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SubjectRe: 3.9+ tty lockdep trace.
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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