drm/i915: Improve lockdep tracking for obj->mm.lock

The shrinker may appear to recurse into obj->mm.lock as the shrinker may
be called from a direct reclaim path whilst handling get_pages. We
filter out recursing on the same obj->mm.lock by inspecting
obj->mm.pages, but we do want to take the lock on a second object in
order to reap their pages. lockdep spots the recursion on the same
lockclass and needs annotation to avoid a false positive. To keep the
two paths distinct, create an enum to indicate which subclass of
obj->mm.lock we are using. This removes the false positive and avoids
masking real bugs.

Suggested-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161101121134.27504-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

drivers/gpu/drm/i915/i915_drv.h

@@ -3287,7 +3287,13 @@ i915_gem_object_unpin_pages(struct drm_i915_gem_object *obj)
 	__i915_gem_object_unpin_pages(obj);
 }
 
-void __i915_gem_object_put_pages(struct drm_i915_gem_object *obj);
+enum i915_mm_subclass { /* lockdep subclass for obj->mm.lock */
+	I915_MM_NORMAL = 0,
+	I915_MM_SHRINKER
+};
+
+void __i915_gem_object_put_pages(struct drm_i915_gem_object *obj,
+				 enum i915_mm_subclass subclass);
 void __i915_gem_object_invalidate(struct drm_i915_gem_object *obj);
 
 enum i915_map_type {

drivers/gpu/drm/i915/i915_gem.c

@@ -491,7 +491,7 @@ i915_gem_object_attach_phys(struct drm_i915_gem_object *obj,
 	if (ret)
 		return ret;
 
-	__i915_gem_object_put_pages(obj);
+	__i915_gem_object_put_pages(obj, I915_MM_NORMAL);
 	if (obj->mm.pages)
 		return -EBUSY;
 
@@ -2181,7 +2181,8 @@ static void __i915_gem_object_reset_page_iter(struct drm_i915_gem_object *obj)
 		radix_tree_delete(&obj->mm.get_page.radix, iter.index);
 }
 
-void __i915_gem_object_put_pages(struct drm_i915_gem_object *obj)
+void __i915_gem_object_put_pages(struct drm_i915_gem_object *obj,
+				 enum i915_mm_subclass subclass)
 {
 	struct sg_table *pages;
 
@@ -2193,7 +2194,7 @@ void __i915_gem_object_put_pages(struct drm_i915_gem_object *obj)
 		return;
 
 	/* May be called by shrinker from within get_pages() (on another bo) */
-	mutex_lock_nested(&obj->mm.lock, SINGLE_DEPTH_NESTING);
+	mutex_lock_nested(&obj->mm.lock, subclass);
 	if (unlikely(atomic_read(&obj->mm.pages_pin_count)))
 		goto unlock;
 
@@ -4283,7 +4284,7 @@ static void __i915_gem_free_objects(struct drm_i915_private *i915,
 
 		if (WARN_ON(i915_gem_object_has_pinned_pages(obj)))
 			atomic_set(&obj->mm.pages_pin_count, 0);
-		__i915_gem_object_put_pages(obj);
+		__i915_gem_object_put_pages(obj, I915_MM_NORMAL);
 		GEM_BUG_ON(obj->mm.pages);
 
 		if (obj->base.import_attach)

drivers/gpu/drm/i915/i915_gem_shrinker.c

@@ -111,7 +111,7 @@ static bool can_release_pages(struct drm_i915_gem_object *obj)
 static bool unsafe_drop_pages(struct drm_i915_gem_object *obj)
 {
 	if (i915_gem_object_unbind(obj) == 0)
-		__i915_gem_object_put_pages(obj);
+		__i915_gem_object_put_pages(obj, I915_MM_SHRINKER);
 	return !READ_ONCE(obj->mm.pages);
 }
 
@@ -225,7 +225,7 @@ i915_gem_shrink(struct drm_i915_private *dev_priv,
 			if (unsafe_drop_pages(obj)) {
 				/* May arrive from get_pages on another bo */
 				mutex_lock_nested(&obj->mm.lock,
-						  SINGLE_DEPTH_NESTING);
+						  I915_MM_SHRINKER);
 				if (!obj->mm.pages) {
 					__i915_gem_object_invalidate(obj);
 					list_del_init(&obj->global_list);

drivers/gpu/drm/i915/i915_gem_userptr.c

@@ -75,7 +75,7 @@ static void cancel_userptr(struct work_struct *work)
 
 	/* We are inside a kthread context and can't be interrupted */
 	if (i915_gem_object_unbind(obj) == 0)
-		__i915_gem_object_put_pages(obj);
+		__i915_gem_object_put_pages(obj, I915_MM_NORMAL);
 	WARN_ONCE(obj->mm.pages,
 		  "Failed to release pages: bind_count=%d, pages_pin_count=%d, pin_display=%d\n",
 		  obj->bind_count,