FAILED: patch "[PATCH] drm: Fixup racy refcounting in plane_force_disable" failed to apply to 3.19-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 3.19-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

>From 220dd2bc43e23f72fcdf6cc6ced3107c7023f123 Mon Sep 17 00:00:00 2001
From: Daniel Vetter <daniel.vetter@ffwll.ch>
Date: Fri, 27 Feb 2015 12:58:13 +0100
Subject: [PATCH] drm: Fixup racy refcounting in plane_force_disable

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.

Cc: stable@vger.kernel.org
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>

diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
index 62f485c952e8..c83e4db0adf7 100644
--- a/drivers/gpu/drm/drm_crtc.c
+++ b/drivers/gpu/drm/drm_crtc.c
@@ -524,17 +524,6 @@ void drm_framebuffer_reference(struct drm_framebuffer *fb)
 }
 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
@@ -1319,7 +1308,7 @@ void drm_plane_force_disable(struct drm_plane *plane)
 		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;