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

[Sagi Grimberg]

> > > > 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.