drm/i915: Remove struct_mutex for destroying framebuffers

We do not need to hold struct_mutex for destroying drm_i915_gem_objects
any longer, and with a little care taken over tracking
obj->framebuffer_references, we can relinquish BKL locking around the
destroy of intel_framebuffer.

v2: Use atomic check for WARN_ON framebuffer miscounting

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/20170216094621.3426-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

drivers/gpu/drm/i915/i915_gem_object.h

@@ -165,7 +165,7 @@ struct drm_i915_gem_object {
 	struct reservation_object *resv;
 
 	/** References from framebuffers, locks out tiling changes. */
-	unsigned long framebuffer_references;
+	atomic_t framebuffer_references;
 
 	/** Record of address bit 17 of each page at last unbind. */
 	unsigned long *bit_17;

drivers/gpu/drm/i915/i915_gem_shrinker.c

@@ -207,7 +207,7 @@ i915_gem_shrink(struct drm_i915_private *dev_priv,
 
 			if (!(flags & I915_SHRINK_ACTIVE) &&
 			    (i915_gem_object_is_active(obj) ||
-			     obj->framebuffer_references))
+			     atomic_read(&obj->framebuffer_references)))
 				continue;
 
 			if (!can_release_pages(obj))

drivers/gpu/drm/i915/i915_gem_tiling.c

@@ -238,7 +238,7 @@ i915_gem_object_set_tiling(struct drm_i915_gem_object *obj,
 	if ((tiling | stride) == obj->tiling_and_stride)
 		return 0;
 
-	if (obj->framebuffer_references)
+	if (atomic_read(&obj->framebuffer_references))
 		return -EBUSY;
 
 	/* We need to rebind the object if its current allocation

drivers/gpu/drm/i915/intel_display.c

@@ -14265,14 +14265,13 @@ static void intel_setup_outputs(struct drm_i915_private *dev_priv)
 
 static void intel_user_framebuffer_destroy(struct drm_framebuffer *fb)
 {
-	struct drm_device *dev = fb->dev;
 	struct intel_framebuffer *intel_fb = to_intel_framebuffer(fb);
 
 	drm_framebuffer_cleanup(fb);
-	mutex_lock(&dev->struct_mutex);
-	WARN_ON(!intel_fb->obj->framebuffer_references--);
+
+	WARN_ON(atomic_dec_return(&intel_fb->obj->framebuffer_references) < 0);
 	i915_gem_object_put(intel_fb->obj);
-	mutex_unlock(&dev->struct_mutex);
+
 	kfree(intel_fb);
 }
 
@@ -14509,7 +14508,7 @@ static int intel_framebuffer_init(struct drm_device *dev,
 		return ret;
 	}
 
-	intel_fb->obj->framebuffer_references++;
+	atomic_inc(&intel_fb->obj->framebuffer_references);
 
 	return 0;
 }