Re: [PATCH v2 3/3] nvme: fire discovery log page change events to userspace

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Aug 26, 2019 at 08:56:39AM +0200, Christoph Hellwig wrote:
> On Thu, Aug 22, 2019 at 12:10:23PM -0700, Sagi Grimberg wrote:
> >> 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?

thanks,

greg k-h