Created attachment 110564 [details]
dmesg drm-intel-nightly 3.18.0-00343-gfc7bddc

Hardware:
- Lenovo ThinkPad T440s + ThinkPad Pro Dock 40A1
- 1. monitor (Dell U2410 1920x1200) at DP of docking station
- 2. monitor (Dell U2413 1920x1200) at VGA of docking station

If I boot this setup with both monitor attached to the console and remove the monitor (unplug the connector) at the DP, then I can reproduce the attached backtrace (not?) always.

This happens with v3.17.6, v3.18 and current drm-intel-nightly (fc7bddc drm-intel-nightly: 2014y-12m-08d-12h-45m-52s UTC integration manifest). The dmesg log has been captured with drm-intel-nightly.

[   18.947852] WARNING: CPU: 1 PID: 128 at include/linux/kref.h:47 drm_dp_mst_get_validated_mstb_ref_locked+0x92/0xa0 [drm_kms_helper]()                          
...
[   22.999859] [drm:drm_dp_mst_wait_tx_reply] timedout msg send ffff88030cd51800 2 1
[   23.002183] ======================================================
[   23.003160] [ INFO: possible circular locking dependency detected ]
[   23.004108] 3.18.0-00343-gfc7bddc #11 Tainted: G        W     
[   23.005041] -------------------------------------------------------
[   23.005941] kworker/u16:2/128 is trying to acquire lock:
[   23.006826]  ((&mgr->work)){+.+.+.}, at: [<ffffffff81065585>] flush_work+0x5/0x260
[   23.007667] 
but task is already holding lock:
[   23.009294]  (&dev->mode_config.mutex){+.+.+.}, at: [<ffffffffa03db365>] drm_fb_helper_hotplug_event+0x25/0xe0 [drm_kms_helper]
[   23.010166] 
which lock already depends on the new lock.

[   23.012646] 
the existing dependency chain (in reverse order) is:
[   23.014307] 
-> #1 (&dev->mode_config.mutex){+.+.+.}:
[   23.015951]        [<ffffffff8109b58c>] lock_acquire+0xac/0x140
...
other info that might help us debug this:

[   23.060273]  Possible unsafe locking scenario:

[   23.061790]        CPU0                    CPU1
[   23.062536]        ----                    ----
[   23.063323]   lock(&dev->mode_config.mutex);
[   23.064096]                                lock((&mgr->work));
[   23.064814]                                lock(&dev->mode_config.mutex);
[   23.065531]   lock((&mgr->work));
[   23.066242] 
 *** DEADLOCK ***

[   23.068373] 3 locks held by kworker/u16:2/128:
[   23.069075]  #0:  ("i915-dp"){.+.+.+}, at: [<ffffffff81065f51>] process_one_work+0x141/0x490
[   23.069823]  #1:  ((&dev_priv->dig_port_work)){+.+.+.}, at: [<ffffffff81065f51>] process_one_work+0x141/0x490
[   23.070561]  #2:  (&dev->mode_config.mutex){+.+.+.}, at: [<ffffffffa03db365>] drm_fb_helper_hotplug_event+0x25/0xe0 [drm_kms_helper]