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

[Thomas Zimmermann]

Hi

Am 17.02.23 um 20:47 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@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     | 6 ++++++
>   drivers/gpu/drm/drm_fbdev_generic.c | 5 +++++
>   2 files changed, 11 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> index 3e17261a12b6..2415a2c7ca44 100644
> --- a/drivers/gpu/drm/drm_fb_helper.c
> +++ b/drivers/gpu/drm/drm_fb_helper.c
> @@ -545,6 +545,9 @@ EXPORT_SYMBOL(drm_fb_helper_alloc_info);
>    * 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_info(struct drm_fb_helper *fb_helper)
>   {
> @@ -558,6 +561,9 @@ EXPORT_SYMBOL(drm_fb_helper_unregister_info);
>    * @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)
>   {
> diff --git a/drivers/gpu/drm/drm_fbdev_generic.c b/drivers/gpu/drm/drm_fbdev_generic.c
> index 365f80717fa1..1618109592ce 100644
> --- a/drivers/gpu/drm/drm_fbdev_generic.c
> +++ b/drivers/gpu/drm/drm_fbdev_generic.c
> @@ -347,7 +347,12 @@ 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);

The unregister code runs as part of the client cleanup.  And the client 
also goes through a number of helpers that handle display hotplug and/or 
restore, which are affected by ->deferred_setup.  But it's all mutually 
exclusive. AFAICT nothing runs parallel to the unregister code.  See the 
use of dev->clientlist_mutex in drm_client.c.  The patch is not 
necessary IMHO.

Best regards
Thomas

> +
>   	if (fb_helper->info) {
> +		/* drm_fbdev_fb_destroy() takes care of cleanup */
>   		drm_fb_helper_unregister_info(fb_helper);
>   	} else {
>   		drm_client_release(&fb_helper->client);

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