On 05/07/2013 02:00 PM, Dave Jones wrote:
On Tue, May 07, 2013 at 01:35:36PM -0400, Peter Hurley wrote:
> On 05/07/2013 11:04 AM, Dave Jones wrote:
> > On Tue, May 07, 2013 at 10:14:09AM -0400, Peter Hurley wrote:
> >
> > > >> Just hit this again, by switching tty's during bootup.
> > > >> The previous time was switching from X to console iirc, so that
seems to be
> > > >> the key to triggering this.
> > > >
> > > > Sorry for not getting back to this sooner, merge window fun...
> > > >
> > > > Anyway, Peter, could this be something that your ldisc patches are
> > > > triggering?
> > >
> > > Yes, although early analysis also suggests the lockdep splat is
possible
> > > regardless, just much less likely.
> > >
> > > I looked at this over the weekend, but I wasn't able to determine (in
> > > the limited time I had available) if the splat was valid; ie., if the
> > > cpu states could actually be concurrent and result in deadlock.
> > >
> > > I need to study the VT_DISALLOCATE ioctl in more detail which I plan
> > > to do this week.
> >
> > I don't know if it proves/disproves anything, but shortly before the dump
> > gets printed, everything is kind of locked up. I can switch tty's, but
> > the gettys on them are dead to the world until after the dump.
> >
> > ISTR a pause for a while when I hit this switching from X too.
>
> It would help to know the steps to reproduce. For example, you mention
> unresponsive gettys but the VT_DISALLOCATE ioctl doesn't work (and isn't
> designed to) on open ttys.
Hmm, dunno. I booted up, and tried to log in on tty1 (no X on that box), and
after
I entered my password everything just hung for a while. I flipped to tty2
to try and log in as root, but after changing tty, I just got a blinking cursor.
10 seconds or so later, lockdep spew.
Ok, Dave, thanks for the extra information; that makes sense now.
systemd optionally uses the VT_DISALLOCATE ioctl to free the VT. So when
getty on tty2 died because of some error, systemd exercises the VT_DISALLOCATE
ioctl which generates the lockdep splat.
I'm still trying to figure out if the states can actually overlap; they're
not supposed to, but the VT_DISALLOCATE ioctl is not particularly
defensive and I suspect that overlap is possible.
It's somewhat unfortunate that systemd chose to use a really antiquated
and fundamentally unsafe ioctl like VT_DISALLOCATE. Even when I fix this
lockdep splat, bad things may still be possible.
I'm certain that the lockdep warning has nothing to do with your
login delay; if you want to send me sysrq+t output when that happens again,
I'd be willing to look over it for the underlying problem.
Regards,
Peter Hurley
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