drm/i915: Drop the deferred active reference

An old optimisation to reduce the number of atomics per batch sadly
relies on struct_mutex for coordination. In order to remove struct_mutex
from serialising object/context closing, always taking and releasing an
active reference on first use / last use greatly simplifies the locking.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190528092956.14910-15-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_vma.c

@@ -112,10 +112,7 @@ static void __i915_vma_retire(struct i915_active *ref)
 	 */
 	obj_bump_mru(obj);
 
-	if (i915_gem_object_has_active_reference(obj)) {
-		i915_gem_object_clear_active_reference(obj);
-		i915_gem_object_put(obj);
-	}
+	i915_gem_object_put(obj); /* and drop the active reference */
 }
 
 static struct i915_vma *
@@ -443,7 +440,7 @@ void i915_vma_unpin_and_release(struct i915_vma **p_vma, unsigned int flags)
 	if (flags & I915_VMA_RELEASE_MAP)
 		i915_gem_object_unpin_map(obj);
 
-	__i915_gem_object_release_unless_active(obj);
+	i915_gem_object_put(obj);
 }
 
 bool i915_vma_misplaced(const struct i915_vma *vma,
@@ -933,12 +930,12 @@ int i915_vma_move_to_active(struct i915_vma *vma,
 	 * add the active reference first and queue for it to be dropped
 	 * *last*.
 	 */
-	if (!vma->active.count)
-		obj->active_count++;
+	if (!vma->active.count && !obj->active_count++)
+		i915_gem_object_get(obj); /* once more for the active ref */
 
 	if (unlikely(i915_active_ref(&vma->active, rq->fence.context, rq))) {
-		if (!vma->active.count)
-			obj->active_count--;
+		if (!vma->active.count && !--obj->active_count)
+			i915_gem_object_put(obj);
 		return -ENOMEM;
 	}