Comment 10 for bug 1185394

Thanks for your investigations!

Stefan Bader [2013-05-29 13:42 -0000]:
> Booting in Xen mode there is
> one systemd-udevd crashing immediatly after calling udevadm control
> --exit and that ends the udevadm command (so it does not wait).

That indeed sounds like the most plausible cause for now, and in any
way udevd should not crash; let's fix the crash first, and then see
how much further this gets us.

Thanks for the core file! I can load it fine with current
202-0ubuntu11 version, and get the same bt as you. The actual crash
seems obvious:

  #0 udev_monitor_allow_unicast_sender udev_monitor=udev_monitor@entry=0x6e0110, sender=0x0)

with

  udev_monitor->snl_trusted_sender.nl.nl_pid = sender->snl.nl.nl_pid;

The caller does

        if (worker_monitor == NULL)
                return;
        /* allow the main daemon netlink address to send devices to the worker */
        udev_monitor_allow_unicast_sender(worker_monitor, monitor);

I. e. "monitor" is not checked for being NULL here; but it is (see
frame #0), thus causing the crash.

The global monitor gets initialized in main(), and after the
initialization there is a NULL check. However, further down in main()
it is unreffed and set to NULL if "udev_exit" becomes true, i. e. the
worker got a SIGTERM. As far as I can see, this is incompatible with
our 0024-avoid-exit-deadlock-for-dm_cookie.patch patch which keeps
processing events even after an exit has been requested; in that case,
we must not access monitor any more.