Messages in this thread | | | Date | Wed, 08 May 2013 17:18:00 -0400 | From | Peter Hurley <> | Subject | Re: 3.9+ tty lockdep trace. |
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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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