[PATCH 3.19 020/101] drm: Fixup racy refcounting in plane_force_disable

Greg Kroah-Hartman
2015-04-17T13:35:53Z

From: Daniel Vetter <daniel.vetter@ffwll.ch>

commit 8218c3f4df3bb1c637c17552405039a6dd3c1ee1 upstream.

Originally it was impossible to be dropping the last refcount in this
function since there was always one around still from the idr. But in

commit 83f45fc360c8e16a330474860ebda872d1384c8c
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date: Wed Aug 6 09:10:18 2014 +0200

drm: Don't grab an fb reference for the idr

we've switched to weak references, broke that assumption but forgot to
fix it up.

Since we still force-disable planes it's only possible to hit this
when racing multiple rmfb with fbdev restoring or similar evil things.
As long as userspace is nice it's impossible to hit the BUG_ON.

But the BUG_ON would most likely be hit from fbdev code, which usually
invovles the console_lock besides all modeset locks. So very likely
we'd never get the bug reports if this was hit in the wild, hence
better be safe than sorry and backport.

Spotted by Matt Roper while reviewing other patches.

[airlied: pull this back into 4.0 - the oops happens there]

Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/gpu/drm/drm_crtc.c | 13 +------------
1 file changed, 1 insertion(+), 12 deletions(-)

--- a/drivers/gpu/drm/drm_crtc.c
+++ b/drivers/gpu/drm/drm_crtc.c
@@ -531,17 +531,6 @@ void drm_framebuffer_reference(struct dr
}
EXPORT_SYMBOL(drm_framebuffer_reference);

-static void drm_framebuffer_free_bug(struct kref *kref)
-{
- BUG();
-}
-
-static void __drm_framebuffer_unreference(struct drm_framebuffer *fb)
-{
- DRM_DEBUG("%p: FB ID: %d (%d)\n", fb, fb->base.id, atomic_read(&fb->refcount.refcount));
- kref_put(&fb->refcount, drm_framebuffer_free_bug);
-}
-
/**
* drm_framebuffer_unregister_private - unregister a private fb from the lookup idr
* @fb: fb to unregister
@@ -1322,7 +1311,7 @@ void drm_plane_force_disable(struct drm_
return;
}
/* disconnect the plane from the fb and crtc: */
- __drm_framebuffer_unreference(plane->old_fb);
+ drm_framebuffer_unreference(plane->old_fb);
plane->old_fb = NULL;
plane->fb = NULL;
plane->crtc = NULL;