systemd-logind can get hung in cg_enumerate_tasks

While looking into some leftover processes after session end, I discovered that logind can get into a hung state. Here's roughly what I did (sorry that I don't have exact steps)

1. Log in to unity, log out
2. ssh in
3. loginctl
4. loginctl session-status <your closing session>
5. log in, log out
6. loginctl session-status <your new closing session>
7. log in
8. loginctl (systemd-logind is hung at this point)

bt of loginctl while it's hung showing that it's going to timeout waiting for a reply from logind over dbus

(gdb) bt
#0 0x00007ffff6e67fa0 in __poll_nocancel () at ../sysdeps/unix/syscall-template.S:81
#1 0x00007ffff757c4b8 in socket_do_iteration (transport=0x6183d0, flags=6, timeout_milliseconds=<optimised out>)
    at ../../dbus/dbus-transport-socket.c:1125
#2 0x00007ffff757b3ff in _dbus_transport_do_iteration (transport=0x6183d0, flags=1, flags@entry=6,
    timeout_milliseconds=timeout_milliseconds@entry=25000) at ../../dbus/dbus-transport.c:976
#3 0x00007ffff75659dc in _dbus_connection_do_iteration_unlocked (connection=connection@entry=0x618ad0,
    pending=pending@entry=0x618320, flags=flags@entry=6, timeout_milliseconds=timeout_milliseconds@entry=25000)
    at ../../dbus/dbus-connection.c:1234
#4 0x00007ffff7566389 in _dbus_connection_block_pending_call (pending=0x618320)
    at ../../dbus/dbus-connection.c:2415
#5 0x00007ffff757572a in dbus_pending_call_block (pending=<optimised out>) at ../../dbus/dbus-pending-call.c:748
#6 0x00007ffff756694d in dbus_connection_send_with_reply_and_block (connection=connection@entry=0x618ad0,
    message=message@entry=0x619000, timeout_milliseconds=timeout_milliseconds@entry=-1,
    error=error@entry=0x7fffffffd610) at ../../dbus/dbus-connection.c:3530
#7 0x000000000040ebf4 in bus_method_call_with_reply (bus=bus@entry=0x618ad0,
    destination=destination@entry=0x40eeb1 "org.freedesktop.login1",
    path=path@entry=0x40ee99 "/org/freedesktop/login1",
    interface=interface@entry=0x40f478 "org.freedesktop.login1.Manager",
    method=method@entry=0x40ef3a "ListSessions", return_reply=return_reply@entry=0x7fffffffd738,
    return_error=return_error@entry=0x0, first_arg_type=first_arg_type@entry=0)
    at ../src/shared/dbus-common.c:1375
#8 0x0000000000404426 in list_sessions (bus=0x618ad0, args=<optimised out>, n=<optimised out>)
    at ../src/login/loginctl.c:82
#9 0x0000000000403b09 in loginctl_main (error=0x7fffffffd890, argv=0x7fffffffd9c8, argc=<optimised out>,
    bus=0x618ad0) at ../src/login/loginctl.c:1543
#10 main (argc=<optimised out>, argv=0x7fffffffd9c8) at ../src/login/loginctl.c:1574

