[PATCH] drm/fb-helper: Try to protect cleanup against delayed setup

[Daniel Vetter <daniel.vetter@ffwll.ch>]

Some vague evidences suggests this can go wrong. Try to prevent it by
holding the right mutex and clearing ->deferred_setup to make sure we
later on don't accidentally try to re-register the fbdev when the
driver thought it had it all cleaned up already.

v2: I realized that this is fundamentally butchered, and CI complained
about lockdep splats. So limit the critical section again and just add
a few notes what the proper fix is.

References: https://intel-gfx-ci.01.org/tree/linux-next/next-20201215/fi-byt-j1900/igt@i915_pm_rpm@module-reload.html
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
---
 drivers/gpu/drm/drm_fb_helper.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 9d82fda274eb..8f11e5abb222 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -598,6 +598,9 @@ EXPORT_SYMBOL(drm_fb_helper_alloc_fbi);
  * A wrapper around unregister_framebuffer, to release the fb_info
  * framebuffer device. This must be called before releasing all resources for
  * @fb_helper by calling drm_fb_helper_fini().
+ *
+ * Note that this is fundamentally racy on hotunload because it doen't handle
+ * open fbdev file descriptors at all. Use drm_fbdev_generic_setup() instead.
  */
 void drm_fb_helper_unregister_fbi(struct drm_fb_helper *fb_helper)
 {
@@ -611,6 +614,9 @@ EXPORT_SYMBOL(drm_fb_helper_unregister_fbi);
  * @fb_helper: driver-allocated fbdev helper, can be NULL
  *
  * This cleans up all remaining resources associated with @fb_helper.
+ *
+ * Note that this is fundamentally racy on hotunload because it doen't handle
+ * open fbdev file descriptors at all. Use drm_fbdev_generic_setup() instead.
  */
 void drm_fb_helper_fini(struct drm_fb_helper *fb_helper)
 {
@@ -2382,6 +2388,10 @@ static void drm_fbdev_client_unregister(struct drm_client_dev *client)
 {
 	struct drm_fb_helper *fb_helper = drm_fb_helper_from_client(client);
 
+	mutex_lock(&fb_helper->lock);
+	fb_helper->deferred_setup = false;
+	mutex_unlock(&fb_helper->lock);
+
 	if (fb_helper->fbdev)
 		/* drm_fbdev_fb_destroy() takes care of cleanup */
 		drm_fb_helper_unregister_fbi(fb_helper);
-- 
2.32.0