Currently drm_bridge_detach() expects that the bridge private_obj is not
locked by a drm_modeset_acquire_ctx, and it warns in case that happens:

  drm_bridge_detach()
  -> drm_atomic_private_obj_fini()
     -> list_del(&obj->head) // removes priv_obj from
                             // dev->mode_config.privobj_list
     -> obj->funcs->atomic_destroy_state()
     -> drm_modeset_lock_fini(&obj->lock)
        -> WARN_ON(!list_empty(&lock->head)) // warn if priv_obj->lock
	                                     // is still in ctx->locked

The expectation is not respected when introducing bridge hot-plugging. In
such case the warning triggers if the bridge is being removed concurrently
to an operation that locks the private object using a
drm_modeset_acquire_ctx, such as in this execution scenario:

  CPU0:
  drm_mode_obj_get_properties_ioctl() // userspace request
  -> DRM_MODESET_LOCK_ALL_BEGIN()
  .  -> drm_for_each_privobj() // loop on dev->mode_config.privobj_list
  .     - lock the privobj mutex
  .	- add priv_obj->lock to ctx->locked
  .	  (list of locks to be released later)
  .
  .                         CPU1:
  .                         drm_bridge_detach() // bridge hot-unplug
  .		            -> WARN triggers!
  .
  -> DRM_MODESET_LOCK_ALL_END()
     -> for each lock in ctx->locked
	- remove priv_obj->lock from ctx->locked
        - unlock the privobj mutex

Prevent this potential deadlock by using DRM_MODESET_LOCK_ALL_BEGIN/END()
around the list removal in drm_atomic_private_obj_fini(). This ensures that
exactly one of these happens:

 * the concurrent code (e.g. drm_mode_obj_get_properties_ioctl()) acquires
   all the locks first, so it can execute fully and release the
   privobj->lock before drm_atomic_private_obj_fini() calls list_del() and
   before the WARN_ON()
 * drm_atomic_private_obj_fini() acquires all the locks first, so it
   removes its privobj->lock from the dev->mode_config.privobj_list; the
   concurrent code will run afterwards and not acquire that lock because it
   is not present anymore

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>

---

Changes in v3:
- Rebased on current drm-misc-next (on 7.0-rc3)
- Small commit message clarification

Changes in v2:
- added 'drm/atomic:' prefix to commit title
- Adapted to work on top of "drm/atomic: Add dev pointer to drm_private_obj"
- Slightly improved commit message
---
 drivers/gpu/drm/drm_atomic.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
index 41c57063f3b4..0f9c2528c9ee 100644
--- a/drivers/gpu/drm/drm_atomic.c
+++ b/drivers/gpu/drm/drm_atomic.c
@@ -962,7 +962,13 @@ EXPORT_SYMBOL(drm_atomic_private_obj_init);
 void
 drm_atomic_private_obj_fini(struct drm_private_obj *obj)
 {
+	struct drm_modeset_acquire_ctx ctx;
+	int ret = 0;
+
+	DRM_MODESET_LOCK_ALL_BEGIN(obj->dev, ctx, 0, ret);
 	list_del(&obj->head);
+	DRM_MODESET_LOCK_ALL_END(obj->dev, ctx, ret);
+
 	obj->funcs->atomic_destroy_state(obj, obj->state);
 	drm_modeset_lock_fini(&obj->lock);
 }

-- 
2.53.0