an event to do a discovery operation (namely to
issue an ioctl to it) so my udev code calls a systemd script.

By the time I actually get to do that, /dev/nvme0 represents now a new
network endpoint (where the event is no longer relevant to). I would
rather the discovery to explicitly fail than to give me something
different, so we pass some arguments that we verify in the operation.

Its a stretch case, but it was raised by people as a potential issue.