[lockdep] drm/i915: possible circular locking dependency in i915 driver init

[Masami Hiramatsu <mhiramat@kernel.org>]

Hello,

I've found a suspicious circular locking dependency in i915 by lockdep.
It seems main driver initialization thread and sub fbdev configuration
thread take locks in different order implicitly. Please check it.

The lockdep report is here.

[    4.254984] ======================================================
[    4.254984] [ INFO: possible circular locking dependency detected ]
[    4.254985] 4.8.0-rc1+ #8 Not tainted
[    4.254985] -------------------------------------------------------
[    4.254986] kworker/u16:1/72 is trying to acquire lock:
[    4.255001]  (&dev->mode_config.mutex){+.+.+.}, at: [<ffffffffc02eafb0>] drm_modeset_lock_all+0x40/0x130 [drm]
[    4.255001] 
               but task is already holding lock:
[    4.255005]  ((fb_notifier_list).rwsem){++++.+}, at: [<ffffffff980b2ea5>] __blocking_notifier_call_chain+0x35/0x70
[    4.255005] 
               which lock already depends on the new lock.

[    4.255005] 
               the existing dependency chain (in reverse order) is:
[    4.255006] 
               -> #1 ((fb_notifier_list).rwsem){++++.+}:
[    4.255009]        [<ffffffff980e7a00>] lock_acquire+0x100/0x1f0
[    4.255011]        [<ffffffff988b29a9>] down_write+0x49/0x80
[    4.255013]        [<ffffffff980b3031>] blocking_notifier_chain_register+0x21/0xb0
[    4.255015]        [<ffffffff984c93c8>] fb_register_client+0x18/0x20
[    4.255016]        [<ffffffff984c8c1e>] backlight_device_register+0x12e/0x250
[    4.255046]        [<ffffffffc04e3842>] intel_backlight_device_register+0xa2/0x160 [i915]
[    4.255070]        [<ffffffffc04b173e>] intel_connector_register+0xe/0x10 [i915]
[    4.255093]        [<ffffffffc04cd5db>] intel_dp_connector_register+0x1b/0x80 [i915]
[    4.255102]        [<ffffffffc02d943a>] drm_connector_register+0x4a/0x80 [drm]
[    4.255110]        [<ffffffffc02ddbe6>] drm_modeset_register_all+0x1c6/0x260 [drm]
[    4.255116]        [<ffffffffc02d4ba2>] drm_dev_register+0xc2/0xd0 [drm]
[    4.255135]        [<ffffffffc04212f1>] i915_driver_load+0x771/0x1460 [i915]
[    4.255153]        [<ffffffffc042c63f>] i915_pci_probe+0x4f/0x70 [i915]
[    4.255155]        [<ffffffff98497905>] local_pci_probe+0x45/0xa0
[    4.255157]        [<ffffffff98498d11>] pci_device_probe+0xe1/0x130
[    4.255159]        [<ffffffff985ae5e4>] driver_probe_device+0x224/0x430
[    4.255160]        [<ffffffff985ae8d3>] __driver_attach+0xe3/0xf0
[    4.255160]        [<ffffffff985ac166>] bus_for_each_dev+0x66/0xa0
[    4.255161]        [<ffffffff985adcee>] driver_attach+0x1e/0x20
[    4.255162]        [<ffffffff985ad7d0>] bus_add_driver+0x200/0x270
[    4.255164]        [<ffffffff985af520>] driver_register+0x60/0xe0
[    4.255165]        [<ffffffff98497230>] __pci_register_driver+0x60/0x70
[    4.255182]        [<ffffffffc035f05b>] i915_init+0x5b/0x62 [i915]
[    4.255183]        [<ffffffff9800217d>] do_one_initcall+0x3d/0x160
[    4.255186]        [<ffffffff981c6d8d>] do_init_module+0x5f/0x1f8
