Stack from kworker:

    [  240.608612] INFO: task kworker/0:2:861 blocked for more than 120 seconds.
    [  240.608617]       Not tainted 3.13.0-17-generic #37-Ubuntu
    [  240.608618] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
    [  240.608620] kworker/0:2     D ffff88001e414440     0   861      2 0x00000000
    [  240.608628] Workqueue: events hot_add_req [hv_balloon]
    [  240.608630]  ffff88001a00fb30 0000000000000002 ffff88001a6f8000 ffff88001a00ffd8
    [  240.608632]  0000000000014440 0000000000014440 ffff88001a6f8000 ffff88001aac6c98
    [  240.608635]  ffff88001aac6c9c ffff88001a6f8000 00000000ffffffff ffff88001aac6ca0
    [  240.608637] Call Trace:
    [  240.608643]  [<ffffffff817159f9>] schedule_preempt_disabled+0x29/0x70
    [  240.608645]  [<ffffffff81717865>] __mutex_lock_slowpath+0x135/0x1b0
    [  240.608647]  [<ffffffff817178ff>] mutex_lock+0x1f/0x2f
    [  240.608651]  [<ffffffff8148a5bd>] device_attach+0x1d/0xa0
    [  240.608653]  [<ffffffff81489a38>] bus_probe_device+0x98/0xc0
    [  240.608656]  [<ffffffff81487895>] device_add+0x4c5/0x640
    [  240.608658]  [<ffffffff81487a2a>] device_register+0x1a/0x20
    [  240.608661]  [<ffffffff8149e000>] init_memory_block+0xd0/0xf0
    [  240.608663]  [<ffffffff8149e141>] register_new_memory+0x91/0xa0
    [  240.608666]  [<ffffffff81700d10>] __add_pages+0x140/0x240
    [  240.608670]  [<ffffffff81055649>] arch_add_memory+0x59/0xd0
    [  240.608672]  [<ffffffff81700fe4>] add_memory+0xe4/0x1f0
    [  240.608675]  [<ffffffffa00411cf>] hot_add_req+0x31f/0x1150 [hv_balloon]
    [  240.608679]  [<ffffffff810824a2>] process_one_work+0x182/0x450
    [  240.608681]  [<ffffffff81083241>] worker_thread+0x121/0x410
    [  240.608683]  [<ffffffff81083120>] ? rescuer_thread+0x3e0/0x3e0
    [  240.608686]  [<ffffffff81089ed2>] kthread+0xd2/0xf0
    [  240.608688]  [<ffffffff81089e00>] ? kthread_create_on_node+0x190/0x190
    [  240.608691]  [<ffffffff817219bc>] ret_from_fork+0x7c/0xb0
    [  240.608693]  [<ffffffff81089e00>] ? kthread_create_on_node+0x190/0x190

kworker looks to be blocked on the device lock in device_attach:

    int device_attach(struct device *dev)
    {
	int ret = 0;

	device_lock(dev);
	[...]
    }

If we follow the call trace we take mem_hotplug_mutex in add_memory():

    int __ref add_memory(int nid, u64 start, u64 size)
    {
	[...]
        lock_memory_hotplug();
    }

We later call device_add which triggers the udev event for this block:

    int device_add(struct device *dev)
    {
        kobject_uevent(&dev->kobj, KOBJ_ADD);
    [...]
    }

Finally, after emitting this event and and while holding that we call
device_attach() above, nesting the device_lock(dev) inside the memory
hotplug lock.


Stack from systemd-udevd:

    [  240.608705] INFO: task systemd-udevd:1906 blocked for more than 120 seconds.
    [  240.608706]       Not tainted 3.13.0-17-generic #37-Ubuntu
    [  240.608707] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
    [  240.608708] systemd-udevd   D ffff88001e414440     0  1906    404 0x00000004
    [  240.608710]  ffff88001a97bd20 0000000000000002 ffff8800170e0000 ffff88001a97bfd8
    [  240.608712]  0000000000014440 0000000000014440 ffff8800170e0000 ffffffff81c620e0
    [  240.608714]  ffffffff81c620e4 ffff8800170e0000 00000000ffffffff ffffffff81c620e8
    [  240.608716] Call Trace:
    [  240.608719]  [<ffffffff817159f9>] schedule_preempt_disabled+0x29/0x70
    [  240.608721]  [<ffffffff81717865>] __mutex_lock_slowpath+0x135/0x1b0
    [  240.608725]  [<ffffffff8115a8ae>] ? lru_cache_add+0xe/0x10
    [  240.608727]  [<ffffffff817178ff>] mutex_lock+0x1f/0x2f
    [  240.608729]  [<ffffffff817019c3>] online_pages+0x33/0x570
    [  240.608731]  [<ffffffff8149dd98>] memory_subsys_online+0x68/0xd0
    [  240.608733]  [<ffffffff814881e5>] device_online+0x65/0x90
    [  240.608735]  [<ffffffff8149da24>] store_mem_state+0x64/0x160
    [  240.608738]  [<ffffffff81485748>] dev_attr_store+0x18/0x30
    [  240.608742]  [<ffffffff8122e698>] sysfs_write_file+0x128/0x1c0
    [  240.608745]  [<ffffffff811b88c4>] vfs_write+0xb4/0x1f0
    [  240.608747]  [<ffffffff811b92f9>] SyS_write+0x49/0xa0
    [  240.608749]  [<ffffffff81721c7f>] tracesys+0xe1/0xe6

udevd seems to be blocked on the hotplug lock:

    int __ref online_pages(unsigned long pfn, unsigned long nr_pages, int online_type)
    {
    [...]
	lock_memory_hotplug();
    [...]
	mutex_lock(&zonelists_mutex);
    [...]
    }

Note that udevd would have taken the device lock in device_online():

    int device_online(struct device *dev)
    {
        int ret = 0;

        device_lock(dev);
    [...]
    }

And while holding this we call online_pages() as above, nesting the memory
hotplug lock inside the device_lock(dev).

This looks to be an ABBA deadlock, assuming dev is the same in these two
cases which seems plausible as we emit the udev event in the middle.