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.
>
> 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.