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

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Am 13.07.21 um 15:59 schrieb Daniel Vetter:
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@xxxxxxxxxxxxxxxxxx
Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
Cc: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
Cc: Maxime Ripard <mripard@xxxxxxxxxx>
Cc: Thomas Zimmermann <tzimmermann@xxxxxxx>
Cc: David Airlie <airlied@xxxxxxxx>
Cc: Daniel Vetter <daniel@xxxxxxxx>

Acked-by: Thomas Zimmermann <tzimmermann@xxxxxxx>

---
  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);


--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer

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