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

[Daniel Vetter]

On Tue, Nov 15, 2022 at 10:30:01AM +0100, Andrzej Hajda wrote:
> On 13.07.2021 15:59, 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>
> 
> Reviewed-by: Andrzej Hajda <andrzej.hajda at intel.com>

I just dropped this one from my patch pile a while ago, because there were
conflicts. If you like it, feel free to resurrect&rebase and then merge it
(but maybe cc intel-gfx so the CI there can test it).
-Daniel

> 
> Regards
> Andrzej
> 
> > ---
> >   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);
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch