drm/mm: Use clear_bit_unlock() for releasing the drm_mm_node()

A few callers need to serialise the destruction of their drm_mm_node and
ensure it is removed from the drm_mm before freeing. However, to be
completely sure that any access from another thread is complete before
we free the struct, we require the RELEASE semantics of
clear_bit_unlock().

This allows the conditional locking such as

Thread A			Thread B
  mutex_lock(mm_lock);		  if (drm_mm_node_allocated(node)) {
  drm_mm_node_remove(node);	    mutex_lock(mm_lock);
  mutex_unlock(mm_lock);	    if (drm_mm_node_allocated(node))
				      drm_mm_node_remove(node);
				    mutex_unlock(mm_lock);
				  }
				  kfree(node);

to serialise correctly without any lingering accesses from A to the
freed node. Allocation / insertion of the node is assumed never to race
with removal or eviction scanning.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191003210100.22250-5-chris@chris-wilson.co.uk

drivers/gpu/drm/drm_mm.c

@@ -424,9 +424,9 @@ int drm_mm_reserve_node(struct drm_mm *mm, struct drm_mm_node *node)
 
 	node->mm = mm;
 
+	__set_bit(DRM_MM_NODE_ALLOCATED_BIT, &node->flags);
 	list_add(&node->node_list, &hole->node_list);
 	drm_mm_interval_tree_add_node(hole, node);
-	__set_bit(DRM_MM_NODE_ALLOCATED_BIT, &node->flags);
 	node->hole_size = 0;
 
 	rm_hole(hole);
@@ -543,9 +543,9 @@ int drm_mm_insert_node_in_range(struct drm_mm * const mm,
 		node->color = color;
 		node->hole_size = 0;
 
+		__set_bit(DRM_MM_NODE_ALLOCATED_BIT, &node->flags);
 		list_add(&node->node_list, &hole->node_list);
 		drm_mm_interval_tree_add_node(hole, node);
-		__set_bit(DRM_MM_NODE_ALLOCATED_BIT, &node->flags);
 
 		rm_hole(hole);
 		if (adj_start > hole_start)
@@ -589,11 +589,12 @@ void drm_mm_remove_node(struct drm_mm_node *node)
 
 	drm_mm_interval_tree_remove(node, &mm->interval_tree);
 	list_del(&node->node_list);
-	__clear_bit(DRM_MM_NODE_ALLOCATED_BIT, &node->flags);
 
 	if (drm_mm_hole_follows(prev_node))
 		rm_hole(prev_node);
 	add_hole(prev_node);
+
+	clear_bit_unlock(DRM_MM_NODE_ALLOCATED_BIT, &node->flags);
 }
 EXPORT_SYMBOL(drm_mm_remove_node);
 
@@ -614,6 +615,7 @@ void drm_mm_replace_node(struct drm_mm_node *old, struct drm_mm_node *new)
 
 	*new = *old;
 
+	__set_bit(DRM_MM_NODE_ALLOCATED_BIT, &new->flags);
 	list_replace(&old->node_list, &new->node_list);
 	rb_replace_node_cached(&old->rb, &new->rb, &mm->interval_tree);
 
@@ -627,8 +629,7 @@ void drm_mm_replace_node(struct drm_mm_node *old, struct drm_mm_node *new)
 				&mm->holes_addr);
 	}
 
-	__clear_bit(DRM_MM_NODE_ALLOCATED_BIT, &old->flags);
-	__set_bit(DRM_MM_NODE_ALLOCATED_BIT, &new->flags);
+	clear_bit_unlock(DRM_MM_NODE_ALLOCATED_BIT, &old->flags);
 }
 EXPORT_SYMBOL(drm_mm_replace_node);