and here's the bt of systemd-logind

(gdb) bt full
#0 0x00007f75545d6fa0 in __poll_nocancel () at ../sysdeps/unix/syscall-template.S:81
No locals.
#1 0x00007f7554ada4b8 in socket_do_iteration (transport=0x2c87000, flags=7, timeout_milliseconds=<optimised out>) at ../../dbus/dbus-transport-socket.c:1125
        socket_transport = 0x2c87000
        poll_fd = {
          fd = 1041,
          events = 1,
          revents = 0
        }
        poll_res = <optimised out>
        poll_timeout = <optimised out>
#2 0x00007f7554ad93ff in _dbus_transport_do_iteration (transport=0x2c87000, flags=1, flags@entry=7, timeout_milliseconds=timeout_milliseconds@entry=-1) at ../../dbus/dbus-transport.c:976
No locals.
#3 0x00007f7554ac39dc in _dbus_connection_do_iteration_unlocked (connection=connection@entry=0x2c87470, pending=pending@entry=0x0, flags=flags@entry=7, timeout_milliseconds=timeout_milliseconds@entry=-1) at ../../dbus/dbus-connection.c:1234
No locals.
#4 0x00007f7554ac3b84 in _dbus_connection_flush_unlocked (connection=0x2c87470) at ../../dbus/dbus-connection.c:3574
No locals.
#5 0x00007f7554ac4331 in _dbus_connection_block_pending_call (pending=0x2c63930) at ../../dbus/dbus-connection.c:2382
        start_tv_sec = 140141908468009
        start_tv_usec = 46691440
        tv_sec = 0
        tv_usec = 140735977390056
        status = <optimised out>
        connection = 0x2c87470
        client_serial = <optimised out>
        timeout = <optimised out>
        timeout_milliseconds = <optimised out>
        elapsed_milliseconds = <optimised out>
#6 0x00007f7554ad372a in dbus_pending_call_block (pending=<optimised out>) at ../../dbus/dbus-pending-call.c:748
        __FUNCTION__ = "dbus_pending_call_block"
#7 0x00007f7554ac494d in dbus_connection_send_with_reply_and_block (connection=0x2c87470, message=0x2c86d60, timeout_milliseconds=-1, error=0x7fffa5f08030) at ../../dbus/dbus-connection.c:3530
        reply = <optimised out>
        pending = 0x2c63930
        __FUNCTION__ = "dbus_connection_send_with_reply_and_block"
#8 0x00007f7555125a85 in cgmanager_ping_sync () from /lib/x86_64-linux-gnu/libcgmanager.so.0
No symbol table info available.
#9 0x0000000000422285 in cgm_dbus_connect () at ../src/shared/cgmanager.c:68
        dbus_error = {
          name = 0x0,
          message = 0x0,
          dummy1 = 1,
          dummy2 = 0,
          dummy3 = 0,
          dummy4 = 0,
          dummy5 = 0,
          padding1 = 0x285
        }
        connection = <optimised out>
        __func__ = "cgm_dbus_connect"
#10 0x00000000004205fc in cg_enumerate_tasks (controller=0x425b4a "name=systemd", path=0x214e3b0 "/user/1000.user/c2.session", _f=_f@entry=0x7fffa5f08158) at ../src/shared/cgroup-util.c:117
        fs = 0x0
        f = <optimised out>
        r = <optimised out>
        value = 0x0
        template = 0x0
        fd = <optimised out>
        __PRETTY_FUNCTION__ = "cg_enumerate_tasks"
#11 0x00000000004207da in cg_is_empty (controller=<optimised out>, path=<optimised out>, ignore_self=false) at ../src/shared/cgroup-util.c:1090
        f = 0x0
        pid = 0
        self_pid = <optimised out>
        found = false
        r = <optimised out>
#12 0x0000000000420932 in cg_is_empty_recursive (controller=controller@entry=0x425b4a "name=systemd", path=0x214e3b0 "/user/1000.user/c2.session", ignore_self=ignore_self@entry=false) at ../src/shared/cgroup-util.c:1136
        d = 0x0
        fn = 0x0
        r = <optimised out>
        children = 0x7f7554ac3de5 <dbus_connection_unref+101>
        p1 = <optimised out>
        __PRETTY_FUNCTION__ = "cg_is_empty_recursive"
#13 0x000000000040fbb9 in session_check_gc (s=s@entry=0x2134f10, drop_not_started=drop_not_started@entry=true) at ../src/login/logind-session.c:976
        r = <optimised out>
        __PRETTY_FUNCTION__ = "session_check_gc"
#14 0x00000000004085e6 in manager_gc (m=m@entry=0x2067010, drop_not_started=drop_not_started@entry=true) at ../src/login/logind.c:1503
        seat = <optimised out>
        session = 0x2134f10
        user = <optimised out>
        i = 0xffffffffffffffff
        __PRETTY_FUNCTION__ = "manager_gc"
#15 0x00000000004093c5 in manager_run (m=m@entry=0x2067010) at ../src/login/logind.c:1728
        event = {
          events = 4294967295,
          data = {
            ptr = 0xffffffff,
            fd = -1,
            u32 = 4294967295,
            u64 = 4294967295
          }
        }
        n = <optimised out>
        msec = -1
        __PRETTY_FUNCTION__ = "manager_run"
        __func__ = "manager_run"
#16 0x00000000004061cc in main (argc=<optimised out>, argv=<optimised out>) at ../src/login/logind.c:1867
        m = 0x2067010
        r = 0
        __func__ = "main"

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: systemd-services 204-5ubuntu15
ProcVersionSignature: Ubuntu 3.13.0-22.44-generic 3.13.8
Uname: Linux 3.13.0-22-generic x86_64
ApportVersion: 2.14-0ubuntu1
Architecture: amd64
CurrentDesktop: Unity
Date: Thu Apr 3 12:04:11 2014
InstallationDate: Installed on 2012-10-07 (542 days ago)
InstallationMedia: Ubuntu 12.10 "Quantal Quetzal" - Beta amd64 (20121007)
SourcePackage: systemd
UpgradeStatus: Upgraded to trusty on 2013-05-07 (331 days ago)