Currently; we're grabbing all of the modesetting locks before adding MST connectors to fbdev. This isn't actually necessary, and causes a deadlock as well:

====================================================== WARNING: possible circular locking dependency detected 4.17.0-rc3Lyude-Test+ #1 Tainted: G O ------------------------------------------------------ kworker/1:0/18 is trying to acquire lock: 00000000c832f62d (&helper->lock){+.+.}, at: drm_fb_helper_add_one_connector+0x2a/0x60 [drm_kms_helper]

but task is already holding lock: 00000000942e28e2 (crtc_ww_class_mutex){+.+.}, at: drm_modeset_backoff+0x8e/0x1c0 [drm]

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #3 (crtc_ww_class_mutex){+.+.}: ww_mutex_lock+0x43/0x80 drm_modeset_lock+0x71/0x130 [drm] drm_helper_probe_single_connector_modes+0x7d/0x6b0 [drm_kms_helper] drm_setup_crtcs+0x15e/0xc90 [drm_kms_helper] __drm_fb_helper_initial_config_and_unlock+0x29/0x480 [drm_kms_helper] nouveau_fbcon_init+0x138/0x1a0 [nouveau] nouveau_drm_load+0x173/0x7e0 [nouveau] drm_dev_register+0x134/0x1c0 [drm] drm_get_pci_dev+0x8e/0x160 [drm] nouveau_drm_probe+0x1a9/0x230 [nouveau] pci_device_probe+0xcd/0x150 driver_probe_device+0x30b/0x480 __driver_attach+0xbc/0xe0 bus_for_each_dev+0x67/0x90 bus_add_driver+0x164/0x260 driver_register+0x57/0xc0 do_one_initcall+0x4d/0x323 do_init_module+0x5b/0x1f8 load_module+0x20e5/0x2ac0 __do_sys_finit_module+0xb7/0xd0 do_syscall_64+0x60/0x1b0 entry_SYSCALL_64_after_hwframe+0x49/0xbe

-> #2 (crtc_ww_class_acquire){+.+.}: drm_helper_probe_single_connector_modes+0x58/0x6b0 [drm_kms_helper] drm_setup_crtcs+0x15e/0xc90 [drm_kms_helper] __drm_fb_helper_initial_config_and_unlock+0x29/0x480 [drm_kms_helper] nouveau_fbcon_init+0x138/0x1a0 [nouveau] nouveau_drm_load+0x173/0x7e0 [nouveau] drm_dev_register+0x134/0x1c0 [drm] drm_get_pci_dev+0x8e/0x160 [drm] nouveau_drm_probe+0x1a9/0x230 [nouveau] pci_device_probe+0xcd/0x150 driver_probe_device+0x30b/0x480 __driver_attach+0xbc/0xe0 bus_for_each_dev+0x67/0x90 bus_add_driver+0x164/0x260 driver_register+0x57/0xc0 do_one_initcall+0x4d/0x323 do_init_module+0x5b/0x1f8 load_module+0x20e5/0x2ac0 __do_sys_finit_module+0xb7/0xd0 do_syscall_64+0x60/0x1b0 entry_SYSCALL_64_after_hwframe+0x49/0xbe

-> #1 (&dev->mode_config.mutex){+.+.}: drm_setup_crtcs+0x10c/0xc90 [drm_kms_helper] __drm_fb_helper_initial_config_and_unlock+0x29/0x480 [drm_kms_helper] nouveau_fbcon_init+0x138/0x1a0 [nouveau] nouveau_drm_load+0x173/0x7e0 [nouveau] drm_dev_register+0x134/0x1c0 [drm] drm_get_pci_dev+0x8e/0x160 [drm] nouveau_drm_probe+0x1a9/0x230 [nouveau] pci_device_probe+0xcd/0x150 driver_probe_device+0x30b/0x480 __driver_attach+0xbc/0xe0 bus_for_each_dev+0x67/0x90 bus_add_driver+0x164/0x260 driver_register+0x57/0xc0 do_one_initcall+0x4d/0x323 do_init_module+0x5b/0x1f8 load_module+0x20e5/0x2ac0 __do_sys_finit_module+0xb7/0xd0 do_syscall_64+0x60/0x1b0 entry_SYSCALL_64_after_hwframe+0x49/0xbe

-> #0 (&helper->lock){+.+.}: __mutex_lock+0x70/0x9d0 drm_fb_helper_add_one_connector+0x2a/0x60 [drm_kms_helper] nv50_mstm_register_connector+0x2c/0x50 [nouveau] drm_dp_add_port+0x2f5/0x420 [drm_kms_helper] drm_dp_send_link_address+0x155/0x1e0 [drm_kms_helper] drm_dp_add_port+0x33f/0x420 [drm_kms_helper] drm_dp_send_link_address+0x155/0x1e0 [drm_kms_helper] drm_dp_check_and_send_link_address+0x87/0xd0 [drm_kms_helper] drm_dp_mst_link_probe_work+0x4d/0x80 [drm_kms_helper] process_one_work+0x20d/0x650 worker_thread+0x3a/0x390 kthread+0x11e/0x140 ret_from_fork+0x3a/0x50

other info that might help us debug this: Chain exists of: &helper->lock --> crtc_ww_class_acquire --> crtc_ww_class_mutex Possible unsafe locking scenario: CPU0 CPU1 ---- ---- lock(crtc_ww_class_mutex); lock(crtc_ww_class_acquire); lock(crtc_ww_class_mutex); lock(&helper->lock);