[    4.255187]        [<ffffffff9812fbdc>] load_module+0x246c/0x2bd0
[    4.255188]        [<ffffffff9813057c>] SYSC_finit_module+0xbc/0xf0
[    4.255190]        [<ffffffff981305ce>] SyS_finit_module+0xe/0x10
[    4.255191]        [<ffffffff988b5680>] entry_SYSCALL_64_fastpath+0x23/0xc1
[    4.255192] 
               -> #0 (&dev->mode_config.mutex){+.+.+.}:
[    4.255194]        [<ffffffff980e732c>] __lock_acquire+0x125c/0x14b0
[    4.255195]        [<ffffffff980e7a00>] lock_acquire+0x100/0x1f0
[    4.255197]        [<ffffffff988b0889>] mutex_lock_nested+0x69/0x3b0
[    4.255206]        [<ffffffffc02eafb0>] drm_modeset_lock_all+0x40/0x130 [drm]
[    4.255212]        [<ffffffffc03f877b>] drm_fb_helper_restore_fbdev_mode_unlocked+0x2b/0x80 [drm_kms_helper]
[    4.255217]        [<ffffffffc03f87fd>] drm_fb_helper_set_par+0x2d/0x50 [drm_kms_helper]
[    4.255240]        [<ffffffffc04c21da>] intel_fbdev_set_par+0x1a/0x60 [i915]
[    4.255242]        [<ffffffff984c2f2f>] fbcon_init+0x57f/0x600
[    4.255244]        [<ffffffff98555026>] visual_init+0xd6/0x130
[    4.255245]        [<ffffffff98556831>] do_bind_con_driver+0x1c1/0x3a0
[    4.255246]        [<ffffffff98556d16>] do_take_over_console+0x116/0x180
[    4.255247]        [<ffffffff984beb67>] do_fbcon_takeover+0x57/0xb0
[    4.255249]        [<ffffffff984c39a8>] fbcon_event_notify+0x658/0x750
[    4.255250]        [<ffffffff980b2c1a>] notifier_call_chain+0x4a/0x70
[    4.255251]        [<ffffffff980b2ebd>] __blocking_notifier_call_chain+0x4d/0x70
[    4.255253]        [<ffffffff980b2ef6>] blocking_notifier_call_chain+0x16/0x20
[    4.255254]        [<ffffffff984c940b>] fb_notifier_call_chain+0x1b/0x20
[    4.255255]        [<ffffffff984cb6c1>] register_framebuffer+0x251/0x330
[    4.255259]        [<ffffffffc03f8a7f>] drm_fb_helper_initial_config+0x25f/0x3f0 [drm_kms_helper]
[    4.255282]        [<ffffffffc04c3498>] intel_fbdev_initial_config+0x18/0x30 [i915]
[    4.255283]        [<ffffffff980b4aa7>] async_run_entry_fn+0x37/0x150
[    4.255284]        [<ffffffff980aa9c8>] process_one_work+0x1e8/0x710
[    4.255285]        [<ffffffff980aaf3b>] worker_thread+0x4b/0x4f0
[    4.255286]        [<ffffffff980b193f>] kthread+0xef/0x110
[    4.255287]        [<ffffffff988b58cf>] ret_from_fork+0x1f/0x40
[    4.255287] 
               other info that might help us debug this:

[    4.255288]  Possible unsafe locking scenario:

[    4.255288]        CPU0                    CPU1
[    4.255288]        ----                    ----
[    4.255289]   lock((fb_notifier_list).rwsem);
[    4.255290]                                lock(&dev->mode_config.mutex);
[    4.255290]                                lock((fb_notifier_list).rwsem);
[    4.255291]   lock(&dev->mode_config.mutex);
[    4.255291] 
                *** DEADLOCK ***

And i915 related kconfigs are here:
----
CONFIG_DRM_I915=m
CONFIG_DRM_I915_PRELIMINARY_HW_SUPPORT=y
CONFIG_DRM_I915_USERPTR=y
CONFIG_DRM_I915_GVT=y
# CONFIG_DRM_I915_WERROR is not set
# CONFIG_DRM_I915_DEBUG is not set
----

I've hit this on my DELL XPS15 which has skylake CPU/GPU.

Thanks,


-- 
Masami Hiramatsu <mhiramat@kernel.org>