drm/i915: On fb alloc failure, unref gem object where it gets refed

Currently when allocating a framebuffer fails, the gem object gets
unrefed at the bottom of the call stack in __intel_framebuffer_create,
not where it gets refed, which is in intel_framebuffer_create_for_mode
(via i915_gem_alloc_object) and in intel_user_framebuffer_create
(via drm_gem_object_lookup).

This invites mistakes: __intel_framebuffer_create is also called from
intelfb_alloc, and as discovered by Tvrtko Ursulin, a double unref
was introduced there with a8bb6818 ("drm/i915: Fix error path leak
in fbdev fb allocation").

As suggested by Ville Syrjälä, fix the double unref and improve code
clarity by moving the unref away from __intel_framebuffer_create to
where the gem object gets refed.

Based on Tvrtko Ursulin's original v2.

v3: On fb alloc failure, unref gem object where it gets refed,
    fix double unref in separate commit (Ville Syrjälä)

v4: Lock struct_mutex on unref (Chris Wilson)

v5: Rebase on drm-intel-nightly 2015y-09m-01d-09h-06m-08s UTC,
    rephrase commit message (Jani Nicula)
Tested-by: NPierre Moreau <pierre.morrow@free.fr>
    [MBP  5,3 2009  nvidia MCP79 + G96        pre-retina]
Tested-by: NPaul Hordiienko <pvt.gord@gmail.com>
    [MBP  6,2 2010  intel ILK + nvidia GT216  pre-retina]
Tested-by: NWilliam Brown <william@blackhats.net.au>
    [MBP  8,2 2011  intel SNB + amd turks     pre-retina]
Tested-by: NLukas Wunner <lukas@wunner.de>
    [MBP  9,1 2012  intel IVB + nvidia GK107  pre-retina]
Tested-by: NBruno Bierbaumer <bruno@bierbaumer.net>
    [MBP 11,3 2013  intel HSW + nvidia GK107  retina]

Fixes: a8bb6818 ("drm/i915: Fix error path leak in fbdev fb
    allocation")
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NLukas Wunner <lukas@wunner.de>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/2161c5062ef5d6458f8ae14d924a26d4d1dba317.1446892879.git.lukas@wunner.deSigned-off-by: NJani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_display.c

@@ -10065,20 +10065,17 @@ __intel_framebuffer_create(struct drm_device *dev,
 	int ret;
 
 	intel_fb = kzalloc(sizeof(*intel_fb), GFP_KERNEL);
-	if (!intel_fb) {
-		drm_gem_object_unreference(&obj->base);
+	if (!intel_fb)
 		return ERR_PTR(-ENOMEM);
-	}
 
 	ret = intel_framebuffer_init(dev, intel_fb, mode_cmd, obj);
 	if (ret)
 		goto err;
 
 	return &intel_fb->base;
+
 err:
-	drm_gem_object_unreference(&obj->base);
 	kfree(intel_fb);
-
 	return ERR_PTR(ret);
 }
 
@@ -10118,6 +10115,7 @@ intel_framebuffer_create_for_mode(struct drm_device *dev,
 				  struct drm_display_mode *mode,
 				  int depth, int bpp)
 {
+	struct drm_framebuffer *fb;
 	struct drm_i915_gem_object *obj;
 	struct drm_mode_fb_cmd2 mode_cmd = { 0 };
 
@@ -10132,7 +10130,11 @@ intel_framebuffer_create_for_mode(struct drm_device *dev,
 								bpp);
 	mode_cmd.pixel_format = drm_mode_legacy_fb_format(bpp, depth);
 
-	return intel_framebuffer_create(dev, &mode_cmd, obj);
+	fb = intel_framebuffer_create(dev, &mode_cmd, obj);
+	if (IS_ERR(fb))
+		drm_gem_object_unreference_unlocked(&obj->base);
+
+	return fb;
 }
 
 static struct drm_framebuffer *
@@ -14530,6 +14532,7 @@ intel_user_framebuffer_create(struct drm_device *dev,
 			      struct drm_file *filp,
 			      struct drm_mode_fb_cmd2 *mode_cmd)
 {
+	struct drm_framebuffer *fb;
 	struct drm_i915_gem_object *obj;
 
 	obj = to_intel_bo(drm_gem_object_lookup(dev, filp,
@@ -14537,7 +14540,11 @@ intel_user_framebuffer_create(struct drm_device *dev,
 	if (&obj->base == NULL)
 		return ERR_PTR(-ENOENT);
 
-	return intel_framebuffer_create(dev, mode_cmd, obj);
+	fb = intel_framebuffer_create(dev, mode_cmd, obj);
+	if (IS_ERR(fb))
+		drm_gem_object_unreference_unlocked(&obj->base);
+
+	return fb;
 }
 
 #ifndef CONFIG_DRM_FBDEV_EMULATION