*** DEADLOCK *** 5 locks held by kworker/1:0/18: #0: 000000004a05cd50 ((wq_completion)"events_long"){+.+.}, at: process_one_work+0x187/0x650 #1: 00000000601c11d1 ((work_completion)(&mgr->work)){+.+.}, at: process_one_work+0x187/0x650 #2: 00000000586ca0df (&dev->mode_config.mutex){+.+.}, at: drm_modeset_lock_all+0x3a/0x1b0 [drm] #3: 00000000d3ca0ffa (crtc_ww_class_acquire){+.+.}, at: drm_modeset_lock_all+0x44/0x1b0 [drm] #4: 00000000942e28e2 (crtc_ww_class_mutex){+.+.}, at: drm_modeset_backoff+0x8e/0x1c0 [drm]

stack backtrace: CPU: 1 PID: 18 Comm: kworker/1:0 Tainted: G O 4.17.0-rc3Lyude-Test+ #1 Hardware name: Gateway FX6840/FX6840, BIOS P01-A3 05/17/2010 Workqueue: events_long drm_dp_mst_link_probe_work [drm_kms_helper] Call Trace: dump_stack+0x85/0xcb print_circular_bug.isra.38+0x1ce/0x1db __lock_acquire+0x128f/0x1350 ? lock_acquire+0x9f/0x200 ? lock_acquire+0x9f/0x200 ? __ww_mutex_lock.constprop.13+0x8f/0x1000 lock_acquire+0x9f/0x200 ? drm_fb_helper_add_one_connector+0x2a/0x60 [drm_kms_helper] ? drm_fb_helper_add_one_connector+0x2a/0x60 [drm_kms_helper] __mutex_lock+0x70/0x9d0 ? drm_fb_helper_add_one_connector+0x2a/0x60 [drm_kms_helper] ? ww_mutex_lock+0x43/0x80 ? _cond_resched+0x15/0x30 ? ww_mutex_lock+0x43/0x80 ? drm_modeset_lock+0xb2/0x130 [drm] ? drm_fb_helper_add_one_connector+0x2a/0x60 [drm_kms_helper] drm_fb_helper_add_one_connector+0x2a/0x60 [drm_kms_helper] nv50_mstm_register_connector+0x2c/0x50 [nouveau] drm_dp_add_port+0x2f5/0x420 [drm_kms_helper] ? mark_held_locks+0x50/0x80 ? kfree+0xcf/0x2a0 ? drm_dp_check_mstb_guid+0xd6/0x120 [drm_kms_helper] ? trace_hardirqs_on_caller+0xed/0x180 ? drm_dp_check_mstb_guid+0xd6/0x120 [drm_kms_helper] drm_dp_send_link_address+0x155/0x1e0 [drm_kms_helper] drm_dp_add_port+0x33f/0x420 [drm_kms_helper] ? nouveau_connector_aux_xfer+0x7c/0xb0 [nouveau] ? find_held_lock+0x2d/0x90 ? drm_dp_dpcd_access+0xd9/0xf0 [drm_kms_helper] ? __mutex_unlock_slowpath+0x3b/0x280 ? drm_dp_dpcd_access+0xd9/0xf0 [drm_kms_helper] drm_dp_send_link_address+0x155/0x1e0 [drm_kms_helper] drm_dp_check_and_send_link_address+0x87/0xd0 [drm_kms_helper] drm_dp_mst_link_probe_work+0x4d/0x80 [drm_kms_helper] process_one_work+0x20d/0x650 worker_thread+0x3a/0x390 ? process_one_work+0x650/0x650 kthread+0x11e/0x140 ? kthread_create_worker_on_cpu+0x50/0x50 ret_from_fork+0x3a/0x50

Taking example from i915, the only time we need to hold any modesetting locks is when changing the port on the mstc, and in that case we only need to hold the connection mutex.

Signed-off-by: Lyude Paul lyude@redhat.com Cc: Karol Herbst kherbst@redhat.com Cc: stable@vger.kernel.org

Signed-off-by: Lyude Paul lyude@redhat.com --- drivers/gpu/drm/nouveau/nv50_display.c | 7 +++---- 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/nouveau/nv50_display.c b/drivers/gpu/drm/nouveau/nv50_display.c index 8bd739cfd00d..2b3ccd850750 100644 --- a/drivers/gpu/drm/nouveau/nv50_display.c +++ b/drivers/gpu/drm/nouveau/nv50_display.c @@ -3264,10 +3264,11 @@ nv50_mstm_destroy_connector(struct drm_dp_mst_topology_mgr *mgr,

drm_connector_unregister(&mstc->connector);

- drm_modeset_lock_all(drm->dev); drm_fb_helper_remove_one_connector(&drm->fbcon->helper, &mstc->connector); + + drm_modeset_lock(&drm->dev->mode_config.connection_mutex, NULL); mstc->port = NULL; - drm_modeset_unlock_all(drm->dev); + drm_modeset_unlock(&drm->dev->mode_config.connection_mutex);

drm_connector_unreference(&mstc->connector); } @@ -3277,9 +3278,7 @@ nv50_mstm_register_connector(struct drm_connector *connector) { struct nouveau_drm *drm = nouveau_drm(connector->dev);

- drm_modeset_lock_all(drm->dev); drm_fb_helper_add_one_connector(&drm->fbcon->helper, connector); - drm_modeset_unlock_all(drm->dev);

drm_connector_register(connector); }