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

[Sam Ravnborg]

Hi Daniel,

On Tue, Jul 13, 2021 at 03:59:22PM +0200, Daniel Vetter wrote:
> 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 at intel.com>
> Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> Cc: Maxime Ripard <mripard at kernel.org>
> Cc: Thomas Zimmermann <tzimmermann at suse.de>
> Cc: David Airlie <airlied at linux.ie>
> Cc: Daniel Vetter <daniel at 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
s/doen't/doesn't/
> + * 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
s/doen't/doesn't/
> + * 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);

I could not find any better spot to clear deferred_setup - so I think
this is OK.

With the two spellign issues fixed:
Acked-by: Sam Ravnborg <sam at ravnborg.org>

No r-b as I an not too fluent in these code paths and all the locking.

	Sam