[PATCH v3] drm/atomic: protect bridge private_obj during bridge removal

[Luca Ceresoli]

This series prevents a race between DRM bridge removal and usage of the
bridge private_obj during DRM_MODESET_LOCK_ALL_BEGIN/END() and other
locking operations.

== Series description

The need for this series emerged during testing of DRM bridge
hot-plugging. Very rarely on hot-unplug the following warning has appeared:

  WARNING: CPU: 0 PID: 123 at include/drm/drm_modeset_lock.h:114 drm_atomic_private_obj_fini+0x64/0x80
  ...
  Call trace:
   drm_atomic_private_obj_fini+0x64/0x80
   drm_bridge_detach+0x38/0x98

This series does not depend on other series.

== Grand plan

This is part of the work to support hotplug of DRM bridges. The grand plan
was discussed in [0].

Here's the work breakdown (➜ marks the current series):

 1. … add refcounting to DRM bridges struct drm_bridge,
      based on devm_drm_bridge_alloc()
    A. ✔ add new alloc API and refcounting (v6.16)
    B. ✔ convert all bridge drivers to new API (v6.17)
    C. ✔ kunit tests (v6.17)
    D. ✔ add get/put to drm_bridge_add/remove() + attach/detach()
         and warn on old allocation pattern (v6.17)
    E. … add get/put on drm_bridge accessors
       1. ✔ drm_bridge_chain_get_first_bridge(), add cleanup action (v6.18)
       2. ✔ drm_bridge_get_prev_bridge() (v6.18)
       3. ✔ drm_bridge_get_next_bridge() (v6.19)
       4. ✔ drm_for_each_bridge_in_chain() (v6.19)
       5. ✔ drm_bridge_connector_init (v6.19)
       6. … protect encoder bridge chain with a mutex
       7. … of_drm_find_bridge
          a. ✔ add of_drm_get_bridge() (v7.0),
	       convert basic direct users (v7.0-v7.1)
	  b. ✔ convert direct of_drm_get_bridge() users, part 2 (v7.0)
	  c. ✔ convert direct of_drm_get_bridge() users, part 3 (v7.0)
	  d. ✔… convert direct of_drm_get_bridge() users, part 4
	        (some v7.1, some pending)
	  e.   convert bridge-only drm_of_find_panel_or_bridge() users
       8. drm_of_find_panel_or_bridge, *_of_get_bridge
       9. ✔ enforce drm_bridge_add before drm_bridge_attach (v6.19)
    F. ✔ debugfs improvements
       1. ✔ add top-level 'bridges' file (v6.16)
       2. ✔ show refcount and list lingering bridges (v6.19)
 2. ➜ handle gracefully atomic updates during bridge removal
    A. ✔ Add drm_bridge_enter/exit() to protect device resources (v7.0)
    B. ➜ protect private_obj removal from list
    C. ✔ Add drm_bridge_clear_and_put() (v7.1)
 3. … DSI host-device driver interaction
 4. ✔ removing the need for the "always-disconnected" connector
 5. … Migrate i.MX LCDIF driver to bridge-connector
 6.   DRM bridge hotplug
    A.   Bridge hotplug management in the DRM core
    B.   Device tree description

[0] https://lore.kernel.org/lkml/20250206-hotplug-drm-bridge-v6-0-9d6f2c9c3058@xxxxxxxxxxx/#t

Signed-off-by: Luca Ceresoli <luca.ceresoli@xxxxxxxxxxx>
---
Changes in v3:
- Rebased on current drm-misc-next (on 7.0-rc3)
- Small commit message clarification
- Link to v2: https://lore.kernel.org/r/20251021-drm-bridge-atomic-vs-remove-private_obj-v2-1-412a18399bac@xxxxxxxxxxx

Changes in v2:
- Adapted to work on top of "drm/atomic: Add dev pointer to drm_private_obj"
- Removed 'To: jessica.zhang@xxxxxxxxxxxxxxxx', invalid address
- Link to v1: https://lore.kernel.org/r/20251013-drm-bridge-atomic-vs-remove-private_obj-v1-0-1fc2e58102e0@xxxxxxxxxxx

---
Luca Ceresoli (1):
      drm/atomic: drm_atomic_private_obj_fini: protect private_obj removal from list

 drivers/gpu/drm/drm_atomic.c | 6 ++++++
 1 file changed, 6 insertions(+)
---
base-commit: 7ea0468380216c10b73633b976d33efa8c12d375
change-id: 20251013-drm-bridge-atomic-vs-remove-private_obj-d792805bebdc

Best regards,
--  
Luca Ceresoli <luca.ceresoli@xxxxxxxxxxx>